PROCESS FOR INCREASING STRENGTH OF STRUCTURAL CERAMIC MATERIALS BY FORMING CERAMIC COATING ON SURFACE AND PRODUCT FORMED THEREBY

Abstract: A method is disclosed for increasing the strength of and repairing surface defects in a structural ceramic material which comprises coating the structural ceramic material with a tractable preceramic coating material capable of pyrolyzing to form a ceramic coating and heating the coated structural ceramic material to a temperature sufficiently high to permit pyrolysis of the preceramic polymer for a period of time sufficient to form a ceramic coating on the surface of the structural ceramic material.

Description: PROCESS FOR INCREASING STRENGTH OF STRUCTURAL CERAMIC MATERIALS BY FORMING CERAMIC COATING ON SURFACE AND PRODUCT FORMED THEREBY Field of the Invention The invention relates to the strengthening of structural ceramic materials by forming a polymer-derived ceramic coating on the surface of the structural ceramic material. The invention additionally relates to a process for improving the strength of and repairing surface defects in structural ceramic materials by coating them with a polymer-derived preceramic layer and then converting the preceramic layer to a ceramic coating. More particularly, the invention relates to a process for strengthening structural silicon nitride and alumina materials by coating them with a polymer-derived preceramic layer and then converting the preceramic layer to a ceramic coating. In addition to strengthening structural ceramic materials and repairing surface defects, the invention can replace elaborate and costly fine machining of ceramic materials. Background of the Invention The use of manufactured structural ceramic products is compromised by the great reduction in strength of the structural material due to surface and near-surface defects induced by processing and machining operations or inherent in the material itself. Attempts to preserve the integrity of manufactured products, for example, glass materials, have been made by coating the product with various materials. Yoshida et al., U.S. Patent No. 4,370,385 describes coating a glass vessel with an organopolysiloxane composition to provide a scuf -masking coating over the glass. Wieczorrek et al., U.S. Patent No. 4,409,266, provides a shatterproof coating on glass by applying to the glass surface a silane adhesion promoter and a two-component system which reacts to form a polyurethane binder. Kurita et al., U.S. Patent No. 4,656,221, conceals graze marks on a glass bottle by coating the glass with a composition formed from polydiorganosiloxane components and a surfactant. The coating is applied to the glass surface as an emulsion and allowed to air dry. In general, the formation of ceramic coatings on substrates through the pyrolysis of preceramic polymer coatings is known. For example, Gaul, in U.S. Patent Nos. 4,395,460 and 4,404,153, forms a silicon carbide coating on a substrate by coating the substrate with a polysilazane polymer and then heating the coated substrate in an inert atmosphere or in a vacuum to an elevated temperature of at least 750°C. Seyferth et al., U.S. Patent Nos.4,482,669; 4,720,532; 4,705,837; and 4,645,807, teach forming an oxidation-resistant coating on otherwise oxidizable materials such as pyrolytic graphite by application of a preceramic polymer coating over the materials followed by pyrolysis of the preceramic coating to form a ceramic coating. The preceramic polymer materials respectively used in the Seyferth et al. patents include: (1) polysilazanes that are synthesized by strong base-catalyzed polymerization of cyclomethylsilazane, which is the ammonolysis product formed by reacting anhydrous ammonia with a mixture of dihalohydridosilanes and trihalosilanes; (2) polymers formed by reacting polysiloxane with a polysilylamide, which is an intermediate potassium salt of the polymer in (1); and (3) polymers formed by reacting an organopolysilane of the formula [(RSiH)χ(RSi) y] ^ with alkali metal amides or silylamides. See also Cramer, Ceramic Bulletin. Vol.68, No.2, 1989, pp.415- 419. Coblenz et al., in an article entitled "Formation of Ceramic Composites and Coatings Utilizing Polymer Pyrolysis" on pp.271-285 of a publication entitled "Emergent Process Methods for High-Technology Ceramics", edited by Davis et al. and published in 1984 by Plenum Publishing Corporation, describe the coating of carbon and silicon nitride materials with a dimethylsiloxydiphenylsiloxycarborane polymer and with a silazane oligomer. They report that the resulting coatings were of poor quality with shrinking and cracking of the coatings noted. Winter et al., U.S. Patent No. 3,892,583, and Verbeck, U.S. Patent No. 3,853,567, describe the formation of shaped articles of homogeneous mixtures of silicon carbide and silicon nitride. The homogeneous mixtures are also said to be useful in forming films, flakes, and coatings. Baney et al., U.S. Patent No.4,666,872, describes coating substrates with R3S-1NH- containing silazane polymers to which have been added certain precious metals, followed by heating the substrate to an elevated temperature of at least 750°C in an inert atmosphere or vacuum to form a ceramic-coated article. Bujalski, U.S. Patent No. 4,668,642, discloses coating substrates with R3S1NH- containing silazane polymers to which have been added certain boron compounds, followed by heating the