CA2036864A1 - A self-reinforced silicon nitride ceramic of high fracture toughness and a method of preparing the same - Google Patents

Abstract

ABSTRACT

A process for preparing a self-reinforced silicon nitride ceramic body of high fracture toughness comprising hot-pressing a powder mixture containing silicon nitride, a densification aid such as sodium oxide, a conversion aid such as lanthanum oxide and a compound, such as gallium oxide, which enhances growth of .beta.-silicon nitride whiskers-under conditions such that densification and the in situ formation of .beta.-silicon nitride whiskers having a high aspect ratio occur. A
novel silicon nitride ceramic of high fracture toughness and high fracture strength is disclosed comprising a silicon nitride crystalline phase wherein at least 20 volume percent of the phase is in the form of whiskers having an average aspect ratio of at least 2.5; a glassy second phase containing the densification aid, the conversion aid, the compound which enhances growth of .beta.-silicon nitride whiskers, and an amount of silica; and not greater than 10 weight percent of the total weight 36,172D-F

as other phases. The glassy phase may also include a minor amount, e.g., up to 5.0 weight percent, based upon total weight of the ceramic, of aluminum nitride or boron nitride. The glassy phase optionally includes an amount of a secondary reinforcing material such as silicon carbide whiskers.

36,172D-F

Description

~ ~ 3 ~ 3~

A SELF-REINFORCED SILICON NITRIDE CERAMIC OF HIGH
FRACTURE TOUGHNESS AND A METHOD OF PREPARING THE SAME

This invention pertains to a silicon nitride (Si3N4) ceramic body and a process for preparing the ceramic body.
Silicon nitride ceramics are recognized for their excellent mechanical and physical properties, including good wear resistance, low coefficient of thermal expansion, good thermal shock resistance, high creep resistance and high electrical resistivity. In addition, silicon nitride ceramics are resistant to chemical attack, particularly to oxidation. ~ecause of these attributes, silicon nitride is useful in a variety of wear and high temperature applications, such as cutting tools and parts in pumps and engines.
Failure of silicon nitride ceramics is generally associated with brittleness and flaws. The object therefore is to prepare a silicon nitride ceramic with high fracture toughness (KIC~ and strength.
Fracture strength is directly proportional to the fracture toughness and inversely proportional to the square root of the flaw size. High fracture toughness combined with small flaw size is therefore highly 36,172D-F -1-203~

desirable. Monolithic silicon ni~ride, however, has a relatively low fracture toughness of about 5 MPa (m)~.
It would be very desirable to have a silicon nitride ceramic of high fracture toughness and high 5 fracture strength. Moreover, it would be highly desirable to have a process which would be reproducible, inexpensive9 and amenable to industrial scale-up for preparing such a tough and strong silicon nitride ceramic.
A first aspect of this invention is a process for preparing a self-reinforced silicon nitride ceramic body containing predominately ~-silicon nitride whiskers having a high average aspect ratio. The process comprices subjecting a powder mixture comprising:
(a) silicon nitride in an amount suffi-cient to provide a ceramic body;
(b) a densification aid, said densification aid being a non-oxide derivative of magnesium or a source of beryllium, calcium, strontium, barium, radium, lithium, sodium, potassium, rubidium, cesium, silicon, hafnium, tantalum, indium9 galium, zinc, titanium or francium, said source being present in an amount sufficient to promote densification of the powder;
(c) a conversion aid, said conversion aid being a non-oxide derivative of yttrium or a source of scandium, actinium, lanthanum, lithium, sodium9 potassium, rubidium, cesium or francium, said source being present in an amount suf~icient to promote the essentially 36,172D-F -2 % ~ 6 ~

complete conversion of the starting silicon nitride to ~-silicon nitride; and (d) at least one whisker growth enhancing compound in an amount su~ficient to promote the formation of ~~silicon nitride whiskers, said compound being a non-oxiue derivative of calcium, or a derivative o~ sodium. potassium, scandium9 titanium, vanadium. chromium9 manganese, iron, cobalt, nickel, copper. zinc, strontium, zirconium, niobium, barium and lanthanum, or mixtures thereof, or an oxide of of gallium, indium. hafnium, tantalum and boron: (b), (c) and (d) being derived from three different elements;
to conditions of temperature and pressure su~icient such that densi~ication and in situ formation of p-silicon nitride whiskers having a high average aspect ratio occur. In this manner a self-reinforced silicon nitride ceramic body having a fracture toughness greater than 6 MPa (m)-~f as measured by the Chevron notch technique described hereinbelow, is formed. For the purposes of the present invention a "high" average aspect ratio means an average aspect ratio of at least 2.5. The powder mixture optionally includes aluminum nitride or boron nitride in an amount of 0.01 to 5 weight percent, based upon total powder mixture weight. Any means may be used to apply pressure and temperature so long as sufficient densification and in situ whisker formation 3 occur. Application of pressure and temperature beneficially occurs by hot-pressing or hot isostatic pressing~ preferably by hot~pressing.

In a related aspect, the powder mixture further comprises a Palmqvist toughness enhancing amoun~ of at 36,172D-F -3_ 2 ~

least one preformed reinforcing material. The material is silicon carbide, titanlum carbide, boron car~ide, titanium diboride, aluminum oxide or zirconium oxide.
~he materials are in forms whiskers, fibers, particles and platelets~
In a second aspect, this invention is a silicon nitride ceramic body having a fracture toughness greater than 6 MPa (m)~, as measured by the Chevron notch technique described hereinbelow, comprising:
(a) a crystalline phase of ~-silicon nitride of which at least 20 volume percent, as measured by viewing one plane of the silicon nitride ceramic body by scanning electron microscopy, is in the form of whiskers having an average aspect ratio of at least 2.5; and (b) a glassy phase, in an amount not greater than 35 weight percent of the total weight, comprising a densification aid, e.g., a non-oxide derivative of magnesium, a conversion aid, e.g., a non-oxide derivative of yttrium, silica, and a beta-silicon nitride whisker growth enhancing compound, said compound being a non-oxide derivative of calcium9 a derivative of sodium, potassium, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, strontium, zirconium, niobium, barium or lanthanum, or an oxide of 3 gallium, indium, hafnium, tantalum and boron.
The glassy phase optionally contains an amount, e.g., up to 15.0 percent by weight of the glassy phase, of aluminum nitride or boron nitride. The densification aid and the conversion aid are the same as those detailed 36,172D-F _4_ above in the first aspect. As in the first aspect, the densi~ication aid, the converxion aid and the whisker growth enhancing compound are each based upon, or derived from, a dif~erent element.
In a related aspect J the glassy phase further comprises a Palmqvist toughness enhancing amount of at least one preformed reinforcing material. The material is ~-silicon nitride fibers or whiskers or silicon carbide, titanium carbide, boron carbide, titanium diboride, aluminum oxide or zirconium oxide in at least one form selected from the group consisting of whiskers, fiberq, particles and plate]ets.

In a third aspect, this invention is a cutting tool comprising the above-iden~ified silicon nitride ceramic body.
Unexpectedly, the silicon nitride ceramic body of this invention exhibits a significantly hi~her frac-ture toughness than the monolithic or whisker-reinforced silicon nitride ceramics of the prior art. Moreover, if the fracture toughness of the silicon nitride ceramic of this invention is normalized with respect to density, the normalized fracture toughness and fracture strength are among the highest known for any ceramic material.
Advantageously, the silicon nitride ceramlc of this invention is self-reinforced. More advantageously~ the process for preparing the novel, self-reinforced silicon nitride ceramic body of this invention is reproducible~
amenable to industrial scale-up, and less expensive than processes using silicon carbide whisker reinforcement.

36,172D~F _5_ The silicon nitride starting material used in preparing ceramic body of this invention can be an~
silicon nitride powder9 including the crystalline forms of ~-silicon nitride and ~-silicon nitride, or noncrystalline amorphous silicon nitride, or mixtures thereof. Preferably, the silicon nitride powder is predominately in the alpha crystalline form or the amorphous form, or mixtures thereof. More preferably, the starting silicon nitride is predominately in the alpha crystalline form. It is also advantageous if the preferred starting powder possesses a high a/~ weight ratio. Preferably, the starting powder contains no greater than about 20 weight percent ~-silicon nitride;
more preferably, no greater than about 10 weight percent ~-silicon nitride; most preferably, no greater than about 6 weight percent ~-silicon nitride.
Generally, the higher the purity of the start-ing silicon nitride powder, the better will be the properties of the finished ceramic body. Depending on the source, however, the silicon nitride powder may contain nonmetallic impurities. Some impurities may be tolerated in the powder, although it ls preferred to minimize these as much as possible. Oxygen, ~or example, is present to some extent in the form of silica, SiO2, which usually is found as a coating on the surface of the silicon nitride particles. The amount of silica varies according to the purity of the starting silicon nitride powder and its method of manufacture.
The silica content may be reduced by leaching or increased by adding free silica in order to attain a desired total silica content. In addition to oxygen, elemental silicon is usually present in amounts ranging up to 0.5 weight percent. The3e amounts of elemental 36,172D-F -6-~36~

silicon are not deleterious an~ can be tolerated. Other nonmetals~ such as carbon which is likely to form silicon carbide during hot-pressing or sintering, are tolerable in small amounts.
The silicon nitride starting powder can be of any size or surface area provided that the ceramic body of this invention is obtained by hot-pressing. Large particles having an average diameter of 15 ~m to 50 ~m, for example, may be in the form of hard agglomerates which cannot be easily broken. Powders containing such agglomerates make poor ceramics. On the other hand, very fine powders having an average diameter less than 0.2 ~m are difficult to obtain uniformly and to process.
Prèferably, the particles have an average diameter of 0.2 ~m to 10.0 ~m; more preferably, from 0.5 ~m to 3.0 ~m. Preferably, the surface area of the silicon nitride particles is 5 m2/g to 15 m2/g, as determined by the Brunauer-Emmett-Teller ~BET) method of measuring surface area, which is described by C. N. Satterfield in HeterogeneousCatalysisinPractice, McGraw Hil.l Book Company, 1980, pp. 102-105. More preferably, the surface area is 8 m2/g to 15 m2/g.
The silicon nitride is present in an amount which is suitably in a range of from 65 to 99.75 weight percent based on total powder mixture weight. The range is desirably from 80 to 97 weight percent based upon total powder mixture weight. When a rein~orcing 3 material is present, the amount of silicon nitride is reduced so that a total of silicon nitride plus reinforcing material falls within these ranges.
Raw silicon nitride powders cannot be densified to high densities in the absence of densification aids.

36,172D-F -7-~hus, at least one densification aid is admixed with the silicon nitride starting powder in a manner describ~d hereinbelow for the purpose of promoting densification of the silicon nitride during processing. The densification aids form a li~uid phase into which the ~-silicon nitride dissolves. The liquid phase forms at a temperature or over a temperature range which varies with the densification aid. The rate of mass transport of the ~-silicon nitride is usually quite rapid in the liquid phase; thus, the silicon nitride density increases until a critical mass is reached and precipitation occurs.
U.S. Patent No. 4,883~776 teaches the use of magnesium oxide as a densificàtion aid. Copending application, Serial No. 07/398,801, filed August 25, 19~9, expands the scope of densification aids to include non-oxide derivatives of magnesium. Non-oxide derivatives of magnesium include magnesium boride, magnesium nitride, and magnesium disilicide. The latter application also demonstrates that sources of beryllium, calcium, strontium, barrium, radium. lithium, sodium, potassium, rubidium. cesium and francium also promote densification of` silicon nitride. The source is suitably an oxide, but acceptable results are obtained with non-oxide derivatives such as borides or nitrides.
Any amount of a densification aid which promotes densification as described herein and produces 3 the tough silicon nitride ceramic body of the invention is acceptable. The densification aid is beneficially a non-oxide derivative of ~agnesium, beryllium oxide, calcium oxide, strontium oxide, barium oxide or radium oxide. The densi~ication aid is desirably calcium oxide or strontium oxide and is present in an amount of 36,172D-F _~_ ~ f~ 3 ~

0.04 to 27.0 weight percent based on the total weight of the powder mlxture. The amount of densification aid is desirably 0.5 to 9.8 weight percent: and preferably, from 0.9 to 4.7 ~eight percent.
In addition to a densification aid, the powder mixture must contain a conversion aid~ The conversion aid forms a glassy phase through which mass transport is, in general, considerably slower than in the densification aid. Thus, a-silicon nitride dissolves in the conversion aid on heating, but is not readily densified. Advantageously, however, the conversion aid promotes the rapid. essentially complete conversion of a-silicon nitride to ~-silicon nitride. This conversion is most desirable because the ~-silicon nitride in the form of elongated, single crystal whiskers or grains is responsible for the high fracture toughness and high fracture strength of the silicon nitride ceramic body of this invention. All references hereinafter to silicon nitride whiskers, single crystal whiskers and single crystal silicon nitride whiskers are intended to be synonomous and may be used innerchangeably. Any amount of conversion aid can be employed in the starting powder providing the quantity is sufficient to cause the essentially complete conversion of the starting silicon nitride to ~-silicon nitride, and is sufficient to produce the tough silicon nitride ceramic body of the invention. Preferably, the amount of conversion aid employed is 0.2 to 29.5 weight percent based on the total weight of the powder mixture. More preferably, the amount of conversion aid employed is 1.0 to 10.0 weight percent; most preferably, 1.7 to 8.5 weight percent.

