《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

V. Garnieret al /Journal of the European Ceramic Sociery 25(2005)3485-3493 ▲Sicw"L·1300°c 0. 6].SICw'L'-1200C sicw"H"-1200°C △△Δ△ Time(Hours Fig 8. Creep curves, at 1200 and 1300"C under 100 MPa, for alumina-SiC whisker composites. creep rate is 7x 10-s for theL composite after 60h of the composite with 'L SiC whiskers has been significantly of exposure under load 100 MPa and the corresponding final reduced by 2 orders of magnitude(=1.5 x 10-8s-)On the strain is 0.15%, same results have been observed earlier by other hand, a short stage of tertiary creep is observed before several authors 28-30 breaking at a strain of about 0.6% and after 40 h Observations of Al2O3/SiCw composites after creep test Concerning the composites with 'H whiskers(Fig 8)the ng have revealed that the Sic whiskers on the surface as specimen has been broken after only 2 h of testing. This creep well as in the composite core have been oxidized. Precise behaviour degradation and the increase of the fracture tough examination of a sample fracture surface, broken at room ness under air, at high temperatures, may be explained by the temperature after creep testing at 1200C in air, has been SiC oxidation and the amount of glassy phase in the grain performed. The SEM micrographs(Fig. 7)show the occur- boundaries rence of the glassy phase on the surface fracture. The I As it has been discussed earlier, toughening by deviation uid phase formed on exposed surfaces could migrate near and crack bridging occur in whisker reinforced materials the surface grain boundaries of alumina composite and ac- Consequently, the process zone size is not negligible and celerate creep deformation process. At higher temperatures non-linear macroscopic fracture behaviour must be observed, (1200C), the volume fraction of liquid phase increases and producing R-curve effect in which toughness (KR)varies with the viscosity of liquid phase decreases. Consequently, grain crack growth. So, the existence of a R-curve behaviour has boundaries sliding are more important and induce the for- been investigated by SENB method At room temperature and mation of grain boundary cavities, the creep behaviour may 1000C, no stable crack propagation could be obtained for be deeply modified. The observed creep deformation for"L' the composites prepared using 'L SiCw, suggesting that there composite at a stress of 100 MPa and 1300C confirms the are no extended R-curve behaviour But hereafter 1200C precedent hypothesis(Fig 8). At 1300C, the creep strength stable crack propagation was observed showing a significant 876 aAaaaa ▲▲A▲▲ ▲Sicw"L a SiCW'H Fig 9. Rising crack-growth resistance(R-curve)of alumina-SiC whisker composites at 1200C

3490 V. Garnier et al. / Journal of the European Ceramic Society 25 (2005) 3485–3493 Fig. 8. Creep curves, at 1200 and 1300 ◦C under 100 MPa, for alumina–SiC whisker composites. creep rate is 7 × 10−10 s−1 for the ‘L’ composite after 60 h of exposure under load 100 MPa and the corresponding final strain is 0.15%, same results have been observed earlier by several authors.28–30 Observations of Al2O3/SiCw composites after creep test￾ing have revealed that the SiC whiskers on the surface as well as in the composite core have been oxidized. Precise examination of a sample fracture surface, broken at room temperature after creep testing at 1200 ◦C in air, has been performed. The SEM micrographs (Fig. 7) show the occur￾rence of the glassy phase on the surface fracture. The liq￾uid phase formed on exposed surfaces could migrate near the surface grain boundaries of alumina composite and ac￾celerate creep deformation process. At higher temperatures (>1200 ◦C), the volume fraction of liquid phase increases and the viscosity of liquid phase decreases. Consequently, grain boundaries sliding are more important and induce the for￾mation of grain boundary cavities, the creep behaviour may be deeply modified. The observed creep deformation for ‘L’ composite at a stress of 100 MPa and 1300 ◦C confirms the precedent hypothesis (Fig. 8). At 1300 ◦C, the creep strength of the composite with ‘L’ SiC whiskers has been significantly reduced by 2 orders of magnitude (=1.5 × 10−8 s−1). On the other hand, a short stage of tertiary creep is observed before breaking at a strain of about 0.6% and after 40 h. Concerning the composites with ‘H’ whiskers (Fig. 8) the specimen has been broken after only 2 h of testing. This creep behaviour degradation and the increase of the fracture tough￾ness under air, at high temperatures, may be explained by the SiC oxidation and the amount of glassy phase in the grain boundaries. As it has been discussed earlier, toughening by deviation and crack bridging occur in whisker reinforced materials. Consequently, the process zone size is not negligible and non-linear macroscopic fracture behaviour must be observed, producing R-curve effect in which toughness (KR) varies with crack growth. So, the existence of a R-curve behaviour has been investigated by SENB method. At room temperature and 1000 ◦C, no stable crack propagation could be obtained for the composites prepared using ‘L’ SiCw, suggesting that there are no extended R-curve behaviour. But hereafter 1200 ◦C stable crack propagation was observed showing a significant Fig. 9. Rising crack-growth resistance (R-curve) of alumina–SiC whisker composites at 1200 ◦C

K Garnier et al. /Journal of the European Ceramic Society 25(2005)3485-3493 improvement of the R-curve behaviour with temperature. At the differences may become more pronounced during sl high temperatures the viscous liquid phase fastens cracks and crack growth leads to the formation of well-developed wake zone, associ- ated with whisker bridging The Fig 9 shows R-curve behaviour of two composites 4. Discussion prepared using"L' and H' SiCw For all composites, R in- creases with crack extension. In the 'L' type composite Thermomechanical properties of alumina were signifi id increase of KR(KR=8.8 MPamo. 5)is observed fol- cantly improved by the addition of Sic whiskers. However, lowed by a plateau value and a slow decrease after the crack this improvement depends on the whisker quality, particularly extension a* has reached the value of 600 um. This Kp d the surface oxygen co value corresponds to the steady state of whiskers and may Thermomechanical properties of the composites prepared be referred as fracture toughness for crack propagation. In with high surface oxygen content whiskers were lower than this state, the bridge generation is balanced by its extension those obtained with the composites prepared with low surface and the length of the well-developed bridging zone is given oxygen content whiskers. This result suggests two possible by a* explanations. First, the superficial silica whiskers and alu- The R-curve behaviour concerning the H"composite is mina matrix may react to form mullite, producing a strong different(Fig 9), the initial K value is much lower and the interface. Consequently, this strong interface minimizes the rising domain is longer than for the L' composite. This dif- usual reinforcement mechanisms: crack deflection along the ference in the behaviour may be attributed to the presence interface, whisker pullout, and resistance to crack opening of high volume fraction of liquid phase in the"H'compos- In this case reducing the amount of surface oxygen contam- te. thus reinforcement mechanisms such as crack deflection ination on the Sic whiskers should increased the composite along the interface, whisker bridging and pullout are disabled toughne It has been shown that AlO3/SiC whisker co If it is not the case a second explanation must be found have higher toughness than monolithic alumina, it is believed The SiC whiskers could be degraded through the following that these composites also have a higher resistance to slow reactions crack growth. a plot of the V-KI curves(crack growth rate SiC(s)→Si(g)+C(g) versus stress intensity factor)obtained from the relaxation test t room temperature is showed in Fig 10 for alumina and two SiC(s)+ 2SiO2(s)-3Sio(g)+co(g) Al2O3/35 vol. SiC composite materials. The crack veloc- measured in the composites also shows a single stage, SiC(s)+O2(g)->Sio(g)+Co(g) corresponding to the first stage, but the slope is much higher Singhal2 lists these reactions as the most thermodynam than alumina(n=432 for Al2O3-35% SiCw and n=35 for ically significant reactions above 1727C, especially under alumina). This suggests that the whisker composite is less dynamic vacuum conditions where the gaseous products are sensitive to slow crack growth than alumina. This difference continuously removed from the system. Reaction (1)rep- in the behaviour can be attributed to the difference in their resents the dissociation of sic into its constituents and is microstructures. For the whisker composite, the crack has strongly dependent on carbon potential in the system. The greater difficulty to move by or through a whisker than only extent of reaction(2 )in degrading the SiC whiskers is signif- to propagate through the polycrystalline Al2O3, in addition icant, especially when the products are constantly removed in 10E02 ▲Sicw"L 10E03 10E04 10E07 10E08 K,(MPa ms) Fig. 10. The slow crack growth resistance of alumina in region I is significantly increase at room temperature with the addition of the Sic whiskers

