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Scripta Metallurgicaet Materialia,Vol. 31, No. 8, pp. 1025-1030,1994 Copyright©1994ElsevierScienceLid Printed in the USA. All rights reserved 0956-716X/94 $6.00 + 00 CONFERENCE SET No. 2
INTERFACES OF POLYMERIC PRECURSORS WITH CERAMICS/METALS Sylvia M. Johnson, Yigal D. Blum, Gregory A. McDermott, and Michael I. Gusman SRI International, Menlo Park, CA 94025-3493
(Received May 6, 1994) Introduction Preceramic polymers that yield ceramic materials on heating have been investigated as precursors to fibers, binders (1), powders, and coatings (2). These materials have also been used as processing additives for ceramics, rheological control agents, and aids for controlling the microstructure of ceramics and coatings. The reactivity of the precursors with ceramic and metal powders or with substrate surfaces opens up many possibilities for improved processing of materials, fabrication of novel materials, and the preparation of materials and coatings with improved properties. Although reactions at interfaces between polymeric precursors and ceramics or metals are not yet well understood, the beneficial effects of using precursors have been demonstrated. Preceramic polymers consist of inorganic skeletons substituted (in most cases) with organic pendant groups. They can be fabricated similarly to organic polymers but are converted to a ceramic on heat treatment. Heat treatment involves an initial curing step in which the polymer is crosslinked to prevent melting during the second heating stage, pyrolysis, and to increase ceramic yields. Curing may be achieved by heating, adding crosslinking additives, or exposing the material to a reactive environment. After curing, the polymer is pyrolyzed (at 500 ° to 900°C). The polymer substituents are removed as volatile compounds, and the polymer skeleton is converted into an amorphous, three-dimensional ceramic network. Ideally, only a small amount of the material and none of the inorganic skeleton is volatilized during the pyrolysis. The pyrolysis time, temperature, and atmosphere affect the final conversion yield, composition, and integrity of the product, and differ for various applications. If the application requires, the amorphous ceramic material can be crystallized by further heat treatment at higher temperature. The crystallization temperature is a function of the ceramic type, composition, impurities, and incorporated additives. Interfacial reactions between polymers and other materials depend on the polymer composition and rheological properties, the ceramic or metallic material composition and surface states, and the heat treatments and environments. Interfaces range from relatively simple interfaces between a pure polymer-derived material and a substrate, as reflected by the formation of coatings or the use of polymers to infiltrate porous materials, to complex interfaces between polymers, various powders, and occasionally substrates. Examples of complex interactions include the use of precursors as binders or microstructural control agents and the direct formation of bulk materials or matrix materials from polymer/powder compositions. This paper gives specific examples of the interactions of polymeric precursors with ceramics to provide controlled microstructure development during sintering of Si3N4 (in-situ growth of elongated grains) and to form near-net-shape mullite and mullite-based coatings with unique properties. Experimental Techniques Preceramic Polymer Development The polymers used in our work are part of ongoing research at SRI International to design, develop, and use a variety of preceramic polymers. We have developed novel synthesis methods for polymeric precursors to Si3N4, SiC, Si3N4]SiC, Si2ON2 SiO2, SiOC, silicates, TiN, TiO2, A1N, and BN. These compounds can be liquids or solids and are soluble in organic solvents, and can be mixed with powders by wet techniques or applied to surfaces from solutions.
