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  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/515—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics
  • C04B35/56—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbides or oxycarbides
  • C04B35/565—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbides or oxycarbides based on silicon carbide
  • C04B35/575—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbides or oxycarbides based on silicon carbide obtained by pressure sintering
  • C04B35/5755—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbides or oxycarbides based on silicon carbide obtained by pressure sintering obtained by gas pressure sintering
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  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/515—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics
  • C04B35/58—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides
  • C04B35/584—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides based on silicon nitride
  • C04B35/593—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides based on silicon nitride obtained by pressure sintering
  • C04B35/5935—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides based on silicon nitride obtained by pressure sintering obtained by gas pressure sintering
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  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/622—Forming processes; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/64—Burning or sintering processes
  • C04B35/645—Pressure sintering
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
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  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/622—Forming processes; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/64—Burning or sintering processes
  • C04B35/645—Pressure sintering
  • C04B35/6455—Hot isostatic pressing

Definitions

• This invention relates to ceramic particles, especially silicon nitride or silicon carbide particles, having a substan ⁇ tially uniform coating thereon of one or more conventional ox ⁇ ide sintering aids, e.g. yttria, or a precursor thereto, in the substantial absence of any individual sintering aid oxide parti ⁇ cles. Due to a uniform coating of sintering aid on each ceram ⁇ ic nitride or carbide particle, the total amount of sintering aid required to produce a sintered body having excellent high temperature properties may be reduced substantially below the a ⁇ ounts now conventionally used.
• the invention also relates to a method of preparing such sintering aid-coated particles in the substantial absence of any free sintering aid particles by means of a surface precipitation of a precursor to the sinter ⁇ ing aid upon the ceramic particles.
• the invention further relates to the general comminution of one material in the presence of another material which is known to deleteriously interact with the freshly formed sur ⁇ faces of the material being comminuted. More particularly, it relates to the comminution of ceramic particles, especially sil ⁇ icon nitride or silicon carbide particles, in water by a pro ⁇ cess which precludes deleterious reactions between the water and the ceramic particles.
• the aqueous comminution process al ⁇ so permits the incorporation of additional components, e.g. sin ⁇ tering aids, onto the freshly formed ceramic particle surfaces.
• sintering aid addition to ceramic materi ⁇ als which undergo a liquid phase densification is performed by simply milling a solid sintering aid powder with the ceramic material in the presence of an alcoholic solvent, normally iso- propanol.
• the milling serves both to comminute the ceramic par ⁇ ticles to a smaller and more uniform particle size and to dis ⁇ tribute particles of the sintering aid somewhat uniformly among the ceramic particles.
• the result is a simple mixture of dif ⁇ ferent particles which inherently means that some portions of the mixture will be richer in sintering aid particles while oth ⁇ er portions will be poorer in them.
• This variation in sinter ⁇ ing aid concentration is believed to be a cause for failure of some ceramic parts, particularly at elevated temperatures.
• a further method of adding a sintering aid is shown in Ja ⁇ panese Publn. 62-187,170 in which a mixture of silicon nitride particles and silicon carbide whiskers is impregnated with a solvent-based solution of yttrium hydroxide or alkoxide and then the impregnated hydroxide or alkoxide is transformed to an oxide.
• the process is similar to that of Shaw et al., discuss ⁇ ed below, and produces a substantial amount of yttrium oxide particles intimately blended with silicon nitride particles and silicon carbide whiskers as well as a small amount of the ni ⁇ tride and carbide having a partial yttria coating.
• the intermediate pulverization step means that many fresh silicon nitride surfaces are produced which can not pos ⁇ sibly have any sintering aid thereon.
• water is sug ⁇ gested as a possible solvent there is no disclosure in the ab ⁇ stract of any step being taken to prevent or even deter the de ⁇ gradation of the silicon nitride that must occur by reaction with the water. Also no steps are taken to prevent the forma ⁇ tion of individual particles of sintering aid oxide which are less efficient than a coating in promoting densification.