36,172D-F _g_ 2 ~

- l o -The conversion aid is suitably a non-oxide derivative of yttrium or a source or derivative of an element selected from the group consisting of scandium, actinium, lanthanum, lithium, sodium, potassium, rubidium, cesium and francium. Non-oxide derivatives of yttrium include yttrium carbide. The conYersion aid is beneficially scandium oxide, lanthanum oxide, actinium oxide or ~odium oxide. The conversion aid pre~erably is lanthanum oxide or sodium oxide.
Surprisingly, the weight ratio of conversion aid to densification aid has been found to af~ect the fracture toughness of the finished ceramic, providing the whisker growth enhancing compound is also present in the powder mixture. Any weight ratio of conversion aid to densification aid is acceptable providing the fracture toughness shows an improvement over the fracture toughness value of 5 MPa (m)~ for nonreinforced, monolithic silicon nitride. The weight ratio is suitably from 0.25 to 8. The weight ratio is beneficially 0.5 to 5; desirably, 1 to 3; and, preferably, 1 to 1.8. In the absence of a whisker growth enhancing compound, the conversion aid/densification aid weight ratio has no signi~icant effect on the fracture toughness.
The third component required to be present in the powder mixture is a whisker growth enhancing compound. This compound helps to provide a ceramic body 3 of superior fracture toughness and high strength. Just how the whisker growth enhancing compound contributes to the excellent physical properties which are observed in the silicon nitride ceramic body of this invention is not completely understood. It is possible that the whisker growth enhancing compound improves the viscosity ~3~

, 1 of the glassy phase thereby facilitating the nucleation of elongated whiskers or grains o~ ~-silicon nitride;
the latter bein~ primarily responsible for the improved ~racture toughness. rhe aforementioned theory is presented with the understanding that such a theory is not to be binding or limiting of th~ scope of the invention. Any amount of the compo-~nd in the starting powder is acceptable providing the amount is su~ficient to promote the formation of ~-silicon nitride whiskers, described hereinbelow, and sufficient to produce the tough sllicon nitride ceramic body of this invention.
Preferably, the amount of the compound employed is 0.01 to 5 weight percent based on the tokal weight of the powder mixture. More preferably, the amount of whisker growth enhancing compound employed is 0.1 to 1.0 weight percent5 most pre~erably~ from 0.2 to 0.5 weight percent.
In U.S. Patent No. 4,883,776, the presence of calcium, particularly calcium oxide, was found to provide advantages when silicon nitride powder compositions were formed by hot-pressing into finished ceramic bodies. Silicon nitride powders doped with up to 5.3 weight percent calcium oxide were ~ound to be desirable. It was believed that commercial silicon nitride powders contained only 100 ppm or le~s of calcium oxide.
i An earlier application in this chain of 3 applications, Serial No. 07/297,627, ~iled January 13, 1989 and now abandoned, included three discoveries.
First, non-oxide derivatives of magnesium worked as densification aids. Second, non-oxide derivatives of yttrium provided satisfactory results when used as conversion aids. Third, elements of The Periodic Table 36,172D-F

~ ,3l~

of the Elements other than calcium also promoted growth of ~-silicon nitride whisker~ and provided hGt-pressed silicon nitride ceramic bodies with a fracture toughness greater than 6 MPa (m)', as measured by ~he Chevron notch technique and a fracture strength in excess of 120 ksi (825 ~Pa). Satisfactory results were obtained when the element was selected from the group consisting of sodium, potassium, scandium, titanium, vanadium.
chromium, manganese, iron, cobalt9 nickel, copper, zinc, strontium7 zirconium, niobium, barium~ lanthanum and mixtures thereof. The elements were beneficially selected from the group consisting of barium, sodium, potassium, titanium, vanadium, strontium, zirconium, niobium, lanthanum, tungsten and mixtures thereof.
Desirable results followed when the element was titanium, niobium, strontium or a mixture of two or more of such elements. The foregoing elements, or mixtures thereof, were suitably added in the form of a derivative, e.g., an oxide, a boride, a carbide, a carbonitride, a nitride or an oxynitride, rather than in their elemental form.
Copending application, Serial No. 07/398,801, filed August 25, 1989, included the discovery that additional substitution~ may be made for calcium oxide without sacrificing either fracture toughness or fracture strength. Particularly suitable results are obtained with an oxide of an element selected from the group consisting of gallium, indium, hafnium9 tantalum and boron.
Acceptable results are believed to be attainable with derivatives of lithium, beryllium, magnesium, ~ilicon, germanium, selenium9 rubidium, molybdenum, technetium, ruthenium9 rhodium, palladium, 36,172D-F -12~

2 ~ 3 ~

silver, cadmium, tin, antimony, tellurium, cesium, cerium, praseodymium, neodymium~ promethium, samarium, europium, gadolinium. terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium. tungsten, rhenium, osmium, iridium, platinum, gold, thallium, lead?
bismuth, polonium, francium, radium, thorium, protactinium, neptunium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, nobelium, and lawrencium. Non-oxide derivatives of gallium, indium, hafnium, tantalum and boron may also produce satisfactory results. Skilled artisans recognize that elements 58-71 and 90-103 of the Periodic Table of the Elements, commonly referred to as "the Lanthanides", are not readily available in their pure ~orm. They are, however, available as mixtures of two or more of such elements. Such mixtures are also believed to be suitable for purposes of the present invention. Skilled artisans also recognize that certain of the elements and their derivatives may be less desirable than the others based upon considerations such as availability and cost.
Suitable results are also obtained with non-oxide derivatives of calcium. Illustrative non-oxide derivatives include calcium boride, calcium carbide, calcium nitride and calcium disilicide.
Skilled artisans will recognize that results will vary depending upon both the element(s) of choice and the particular derivative(s) of that element or mixture of elements. The whisker growth enhancing aid is beneficially a source or derivative of niobium or an oxide of an element selected from the group consisting of potassium9 sodium, strontium, barium, scandium, lanthanum, titanium, zirconium, vanadium, chromium, 36,172D-F 13-~ a ~

tungsten, manganese, iron, cobalt, nickel, copper, zinc, calcium, gallium, indium, hafnium. tantalum and boron.
The whisker growth enhancing aid is desirably niobium stannate~ nio~ium galliate, niobium boride, or an oxide o~ potassium, barium, scandium, niobium, titanium, chromium, tungsten, manganese, cobalt, nickel, zinc, calcium, gallium, indium, hafnium, tantalum. The whisker growth enhancing aid is pre~erably an oxide of an element selected from the group consisting of gallium, indium, hafnium, tantalum and boron.
It is desirable to use whisker growth enhancing compounds and derivatives of magnesium and yttrium in the form of powders which are pure and sufficiently small in size. Purity is not typically a problem.
because commercially available materials used as whisker growth enhancing compounds, densification aids and conversion aids, particularly the oxide powders, generally contain less than 20 ppm each of assorted impurities. These levels of impurities are tolerable.
Larger amounts of impurities, as for example in the 0.5 weight percent range, ar-e not recommended as they may cause a change in the final ceramic composition and properties. A small powder particle size is favored, because dispersion is enhanced by smaller particles.
Preferably, the oxide powders have an average particle size no greater than 5 ~m in diameter.
Certain elements, e.g., sodium and potassium, may be used as a densification aid, a conversion aid or a whisker growth enhancing compound in a given powder mixture. No single element may 9 however, be used in an amount suf~icient to function as two or more of thesie 36~172D-F _14_ functions, e.g., as both a densification aid and a conversion aid.
It has now been found that incorporation into the powder mixture of a Palmqvist toughness enhancing amount of at least one preformed reinforcing material improves properties of the resultant silicon nitride body. Properties which are improved include room temperature toughness and high temperature strength and stiffness. The material is selected from the group consisting ~-silicon nitride fibers or whiskers or silicon carbide, titanium carbide, boron carbide, titanium diboride, aluminum oxide or zirconium oxide in at least one form selec~ed from the group consisting of whiskers, fibers, particles and platelets.
The reinforcing material must be chemically compatible with the glassy phase and its components.
Chemically compatible materials are selected from the 2~ group consisting of boron carbide, silicon carbide~
titanium carbide, aluminum oxide~ zirconium oxide and titanium diboride. Materials which are normally chemically incompatible with the glassy phase and its components may be rendered compatible by coating them with one o~ the a~orementioned chemically compatib]e elements. Normally incompatible materials include aluminum nitride, magnesium oxide and mullite. Titanium carbide provides a satisfactory coating for the latter group of materials.
3o The reinforcing material must be present in an amount which is sufficient to enhance the Palmqvist toughness of the resultant silicon nitride body without substantially interfering with or eliminating the ~ormation of elongated silicon nitride grains The 36,172D-F -15~

~03~
,~.

reinforcing material, when present, occupies space in the glassy phase in which elongated silicon nitride grains would otherwise grow in its absence. The amount of reinforcing material varies with the size of reinforcing material pieces, particles. fibers or whiskers as well as the volume occupied by the reinforcing material. Polycrystalline fibers provide acceptable results as a nominally continuous reinforcing material. Single crystal whiskers also provide acceptable results, albeit as a nominally discontinuous reinforcing material.
As a general rule, a given volume percentage of large particles, fibers. etc., will interfere less with formation of the elongated silicon nitride grains than an equal volume percentage of smaller particles, etc.
Accordingly, satisfactory results are obtained with any of the following combinations of reinforcement material diameter and volume percentages, based upon glassy phase volume: (a) less than 0.2 ~m diameter, up to 10 volume percent; (b) from 0.2 ~m to 0.5 ~um diameter, up to 15 volume percent; (c) from 0.5 ym to 1.5 ~m diameter, up to 25 volume percent; (d) from 1.5 ~m to 2.5 ~m diameter, up to 30 volume percent; (e) from 2.5 ~m to 5.0 ~m diameter, up to 35 volume percent; (f) from 5.0 ,um to 15.0 lum diam0ter, up to 45 volume percent; (g) from 15.0 ~m to 25.0 ~m diameter, up to 50 volume percent; and (h) greater than 25.0 ~m diameter, up to 65 volume percent. Irrespective of the reinforcing material diameter, the amount of reinforcing material~
where used, is beneficially greater than 5 volume percent. The reinforcing materials have different densities, As such, a universal weight percentage is not applicable. The weight percentages corresponding to 36,172D-F -16-2 ~3 3 ~

the foregoing volume percentages are readily determined given the density of a particular reinforcing material.
In the process of this invention. it ls required to mix the starting silicon nitride powder, described hereinabove, with a combination of a densification aid, a conversion aid, a whisker growth enhancing compound and, optionally, a reinforcing material to obtain a powder mixture 9 which is used in preparing the tough silicon nitride ceramic body of this invention. The reinforcing material, when present, is beneficially added to the remaining components of the powder mixture a~ter they are well mixed to minimize breakage or comminution of reinforcement material pieces, etc. Suitable densification aids, conversion aids and whisker growth enhancing components are disclosed hereinabove. Ordinarily, the total quantity of the derivative of magnesium, the derivative of yttrium densification aid, conversion aid and the whisker growth enhancing compound is no greater than 35 weight percent of the total weight of the powder mixture. The 35 weight percent limit is also appropriate when a reinforcing material is used. The total quantity will depend, however, on probable end use applications for fired ceramics prepared from the powder mixture. ~or some applications, total quantities in excess of 35 weight percent will provide acceptable re3ults. Preferably, however, the total quantity is 5 to 35 weight percent for medium temperature and/or the highest fracture toughness applications. By "medium temperaturel', it is meant temperatures from 900C to 1200C. Ceramic cutting tools are an exarnple of a medium temperature and very high fracture toughness application. Preferably, the total quantity is 0~2S to 36,172D-F _17-2 ~ 3 ~