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Vol. 31, No. 8
SRI has developed a novel chemical method for synthesizing, modifying, and curing silicon-based preceramic polymers that allows the preparation of precursors to many ceramic compositions and synthesis of polymers with a wide range of viscosities and high ceramic yields. Variations in heat treatment conditions control the ceramic yield and composition. The approach is based mainly on a catalyst-induced dehydrocoupling reaction between compounds containing Si-H bonds and compounds containing either N-H or O-H bonds to form Si-N or Si-O bonds, respectively (3). Currently, 10 to 50 ppm of a transition metal catalyst, Ru3(CO)12, is used for polymer synthesis and modification, control of rheology and product compositions, and improvement in curing behavior. This approach yields highly processable preceramic polymers that have ceramic yields of 70 to 95 wt%. The polymers used in this study were polycyclomethylsilazane (PCMS, [CH3SiHNH]x ) a precursor to silicon nitride and polyhydridomethylsiloxane (PHMSO, [CH3SiHO]x) a precursor to silica. As the polysilazane is moderately sensitive to moisture in the atmosphere and to oxidation, mixtures containing this polymer were stored and handled in inert atmospheres as much as possible. The PHMSO technology uses a very inexpensive by-product of the silicone industry that is not useful as a ceramic precursor because of the negligible ceramic yields obtained on pyrolysis. However, after the addition of 50 ppm of catalyst and exposure to atmospheric moisture, the polymer cross-links rapidly and the product can be converted to silica, silicon-oxycarbide (so-called "Black Glass"), or silicon oxynitride with ceramic yields of 80 to 95 wt%. ln-situ Reinforced Silicon Nitri~l~
Silicon nitride has emerged as the leading candidate for structural applications in heat engines. It is strong, relatively tough for a monolithic ceramic, and its thermal properties (shock, conductivity, expansion) properties are acceptable for many applications. It also has excellent wear and erosion resistance and good oxidation resistance. Although silicon nitride is the topic of considerable research, there are still problems associated with inadequate properties and the high cost of fabrication. Composite materials have higher toughnesses than the monolithic materials, but the fabrications costs are generally too high for applications such as automotive engines. One technique for improving the toughness of silicon nitride has been the concept of "in-situ" reinforcement by the growth of whisker-like grains of silicon nitride within the microstructure. Current techniques to induce growth of elongated 13-Si3N4 grains require high sintering temperatures and pressures or incorporation of large amounts of oxides, which can reduce the strength and increase creep at elevated temperatures. We have explored the use of preceramic polymers as processing aids for low cost fabrication of silicon nitride. The precursors offer the advantages of ease of processing during shape-forming and handling, sintering at modest temperatures and pressures, and the formation of a desirable elongated grain microstructure. Silicon nitride pellets and bend bars were die-pressed from dry mixtures of 20 wt% PCMS mixed with 80% powder [92 wt% Si3N4 (Ube SNE-10), 5 wt% Y203, and 3 wt% A1203]. The pressed pellets were cured at 150°C, pyrolyzed by heating to 900°C in NH 3 and sintered at 1750 ° to 1800°C for 4 to 6 hours under -700 kPa of N2. Sintered samples were sectioned, polished, and examined by scanning electron microscopy (SEM). Density was measured by Archimedean techniques or by helium pycnometry. Shrinkage and weight loss were measured at various stages in the heat treatment. Strength was determined by 4-point bend tests on bars - 3 mm x 4 m m x 50 ram. Near-Net-Shaoe Mullite Mullite, 3A1203.2SIO2, has both high temperature structural and electronic applications. It is refractory, has a good thermal shock resistance, low thermal expansion coefficient, and a low dielectric constant. The properties of mullite depend on the composition (the alumina/silica ratio), the synthesis method, and the processing conditions. Many approaches have been taken to the formation of mullite, including formation from clays, sol-gel routes, organometallic compounds, coprecipitation, and reaction sintering (4,5). Oxide composites are of interest for applications such as turbines engines because of their oxidation resistance. Toughened mullite is also of interest for electronic applications where toughened ~ubstrates are required. Inf'fltration of fiber preforms with matrix materials requires precursor mixtures with suitable rheological properties. Preceramic polymers provide a very promising route to forming mullite matrix materials for oxide composites.
