MATERIALS CIENCE EIEERIG ELSEVIE Materials Science and Engineering A 428 (2006)346-350 www.elsevier.com/locate/msea Silicon carbide whisker reinforced silicon carbide composites by chemical vapor infiltration Yunfeng hua Litong Zhang, Aifei Cheng, Jing wang National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi'an 710072, PR China Received 21 February 2006: received in revised form 23 April 2006: accepted 16 May 2006 Silicon carbide (SiC) whisker reinforced SiC ceramic matrix composites were prepared by chemical vapor infiltration. Microstructure and mechanical properties of the composites were investigated. The fracture toughness of the composites was improved to 5.65-7.21 MPam", and its flexural strength value was between 196 MPa and 305 MPa, depending on the relative density. The pullout of rough whisker, crack defection and whisker bridging are responsible for the improvement in fracture toughness. The randomly oriented whiskers that paralleled to the crack plane, and the weak bonding strength between whisker and matrix could not have much stress transfer from the matrix, leading to the low flexural strength C 2006 Elsevier B. V. All rights reserved. Keywords: Chemical vapor infiltration; Organic binder; Reinforcement clusters; Preform; SiCw/SiC 1. Introduction forcements. Both of them have a negative effect on high temper ature mechanical and chemical properties of composites [6-8 Silicon carbide (SiC) is one of the promising candidate Incorporation of other materials has been accompanied by the eramic materials for a variety of high temperature, high stress degradation of desirable properties of SiC. Although isotropic and severe erosion applications in aerospace, hot engine and SiC whisker reinforced SiC matrix composites(SiCw/SiC)with energy conversion devices because of its excellent high tem- minor sintering aids can be prepared by hot isostatic pressing, it perature properties. However, its low fracture toughness(Kic) needs very high temperature and pressure and ultra-fine and high has long hindered this material from being used for wide appli- purity powders with the drawback of technological difficulties cations. One major research direction has been to increase its in manufacturing as well as with respect to costs, and the Sic fracture toughness by changing microstructures through the whiskers with high volume fraction and large aspect ratio are incorporation of other materials, and substantial toughening with difficult to be introduced into the SiC matrix. The reduction in isotropic property can be achieved by introducing whisker [1, 2]. whisker aspect ratio and the extensive growth of grains can mask he conventional methods for preparation of whisker rein- any benefit the whisker has imparted [9-11]. The contribution forced ceramic matrix composites are sintering, often involving of whisker on toughening also can be substantially decreased aids. The whisker can be damaged mechanically during ball by the strong whisker-matrix bonding strength due to sintering. milling dispersion, and it also can be damaged chemically via and appropriately strong bonding can facilitate the debonding at a solution and reprecipitation process during the densification the whisker-matrix interface and activate the wake-toughening because of the liquid sintering aids. The toughening effect of the [12-14 whiskers can be decreased due to its damage [2-5]. The residue Based on these research results, we have investigated the pos of sintering aids exists as continuous intergranular glassy phase, sibility for improving the fracture toughness of the SiCw/SiC which will soften and volatilize at elevated temperature. And composites by increasing whisker volume fraction and aspect microcracks also can be induced by thermal stress due to the ratio and improving working temperature by avoiding sintering mismatch of matrix, intergranular phase and heterogeneous rein- aids. Chemical vapor infiltration(CVi) has been demonstrated to be an effective and mature enough method to prepare Sic matrix with ultra-pure and controllable grain sizes [15-20]. In Corresponding author. Tel +862988486068x827: fax: +8629 88494620. this study, SiCw/SiC is prepared by depositing SiC matrix on the E-mailaddress:nanjing@163.com(.Hua) internal surfaces of the porous SiC whisker preforms by CVI, 0921-5093 )6 Elsevier B v. All rights reserved doi:10.1016