• Japanese Publn. 61-281,069 mixes silicon carbide and sili ⁇ con nitride powders in a solution containing a metal alkoxide and then calcines the mixture until the alkoxide is hydroliz- ed.
• the result is predominantly a mixture of particles of sil ⁇ icon carbide, silicon nitride, and metallic oxide, with a small amount of metal oxide particles formed on the surfaces of the carbide and nitride ⁇ powders.
• Japanese Publn. 61-251,578 forms an alcoholic solution of a metal alkoxide and uses that solution in place of a metal ox ⁇ ide powder as a sintering aid source for silicon nitride.
• the result is a mere blend of silicon nitride and metal oxide parti ⁇ cles with a slight amount of the metal oxide possibly adhering to some of the silicon nitride particles, but not completely, coating them.
• T.M. Shaw and B.A. Pethica in "Preparation and Sintering of Homogeneous Silicon Nitride Green Compacts", 69 Journal of the American Ceramic Society 88-93 (1986) teach a means to ob ⁇ tain a more uniform distribution of sintering aid than by con ⁇ ventional milling. Specifically, they teach the precipitation of yttrium, magnesium, and/or aluminum hydroxides by adding so ⁇ lutions of the corresponding metal nitrates to a suspension of silicon nitride powder in water which also contains tetraethyl- ammonium hydroxide in sufficient quantity to cause precipita ⁇ tion of the metal hydoxide from the dispersion medium.
• the pre ⁇ cipitate and the suspended silicon nitride particles are jointly flocculated to produce a well-mixed powder of silicon nitride and densification aiding metal hydroxides.
• Such mixtures were found to sinter to higher final densities under the same sinter ⁇ ing conditions than mixtures of the same chemical composition formed by conventional joint milling of constituents initially introduced into the suspension in powder form.
• SiC + 2 H 2 0 Si0 2 + CH 4 which again results in the presence of substantial amounts of silica on the particle surfaces.
• the silica in each case can reduce the usefulness of the particles particularly at high tem ⁇ peratures.
• the water can generate dangerous amounts of am ⁇ monia or methane.
• milling of sili ⁇ con nitride and silicon carbide in water while having been at ⁇ tempted, has previously generally been avoided.
• milling in water has been found to cause extreme morphological changes in the powder by forming extensive hard agglomerates which have impeded densification and adversely affected the mechanical pro ⁇ perties of the resultant dense body.
• ceram ⁇ ics particularly nitride and carbide ceramics, and most partic ⁇ ularly silicon nitride and silicon carbide, can be milled in wa ⁇ ter in the presence of suitable surfactant materials to produce a readily densifiable powder without an undesirably high silica content and without causing any substantial agglomeration.
• the ability of being able to place the ceramic powders in water, without causing a deleterious reaction, in combination with control of the surface chemistry enables a more effective and economical mixing of sintering aids with the ceramic parti ⁇ cles which results in surface precipitation of a densification aid, or more usually a precursor of it, onto the surfaces of the suspended ceramic particles, without simultaneously precipi ⁇ tating any of the sintering aid or its precursor in the bulk of the suspension medium.
• the resultant ceramic particles are es ⁇ sentially completely coated with the sintering aid or its pre- cursor in the substantial absence of free sintering aid or pre ⁇ cursor particles.
• a metal hydroxide that will readily convert to a densification-aiding metal oxide is con- trollably precipitated onto the surfaces of suspended nitride or carbide particles, while avoiding simultaneous precipitation of hydroxide particles in the bulk.
• a desirable inter- granular phase constituent such as yttria, is distributed over the nitride or carbide powder surfaces with exceptional uniform ⁇ ity, thereby reducing the total amount of sintering aid requir ⁇ ed for densification.
• the process is particularly ad ⁇ vantageous when two or more sintering aids are used simultane ⁇ ously, since they can be deposited either simultaneously or, more preferrably, sequentially thereby providing additional con ⁇ trol of the subsequent densification.
• Figure 1 is a scanning transmission electronmicrograph of silicon nitride particles which have been milled in water con ⁇ taining 2% Cavco Mod APG surfactant.