Vol. 31, No. 8
POLYMERICPRECURSORS
1027
Various proprietary mixtures of powders [including mullite (Baikowski), A1203 (Alfa), and other powders] with PHMSO were prepared to produce mixtures with an A1/Si ratio of 3:1. The A1 component was derived from powders, and the Si component was derived from the polymer and the powders. Polymer/powder mixtures were die-pressed into pellets of 1 to 2 cm diameter or bend bars. The pellets were cured in air at 200°C for 4 hours, pyrolyzed in air at 500°C for 4 hours followed by heating to 900°C in air, and sintered at 1600°C in air for 2 to 10 hours. The density was measured by helium pycnometry. Samples were sectioned, polished, and examined by SEM. Samples were also ground for X-ray analysis. Mullite Coatines Coatings were prepared by dipping or painting slurries or solutions onto various substrates, including Si3N4, aluminum foil, glass slides, quartz, steel, and A!203. After application, coatings were dried, cured, and heated to various temperatures depending on the substrate (AI: 400°-550°C, A1203: 1600°C, glass: 650°C, and quartz: 1500°C). Surfaces and cross sections of the coated materials were examined by SEM. A1 foil samples were coated on one side and immersed in HCI to test resistance to corrosion. Various coated samples were heated to higher temperatures to examine the behavior of the coating interface at high temperatures. Results and Discussion In-situ Reinforced Silicon Nitride Figure 1 shows powder formed from a mixture of PCMS and Si3N4 powder and oxide additives as described above. A proprietary mixing technique was used to form these spheres, which are free flowing and homogeneously mixed. Materials pressed from these powders were uniform, with no evidence of segregation of the polymer. The samples containing polymer were hard and were easy to handle after pyrolysis. The pyrolyzed and sintered densities (g/cm3) are: 1.92 and 3.13 (0 wt% PCMS); 1.92 and 3.16 (15 wt% PCMS); and 1.99 and 3.16 (20 wt% PCMS). The as-pyrolyzed and sintered densities are similar for all three mixtures. The amount of polymer added was not selected to exactly fill the spaces between the powder particles, and thus the densities of the samples containing the precursor are not significantly higher than that for the sample without precursor, as might be expected. In these initial experiments, the presence or amount of precursor added has little effect on the sintered density, which is >98% of theoretical density. The microstructure of these three samples is shown in Figure 2. The number and size of the elongated grains increase with the amount of precursor. Very few such grains are apparent in the sample without precursor. The sample containing 20 wt% polymer has the largest elongated grains and the largest number of such grains. The optimum amount of polymer for grain growth appears to be around 20 wt%. Preliminary strength data from the material containing 15 wt% polymer indicated an average strength of -800 MPa. Most significant is the effect of polymer on the microstructure. High density silicon nitride with desirable microstructures can be fabricated from these mixtures at modest sintering temperatures and pressures. On pyrolysis, the polymer loses any organic material and forms an amorphous "preceramic." The density increases from -1 g/cm3 for the polymer to - 2 g/era 3 for this amorphous material, which covers the particles uniformly, as a result of the mixing. On further heating, the polymer-derived material crystallizes to Si3N4 at -1600°C. Si3N4 powder densities by liquid phase sintering. We speculate that the amorphous material is very active and promotes low temperature intergrain diffusion. The increased rate of diffusion allows the [~-Si3N4grains to grow at modest sintering temperatures and overpressures. (High gas overpressures are required to minimize dissociation of Si3N4 at the high temperatures required for grain growth in other systems.) Near-Net-Shaoe Mullite The mullite precursor mixtures could be pressed into bulk samples. The viscosity was adjusted by adding mullite powder or solvent. Bulk samples were hard and were easily handled after the curing and pyrolysis treatment.
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Vol. 31, No. 8
XRD indicates that muilite is the major phase after heat treatment, and mullite peaks were observed after heat treatments as low as 1300°C. Alumina and cristobalite peaks were observed in samples where AI or Si were present in excess in the polymer or in the powder. Carbon was not detected by elemental analysis. The microstructure of the mullite materials indicated some residual porosity, and cracking was a problem in some samples. However, the most important observation was the low shrinkage (-4%) observed in these materials. Polymeric precursor/powder mixtures can be used to form mullite directly. The polymers and powders react to form a homogenous mullite. The bulk of this reaction occurs above 1300°C where mullite is first observed by XRD. The reaction is probably analogous to that for silicon nitride described above. A uniform amorphous Si-O phase is uniformly distributed around the powder panicles. The uniformity and activity of this phase promote the formation of mullite with relatively high density and very low shrinkage. The advantage of this approach is the ability to tailor the compositions and powder formulations to yield near-net-shape muUite bodies.