5 weight percent for high temperature and/or moderately high fracture toughness applications. By "high ~emperature", it is meant temperatures from 1200C to 1400C~ Parts for ceramic engines are an example of a high temperature and moderately high fracture toughness application-The preparation of the powder mixture containing silicon nitride, densification aid(s), conversion aid(s) and whisker growth enhancing 0 compound(s), is accomplished in any suitable manner.
~all-milling of the components in powder form is one acceptable manner of preparation.
The preferred method of preparing the powder mixtur~ comprises use of an attritor with zirconia balls to prepare a finely-divided suspension o~ sillcon nitride and a powdered combination of the densification aid(s), the conversion aid(s) and the whisker growth enhancing compound(s) in a carrier medium, drying an admixture of the suspension and the attritor balls, beneficially after removing, by filtration or otherwise, excess carrier medium, and thereafter separating the attritor balls to obtain the powder mixture.
The preparation of the finely-divided suspension of silicon nitride and the combination of the densification aid(s), the conversion aid(s) and the whisker growth enhancing compound(s) in a carrier medium requires no particular order of addition of the compo-nents. For example, it is possible to add the powdered combination or powdered components thereof to a colloidal suspension of silicon nitride in a carrier medium or vice versa. Alternatively, all components oP
the powder mixture ma~ be added simultaneousl~ to the 36,172D-F -18-2~3~,3$~

carrier medium prior to attritor milling. The latter method is preferred, particularly when an organic carrier m~dium such as toluene or an alcohol is employed.
The carrier medium may be any inorganic or organic compound which is a liquid at room temperature and atmospheric pressure. Exarnples of suitable carrier media include water; alcohols, such as methanol, ethanol and isopropanol; ketones, such as acetone and methyl 0 ethyl ketone; aliphatic hydrocarbons, such as pentanes and hexanes; and aromatic hydrocarbons, such as benzene and toluene. The carrier medium is desirably an organic liquid~ preferably toluene or an alcohol such as ~ethanol. The function of the carrier medium is to impart a viscosity suitable for mixing to the solid powders. Any quantity of carrler medium which achieves this purpose is sufficient and acceptable. Preferably, a quantity of carrier medium is employed such that the solids content is 20 volume percent to 50 volume percent. More preferably, a quantity of carrier medium is employed such that the solids content is 35 volume percent to 45 volume percent. Below the preferred lower limit the viscosity of the solid suspension may be too low and the deagglomeration mixing may be ineffective.
Above the preferred upper limit the viscosity may be too high, and the deagglomeration mixing may be difficult.
The components of the powdered combination are 3 added to the carrier medium in any manner which gives rise to a finely dispersed suspension of the components.
Typically, the process is conducted in a large vessel at room temperature (taken as 23~) under air with vigorous stirring. Any common stirring means is suitable, such as a ball milling device or an attrition mixer. An 36,172D-F -19-r~

ultrasonic vibra~or may be used in a supplementary manner to break down smaller agglornerates. The attrition mixer is preferred.
To aid in the dispersion of components of the powder mixture, optionally one or more surfactants or dispersants can be added to the suspension. The choice oP surfactant(s) or dispersant(s) c~n vary widely as is well-known in the art.
If the carrier medium is toluene, a coupling agent, such as an aluminate coupling agent commercially available from Kenrich Petrochemicals under the trade designation KEN-REACT KA 322, may be used to aid in forming a suspension. When using an alcohol such as methanol, a dispersant such as a polyethyleneimine may be used to facilitate mixing and a flocculant such as olelc acid may be used to ease recovery of the powder mixture.
Any amount of surfactant or dispersant is acceptable providing dispersion of powder mixture components is improved. Typically, the amount of surfactant is 0.01 to 1.0 welght percent of the powder mixture.
The finely-divided suspension is now ready for processing into greenware. For example, the suspension can be slip-cast by techniques well-known in the art for eventual sintering, Alternatively, the suspension can be dried into a powder and ground for use in hot--pressing processes. Drying is accomplished by standard drying means, such as by spray-drying or oven drying under a nitrogen purge. Preferably, drying of the admixture o~ the powder mixture and the attritor balls 36,172D-F ~20-2 ~ 6~ ~

is accomplished in an oven under a nitrogen purge after removal of excess carrier medium. During the drying process~ additional free carrier medium is removed. The temperature of the drying depends on the boiling point of the carrier medium employed. Typically, the drying process is conducted at a temperature just below the boiling point of the carrier medium under atmospheric pressure. Preferably, the carrier medium is toluene or an alcohol and the temperature of drying is 50C. After drying, the resulting powder is separated from the attritor balls and sieved through a screen to obtain a powder having a maximum agglomerate diameter of 100 ~m.
The screen size is usually less than 60 mesh (250 ~m);
more preferably, less than 80 mesh (180 ~m). The powder which is obtained on sieving is the powder mixture which is used in the hot-pressing process of this invention.
When reinforcing materials are included in the powder mixture, the foregoing procedure is modified depending upon the form of the reinforcing material. If the reinforcing material is in a form other than long or continuous fibers, the reinforcing material is added to the finely dispersed suspension of components and mixed or attrited for a suitable length of time. Attrition time largely depends upon a balancing of the extent of reinforcing material agglomeration with its friability or fragility. In other words, attrition time is long enough to break up most, if not all, of the agglomerates. It is also short enough to maintain sufficient reinfarcing material integrity to provide a desired degree of reinforcement. The additional attritlon time will also depend upon the reinforcing material. A typical time need to accomplish dispersion of the reinforcing material will vary from 10 minutes to 36,172D-F -21-~3~

45 minutes. The time i~ beneficially ~rom 10 to 20 minutes. If the reinforcing material is in the form of fibersO also known as continuous fibers, no additional attrition time is required. The fibers are suitably immersed in the finely dispersed suspension to deposit a coating of the suspension on their outer surface. The fibers are then removed from the suspension and dried before further processing. If desired, multiple coatings may be applied in this manner. The dried, coated fibers, whether in the form of single fibers or a fiber mat or fabric, are beneficially surrounded by the powder mixture in a hot-pressing die and then hot--pressed as described herein. Other known methods of proces~ing fibers may also be used.
The preferred method of processing the powder mixture is by hot-pressing, which comprises heating the powder under pressure to obtaln the densified ceramic body~ Any standard hot-pressing equipment is acceptable, such as a graphite die equipped with a heating means and a hydraulic press. Particularly suitable results are obtained when the die is fabricated from a material which is substantially non-reactive with components o~ the powder mixture at hot-pressing temperatures and has a mean linear coefficient of expansion greater than silicon nitride. The use of such a die material aids in the preparation of near net shapes without post-densification machining operationsO
The die material is desirably titanium carbide. R.
Morrell, Handbook of Properties of Technical and Engineering Ceramics, pages 82-83 (1985)~ lists the mean linear coefficients respectively for silicon nitride and titanium carbide as 3.6 x 1c-6 K-1 and 8.2 x 10-6 K-1.
The hot-pressing is conducted under an inert atmosphere, 36,172D-F -22-2~313~

such as nitrogen. to prevent the oxidation and decomposition of silicon nitride at high temperatures.
The direction of pressing is uniaxial and perpendicular to the plane of the die plates.
Any processing temperature and pressure will suffice providing the novel silicon nitride ceramic of this invention, described hereinbelow, is obtained.
Typically, however, the temperature must be carefully controlled, because the elongated ~-silicon nitride whiskers are found to form in a narrow temperature range. Preferably? the temperature is maintained during pressurizing from 1750C to 1870C. More preferably, the temperature is maintained from 1800C to 1850C. Most pre~erably, the temperature is maintained from 1820C to 18~0C. Belo~ the preferred lower temperature limit the formation of elongated ~-silicon nitride whiskers may be retarded. Above the preferred upper temperature limit the silicon nitride may decompose, and special pressure equipment may be required to conduct the densification.
In the absence o~ a reinforcing material, the use of high pressure techniques such as hot isostatic pressing may allow use of higher temperatures ? e.g. up to 2000C
or even 2100C. It is noted that the accurate measurement of high temperatures, such as those quoted hereinabove, is technically dif~icult. Some variation in the preferred temperature range may be observed depending on the method employed in measuring the temperature. The preferred temperatures of this invention are measured by use of a tungsten-rhenium thermocouple, obtained ~rom and calibrated by the Omega Company.
While the pressure during hot-pre~sing is important, it is not quite as critical a parameter as 36,172D-F -23-$; ~

temperature. Typically, the pressure should be suf-ficient to cause densification of the green body.
Pre~erably, the pressure is 3000 psig (20.8 MPa) to 6000 psig (41.5 MPa); more preferably, from 4000 psig (27.7 MPa) to 5500 psig (38.o MPa); most pre~erably, 4500 psig (31.1 MPa) to 5200 psig (36.0 MPa). Below the preferred lower pressure limit the powder will not be sufficiently densified. Above the preferred upper pressure limit the powder will densify in a shorter time and at a lower temperature. Although less rigorous processing conditions seem on the surface to be desirable, the formation of elongated ~-silicon nitride crystals may be inhibited at lower temperatures and shorter pressing times.
The amount of time that the powder mixture is heated under pressure should be sufficient to bring the powder to essentially complete densification. Gener-ally, ram movement is a good indicator of the extent of densification. As long as the ram continues to move, the densification is incomplete. When the ram has stopped moving for at least 15 minutes, the densification is essentially complete at 99 percent or ~reater of the theoretical value. Thus, the time required for hot-pressing is the time during ram movement plus an additional 15 to 30 minutes.
Preferably, the time is 15 minutes to 2 hours; more preferably, from 30 minutes to 90 minutes; most preferably, 45 minutes to 75 minutes.
The hot-pressing method of densification, described hereinbefore, allows for the formation of silicon nitride ceramic articles which can be used as cutting tools. A variety of shapes can be made by hot--pressing, one common shape being a flat plate~ These 36,172D-F -24-2 ~ tl l1 plates may range in size from 2 inches in length by 1.5 inches in width by 0.45 inch in thickness to 16 inches (40.6 cm) in length by 16 inches (40.6 cm) in width by 1.0 inch (2.5 cm) in thickness. Smaller and larger plates can also be fabricated, as determined by the size of the hot-pressing plaques. Cutting tooLs can be fabricated by slicing and grinding these plates into a variety of cutting tool shapes.
The silicon nitride ceramic body which is pro-0 duced by the hot-pressing pr-ocess of this invention is a dense material having no significant porosity.
Preferably, densification proceeds to greater than 95 percent of the theoretical value; more preferably, to greater than 97 percent of the theoretical value; most preferably, to greater than 99 percent of the theoretical value. Moreover, as measured by X-ray diffraction9 the silicon nitride is present in the ~-crystalline form, indicating essentially complete alpha to beta conversion during processing. Quite unexpectedly, the ~-silicon nitride is present predominately as single crystal, ~needle-like" whiskers or elongated grains, as determined by both scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The size of the hexagonal ~-silicon nitride grains is usually 1 ~m to 20 ~m in length with a mean diameter of from 0.2 ~m to 1.5 ~m; preferably from 3 ~m to 10 ~m in length with a mean diameter from 0.3 ~m to 1.0 ~m.
Since the whiskers are oriented randomly, it is difficult to determine exactly the percentage of silicon nitride which exists as whiskers, as opposed to equiaxed particles. The measurement is made by viewing one plane of the silicon nitride ceramic in a scanning 36,172D-F -25~

2~3~

electron microscope (SEM) and measuring the percentage by volume occupied by whiskers having an aspect ratio between 2 and 16. It is observed that the whiskers are homogeneously distributed and randomly oriented throughout the ceramic body, and that the volurne occupied by the whiskers is approximately the same in all planes. Typically, the percentage of silicon nitride whiskers having an aspect ratio of between 2 and 16 is at least 20 volume percent as measured in a plane.
Preferably, the percentage of silicon nitride whiskers having an aspect ratio between 2 and 16 i~ at least 35 volume percent as measured in a plane. Unexpectedly, the whiskers are found to have a high average aspect ratio. Typically, the average aspect ratio of the silico~ nitride whiskers is at least 2.5; preferably, at least 5.5. It is noted that because the aspect ratio is measured in a plane, the average aspect ratio is a lower bound. For example, a whisker which is perpendicular to the plane may have an apparent aspect ra~io oE less than 2 7 whereas the true aspect ratio may be very much greater than 2.
In addition to the ~-silicon nitride phase, the ceramic body of this invention contains a glassy second phase, which constitutes no greater than 35 weight percent of the total weight. The glassy second phase has a bulk chemical composition consisting essen-tially of from 8 to 60 weight percent of a densification aid, from 15 to 64 weight percent of a conversion aid, from 7 to 77 weight percent silica~ and from 0.1 to 25 weight percent of at least one whisker growth enhancing compound, as determined by neutron activation analysis;
and wherein the conversion aid to densification aid weight ra~tio is 0.25 to 8.

