J. An ceran.Soc,83[2]2961-6602000) urna In situ growth of sic whisker in Pyrolyzed Monolithic Mixture of AHPCS and Sic Jing Zheng, Matthew J. Kramer, and Mufit Akinc Ames Laboratory and Department of Material Science and Engineering, lowa State University, Ames, lowa 50011 In situ whisker growth was observed during heat treatment of Methods of in situ growth of Sic whisker in carbon and si,N4 allylhydridopolycarbosilane(AHPCS)and Sic powder in the to produce composites have been studied recently. In the study by temperature range of 1250%-1350oC. tEM equipped with an Chen et a ders of calcined rice husks, graphite, and electron energy loss spectrometer (EELs) verified that the analyst were mixed with asphalt and compacted. In situ growth of banded whiskers are twinned single-grained B-SiC. Conver- SiC whiskers in the compacts was observed after heat treatment at gent beam electron diffraction patterns(CBED)of the whisker the appropriate temperature. The growth of these whiskers was tips were consistent with formation by a vapor-solid (vS) attributed to the reduction of SiO, by the carbon sources in the mechanism. The effects of process variables on whisker growth wder mixture. However, the residual impurity, such as Si, SiO2, were addressed. The mechanism of whisker growth was dis- and catalyst, influenced the properties of the composites.In cussed and attributed to the reaction between the gaseous another study by Wang and Fischman, in situ synthesis of Sic products of the polymers AHPCS and polycarbosilane(PCS) whiskers by direct carbothermal reduction of Si,N4 with graphite Thermal decomposition behavior of the polymers was followed was reported. The formation of SiC whiskers was believed to be to relate gas evolution to whisker formation. elated to the decomposition of Si, NA. This so-called chemical mixing process has been used to produce Si3N4 matrix/SiC whisker composites. Because no catalyst was used in the process, L. Introduction the effect of impurities was minimized. Although SiC whisker USE of ceramics in high-temperature structural applications einforced Sic composites produced by mixing the preformed SiC has been severely limited by the low fracture toughness and the whisker with the Sic powder show potential for improving the lack of predictable service life. Any flaw in the ceramic can lead toughness of the monolithic ceramics, in situ growth of Sic to catastrophic failure. However, by using high-strength whiskers whiskers in Sic has not been reported to reinforce the ceramic matrix, fracture toughness has been In the present work, a method of in situ growth of Sic in a significantly improved. Of all the candidate whiskers, SiC is pyrolyzed mixture of ally lhydridopolycarbosilane(AHPCS) poly er and Sic powder is described. The in situ grown whiskers are recognized for its favorable properties, high strength, high elastic characterized by SEM and TEM, and are identified to be B-Sic modulus, heat resistance, and chemical stability ingle crystal. The process described here provides a potential way Because of its potential as a reinforcement material for ceram- for producing either SiC whiskers or SiC whisker-reinforced SiC ics, SiC whiskers have been prepared by various techniques, such as crystallizing the whiskers from liquefied Sic under high temperature and pressure, pyrolyzing the fibrous polymer precur- sor of SiC, and reacting Sio with graphite or rice husk at high for growing SiC whiskers, most of them can be described by (1) Material Preparation The vls mechanism invol es the se of liquid camaysts iems. watervliet Ny a liquid polymer precursor of sic in ae vial and iron)to act on the whisker tip to absorb silicon and carbon species stirred with a spatula. After mixing, the slurry in the vial was from the surrounding gases to grow whiskers at the interface placed in an ultrasonic bath for - 40 min to homogenize the between the liquid catalyst and SiC crystal. These whiskers are mixture. The slurry was then transferred to an alumina plate or easily recognized by the iron-rich semispherical whisker tip. In the crucible and pyrolyzed in a tube furnace VS mechanism though, the whisker tip is sharp Green compacts of Sic were prepared by mixing 95 parts by Preformed SiC whiskers are commonly mixed with the ceramic, volume of Sic powder (p-SiC, 059(S), Superior Graphite, Chi metal, or polymer powders to produce whisker-reinforced com- cago, IL) with 5 parts polycarbosilane(molecular weight 1470, posites. However, previous research has shown that inhomoge- PCS, Nippon Carbon Co., Tokyo, Japan) and blending with 300 neous distribution of the preformed whiskers in the final product parts toluene to form a suspension. The suspension was homoge- limits the potential benefit of fiber reinforcement. Because of the nized with an ultrasonic horn for 5 min. The resulting suspension homogeneity in whisker distribution, the samples also was dried in the hood and sieved through No. 100 mesh. Green patial variation in properties. Furthermore, the preformed compacts were produced by uniaxially pressing the powders in a inhalindsi possessing a size that has been shown to cause cancer on stainless steel die with a diameter measuring 25.4 mm at an applied pose a potential health risk. To achieve the full pressure of 34.5 MPa followed by isostatic pressing at a pressure tential in fiber-reinforced composites, an in situ method for fiber of 138 MPa. Green densities of these compacts ranged between formation must be realized 65%60% of theoretical so that, after pyrolysis, the compacts d-50% porosity. During whisker growth, several crucibles containing the slt G. Grathwohk--contributing editor of AHPCS and Sic were placed between Sic green compacts and placed in tube fumace for pyrolysis. This arrangement made it possible for gaseous pyrolysis products to be mixed thore us products of AhPCS to diffuse into anuscript No 188983. Received November 1, 1999, approved June 1, 2000. the green compacts. Besides green compacts of Sic (which had lember. American Ceramic Socie PCS as binder), bulk PCs powder was also used to form whiskers. 2961