FIG. 1. Dried Preceramic Polymer/Si3N4 Powder Spheres. Mullite Coatings Coatings applied by painting or air-brushing the mullite precursor mixture were adherent to all the substrates investigated. The thicknesses varied from 10 gm to 30 gm, depending on the application technique. The appearance of the coating varied with the heat treatment temperature. Coatings on the AI substrates heated to 400 ° to 550°C were gray. Coatings heated to higher temperatures, such as those on A1203 substrates, were white and almost indistinguishablefrom the substrate. Figure 3 shows the surface and coating/substrate interface of AI foil (1 mm thick) coated with the p.mcursor mixture and heated to 400°C. Some organic material (<5 wt%) remains after treatment to 400°C. The coaung is an homogeneous composite consisting of various phases, but it is relatively dense and very adherent to the substrate. Corrosion tests on coated material showed that the AI foil was dissolved in a few hours. The coating remained intact, and the initial brush marks could still be seen even after exposure to HCI for 3 days. The coated AI foils were bent repeatedly without the coating spalling. Eventually, after repeated severe deformation, cracks appeared in the coating and some coating was removed. However, even after such deformation and damage, a thin layer of the coating remained on the AI. Figure 4 shows a coating on A1 foil treated at 550°C and repeatedly bent; the cracks shown appeared after about 100 cycles. The interface area shows that, although some coating chipped off when the deformed material was cut, an adherent layer is still chemically bonded to the substrate. Coatings treated to 550°C contain no organic species. Coatings treated to 400°C have very similar behavior. Simple electrical measurements indicated that the coatings have very high resistances and are essentially nonconducting.
Vol. 31, No. 8
POLYMERICPRECURSORS
.,
1029
(a) No polymer
(a) No polymer
(b) 15 wt% polymer
(b) 15 wt% polymer
. . . .
(¢) 20 wt% polymer
.j~+
(c) 20 wt% polymer
FIG. 2. Microstructure of Silicon Nitride Materials Containing 0, 15, and 20 wt% Polymer Initially.
FIG. 3. Surface and Interface of Mullite-Type Coating on A1 Foil Treated to 400°C.
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Conclusions The interactions between preceramic polymers and ceramics and metals allow for the development of new processing methods, new materials, and materials with desirable properties. All the material and coating fabrication techniques described here use potentially low cost polymers and modest heat treatment conditions, making the processes economically feasible. The activity of the polymer-derived material leads to enhanced chemical reactions between the polymer-derived material and other materials. This reactivity at the interfaces can lead to very adherent coatings such as those on AI foil and other materials, the development of desirable microstructures such as elongated grains in Si3N4, and the formation of materials directly from powder/polymer mixtures as in the bulk mullite materials. These are only a few of the applications for preceramic polymers in processing of ceramics and metals. Interfaces and the reactions between polymer-derived material and powders and bulk materials have significant effects on the properties and require further study to understand the process and to develop applications for these materials.
FIG. 4. Surfaces and Interfaces of Coatings on A1 Heated to 550°C and Later Deformed. References 1. 2. 3. 4. 5.
K.B. Schwartz, D. J. Rowcliffe, and Y. D. Blum, Adv. Ceram. Mat. 3, 320 (1988). Y.D. Blum, R. M. Platz, and E. J. Crawford, J. Am. Ceram. Soc. 73, 170 (1990). R.M. Laine and Y. D. Blum, Method of Forming Compounds Having Si-N Groups and Resulting Products, International Patent Application, PCT Publication WO 86/06377 (1986); U.S. Patent 4,788,309 (Nov. 29, 1988). Cer. Trans. 6, S. Somiya, R. F. Davis, and J. A. Pask, Eds. (Am. Cer. Soc., 1990) and references therein. J. Am. Ceram. Soc., 74 (1991) and references therein.
