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±0.2V. 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