In situ Growth of SiC Whisker in Pyrolyzed Monolithic Mixture of AHPCS and SiC Jing Zheng,* Matthew J. Kramer, and Mufit Akinc* Ames Laboratory and Department of Material Science and Engineering, Iowa State University, Ames, Iowa 50011 In situ whisker growth was observed during heat treatment of allylhydridopolycarbosilane (AHPCS) and SiC powder in the temperature range of 1250°–1350°C. TEM equipped with an electron energy loss spectrometer (EELS) verified that the banded whiskers are twinned single-grained b-SiC. Convergent beam electron diffraction patterns (CBED) of the whisker tips were consistent with formation by a vapor–solid (VS) mechanism. The effects of process variables on whisker growth were addressed. The mechanism of whisker growth was discussed and attributed to the reaction between the gaseous products of the polymers AHPCS and polycarbosilane (PCS). Thermal decomposition behavior of the polymers was followed to relate gas evolution to whisker formation. I. Introduction THE USE of ceramics in high-temperature structural applications has been severely limited by the low fracture toughness and the lack of predictable service life. Any flaw in the ceramic can lead to catastrophic failure. However, by using high-strength whiskers to reinforce the ceramic matrix, fracture toughness has been significantly improved.1 Of all the candidate whiskers, SiC is recognized for its favorable properties, high strength, high elastic modulus, heat resistance, and chemical stability. Because of its potential as a reinforcement material for ceramics, SiC whiskers have been prepared by various techniques, such as crystallizing the whiskers from liquefied SiC under high temperature and pressure,2 pyrolyzing the fibrous polymer precursor of SiC,3 and reacting SiO with graphite or rice husk at high temperature.4,5 Despite the large variety of techniques that exist for growing SiC whiskers, most of them can be described by vapor–liquid–solid (VLS)6,7 and vapor–solid (VS)8 mechanisms. The VLS mechanism involves the use of liquid catalysts (e.g., iron) to act on the whisker tip to absorb silicon and carbon species from the surrounding gases to grow whiskers at the interface between the liquid catalyst and SiC crystal. These whiskers are easily recognized by the iron-rich semispherical whisker tip. In the VS mechanism though, the whisker tip is sharp. Preformed SiC whiskers are commonly mixed with the ceramic, metal, or polymer powders to produce whisker-reinforced composites. However, previous research has shown that inhomogeneous distribution of the preformed whiskers in the final product limits the potential benefit of fiber reinforcement.9 Because of the spatial inhomogeneity in whisker distribution, the samples also exhibit spatial variation in properties. Furthermore, the preformed whiskers, possessing a size that has been shown to cause cancer on inhaling,10 pose a potential health risk. To achieve the full potential in fiber-reinforced composites, an in situ method for fiber formation must be realized. Methods of in situ growth of SiC whisker in carbon and Si3N4 to produce composites have been studied recently. In the study by Chen et al., 11 powders of calcined rice husks, graphite, and catalyst were mixed with asphalt and compacted. In situ growth of SiC whiskers in the compacts was observed after heat treatment at the appropriate temperature. The growth of these whiskers was attributed to the reduction of SiO2 by the carbon sources in the powder mixture. However, the residual impurity, such as Si, SiO2, and catalyst, influenced the properties of the composites. In another study by Wang and Fischman,12 in situ synthesis of SiC whiskers by direct carbothermal reduction of Si3N4 with graphite was reported. The formation of SiC whiskers was believed to be related to the decomposition of Si3N4. This so-called chemical mixing process has been used to produce Si3N4 matrix/SiC whisker composites. Because no catalyst was used in the process, the effect of impurities was minimized. Although SiC whiskerreinforced SiC composites produced by mixing the preformed SiC whisker with the SiC powder show potential for improving the toughness of the monolithic ceramics,13 in situ growth of SiC whiskers in SiC has not been reported. In the present work, a method of in situ growth of SiC in a pyrolyzed mixture of allylhydridopolycarbosilane (AHPCS) polymer and SiC powder is described. The in situ grown whiskers are characterized by SEM and TEM, and are identified to be b-SiC single crystal. The process described here provides a potential way for producing either SiC whiskers or SiC whisker-reinforced SiC composites. II. Experimental Procedure (1) Material Preparation SiC powder was mixed with AHPCS (Starfire System, Inc., Watervliet, NY), a liquid polymer precursor of SiC, in a vial and stirred with a spatula. After mixing, the slurry in the vial was placed in an ultrasonic bath for ;40 min to homogenize the mixture. The slurry was then transferred to an alumina plate or crucible and pyrolyzed in a tube furnace. Green compacts of SiC were prepared by mixing 95 parts by volume of SiC powder (b-SiC; 059(S), Superior Graphite, Chicago, IL) with 5 parts polycarbosilane (molecular weight 1470; PCS, Nippon Carbon Co., Tokyo, Japan) and blending with 300 parts toluene to form a suspension. The suspension was homogenized with an ultrasonic horn for 5 min. The resulting suspension was dried in the hood and sieved through No. 100 mesh. Green compacts were produced by uniaxially pressing the powders in a stainless steel die with a diameter measuring 25.4 mm at an applied pressure of 34.5 MPa followed by isostatic pressing at a pressure of 138 MPa. Green densities of these compacts ranged between 55%–60% of theoretical so that, after pyrolysis, the compacts contained ;50% porosity. During whisker growth, several crucibles containing the slurry of AHPCS and SiC were placed between SiC green compacts and placed in tube furnace for pyrolysis. This arrangement made it possible for gaseous pyrolysis products to be mixed thoroughly, especially allowing the gaseous products of AHPCS to diffuse into the green compacts. Besides green compacts of SiC (which had PCS as binder), bulk PCS powder was also used to form whiskers. G. Grathwohl—contributing editor Manuscript No. 188983. Received November 1, 1999; approved June 1, 2000. *Member, American Ceramic Society. J. Am. Ceram. Soc., 83 [12] 2961–66 (2000) 2961 journal