V. Garnier et al. / Journal of the European Ceramic Society 25 (2005) 3485–3493 3491 improvement of the R-curve behaviour with temperature. At high temperatures the viscous liquid phase fastens cracks and leads to the formation of well-developed wake zone, associ￾ated with whisker bridging. The Fig. 9 shows R-curve behaviour of two composites prepared using ‘L’ and ‘H’ SiCw. For all composites, R in￾creases with crack extension. In the ‘L’ type composite, a rapid increase of KR (KR = 8.8 MPa m0.5) is observed fol￾lowed by a plateau value and a slow decrease after the crack extension a* has reached the value of 600
m. This KR value corresponds to the steady state of whiskers and may be referred as fracture toughness for crack propagation. In this state, the bridge generation is balanced by its extension and the length of the well-developed bridging zone is given by a*. The R-curve behaviour concerning the ‘H’ composite is different (Fig. 9), the initial K value is much lower and the rising domain is longer than for the ‘L’ composite. This dif￾ference in the behaviour may be attributed to the presence of high volume fraction of liquid phase in the ‘H’ compos￾ite, thus reinforcement mechanisms such as crack deflection along the interface, whisker bridging and pullout are disabled. It has been shown that Al2O3/SiC whisker composites have higher toughness than monolithic alumina, it is believed that these composites also have a higher resistance to slow crack growth. A plot of the V–KI curves (crack growth rate versus stress intensity factor) obtained from the relaxation test at room temperature is showed in Fig. 10 for alumina and two Al2O3/35 vol.% SiCw composite materials. The crack veloc￾ity measured in the composites also shows a single stage, corresponding to the first stage, but the slope is much higher than alumina (n = 432 for Al2O3–35% SiCw and n = 35 for alumina). This suggests that the whisker composite is less sensitive to slow crack growth than alumina. This difference in the behaviour can be attributed to the difference in their microstructures. For the whisker composite, the crack has greater difficulty to move by or through a whisker than only to propagate through the polycrystalline Al2O3, 31 in addition the differences may become more pronounced during slow crack growth. 4. Discussion Thermomechanical properties of alumina were signifi- cantly improved by the addition of SiC whiskers. However, this improvement depends on the whisker quality, particularly the morphology and the surface oxygen content. Thermomechanical properties of the composites prepared with high surface oxygen content whiskers were lower than those obtained with the composites prepared with low surface oxygen content whiskers. This result suggests two possible explanations. First, the superficial silica whiskers and alu￾mina matrix may react to form mullite, producing a strong interface. Consequently, this strong interface minimizes the usual reinforcement mechanisms: crack deflection along the interface, whisker pullout, and resistance to crack opening. In this case reducing the amount of surface oxygen contam￾ination on the SiC whiskers should increased the composite toughness. If it is not the case a second explanation must be found. The SiC whiskers could be degraded through the following reactions: SiC(s) → Si(g) + C(g) (1) SiC(s) + 2SiO2(s) → 3SiO(g) + CO(g) (2) SiC(s) + O2(g) → SiO(g) + CO(g) (3) Singhal32 lists these reactions as the most thermodynam￾ically significant reactions above 1727 ◦C, especially under dynamic vacuum conditions where the gaseous products are continuously removed from the system. Reaction (1) rep￾resents the dissociation of SiC into its constituents and is strongly dependent on carbon potential in the system. The extent of reaction (2) in degrading the SiC whiskers is signif￾icant, especially when the products are constantly removed in Fig. 10. The slow crack growth resistance of alumina in region I is significantly increase at room temperature with the addition of the SiC whiskers