• Figure 2 is a- scanning transmission electronmicrograph of the particles of Figure 1 wherein 4% yttria has been surface precipitated thereon.
• Figure 3 is a scanning transmission electronmicrograph of conventionally prepared mixture of silicon nitride particles which have been milled in isopropanol with yttria present.
• the present invention is applicable to nitride and ca ⁇ rbide ceramic powders which are reduced in size to form very fine powders which are then used to produce useful articles. More particularly it is applicable to ceramic powders which un ⁇ dergo a liquid phase densification.
• suitable such nitride and carbide powders include, but are not limited to, silicon nitride, aluminum nitride, boron nitride, titanium nitride, silicon carbide, boron carbide, and titanium carbide.
• the ceramic material is comminuted, preferably in water, to a desired fine particle size
• a metal salt which will gener ⁇ ate metal ions of the desired sintering aid is added to the cer ⁇ amic particles in the form of an aqueous solution, generally af ⁇ ter the comminution
• the metal ions combine with hydroxyl ions and a metal hydroxide precursor to the desired oxide is surface-precipitated onto the ceramic particles from the aque ⁇ ous solutions under conditions which preclude the precipitation of the precursor in the bulk, and finally (iv) the precursor is converted to the desired final oxide.
• the comminution (milling) of the nitride or carbide parti ⁇ cles to the desired fine particle size may be performed in a conventional manner in the presence of an alcoholic solvent, such as isopropanol, and then dried to remove the solvent.
• an alcoholic solvent such as isopropanol
• the milling is performed in the presence of water and a surfactant which is more reactive with nitride or carbide than is the water so that the ceramic materi ⁇ al is protected from attack by the water and thus reaction with the water is prevented.
• Suitable surfactants useful herein in ⁇ clude amino-functional zircoaluminate surfactants having an in ⁇ organic polymer backbone (from Cavedon Chemical, now Manchem) , silanes, titanium alkoxides, aluminum alkoxides, zirconium al- koxides, iridium-based surfactants, and the like.
• the selected surfactant must not detrimentally effect the per ⁇ formance of the ceramic material in its intended use.
• the surfactant In addi ⁇ tion, the surfactant must not generate any substantial amount of foam which could deleteriously effect the comminuting opera ⁇ tion.
• it is an organofunctional zircoaluminate.
• the organofunctional groups are selected from amino, mercapto, carboxy, and oleophilic groups.
• the surfactants useful herein are those surface active agents which are sufficiently attracted to the nitride or carbide particles to react therewith more rapidly than can water.
• the surfactant serves to prevent, or at least substan- tially retard, reaction of the nitride or carbide with water while simultaneously not generating an excessive amount of foam which would interfere with the milling process.
• the surfactant is normally used in an amount of about 0.5 to about 10, prefer ⁇ ably about 1 to about 5, weight % based on the water.
• the mill ⁇ ing is normally conducted for an extended period of at least about 10 hours, though the time has not been found critical pro ⁇ vided that the desired particle size is achieved.
• the water milling in the presence of the surfactant has been found to sub ⁇ stantially increase the milling efficiency such that the total milling time required to produce a particular average size par ⁇ ticle has been found to be reduced by 50 to 70 % or more.
• the comminution of the relatively large nitride or car ⁇ bide particles to the desired particle size may be performed in any conventional mill or other particle comminution device that operates on the principle of suspending solid particles in a liquid medium.
• suitable such devices include vibra ⁇ tory, ball, and attritor mills, and the like.
• the sintering/densification aids added to the nitride or carbide include those conventional oxides which are known to be useful for densifying the ceramic material involved.
• suitable oxides include those of rare earth metals such as yttrium, as well as magnesium, zirconium, aluminum, cerium, haf ⁇ nium, and mixtures thereof.
• the sintering aid is selected from yttria, magnesia, and mixtures thereof. Most pre ⁇ ferably the sintering aid is yttria.