substrate to an elevated temperature of at least 750°C in an inert atmosphere or vacuum to form a ceramic-coated article. While the use of such ceramic materials may generally protect the surface of a substrate against scratching or other abrasive action, as well as imparting some oxidation protection for oxidizable substrates such as the above-described carbonaceous materials, such coatings are not generally known to impart any physical strength to the substrate itself when applied as thin layers, e.g., less than 10 microns in thickness. Structural ceramic materials, however, in addition to needing surface protection such as afforded by the above discussed organic coating materials, need physical strengthening as well. Barbee et al., U.S. Patent No.4,781,970 describes strengthening of ceramics and glass- ceramics such as spumodumene (LΪ2θ-Al2θ3-4Siθ2) an( ^ cordierite (2MgO-2Al2θ3 * 5Siθ2) by chemical vapor deposition or sputtering deposition of Siθ2- The strengthening of the ceramic substrates by the invention of Barbee et al. is a result of the differential thermal coefficient of expansion (TCE) between the coating material and the substrate, with the coating material having a requisite lower TCE than the substrate, rather than being the result of filling in fine surface flaws. Furthermore, the deposition technique is time-consuming and expensive. It should be noted that certain aspects of the present invention are discussed in the following patents of common assignment herewith. Laine et al., U.S. Patent No. 4,612,383, describe a method of preparing polysilazanes by reaction of a starting material containing Si-H, Si-N, or Si-Si groups with hydrogen or an amine in the presence of a catalyst. Laine et al., U.S. Patent No. 4,788,309, detail a method of carefully controlling the reaction products obtained by reaction of a starting material containing Si-H, Si-N, or Si-Si groups with hydrogen or an amine in the presence of a catalyst and enabling further reaction of the products to give an additional set of compounds. This patent provides methods of making silazane compounds that are not necessarily polymeric. Blum et al., U.S. Patent No. 5,008,422, describe new precursors to high molecular weight polysilazanes and polysiloxazanes, the new high molecular weight polymers themselves, and methods of formation of the preceramic coating materials when such coating materials comprise polymers, for example, of polysilazane, polysiloxazane, and polysiloxane suitable for use as coating materials in the practice of the present invention. See also Blum at al., U. S. Patents 5,017,529 and 5,162,136. U. S. Patent 5,162,136 provides a process for strengthening glass by coating it with a preceramic layer and then coverting the preceramic layer to a ceramic coating. Blum et al., U.S. Patent No. 4,952,715, delineate a novel class of silazanes including cyclomeric silazane units within an oligomeric or polymeric structure. Blum et al., U.S. Patent No. 5,055,431, describe the synthesis of silazane and polysilazane compounds which have more than one cyclomeric silazane in their structure, the pyrolysis of these compounds upon fabrication to give ceramic coatings, fibers and articles as well as the use of the compounds as binders. The disclosures of these related patents are hereby incorporated by reference in their entirety. Additional aspects of the present invention are discussed in the following co-pending U.S. patent applications of common assignment herewith, the disclosures of which are also incorporated by reference herein. U.S. Application Serial No. 541,331, filed 19 June 1990, sets forth certain details of the present coating procedures and identifies specific preceramic materials useful therewith. Disclosure of the Invention It is an object of this invention to provide a process for treating the surface of a structural ceramic material by wet-coating with a ceramic precursor solution and converting the precursor at the surface to a ceramic material by heat process and thereby, to increase the strength of the structural ceramic material. The idea behind this invention is that deposition of ceramic precursor solutions at surface and subsurface defects, followed by pyrolysis, can restore or even improve the original strength of the material and repair damage caused by machining. The invention can replace elaborate and costly fine machining of ceramic materials. It is another object of the invention to provide a process for treating the surface of a structural ceramic material to increase the strength of the structural ceramic material wherein a ceramic coating is formed on the structural ceramic material surface. It is yet another object of this invention to provide a process for treating the surface of a structural ceramic material to repair surface defects and thereby, increase the strength of the structural ceramic material by pyrolyzing or heating a ceramic precursor coating material applied to the surface of the structural ceramic material by wet techniques, for example, dipping, drawing, spraying, painting and the like, to form a ceramic coating on the structural ceramic material surface. It is a further object of this invention to provide a process for treating the surface of a structural ceramic material to increase the strength of the structural ceramic material which has been preselected to provide optimal viscosity, wetting and adhering