36,172D-F -26 8 ~ ~

Small quantities of other phases may be present in a total amount not exceeding 10 weight percent. One of the phases. enstatite. possesses a fiber-like~
layer~d and ordered structure. The typicaL size of the particles of this phase is 500 nanometers (nm) in width by 0.7 ~m to 1.0 ~m in length. This phase. being distributed throughout the glassy phase, connects and bridges Si3N4 whis~ers. In other words, unique microstructures can be created with small needles or fibers of enstatite situate~ between comparatively large Si3N4 whiskers or particles.
The mechanical properties of the self-rein-forced silicon nitride ceramic body are readily measured by use of standard tests. In particular, fracture toughness (KIC) is measured according to the Chevron notch and the Palmqvist methods ~escribed hereinafter.
Fracture strength (modulus of rupture) is measured according to the Military Standard 1942b test. Hardness is measured according to the Vickers indentation test.
Fracture strength (modulus of rupture) measures the resistance of the material to fracture under a steady ]oad. Fracture strength is defined as the maximum unit stress which the material will develop before fracture occurs. Test bars are prepared by cutting rectangular bars (~5 mm x ~ mm x 3 mm) of the silicon nitride eeramic in a plane perpendicular to the pressing direction. The bars are ground on the surfaces parallel to the pressing plates using a 500 grit grinding wheel (Military Standard 1974). The fracture strength is measured at room temperature using a 4-point bend test with 20 mm span and crosshead speed of 0.5 mm/min. Typically, the fracture strength at room temperature is at least 650 MPa. Preferably, the 36,172D-F -27-~racture strength at room temperature ranges ~rom 825 MPa to 1250 MPa; more preferably, ~rom 1000 MPa to 1150 MPa. High temperature strength is measured using a 3-point bend test with 20 mm span and crosshead speed of 0.5 mm/min. Typically, at 1000~C the fracture strength is at least 550 MPa. Typically, a~ 1300C the fracture strength is at least 300 MPa.
Toughness measures the resistance of the material to fracture under a dynamic load. More specifically9 fracture toughness is defined as the maximum amount of energy which a unit volume of material will absorb without fracture. In the present invention two methods are employed to measure ~racture toughness.
The first of these is the Chèvron notch test. Test bars are prepared as described hereinabove, and additionally scored with a Chevron notch. The test bars are then subjected to a 3-point bend test with 40 mm span and crosshead speed of Q.5 mm/min. Typically, the fraeture toughness oP the silicon nitride ceramic body of this invention, as measured at room temperature (taken as 23C) by the Chevron notch technique, is greater than 6 MPa (m)~. Preferably, the room temperature fracture toughness is greater than 7 MPa (m)~; more preferably, greater than 8 MPa (mjS. Most preferably, the room temperature fracture toughness ranges from 9 MPa (m)~ to 14 MPa (m)~. Preferably, at 1000C the fracture toughness is greater than 6 MPa (m)~. More preferably, at 1000C the fracture toughness ranges from 7 MPa (m)~
to 12 MPa (m)~.
In the evaluation of cutting tool materials it is useful to measure the Palmqvist toughness and the Vickers hardness. Both measurements can be made 36,172D-F -28-~ o ~

simultaneously on one test sample, and therefore these tests are very convenient.
The Vickers hardness test measures the resistance of the ceramic material to indentation. A
sample, approximately 1 cm in length by 1 cm in width by 1 cm in height, is placed on a flat surface, and indented with a standard Vickers diamond indentor at a crosshead speed of 0.02 in/min. The Vickers hardness number is calculated from the applied load, in this case 14 kg, and the cross~sectional area of the indentation.
Prior to making the test, the test sample is polished in a special manner. First, the sample is cleaned an rough spots are flattened by use of a 220-grid diamond wheel. Next7 a 45-micron diamond wheel is used to start the polishing. Next, the sample is treated to a series of polishings at 30 psi and 200 rpm in the following consecutive manner: three five-minute intervals with 30 micron diamond paste, three five-minute intervals with 15-micron diamond paste, three five-minute intervals with 6-micron diamond paste, two five-minute intervals with l-micron dlamond paste, and one five--minute interval with 0.25-micron dLamond paste.
Between each interval the sample is thoroughly cleansed by washing with water and sonicating for two minutes~
The Vickers hardness number of the silicon nitride ceramic of this invention is at least 1325 kg/mm2 at room temperature. Preferably, the Vickers hardness number ranges from 1340 kg/mm2 to 1600 kg/mm2 at room temperature; more preferably, from 1450 kg/mm2 to 1600 kg/mm2 .
The Palmqvist toughness test is an extension of the Vickers test. (See S. Palmqvist in Jerndontorets Annalen, 141 (1957), 300, for a description o~ the 36,172D-F -29=

Palmqvist toughness test.) The test sample is prepared and indented as in the Vicker~ test, but the 14-kg load is additionally held for 15 seconds. The sample cracks.
The measurements of the indented diagonalS and the crack lengths are made on a Nikon UM2 microscope at 1000x 5 magnification. The Palmqvist toughness (W) is directly proportional to the applied load (P) and inversely proportional to the crack length (L). Preferably, the silicon nitride ceramic body of this invention exhibits a Palmqvist toughness at room temperature o~ at least 37 kg/mm. Preferably, the silicon nitride ceramic body of this invention exhibits a Palmqvist toughness at room temperature from 37 kg/mm to 52 kg/mm; more preferably, from 4'; kg/mm to 52 kg/mm.
The following examples serve to illustrate the novel self-reinforced silicon nitride composition of this invention, the method of preparing the novel sili-con nitride ceramic7 and the utility of the composition as a cutting tool. The examples are not intended to be limiting of the scope of this invention. All percentages are weight percent unless otherwise noted.

3o 36,172D-F _30_ %~3~

Example l Materials: Silicon nitride (KemaNord P95-H) is employed containing 1.81 percent oxygen, 0.6 percent carbon, and the following major metallic impurities: 641 ppm iron, 315 ppm Al, and 25 ppm ri. The silicon nitride is present in the alpha and beta crystalline forms in an ~/~ weight ratio of 95/5. The BET surface area of the silicon nitride powder is 10.15 m2/g and the aver~ge particle size is l ~m in diameter. Magnesium oxide (J. T. BakeY-) is employed containing less than 5 ppm each of boron, zinc, and iron. Greater than 80 percent of the MgO particles range in size from 0.2 ~m to 0.7 ~m in diameter. Yttrium oxide (Molycorp) is employed containing less than lO ppm each of sodium and iron. The Y203 particles range in size from 2 ~um to 5 ~um in diameter. Calcium oxide (Aldrich Chemical Co.) is employed containing less than 0.002 percent each of lead and iron. The average CaO particle size is 3 ~m in diameter.
The above-identified magnesium oxide (4.7 g) and calcium oxide powders (0.2 g) are suspended in 80 ml of water-, and agitated at room temperature under air by means of a mechanical stirrer to form a two-oxide suqpension. The pH of the suspension is adjusted to 11.35 by the addition of aqueous sodium silicate (7 drops). A~ter adjustment of the pH, the suspension is ultrasonicated for 30 seconds to break down fine 3 agglomerates. After sonication the pH is observed to drop. The pH is readjusted to 11.5 by adding 5 drops of 5 M sodium hydroxide. The suspension is mixed for 30 minutes. Yttrium oxide powder (8.5 g), described hereinabove, is added to the suspension, and the suspension is sonicated for 30 seconds and mixed with a 36~172D-F -31-2 ~

mechanical stirrer for 30 minutes. Silicon nitride powder (86.6 g), described hereinabove, is added to the suspension, and the suspension is mixed in an attrition mixer for 30 minutes to ensure complete dispersion of all components. The resulting suspension is poured through a 100 mesh nylon sieve. The pH is adjusted to 9.8 by adding lO ml of 50 percent nitric acid to increase the flocculation slightly. The finely divided suspension is dried in an oven at 90C for a period of 12 hours under a flow of dry nitrogen gas. After drying, the resulting powder mixture is passed through a 60 mesh sieve. The powder mixture is composed of 86.6 percent silicon nitride, 4.~ percent magnesium oxide, 8.5 percent yttrium oxide, and 0.2 percent calc-ium oxide.
The powder mixture ~80 g), described hereinabove, is poured into a graphite die in the shape of plates measuring 2 inches in length by 1.5 inches in width by 0.5 inches in depth. A pressure of lO00 psig is applied to the die, while the temperature is raised from ambient to 1200C in 30 minutes. At 1200C the pressure is gradually increased to 5000 psig and maintained thereat. The temperature is then increased to 1825C over a 40-minute period. The die is maintained at 1825C and a pressure of 5000 psig for 45 minutes. Afterwards the die is cooled over a 2 hour period to lOO~C. At 1500C the pressure is slowly released. When the die reaches room temperature, it is opened, and a silicon nitride ceramic body in the shape of a plate having the above-identified dimensions is retrieved.
The density of the silicon nitride ceramic body, prepared hereinabovel is measured by the water 3~,172D-F -32-2~3~

immersion method, as described in "Modern Ceramic Engineering" by D. W. Richerson, Marcel Dekker, 1982, and by stereology analysis from SEM photomicrographs.
The density is essentially 100 percent of theoretical~
and therefore the material is essentially nonporous.
Silicon nitride is present essentially in the ~
crystalline phase, as determined by X-ray diffraction.
The bulk chemical composition of the ceramic is determined by neutron activation analysis, and is found to contain 77.2 percent silicon nitride, 20.4 percent glassy second phase, and 2.4 percent silicon carbide.
The glassy second phase is found to consist of 32.4 percent magnesium oxide, 42.2 percent yttrium oxide, 2.5 percent calcium oxide, and 23.0 percent silicon oxide.
Two unidentified phases are found. The first is present in a quantity of 3.9 percent, and possesses a com-position of 9 percent magnesium, 59 percent silicon and 32 percent nitrogen. The second is present in a quantity of 1 percent, and possesses a fiber-like, layered and ordered structure typically 500A in width and 0.7 ~m in length~ The microstructure of the silicon nitride ceramic, prepared hereinabove, is determined by scanning electron microscopy (SEM), as viewed in a plane. 35 volume percent of the silicon nitride appears in the form of elongated whiskers having an aspect ratio ranging from 2 to 16. The average aspect ratio is 5.6 The fracture strength of the above-identified silicon nitride ceramic body, measured by the 4-point bend test described hereinbefore, is 130 ksi (890 MPa~
at room temperature and 90 ksi (616 MPa) at 1000C. The fracture toughness measured by the Chevron no~ch technique is 13.9 MPa (m~-~ at room temperature and 11.5 MPa (m)~ at 1000C. The Vickers hardness measured at 36,172D-F _33_ -3~-room temperature and under a 14-kg load ranges from 1~50 kg/mm2 to 1400 kg/mm2 and averages 1375 kg/mm2. The Palmqvist toughness measured at room temperature ranges ~rom 49.3 kg/mm to 51.1 kg/mm. It is seen that the ~racture toughness of this silicon nitride ceramic body is very high.
ExamDles 2 (a n) A series of hot-pressed silicon nitride ceramic compositions is prepared according to the procedure of Example 1, except that the composition of the powder mixture is varied as described in Table I. The Vickers hardness and the Palmqvist toughness measured at room temperature are presented in Table I for each composition.

