METHOD OF PREPARING POWDER INGREDIENTS FOR SUBSEQUENT CONSOLIDATION

Abstract: A method of making a compacted body useful in the fabrication of an improved, fully densified Si3N4 ceramic. Silicon powder, oxygen carrying agents, and a dry lubricant are admixed. The lubricant is selected to have a low surface energy value, less than 15 dyne/cm, and have a glass transition temperature of -54 to -82<o>C. The admixture is milled to a desired average particle size no greater than 35 microns. The milled mixture is then formed into a compact which can be nitrided and hot pressed to form a fully dense Si3N4 object.

Description: METHOD OF PREPARING POWDER INGREDIENTS FOR SUBSEQUENT CONSOLIDATION BACKGROUND OF THE INVENTION In the manufacture of ceramic parts, powder ingredients are first admixed and dry milled to uniformly mix the ingredients and to create the proper proportions if attrition or wear from the milling media is desired as part of the ingredients. The milled powder mixture can then be subjected to cold compaction, gas chemical treatment, and hot pressing to produce the desired product. If the milling is not complete, that is, the powder ingredients are not reduced to a desired average or mean particle size and desired homogeniety, then incomplete or unreacted ingredients may result from subsequent gas chemical treatment, resulting in a hot pressed product with internal flaws. In the case of the manufacture of a silicon nitride cutting tool material, these flaws may originate by the oxidation of free silicon and impurities, such as iron oxide, or unreacted additives to form suicide particles, such as iron suicide. The suicide forms a solute which dissolves silicon nitride during hot pressing and, when cooled, recrystallization results in large beta Si3N4 particles adjacent the suicide along with porosity. These are soft spots in the ceramic which pull out or degrade during use of the ceramic, particularly as a cutting tool. What is needed is a method of milling the. starting powder ingredients to insure that the particles of the milled mixture are not above a critical average or mean particle size, such as 35 microns, thus preventing packing of the mixture along the sides of the milling chamber and thus insuring a constant milling action. Such method should not incorporate materials to achieve such object that will provide for undesirable side reactions should optimally reduce the milling time, should improve the green strength of the milled mixture when compacted, and should minimize, if not totally eliminate, contamination that results in flaws in the final ceramic product. Heretofore, prior art methods have incorporated milling aids in a high amount of about 5% by weight, such as stearic acid, Zn stearate, oleic acid and carbowax, each of which have a high carbon content which can cause contamination. SUMMARY OF THE INVENTION The invention is a method of making a compacted body useful in the fabrication of an improved, fully densified, silicon nitride ceramic product which has par¬ ticular utility as a cutting tool. The method comprises mixing a supply of silicon powder and a supply of oxygen carrying agents with an effective but small amount of a dry lubricant of the type having a low surface energy value less than 15 dyne/cm, and further characterized by a glass transition temperature of -54 to -82°C and a molecular weight of 5000. The admixture is then milled for a sufficient period of time to insure that the average particle size of the mixture is no greater than 35 microns. Finally, the milled mixture is formed into a compact with pressure sufficient to provide a green strength of at least 42% of full theoretical. It is preferable that the dry lubricant be selected from the group consisting of polybutylacrylate, poly 2-ethylhexylacrylate and polyisobutyacrylate. The dry lubricant may be further characterized by its ability to render lubrication even though added in an extremely small amount, thus avoiding carbon contamination. The lubricant is effective to avoid packing of the powder mixture along the sides of the milling chamber during the milling opera¬ tion, thereby permitting said milling to reduce the particles to the desired mean size of no greater than 35 microns within a milling period equal to or less than 48 hours. It is advantageous to add the dry lubricant in an amount of .001-.006% by weight of the mixture or about .001 grams per 100 grams of mixture. It is preferable if the starting materials have defined purities. For example, silicon powder may have a purity of 97% or