properties, and ceramic yield, and which is applied to the surface of the structural ceramic material to form a ceramic coating on the structural ceramic material surface upon pyrolysis. It is yet a further object of this invention to provide a structural ceramic material having a ceramic coating formed thereon to increase the strength of the structural ceramic material by pyrolyzing or heating a ceramic precursor coating material, which has been preselected to provide optimal viscosity, wetting and adhering properties and ceramic yield, and applied to the surface of the structural ceramic material to form a ceramic coating on the structural ceramic material surface upon pyrolysis. It is yet a further object of this invention to provide a structural silicon nitride material having a ceramic coating formed thereon to increase the strength of the structural silicon nitride material by pyrolyzing or heating a ceramic precursor coating material which has been preselected to provide optimal viscosity, wetting and adhering properties and ceramic yield, and applied to the surface of the structural silicon nitride material to form a ceramic coating on the structural silicon nitride material surface upon pyrolysis. These and other objects of the invention will be apparent from the following description. In accordance with the invention, a method is provided for improving the strength of a structural ceramic material which comprises: coating the structural ceramic material with a tractable ceramic precursor coating capable of pyrolyzing to form a ceramic coating; and heating the coated structural ceramic material to a temperature sufficiently high to permit conversion of the ceramic precursor for a period of time sufficient to form the ceramic coating on the surface of the structural ceramic material. Detailed Description of the Invention Before the present methods are disclosed and described, it is to be understood that this invention is not limited to the specific coating process and subsequent pyrolysis of coated structural ceramic materials, or to the specific structural ceramic materials or ceramic coated structural ceramic materials, i.e., polycyclomethylsilazane-coated silicon nitride, or the like, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. It must be noted that, as used in the specification and the appended claims, the singular forms "a", "an" and "the" include plural references unless the context clearly dictates otherwise. Thus, for example, reference to "a layer" includes multiple layers of ceramic material, reference to "a coating" includes multiple coatings with a ceramic coating material, and the like. In the specification and in the claims which follow reference will be made to a number of terms which shall be defined to have the following meanings: The term "tractable", as used herein with respect to the preceramic coating material used in the practice of the invention, is intended to define a material which is soluble in organic or inorganic solvents, meltable, or malleable, or which can be processed like an organic polymer to form a desired shape, i.e., in this case, a coating on a ceramic substrate. By use of the terms "pyrolysis" or "pyrolyze" is meant the transformation of the preceramic coating material or precursor, to a ceramic product by a heating process in which a multiple thermal reactivity occurs, or the reaction of such materials with oxygen or nitrogenous gas or other gases present during the pyrolysis to form materials separable from the resulting ceramic coating on the structural ceramic material. It also may be defined as the minimum temperature at which formation of a ceramic coating occurs for any given preceramic coating material, e.g., polymer or precursor, by losing its functional groups or nature coincidentally with the extensive formation of crosslinking. The "ceramic" which is formed by the pyrolysis may be defined as an inorganic material that forms a highly crosslinked network of covalent (sigma) bonds which may contain additional coordination bonds. In most cases, except carbon, the ceramic material contains at least two chemical elements. In some cases both are metals or metalloids, e.g., borides and suicides, or non-metals, e.g., PN, ASS3, or S.1PO4. The ceramic material can be in an amorphous, crystalline, glass-ceramic, or solid solution form and usually is stable at high temperature. The terms "structural ceramic material" or "ceramic substrate" are used interchangeably herein to define the material or substrate onto the surface of which the polymer-derived ceramic coating is formed by pyrolyzing a preceramic coating material applied to said surface as herein disclosed. The term "ceramic" to describe the substrate is as defined above. By way of example, and not of limitation, the structural ceramic materials may include Si3N SiC, AI2O3, Z1 2, aluminum silicates, aluminum titanates, mullite, aluminum borosilicates, A1N, and TiN, or mixtures thereof. The term "ceramic yield" of the ceramic precursor coating material, such as a precursor or preceramic polymer, upon pyrolysis, as used herein, is intended to define the ratio of the weight of the ceramic coating after pyrolysis to the weight of the coating before pyrolysis. The term "ceramic precursor" as used herein is intended to include inorganic and organometallic compounds, inorganic polymers, and organometallic polymers, while the term "preceramic