Scripta Metallurgicaet Materialia,Vol. 31, No. 8, pp. 1025-1030,1994 Copyright©1994ElsevierScienceLid Printed in the USA. All rights reserved 0956-716X/94 $6.00 + 00 CONFERENCE SET No. 2
INTERFACES OF POLYMERIC PRECURSORS WITH CERAMICS/METALS Sylvia M. Johnson, Yigal D. Blum, Gregory A. McDermott, and Michael I. Gusman SRI International, Menlo Park, CA 94025-3493
(Received May 6, 1994) Introduction Preceramic polymers that yield ceramic materials on heating have been investigated as precursors to fibers, binders (1), powders, and coatings (2). These materials have also been used as processing additives for ceramics, rheological control agents, and aids for controlling the microstructure of ceramics and coatings. The reactivity of the precursors with ceramic and metal powders or with substrate surfaces opens up many possibilities for improved processing of materials, fabrication of novel materials, and the preparation of materials and coatings with improved properties. Although reactions at interfaces between polymeric precursors and ceramics or metals are not yet well understood, the beneficial effects of using precursors have been demonstrated. Preceramic polymers consist of inorganic skeletons substituted (in most cases) with organic pendant groups. They can be fabricated similarly to organic polymers but are converted to a ceramic on heat treatment. Heat treatment involves an initial curing step in which the polymer is crosslinked to prevent melting during the second heating stage, pyrolysis, and to increase ceramic yields. Curing may be achieved by heating, adding crosslinking additives, or exposing the material to a reactive environment. After curing, the polymer is pyrolyzed (at 500 ° to 900°C). The polymer substituents are removed as volatile compounds, and the polymer skeleton is converted into an amorphous, three-dimensional ceramic network. Ideally, only a small amount of the material and none of the inorganic skeleton is volatilized during the pyrolysis. The pyrolysis time, temperature, and atmosphere affect the final conversion yield, composition, and integrity of the product, and differ for various applications. If the application requires, the amorphous ceramic material can be crystallized by further heat treatment at higher temperature. The crystallization temperature is a function of the ceramic type, composition, impurities, and incorporated additives. Interfacial reactions between polymers and other materials depend on the polymer composition and rheological properties, the ceramic or metallic material composition and surface states, and the heat treatments and environments. Interfaces range from relatively simple interfaces between a pure polymer-derived material and a substrate, as reflected by the formation of coatings or the use of polymers to infiltrate porous materials, to complex interfaces between polymers, various powders, and occasionally substrates. Examples of complex interactions include the use of precursors as binders or microstructural control agents and the direct formation of bulk materials or matrix materials from polymer/powder compositions. This paper gives specific examples of the interactions of polymeric precursors with ceramics to provide controlled microstructure development during sintering of Si3N4 (in-situ growth of elongated grains) and to form near-net-shape mullite and mullite-based coatings with unique properties. Experimental Techniques Preceramic Polymer Development The polymers used in our work are part of ongoing research at SRI International to design, develop, and use a variety of preceramic polymers. We have developed novel synthesis methods for polymeric precursors to Si3N4, SiC, Si3N4]SiC, Si2ON2 SiO2, SiOC, silicates, TiN, TiO2, A1N, and BN. These compounds can be liquids or solids and are soluble in organic solvents, and can be mixed with powders by wet techniques or applied to surfaces from solutions.
