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, respectivelyV. 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