VoL. 83. No. 12 held at the temperature for 2 h. The samples were then cooled to room temperature at a rate of 150.C/h SEM was used to observe the morphology of the whiskers. The phase composition of the individual whiskers was identified using electron diffraction and electron energy loss spectroscopy(EeLs) in a TEM(Model CM30, Philips, Eindhoven, The Netherlands) operated at 300 ke V. Samples for TEM were prepared by scraping fibers from the fractured surface of a pyrolyzed compact directly onto a holey carbon grid. Because the whiskers were only a few tens of nanometers in diameter with a very large aspect ratio, no other sample preparation was necessary. Gases evolved during decomposition of the PCs polymer were analyzed by quadrupole Fig. 1. Typical logy of in situ whiskers observed on fracture surface of pyrolyzed re of AHPCS and Sic powder II. Results (I Morphology and Characterization of In Situ Grown Whiskers (2) Heat Treatment nent of 60 parts by volume of Samples were heated in a tube fumace in flowing argon while AHPCS and 40 parts by volume of Sic powder at 1250C in the the temperature was raised to 150.C and held for I h. The furnace presence of PCs were studied by sEM. a typical micrograph was then ramped to 400C and held at this temperature for 2 h and obtained from the fracture surface of the compact exhibited an heated to the pyrolysis temperature of 1100C, at a rate of 60%C/h, abundance of whiskers, as illustrated in Fig. I. The length of the and held for 2 h. The samples were then heated to various pyrolysis needlelike whiskers ranged from 20 to 40 um, while the diameter emperatures between 1050% and 1350C, at a rate of 150C/h, and ranged from 50 to 200 nm. For the tens of fibers observed (b) Fig. 2. TEM micrograph of individual whiskers: (a) morphology of SiC whisker with band and straight region, (b) band and straight region with CBED and SADP, and (c) morphology and SADP of whisker tip
(2) Heat Treatment Samples were heated in a tube furnace in flowing argon while the temperature was raised to 150°C and held for 1 h. The furnace was then ramped to 400°C and held at this temperature for 2 h and heated to the pyrolysis temperature of 1100°C, at a rate of 60°C/h, and held for 2 h. The samples were then heated to various pyrolysis temperatures between 1050° and 1350°C, at a rate of 150°C/h, and held at the temperature for 2 h. The samples were then cooled to room temperature at a rate of 150°C/h. (3) Whisker Characterization SEM was used to observe the morphology of the whiskers. The phase composition of the individual whiskers was identified using electron diffraction and electron energy loss spectroscopy (EELS) in a TEM (Model CM30, Philips, Eindhoven, The Netherlands) operated at 300 keV. Samples for TEM were prepared by scraping fibers from the fractured surface of a pyrolyzed compact directly onto a holey carbon grid. Because the whiskers were only a few tens of nanometers in diameter with a very large aspect ratio, no other sample preparation was necessary. Gases evolved during decomposition of the PCS polymer were analyzed by quadrupole mass spectroscopy. III. Results (1) Morphology and Characterization of In Situ Grown Whiskers Whiskers produced by heat treatment of 60 parts by volume of AHPCS and 40 parts by volume of SiC powder at 1250°C in the presence of PCS were studied by SEM. A typical micrograph obtained from the fracture surface of the compact exhibited an abundance of whiskers, as illustrated in Fig. 1. The length of the needlelike whiskers ranged from 20 to 40 mm, while the diameter ranged from 50 to 200 nm. For the tens of fibers observed, no Fig. 1. Typical morphology of in situ whiskers observed on fracture surface of pyrolyzed monolithic mixture of AHPCS and SiC powder. Fig. 2. TEM micrograph of individual whiskers: (a) morphology of SiC whisker with band and straight region, (b) band and straight region with CBED and SADP, and (c) morphology and SADP of whisker tip. 2962 Journal of the American Ceramic Society—Zheng et al. Vol. 83, No. 12
In situ Growth of sic Whisker in Pyrolyzed Monolithic Mixture of AHPCs and Sit spherical caps at the tips of the whiskers, a typical feature for Table L. Effect of siC Content on the growth of whiskers grown by the VLs mechanism, were observed. Instead, it Sic Whiskers appeared that the whiskers were formed by the Vs mechanism The morphology of an individual whisker observed in the tEM (vol%) showed that the whiskers were highly uniform over a long length but showed alternating bands of light and dark contrasts(Fig. Sample AHPCs powder Observation 2(a). The crystal structure was identified using both selected area nd convergent beam electron diffraction(Fig. 2(b). The selected ea diffraction pattern(SADP) was consistent with a twinned observed ign alumina crucible B-SiC, while the streaks in the diffraction spots along the [111 28 Same as sample 1 growth direction indicated a highly defective structure. The orien tations of the twins, established using convergent beam electron 2345 0205 40 Several whiskers observed, especially