《复合材料 Composites》课程教学资源（学习资料）第二章 增强体_SiC WHISKER-14 Influence of SiC whisker morphology and nature of SiC/Al2O3 interface on thermomechanical properties of SiC reinforced Al2O3 composites

Availableonlineatwww.sciencedirect.com SCIENCE DIRECT● E噩≈S ELSEVIER Journal of the European Ceramic Society 25 (2005)3485-3493 www.elsevier.com/locate/jeurceramsoc Influence of SiC whisker morphology and nature of SiC/Al2O3 interface on thermomechanical properties of SiC reinforced Al2 O3 composites V Garnier G. Fantozzi, D. nguyen, J. Dubois, G. Thollet INSA de Lyon, GEMPPM UMR CNRS 5510, Villeurbanne 69621, france Received 9 July 2004; received in revised form 13 September 2004: accepted 18 September 2004 Available online 20 June 2005 Abstract Thermomechanical properties of a 35 vol. SiC whiskers/Al,O3 matrix composite were investigated as a function of whisker surface quality. Two batches of Sic whiskers(Tateho-SCw-l-S)were studied. Whisker surface chemistry, as determined by X-ray photoelectron pectroscopy and whisker morphology, as determined by SEM or TEM, was correlated to the thermomechanical properties of the composites The surface oxygen content of the whiskers was shown to strongly affect the composite thermomechanical properties. High oxygen surface ontent appears to affect the whisker/matrix interfacial bonding thus decreasing the amount of crack deflection, whisker pullout and whisker bridging which are required to reach high fracture toughness values C 2004 Elsevier Ltd. All rights reserved. Keywords: SiC; Whiskers; Interfaces; Composites; Thermal properties; Mechanical properties; Al2O3/SiC 1. Introduction Recent works on ceramic matrix composites have demon strated that fracture toughness and flexural strength of poly In ceramic matrix composites, whisker reinforcements crystalline Al2O3 can be significantly improved by addi are primarily used to enhance the fracture toughness and tion of SiC whiskers. Becher and Wei, 3 Wei and Becher, 14 the flexural strength of the composite at temperatures Becher et al., 15 and Homey et al. 6 have achieved frac- 1000C. Essentially, the whisker reinforcement prevents ture toughness values approaching 10 MPam.and flexural catastrophic brittle failure by providing processes that dis- strength values approaching 800 MPa. Furthermore, Homeny spate energy during the fracture process. Toughening mech- and Vaughn have demonstrated that the fracture toughness anisms, such as crack deflection, -whisker pullout, -and could vary with whisker type from 4 to 9 MPam>when whisker bridging 8-10 depends to a large extent on the na- utilising whiskers that were similar in all aspects, except for ture of the whisker/matrix interface. Several factors affect surface chemistry. They have associated the high fracture the whisker/matrix interface, including matrix chemistry, toughness with the presence of carbon and silicon oxycarbide whisker surface chemistry, whisker morphology and ther- phases on the whiskers surface Tiegs et al. I8 have also per- mal expansion mismatches. The internal stresses are also formed a detailed study on whiskers from numerous sources pected to affect the toughening behaviour of SiC-whiskers- and have correlated the oxygen and carbon concentrations of et al. and Li and Brad. 2 omposite as shown by Predecki reinforced alumina matrix the whisker surfaces with the fracture toughness. According to them as well, the high fracture toughness is associated with of carbon excess on the surfa fracture toughness is attributed to oxygen excess Corresponding author The present work deals specifically with the effect of E-mail address: Vincent Garnier@insa-lyon fr(V. Garnier) hiskers quality on the thermomechanical properties of Sic 0955-2219/S-see front matter c 2004 Elsevier Ltd. All rights reserved doi: 10. 1016/j. jeurceramsoc. 2004.09.026