36,172D-F _34-", ~
~D OIr~ N ~ C a~ 0 ~ ~ 3 , ~ ~ _ ~~ ~ ~ ~ ~) ~) -- ~) r~ ~ ~ r'q O
~C--~ c~ E
r-~ o 07 J~ O
^
~ C E a~ N ~ N O ~O N
E bO f` ~L) o cr~ ~J o N CO ~r ~ O ~ ~-- ~ O
3 Y ~ ~ ~~
t~ o_ ~V
~ V~
O I

s:~ ~ U~ O ~ ~ O O Lr~ ~ O ~ '~J N ~J 5 e ON =1- N N ~ ~ O N ~O ~ ~ '~
* ' 2) 0 Ln u~ n o o o oo o o ~ O O O O O O O ~ ~ N Ln a~ ~ -m ~. O O C O O O O O O O O O ~ ~

E~ n oNO oO o o o oo Oo o o o o O O a s~
N. , O Ln Ln ~O~ Ln Ln t~l N Ol ~J
r ~D Ln ~o Ln CO a~ CO 0 Vl S

o Ln o o o o o o o o o o o 0 0 v~
~ ~D Ln ~ r- ~ ~ N Ln r- N r- r- Ln Ln t3~ N r~ r Ln ~D r- ~ =r ~ ~ C1 ~ b~
=r o Z o L~ Ln ~n Ln Ln Ln C O o u~
C/l ~ o ~ ~ r~ o ~ Ln ~ ~o 0 0 co ~0 0 00 ~ ~ 0 ~ a a"a r N t ::1 tD
.(a ~ ~ Y ~ ~ X
X tL, E

2(~3~
-3~-The data show that the Palmqvist toughness and the ~Jickerq hardness vary as a function of the calcium oxide concentration and the f203/MgO ~eight ratio in the powder mixture. For example. it is seen in Examples 2(d,i,k,1,m,n) that as the calcium oxide concentration increases at constant ~203/MgO ratio, the Yracture toughness passes through a maximum value of 51 kg/mm at a calcium oxide concentration of 0.20 weight percent.
As the calcium oxide concentration increases at constant Y203/MgO ratio. the hardness decreases.
ComDarative Ex~eriments l(a-d) Four hot-pressed silicon nitride ceramic bodies are prepared as in Example 1, except that calcium oxide is omitted from the preparation. The powder com-positions are listed in Table II. The Vickers hardness and the Palmqvist toughness are measured as described in Example 2, and the values are tabulated in Table II.

3o 36~172D-F -36-2~3~fi~

E O ~ ~ O~ ~
a~ ~ ~~ ~ ~ ~a ,- , ~ ~ a~
Y ,s:
_. ~

S ~ `D u~ o ~
o ~, a~ ~a E~ CL
o ~ :~
bO~
~: 0 ~ Q
CO ~ C~
. . . . 3 O ~ ~ o ~ V~
~ C V~
* ~ o ~
C
o o U~
~3 o oLn o ~ U~
_l N .. . ~ S
m ~ Q CO U~ =r ,a ¢ _ 'O
E~ ~ ~ C
S.
C O O O ~ ~ V~
~o r~
. . . . C C
r~
C bO
O ~
0~ 0 ~ C) o U~ O o Z I J ~
~O ~O ~ (Y~ ~, ) V~ I ) ~, .- a~ X a) ._1 D, . C)~ ~
s~ 3 o e a) O O

U ~ ~L.
*

-38~

When Comparative Experiments 1~b,d) are compared with Examples 2(d~i,k,1,m,n) it is seen that the self--rein~orced silicon nitride ceramic body of this invention possesses a significantly higher Palmqvist toughness than the samples which ~o not contain calcium oxide. The same conclusion holds on comparirg Compara-tive Experiment l(a) with Examples 2(b and h). Even at a low Y203/MgO ratio and a low calcium oxide concentra-tion, the improvement in the ceramic body of this in-vention is noticeable, as seen in the comparison betweenComparative Experiment l(c~ and Examples 2(g and j).
ExamDles 3(a-c) .
Three hot-pressed silicon nitride compositions are pr~pared according to the procedure of Example 1, except that the powder compositions are varied as deqcribed in Table III. The Y203/MgO ratio in these powder compositions is 1.82. The Palmqvist toughness and the Vickers hardness are measured at room temperature according to the procedure in Example 2.
The values obtained are presented in Table III.

36,172D-F -38-~03~8~

F ~
, ~ E I ~ O' 0 `' ~ h~ ~ ~ r-- 3 Y I O
_,1 a~
V ~s ~a c E ~ ~ c a~
~C ~ ~O 0 '-1 O
C~ ~
r o O O ~
Q~ ~0 ~ ~D
O .~
C~ .

*
r O O O
~ C~ ~ ~
m ~: o o o ~: n U~
~ Ln o o s~
o . . .
.
a) v~
~ 'J1 O O
~:
' ~ o s~
o ~r~ moo a) S
C~ ~o _ ~
. ~ ~ C~ ~ o X E~
~C

2~3~
.. ..

~he data show that as the glass content inereases. the Palmqvist toughness also increases: whereas the iliekers hardness varies in a non-linear fashion.
ExamDle 4 -The hot-pressed silicon nicride eeramie body of Example 1 is diamond ground into a cutting tool insert.
The cutting tool insert is made aecording to the ANSI
standards in the SNG 433 style. The cutting edge is chamfered at a 20 angle by 0.008-inch width. The insert is tested in a faee milling applieation using a 40 HP Cineinnati #5 single spindle. knee and saddle.
vertical milling maehine with a 5 HP variable speed table. The work material is a nodular "ductile" cast iron measuring 2 inehes in diameter and having a measured hardness of 207 BHN. ~ milling cutter having a 6-ineh diameter is used with a -5 axial rake and a -5 radial rake. A 15 lead angle is employed. The maehine is run at a eutting speed of 1360 surface feet per minute, a 0.060-ineh depth of cut, and a feed rate of 0.005 ineh per revolution (or tooth). The center line of the cutter and the cen~er- Line of the workpiece are coineident. No cutting fluid is used. Successive passes are taken on the cast iron, and the cutting edge is examined for flank wear and chippage after every 8 passes~ Te~ting is terminated when the flank wear or chippage exeeeds 0.015 inch in depth as measured with a 30-power mieroseope. It is found that an average of 26.0 passes are aehieved prior to failure. The flank wear is uniform.

36,172D-F -40-2~3~

Comparative ExDeriment 2 (a-b) A commercial silicon nitride ceramic body is obtained from each o~ the following sources: (a) Boride Products (Product No. US-20) and (b) GTE Valeron Corporation (Product No. Q6). Each sample is diamond ground into a cutting tool in the manner described in Example 4. The cuttlng tools are used to cut nodular "ductile" cast iron in the manner deqcribed in Example 4. It is found that an average of 13.5 passes of the Boride Products sample are achieved prior to failure, and an average of 10.5 passes of the GTE sample are achieved prior to failure. In both cases the flank wear is uniform. When Comparative Experiments 2(a and b) are compared with Example 4, it is seen that the silicon nitride ceramic body of this invention significantly outperforms the commercial products.
Examples 5(a-w) - Effect of Various Whisker Growth Enhancin~ Compounds Upon the Properties of Self--~einforced Si~N4 A series of hot-pressed silicon nitride compositions is prepared using a variation of the procedure described in Example 1 as well as a different silicon nitride powder and a variety of elements to be evaluated for use as whisker growth enhancing compounds.
The elements and the reYultant physical properties are listed in Table IV. The physical propertie~ include fracture strength (modulus of rupture) measured according to Military Standard 1942b 9 ~racture toughness (Chevron notch technique) and Hardness (Vickers indentation test).

36,172D-F -41-2~3~

The silicon nitride powder is commercially available from Ube Industries, Ltd. under the trade designation SN-E10. It contains 1.46 percent oxygen, less than 100 ppm Cl, less than 100 ppm Fe~ less than 50 ppm Ca, and less than 50 ppm Al. Lt has a crystallinity of greater than 99.5 percent, a ratio of ai(a+~) of less than five and a surface area of 11.2 m2/g.

K2CO3 and Na2co3~ both commercially available from Fisher Scientific, are suitable sources, respectivelr for K2O and Na2O. Fisher Scientific also supplies La2O3, Zr02, Cr2O3, WO3, MnO2 and Fe23-Morton Thiokol supplies SrO and V2O4. Matheson, Coleman and BeLl supply BaO. 3aker Incorporated supplies TiO2, NiO and CaO. Alfa Products supplies Sc2O3. NbB2, CoQ, ZnO~ G'l23~ In2O3, HfO2, Ta2O5, B203 and C
The materials are mixed in an attritor (Union Process batch attritor, Model 01HD - 750 cc capacity with polytetrafluoroethylene coated tube and stirrer) containing zirconia balls with a stirring rate of 330 revolutions per minute (rpm). A mixture of 225 ml toluene and 25 drops of an aluminate coupling agent.
commercially available from Kenrich Petrochemicals under the trade designation Kenreact KA 322, is used as a mixing medium. The silicon nitride powder (86.3 g), the magnesium oxide powder (4.7 g), the yttrium oxide powder (8.5 g) and the powdered elemental derivative identified in Table IV (0.5 g) are added to the mixing medium and agitation is started. After thirty minutes of mixing~
an additional 100 ml of toluene is added and the agitation stirring rate is increased to 630 rpm for a period of two minutes to thin the mix~ure before removing it from the attritor together with ~he zirconia balls. Excess toluene is removed by vacuum filtration.

36,172D-F -42~

2~l3~3~

The mixture is then dried under a flow of dry nitrogen gas. APter drying, the mixture is separated from the zirconia balls using a 30 mesh stainless steel sieve and subsequently passed through 40 mesh and 60 mesh stainless steel sieves to provide a dried powder mixture composed of 86.3 percent silicon nitride, 4.7 percent magnesium oxide. 8.5 percent yttrium oxide and 0.5 percent derivative. The powder is then hot-pressed into plates and subjected to physical property testing as described in Example 1.

3o 36,172D-F -43 203~

TABLE IV
___ ~_ ~_ ~
. . Fracture Fracture Vickers Ex.5 Addltlve/ Strength ToughnessHardness 5erivative (MPa~ (MPa m~) (kg/mm2) ~_ _. ___ __ ___ a K2O 1062 8.43 1516 __ __ ___ __ ___ b Na2O 869 8.65 l 555 __ ~ ~ ___ c SrO 869 8.56 1604 l 0 d BaO 917 8.83 1486 __ ~ ~ ~ ~
e 5C2O3 896 8.36 1520 f La23 862 7.71 1515 __ =~ ~ __ __ __ 9 TiO2 1145 l 0.32 1526 ZrO2 972 8.0 1512 l 5 __ . . ~ __ . . .
l V2O4 1069 8.36 1514 J NbB2 1034 8 21 1500 k Cr23 1034 8.1 1512 __ ~ ~
I WC)3 958 7.541482 . . .-. ____ _ _~ __ m MnO2 972 1528 __ . ____~ ___ n Fe23 841 7.74 l 519 __ ~ ~ ~ __ o CoO 972 7.75 149G
__ ___ _._ ___ ___ P NiO 979 7.39 1538 __ ~ ~ ~ __ q CuO B55 7.05 l 496 __ ___ ... . ~ ___ ~
r ZnO 945 6.83 1548 __,. __ ~ ___ __ s CaO 896 g.5 1430 .__ ~ ___ ____ _~_ t Gaz03 986 10.16 __ __ ~ ~
3o u In23 1015 9.92 __ __. __ ~ __ v HfO2 1 û05 9.85 __ ~
w Ta25 1075 10.86 __ __ __ ~ ~
x B2C)3 915 9.93 __ ~ __ _ ~ __ -- means not measured 36,172D F -44_ _~5_ The data presented in Table IV demonstrate that a number of elemental derivatives4 when used as whisker growth enhancing compounds, provide self-reinforced silicon nitride compounds with satisfactory physical properties. Similar results are expected with other compositions ~hich are disclosed herein.
ExamDle 6(a-g) - Ef~ect of Adding Aluminum Nitride or 8Oron Nitride A series of hot-pressed silicon nitride ceramic compositions is prepared according to the procedure of Example 5 except, in the case of Examples 6a - 6d. for a variation in the mixing technique due to a change in mixing medium from toluene to methanol and the substitution of a small amount (see Table Va) of aluminum nitride (Example 6a) or boron nitride (Example 6b) for silicon nitride in the compositions shown respectively for Examples 5c and 5j.
80 ml of methanol are added to the same attritor as used in Example 5 with stirring at a rate of 200 rpm. The powder components are added slowly over a period of 18 minutes with additional methanol (50 ml) as needed to wash the powder components from the walls of the attritor. If deqired for more uniform and rapid dispersion, a dispersant, such as a 50 percent mixture of polyethyleneimine in methanol, commercially available under the trade de~ignation Corcat P-12 ~rom Virginia Chemicals, may be added in an amount of, for example, 26 dropsO After mixing for an hour at 220 rpm, a small amount (14 drops) o~ oleic acid is added to flocculate the slurry. The flocculated slurry is then recovered as described in Example 5.