polymer" is intended to define a tractable compound with any number of monomeric units which is sufficient to be deposited as a coating on substrates and to form ceramic compositions upon pyrolysis or heat treatment. Both terms are intended to be included in the term "ceramic precursor coating material". The invention provides a process for improving the strength of a structural ceramic material by forming a ceramic coating on the surface of the structural ceramic material. The ceramic coating is formed by applying a preceramic coating material to the surface of the structural ceramic material and then pyrolyzing or heating the preceramic coating material at a temperature sufficiently high to permit the conversion of the precursor coating to a ceramic coating. When structural ceramic substrates are coated with ceramic, in accordance with the practice of the invention, the strength of the structural ceramic material may be increased, on the average, by 15%. A few examples have shown increases of up to 20% to 30%. While not wishing to be bound by any particular theory as to why such strength increases occur, we have theorized that the ceramic coating material may act to increase the strength of the substrate material by healing surface defects in the substrate material. The present inventors further postulate that chemical or physical interactions between the coating and the substrate play a role in the strengthinging mechanism. Stress-corrosion effect may be another mechanism to improve the strength. Chemical interactions between the substrate and the coating may be advantageous but migration of material from the substrate surface area to the coating should be prevented if it will cause the formation of undercoating holes and pinholes and/or cracks in the coatings. The preceramic coating material, e.g., a polymer or precursor, from which the ceramic coating will be formed may comprise any tractable inorganic or organometallic polymer or compound capable of being in a liquid form or in solution and of wetting and adhering to the surface of the structural ceramic material and capable of converting to a ceramic coating on a ceramic substrate by heat treatment. By way of example, and not of limitation, the ceramic coatings which may be formed from such precursors may contain Si3N SiCN, SiC, C, SiON 2 , SiO 2 , 2kO 2 , Al 2 O 3 , Y 2 O 3 , A1N, B 4 C, BN, TiC, WC, W 2 C, Mo 2 C, TiN, TiO 2 , CaP2θg, and others, metal phosphates, metal silicates, metal borates, as well as other ceramics known for their strength and hardness, including other suicides, borides, nitrides, carbides, or oxides, or mixtures thereof. Examples of preceramic coating materials such as polymers or precursors from which such ceramic coatings may be formed comprise polysilazanes, such as poly-N-methylsilazane and/or polycyclomethylsilazane, from which ceramic compositions of SiN, SiCN, Siθ2, SiOC, SiON and SiOCN (depending upon the atmosphere used during pyrolysis) may be formed; polysiloxazane; polysiloxane, including polymethyl-silsesquioxane and polyhydridomethylsiloxane from which ceramic coating containing Si and O and potentially other elements such as C and N can be formed; polysilanes; polycarbosilanes; polyboranes; polycarboranes; polyaminoboranes; polyaminotitanium; and combinations thereof. Examples may also include precursors to Tiθ2, A1N, AI2O3, Zrθ2, meta l phosphates, and others. The above described preceramic coating materials either may be obtained commercially or may be readily synthesized using methods know to those skilled in the art. Reference may also be had to U.S. Patent Nos. 4,612,383, 4,788,309, 4,952,715, 5,008,422 and 5,055,431, cited and incorporated by reference above. In addition, ceramic precursor coating materials may include compounds and polymers which are totally inorganic and soluble in water. By way of example, and not of limitation, inorganic ceramic precursor coating materials may include Caζ^PO^ and AIO^PO^. Precursors made by sol-gel technology may also be used. However, they are expected to be less favorable due to short term stability in solution or on the shelf, and low ceramic yields and high shrinkage are obtained upon their pyrolysis. It is preferable that the ceramic yield of the precursor be at least 50 wt.%. More preferably, it should be above 70 wt.% and most preferably, it should be above 85 wt.%. The structural ceramic material which may be strengthened by formation of the ceramic coating thereon, in accordance with the invention, may comprise any particular shape, including flat sheets, and shaped objects, such as bars, rods, fibers or the like. The structural ceramic material which may be strengthened by formation of the ceramic coating thereon, in accordance with the invention, may include, by way of example and not of limitation, silicon nitride and AI2O3. It may sometimes be preferred or necessary to pretreat the structural ceramic material with HF or other acids or tetrachlorosilane in order to hydroxylate the surface and thereby to optimize the coating of the ceramic substrate with the preceramic coating material. Whether or not it is necessary to pretreat the structural ceramic material and the methods whereby such pretreatment is to be effected may be determined using routine experimental techniques. The choice of the preceramic coating material and the pyrolysis schedule may vary depending on the substrate material. It is not necessary that a type of polymer that performed well for one kind of material will strengthen another type of material. The chemical interactions at the interface between the coating and the substrate is assumed to play a role in the strengthening mechanism. Therefore, the development of a strengthening process for various materials requires screening of polymer and process conditions individually for each material (see Examples 1-3). The preceramic coating material may be applied to the ceramic substrate by any convenient method such as by dipping in a selected coating solution or by spraying, painting, spinning, or the like, with such coating solution, the solution having a predetermined concentration preferably between 0.1 and 100 wt.