V Garnier et al./Journal of the European Ceramic Sociery 25(2005)3485-3493 a dynamic vacuum. Reaction(3), which represents active oxi- Mechanical behaviour of whisker-reinforced material was dation of the Sic whiskers, is severely inhibited by the Al2O3 also found to be strongly dependent on the whisker con- matrix, which protects the whiskers, and by the reaction of tent. The creep rate of alumina at 1200oC could be reduced ambient oxygen with the graphite die which significantly re- by one or two orders of magnitude with the addition of duces the oxygen partial pressure in the hot-pressing cham- 35 vol. whiskers. However, an optimum exists where the ber. Considering the above discussion, degradation of the Sic deformation rate starts to decrease with increasing whiskers whiskers by reaction(2)is the most plausible explanation for content the thermomechanical properties decrease of composites pre- pared with high surface oxygen content whiskers TEM observations of the two SiC whiskers after hot press- Acknowledgments ng are in agreement with this hypothesis. Whiskers with high surface oxygen content have a polycrystalline structure, This work was supported by the BRITE/EURAM program while whiskers with a low surface oxygen content have a (project BREU 0181-C) Composites ceramic reinforced by monocrystalline structure(Fig. 11). In addition, for I comi whiskers and platelets for cutting tools and wear applications ite with low oxygen content, the presence of a thin glass layer along alumina/whiskers interface is observed(Fig. 5). This is not the case for the composite with high surface oxygen References content whiskers 1. Faber, K. T and Evans, A. G, Crack deflection processes-1. Theory Acta. Metall.,1983,31(4),565-576 5. Conclusion 2. Faber, K. T. and Evans, A G, Crack deflection processes-lI.Ex- periment. Acta Metall, 1983, 31(4) 577-584 SiC whisker reinforced alumina composites containing up 3. Rice, R. w, Mechanisms of toughening in ceramic matrix composites Ceram. Eng. Sci. Proc., 1981, 2(7/8), 661-701 to 35 vol. SiC whiskers were hot-pressed to more than 99% 4. Rice, R. W, Ceramic matrix composites toughening mechanisms: an of the theoretical density and were shown to have substan- pdate. Ceram. Eng. Sci. Proc., 1985, 6(7/8), 589-607 tial improved fracture toughness and strength compared to 5. Rice, R. W, A material opportunited: ceramic composites monolithic alumina. In addition these mechanical properties CHEMTECH,1983,4,230-239 remain relatively constant to 1000C. Above 1000C in air 6. Shetty, D. K, Ceramic Matrix Composites. In Current Awareness Bulletin (ol 118, No 12). Metals and Ceramics Infrormation Center, the low viscosity of the liquid phase, resulting from the Ox- Battelle Columbus Laboratories. Columbus OH. 1982 idation of the SiC whiskers, leads to crack-shielding, thus 7. Kelly. A. Interface effects and the work of fracture of a fibrous enhancing mechanical properties and R-curve behaviour. Ob- posite. Proc. R. Soc. London, 1970, 319, 95-116 servations have shown that crack deflection by the whisker 8. Jenkins, M. G, Kobayashi, A.S., White, K. w. and Bradt, R. C and whisker bridging and pullout are the significant tough Cracks initiation and arrest in SiC whisker/AlO3 matrix composite J.Am. Ceram.Soc.,1987,70(6,393-395 ening mechanisms 9. Piggot, M.R., Theoretical estimation of fracture toughness of fibrous The surface oxygen content of Sic whiskers has a major composites. J. Mater: Sci., 1970, 5, 669-675 impact on the mechanical properties of Al2O3/SiCw matrix 10. Lewis Ill. D. Whisker reinforced ceramics. In Proce composites. The oxygen surface content appeared to affect anced ceramics, ed. J. S. Moya and S. D. Aza. Socidad the whisker/matrix interfacial bonding thus decreasing the de Ceramica Y Vidro, Madrid, Spain, 1987, pp. 49-72. I1. Predecki, P, Abuhasan, A and Barrett, C. S, Residual stress determi- amount of deflection, whisker pullout and whisker bridging ation in Al203/SiC (whisker) composites by X-ray diffraction. Adv which are required to achieve high fracture toughness values Y-ramy Ana,1988,31,231-243