• Precursor hydroxides of these sintering aids which can be surface-precipitated in accor ⁇ dance with this invention are prepared in situ from metal salts which produce metal ions when placed in water.
• Salts which can generate the desired metal hydroxide precursors include yttrium nitrate, yttrium chloride, yttrium acetate, magnesium nitrate, magnesium bromide, magnesium acetate, and mixtures thereof.
• the salt used is the nitrate of the particular metal because the product of neutralization, ammonium nitrate, is readily decomposable at low temperatures with no deleterious effect on the ceramic products.
• the amount of sintering aid used will depend upon its particular efficiency and the use to which the final body will be put, but generally an amount of about 0.5 to about 10 % by weight of the ceramic material will be appropriate. Preferably, the amount of sintering aid will be as low as possible, particularly for high temperature appli ⁇ cations.
• the amount of sintering aid can be less than about 2%, since silicon nitride bodies prepared with as little as 0.7 weight % yttria in accordance with the present invention have shown essentially equivalent performance to con ⁇ ventionally prepared bodies having 4% yttria.
• the sintering aid is yttria which is used in an amount of about 0.5 to about 1.2 weight %.
• the milled par ⁇ ticles can be slurried in water which contains the surfactants described above, in substantially similar amounts, along with the necessary metal salt.
• the desired metal salt may simply be add ⁇ ed to the mill after the milling is completed and, preferably, after the milled suspension has been filtered to remove any ni ⁇ tride or carbide particles larger than about 2 microns.
• the slurry may also contain an alcohol in up to about 50% of the weight of the water.
• the deposition of the sintering aid pre ⁇ cursor hydroxide upon the milled particles by surface precipi ⁇ tation is then performed by adding the desired metal salt and then slowly increasing the pH of the solution by the addition of a base such as aqueous ammonia.
• the deposition conditions are selected to preclude any bulk precipitation of the sinter ⁇ ing aid hydroxide precursor so that the final composition is substantially free of independent sintering aid oxide parti ⁇ cles.
• pH(s) pH(b) + ⁇ e y>y/2.3kT
• ⁇ e the electronic charge of the metal
• f y the surface potential of the particle suspension
• k the Boltz- mann constant
• T the absolute temperature
• the pH of the entire system is controlled to be at least about 1.5, preferably at least about 2, and most preferably at least about 2.5 pH units below the hydrolysis constant of the specific metal ion until almost all, i.e. at least, about 90 percent, of the metal has- been con ⁇ sumed in the surface precipitation. Thereafter, the pH can be increased to slightly below the hydrolysis constant to assure that all of the metal is precipitated onto the nitride or car ⁇ bide surfaces and that essentially none of it is precipitated in the bulk.
• two or more sintering aids are desired for a particu ⁇ lar ceramic material, they may be deposited from a single solu ⁇ tion containing metal ions of each. More preferably, the sin ⁇ tering aids will be deposited in a sequential manner, thereby providing an additional means of controlling the interaction of the materials to obtain optimum densification results and physi ⁇ cal properties of the resultant ceramic body.
• the precursor hydroxide coating may be converted to the desired oxide by simply heating the particles. Generally this will be performed at temperatures generally greater than about 20*C. and under conditions which will not cause degradation of the nitride or carbide. Thus the maximum temperature may be as high as conventional sintering temperatures if the conversion is performed in an inert atmosphere, but normally only up to about 250*C. when performed in the presence of oxygen to mini ⁇ mize any reaction with oxygen. Preferably, the conversion is performed by placing the material in a vacuum oven at about 180 °C. It should be noted that these temperatures are substantial ⁇ ly below those at which the corresponding bulk hydroxides con ⁇ vert to oxides.
• the nitride or carbide particles having a coating of sin ⁇ tering aid or precursor may also be used directly for slip cast ⁇ ing or shape forming procedures without drying or they may be dried in any conventional manner. Freeze-drying has been found to be particularly advantageous since it minimizes particle ag ⁇ gregation. Thereafter the sintering aid coated particles may be used in the same manner as previous ceramic material/sinter ⁇ ing aid mixtures, i.e. conventional forming procedures such as cold pressing, hot pressing, gas pressure sintering, hot isosta- tic pressing, and the like may be used. Further details on sin ⁇ tering procedures are not included herein as they readily exist in the open literature.