Journal of the European Ceramic Society 25 (2005) 3485–3493 Influence of SiC whisker morphology and nature of SiC/Al2O3 interface on thermomechanical properties of SiC reinforced Al2O3 composites V. Garnier∗, G. Fantozzi, D. Nguyen, J. Dubois, G. Thollet INSA de Lyon, GEMPPM UMR CNRS 5510, Villeurbanne 69621, France Received 9 July 2004; received in revised form 13 September 2004; accepted 18 September 2004 Available online 20 June 2005 Abstract Thermomechanical properties of a 35 vol.% SiC whiskers/Al2O3 matrix composite were investigated as a function of whisker surface quality. Two batches of SiC whiskers (Tateho-SCW-1-S) were studied. Whisker surface chemistry, as determined by X-ray photoelectron spectroscopy and whisker morphology, as determined by SEM or TEM, was correlated to the thermomechanical properties of the composites. The surface oxygen content of the whiskers was shown to strongly affect the composite thermomechanical properties. High oxygen surface content appears to affect the whisker/matrix interfacial bonding thus decreasing the amount of crack deflection, whisker pullout and whisker bridging which are required to reach high fracture toughness values. © 2004 Elsevier Ltd. All rights reserved. Keywords: SiC; Whiskers; Interfaces; Composites; Thermal properties; Mechanical properties; Al2O3/SiC 1. Introduction In ceramic matrix composites, whisker reinforcements are primarily used to enhance the fracture toughness and the flexural strength of the composite at temperatures to 1000 ◦C. Essentially, the whisker reinforcement prevents catastrophic brittle failure by providing processes that dis￾sipate energy during the fracture process. Toughening mech￾anisms, such as crack deflection,1–2 whisker pullout,3–7 and whisker bridging,8–10 depends to a large extent on the na￾ture of the whisker/matrix interface. Several factors affect the whisker/matrix interface, including matrix chemistry, whisker surface chemistry, whisker morphology and ther￾mal expansion mismatches. The internal stresses are also ex￾pected to affect the toughening behaviour of SiC-whiskers￾reinforced alumina matrix composite as shown by Predecki et al.11 and Li and Bradt.12 ∗ Corresponding author. E-mail address: Vincent.Garnier@insa-lyon.fr (V. Garnier). Recent works on ceramic matrix composites have demon￾strated that fracture toughness and flexural strength of poly￾crystalline Al2O3 can be significantly improved by addi￾tion of SiC whiskers. Becher and Wei,13 Wei and Becher,14 Becher et al.,15 and Homeny et al.16 have achieved frac￾ture toughness values approaching 10 MPa m0.5 and flexural strength values approaching 800 MPa. Furthermore, Homeny and Vaughn17 have demonstrated that the fracture toughness could vary with whisker type from 4 to 9 MPa m0.5 when utilising whiskers that were similar in all aspects, except for surface chemistry. They have associated the high fracture toughness with the presence of carbon and silicon oxycarbide phases on the whiskers surface. Tiegs et al.18 have also per￾formed a detailed study on whiskers from numerous sources and have correlated the oxygen and carbon concentrations of the whisker surfaces with the fracture toughness. According to them as well, the high fracture toughness is associated with the presence of carbon excess on the surfaces, while the low fracture toughness is attributed to oxygen excess. The present work deals specifically with the effect of whiskers quality on the thermomechanical properties of SiC 0955-2219/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2004.09.026

3486 V. Garnieret al /Journal of the European Ceramic Sociery 25(2005)3485-3493 hikers/Al2O3 Tateho-SCW-1- phologies and surf S composites. Two whiskers batches rence of an oxygen layer on the whisker surface. SEM ob. been used to obtain different mor- servations allow making comparisons between the different xidation states morphologies of the batches. The whiskers 'L have mainly small cross section (Fig 2a)whereas the whiskers 'H show mainly large cross section with undulated surfaces(Fig 2b) 2. Experimental procedures The polycrystalline alumina powder utilized for the matrix is SM8 Baikowski Chimie, france, 99.9% alumina, <50 2. 