36,172D-F -45-Composition and phy~ical property data for Examples 6a - 6h as well as 5c and 5j are summarized in Tables Va - Vc.
Table Va _~ ~. . ~ .
Additives (wt-%) Fracture Fracture Vickers Ex. ~ ~ ~ Strength Toughness Hardness No. SrO NbB2 AIN BN (MPa) (MPa m~) (kg/m2) _ __ _~ _~ ______ 5c 0.5 0.0 0.0 0.0 869 8.56 1604 _ __ __ _ _ _ _._ _ 6a 0.5 0.0 1.0 0.0 1096 10.26 1541 _ __ , __ _ ~_ __ 5j 0.0 0.5 0.0 0.0 1034 8.21 1512 ___ , ......... . _ _.
6b 0.0 0.5 0.01.0 1076 8.721482 . . . __ _ . . . . _ . - . .

Table Vb _ _ . . -- . .__. ~
Additives (wt-%) Fracture Fracture Vickers Ex. ~ ~ ~ Strength Toughness llardness No. ZnO AIN BN (MPa) (MPa-mz) (kg/m2) _._ ~ __ _ __ _ 6c 0.5 1.00.0 1014 8.44 1496 _ ___ ___ . _~ _~_ 6d 0.5 0.01 .0 917 8.52 1537 ___ ~ ._. _ __ _ Table Vc _ _ E BN Fracture Fracture Vickers x. aStrength Toughness Hardness No. (wt- /a)(MPa) (MPa m*) (kg/m2) _ ____ _ 5c 0.0 869 8.56 1 60 _ _ __,_____ 3o 6e 0 5 896 9 45 1450 6f 2.0 931 9.39 1365 _ ___ ~ _._ 69 3 ~ 95~ 8.76 1377 _ _ ~ __ ___ 36 ,172D-F ~46-2 0 3 ~

A review of the data presented in lables Va -~/c highlights ~everal points. First. the substitution of a minor amount of aluminum nitride or boron nitride for a corresponding amount of silicon nitride provides improvements in both fracture strength and ~racture toughness at the expense of a limited reduction in hardness (Table Va). Second~ the trend shown in Table Va wherein AlN appears to provide a greater improvement in fracture strength and fracture toughness than BN does not hold true for all compositions. Table Yb shows that the reverse is true when the whisker growth enhancing compound is ZnO. Third. substantial increases in the amount o~ BN give limited improvement in fracture strength at the expense of both fracture tough~ess and hardness. In other words, large amounts oP AlN or BN
are unnecessary. In addition, a certain amount of physical property tailoring is possible with limited compositional variation. Similar results are attained with other compositions of the present invention.
ExamDles 7a - 7e - Replication of Example 6b with Varving_~ ounts of the Glassy Phase Using the procedure of Example 6, the components of Example 6b (Si3N4, Y203, MgO, NbB2 and RN) and the component ratios of components other that silicon nitride, a serie~ of four hot-pressed silicon nitride ceramic bodies are prepared with varying percentages o~ the Si3N4 phase (components other than silicon nitride). The component ra~ios are 1.8 (Y203/MgO). 14.7 (total glassy phase/BN) and 29.4 (total glassy phase/NbB2). The Si3N4 content and physical properties are summarized in Table VI.

36,172D-F -47-2~3~g~

Table Vl _ Fracture FractureVickers Ex.Sl~N,~ Strength Toughness I tardness No. (w - ) ~MPa) (MPa m2) (kg/m2) _ __ __ __ __ 7a 80 993 9.17 14~4 _ _ __~ __ 7b 85 1076 8.72 1482 __ ~_ __ 7c 90 1069 9.92 1526 _ -, ~ _ ~
7d 95 1096 9.47 1548 _ _ _ ___ ~_ 7e 97 875 8.45 1601 The data presented in rable VI suggest-that the Si3N4 content may be varied considerably without undue adverse ef~ects upon physical properties of the resultant hot-pressed silicon nitride ceramic bodies.
Similar results are attainable with other compositions of the present invention.
ExamDles 8a -_8d -_~eplication of ExamDle 6a with Varyln~ Amounts of the Glassv Phase Using the procedure of Example 6, ~he components of Example 6a (Si3N4, Y203, MgO, SrO and AlN) and the component ratios of components other that silicon nitride, a series of four hot-pressed silicon nitride ceramic bodies are prepared with varying percentages of the Si3N4 phase (components other than silicon nitride). The component ratios are 1.8 (Y203/MgO), 1407 (total glassy phase~AlN) and 29.4 (total glassy phase/SrO). The Si3N4 content and physical properties are summarized in Table VII.

~3~8~L

Table Vll ~ . , _ ___ ~ _ ~
E . Fracture Fracture Vickers x. Sl~N; Strength Toughness Hardness No. (w- ) (MPa) (MPa-m~ (kg/m2) __ _ __ __ _____ 8a 85 1089 93 1591 8b 90 l125 8.7 1622 __~ ~ ~__ 8c 95 1102 8.6 1631 8d 97 993 88 1651 The data presented in Table VII suggest that the trend in physical property changes observed in Examples 7a - 7e as a result of an increase in Si3N4 conten~ holds true when AlN is substituted for BN and NbB2 is replaced by SrO. Similar results are attainable with other compositions of the present invention.
Comparative Examples 3a - 3e_- Effect of Omittin~ One or More Co~mponents of the Com~osition of Exam~le 5~
Using the procedure of Example 5, a series of hot-pressed silicon nitride ceramic bodies are prepared by omitting one or more components of the composition of Example 5g and adjusting the remaining components as shown in Table VIII. Table VIII also shows the requltant physical properties. Example 5g is included in Table VIII for ea~e of comparison.

36,172D-F -49-~6~
o Table Vlll 7~ ~1 n~os ~ c~ -- Frac-Ex/ (v\lt-% ) Fracture ture Vickers Comp Strength Tough- Hardness Ex __ ;; . ~MPa) (MPa.- ~kg/m2) 5i3N4 Y203 MgO CaO m2) __ _~; __ _ _~ __ ___ 5g 86.3 8.5 4.7 0.5 896 9 5 1430 _ ~ __ __ _ _ __ __ _ 3a 86.8 8.5 4.7 0.0 607 5.0 1480 __ __ ~ _ __ ~ ~ __ 3b 86.3 0.0 13.2 0.5 386 5.57 1449 ~ ~ ~ _ . . _~ ~
l O 3c 86.8 0.0 1 3.~ 0.0 352 4.65 1450 . ... . . . ~ _~ . ., 3d 86.3 13.2 0.0 0.5 510 3.83 146l __ __ . -. . _ _ _~___ 3e 86.3 ¦ 132 00 00 572 554 1534 The data presented in Table VIII clearly demonstrate the effect of omitting one or more components, as in Comparative Examples 3a -3e, from the composition of Example 5g. Although ~he data contained in this table relates to the use of a calcium derivative as the additive component, similar results are attainable without undue experimentation using other derivatives of elements listed in The_Periodic Table of the Elements and included within khe scope of the . _ present invention.
_xameles 9(a-c) - Effect of VarYin~ the Source of Niobium U~ing the procedure of Example 6, a series of three hot-pre3sed silicon nitride ceramic bodies are prepared using sources of niobium other than that used in Example 5j. The sources of niobium are as follows:
Example 9a - Nb3Sn 36,172D-F _50_ ~3~

Example 9b - Nb~Ga Example 9c - NbO
?hysical property data for the three bodies and for Example 5j are summarized in Table IX.
Table IX
~ _, Fra~u re Fractu re Vickers Example Strength Toughness Hardness No. ~MP~) (MPa m~) (kg/m2) 10__~_ ___ Sj I034 8 21 1500 9a 1000 8.03 1529 _ __ ~_ ~
9b 938 8.20 1538 9c 896 7.26 I 533 The data presented in Table IX demonstrate that non-oxide derivatives perform as well as, if not better, than the oxides. Similar results are attainable with other compositions of the present invention without undue experimentation.
ExamDles 10(a-b) - Ef~ect of ReDlacing M~O with SrO or Using the procedure of Example 5, two hot pressed silicon nitride ceramic bodies are prepared by substituting equal weight percentages o~ strontium oxide (SrO) (Example 10a) or calcium oxide (CaO) (Example lOb) for the magnesium oxide (MgO) used in Example 5.
Physical property data for the two bodies and for Examples 5c and 5s are summarized in Table X. CaO is the whisker growth enhancing compound for Examples 1Oa 36,172D F -51-2~3~

and 5s. SrO is the whisker growth enhancing compound for Examples 10b and 5c.

TableX
~ ~_ ~_ _~
Fracture Fracture Vickers Example Strength Toughness Hardness No. (MPa~ (MPa-mi-) (kg/m2) __ ___ ___ _ 1Oa 1027 6.67 1634 _ _ ___ ._~_ l O Ss 896 . . _ 1430 10b 1014 6.61 1556 ___ ~
Sc 869 8.56 1 604 The data presented in Table X demonstrate that strontium oxide and calcium oxide are suitable substitutes for magnesium oxide. Similar res~lts are expected with other oxides from Group II~ of the 2a Periodic Table of the Elements, namely beryllium oxide.
barium oxide and radium oxide.
ExamDle_11 - Effect of Usin~_~a~nesium Oxid_ as a Whisker Growth Enhancing Compound.
Using the procedure of Example 10 and the composition of Example 10b, save for the substitution of 0.5 weight percent MgO for a like amount of SrO, a hot-pressed silicon nitride ceramic bo~y is prepared.
Physical property data for the body are as follows:
Fracture Strength (MPa) - 800 Fracture Toughress (MPa-m~) - 7.94 36,172D-F -52 2 ~

Vickers Hardness (kg/m2) - 1545 The data demonstrate the suitability of MgO as a whisker growth enhancing compound.
Example 12(a b) - Ef~ect of Substituting SrO for M,~ in Com~ounds Containing a Small Amount of Aluminurn Nitride.
Using the proce~ure of Example 6, two hot--pressed silicon nitride ceramic bodies are prepared from the compositions shown in Table XIA. Physical property data ~or the bodies are shown in Table XIB.
TableXIA
___ , _ ._ __ ... ___ EXaNm~le 5i3N4 Y2O3 SrO AIN Tio2 __ __ _ __ __ __ 12a 96 2.3Z 1.Z8 0.27 0.13 .. ... . __. __ . _ , ~ ..
1 2b 97 1 74 0 96 0 2 0 1 2QTable Xl B
__ ~
E I Fracture Fracture Vickers xamp e Strength Toughness Hardness . (MPa) (MPa-m~) (Kg/mm2) __ __ ___ __ 1 2a 820 8.52 1650 __ __ __ _ 1 2b 770 8.29 1640 ~ _ _ ~__ The data shown in Table XIB, like that of Table IX9 demonstrate the suitability o~ SrO a~ a substitute for MgO in the preparation of selP-reinforced silicon nitride bodie~ Similar results are expected with other oxides from Group IIA of the Periodic Table_of the Elements.
~ . . . .