• the resultant nitride or carbide particles have surfaces which consist essentially of the sintering aid oxide or its hy ⁇ droxide precursor.
• silicon nitride particles coated with about 0.7 weight % yttria produced a coating about 2 angstroms thick (about a monolayer) .
• the yttria amount was increased to about 1%, the layer was about 2-3 angstroms; when the yttria amount was 2%, the coating was about 6-7 ang ⁇ stroms (about two monolayers) .
• the silicon nitride particles are believed to be uniformly coated even at the very low levels of sintering aid, but no direct means for confirming this is currently available.
• the pre ⁇ sence of a uniform coating is supported by relative acoustophor- etic mobility techniques (RAM) and by electron spectroscopy for chemical analysis (ESCA) , scanning transmission electron micros ⁇ copy (STEM) , and most importantly by the successful preparation of densified silicon nitride bodies from coated particles hav ⁇ ing less yttria than is required for conventionally prepared particle mixtures.
• RAM relative acoustophor- etic mobility techniques
• ESA electron spectroscopy for chemical analysis
• STEM scanning transmission electron micros ⁇ copy
• the coated nitride or carbide particles produced are in the substantial absence of any free sintering aid particles. It is believed that no free sintering aid particles are produc ⁇ ed by the surface precipitation process of the invention since none were detectable by examination of scanning transmission electronmicrographs of the products. However, this does not preclude the presence of trace amounts of some free sintering aid particles.
• the coated particles will be used in this condition, i.e. in the substantial absence of free sintering aid particles, since this is considered the most efficient way to utilize the sintering aid, it is also within the scope of the present invention to intentionally add free particles of one or more sintering aids to the sintering aid- coated particles. While this would partially defeat the pur ⁇ pose and some of the benefits of the surface precipitation pro ⁇ cess, it will not prevent the use of the sintering aid oxide- coated ceramic particles in many applications.
• the surface precipitation process also avoids any contami ⁇ nation problem which could occur when dealing with fine parti ⁇ cles of different sintering aids. Since no particles of sinter ⁇ ing aid oxides are milled to an ultrafine particle size, dedi ⁇ cated equipment for each particular sintering aid is avoided.
• While one aspect of the present invention is to form sin- tering aid-coated nitride and carbide particles in the substan ⁇ tial absence of free sintering aid particles, the process of preparing suspensions of finely divided particles of solid ma ⁇ terials when the finely divided materials will undergo a dele ⁇ terious reaction with the suspension medium is of more general application. It has been found that reactions between a fluid, i.e. water, and a freshly prepared finely divided particle may be precluded by the addition of surfactants which are soluble in the fluid and which protect the finely divided particles from attack by the fluid.
• Examples of materials which may be protected include silicon nitride, silicon carbide, boron ni ⁇ tride, aluminum nitride, and other carbides and nitrides.
• silicon nitride silicon carbide
• boron ni ⁇ tride silicon carbide
• aluminum nitride aluminum nitride
• other carbides and nitrides examples include silicon nitride, silicon carbide, boron ni ⁇ tride, aluminum nitride, and other carbides and nitrides.
• the best mode known for using this invention involves com ⁇ minution of materials comprising either silicon nitride or sili ⁇ con carbide in water to prepare powders upon which a sintering aid precursor hydroxide is surface precipitated so that the re ⁇ sultant powders are useful for molding and eventual densifica ⁇ tion to near-theoretical density.
• Examples 1-6 and Comparative Examples C-l - C-9 These examples and comparative examples demonstrate the comminution of silicon nitride particles in water which would normally be expected to attack the surfaces of the silicon ni ⁇ tride and convert them to silica.
• 250 g of silicon nitride powder and 750 g of dispersion medium were milled together with 3 kg of dense silicon nitride-yttria milling media in a vibratory mill (Sweco mill) . Each such lot was milled for about 18-20 hours.