%, more preferably between about 5 and 30 wt.% for most applications. The preceramic coating material is applied to the ceramic substrate in an amount sufficient to provide, upon subsequent pyrolysis, a ceramic coating of from about 0.01 to about 20 microns and preferably from about 0.05 to about 4 microns. Typically, the desired ceramic coating thickness is achieved in a single coating layer, i.e., without the use of several built-up layers. However, if necessary, the coating process may be repeated to build up the desired thickness without the formation of a substantial level of cracks in the developed ceramic coating. Li addition, cracked coatings can be healed by additional coating procedure. Achieving the desired thickness of the eventual ceramic coating layer on the structural ceramic material is related to both the initial coating thickness of the preceramic coating material and the ceramic yield of the preceramic coating material, e.g., polymer or precursor. The coating thickness of the preceramic coating material is, in turn, related to the viscosity of the coating material solution and the amount of solvent added to it to permit it to be applied to the surface of the ceramic substrate. The ceramic yield is related to: (1) the molecular structure with reference to whether the coating material comprises a branched, crosslinked, or ring-type polymer; (2) the molecular weight of the coating material with higher molecular weight polymers favored because of the higher ceramic yield of such materials; (3) the latent reactivity of the coating material that allows thermosetting properties and further crosslinking as desired in (2); and (4) small amounts of extraneous organic groups if needed to provide tractability and shelf stability. Generally speaking, to achieve the desired ceramic coating thickness uniformly across the surface of the ceramic substrate, the preceramic coating material should have a sufficient molecular weight, e.g., be sufficiently polymerized, to provide a minimum viscosity, after removal of solvent, of at least 1-3 poise so that the coating will remain in a uniform thickness on the structural ceramic material as the coating is heated to the pyrolysis temperature. Alternatively, the precursor can be a monomeric compound capable of condensation or cross- linking at the substrate surface prior to evaporation. In some instances the preceramic coating material, although sufficiently polymerized to achieve the desired ceramic yield, will have a viscosity sufficiently low to permit it to be applied to the structural ceramic material as a uniform and homogeneous coating without the necessity of diluting the coating material in a solvent, i.e., the "tractable" coating material, such as a polymer, will be liquid or, by application of heat, meltable. However, in other instances, the preceramic coating material will need to be diluted to lower the viscosity sufficiently to facilitate application of the preceramic coating material as a coating on the surface of the ceramic substrate and to control the thickness of the coating. In such cases, the preceramic coating material must be capable of being dissolved in a solvent which may later be removed from the coating without negative impact on the desired formation of the ceramic coating layer, i.e., the "tractable" coating material must be soluble. . Thus, in any event, the preceramic coating material, such as a polymer or precursor, must not only be of sufficiently high molecular weight or non-volatile, or cross-linkable, to achieve the desired minimum ceramic yield, but it must be "tractable" as well, as previously defined. The preceramic coating material may be applied to the structural ceramic material at ambient temperature, i.e., about 20-25°C, or either the coating material or the structural ceramic material may be at an elevated temperature. In cases where the preceramic polymer is sensitive to moisture and/or oxygen, it is preferable to deposit the solution coating under a dry and/or oxygen- free environment. This is especially preferable in cases where it is desireable to exclude oxygen from the final coating composition. However, from a practical processing point of view, it is preferable to use a polymer that is not air sensitive or only slightly air sensitive as this will not require special costly equipment and more expensive operation. In cases where oxygen is desired to be in the final coating ceramic composition, polymers sensitive to air may be preferably coated in air or exposed to air immediately after the coating procedure and prior to heat treatment to allow curing at low temperature. The minimum temperature to which the