on surface and interior of sample 3 diffraction(CBED), were [110] and [112] To investigate the possible mechanism for whisker growth, the 28 Several whiskers observed on surface tips of individual whiskers were examined (Fig. 2(c). No and interior of compact. p was observed, and the diffraction pattern indicated AHPCS was cured at 400C. Cured solid was then ground and mixed with SiC rystalline B-SiC. The absence of the low-melting-point wder. Powder mixture was loosely compressed into compacts phases of SiC and impurities, such as iron or nickel, at tip indicated that the whiskers were formed by a vs rather than a VLS mechanism formed by slurries with lower SiC loading were covered by a dense EELS was used to identify the composition of the individual layer of pyrolyzed AHPCS residue, which isolated the Sic powder whiskers. The carbon edge(284 eV)results obtained by EELS on from interacting with the gas environment. The dense layer could n individual whisker and the amorphous carbon support mesh are be attributed to the enrichment of ahPcs at the surface of the shown in Fig. 3. While the carbon edge was quite strong, an pyrolyzed mixture. To prevent this dense layer formation, AHPCS oxygen edge was not observed, indicating that its presence, if any, was first cured at 400oC to form a solid material and then ground lay below the detection limits(<0. 1%). An advantage of EELS nto powders and mixed with SiC powder. The powder mixture was that the edge shape reflected the bonding state of the omprised 28 vol% SiC and 72 vol% cured AHPCS, and was material.4 The striking difference in the shape of the two carbon loosely pressed into a compact(sample 6 in Table D). The same edges was the strong evidence that the carbon detected by EEls in heat treatment as for whisker growth was followed. Although this the whiskers was not from the amorphous carbon residual that sample had a similar Sic volume percentage as sample 2, whiskers might have been present in the pyrolyzed polymer. Furthermore, were observed on the surface as well as in the interior of the the shape of the carbon peak from whiskers suggested strong o sample. This observation supported the idea that for samples with bonding. which exists in SiC high AHPCS fraction, the formation of a dense layer on the surface prevented SiC whisker growth 2) Influence of Process Variables on Whisker Growth SiC loading may also play a critical role in whisker growth by To address the essential factors that influence Sic whisker bon(sic is predoped with 0.8% growth in the consolidated AHPCS and Sic mixture, process sintering aid) to the reaction medium, which reduces CO, and ariables, such as Sic content in the mixture, gas atmosphere SiO to their respective monoxides, as is discussed in more detail during pyrolysis, and temperature at which the whiskers grow, in the next section (B) Temperature Regime of Whisker Growth: Slurries were prepared with different Sic contents and then heat-treated in various temperatures from 1050 to 1350 C(Table m) to determine laced next to the slurries The of PCS was presumed to tal results. listed in table l. observed at =1250C. Meanwhile, there seemed to be a slight S and sic with≥40vol difference(~100°C)in SiC resulted in whisker formation. The effect of Sic loading o between using the green compact (where PCs was used whisker growth was not completely understood. SEM observation binder)and using the PCS powder during heat treatment. This was of the samples after pyrolysis revealed that the green compacts attributed to the difference in the decomposition temperature of the two materials. A shift in the decomposition temperature was also observed by Okamura during gas evolution of PCS fiber and cured PCS fiber. The difference was attributed to the preheat process conducted on the cured PCS fiber in air, which was also applicable to the PCs in the green compacts C) Critical Reactants for Whisker Growth: The compo. nents used in the process, such as AHPCS, green compact/PCS Table Il. Influence of Temperature on SiC Whisker B10 Source of gas Whisker growth 5 From carbon mesh Green compact PCS powder PCS powder 1350 Fig. 3. EELS of SiC whiskers and carbon. Green compact contains 5 vol% PCS and 95 vol% SiC
spherical caps at the tips of the whiskers, a typical feature for whiskers grown by the VLS mechanism, were observed. Instead, it appeared that the whiskers were formed by the VS mechanism. The morphology of an individual whisker observed in the TEM showed that the whiskers were highly uniform over a long length but showed alternating bands of light and dark contrasts (Fig. 2(a)). The crystal structure was identified using both selected area and convergent beam electron diffraction (Fig. 2(b)). The selected area diffraction pattern (SADP) was consistent with a twinned b-SiC, while the streaks in the diffraction spots along the [111] growth direction indicated a highly defective structure. The orientations of the twins, established using convergent beam electron diffraction (CBED), were [11#0] and [1#12#]. To investigate the possible mechanism for whisker growth, the tips of individual whiskers were examined (Fig. 2(c)). No spherical cap was observed, and the diffraction pattern indicated perfect crystalline b-SiC. The absence of the low-melting-point eutectic phases of SiC and impurities, such as iron or nickel, at the whisker tip indicated that the whiskers were formed by a VS rather than a VLS mechanism. EELS was used to identify the composition of the individual whiskers. The carbon edge (284 eV) results obtained by EELS on an individual whisker and the amorphous carbon support mesh are shown in Fig. 3. While the carbon edge was quite strong, an oxygen edge was not observed, indicating that its presence, if any, lay below the detection limits (,0.1%). An advantage of EELS was that the edge shape reflected the bonding state of the