3492 V. Garnier et al. / Journal of the European Ceramic Society 25 (2005) 3485–3493 Fig. 11. TEM micrographs of SiC whiskers after hot-pressing: (a) ‘L’ whisker and (b) ‘H’ whisker. a dynamic vacuum. Reaction (3), which represents active oxi￾dation of the SiC whiskers, is severely inhibited by the Al2O3 matrix, which protects the whiskers, and by the reaction of ambient oxygen with the graphite die which significantly re￾duces the oxygen partial pressure in the hot-pressing cham￾ber. Considering the above discussion, degradation of the SiC whiskers by reaction (2) is the most plausible explanation for the thermomechanical properties decrease of composites pre￾pared with high surface oxygen content whiskers. TEM observations of the two SiC whiskers after hot press￾ing are in agreement with this hypothesis. Whiskers with high surface oxygen content have a polycrystalline structure, while whiskers with a low surface oxygen content have a monocrystalline structure (Fig. 11). In addition, for compos￾ite with low oxygen content, the presence of a thin glass layer along alumina/whiskers interface is observed (Fig. 5). This is not the case for the composite with high surface oxygen content whiskers. 5. Conclusion SiC whisker reinforced alumina composites containing up to 35 vol.% SiC whiskers were hot-pressed to more than 99% of the theoretical density and were shown to have substan￾tial improved fracture toughness and strength compared to monolithic alumina. In addition these mechanical properties remain relatively constant to 1000 ◦C. Above 1000 ◦C in air, the low viscosity of the liquid phase, resulting from the ox￾idation of the SiC whiskers, leads to crack-shielding, thus enhancing mechanical properties and R-curve behaviour. Ob￾servations have shown that crack deflection by the whiskers and whisker bridging and pullout are the significant tough￾ening mechanisms. The surface oxygen content of SiC whiskers has a major impact on the mechanical properties of Al2O3/SiCw matrix composites. The oxygen surface content appeared to affect the whisker/matrix interfacial bonding thus decreasing the amount of deflection, whisker pullout and whisker bridging which are required to achieve high fracture toughness values. Mechanical behaviour of whisker-reinforced material was also found to be strongly dependent on the whisker con￾tent. The creep rate of alumina at 1200 ◦C could be reduced by one or two orders of magnitude with the addition of 35 vol.% whiskers. However, an optimum exists where the deformation rate starts to decrease with increasing whiskers content. Acknowledgments This work was supported by the BRITE/EURAM program (project BREU 0181-C). Composites ceramic reinforced by whiskers and platelets for cutting tools and wear applications. References 1. Faber, K. T. and Evans, A. G., Crack deflection processes—I. Theory. Acta. Metall., 1983, 31(4), 565–576. 2. Faber, K. T. and Evans, A. G., Crack deflection processes—II. Ex￾periment. Acta. Metall., 1983, 31(4), 577–584. 3. Rice, R. W., Mechanisms of toughening in ceramic matrix composites. Ceram. Eng. Sci. Proc., 1981, 2(7/8), 661–701. 4. Rice, R. W., Ceramic matrix composites toughening mechanisms: an update. Ceram. Eng. Sci. Proc., 1985, 6(7/8), 589–607. 5. Rice, R. W., A material opportunited: ceramic composites. CHEMTECH, 1983, 4, 230–239. 6. Shetty, D. K., Ceramic Matrix Composites. In Current Awareness Bulletin (Vol 118, No 12). Metals and Ceramics Infrormation Center, Battelle Columbus Laboratories, Columbus, OH, 1982. 7. Kelly, A., Interface effects and the work of fracture of a fibrous composite. Proc. R. Soc. London, 1970, 319, 95–116. 8. Jenkins, M. G., Kobayashi, A. S., White, K. W. and Bradt, R. C., Cracks initiation and arrest in SiC whisker/Al2O3 matrix composite. J. Am. Ceram. Soc., 1987, 70(6), 393–395. 9. Piggot, M. R., Theoretical estimation of fracture toughness of fibrous composites. J. Mater. Sci., 1970, 5, 669–675. 10. Lewis III, D., Whisker reinforced ceramics. In Processing of Ad￾vanced ceramics, ed. J. S. Moya and S. D. Aza. Socidad Espanola de Ceramica Y Vidro, Madrid, Spain, 1987, pp. 49–72. 11. Predecki, P., Abuhasan, A. and Barrett, C. S., Residual stress determi￾nation in Al2O3/SiC (whisker) composites by X-ray diffraction. Adv. X-ray Anal., 1988, 31, 231–243

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