1025
1026
POLYMERIC PRECURSORS
Vol. 31, No. 8
SRI has developed a novel chemical method for synthesizing, modifying, and curing silicon-based preceramic polymers that allows the preparation of precursors to many ceramic compositions and synthesis of polymers with a wide range of viscosities and high ceramic yields. Variations in heat treatment conditions control the ceramic yield and composition. The approach is based mainly on a catalyst-induced dehydrocoupling reaction between compounds containing Si-H bonds and compounds containing either N-H or O-H bonds to form Si-N or Si-O bonds, respectively (3). Currently, 10 to 50 ppm of a transition metal catalyst, Ru3(CO)12, is used for polymer synthesis and modification, control of rheology and product compositions, and improvement in curing behavior. This approach yields highly processable preceramic polymers that have ceramic yields of 70 to 95 wt%. The polymers used in this study were polycyclomethylsilazane (PCMS, [CH3SiHNH]x ) a precursor to silicon nitride and polyhydridomethylsiloxane (PHMSO, [CH3SiHO]x) a precursor to silica. As the polysilazane is moderately sensitive to moisture in the atmosphere and to oxidation, mixtures containing this polymer were stored and handled in inert atmospheres as much as possible. The PHMSO technology uses a very inexpensive by-product of the silicone industry that is not useful as a ceramic precursor because of the negligible ceramic yields obtained on pyrolysis. However, after the addition of 50 ppm of catalyst and exposure to atmospheric moisture, the polymer cross-links rapidly and the product can be converted to silica, silicon-oxycarbide (so-called "Black Glass"), or silicon oxynitride with ceramic yields of 80 to 95 wt%. ln-situ Reinforced Silicon Nitri~l~
Silicon nitride has emerged as the leading candidate for structural applications in heat engines. It is strong, relatively tough for a monolithic ceramic, and its thermal properties (shock, conductivity, expansion) properties are acceptable for many applications. It also has excellent wear and erosion resistance and good oxidation resistance. Although silicon nitride is the topic of considerable research, there are still problems associated with inadequate properties and the high cost of fabrication. Composite materials have higher toughnesses than the monolithic materials, but the fabrications costs are generally too high for applications such as automotive engines. One technique for improving the toughness of silicon nitride has been the concept of "in-situ" reinforcement by the growth of whisker-like grains of silicon nitride within the microstructure. Current techniques to induce growth of elongated 13-Si3N4 grains require high sintering temperatures and pressures or incorporation of large amounts of oxides, which can reduce the strength and increase creep at elevated temperatures. We have explored the use of preceramic polymers as processing aids for low cost fabrication of silicon nitride. The precursors offer the advantages of ease of processing during shape-forming and handling, sintering at modest temperatures and pressures, and the formation of a desirable elongated grain microstructure. Silicon nitride pellets and bend bars were die-pressed from dry mixtures of 20 wt% PCMS mixed with 80% powder [92 wt% Si3N4 (Ube SNE-10), 5 wt% Y203, and 3 wt% A1203]. The pressed pellets were cured at 150°C, pyrolyzed by heating to 900°C in NH 3 and sintered at 1750 ° to 1800°C for 4 to 6 hours under -700 kPa of N2. Sintered samples were sectioned, polished, and examined by scanning electron microscopy (SEM). Density was measured by Archimedean techniques or by helium pycnometry. Shrinkage and weight loss were measured at various stages in the heat treatment. Strength was determined by 4-point bend tests on bars - 3 mm x 4 m m x 50 ram. Near-Net-Shaoe Mullite Mullite, 3A1203.2SIO2, has both high temperature structural and electronic applications. It is refractory, has a good thermal shock resistance, low thermal expansion coefficient, and a low dielectric constant. The properties of mullite depend on the composition (the alumina/silica ratio), the synthesis method, and the processing conditions. Many approaches have been taken to the formation of mullite, including formation from clays, sol-gel routes, organometallic compounds, coprecipitation, and reaction sintering (4,5). Oxide composites are of interest for applications such as turbines engines because of their oxidation resistance. Toughened mullite is also of interest for electronic applications where toughened ~ubstrates are required. Inf'fltration of fiber preforms with matrix materials requires precursor mixtures with suitable rheological properties. Preceramic polymers provide a very promising route to forming mullite matrix materials for oxide composites.