1. Material preparation Na, Mg and Ca). The mean particle size diameter of the mina powder is about 0.25 um and the specific surface area Two batches ofscw-I grade SiC whiskers (Tateho, Japan) is 10.4 m/g have been used as reinforcement material one batch with a The details of processing technique are described low surface oxygen content of 6 at oxygen as received and elsewhere. - -4 Briefly, the Sic whiskers(35 vol %)and another batch with a high surface oxygen content of 39 at AlO3 powder are mixed using a water-based slurry method oxygen as received, respectively, labelled'L' andH. TEM First, a slurry of alumina powder is prepared in distilled wa- analysis of the 'H' batch sample(Fig. 1)reveals the occur- ter, the pH of the slurry is adjusted to 4, and the suspension Fig 1. TEM micrograph of Tateho SiC whiskers'H a)90207 220M Fig 2 SEM micrographs of Tateho SiC whiskers: (a)"L whisker and (b)'H whisker

3486 V. Garnier et al. / Journal of the European Ceramic Society 25 (2005) 3485–3493 whiskers/Al2O3 matrix composites. Two whiskers batches (Tateho-SCW-1-S) have been used to obtain different mor￾phologies and surface oxidation states. 2. Experimental procedures 2.1. Material preparation Two batches of SCW-1 grade SiC whiskers (Tateho, Japan) have been used as reinforcement material: one batch with a low surface oxygen content of 6 at.% oxygen as received and another batch with a high surface oxygen content of 39 at.% oxygen as received, respectively, labelled ‘L’ and ‘H’. TEM analysis of the ‘H’ batch sample (Fig. 1) reveals the occur￾rence of an oxygen layer on the whisker surface. SEM ob￾servations allow making comparisons between the different morphologies of the batches. The whiskers ‘L’ have mainly small cross section (Fig. 2a) whereas the whiskers ‘H’ show mainly large cross section with undulated surfaces (Fig. 2b). The polycrystalline alumina powder utilized for the matrix is SM8 (Ba¨ıkowski Chimie, France, 99.9% alumina, <50 ppm Na, Mg and Ca). The mean particle size diameter of the alu￾mina powder is about 0.25
m and the specific surface area is 10.4 m2/g. The details of processing technique are described elsewhere.23–24 Briefly, the SiC whiskers (35 vol.%) and Al2O3 powder are mixed using a water-based slurry method. First, a slurry of alumina powder is prepared in distilled wa￾ter, the pH of the slurry is adjusted to 4, and the suspension Fig. 1. TEM micrograph of Tateho SiC whiskers ‘H’. Fig. 2. SEM micrographs of Tateho SiC whiskers: (a) ‘L’ whisker and (b) ‘H’ whisker

K Garnier et al. /Journal of the European Ceramic Society 25(2005)3485-3493 dispersed ultrasonically. Concerning the SiC whiskers, the Slow crack growth behaviour was determined by a double slurry is prepared using a basic solution and is subjected to torsion method. The specimens, 40 mm x 20 mm x 2 mm ultrasonic dispersion for 10 min. The final composite is ob- were centre notched using a diamond saw. The notch length tained by adding these slurries, each of them having a pH was about 10 mm and subsequently was precracked at a low value corresponding to a maximum zeta potential. The mix- cross-head speed of 4 um/min. The relaxation tests have been ture is subsequently dried through evaporation of water and made on these samples to obtain the variation of the load as filtered successively through 60 and 250 mesh sieves function of time and finally to allow the determination of Hot pressed discs of alumina and Al2O3/SiCw have been the V-K curves 20-21 obtained under a pressure of 45 MPa in an argon atmosphere atl850°C/lh 3. Results and discussion 2.2. Experimental techniques Table I shows mechanical properties at room temperature of monolithic alumina and Al2O3/SiCw composites prepared Final densities of the sintered samples have been measured with whiskers'L orH. After hot pressing, all samples show using the Archimede's principle. Vickers hardness, load of relative density close to the theoretical value. The microstruc- 100N, has been determined on polished surfaces ture of Al2O3/SiCw composites was observed by an optical Flexural strength and fracture tough micrograph on polished