coated structural ceramic material must be heated will be determined by the particular preceramic coating material used and the minimum temperature at which a ceramic material may be formed from such a preceramic coating material. Thus, the minimum pyrolysis temperature must be at least at, and preferably above, the temperature at which the preceramic coating material converts to a ceramic material. This determination of the conversion of the preceramic coating material to a ceramic material may be TGA, IR, XRD, NMR, and elemental analysis. The minimum temperature, however, must be high enough to permit organic groups, if contained in the preceramic coating material, e.g., polymer or organometallic compound, to be pyrolyzed, leaving only an inorganic ceramic network. For example, when a poly-N- methylsilazane polymer having a molecular weight of 800-3000 Daltons is used, the minimum temperature needed to remove extraneous methyl groups from the coating and to form the ceramic material is about 300°C to 550°C. However, higher temperatures may be needed to allow desired chemical or physical interactions between the coating and the substrate or to densify or crystalize the ceramic coating materials. The preceramic coating material may be pyrolyzed to convert it into a ceramic coating using a variety of heating schedules, including different heating rates, dwell times at intermediate and maximum temperatures, and cooling rates, depending upon the specific material used. In any of the embodiments used for heating the coated ceramic substrate to the pyrolysis temperature, the coated ceramic substrate is held at the pyrolysis temperature for a period of time sufficient to permit formation of the ceramic coating and to develop the desired strength. It will be understood that if less than the optimal dwell time is used for the particular preceramic coating and structural ceramic material, a ceramic coating may be formed over the ceramic substrate in accordance with the invention, but the coated ceramic substrate may not have as much strength or hardness as is possible if the dwell time is extended. There will then be a trade-off between desired strength and process economics. The holding or dwell time of the coated ceramic substrate at the pyrolysis temperature may, therefore, vary from a minimum of 0 minutes, preferably at least about 5 minutes, and most preferably at least about 90 minutes, up to a maximum time of about 2 hours. Longer time periods may be used, but are usually unnecessary and, therefore, not economically justifiable. In some instances shorter dwell times of, for example, 30-60 minutes may be used, even though maximum strength has not been developed, if the economics of the process justifies the trade-off of reduced dwell time with reduced strength. It should be further noted, with respect to the period of time used to initially heat the coated structural ceramic material up to the pyrolysis temperature, that it is desirable, from a standpoint of economics, to conduct the warm-up period, dwell period at the pyrolysis temperature, and cooling-off period in as short a period of time as possible. Preferably, the entire pyrolysis period is carried out in a period of about 2 hours. By "the entire pyrolysis period" is meant from the time the coated ceramic substrate is placed in the furnace at room temperature until the temperature during cooling again reaches 200"C. The coated-structural ceramic material, during both the temperature ramp-up period and the dwell period, may be maintained in an inert atmosphere such as argon, a nitrogen- containing atmosphere such as N2 or ammonia, a hydrogen atmosphere, or mixtures of the aforementioned atmospheres, and a dry- or wet-air atmosphere, depending upon the chemical constituents of the preceramic polymer and the desired ceramic-coating composition. For example, the formation of a ceramic with Si-O bonds is favored when using an air atmosphere whereas the formation of a ceramic with Si-N bonds is favored when the preceramic polymer contains both silicon and nitrogen and either an inert or a nitrogen-containing atmosphere is used during the entire pyrolysis period. The choice of the pyrolysis environment in conjunction with the preceramic polymer structure will determine the final ceramic composition. Any precursor that is pyrolyzed in air will provide a final composition consisting mainly of oxide material. If the pyrolysis is conducted in an inert atmosphere such as nitrogen or argon, the final composition will contain substantial amounts of carbon if the precursor contained organic pendant groups or carbon was an element in the polymer skeleton (e.g., polycarbosilane). If nitride coating is desired then the polymers should be preferably pyrolyzed in ammonia. Oxynitride and oxycarbide coatings can also be formed if oxygen is incorporated in the skeleton of the original polymer or during the pyrolysis period. The choice of the final coating material might vary based on the application and the physical and/or chemical conditions at which the product is going to perform. However, for economic reasons, it is preferable to conduct the pyrolysis in air. The following examples will serve to further illustrate the invention, including some of the parameters of the process. It is to be understood that while the invention has been described in conjunction with the preferred embodiments thereof, that the foregoing description and the examples which follow are intended to illustrate and not limit the scope of the invention. Other aspects, advantages and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains. Example 1 Five ground, highly polished beveled silicon nitride bars, the dimensions of which were approximately 50 mm x 4 mm x 3 mm, were ultrasonically cleaned and then dehydrated by heating at 500"C under nitrogen for 1 hour. Bars were then dipped into a 10 wt.