Materials Science and Engineering A 428 (2006) 346–350 Silicon carbide whisker reinforced silicon carbide composites by chemical vapor infiltration Yunfeng Hua ∗, Litong Zhang, Laifei Cheng, Jing Wang National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi’an 710072, PR China Received 21 February 2006; received in revised form 23 April 2006; accepted 16 May 2006 Abstract Silicon carbide (SiC) whisker reinforced SiC ceramic matrix composites were prepared by chemical vapor infiltration. Microstructure and mechanical properties of the composites were investigated. The fracture toughness of the composites was improved to 5.65–7.21 MPa m1/2, and its flexural strength value was between 196 MPa and 305 MPa, depending on the relative density. The pullout of rough whisker, crack deflection and whisker bridging are responsible for the improvement in fracture toughness. The randomly oriented whiskers that paralleled to the crack plane, and the weak bonding strength between whisker and matrix could not have much stress transfer from the matrix, leading to the low flexural strength. © 2006 Elsevier B.V. All rights reserved. Keywords: Chemical vapor infiltration; Organic binder; Reinforcement clusters; Preform; SiCW/SiC 1. Introduction Silicon carbide (SiC) is one of the promising candidate ceramic materials for a variety of high temperature, high stress and severe erosion applications in aerospace, hot engine and energy conversion devices because of its excellent high temperature properties. However, its low fracture toughness (KIC) has long hindered this material from being used for wide applications. One major research direction has been to increase its fracture toughness by changing microstructures through the incorporation of other materials, and substantial toughening with isotropic property can be achieved by introducing whisker [1,2]. The conventional methods for preparation of whisker reinforced ceramic matrix composites are sintering, often involving aids. The whisker can be damaged mechanically during ball milling dispersion, and it also can be damaged chemically via a solution and reprecipitation process during the densification because of the liquid sintering aids. The toughening effect of the whiskers can be decreased due to its damage [2–5]. The residue of sintering aids exists as continuous intergranular glassy phase, which will soften and volatilize at elevated temperature. And microcracks also can be induced by thermal stress due to the mismatch of matrix, intergranular phase and heterogeneous rein- ∗ Corresponding author. Tel.: +86 29 88486068x827; fax: +86 29 88494620. E-mail address: nanojing@163.com (Y. Hua). forcements. Both of them have a negative effect on high temperature mechanical and chemical properties of composites [6–8]. Incorporation of other materials has been accompanied by the degradation of desirable properties of SiC. Although isotropic SiC whisker reinforced SiC matrix composites (SiCW/SiC) with minor sintering aids can be prepared by hot isostatic pressing, it needs very high temperature and pressure and ultra-fine and high purity powders with the drawback of technological difficulties in manufacturing as well as with respect to costs, and the SiC whiskers with high volume fraction and large aspect ratio are difficult to be introduced into the SiC matrix. The reduction in whisker aspect ratio and the extensive growth of grains can mask any benefit the whisker has imparted [9–11]. The contribution of whisker on toughening also can be substantially decreased by the strong whisker–matrix bonding strength due to sintering, and appropriately strong bonding can facilitate the debonding at the whisker–matrix interface and activate the wake-toughening [12–14]. Based on these research results, we have investigated the possibility for improving the fracture toughness of the SiCW/SiC composites by increasing whisker volume fraction and aspect ratio and improving working temperature by avoiding sintering aids. Chemical vapor infiltration (CVI) has been demonstrated to be an effective and mature enough method to prepare SiC matrix with ultra-pure and controllable grain sizes [15–20]. In this study, SiCW/SiC is prepared by depositing SiC matrix on the internal surfaces of the porous SiC whisker preforms by CVI, 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.05.050