surface and has shown a homoge- mined in temperature range from 25 to 1300C in air atmo- neous dispersion of the whiskers into the alumina matrix ei- sphere, using the 4-point bending technique with a cross-head ther perpendicular or parallel to the hot pressing axis d ofo. I mm/min. The outer and inner spans were, respec No significant variation of Young's modulus is observed tively, 35 and 10 mm. The dimensions of flexural strength bars between alumina and composites samples On the opposite, were 3 mm x 4 mm x 40 mm and their tensile surfaces were Vickers hardness, flexural strength and fracture toughness of olished with a 3 um diamond-grinding wheel in the direc- the Al2O3/SiCw composites are higher than for monolithic tion of tensile axis to avoid the effect of machining defects on alumina. For the composites containing 'L SiC whiskers the intrinsic characteristic material. The edges on the tensile the fracture toughness is twice that of monolithic alumina surface were rounded. Thereafter, Young's modulus has been (4MPamo.). These results are quite comparable with the measured by the grindo-Sonic technique perties values reported by becher and co- The fracture toughness measurement has been perform workers 3-15,22-25 for a similar material using centre notched bars(6mm x 4 mm x 40 mm)to less Fracture surfaces of the two composites were also ob- one half of the thickness with a 0.3 mm thick diamond blade. served, micrographs are shown in Fig 3a and b. The fracture Creep tests have been conducted in air under 100 MPa surfaces generally exhibit both intergranular and intragranu- stress level at several temperatures(1000, 1200 and 1300C lar mode of failure, with some appearance of whiskers pull Specimens have been deformed in a 4-point bending de- out. Many observations performed on polished surfaces have vice whose inner and outer spans were, respectively, 18 and been made on fracture surface and indentation crack. It was 36 mm. The applied stress and resulting strain have been cal- pointed out that several toughening mechanisms occur in the edure described by Hollenberg et al., 9 the secondary creep pullout. Nevertheless, the main contribution of the alumina rates were determined from the variation of the displacement matrix reinforcement is due to cracks deflection as it can be versus time when the values are stabilized seen in Fig 4 Fracture resistance curves(R-curves)have been deter- As previously noted, flexural strength and fracture tough mined following single edge notched beam(SENB) tech- ness of polycrystalline alumina are improved by addition of nique in 4-point bending at a cross-head speed of 4 um/min. SiC whiskers. However, this improvement closely depends on The samples are machining with a 300 um diamond saw con- the Sic whiskers surface oxygen content. For the composite tinued by a thin notch made with a 70 um saw. The initial H, flexural strength and fracture toughness, are observed ratio of the precrack depth(ao) to sample width(w), ao/w, to be higher than for alumina but lower than for composite was chosen as 0.6 containing"L' SiC whiskers(Table 1) Table 1 Mechanical properties at room temperature of monolithic material and AlO3-35 vol % SiC whiskers and"H Mechanical properties Al2O3+35vol.%SiCw“L l2O3+35vol.%SiCw“L Relative density(dth % Youngs modulus(GPa) 406±10 421±10 407±9 Hardness Vickers(10kg) 854士38 2107±32 Flexural strength( MPa) 488±151 639±21 49±4 Fracture toughness(MPam.) 5.4±0.4 79±0.3 6.9±0.2