Vol. 31, No. 8
POLYMERICPRECURSORS
1027
Various proprietary mixtures of powders [including mullite (Baikowski), A1203 (Alfa), and other powders] with PHMSO were prepared to produce mixtures with an A1/Si ratio of 3:1. The A1 component was derived from powders, and the Si component was derived from the polymer and the powders. Polymer/powder mixtures were die-pressed into pellets of 1 to 2 cm diameter or bend bars. The pellets were cured in air at 200°C for 4 hours, pyrolyzed in air at 500°C for 4 hours followed by heating to 900°C in air, and sintered at 1600°C in air for 2 to 10 hours. The density was measured by helium pycnometry. Samples were sectioned, polished, and examined by SEM. Samples were also ground for X-ray analysis. Mullite Coatines Coatings were prepared by dipping or painting slurries or solutions onto various substrates, including Si3N4, aluminum foil, glass slides, quartz, steel, and A!203. After application, coatings were dried, cured, and heated to various temperatures depending on the substrate (AI: 400°-550°C, A1203: 1600°C, glass: 650°C, and quartz: 1500°C). Surfaces and cross sections of the coated materials were examined by SEM. A1 foil samples were coated on one side and immersed in HCI to test resistance to corrosion. Various coated samples were heated to higher temperatures to examine the behavior of the coating interface at high temperatures. Results and Discussion In-situ Reinforced Silicon Nitride Figure 1 shows powder formed from a mixture of PCMS and Si3N4 powder and oxide additives as described above. A proprietary mixing technique was used to form these spheres, which are free flowing and homogeneously mixed. Materials pressed from these powders were uniform, with no evidence of segregation of the polymer. The samples containing polymer were hard and were easy to handle after pyrolysis. The pyrolyzed and sintered densities (g/cm3) are: 1.92 and 3.13 (0 wt% PCMS); 1.92 and 3.16 (15 wt% PCMS); and 1.99 and 3.16 (20 wt% PCMS). The as-pyrolyzed and sintered densities are similar for all three mixtures. The amount of polymer added was not selected to exactly fill the spaces between the powder particles, and thus the densities of the samples containing the precursor are not significantly higher than that for the sample without precursor, as might be expected. In these initial experiments, the presence or amount of precursor added has little effect on the sintered density, which is >98% of theoretical density. The microstructure of these three samples is shown in Figure 2. The number and size of the elongated grains increase with the amount of precursor. Very few such grains are apparent in the sample without precursor. The sample containing 20 wt% polymer has the largest elongated grains and the largest number of such grains. The optimum amount of polymer for grain growth appears to be around 20 wt%. Preliminary strength data from the material containing 15 wt% polymer indicated an average strength of -800 MPa. Most significant is the effect of polymer on the microstructure. High density silicon nitride with desirable microstructures can be fabricated from these mixtures at modest sintering temperatures and pressures. On pyrolysis, the polymer loses any organic material and forms an amorphous "preceramic." The density increases from -1 g/cm3 for the polymer to - 2 g/era 3 for this amorphous material, which covers the particles uniformly, as a result of the mixing. On further heating, the polymer-derived material crystallizes to Si3N4 at -1600°C. Si3N4 powder densities by liquid phase sintering. We speculate that the amorphous material is very active and promotes low temperature intergrain diffusion. The increased rate of diffusion allows the [~-Si3N4grains to grow at modest sintering temperatures and overpressures. (High gas overpressures are required to minimize dissociation of Si3N4 at the high temperatures required for grain growth in other systems.) Near-Net-Shaoe Mullite The mullite precursor mixtures could be pressed into bulk samples. The viscosity was adjusted by adding mullite powder or solvent. Bulk samples were hard and were easily handled after the curing and pyrolysis treatment.
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XRD indicates that muilite is the major phase after heat treatment, and mullite peaks were observed after heat treatments as low as 1300°C. Alumina and cristobalite peaks were observed in samples where AI or Si were present in excess in the polymer or in the powder. Carbon was not detected by elemental analysis. The microstructure of the mullite materials indicated some residual porosity, and cracking was a problem in some samples. However, the most important observation was the low shrinkage (-4%) observed in these materials. Polymeric precursor/powder mixtures can be used to form mullite directly. The polymers and powders react to form a homogenous mullite. The bulk of this reaction occurs above 1300°C where mullite is first observed by XRD. The reaction is probably analogous to that for silicon nitride described above. A uniform amorphous Si-O phase is uniformly distributed around the powder panicles. The uniformity and activity of this phase promote the formation of mullite with relatively high density and very low shrinkage. The advantage of this approach is the ability to tailor the compositions and powder formulations to yield near-net-shape muUite bodies.