Y Hua et al. Materials Science and Engineering A 428(2006)346-350 3 mm x 4 mm x 40 mm were used to evaluate the three-point flexural strength using a cross-head speed of 0.5 mm min and a span length of 30 mm at room temperature. Five specimens were tested to obtain an average value. Micro-hardness and frac ture toughness were determined at room temperature by Vickers indentation method with 196N load for 30s. 10 times on each of five samples per data point 3. Results and discussion 3. 1. Phase analysis and density The relative density of 25 vol. SiCw/SiC composite to deposition time is summarized in Table 1. As shown, the density Fig. 1. SEM micrograph of the silicon carbide whiskers of the specimens is low compared to that of the whisker rein- forced ceramic matrix composites prepared by sintering method and the purpose of this work is to evaluate the employment of [9, 11]. CVI process results in composites intrinsically display CVI, as a novel method, in fabricating the SiCw/SiC from Sic significant residual porosity(typically, about 10%6), which is the major factor inducing the low density [15]. Fig. 2 shows the whisker preforms, and its microstructure and mechanical prop- XRD patterns of Sic whisker, pure Sic matrix and composites with deposition time of 450h. As can be seen, the content of mperature a-SiC phase in the composites increases, and 2. Experimental procedure the composites consist of pure SiC. spersing and large aspect ratio of whiskers can create ig. I shows the SEM micrograph of the Sic whisker with enough space in clusters. The spacing structure among and in the rough surface. B-SiC whisker(99% pure, Alfa Aesar, MA, USA) clusters is similar to that of the fiber preforms, creating path for had an average size of 18 pm in length and 1.5 um in diameter. the CH3SiCl3 and H2 to infiltrate. After depositing SiC matrix However, SiC whiskers as long as 50-100 um were observed. for 450h, the Sic coating(shell)still does not form, and the rel- The procedure for whisker surface treatment was as follows. ative density of the composites is 88.72%. The relative density As-purchased Sic whiskers were first washed with concen- of 88.72% is close to its limit that can be achieved by CVl.It trated hydrochloric acid (36%)to remove metallic impurities After repeatedly washing with distilled water, the whisker was Table I dipped into 20%o hydrofluoric acid to remove SiO2, and washed Deposition time and relative density of the 25% SiCw/SiC composites again with distilled water. Then, after drying, it was dipped into Deposition time (h) Relative density (%o) acetylacetonate to pre-disperse for 24 h. Then the solvent was extracted by a rotary evaporator. As-treated whisker was then 300 84.30 stirred into the 10% acetone solution of epoxy resin and poly- 350 8795 imide, and mixed using heavy duty laboratory mixer(Silverson 400 Machine Ltd, Model L2R) for 24 h After faster drying to main-450 ain the uniformly dispersed state, the resultant SiC whisker was packed in a stainless steel die, and cold-pressed at room tempera- ture and 30 MPa. The green body was then crushed and screened to select clusters with size of 0.3-0.6 mm. again the selected · clusters were mixed with 5% water solution of poly-vinyl alco- hol, and cold-pressed at room temperature and 15 MPa to mold the preforms. The volume fraction of the whiskers was about 5 vol %. After burning out the organic, CVI was performed to deposit SiC matrix from CH3 SiCl3(MTS)/H2= 1/10 for 300 h at P=3 kPa, Ar=350 m/min and T=1100C. The prepared com- posites were machined, and cut into specimens with dimension 3 mm x 4 mm x 40 mm to further deposit Sic matrix for 50h, 100 h and 150h, respectively pecific mass was determined according to Archimedes method. The phase analysis of Sic whisker, SiC matrix 253035404550556065 and SiCw/Sic composites was identified by X-ray diffrac tion(XRD)and microstructure was studied using scanning Fig. 2. XRD patterns of: (a)Sic whisker, (b) SiCw/SiC composites and (c)CVI electron microscopy (SEM). Bend specimens with dimension SiC()a-SiC and(O)B-Sic
Y. Hua et al. / Materials Science and Engineering A 428 (2006) 346–350 347 Fig. 1. SEM micrograph of the silicon carbide whiskers. and the purpose of this work is to evaluate the employment of CVI, as a novel method, in fabricating the SiCW/SiC from SiC whisker preforms, and its microstructure and mechanical properties. 