greater, and the oxygen carrying powder agents may have a purity such as 99.99% for Ϊ2°3 and 99.5% for AI2O3. All of the starting powder materials can have an average particle size greater than 270 mesh (10 microns), and operably the starting average particle size can be as high as 10-60 microns. Optimally, the starting powder ingredients may comprise silicon powder, y 2 ( - ) 3' anc -- A -*-2°3' tn e powder ingredients containing trace contaminants of up to 1.0% Fe, .3% Al, 1.0% Mn, and 0.1% Ca, and an O2 content of no greater than .5%. DETAILED DESCRIPTION A preferred method for making a compacted body useful in the fabrication of an improved, fully densified, silicon nitride ceramic product according to this invention is as follows. 1. Mixing An admixture of powder ingredients comprising silicon powder, reactive oxygen carrying powder agents, and a dry milling lubricant is prepared. The reactive oxygen carrying powder agents is defined herein to mean powder ingredients that are effective to form second phase crystallites, particularly oxynitrides, when reacted with the silicon powder under a heated nitrogen atmosphere. The oxygen carrying agents can be advantageously selected from the group consisting of Si0 2 , Y2°3' e0 2' Zr0 2' Hf0 2' and other rare earths. Use of these agents will improve physical characteristics and formation of a second phase crystallite which will be uniformly disbursed and which substantially displaces a detrimental glassy silicate phase normally formed except for a controlled and limited amount of the oxygen carrying agent. For purposes of the preferred method, a uniform powder mixture is typically prepared with 2000 grams of silicon (86.6 weight percent of mixture), 278 grams of Y2O3 (12 weight percent of mixture and 13.9% of silicon), and 32 grams AI2O3 (1.4 weight percent of mixture and 1.6% of silicon). The usable range for the oxygen carrying agent is .4-2.3 molar percent of the mixture and 0.4-2.4 molar percent of silicon. Y2O3 -*- s normally used in the range of 3-19% by weight of the silicon and 3-16% by weight of the mixture. A glass forming oxide, such as AI2O3, is used in the range of .4-5% by weight of the silicon, .4-4% by weight of the mixture. Siθ2 is present usually as an oxide on the silicon powder and increased to 1-3% by weight of the silicon by milling. The oxide that is added to be reactive with Siθ2 and Y2O3/ such as AI2O3, can be selected from the group consisting of MgO, CeO, BO, I2O3, Fe2θ3, Caθ2, Cr2θ3, ZrO , BeO, and rare earth oxides. Silicon is preferably selected to have 98% or greater purity and a starting average particle size of about 10-20 microns with random particles in the 100-540 micron range. The major trace metal contaminants experienced with such purity include: iron up to 1.0%, aluminum up to .5%, and manganese up to .09%. Nonmetallic contaminants include: carbon up to .05%, and oxygen less than .5%. The average powder size of the Y2°3 powder is o about .0438 microns (438A) with random particles as large as 40 microns. The average particle size of the AI2O3 powder is .3-.5 microns with random particles as large as 50 microns. To the above ingredients is added a dry milling lubricant which must have a low surface energy value less than dyne/cm, and is further characterized by a molecular weight of 5000. The dry milling lubricant is preferably selected from the group consisting of polybutylacrylate, poly 2-ethylhexylacr late, and polyisobutlyl. The dry milling lubricant is added in an effective small amount to insure that the powder mass will not collect and pack along the sides of the milling chamber during the milling operation, thereby permitting the milling to reduce all particles to the desired size of no greater than 40 microns within a milling period equal to or less than 48 hours. An effective small amount is in the range of .001-.006% by weight of the mixture; this amount is insufficient to cause a problem by carbon contamination. 2. Milling The mixture is then comminuted and blended by being charged into an inert milling jar along with grinding media in the form of Burundum cylinders (85% AI2O3 and 15% Siθ2, which adds AI2O3 to the mixture by attrition), and is milled for 48 hours at 64 rpm. Thereafter the mixture is separated from the media by use of a #10 mesh screen. The resulting milled mixture must have no particle of a size greater than 35 microns. If any of the particles in the milled mixture are greater than 35 microns, the deleterious effects indicated earlier would ensue. Such effects include (a) insufficient nitriding of all particles leaving free silicon which leads to flaws in the final product and thus decreases strength, (b) local concentrations of 2O3, AI2O3, or silicon may appear causing strength variations in the final product. The oxygen level after milling in air will be increased to about 1.6 weight percent of the silicon, and be present as an oxide coating on the silicon in an amount of about 3 weight percent. The oxide coating should never be stripped off for milling purposes. The ratio of Y2°3 Si0 2 is controlled to be in the range of 1.1-6.4 and preferably about 4. 