FIG. 1. Dried Preceramic Polymer/Si3N4 Powder Spheres. Mullite Coatings Coatings applied by painting or air-brushing the mullite precursor mixture were adherent to all the substrates investigated. The thicknesses varied from 10 gm to 30 gm, depending on the application technique. The appearance of the coating varied with the heat treatment temperature. Coatings on the AI substrates heated to 400 ° to 550°C were gray. Coatings heated to higher temperatures, such as those on A1203 substrates, were white and almost indistinguishablefrom the substrate. Figure 3 shows the surface and coating/substrate interface of AI foil (1 mm thick) coated with the p.mcursor mixture and heated to 400°C. Some organic material (<5 wt%) remains after treatment to 400°C. The coaung is an homogeneous composite consisting of various phases, but it is relatively dense and very adherent to the substrate. Corrosion tests on coated material showed that the AI foil was dissolved in a few hours. The coating remained intact, and the initial brush marks could still be seen even after exposure to HCI for 3 days. The coated AI foils were bent repeatedly without the coating spalling. Eventually, after repeated severe deformation, cracks appeared in the coating and some coating was removed. However, even after such deformation and damage, a thin layer of the coating remained on the AI. Figure 4 shows a coating on A1 foil treated at 550°C and repeatedly bent; the cracks shown appeared after about 100 cycles. The interface area shows that, although some coating chipped off when the deformed material was cut, an adherent layer is still chemically bonded to the substrate. Coatings treated to 550°C contain no organic species. Coatings treated to 400°C have very similar behavior. Simple electrical measurements indicated that the coatings have very high resistances and are essentially nonconducting.
Vol. 31, No. 8
POLYMERICPRECURSORS
.,
1029
(a) No polymer
(a) No polymer
(b) 15 wt% polymer
(b) 15 wt% polymer
. . . .
(¢) 20 wt% polymer
.j~+
(c) 20 wt% polymer
FIG. 2. Microstructure of Silicon Nitride Materials Containing 0, 15, and 20 wt% Polymer Initially.
FIG. 3. Surface and Interface of Mullite-Type Coating on A1 Foil Treated to 400°C.
1030
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Vol. 31, No. 8
Conclusions The interactions between preceramic polymers and ceramics and metals allow for the development of new processing methods, new materials, and materials with desirable properties. All the material and coating fabrication techniques described here use potentially low cost polymers and modest heat treatment conditions, making the processes economically feasible. The activity of the polymer-derived material leads to enhanced chemical reactions between the polymer-derived material and other materials. This reactivity at the interfaces can lead to very adherent coatings such as those on AI foil and other materials, the development of desirable microstructures such as elongated grains in Si3N4, and the formation of materials directly from powder/polymer mixtures as in the bulk mullite materials. These are only a few of the applications for preceramic polymers in processing of ceramics and metals. Interfaces and the reactions between polymer-derived material and powders and bulk materials have significant effects on the properties and require further study to understand the process and to develop applications for these materials.
FIG. 4. Surfaces and Interfaces of Coatings on A1 Heated to 550°C and Later Deformed. References 1. 2. 3. 4. 5.
K.B. Schwartz, D. J. Rowcliffe, and Y. D. Blum, Adv. Ceram. Mat. 3, 320 (1988). Y.D. Blum, R. M. Platz, and E. J. Crawford, J. Am. Ceram. Soc. 73, 170 (1990). R.M. Laine and Y. D. Blum, Method of Forming Compounds Having Si-N Groups and Resulting Products, International Patent Application, PCT Publication WO 86/06377 (1986); U.S. Patent 4,788,309 (Nov. 29, 1988). Cer. Trans. 6, S. Somiya, R. F. Davis, and J. A. Pask, Eds. (Am. Cer. Soc., 1990) and references therein. J. Am. Ceram. Soc., 74 (1991) and references therein.