material.14 The striking difference in the shape of the two carbon edges was the strong evidence that the carbon detected by EELS in the whiskers was not from the amorphous carbon residual that might have been present in the pyrolyzed polymer. Furthermore, the shape of the carbon peak from whiskers suggested strong s bonding, which exists in SiC. (2) Influence of Process Variables on Whisker Growth To address the essential factors that influence SiC whisker growth in the consolidated AHPCS and SiC mixture, process variables, such as SiC content in the mixture, gas atmosphere during pyrolysis, and temperature at which the whiskers grow, were examined. (A) SiC Content in the Slurry: Slurries of AHPCS and SiC were prepared with different SiC contents and then heat-treated in a tube furnace at 1250°C. SiC compacts with PCS binders were placed next to the slurries. The presence of PCS was presumed to control the gas atmosphere. Experimental results, listed in Table I, show that only a combination of AHPCS and SiC with $40 vol% SiC resulted in whisker formation. The effect of SiC loading on whisker growth was not completely understood. SEM observation of the samples after pyrolysis revealed that the green compacts formed by slurries with lower SiC loading were covered by a dense layer of pyrolyzed AHPCS residue, which isolated the SiC powder from interacting with the gas environment. The dense layer could be attributed to the enrichment of AHPCS at the surface of the pyrolyzed mixture. To prevent this dense layer formation, AHPCS was first cured at 400°C to form a solid material and then ground into powders and mixed with SiC powder. The powder mixture comprised 28 vol% SiC and 72 vol% cured AHPCS, and was loosely pressed into a compact (sample 6 in Table I). The same heat treatment as for whisker growth was followed. Although this sample had a similar SiC volume percentage as sample 2, whiskers were observed on the surface as well as in the interior of the sample. This observation supported the idea that for samples with high AHPCS fraction, the formation of a dense layer on the surface prevented SiC whisker growth. SiC loading may also play a critical role in whisker growth by providing free carbon (SiC is predoped with 0.8% free carbon as a sintering aid) to the reaction medium, which reduces CO2 and SiO2 to their respective monoxides, as is discussed in more detail in the next section. (B) Temperature Regime of Whisker Growth: Slurries containing 40 vol% SiC powder and AHPCS were heat-treated at various temperatures from 1050° to 1350°C (Table II) to determine the temperature regime that favors whisker growth. Table II shows that no whiskers grew below 1150°C, while whiskers were observed at $1250°C. Meanwhile, there seemed to be a slight difference (;100°C) in the temperature regime of whisker growth between using the green compact (where PCS was used as a binder) and using the PCS powder during heat treatment. This was attributed to the difference in the decomposition temperature of the two materials. A shift in the decomposition temperature was also observed by Okamura15 during gas evolution of PCS fiber and cured PCS fiber. The difference was attributed to the preheat process conducted on the cured PCS fiber in air, which was also applicable to the PCS in the green compacts. (C) Critical Reactants for Whisker Growth: The components used in the process, such as AHPCS, green compact/PCS Fig. 3. EELS of SiC whiskers and carbon. Table I. Effect of SiC Content on the Growth of SiC Whiskers Sample Composition (vol%) AHPCS Observation SiC powder 1 100 0 No whisker growth observed. Whiskers observed in alumina crucible. 2 72 28 Same as sample 1. 3 60 40 Several whiskers observed, especially on surface and interior of compact. 4 55 45 Same as sample 3. 5 0 100 No whisker growth observed. 6 72† 28 Several whiskers observed on surface and interior of compact. † AHPCS was cured at 400°C. Cured solid was then ground and mixed with SiC powder. Powder mixture was loosely compressed into compacts. Table II. Influence of Temperature on SiC Whisker Growth Sample Source of gas atmosphere Temperature (°C) Whisker growth 1 Green compact† 1050 No 2 Green compact† 1150 No 3 Green compact† 1250 Many 4 Green compact† 1350 Many 5 PCS powder 1150 No 6 PCS powder 1250 No 7 PCS powder 1350 Many † Green compact contains 5 vol% PCS and 95 vol% SiC. December 2000 In situ Growth of SiC Whisker in Pyrolyzed Monolithic Mixture of AHPCS and SiC 2963
Journal of the American Ceramic Sociery-Zheng et al. Vol. 83. No. 12 Table ill. role of reactants on whisker growth Materials in the system Temperature Gas source 40 SIC None 1250,1350No 60 AHPCS 2 40 SIC AHPCS 60 AHPCS PCS Powder 1350 0 AHPCS 250,1350 60 AHPCS 1250.1350 ompact 60 6 40 SIC 1350 60 Varcum compact um( Oxychem), a liquid phenolic resin as source of carbon; Compacts vol% PCS as binde Fig. 5. Thermal decomposition behavior of AHPCS polymer role each reactant played in forming Sic whiskers. Acting as the io, Direct observation of Sio is complicated because its mass gas source, SiC compacts containing PCS or vials of additional AHPCS or PCS were pyrolyzed side by side with AHPCS and Sic additional complication with the detection of Sio gas is the lurries. Table Ill summarizes the sources of reactive gases and Sio fror etection. These difficulties have been addressed by many in the of AHPCs in the Sic slurry and PCS in the form of powder or iterature green compacts (5 vol% PCS SiC)is essential for whisker B) PCS Polymer and Green Compacts: As shown in Table growth. No noticeable whisker growth was observed in the Ill, the conditions under which growth of Sic whiskers was absence of AHPCS or PCS, as illustrated in micrographs of the uired either PCs powder or green compacts contain- mixtures after pyrolysis(Fig. 4). Because neither the green ng PCs during the heat treatment. To elucidate