36,172D-F _53_ ~3~
-~4-Evaluation of Cuttin~ Tool Effectiveness The hot-pressed silicon nitride body of Example 12 is diamond ground according to American National Standards Ins~itute (A.N.S.I.) standards into an SNG 434 5 style cutting tool insert. The cutting edge is chamfered at a 30 angle with a 0.006 inch (0.015 cm) width.
The insert is tested in a face milling application using a 40 Horsepower Cincinnati #5 single spindle, knee and saddle, vertical milling machine with a 5 Horsepower variable speed table. The work material is Class 30 grey cast iron, four inches (10.16 cm) wide and 12 inches (30.48 cm) long with a measured hardness of 170 BHN (220 kg/mm2). A milling cutter having a 12 inch ~30.~8 cm) diameter, one tooth milling cutter, is used with a -5 axial rake and a -5 radial rake. A 15 lead angle is employed. The machine is run at a cutting speed of 3000 surface feet per minute (914 meters per minute), a 0.060 inch (0.152 cm) depth of cut, and a feed rate of 0~013 inch (0.330 mm~ per revolution (or tooth). The center line of the cutter and the center line of the workpiece are coincident. No cutting fluid is used.
Successive passes are taken on the cast iron work material. The cutting edge is examined for flank wear and chippage after each pass. Testing is terminated when the flank wear or the chippage exceeds 0.010 inch (0.025 cm) in depth as measured with a 30-power microscope. The insert removes 590 cubic inches (9668 cm3) o~ iron prior to failure. An SNG 434 style cutting tool insert prepared from a nominal 36,172D-F -54-2 ~ 3 ~
-~5 silicon nitride material (actually a silicon-alumina-nitride or sialon) marketed by Kennametal Corporation under the trade designation Kyon~ 3000 removes 340 cubic inches (5572 cm3) of iron prior to failure. An SNG 434 style cutting tool insert prepared from a silicon nitride material marketed by Boride Products under the trade designation US-20 removes 311 cubic inches (5096 cm3) of iron prior to failure.
The foregoing illustration demonstrates the suitability of the self-reinforced silicon nitride material prepared in Example 12b in cutting tool applications. Similar results are expected with other compositions disclosed herein.
ExamDle 13 - Substitution_of Lanthanum Oxide for Yttrium Oxide Using the procedure of Example 5, and a composition identical to that of Example 5s, save for the substitution of lanthanum oxide for yttrium oxide. a hot-pressed silicon nitride body is prepared. Physical property data for the body are as follows:
Fracture Strength (MPa) - 930 Fracture Toughness (MPam~) - 9.0 Vickers Hardness (kg/m2) - 1526 The data demonstrate the suitability of lanthanum oxide as a conversion aid. Similar results are expected with other oxides of elements ~rom Group IIIA of the Periodic Table of the Elements.
Satis~actory results are also expected with non-oxide 36,l72D-F -55~

2~3~

sources of the Group IIIA elements as well as sources of Group IIA elements.
Examole 14(a-b) - Substitution of Sodium Oxide for Yttrium Oxide or Ma~nesium Oxide Using the procedure of Example 5, and a composition identical to that of Example 5s, save for the substitution of sodium oxide for yttrium oxide in Example 14a and the substitution of sodium oxide for magnesium oxide in Example 14b, two hot-pressed silicon nitride bodies are prepared. Physical property data for the bodies are shown in Table XII.
TableXII

E t Fracturè Fracture xamp e Strength Toughness o. ~MPa~ (MPa-m~) ____ __ ~
14a 925 8.54 14b 720 74 The data presented in Table XII demonstrate that sodium oxide is an effective substitute either as a den ification aid (Example 14b) or a conversion aid (Example 14a). Similar results are obtained by subqtituting zinc oxide (ZnO)~ hafnium oxide (HfO) and tantalum oxide (Ta20s) for magnesium oxide. Similar results are expected with other sources of sodium as well as sources of other elements of Group IA of the Periodic Table of the Elements.

35,172D-F -56~

~3~8~ --~7 ExamDle 15(a-h) - Variation_in the ~atio of Conversion Aid to Densification Aid A series of hot-pressed silicon nitride ceramic ~ompositions is prepared according to the procedure of Example 6, except that the ratio of conversion aid (yttrium oxide) to densification aid (magnesium oxide) is varied as shown in Table XIII. The basic composition of the powder mixture is 86.3 percent Si3N~, 0.5 percent CaO and 13.2 percent of a combination of MgO and Y203.
The fracture toughness and the flexure strength are shown in Table ~III for each composition.

TABLE XIII*

. . .. .. __. __ __ Fra~ure Fracture Ex. Y O /M O Toughness Strength 2 3 g (MPa-m~P (MPa) .~ ~ .c.~
a 8-1 7.15 827 b 5:1 10.02 944 . ._ .. .
c 3:1 ~.53 1006 . __ d 1.8:1 10.37 1034 _ ..... . .__ e 1:110.50 1034 __ _ _ . __ f 1:2 9.46 930 . ~ __ _ __ ____ -g __ 1:3 _ 7.68_ 703 h 1:4 6.54 701 _~ _ _. ~
3 * Percentages o~ components are based on weight percent in the powder mixture.
The data presented in Table XIII demonstrate that a wide range of compositional variation produces hot-pressed silicon nitride ceramic bo~ies having a 36~172D-F -57_ 2~3~
~8-fracture toughness in excess of 6 MPam~ and a ~lexure strengths in excess of 700 MPa. Similar results are expected with other compositions disclosed herein.
Example 16(a-f) - Effect of Hot-Pressing Temperature Upon Physical Properties A series of hot-pressed silicon nitride ceramic compositions is prepared according to the procedure of Example 6, save for one modificationO The modification is the maximum temperature of 1825 C. used in Example 1 is varied as shown in Table XIV. All compositions are identical to that of Example 15d~ The Palmqvist Toughness of each composition is also shown in Table XIV.
TableXIV
Palmqvist Example Tem (ercj Toughness ~ ~
16a 1650 36.9 ~ 1.7 .~~ ___ 16b 1700 40.1 i 1.5 16c 1750 42.4 ~ 1.8 __ __.__ 1 6d 1785 44.4 ~t 1.8 ___ __ __ 16e 1825 44.9 i 1.9 16f 1850 43.~ i 0.6 The data presented in Table XIV demonstrate that acceptable Palmqvist Toughness is attainable over a relatively broad temperature range. Similar results are expected with other physical properties such as ~racture strength and fracture toughness as well as with other compositions disclosed herein.

36,172D-F -58-~3~8~
_~9_ ExamDle 17(a-c) - Ef~ect o~ Hot-Pressin~ Time U~on Phvsical ProDerties A series of hot-pressed silicon nitride ceramic compositions is prepared according to the procedure of ~xample 16e9 save for one modification. The sole modification is a change in the hot-pressing time from 60 minutes to that shown in Table XV. Accordingly, all hot-pressing is done at a temperature of 1825 C. The Palmqvist Toughness of the compositions is also shown in 0 Table XVO The Palmqvist Toughness and hot-pressing time for Example 16e is shown for comparison.
TableXV
~ ___ __ Example Time Palmqvist Number(minutes) (kg/mm) ~ ~__ 16e60 ,44.9 + 1.9 .... _ 1 7a5 40.4 + 1.2 _,______ 17b15 41.3 + 1.5 17c1 2a 39.6 + 17 ~ . _ _ . ~_ .

The data presented in Table XV demonstrate that considerable latitude in terms of time produces acceptable Palmqvist Toughness values. Similar results are expected with other physical properties such as fracture strength and fracture toughness as well as with other compositions disclosed herein.

36,172D-F _59_ ~3~

--~o--Example 18(a-g) - Ef~ect of Silica (SiO?) Content UDon Physical Properties A series of hot-pressed silicon nitride ceramic compositions is prepared using the procedure and composition of Example 1 with modifications of the silica (SiO2) content of the composition. Silica (SiO2) is inherently present in Si3N4. The actual SiO2 content of a particular powder depends upon variables such as particle size and method of preparation. Typical commercially available Si3N4 powder has an oxygen content in excess of two percent by weight of powder which equates to an SiO2 content of 3.75 percent by weight of powder or more. The Si3N4 powder used in this example is commercially available from UBE Industries America under the trade designation UBE-SN-10 and has an oxygen content of 1.3 percent by weight of powder which equates to a silica content of 2.43 percent by weight of pcwder. Free silica is added to increase the silica content and thereby the oxygen content of a given composition. The total silica content (weight percent)~
fracture strength and fracture toughness ~or each composition is shown in Table XYI.

36,172D-F -60-2 ~ 3 ~
--6 l--Tabte XVI
.. __. _~_ Ex (wjot%) 5Ft(raMCP9)h ness __ __ _ __ 1 8a 2.34 890 9.1 1 8b 2.59 1048 9.6 __ __ __ __ 1 8c 2.84 1082 9.4 o 18~ 3.09 1254 9.7 __ ~ _ __ 18e 3.59 1254 9.8 __ __ _ __ 18~ 4 09 1034 1 8g 5.09 1069 _ _ _ -- means not measured The data presented in Table XVI demonstrate that control of composition oxygen content, at least in the form of silica, has a marked e~fect upon fracture strength of resultant hot pressed bodies. Similar results are expected with other compositions disclosed herein.
Example 19_- Evaluation_ f Chemical Com~atibi.litY of Potential Reinforcin~ Materials Dense pieces of a number o~ different ceramic 3 materials are polished and placed in~o the cavity of a graphite die similar to that of Example 1. The ceramic materials are titanium carbide, boron carbide, silicon carbide, aluminum oxide, zirconium oxide, magnesium oxide, aluminum nitride, titanium diboride and mullite.
A powder mixture~ prepared as in Example 1, is poured 36,172D-F -61-~33~
-o2-into the die cavity and hot-pressed as in Example 1 save for increasing the hot-pressing temperature to 1850C
and the time at pressure to one hour. rhe powder ~ixture contains 90 percent silicon nitride, 5.8 percent yttria, 3.2 percent magnesia. 0.33 percent calcia and 0.67 percent silica.
The hot-pressed material is sectioned to provide interfaces between the sillcon nitride composition and each of the previously densified ceramic 0 materials. The interfaces are polished and examined by scanning electron microscopy ~SEM) for the presence of elongated grains o~ ~-silicon nitride. The SEM
examination shows that elongated grains of ~-silicon nitride are found at or near ~he interfaces of all previously densified ceramic materials save for aluminum nitride, magnesia and mullite. Based upon this preliminary examination, the latter three materials are believed to be unsuitable for use as reinforcing materials in preparing the silicon nitride ceramic bodies of the present invention. Similar results are attainable with other ~orms of the ceramic materials evaluated in this example.
ExamDle 20 - Evalu_tion of Silicon Carbide Whiskers as a ~einforcin~ Mate__al The powder mixture of Example 19 is admixed with varying amounts o~ silicon carbide whiskers (American Matrix) and hot pressed as in Example 19. Th~
whiskers have a number average diameter of 0.9 micrometer and an average aspect ratio of 11. The re~ultant hot-pressed bodies are tested for Vickers hardness and Palmqvist toughness as in Example 1. The 36,172D-F -62-203~
-~3 amounts of silicon carbide whiskers and the ~st results are shown in Table XVII.
TableXVII
__ .__ __ _._ Sample PercentHardnessT almqvist Num er Whiskers(kg/mm2) (kg/mm) 20a 10 1580 44.3 ~ ~ ~ _. ~ ___ 20b 20 1587 37.2 __ __ ~ __ l 0 20c 25 l 587 37.0 __ ~ _____ 20d 30 1595 36.1 __ ~ ~_ The data presented in Table XVII show that SiC
whisker loadings of 30 volume percent provide a Palmqvist Toughness which approaches the values of 30 to 36 kg/mm reported for hot-pressed silicon nitride having neither elongated silicon nitride grains nor reinforcing materials such as silicon carbide whiskers admixed therewith. All of the samples in Table XVII, when examined by SEM as in Example 19, show the presence of elongated silicon nitride grains. By way of contrast.
hot-pressed bodies having SiC whisker loadings in excess of 30 volume percent contain no elongated silicon nitride grains. Similar results are obtained with other reinforcing materials and compositions all of which are disclosed herein.
L 21 - Evaluation of_Multi~le Reinforcement at Various TemDeratures A powder mixture containing 98 percent silicon nitride, 0.74 percent yttria, 0.7 percent silica, 0.46 percent magnesia and 0.1 percent tantalum oxide is 36,172D-F 63-2~3~

prepared as in Example 1. ~ portion of the powder mixture is mixed with an amount of the same silicon carbide whiskers as in Example 20 to provide an admixture containing 25 weight percent silicon carbide whiskers. Equal volumes of the powder mixture and the admixture are converted to hot-pressed silicon nitride bodies using the procedure of Example 20. The resultant bodies are subjected to Vickers Hardness (VH) (kg/mm2) and Fracture Strength (FS) (MPa) testing as in Example 1. The bodies are also tested for Young's Modulus (YM~) (GPa) in accordance with Military Standard 1942b at elevated temperatures. The test re~ults are shown in Table XVTII.
Table XVIII
1 5 ~ ~ . - ~ __. _-. =
Sam- SiC 20C 1 200C l 375C
ple Wh is- ~. ~ __ ~ __ ~ . ~_ No.kers VH F5 F5 YM F5 YM
21a yes 1810 730 475 278 285 71 2 0 ~ _ ~ __ __ __ 21 b no 1614 792 543 200 206 22 ~_ _ .__ __ , _. __ .~ ~__ The data presented in Table XVIII demonstrate 25 that the presence of a reinforcing material in addition to the elongated silicon nitride grains grown in situ provides improvements in high temperature strength.
e.g., at 1375C, and Young's Modulus, e.g., at temperatures of 1 200C or above. The data also show 3 that hardne~s at room temperature (20C) is improved by the addition of such a reinforcing material. Similar results are obtained with other reinforcing materials and compositions all of which are disclosed herein.