V. Garnier et al. / Journal of the European Ceramic Society 25 (2005) 3485–3493 3487 is dispersed ultrasonically. Concerning the SiC whiskers, the slurry is prepared using a basic solution and is subjected to ultrasonic dispersion for 10 min. The final composite is ob￾tained by adding these slurries, each of them having a pH value corresponding to a maximum zeta potential. The mix￾ture is subsequently dried through evaporation of water and filtered successively through 60 and 250 mesh sieves. Hot pressed discs of alumina and Al2O3/SiCw have been obtained under a pressure of 45 MPa in an argon atmosphere at 1850 ◦C/1 h. 2.2. Experimental techniques Final densities of the sintered samples have been measured using the Archimede’s principle. Vickers hardness, load of 100 N, has been determined on polished surfaces. Flexural strength and fracture toughness have been deter￾mined in temperature range from 25 to 1300 ◦C in air atmo￾sphere, using the 4-point bending technique with a cross-head speed of 0.1 mm/min. The outer and inner spans were, respec￾tively, 35 and 10 mm. The dimensions of flexural strength bars were 3 mm × 4 mm × 40 mm and their tensile surfaces were polished with a 3
m diamond-grinding wheel in the direc￾tion of tensile axis to avoid the effect of machining defects on the intrinsic characteristic material. The edges on the tensile surface were rounded. Thereafter, Young’s modulus has been measured by the Grindo-Sonic technique. The fracture toughness measurement has been performed using centre notched bars (6 mm × 4 mm × 40 mm) to less one half of the thickness with a 0.3 mm thick diamond blade. Creep tests have been conducted in air under 100 MPa stress level at several temperatures (1000, 1200 and 1300 ◦C). Specimens have been deformed in a 4-point bending de￾vice whose inner and outer spans were, respectively, 18 and 36 mm. The applied stress and resulting strain have been cal￾culated from the load and displacement data using the pro￾cedure described by Hollenberg et al.,19 the secondary creep rates were determined from the variation of the displacement versus time when the values are stabilized. Fracture resistance curves (R-curves) have been deter￾mined following single edge notched beam (SENB) tech￾nique in 4-point bending at a cross-head speed of 4
m/min. The samples are machining with a 300
m diamond saw con￾tinued by a thin notch made with a 70
m saw. The initial ratio of the precrack depth (a0) to sample width (w), a0/w, was chosen as 0.6. Slow crack growth behaviour was determined by a double torsion method. The specimens, 40 mm × 20 mm × 2 mm, were centre notched using a diamond saw. The notch length was about 10 mm and subsequently was precracked at a low cross-head speed of 4
m/min. The relaxation tests have been made on these samples to obtain the variation of the load as a function of time and finally to allow the determination of the V–KI curves.20–21 3. Results and discussion Table 1 shows mechanical properties at room temperature of monolithic alumina and Al2O3/SiCw composites prepared with whiskers ‘L’ or ‘H’. After hot pressing, all samples show relative density close to the theoretical value. The microstruc￾ture of Al2O3/SiCw composites was observed by an optical micrograph on polished surface and has shown a homoge￾neous dispersion of the whiskers into the alumina matrix ei￾ther perpendicular or parallel to the hot pressing axis. No significant variation of Young’s modulus is observed between alumina and composites samples. On the opposite, Vickers hardness, flexural strength and fracture toughness of the Al2O3/SiCw composites are higher than for monolithic alumina. For the composites containing ‘L’ SiC whiskers, the fracture toughness is twice that of monolithic alumina (4 MPa m0.5). These results are quite comparable with the mechanical properties values reported by Becher and co￾workers13–15,22–25 for a similar material. Fracture surfaces of the two composites were also ob￾served, micrographs are shown in Fig. 3a and b. The fracture surfaces generally exhibit both intergranular and intragranu￾lar mode of failure, with some appearance of whiskers pull￾out. Many observations performed on polished surfaces have been made on fracture surface and indentation crack. It was pointed out that several toughening mechanisms occur in the material such as: crack deflection, debonding, bridging and pullout. Nevertheless, the main contribution of the alumina matrix reinforcement is due to cracks deflection as it can be seen in Fig. 4. As previously noted, flexural strength and fracture tough￾ness of polycrystalline alumina are improved by addition of SiC whiskers. However, this improvement closely depends on the SiC whiskers surface oxygen content. For the composite ‘H’, flexural strength and fracture toughness, are observed to be higher than for alumina but lower than for composite containing ‘L’ SiC whiskers (Table 1). Table 1 Mechanical properties at room temperature of monolithic material and Al2O3–35 vol.