the role of the PCs lymer during whisker growth, mass spectroscopy was used to slurry, the gas products of AHPCS and PCS during pyrolysis could determine the nature of the gases evolving from PCS at elevated be the critical reactants for the formation of sic whiskers temperatures <.C(Fig. 6). The evolution of CH4 gas was ( Thermal Decomposition Behavior of the Polymers bserved from 550 to 950c because of the demethanation of PCS, while CO gas evolution was observed at 21250C. It is or a) AHPCS Polymer: The thermal decomposition behavior noteworthy that the temperature of Co gas evolution is in good AHPCS is followed by determining the weight loss at preset agreement with that of SiC whisker growth, implying that CO gas temperatures in an inert atmosphere. Figure 5 shows the mass loss evolution from PCS at high temperature is related to Sic whisker of the polymer at different temperatures. The majority of mass loss growth in the system 18%)takes place below 400C, which is the stated curing temperature. Mass loss is mainly due to the loss of low molecular weight oligomers and H,, resulting in cross-linking of the poly- mer. Mass loss continues until 650%C because of further losses IV. Discussion f H, and some silicon- and carbon-containing species. There is d for th hardly any mass change between 650 to 1250C, indicating the Sic whiskers by a vS mechanism, but the essential reaction is end of thermal decomposition. Further mass loss between 1250to 1 600C is attributed to the evolution of Sio gas, which is caused given by by the oxygen content in the polymers incorporated during Sio(g)+3C0(g)- SiC(s)+ 2Co(g) polymer synthesis. Chemical analysis of the AHPCS residue after pyrolysis at 1100C(conducted at Material Preparation Center, AG°=465568+379T (1) Ames Lab, Ames, IA)indicates that the average oxygen content 3 wt%, agreeing with the data reported by Interante et al.6However of the During crystallization of the amorphous residue of AHPCS at 4.3at is too small to explain th elevated temperature, the oxygen content can promote evolution of whiskey nly. it is believed that the (a) (c) Fig 4. Whisker growth with various sources of gas atmosphere. Slurries comprised 40 vol% SiC and 60 vol% AHPCS. (a) Argon atmosphere, no whisker formation observed. (b)AHPCS as source of gas atmosphere; few whiskers observed (indicated by A).(c)5 vol% PCS Sic (in form of compacts)as source
powder, and SiC powder, were isolated or combined to assess the role each reactant played in forming SiC whiskers. Acting as the gas source, SiC compacts containing PCS or vials of additional AHPCS or PCS were pyrolyzed side by side with AHPCS and SiC slurries. Table III summarizes the sources of reactive gases and temperature leading to whisker growth. It shows that the presence of AHPCS in the SiC slurry and PCS in the form of powder or green compacts (5 vol% PCS 1 SiC) is essential for whisker growth. No noticeable whisker growth was observed in the absence of AHPCS or PCS, as illustrated in micrographs of the mixtures after pyrolysis (Fig. 4). Because neither the green compact nor PCS powder came in contact with the AHPCS in the slurry, the gas products of AHPCS and PCS during pyrolysis could be the critical reactants for the formation of SiC whiskers. (3) Thermal Decomposition Behavior of the Polymers (A) AHPCS Polymer: The thermal decomposition behavior of AHPCS is followed by determining the weight loss at preset temperatures in an inert atmosphere. Figure 5 shows the mass loss of the polymer at different temperatures. The majority of mass loss (;18%) takes place below 400°C, which is the stated curing temperature. Mass loss is mainly due to the loss of low molecular weight oligomers and H2, resulting in cross-linking of the polymer.16 Mass loss continues until ;650°C because of further losses of H2 and some silicon- and carbon-containing species. There is hardly any mass change between 650° to 1250°C, indicating the end of thermal decomposition. Further mass loss between 1250° to 1600°C is attributed to the evolution of SiO gas,16 which is caused by the oxygen content in the polymers incorporated during polymer synthesis. Chemical analysis of the AHPCS residue after pyrolysis at 1100°C (conducted at Material Preparation Center, Ames Lab, Ames, IA) indicates that the average oxygen content is ;3 wt%, agreeing with the data reported by Interante et al.16 During crystallization of the amorphous residue of AHPCS at elevated temperature, the oxygen content can promote evolution of SiO.17 Direct observation of SiO is complicated because its mass is identical to that of CO2, making the distinction difficult. An additional complication with the detection of SiO gas is the difficulty in keeping SiO from condensing or reacting before detection. These difficulties have been addressed by many in the literature. (B) PCS Polymer and Green Compacts: As shown in Table III, the conditions under which growth of SiC whiskers was observed required either PCS powder or green compacts containing PCS during the heat treatment. To elucidate the role of the PCS polymer during whisker growth, mass spectroscopy was used to determine the nature of the gases evolving from PCS at elevated temperatures #1350°C (Fig. 6). The evolution of CH4 gas was observed from 550° to 950°C because of the demethanation of PCS, while CO gas evolution was observed at $1250°C. It is noteworthy that the temperature of CO gas evolution is in good agreement with that of SiC whisker growth, implying that CO gas evolution from PCS at high temperature is related to SiC whisker growth in the system. IV. Discussion Several methods have been proposed for the synthesis of the SiC whiskers by a VS mechanism, but the essential reaction is given by: SiO~ g! 1 3CO~ g! 3 SiC~s! 1 2CO2~ g! DG8 5 465568 1 379T (1) However, the equilibrium constant of the reaction, log [Kp] 5 24.3 at 1300°C, is