2. Experimental procedure Fig. 1 shows the SEM micrograph of the SiC whisker with rough surface. -SiC whisker (99% pure, Alfa Aesar, MA, USA) had an average size of 18m in length and 1.5m in diameter. However, SiC whiskers as long as 50–100 m were observed. The procedure for whisker surface treatment was as follows. As-purchased SiC whiskers were first washed with concentrated hydrochloric acid (36%) to remove metallic impurities. After repeatedly washing with distilled water, the whisker was dipped into 20% hydrofluoric acid to remove SiO2, and washed again with distilled water. Then, after drying, it was dipped into acetylacetonate to pre-disperse for 24 h. Then the solvent was extracted by a rotary evaporator. As-treated whisker was then stirred into the 10% acetone solution of epoxy resin and polyimide, and mixed using heavy duty laboratory mixer (Silverson Machine Ltd., Model L2R) for 24 h. After faster drying to maintain the uniformly dispersed state, the resultant SiC whisker was packed in a stainless steel die, and cold-pressed at room temperature and 30 MPa. The green body was then crushed and screened to select clusters with size of 0.3–0.6 mm. Again, the selected clusters were mixed with 5% water solution of poly-vinyl alcohol, and cold-pressed at room temperature and 15 MPa to mold the preforms. The volume fraction of the whiskers was about 25 vol.%. After burning out the organic, CVI was performed to deposit SiC matrix from CH3SiCl3 (MTS)/H2 = 1/10 for 300 h at P = 3 kPa, Ar = 350 ml/min and T = 1100 ◦C. The prepared composites were machined, and cut into specimens with dimension 3 mm × 4 mm × 40 mm to further deposit SiC matrix for 50 h, 100 h and 150 h, respectively. Specific mass was determined according to Archimedes method. The phase analysis of SiC whisker, SiC matrix and SiCW/SiC composites was identified by X-ray diffraction (XRD) and microstructure was studied using scanning electron microscopy (SEM). Bend specimens with dimension 3 mm × 4 mm × 40 mm were used to evaluate the three-point flexural strength using a cross-head speed of 0.5 mm min−1 and a span length of 30 mm at room temperature. Five specimens were tested to obtain an average value. Micro-hardness and fracture toughness were determined at room temperature by Vickers indentation method with 196 N load for 30 s, 10 times on each of five samples per data point. 3. Results and discussion 3.1. Phase analysis and density The relative density of 25 vol.% SiCW/SiC composite to deposition time is summarized in Table 1. As shown, the density of the specimens is low compared to that of the whisker reinforced ceramic matrix composites prepared by sintering method [9,11]. CVI process results in composites intrinsically display significant residual porosity (typically, about 10%), which is the major factor inducing the low density [15]. Fig. 2 shows the XRD patterns of SiC whisker, pure SiC matrix and composites with deposition time of 450 h. As can be seen, the content of high temperature -SiC phase in the composites increases, and the composites consist of pure SiC. Dispersing and large aspect ratio of whiskers can create enough space in clusters. The spacing structure among and in the clusters is similar to that of the fiber preforms, creating path for the CH3SiCl3 and H2 to infiltrate. After depositing SiC matrix for 450 h, the SiC coating (shell) still does not form, and the relative density of the composites is 88.72%. The relative density of 88.72% is close to its limit that can be achieved by CVI. It Table 1 Deposition time and relative density of the 25% SiCW/SiC composites Deposition time (h) Relative density (%) 300 84.30 350 87.95 400 88.40 450 88.72 Fig. 2. XRD patterns of: (a) SiC whisker, (b) SiCW/SiC composites and (c) CVI SiC. () -SiC and () -SiC.
Y Hua et al. Materials Science and Engineering A 428(2006)346-350 30 HArdness Relative density (% Fig. 3. Fracture tough ss value as a function of relative density of 25% SiC w/SiC composi can be concluded that the CVI path in the whisker preforms is expedite 3. 2. Microstructure and mechanical propertie Fracture toughness and micro-hardness values. determined by means of vickers indentation method with a load of 196N are shown in Fig. 3. Fracture toughness and micro-hardness value increase insignificantly with increasing the relative density from 84. 30% to 87.95%6. while the fracture toughness increases significantly with increasing the relative density from 87.95%o to 88.72%o Micro-hardness value increases significantly with V8. 5mm x5, 0Ok SE increasing the relative density from 87. 95% to 88.40%o, but ther increases slowly. Fracture toughness and micro-hardness value depend on the relative density significantly when it is higher than 87.95%0 he pores among whisker clusters of the composites with rel- ative density of 84.30%0, namely the pores at the triple-junction pockets, are larger than those with relative density of 87.95%0 and 88.72%. The pores at the triple-junction pockets contribute to the relative density mainly(see Fig 4). There are small pores in whisker clusters of the composites with relative density of 84.30%0, and there are no pores in the whisker clusters of the composites with relative density of 87.95% and 88. 72%0. Micro- hardness is determined by vickers indentation method within the clusters of the composites. The pores in the clusters dominate the micro-hardness, the fewer the pores, the higher the hardness (see Fig. 5). The pores can shield the main cracks, and contribute to the increase of fracture toughness [21 ]. The effect of pores posites with the relative density: (a)84.30%, (b)87.95%and(c)88.72% on fracture toughness is secondary compared to the role of SiC Fig. 4. Typical microstructure of cross-section surface of 25% SiCw/SiC composites with the relative density:(a)84.30%,(b)87.95% and (c)88.72%