% SiC whiskers ‘L’ and ‘H’ Mechanical properties Al2O3 Al2O3 + 35 vol.% SiCw ‘L’ Al2O3 + 35 vol.% SiCw ‘L’ Relative density (dth %) 99.1 100 99.6 Young’s modulus (GPa) 406 ± 10 421 ± 10 407 ± 9 Hardness Vickers (10 kg) 1854 ± 38 2107 ± 32 2032 ± 62 Flexural strength (MPa) 488 ± 151 639 ± 21 549 ± 41 Fracture toughness (MPa m0.5) 5.4 ± 0.4 7.9 ± 0.3 6.9 ± 0.2

3488 V. Garnieret al /Journal of the European Ceramic Sociery 25(2005)3485-3493 Fig. 3. SEM micrographs of a fracture surface of Al2O3/35 vol. SiC whiskers, showing inter and intragranular mode failure and whisker pullout, (a)"L whisker and(b)H whisker. 29nm Fig 4. TEM micrograph of Al2O3/35 vol. SiC whisker "L' composite, showing crack deflection along whisker/matrix interface snm Fig. 5. TEM micrograph of Al2 O3/35 vol. SiC whiskers'L' composite, showing the appearance of a glass layer along the alumina/whisker interface

3488 V. Garnier et al. / Journal of the European Ceramic Society 25 (2005) 3485–3493 Fig. 3. SEM micrographs of a fracture surface of Al2O3/35 vol.% SiC whiskers, showing inter and intragranular mode failure and whisker pullout, (a) ‘L’ whisker and (b) ‘H’ whisker. Fig. 4. TEM micrograph of Al2O3/35 vol.% SiC whisker ‘L’ composite, showing crack deflection along whisker/matrix interface. Fig. 5. TEM micrograph of Al2O3/35 vol.% SiC whiskers ‘L’ composite, showing the appearance of a glass layer along the alumina/whisker interface

K Garnier et al. /Journal of the European Ceramic Society 25(2005)3485-3493 3489 A SICw'L'-D SICw'H' 8765 Temperature('C) Fig. 6. Flexural strength and fracture toughness as a function of temperature The lower of and kic values obtained when increasing and agree with the above assumption. However, the decrease SiC whiskers surface oxygen content can be explained by the of the fexural strength as well as the increase of the fracture degradation of silicon carbide in presence of oxygen and/or toughness started about 200 C earlier than the former. These by the surface chemical reaction between SiO and AlzO3. As observations confirm an effect of the surface oxygen content a consequence, a strong interface whisker-matrix is created of the original Sic whiskers on the mechanical properties of (see Fig. 5), which minimizes the amount of cracks deflection the final composites at high temperatures along the interface, whisker bridging and pullout 25-26 According to Becher and Tiegs, 7 the marked strength Flexural strength and fracture toughness were also mea- degradation, which occurs above 1000oC in air, is associated sured at higher temperatures in air atmosphere(from 800 with creep. At this temperature, the viscosity of the glassy to 1300C). The variation of flexural strength and fracture phase must be sufficiently low to allow the liquid phase to toughness for the two composites are shown in Fig. 6 as a penetrate along the matrix grain boundaries and enhanced function of temperature. For the ' L' composite, KiC and creep and associated crack generation. Observations of the decrease slowly with increasing temperature up to 1000C. fracture surface sample tested at 1200C support this con- At temperatures above 1000C, of significantly decreases clusion(see Fig. 7) suggesting that fracture is governed by a different mecha- Fig. 8 shows the creep deformation for composites at nism. Then, the fracture toughness remains constant up to 1200 C under 100 MPa. For the 'L composites, the be- 200C, and at higher temperatures, above 1200oC, the frac- haviour indicates that the creep deformation involves a short ture toughness increases rapidly up to 10 MPam.s. During primary stage ofcreep, during which the strain rate decreases high-temperature