• the surface areas of the pow ⁇ ders were determined before and after milling by conventional BET adsorption using nitrogen gas.
• the dispersion media used are shown in Table 1.
• Cavco Mod APG is an amino-functional zirocoaluminate surfactant having an inorganic polymer backbone, dissolved in propylene glycol, available from Cavedon Chemical Co., Inc., Woonsocket, R.I. Its total metal content is about 4.1 to 4.4 %, with the surface active component being about 20 % of the to ⁇ tal.
• the synthesis and chemical structure of Cavco Mod APG are set forth in U.S. Pat. No. 4,539,049, particularly in columns 2-4 thereof, and more particularly in lines 21-23 and lines 47- 52 of column 3 thereof, the subject matter of which is hereby incorporated by reference.
• “Coco” is an abbreviation for a mix ⁇ ture of coco alkylamine acetates. Chemical Abstracts Registry No. 6179C-57-6, available from Armak Chemicals, Chicago, IL. Table 1
• the milling conditions were determined by having perform ⁇ ed a preliminary screening for attractive operating conditions which would minimize the amount of ammonia generated during the milling.
• All of the comparative examples except C-6 which re ⁇ presents the currently established conventional art, generated substantial amounts of ammonia during milling.
• C-5 with some of the water replaced by isopropanol, produced less ammonia than the others but still did not make satisfactory powder in comparison with C-6.
• All of the Examples 1-6 generated less ammonia during the milling than did any of the comparative examples except C-4 or C-6.
• Examples 2, 4, and 5 all generated approximately equal amounts of ammonia during milling, smaller amounts of ammonia than the other examples, while Example 3, with more surfactant, generated more foam and was thus considered less desirable.
• Example 2 Several replications of Example 2 were made, and it was always found that the surface area of the resulting powder was within the range of 9.7 - 10.7 m 2 /g and the oxygen content 1.5 plus or minus 0.1%. These values are the same as those ob ⁇ tained for silicon nitride conventionally milled in alcohol as shown in Comparative Example C-6.
• Example 2 After milling, the slurry of Example 2 had a pH of about 9.4 in the bulk. The slurry is very stable and was readily fil- tered through conventional filters with opening sizes as small as 2 microns, without loss of more than 5% of the silicon ni ⁇ tride content. This is a further demonstration of the high quality of the dispersion obtained, and represents a useful technique for removing hard agglomerates or foreign particles that could be a source of weakness in bodies made by densifying the powders. The powder after drying by a preferred freeze dry ⁇ ing technique is fluffy and readily redispersible, at least as well as that powder prepared by conventionally alcohol milling.
• Examples 7 - 10 These examples illustrate the selective precipitation of one solid (yttrium hydroxide) on the surface of another (sili ⁇ con nitride) by control of surface activity of reagents with re ⁇ spect to the bulk activity.
• a slurry prepared as in Example 2 was filtered successively through filters having average open ⁇ ings of 10 microns, 5 microns, and 2 microns. Less than 2% of the slurry was retained which confirms that the suspension is very well deflocculated and that the suspension is well-stabil ⁇ ized by the surfactant covering the silicon nitride particles.
• nitric acid was added slowly with stirring to reduce the pH to about 7.
• An aqueous solution of yttrium nitrate having the desired amount of yttrium was then added slowly with stirring.
• an aqueous ammonia solution with con ⁇ stant monitoring of the pH of the bulk suspension.
• Precipita ⁇ tion of yttrium hydroxide on the surfaces of the silicon ni ⁇ tride particles commenced immediately.
• the amount of yttrium ion added to the suspension was sufficient to form yttria in an amount of 0.7% of the silicon nitride powder for Example 7, 1.4 % for Example 8, 2.8% for Example 9, and 4.0% for Example 10.
• the pH was gradually raised in the bulk of the dispersion medium by the addition of aqueous ammonia until the pH reached about 10.5. This was done to assure essentially complete precipitation of all the added yttrium ion as yttrium hydroxide on the surface of the suspended solid powder.