% polymer solution in tetrahydrofuran (unless otherwise indicated) under a selected atmosphere and pyrolyzed as described below. (Pyrolysis conditions are set forth in Table 1 below.) The pyrolysis process was caπied out under the selected atmosphere (1 atm) according to the following schedule: (a) heating rate of 5°C or 10°C/minute (see Table 1), (b) dwell period at the maximum temperature for 1 hour, and (c) cooling rate of 10°C/min. The entire coating and pyrolysis procedure were repeated 3 times. The bars were tested for strength in accordance with the four-point bend test of ASTM C158. Results are set forth in Table 1. These results demonstrate that the strength of ground and lapped silicon nitride bars can be significantly improved by forming a ceramic coating on the surface by the process described above. The results also show that similar or greater strength can be achieved when the precursor coating material is converted to a ceramic by heating in air or a nitrogenous gas. a Rate of temperature increase in β C per min/dwell temperature in β C/duration at dwell temperature in hours. Each line entry represents a sequential step in the heating schedule. k Heat treated 3 times to 900°C at a rate of 10 β C per min with a dwell period of 2 hours each time. c Polycarbosilane in tetrahydrofuran solution. The polycarbosilane was treated with 50 ppm Ru3(CO)2 as a crosslinking catalyst found to effectively increase the ceramic yields. " ■ Coated and dried twice in air. e Coated and dried three times in N2. * Polycyclomethylsilazane in tetrahydrofuran solution. S Coated and dried twice in N2. h Solution in H 2 O. 1 Coated and dried three times in air. . Two extremely weak bars were included in this batch of five bars indicating major internal flaws, the other three bars obtained strength values of 887, 941, and 978 MPa. ^ Poly-N-methylsilazane in tetrahydrofuran solution. Examnle 2 The procedures of Example 1 can be followed using as-ground silicon nitride bars and thereby achieve essentially identical results (see Table 2). Table 2 Results of Four-Point-Strength Measurements of Coated As-ground Si3N4 Bars a Rate of temperature increase in β C per min dwell temperature in "C/duration at dwell temperature in hours. Each line entry represents a sequential step in the heating schedule. ° Polycyclomethylsilazane in tetrahydrofuran solution. Example 3 The procedures of Example 1 can be followed using Al 2 O3 bars and thereby achieve essentially identical results (see Table 3). Table 3 Results of Four-Point-Strength Measurements of Coated Al 2 O3 Bars a Rate of temperature kcrease in per πiin/dwell te temperature in hours. Each line entry represents a sequential step in the heating schedule. D Heat treated 3 times to 900"C at a rate of 10 β C per min with a dwell period of 2 hours each time. c Poly-N-methylsilazane in tetrahydrofuran solution. " Polycarbosilane in tetrahydrofuran solution.

Claims: WHAT TS CLAIMED TS: 1. A method for improving the strength of and repairing surface defects in a structural ceramic material which comprises: a) forming a tractable ceramic precursor coating solution by dissolving in an organic solvent or in water an inorganic or organometallic compound or an inorganic or organometallic polymeric preceramic coating material to provide a liquid preceramic coating solution: 1 ) capable of convertng by heat treatment after solvent evaporation to a ceramic coating selected from the group consisting of nitrides, carbides, oxides, silicides, and borides; 2) capable of providing a ceramic yield of at least 50 wt.%; 3) being a solid or having a viscosity, after application to the structural ceramic material and removal of solvent, of at least 1 poise; and 4) capable of wetting and adhering to the structural ceramic material to form a uniform coating on the surface of the structural ceramic material; b) coating said structural ceramic material with said preceramic liquid coating solution; c) heating said coated structural ceramic material in a gaseous environment selected from the class consisting of an inert atmosphere, a nitrogen-containing atmosphere, a hydrogen atmosphere, an oxygen atmosphere, and a dry- or wet-air atmosphere, or mixtures of the aforementioned atmospheres, to a pyrolysis temperature at least capable of converting said preceramic liquid coating solution to a ceramic coating; and d) maintaining said coated structural ceramic material at said pyrolysis temperature for a period of time sufficient to form the ceramic coating and remove those portions of said coating material which do not participate in forming said ceramic coating. 2. The method of claim 1 wherein said step of coating said structural ceramic material with said preceramic liquid coating solution further comprises coating said structural ceramic material with an amount of said preceramic liquid coating solution sufficient to form a ceramic coating, after said pyrolysis step, having a thickness of from about 0.01 to about 10.0 microns. 