too small to explain the growth of SiC whiskers. Commonly, it is believed that the CO2 gas evolved Table III. Role of Reactants on Whisker Growth Sample Materials in the system Temperature Slurry Gas source (°C) Whisker growth 1 40 SiC None 1250, 1350 No 60 AHPCS 2 40 SiC AHPCS 1250 Few 60 AHPCS 3 40 SiC PCS Powder 1350 Yes 60 AHPCS 4 40 SiC SiC 1250, 1350 Yes 60 AHPCS compact‡ 5 None SiC 1250, 1350 No compact 6 40 SiC SiC 1350 No 60 Varcum† compact † Varcum (Oxychem), a liquid phenolic resin as source of carbon; ‡ Compacts contain 5 vol% PCS as binder. Fig. 4. Whisker growth with various sources of gas atmosphere. Slurries comprised 40 vol% SiC and 60 vol% AHPCS. (a) Argon atmosphere; no whisker formation observed. (b) AHPCS as source of gas atmosphere; few whiskers observed (indicated by ‚). (c) 5 vol% PCS 1 SiC (in form of compacts) as source of gas atmosphere; extensive whisker growth. Fig. 5. Thermal decomposition behavior of AHPCS polymer. 2964 Journal of the American Ceramic Society—Zheng et al. Vol. 83, No. 12
December 2000 In situ Growth of sic Whisker in Pyrolyzed Monolithic Mixture of AHPCs and Si CH4/Ar7 1500 1250 1000 10 500 a 2 0 T im e(hr. Fig. 6. CH4 and CO gas evolution from PCS as function of temperature. during the reaction is immediately removed by the reaction with It should be noted that (5)and(6) do not necessarily epresent the actual dec ition reaction or include all the reaction products. Howeve reactions show the predominant CO2(g)+C(s)→2CO(g) products at a given temperature The presence of CO from PCS and Sio from AHPCS in the Therefore, the overall reaction of Sic formation can be written as: system leads to reaction 3, forming SiC. The SiC deposits on the Sio(g)+ 2C- SiC(s)+ co(g) pyrolyzed mixture, presumably at the site of SiC powders, because of the low surface energy due to epitaxy, leading to the in situ 132770+339T; log[K]=2.6(1300 C)(3) growth of Sic whiskers though the essential reactions are the same, the sources of the Sio and co in the reactions are different in the various processes Usually, SiO, is mixed with graphite or carbon black to form Sio nd Ce In situ whisker growth was observed in the decomposed mixture f AHPCS and Sic in a controlled Pco and psio atmosphere during Sio(s)+C(s)- Sio(g)+co(g) (4) heat treatment at a temperature regime between 1200 to 1350 TEM and EELs verified that the whiskers were single-crystal However, unlike in the SiO, reduction method, the Sio and co B-SiC with a length of 20-40 um and a diameter of 50-200 nm gases involved in SiC whisker growth in the current study are The whisker tip was also crystalline Sic, suggesting that the sic believed to form from the polymers present in the system during whiskers grew by a Vs mechanism. The process variables for whisker growth, such as temperature and essential reactants, were Analysis of mass spectroscopy supports the notion that the PCs determined. Investigation of the thermal decomposition of the powder or green compact containing PCs is the source of the Co polymers revealed that Co gas was evolved from PCS at 1200% gas during the growth of SiC whiskers. The following thermal 1350%C, while Sio gas possibly evolved from AHPCS at 1250 decomposition occurs at the SiC whisker growth temperature of 1600oC. The reaction of the evolving SiO and CO was believed to 1250°to1350°C be the predominant reaction leading to formation of in situ SiC 1250°-1350° PCS residue SiC(amorphous)+Co(g)(5) References ' P. F. Becher, "Toughening Behavior in Whisker-Reinforced Ceramic Matrix Note that the mass of sio is identical to that of CO2, therefore, we Composites,"J.Am. Ceram Soc, 71, 1050-61(199 cannot preclude the possibility that Sio gas could be evolved a,“ Method for Manuf during the thermal decomposition of PCS at high temperature as turing Silicon Carbide Whisker, U.S. Pat. No, 4975 392, Dec. 4, 1990 reported by Otoshi et al. However, no whisker formation is licon Carbide Fiber of High Tensile Strength, J. An. Ceram. Soc., 59, 324-27(1975). F. J. Narciso- Romero and F. Ro by pCs is not significant or does not lead to whisker formation Husks Catalysed by Iron, Cobalt or Nickel, " J. Mater. Sci., 31, 779-84(1996) On the other hand, the mass loss of AHPCS at a temperature A. Selvam, N. G. Nair, and P. Sing ange of 12500 and 1600 C(Fig. 5)suggests the evolution of Sio bW. E Hollar Jr and J Review of vls SiC Whisker Growth Technology according to: Sti.Proc.,12,979-9(1991) Urretavizcaya and J. M. Porto Lopez, " Growth of SiC Whiskers by VLS 1250°-1600°C s,"J. Mater.Rex.,9,2981-86(1994) AHPCS residue Sic(amorphous)+ Sio(g) N足1k3swh Reinforced Si,N, Composite and Related Microstructural Characteristics, B Ceram. Trans., 93, 11-15(1994)
during the reaction is immediately removed by the reaction with free carbon: CO2~ g! 1 C~s! 3 2CO~ g! (2) Therefore, the overall reaction of SiC formation can be written as: SiO~ g! 1 2C 3 SiC~s! 1 CO~ g! DG8 5 2132770 1 33.9T; log@Kp# 5 2.6~1300°C! (3) Although the essential reactions are the same, the sources of the SiO and CO in the reactions are different in the various processes. Usually, SiO2 is mixed with graphite or carbon black to form SiO and CO. SiO2~s! 1 C~s! 3 SiO~ g! 1 CO~ g! (4) However, unlike in the SiO2 reduction method, the SiO and CO gases involved in SiC whisker growth in the current study are believed to form from the polymers present in the system during heat treatment. Analysis of mass spectroscopy supports the notion that the PCS powder or green compact containing PCS is the source of the CO gas during the growth of SiC whiskers. The following thermal decomposition occurs at the SiC whisker growth temperature of 1250° to 1350°C: PCS residueO¡ 12508–1350°C SiC~amorphous! 