348 Y. Hua et al. / Materials Science and Engineering A 428 (2006) 346–350 Fig. 3. Fracture toughness and micro-hardness value as a function of relative density of 25% SiCW/SiC composites. can be concluded that the CVI path in the whisker preforms is expedite. 3.2. Microstructure and mechanical properties Fracture toughness and micro-hardness values, determined by means of Vickers indentation method with a load of 196 N, are shown in Fig. 3. Fracture toughness and micro-hardness value increase insignificantly with increasing the relative density from 84.30% to 87.95%, while the fracture toughness increases significantly with increasing the relative density from 87.95% to 88.72%. Micro-hardness value increases significantly with increasing the relative density from 87.95% to 88.40%, but then increases slowly. Fracture toughness and micro-hardness value depend on the relative density significantly when it is higher than 87.95%. The pores among whisker clusters of the composites with relative density of 84.30%, namely the pores at the triple-junction pockets, are larger than those with relative density of 87.95% and 88.72%. The pores at the triple-junction pockets contribute to the relative density mainly (see Fig. 4). There are small pores in whisker clusters of the composites with relative density of 84.30%, and there are no pores in the whisker clusters of the composites with relative density of 87.95% and 88.72%. Microhardness is determined by Vickers indentation method within the clusters of the composites. The pores in the clusters dominate the micro-hardness, the fewer the pores, the higher the hardness (see Fig. 5). The pores can shield the main cracks, and contribute to the increase of fracture toughness [21]. The effect of pores on fracture toughness is secondary compared to the role of SiC Fig. 5. Typical microstructure of cross-section surface of 25% SiCW/SiC composites with the relative density: (a) 84.30%, (b) 87.95% and (c) 88.72%. Fig. 4. Typical microstructure of cross-section surface of 25% SiCW/SiC composites with the relative density: (a) 84.30%, (b) 87.95% and (c) 88.72%
Y Hua et al. Materials Science and Engineering A 428(2006)346-350 Fig. 6. SEM morphologies of indentation crack path of the 25% SiCw/SiC composites:(a) crack deflection and bridge; (b) whisker pullout. whiskers, because the composites with the most pores(relative and the flexural strength is 196 MPa and 305 MPa, respectively density of 84.30%)have the lowest fracture toughness The relative density(residual porosity) has an effect on the Fig. 5 shows typical microstructures of the cross-section flexural strength, the lower the residual porosity, the higher surface of 25% SiCw/SiC composites with relative density of the flexural strength, while it is the bonding strength of grain 64.30%0,87.95% and 88.72%, respectively. The whisker retains boundary or interfacial phases(reinforcement phases) as well its original appearance, and its mechanical properties could be as volume fraction of reinforcements, not the residual porosity, maintained after the CVI process. The physical and mechan- that is the main factor influencing the strength of the compos ical properties of whiskers can be damaged by sintering. The ites [22-24]. Table 2 shows the flexural strength of SiCw/SiC whisker pullout and cavity could be observed in the rough frac- composites prepared by CVl, hot pressing, hot isostatic press ture surface. The crack path generated by indentation is shown ing sintering and chemical transformation from polycarbosilane in Fig. 6, and the matrix and whisker interaction zone behind the [9, 25, 26]. As Table 2 and Fig. 7 show, for the composites with crack tip could be seen. It is believed that this interaction effect same whisker volume fraction, the density of the composites between the propagating crack and microstructure leads to the prepared by hot isostatic pressing