3. The method of claim 2 wherein said thickness is from about 0.05 to about 5.0 microns. 4. The method of claim 1 wherein said preceramic liquid coating solution is comprised of an organic based polymeric preceramic liquid coating solution. 5. The method of claim 1 wherein said preceramic liquid coating solution is comprised of an inorganic based polymeric preceramic liquid coating solution. 6. The method of claim 1 wherein said inorganic compound is comprised of an inorganic salt. 7. The method of claim 6 wherein said inorganic salt is comprised of a metal hydridophosphate salt 8. . The method of claim I wherein said tractable ceramic precursor coating solution is comprised of an organometallic polymeric preceramic liquid coating solution. 9. The method of claim 8 wherein said organometallic polymeric preceramic liquid coating solution is comprised of a solution of an organometallic compound capable of crosslinking or decomposition prior to evaporation. 10. The method of claim 1 wherein the concentration of said liquid preceramic coating solution is between 0.1 and 100 wt.%. 11. The method of claim 10 wherein the concentration of said liquid preceramic coating solution is between 5 and 30 wt.%. 12. The method of claim 1 wherein the ceramic yield is at least 70 wt.%. 13. The method of claim 12 wherein the ceramic yield is at least 85 wt. % . 14. The method of claim 1 wherein said step of coating said structural ceramic material with said preceramic liquid coating solution is repeated at least one additional time. 15. The method of claim 14 wherein said step of coating said structural ceramic material with said polymeric preceramic liquid coating solution is repeated 2 to 5 times. 16. The method of claim 1 wherein said gaseous environment comprises a hydrogen atmosphere . 17. The method of claim 1 wherein said gaseous environment comprises an oxygen atmosphere. 18. The method of claim 1 wherein said pyrolysis step further comprises heating said coated structural ceramic material at a rate of from about 30°C per hour up to a rate of about 3300°C per hour up to at least said pyrolysis temperature. 19. The method of claim 18 wherein said pyrolysis step comprises heating said coated structural ceramic material at a rate of from 100 β C per hour to about 600°C per hour. 20. The method of claim 1 wherein said pyrolysis step further comprises heating said coated structural ceramic material up to said pyrolysis temperature by inserting said coated structural ceramic material into a pyrolysis zone already preheated to said pyrolysis temperature. 21. The method of claim 1 wherein said maintaining step further comprises maintaining said coated structural ceramic material at said pyrolysis temperature for a period of time of at least about 10 minutes. 22. The method of claim 21 wherein said maintaining step further comprises maintaining said coated structural ceramic material at said pyrolysis temperature for a period of time of at least about 60 minutes. 23. The method of claim 1 including the further step of cooling said coated structural ceramic material from said pyrolysis temperature down to a temperature of about 200 * C at a rate not exceeding about 60°C per minute. 24. The method of claim 1 including the further step of cooling said coated structural ceramic material from said pyrolysis temperature down to a temperature of about 200°C at a rate not exceeding about 30 β C per minute. 25. The method of claim 1 including the further step of cooling said coated structural ceramic material from said pyrolysis temperature down to a temperature of about 200 β C at a rate not exceeding about 10 β C per minute. 26. The method of claim 1 wherein said structural ceramic material is silicon nitride. 27. The method of claim 1 wherein said structural ceramic material is A-2O3. 28. The method of claim 1 further comprising pretreating said structural ceramic material with acid prior to coating with said preceramic liquid coating solution. 29. The method of claim 28 wherein said acid is comprised of HF. 30. The method of claim 1 further comprising pretreating said structural ceramic material with tettachlorosilane prior to coating with said preceramic liquid coating solution. 31. A structural ceramic material coated with a layer of ceramic material to increase the strength of said structural ceramic material and formed by coating said structural ceramic material and heating said coated structural ceramic material at a temperature sufficient to form said ceramic coating on said structural ceramic material. 32. The ceramic coated structural ceramic material of claim 31 wherein said ceramic-coated structural ceramic material comprises silicon nitride coated with ceramic. 33. The method of claim 1 wherein said gaseous environment comprises an inert atmosphere. 34. The method of claim 1 wherein said gaseous environment comprises a nitrogen-containing atmosphere. 35. The method of claim 33 wherein said inert atmosphere is comprised of Ar. 36. The method of claim 34 wherein said nitrogen-containing atmosphere is comprised of N . 37. The method of claim 34 wherein said nitrogen-containing atmosphere is comprised of NH3.

Inventors: BLUM YIGAL D, MCDERMOTT GREGORY A, BLUM, YIGAL, D., MCDERMOTT, GREGORY, A.
Applicants: STANFORD RES INST INT, SRI INTERNATIONAL
Application year: 1993
Main classification: C04B41
IPC codes: C04B41