1 CO~ g! (5) Note that the mass of SiO is identical to that of CO2; therefore, we cannot preclude the possibility that SiO gas could be evolved during the thermal decomposition of PCS at high temperature as reported by Otoishi et al.18 However, no whisker formation is observed in sample 4 in Table III, which implies that SiO evolution by PCS is not significant or does not lead to whisker formation. On the other hand, the mass loss of AHPCS at a temperature range of 1250° and 1600°C (Fig. 5) suggests the evolution of SiO gas according to: AHPCS residueO¡ 12508–1600°C SiC~amorphous! 1 SiO~ g! (6) It should be noted that reactions (5) and (6) do not necessarily represent the actual decomposition reaction or include all the reaction products. However, these reactions show the predominant products at a given temperature. The presence of CO from PCS and SiO from AHPCS in the system leads to reaction 3, forming SiC. The SiC deposits on the pyrolyzed mixture, presumably at the site of SiC powders, because of the low surface energy due to epitaxy, leading to the in situ growth of SiC whiskers. V. Conclusion In situ whisker growth was observed in the decomposed mixture of AHPCS and SiC in a controlled pCO and pSiO atmosphere during heat treatment at a temperature regime between 1200° to 1350°C. TEM and EELS verified that the whiskers were single-crystal b-SiC with a length of 20–40 mm and a diameter of 50–200 nm. The whisker tip was also crystalline SiC, suggesting that the SiC whiskers grew by a VS mechanism. The process variables for whisker growth, such as temperature and essential reactants, were determined. Investigation of the thermal decomposition of the polymers revealed that CO gas was evolved from PCS at 1200°- 1350°C, while SiO gas possibly evolved from AHPCS at 1250°- 1600°C. The reaction of the evolving SiO and CO was believed to be the predominant reaction leading to formation of in situ SiC whiskers. References 1 P. F. Becher, “Toughening Behavior in Whisker-Reinforced Ceramic Matrix Composites,” J. Am. Ceram. Soc., 71, 1050–61 (1988). 2 M. Yamada, K. Numanami, T. Iizuka, and A. Hayashida, “Method for Manufacturing Silicon Carbide Whisker,” U.S. Pat. No. 4 975 392, Dec. 4, 1990. 3 S. Yajima, J. Hayashi, and M. Omori, “Continuous Silicon Carbide Fiber of High Tensile Strength,” J. Am. Ceram. Soc., 59, 324–27 (1975). 4 F. J. Narciso-Romero and F. Rodrigues-Reinoso, “Synthesis of SiC from Rice Husks Catalysed by Iron, Cobalt or Nickel,” J. Mater. Sci., 31, 779–84 (1996). 5 A. Selvam, N. G. Nair, and P. Singh, “Synthesis and Characterization of SiC Whiskers from Coconut Shells,” J. Mater. Sci. Lett., 17, 57–60 (1998). 6 W. E. Hollar Jr. and J. J Kim, “Review of VLS SiC Whisker Growth Technology,” Ceram. Eng. Sci. Proc., 12, 979–91 (1991). 7 G. Urretavizcaya and J. M. Porto Lopez, “Growth of SiC Whiskers by VLS Process,” J. Mater. Res., 9, 2981–86 (1994). 8 A. P. Levitt, Whisker Technology, Vol. 37. Wiley-Interscience, New York, 1970. 9 J. XinXiang and R. Taylor, “Dispersion of SiC Whiskers in SiC Whisker Reinforced Si3N4 Composite and Related Microstructural Characteristics,” Br. Ceram. Trans., 93, 11–15 (1994). Fig. 6. CH4 and CO gas evolution from PCS as function of temperature. December 2000 In situ Growth of SiC Whisker in Pyrolyzed Monolithic Mixture of AHPCS and SiC 2965
oT. E. Walters, "Refractory Ceramic Fibres Update, "Am. Ceram Soc. Bull., 74, Composites", pp. 273-78 in Ceramic Transactions, Vol. 57, High-Te 61-64(1994) ic whiskers n, bu pe pe乙 a gr ceres trametes or 8-22 gg)以 aslan, American Ceramic SocI OH,199 -H. Wang and G. S. Fischman, "In Situ Synthesis of Silicon Carbide Whiskers Carbide Ceramics-1. Edited by S. Somiya and Y. Inomata. Elsevier Applied Science, trom Silicon Nitnde Powders, J. An. Ceram Soc. 74, 1519-22(199). I6L. V. Interrante. C. w norwood, H. J. Wu, R. Lewis, and G. SiC-SiC Composites"; pp. 363-74 in Ceramic Transactions, VoL. 38, Advances in Mciel, "High Yield Polycarbosilane Precursors to Stoichiometric SiC Synthesis, Ceramic-Matrir Camposites. Edited by N. P. Bansal. American Ceramic Society Pyrolysis and Application, " Proc. Mater, Res. Soc., 346, 593-603( 1994). Westerville. OH. 1993 E. Pippel, J. Woltersdorf, A Hahnel, and R Schneider, "On the Role Otoshi and Y. Tang ion of Sic Whiskers from Polycarbosilane Structure and Composition in anical Behaviour of fibre Ceran.Soc.Jp.,104,1107-12(1996)
10T. E. Walters, “Refractory Ceramic Fibres Update,” Am. Ceram. Soc. Bull., 74, 61–64 (1994). 11K. H. Chen, D. Peng, and Z. Q. Xiao, “Process Parameters for In Situ Growth of SiC Whiskers in Bulk Porous Carbon,” Br. Ceram. Trans., 94, 118–22 (1995). 12H. Wang and G. S. Fischman, “In Situ Synthesis of Silicon Carbide Whiskers from Silicon Nitride Powders,” J. Am. Ceram. Soc., 74, 1519–22 (1991). 13V. V. Pujar and J. D. Cawley, “Microstructural Evolution in Whisker Reinforced SiC–SiC Composites”; pp. 363–74 in Ceramic Transactions, Vol. 38, Advances in Ceramic-Matrix Composites. Edited by N. P. Bansal. American Ceramic Society, Westerville, OH, 1993. 14E. Pippel, J. Woltersdorf, A. Hahnel, and R. Schneider, “On the Role of Interface Structure and Composition in the Mechanical Behaviour of Fibre Reinforced Composites”; pp. 273–78 in Ceramic Transactions, Vol. 57, High-Temperature Ceramic-Matrix Composites I: Design, Durability, and Performance. Edited by A. G. Evans and A. Naslain. American Ceramic Society, Westerville, OH, 1995. 15K. Okamura, “Continuous Silicon Carbide Fibers”; pp. 99–119 in Silicon Carbide Ceramics-1. Edited by S. Somiya and Y. Inomata. Elsevier Applied Science, New York, 1990. 16L. V. Interrante, C. W. Whitmarsh, W. Sherwood, H. J. Wu, R. Lewis, and G. Mciel, “High Yield Polycarbosilane Precursors to Stoichiometric SiC Synthesis, Pyrolysis and Application,” Proc. Mater. Res. Soc., 346, 593–603 (1994). 17Private talk with Dr. Walter Sherwood. 18S. Otoishi and Y. Tange, “Preparation of SiC Whiskers from Polycarbosilane,” J. Ceram. Soc. Jpn., 104, 1107–12 (1996). M 2966 Journal of the American Ceramic Society—Zheng et al. Vol. 83, No. 12