• the hydrolysis constant for yt ⁇ trium is about 12 and thus no yttrium hydroxide should have pre ⁇ cipitated in the bulk.
• the RAM measurements indicate that the surface layer form ⁇ ed in these Examples was capable of reversible hydration-dehy- dration cycles between hydroxide and oxide on drying at room temperature and redispersion in water.
• the RAM behavior in each case was closely analogous to that of colloidal yttria.
• the redispersed powder remained in contact with water, its RAM be ⁇ havior returned to that characteristic of yttrium hydroxide in ⁇ stead.
• the material present on the surface may have a somewhat different chemical reactiv ⁇ ity than conventional bulk yttrium hydroxide or yttrium oxide, but the material was capable of being converted to the oxide by drying and heating, such as would automatically occur if the coated powder were directly formed into a green body and densi ⁇ fied. Any such material is regarded herein as a precursor to the desired oxide.
• the oxide formed may also have a somewhat different chemical reactivity from bulk yttrium oxide because there was some evidence that the coated powders formed by this method densified more readily at a given temperature than would be expected for the amount of oxide present in the powder.
• the products were also studied by electron spectroscopy for chemical analysis ("ESCA").
• the spectra for powders with yttrium compounds coated on the surface are significantly dif ⁇ ferent from those for uncoated powders only in the presence of peaks ascribable to the yttrium compounds and in attenuation of the peaks ascribable to the 2p orbital electrons of silicon and the Is orbital electrons of nitrogen in samples with coatings.
• the thickness of the coating can be esti ⁇ mated from the equation:
• STEM images for powder from Examples 2 and 10 are shown in Figures 1 and 2 respectively.
• the powder shown in Figure 1 contains no yttrium coating and is relatively smooth, while that of Figure 2 with an yttrium surface precipitated coating is noticeably rough in comparison. This is consistent with a coverage of very fine microcrystals of yttrium hydroxide or oxide.
• the surface area of the yttria-coated powders increases with increasing yttria content and is substantially greater than silicon nitride with no yttria coating, as shown in Table 2.
• the powders prepared according to Examples 7-10 were con ⁇ ventionally cold pressed into green bodies and then densified in a hot isostatic press ("HIP") according to the teachings of U.S. Pat. No. 4,446,100.
• the densities achieved thereby were (in megagrams per cubic meter) 3.218 for Example 10, 3.186 for Example 9, 3.178 for Example 8, and 3.171 for Example 7.
• Scanning electron micrograph of chemically etched surfac ⁇ es of material densified as described above show that the sili- con nitride microcrystals have exceptionally long, asymmetric shapes with aspect ratios up to 8. The micrographs do not show any porosity even at 10,000 x magnification.
• X-ray diffraction patterns show that the conversion to beta silicon nitride ap ⁇ pears complete, with no other phases detectable; the sensitiv ⁇ ity is too low to detect the yttrium containing intergranular phase expected to be present.
• Comparative Example C-10 To evaluate the sintering ability of low levels of yttria added to silicon nitride conventionally, i.e. as particles dur ⁇ ing milling in isopropanol, as compared to the yttria-coated silicon nitride powders of the present invention, samples were prepared by each method at a rate of 0.7 and 1.0% yttria.
• the conventional silicon nitride compositions were prepar ⁇ ed by mixing the compositions with a small amount of isopropa ⁇ nol to form a slurry.
• the two slurries were each placed in a mill jar together with several silicon nitride balls and mixed for approximately twenty-four hours. Each slurry was removed and dried. About 55 g of each of the dried powders was formed into a billet by cold isostatic pressing at 30,000-50,000 psi.
• the samples were then degassed at 1350 ⁇ C. and then hot isostat- ically pressed in a glass encapsulant at 1860-1900*C. for one hour. Neither of these samples densified to full density, hav ⁇ ing densities of less than 3.17.
• machining of standard modulus of rupture bars from the billets was attempted. No such bars could be ma ⁇ chined because the billets were so cracked and porous.