is 9.5% higher than that of high fracture toughness. The extensive crack interactions with the composites prepared by CVl, while the flexural strength of the whisker, such as crack deflection, whisker bridging and pull- the former is much higher(486 MPa) than that of the latter. The out, are observed more clearly by the crack propagating paths relative density of 20 vol. SiCw/Si3 N4 composites prepared produced by Vickers indentation. The interaction can devalue by hot pressing increases by 9%(from 88%0 to 97%0), but the the intensity of crack tip. The greater toughening is attributed to the longer whisker aspect ratio because of the increased crack deflection and component pullout [2, 10]. The rough surface and arge aspect ratio of whiskers are effective in improving the resis- tance of whisker pulling out and crack propagating ot isostatic pressing sintering can engender strong bond- ing strength between matrix and homogenous whisker without sintering aids, and the contribution of crack-wake region is inhibited, leading to the brittle fracture [12]. As can be seen 5杀 from Fig. 5, the bonding strength between the Sic whisker and CVI SiC matrix is not too strong to promote the significant pullout of chaotically oriented whisker. The preparation temper- ature of 1 C cannot induce strong bonding strength betweer whiskers and matrix. Fig. 7 shows the flexural strength of Relative density (% 25 vol% SiCw/SiC composites as a function of relative density. Fig. 7. Flexural strength as a function of relative density of 25%SiCw/SiC The composites have a relative density of 84.30% and 88.72%o, composites. ral strength of SiCw/SiC composites prepared by different method Fabrication method Reference Relative density (%) orcement(vol %) 88.72 7.21 Hot pressing Hot isostatic pressing 25 Chemical transformation from polycarbosilane [26] 68.22
Y. Hua et al. / Materials Science and Engineering A 428 (2006) 346–350 349 Fig. 6. SEM morphologies of indentation crack path of the 25% SiCW/SiC composites: (a) crack deflection and bridge; (b) whisker pullout. whiskers, because the composites with the most pores (relative density of 84.30%) have the lowest fracture toughness. Fig. 5 shows typical microstructures of the cross-section surface of 25% SiCW/SiC composites with relative density of 84.30%, 87.95% and 88.72%, respectively. The whisker retains its original appearance, and its mechanical properties could be maintained after the CVI process. The physical and mechanical properties of whiskers can be damaged by sintering. The whisker pullout and cavity could be observed in the rough fracture surface. The crack path generated by indentation is shown in Fig. 6, and the matrix and whisker interaction zone behind the crack tip could be seen. It is believed that this interaction effect between the propagating crack and microstructure leads to the high fracture toughness. The extensive crack interactions with the whisker, such as crack deflection, whisker bridging and pullout, are observed more clearly by the crack propagating paths produced by Vickers indentation. The interaction can devalue the intensity of crack tip. The greater toughening is attributed to the longer whisker aspect ratio because of the increased crack deflection and component pullout [2,10]. The rough surface and large aspect ratio of whiskers are effective in improving the resistance of whisker pulling out and crack propagating. Hot isostatic pressing sintering can engender strong bonding strength between matrix and homogenous whisker without sintering aids, and the contribution of crack–wake region is inhibited, leading to the brittle fracture [12]. As can be seen from Fig. 5, the bonding strength between the SiC whisker and CVI SiC matrix is not too strong to promote the significant pullout of chaotically oriented whisker. The preparation temperature of 1100 ◦C cannot induce strong bonding strength between whiskers and matrix. Fig. 7 shows the flexural strength of 25 vol.% SiCW/SiC composites as a function of relative density. The composites have a relative density of 84.30% and 88.72%, and the flexural strength is 196 MPa and 305 MPa, respectively. The relative density (residual porosity) has an effect on the flexural strength, the lower the residual porosity, the higher the flexural strength, while it is the bonding strength of grain boundary or interfacial phases (reinforcement phases) as well as volume fraction of reinforcements, not the residual porosity, that is the main factor influencing the strength of the composites [22–24]. Table 2 shows the flexural strength of SiCW/SiC composites prepared by CVI, hot pressing, hot isostatic pressing sintering and chemical transformation from polycarbosilane [9,25,26]. As Table 2 and Fig. 7 show, for the composites with same whisker volume fraction, the density of the composites prepared by hot isostatic pressing is 9.5% higher than that of the composites prepared by CVI, while the flexural strength of the former is much higher (486 MPa) than that of the latter. The relative density of 20 vol.