air annealing of alumina silicon carbide and then a long steady-state region follows this stage. Tertiary composites, silicon carbide is oxidizing. This oxidation pro- creep is not observed at all and the specimen is not broken duces an amorphous phase that softens above 1200C and is after a testing period of 80 h responsible for the composite behaviour at 1300C. Results The creep resistance of polycrystalline alumina, >3% of flexural strength and fracture toughness obtained for H without failure limited only by test fixture, can be signifi composites are similar with those obtained forL composites cantly improved through the addition of Sic whiskers. The Fig. 7. SEM micrographs of (a) the tensile surface and (b)room temperature fracture surface of the Al2O3/35 vol. SiC whisker '"L' composite after creep esting at 1200C in air, showing the Sic oxidation and the liquid phase, respectively

V. Garnier et al. / Journal of the European Ceramic Society 25 (2005) 3485–3493 3489 Fig. 6. Flexural strength and fracture toughness as a function of temperature. The lower σf and KIC values obtained when increasing SiC whiskers surface oxygen content can be explained by the degradation of silicon carbide in presence of oxygen and/or by the surface chemical reaction between SiO2 and Al2O3. As a consequence, a strong interface whisker-matrix is created (see Fig. 5), which minimizes the amount of cracks deflection along the interface, whisker bridging and pullout.25–26 Flexural strength and fracture toughness were also mea￾sured at higher temperatures in air atmosphere (from 800 to 1300 ◦C). The variation of flexural strength and fracture toughness for the two composites are shown in Fig. 6 as a function of temperature. For the ‘L’ composite, KIC and σf decrease slowly with increasing temperature up to 1000 ◦C. At temperatures above 1000 ◦C, σf significantly decreases suggesting that fracture is governed by a different mecha￾nism. Then, the fracture toughness remains constant up to 1200 ◦C, and at higher temperatures, above 1200 ◦C, the frac￾ture toughness increases rapidly up to 10 MPa m0.5. During high-temperature air annealing of alumina silicon carbide composites, silicon carbide is oxidizing. This oxidation pro￾duces an amorphous phase that softens above 1200 ◦C and is responsible for the composite behaviour at 1300 ◦C. Results of flexural strength and fracture toughness obtained for ‘H’ composites are similar with those obtained for ‘L’ composites and agree with the above assumption. However, the decrease of the flexural strength as well as the increase of the fracture toughness started about 200 ◦C earlier than the former. These observations confirm an effect of the surface oxygen content of the original SiC whiskers on the mechanical properties of the final composites at high temperatures. According to Becher and Tiegs,27 the marked strength degradation, which occurs above 1000 ◦C in air, is associated with creep. At this temperature, the viscosity of the glassy phase must be sufficiently low to allow the liquid phase to penetrate along the matrix grain boundaries and enhanced creep and associated crack generation. Observations of the fracture surface sample tested at 1200 ◦C support this con￾clusion (see Fig. 7). Fig. 8 shows the creep deformation for composites at 1200 ◦C under 100 MPa. For the ‘L’ composites, the be￾haviour indicates that the creep deformation involves a short primary stage of creep, during which the strain rate decreases, and then a long steady-state region follows this stage. Tertiary creep is not observed at all and the specimen is not broken after a testing period of 80 h. The creep resistance of polycrystalline alumina, >3% without failure limited only by test fixture, can be signifi- cantly improved through the addition of SiC whiskers. The Fig. 7. SEM micrographs of (a) the tensile surface and (b) room temperature fracture surface of the Al2O3/35 vol.% SiC whisker ‘L’ composite after creep testing at 1200 ◦C in air, showing the SiC oxidation and the liquid phase, respectively