• coated silicon nitride particles were prepared in accordance with the procedure of Examples 7- 10, i.e. having yttria deposited upon the particles as a sub ⁇ stantially uniform surface layer coating, but having 0.7 and 1.0% yttria respectively.
• the particles were formed into bill ⁇ ets by cold isostatic pressing and then hot isostatically press ⁇ ed in a glass encapsulant as were the conventional samples.
• the resultant billets were fully dense and uncracked. Modulus of rupture bars were machined from the billets of the 1.0% yt ⁇ tria composition and were found to have a room temperature flex ⁇ ure strength of 132 ⁇ 19 ksi.
• Example 11 The procedure of Examples 1-6 was repeated to demonstrate that materials other than silicon nitride can be protected from deleterious reactions with a suspension fluid during comminu ⁇ tion.
• the material comminuted was silicon carbide and the flu ⁇ id was water.
• Cavco Mod APG was used in one sample as the sur ⁇ factant at a level of 2% while the other sample contained no surfactant. Milling was continued for about 15 hours at which point the samples were evaluated for the presence of oxygen which was found at a level below about 1.8% in the sample con ⁇ taining the surfactant. The oxygen content was about 6% in the corresponding sample not containing the surfactant.
• Comparative Example C-ll The procedure of Examples 1-6 is repeated with a variety of different surfactants which were found either to cause exces ⁇ sive foam or to insufficiently protect the silicon nitride from degradation due to its reaction with the water dispersion medi ⁇ um.
• the surfactants tested and results of the milling tests were as shown in Table 4 which follows:
• Pluronic P123 is a polyol formed by the addi ⁇ tion of propylene oxide to the two hydroxyl groups of a propyl- ene glycol, having an average molecular weight of 4,400, and available from BASF;
• Daxad is a sodium salt of a polymerized alkyl naphthalene sulfonic acid and available from Diamond Sham ⁇ rock;
• Colloid 111M is an amine salt of a polycarboxylate and available from Calgon.
• Example 12 The procedure of Examples 7-10 is repeated except the yt ⁇ trium nitrate is replaced by magnesium nitrate which is used in an amount to produce a coating of about 0.5% surface precipitat ⁇ ed magnesium hydroxide on the silicon nitride particles.
• the hydrolysis constant for magnesium ion is about 12. According ⁇ ly, the pH of the deposition solution is maintained below about 10.5 until most of the magnesium has been deposited. Thereaf ⁇ ter, the pH is increased to about 11 to assure complete utilizat ⁇ tion of the magnesium.
• the resultant magnesium hydroxide coat ⁇ ed silicon nitride particles are then used directly to cast mod ⁇ ulus of rupture test bars.
• Example 13 The procedure of Example 11 was repeated to evaluate the effectiveness of other surfactants (1% by weight) at protecting silicon carbide from reaction with water milling medium while milling for 24 hours.
• the surfactants evaluated and results obtained are shown in Table 5.
• Example 14 The procedure of Examples 1-6 was repeated with a variety of different surfactants which were found to protect silicon ni ⁇ tride from undue deleterious reactions with water.
• the surfac ⁇ tants tested and the results of the milling tests are shown in Table 6 below.
• Example 1-6 The procedure of Examples 1-6 was repeated several times to produce water-milled silicon nitride particles which were dried and their particle sizes determined. The particles were then coated with 1% yttria by the procedure of Example 7 which were dried and their particle sizes determined. The average re ⁇ sults of the multiple different determinations were:
• Example 16 The procedure of Examples 7-10 was repeated to produce a 1% magnesia-coated silicon nitride.
• NBD-200 a conventional 1% magnesia-silicon nitride mixture, was used for comparative pur ⁇ poses.
• the coated material was sintered by the standard sinter ⁇ ing cycle used for the conventional NBD-200 and reached full density.
• the material was evaluated for hardness and toughness by Vickers indentation and for phase conversion.
• the results and typical values for NBD-200 were:

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• 1989
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