% SiCW/Si3N4 composites prepared by hot pressing increases by 9% (from 88% to 97%), but the Fig. 7. Flexural strength as a function of relative density of 25% SiCW/SiC composites. Table 2 Flexural strength of SiCW/SiC composites prepared by different methods Fabrication method Reference Relative density (%) Flexural strength (MPa) Volume fraction of reinforcement (vol.%) Fracture toughness (MPa m1/2) CVI 88.72 305 25 7.21 Hot pressing [9] – 501 – 5.96 Hot isostatic pressing [25] 98.2 791 25 5.66 Chemical transformation from polycarbosilane [26] 68.22 250 54.5 –
350 Y. Hua et al. Materials Science and Engineering A 428(2006)346-350 flexural strength only increases by 97 MPa(from 470 MPa to with increasing the relative density from 88.45% to 88.72%0 67 MPa)[22]. The relative density of 25 vol %o SiCw/SiC com- Fracture toughness and flexural strength increase significantly posites prepared by CVI increases by 4.42%(from 84.30% to with increasing the relative density from 84.30% to 88.72%6 88.72%0), and the flexural strength increases by 109 MPa(from The improved fracture toughness is attributed to the pullout of 96 MPa to 305 MPa). The flexural strength of the composites rough whisker, crack deflection and whisker bridging. The ran- prepared by same method does not increase significantly with domly oriented whiskers that are parallel to the crack plane and increasing its relative density. Vice verse, the flexural strength of the weak bonding strength between whiskers and matrix cannot the composites prepared by different methods increases signifi- have much stress transfer from the matrix, leading to the low cantly with increasing its relative density. The residual porosity flexural strength does not influence the flexural strength significantly. The com- posites prepared by different methods have different bonding Acknowledge trength between whiskers and matrix. It can be concluded that the bonding strength between whiskers and matrix influences the The authors acknowledge support of this work by the Natural flexural strength significantly, and the bonding strength between Science Foundation of China under Grant No. 90405015 whiskers and matrix in the composites prepared by Cviis lower than that in the composites prepared by hot isostatic pressing. 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Kim, tation, as Fig. 5 shows, the randomly oriented whiskers that are parallel to the crack plane cannot have much stress transfer from 9].M. Dong, D.L. Jiang, S.H. Tan, J.K.Guo, J Inorg. Mater. 14(1999) the matrix, therefore the strength of composites can be dropped [10] P.R. Sneary, Z Yeh, M.J. Crimp, JMater. Sci. 36(2001)2529 significantly [23, 27]. The composites prepared by hot pressing [11] T.M. Lillo, D.W.Bailey, D.A. Laughton, H.S. Chu, W.M. Harrison, and hot isostatic pressing undergo degradation at high tempera- Ceram. Eng. Sci. Pro. 24(3)(2003)359 tures due to the thermally weak intergranular glassy phase, while [12] 1. Tanaka, G. Pezzotti,Y. Miyamoto, T.Okamoto, J.Mater. Sci.26 the composites prepared by Cvi will not because sintering aids (1991)208 are not introduced, namely the intergranular glassy phase free [3JJ Petrovic, O. Unal, T.E. Mitchell, J. Mater. Sci. 27(1992)3770 grain boundaries[8, 25 14] K. Ueno, S. Kose, M. Kinoshita. J. Mater. Sci. 28(1993)5770. [15]R. Naslain, R. Pailler, X. Bourrat, G. Vignoles, Key Eng. Mater. 159-160 The fracture toughness of the composites depends on the rel- (1999)359 ative density. One of the reasons for the increase of fracture [16]R. Naslain, Compos. Sci. Technol. 64(2004)155 toughness is considered to be the increase of the relative density [171 Y D. Xu, L.T. Zhang, J. Am. Ceram. Soc. 80(1997)1897 and resultant increase of the flexural strength[5]. Both the rel- [18] KJ. Probst, T.M. Besmann, D.P. Stinton, R.A. Lowden, T.J. Anderson, ative weak bonding strength and large aspect ratio, which lead T.L. Star, Surf. Coat. Technol. 120-121(1999)250. [19 A Hiroshi, N. Tetsuji, Y Wen, K. Akira, J Nucl. Mater. 