36~172D-F -64-2 ~ 3 ~

ExamDle 22 - Evaluation of Platelets as Reinforcin~
Medla A powder mixture containing 71.69 percent silicon nitride, 4.6 percent yttria, 2~54 percent magnesia9 1.09 percent silica, 0.4 percent zirconia, and 19.68 percent silicon carbide platelets (C-Axis Corp.) is prepared as in Example 1 save for adding the platelets after the other components are well dispersed.
Prior to adding the platelets, a portion of the mixture is removed from the mixing apparatus and recovered as in Example 1. Mixing is continued for the remainder of the mixture for a period of ten minutes to provide an admixt~lre which is recovered as in Example 1. The platel~ts have an aspect ratio of 8 to 10 and an average diamet~r of 24~m. Equal volumes of the powder mixture and the admixture are converted to hot-pressed silicon nitride bodies using the procedure of Example 20. The requltant bodies are subjected to Vickers Hardness (VH~
(kg/mm2) and Palmqvist Toughness (kg/mm) teqting. Test results are shown in Table XIX.
Table XIX
~_ ~ ___.___ Sample SiC Vickers Palmqvist Number Platelets (kg/mm2) (kglmm) ___ ___ __ ___ 22a yes 1542 49.4 22b no 1525 45.1 ~_ ~ ~ __ The data presented in Table XIX show that platelets, like whiskers, provide satis~actory resultS
when used as reinforcing media in conjunction with elongated silicon nitride grains found in silicon nitride bodie~ of the present invention. Similar 36,17~D-F -65-2~3~
--o6--results are obtained with other reinforcing materials and compositions all of which are disclosed herein.
ExamDle 23 - Evaluation of Different Tvpes and Amounts ol' SiC Reinforcin~ Media Example 22 is replicated save for changes in the type o~ reinforcing media, the amount of reinforcing media or both. The reinforcing media are the platelets of Example 22, the whiskers of Example 21 or 0.25 ~1m average diameter silicon carbide whiskers (Tateho Chemical Industries Co.~ Ltd.). The resultant silicon nitride bodies are subjected to Palmqvist Tougnness (kg/mm) testing. Test results are shown in Table XX
together with the average diameter and weight percent of reinforcing media.
TableXX
___ _ __ Sam- SiC Palmqvist No Diameter Amo)nt (kg/mm) . ... .. ~ . ~_ ~
23a .25 10 374 __ ._ . ___._ 23b 25 20 35.7 __ ___ _~_ ___ 23c 0.9 2~ 37.2 __ ~ ___ __ 23d 0.9 30 36.1 __ ~. , .__ __ __ 23e 24.0 2n 49.~
~_ ~ .~ ___ __ 23f 240 ~0 373 The results shown in Table XX demonstrate that reinforcing media size plays an important part in determining the amount of reinforcing media which is suitable ~or a given composition. Similar re~ults are 36,172D-F -66-;2~3 -~ ~ 6 ~

obtained with other reinforcing materials and compositions all of which are disclosed herein~

36.172D-F -67-

Claims (36)

1. A process for preparing a silicon nitride ceramic body having a fracture toughness greater than 6 MPa (m)? and containing predominantly .beta.-silicon nitride whiskers having a high average aspect ratio, comprising:
subjecting a powder mixture comprising (a) silicon nitride in an amount suffi-cient to provide a ceramic body;
(b) a densification aid, said densification aid being a non-oxide derivative of magnesium or a source of beryllium, calcium, strontium, barium, radium, lithium, sodium, potassium, rubidium, cesium, silicon, hafnium, tantalum, indium, galium, zinc, titanium or francium, said source being present in an amount sufficient to promote densification of the powder;
(c) a conversion aid, said conversion aid being a non-oxide derivative of yttrium or a source of scandium, actinium, lanthanum, lithium, sodium, potassium, rubidium, cesium or francium, said source being present in an amount sufficient to promote the essentially 36,172D-F -68-complete conversion of the starting silicon nitride to .beta.-silicon nitride; and (d) at least one whisker growth enhancing compound in an amount sufficient to promote the formation of .beta.-silicon nitride whiskers, said compound being a non-oxide derivative of calcium, a derivative of sodium, potassium, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, strontium, zirconium, niobium. barium, lanthanum, or mixtures thereof, or an oxide of gallium, indium, hafnium, tantalum and boron.;
(b), (c) and (d) being derived from three different elements;
to conditions of temperature and pressure sufficient to provide for densification and in situ formation of .beta.-silicon nitride whiskers having an average aspect ratio of at least 2.5 occur and such that the silicon nitride ceramic body having a fracture toughness greater than 6 MPa (m)? is formed.

2. The process of Claim 1 wherein the .beta.-silicon nitride whiskers have an aspect ratio of 2 to 16 and are present in an amount of at least 20 volume percent, as measured by viewing one plane of the silicon nitride ceramic body by scanning electron microscopy.

3. The process of Claim 1 wherein the starting silicon nitride contains no greater than 10 weight percent .beta.-silicon nitride.

4. The process of Claim 1 wherein the densification aid is present in an amount of 0.04 to 36,172D-F -69-27.0 weight percent based on the total weight of the powder mixture.

5. The process of Claim 4 wherein the densification aid oxides of strontium, calcium or sodium.

6. The process of Claim 1 wherein the conversion aid is present in an amount of from 0.2 to 29.5 weight percent based on the total weight of the powder mixture.

7. The process of Claim 1 wherein the densification aid and the conversion aid are present in amounts sufficient to provide a weight ratio of densification aid to conversion aid of 0.25 to 8.

8. The process of Claim 7 wherein the weight ratio is 1:1 to 1.8:1.

9. The process of Claim 1 wherein the whisker growth enhancing compound is present in an amount of 0.01 to 5 weight percent based upon total weight of the powder mixture.

10. The process of Claim 1 wherein the whisker growth enhancing compound is a non-oxide derivative of calcium.

11. The process of Claim 1 wherein the whisker growth enhancing compound is an oxide of an element selected from the group consisting of gallium, indium, hafnium, tantalum and boron.

12. The process of Claim 1 wherein the powder mixture further comprises silica in an amount of from 36,172D-F -70-2.3 to 6 weight percent based upon total weight of the powder mixture.

13. The process of Claim 1 wherein the powder mixture further comprises aluminum nitride or boron nitride in an amount of from 0.01 to 5 weight percent based upon total weight of the powder mixture.

14. The process of Claim 1 wherein the powder mixture further comprises a Palmqvist toughness enhancing amount of at least one preformed reinforcing material of .beta.-silicon nitride in whisker or fiber form, or silicon carbide, titanium carbide, boron carbide, titanium diboride, aluminum oxide or zirconium oxide in whisker, fiber, particle or platelet form.

15. The process of Claim 14 wherein a total of the amount of silicon nitride plus the amount of preformed reinforcing material is 65 to 99.75 weight percent based on the total weight of the powder mixture.

16. The process of Claim 1 wherein the powder mixture further comprises a Palmqvist toughness enhancing amount of at least one preformed, coated reinforcing material of magnesium oxide, aluminum nitride or mullite and having a coating of silicon carbide, titanium carbide, boron carbide, titanium diboride, aluminum oxide or zirconium oxide, said coated material being in a physical form of whiskers, fibers, particles and platelets.

17. The process of Claim 16 wherein a total of the amount of silicon nitride plus the amount of preformed, coated reinforcing material is 65 to 99.75 36,172D-F -71-weight percent based on the total weight of the powder mixture.

18. The process of Claim 1 wherein the temper-ature is 1750°C to 1870°C.

19. The process of Claim 1 wherein the pressure is 3000 psig (20.8 MPa) to 6000 psig (41.5 MPa).

20. The process of Claim 1 wherein the density of the silicon nitride ceramic is greater than 95 percent of the theoretical value.

21. A silicon nitride ceramic body having a fracture toughness greater than 6 MPa (m)?, as measured by the Chevron notch technique at 23°C, comprising:
(a) a crystalline phase of .beta.-silicon nitride of which at least 20 volume percent, as measured by viewing one plane of the silicon nitride ceramic body by scanning electron photomicrographs, is in the form of whiskers having an average aspect ratio of at least 2.5;
and (b) a glassy phase in an amount not greater than 35 weight percent of the total weight comprising a densification aid, a conversion aid, silica, and a beta-silicon nitride whisker growth enhancing compound, said compound being a non-oxide derivative of calcium, a derivative of sodium, potassium, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, strontium, zirconium, niobium, 36,172D-F -72-barium, or lanthanum, or an oxide of gallium, indium, hafnium, tantalum or boron.

22. The body of Claim 21 wherein the densification aid is beryllium oxide, calcium oxide, strontium oxide, tantalum oxide, hafnium oxide, indium oxide, gallium oxide, zinc oxide, barium oxide and radium oxide and the conversion aid is scandium oxide, lanthanum oxide, sodium oxide and actinium oxide, provided, however, than the densification aid is different than the conversion aid.

23. The body of Claim 22 wherein the glassy phase comprises 8 percent to 60 percent densification aid, 15 to 64 percent conversion aid in an amount of 0.1 to 25 percent beta-silicon nitride whisker growth enhancing compound, and silica in an amount of 7 to 77 percent by weight; and wherein the weight ratio of conversion aid to densification aid is 0.25 to 8; and wherein not greater than 10 weight percent of the total weight is present as other phases.

24. The body of Claim 23 wherein one of the other phases is a enstatite which possesses a fiber--like, layered and ordered structure.

25. The body of Claim 24 wherein enstatite fibers have a diameter of 500 nanometers and a length of from 0.7 to 1.0 micrometer.

26. The body of Claim 24 wherein the silicon nitride whiskers of the crystalline phase are intermixed with the enstatite fibers of the glassy phase.

27. The body of Claim 21 wherein the glassy phase further comprises aluminum nitride or boron 36,172D-F -73-nitride in an amount of from 0.01 to 15.0 weight percent based upon total glassy phase weight.

28. The body of Claim 21 wherein the glassy phase further comprises a Palmqvist toughness enhancing amount of at least one preformed reinforcing material selected from the group consisting of .beta.-silicon nitride in whisker or fiber form, or silicon carbide, titanium carbide, boron carbide, titanium diboride, aluminum oxide or zirconium oxide in whisker, fiber, particle or platelet form.

29. The body of Claim 21 wherein the glassy phase further comprise a Palmqvist toughness enhancing amount of at least one preformed, coated reinforcing material of magnesium oxide, aluminum nitride or mullite and having a coating of silicon carbide, titanium carbide, boron carbide, titanium diboride, aluminum oxide or zirconium oxide, said coated material being in a physical form selected from whiskers, fibers, particles or platelets.

30. The body of Claim 28 wherein the amount of reinforcing material is a combination of reinforcement material diameter and volume percentages, based upon glassy phase volume, of (a) up to 10 volume percent of material having a diameter less than 0.2 µm;
(b) up to 15 volume percent of material having a diameter of 0.2 µm to 0.5 µm; (c) up to 25 volume percent of material having a diameter of 0.5 µm to 1.5 µm; (d) up to 30 volume percent of material having a diameter of 1.5 µm to 2.5 µm; (e) up to 35 volume percent of material having a diameter of 2.5 µm to 5.0 µm; (f) up to 45 volume percent of material having a diameter of from 5.0 µm to 15.0 µm; (g) up to 50 volume 36,172D-F -74-percent of material having a diameter of 15.0 µm to 25.0 µm; or (h) up to 65 volume percent of material having a diameter of greater than 25.0 µm.

31. The body of Claim 21 wherein the percentage of silicon nitride whiskers is at least 35 volume percent.

32. The body of Claim 21 wherein the fracture toughness is greater than 7 MPa (m)?.

33. The body of Claim 21 wherein the fracture toughness as measured by the Chevron notch technique at 1000°C is 9 MPa (m)? to 14 MPa (m)?.

34. The body of Claim 21 wherein the Palmqvist toughness measured at 23°C is 37 to 52 kg/mm.

35. A cutting tool fabricated from the body of Claim 21.

36. A cutting tool fabricated from the body of Claim 28.

36,172D-F -75-