3. Cold Compaction A measured quantity of the milled mixture is then loaded into a cold pressed die arrangement and pressed at ambient conditions by use of 1400-1500 psi to form a compact of a size about 6 π x .6", and a density of about 1.4 grams/cm*-*. The pressure of compaction should be sufficient to provide a green strength of at least 1.4 gm/crn-^ (42%) in the compact. 4. Heating to Nitride The compact is then heated in a nitriding atmosphere, without the use of pressure normally associated with hot pressing, to produce a silicon nitride comprising body consisting of silicon nitride, at least one dispersed second phase crystallite (silicon oxynitride), .2-1% silicate (by weight of the body), and up to .5% by weight of free silicon and unreacted oxygen carrying agents (here Y2O3 and AI2O3). The body will have a size greater than and a density less than the object to be formed. For further details on nitriding see the disclosure of co- pending U.S. application Serial No. , the disclosure of which is incorporated herein by reference. 5. Hot Pressing The nitrided body is then hot pressed to produce a silicon nitride comprising object of required dimension and density. A pressing fixture having graphite walls is typically used to carry out the hot pressing. The walls and nitrided body are both coated with a slurry of boron nitride and dried. The details of effective hot pressing are further set forth in copending U.S. application Serial No. , cited above. The resulting product will have a strength (modulus of rupture) in a 4-point bend test of 110-120 hsi, as compared to 80-100 hsi for material prepared according to the prior art. The coelesced particles in the product will have a Weilbull Slope for variation of strength data which is 10-18 as opposed to 7-12 for the prior art. This shows greater uniformity. The hardness will be increased to 92 (45-N scale) as opposed to 90 for the prior art.

Claims: We claim: 1. A method of making a compacted body useful in the fabrication of an improved, fully densified, silicon nitride ceramic body, comprising: (a) mixing a supply of silicon powder and a supply of oxygen carrying agents with an effective small amount of a dry lubricant having a low surface energy value less than 15 dyne/cm, and which is characterized by a glass transition temperature of -54 to -82°C; (b) milling said mixture for a sufficient period of time to insure that no particle in said mixture is greater in size than 40 microns; and (c) forming a compact of said milled mixture with pressure sufficient to provide a green strength of at least 42% of full theoretical. 2. The method as in Claim 1, in which said dry milling lubricant is selected from the group consisting of polybutylacrylate, poly 2-ethylhexylacrylate, and polyisobutyacrylate. 3. The method as in Claim 1, in which said effective small amount of dry milling lubricant is in the range of .001-.006% by weight of the mixture. 4. The method as in Claim 1, in which the supply of silicon powder has a purity of 97% or greater and a starting average particle size as great as 25-60 microns. 5. The method as in Claim 1, in which said dry milling lubricant is added in an amount effective to avoid packing of said powder mixture along the sides of the milling chamber during the milling operation and thereby permitting said milling to reduce all particles to the desired size of no greater than 40 microns within a milling period equal to or less than 48 hours. 6. The method as in Claim 1,. in which said supply of oxygen carrying agents is selected from the group consisting of Y2°3 AI2O3, and Siθ2, the purity of said oxygen carrying agents is characterized by at least 99.99% for said 2O3 and at least 99.5% for said AI2O3.

Inventors: BECKWITH ELAINE C, EZIS ANDRE, BECKWITH, ELAINE, C., EZIS, ANDRE
Applicants: FORD MOTOR CO, FORD WERKE AG, FORD MOTOR COMPANY, FORD MOTOR COMPANY LIMITED, FORD-WERKE AKTIENGESELLSCHAFT
Application year: 1982
Main classification: C04B35
IPC codes: C04B35