307-311(2002) to the pullout of Sic whisker, contribute to the higher fracture The amount and length of pullout of Sic whisker [20] N. Igawaa, T.Taguchi, T Nozawa, L L.Snead, THinoki, JC.McLaugh- with increasing the relative density, so the fracture lin, Y. Katoh, S. Jitsukawa, A. Kohyama, J. Phys. Chem. Solids 66 s Increases (2005)551 [21]AG. Evans, J. Am. Ceram Soc. 73(2)(1990)187. [221 P. Go, C. Sung, J.J. Kostetsky, T. Vasilos, J. Mater. Sci. 27(2002) 4. Conclusions 2587 [23]SA. Baldacim, C.A.A. Cairo, C.R.M. Silva, J. Mater. Process. Technol. Twenty-five volume percent SiCw/SiC composites can be 119(2001)273. prepared by depositing SiC matrix in the Sic whisker pre- 24 B. Chol, H E. Kim, J. Am. Ceram Soc. 81(8)(1998)2191 forms using CVI. Sintering aids can be virtually eliminated [25H Mahfuz, D. Zadoo, E. wilks,M.Maniruzzaman,S. Jeelani, J.Mater. and whisker morphology can be maintained. Micro-hardness of [26] C.X. Feng, K.B. Huang, C.R. Zhang C.H. Chen, Gao Ji Shu Tong Xun the composites increases significantly with increasing the rela 6(1994)29 tive density from 84.30% to 88.45%, and then increases slowly [271 Y. Goto, A. Tsuge, J. Am. Ceram. Soc. 76(6)(1993)1420
350 Y. Hua et al. / Materials Science and Engineering A 428 (2006) 346–350 flexural strength only increases by 97 MPa (from 470 MPa to 567 MPa) [22]. The relative density of 25 vol.% SiCW/SiC composites prepared by CVI increases by 4.42% (from 84.30% to 88.72%), and the flexural strength increases by 109 MPa (from 196 MPa to 305 MPa). The flexural strength of the composites prepared by same method does not increase significantly with increasing its relative density. Vice verse, the flexural strength of the composites prepared by different methods increases signifi- cantly with increasing its relative density. The residual porosity does not influence the flexural strength significantly. The composites prepared by different methods have different bonding strength between whiskers and matrix. It can be concluded that the bonding strength between whiskers and matrix influences the flexural strength significantly, and the bonding strength between whiskers and matrix in the composites prepared by CVI is lower than that in the composites prepared by hot isostatic pressing. The bonding strength between whiskers and matrix is not strong enough for the composites prepared by CVI to obtain a high flexural strength. For the composites with different whisker volumes, the flexural strength of the composites prepared by CVI is 55 MPa higher than that of the composites prepared by chemical transformation, while the density of the former is much higher (22.5%) than that of the latter. The enhancement due to the increased whisker content offsets the decreased overall composite density. The flexural strength of the composites is also influenced by grain boundary phases as well as whisker orientation, as Fig. 5 shows, the randomly oriented whiskers that are parallel to the crack plane cannot have much stress transfer from the matrix, therefore the strength of composites can be dropped significantly [23,27]. The composites prepared by hot pressing and hot isostatic pressing undergo degradation at high temperatures due to the thermally weak intergranular glassy phase, while the composites prepared by CVI will not because sintering aids are not introduced, namely the intergranular glassy phase free grain boundaries [8,25]. The fracture toughness of the composites depends on the relative density. One of the reasons for the increase of fracture toughness is considered to be the increase of the relative density and resultant increase of the flexural strength [5]. Both the relative weak bonding strength and large aspect ratio, which lead to the pullout of SiC whisker, contribute to the higher fracture toughness. The amount and length of pullout of SiC whisker increase with increasing the relative density, so the fracture toughness increases. 4. Conclusions Twenty-five volume percent SiCW/SiC composites can be prepared by depositing SiC matrix in the SiC whisker preforms using CVI. Sintering aids can be virtually eliminated and whisker morphology can be maintained. Micro-hardness of the composites increases significantly with increasing the relative density from 84.30% to 88.45%, and then increases slowly with increasing the relative density from 88.45% to 88.72%. Fracture toughness and flexural strength increase significantly with increasing the relative density from 84.30% to 88.72%. The improved fracture toughness is attributed to the pullout of rough whisker, crack deflection and whisker bridging. 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