Q. Tai. A. Mocellin/Ceramics International 25(1999)393-408 diffusion of the oxygen anions. So, the disloc canon controlled creep mechanism can not be ruled out under some conditions (for example: lower SiC(w) contents and imposed stress levels, higher tempera tures and strains a grain boundary sliding mechanism accommodated by diffusion to a larger or lesser extent was proposed by many authors 143, 45-49, 52]. Some experiments pointed to the existence of two stress-dependent plastic defor-s mation mechanisms [43, 45, 47, 50]. At low stress, grain boundary sliding controls the deformation, while at cavitation and microcracking. Lipetzky et al. [43] pro- posed that at low stress levels the primary creep defor- mation mechanism was diffusional creep and grain boundary sliding facilitated by viscous flow of inter- granular silica glass. Swan et al. [ also indicated that the introduction of sic whiskers into the matrix slowed the creep rate by diffusion and increased the amount of grain boundary sliding. The experiments of De Arellan- Lopez et al. [50] showed that when the SiC(w)content was <15 vol%, the deformation was controlled by grain boundary sliding, but when the SiC(w) content was >15 vol%, the whiskers impeded grain bor undar Applied Stra MIra ing effectively, causing a change in the deformation mechanism to pure diffusional creep Fig. 12. Variation in strain rate with stress for AlO3 and AlO 17 vol% SiC(p) nanocomposite(filled symbol: in the tension test; 3.2.2. A12Or-SiC(p) composites open symbol: in the flexture test)[53 Since Sic whiskers are expensive for practical appli cations and raise health hazards during material pro- cessing, Al2O3-SiC particle composites were developed with most of them lying at the grain boundaries or tri- more recently. Studies on their creep behaviours showed ple-grain junctions did not increase the creep resistance that these composites exhibited excellent resistance The authors attributed it to the serious oxidation of Sic [53, 54] as Al2O3-SiC(w) composites did. By dispersing particles on the grain boundaries. The presence of a 17 vol% SiC nanoparticles into AlO3, creep life was 10 viscous amorphous film decreased the interface bond times longer, creep strain was 8 times smaller and creep strength and decreased the creep resistance [54] rate was 3-4 orders lower than those of monolithic The creep mechanism of Al2O -SiC (p)composites 12O3[53(Fig. 12). The authors attributed it to rotat- was proposed to be grain boundary sliding more or less ng and plunging of intergranular SiC nanoparticles into fully accommodated by diffusion as that of Al2O3- Al2O3 grain which were observed on microstructures of SiC(w) composites deformed specimens such as strain contrast contours From the above. the deformation behaviours of small cavities around the Sic nanoparticles, curve AlO -SiC composites can be briefly summarized as grain boundaries of AlO3, etc. Further study [55, 56 follows revealed that SiC/Al2O3 interfaces possessed much The introduction of Sic into Al,O3 increases the stronger bonding than AlO3/ AlO3 boundaries. The creep resistance of composites to about one or two magnitude of interfacial fracture energy between Sic orders of magnitude superior to that of Al2O3 matrix and Al2O3 was over twice the Al2O3/Al2O3 grain resulting from the pinning or penetrating role of the Sic boundary fracture energy, i.e. grain boundary of Al2O3/ whiskers or particles at grain daries. The creep iC composite was strengthened by SiC nanoparticles resistance of the composites increase with increasing SiC due to the stronger interfaces. Deng et al. introduced content, but too high a Sic content is unfavourable to 10 vol% of 2.7 um SiC particles into Al2O3, and the the creep resistance. Higher temperatures and higher creep rate was markedly decreased [54]. Their observa- stresses often result in both a higher value of n and a tions of microstructure showed that, when Sic particles higher creep rate due to more extensive cavitation and were irregular and elongated, and most of them were microcracks caused by the stress concentration. The Sic entrapped into Al2O3 matrix grains, the creep resistance whiskers or particles are easily oxidised in air at elevated of the composite was higher, but equiaxed SiC particles temperatures, forming grain boundary glassy phasedi€usion of the oxygen anions. So, the dislocation￾controlled creep mechanism can not be ruled out under some conditions (for example: lower SiC(w) contents and imposed stress levels, higher tempera￾tures and strains). A grain boundary sliding mechanism accommodated by di€usion to a larger or lesser extent was proposed by many authors [43,45±49,52]. Some experiments pointed to the existence of two stress-dependent plastic defor￾mation mechanisms [43,45,47,50]. At low stress, grain boundary sliding controls the deformation, while at high stress, the creep deformation is controlled by creep cavitation and microcracking. Lipetzky et al. [43] pro￾posed that at low stress levels the primary creep defor￾mation mechanism was di€usional creep and grain boundary sliding facilitated by viscous ¯ow of inter￾granular silica glass. Swan et al. [49] also indicated that the introduction of SiC whiskers into the matrix slowed the creep rate by di€usion and increased the amount of grain boundary sliding. The experiments of De Arellan￾Lopez et al. [50] showed that when the SiC(w) content was <15 vol%, the deformation was controlled by grain boundary sliding, but when the SiC(w) content was 515 vol%, the whiskers impeded grain boundary slid￾ing e€ectively, causing a change in the deformation mechanism to pure di€usional creep. 3.2.2. Al2O3±SiC(p) composites Since SiC whiskers are expensive for practical appli￾cations and raise health hazards during material pro￾cessing, Al2O3±SiC particle composites were developed more recently. Studies on their creep behaviours showed that these composites exhibited excellent resistance [53,54] as Al2O3±SiC(w) composites did. By dispersing 17 vol% SiC nanoparticles into Al2O3, creep life was 10 times longer, creep strain was 8 times smaller and creep rate was 3±4 orders lower than those of monolithic Al2O3 [53] (Fig. 12). The authors attributed it to rotat￾ing and plunging of intergranular SiC nanoparticles into Al2O3 grain which were observed on microstructures of deformed specimens such as strain contrast contours, small cavities around the SiC nanoparticles, curved grain boundaries of Al2O3, etc. Further study [55,56] revealed that SiC/Al2O3 interfaces possessed much stronger bonding than Al2O3/Al2O3 boundaries. The magnitude of interfacial fracture energy between SiC and Al2O3 was over twice the Al2O3/Al2O3 grain boundary fracture energy, i.e. grain boundary of Al2O3/ SiC composite was strengthened by SiC nanoparticles due to the stronger interfaces. Deng et al. introduced 10 vol% of 2.7m SiC particles into Al2O3, and the creep rate was markedly decreased [54], Their observa￾tions of microstructure showed that, when SiC particles were irregular and elongated, and most of them were entrapped into Al2O3 matrix grains, the creep resistance of the composite was higher, but equiaxed SiC particles with most of them lying at the grain boundaries or tri￾ple-grain junctions did not increase the creep resistance. The authors attributed it to the serious oxidation of SiC particles on the grain boundaries. The presence of a viscous amorphous ®lm decreased the interface bond strength and decreased the creep resistance [54]. The creep mechanism of Al2O3±SiC (p) composites was proposed to be grain boundary sliding more or less fully accommodated by di€usion as that of Al2O3± SiC(w) composites. From the above, the deformation behaviours of Al2O3±SiC composites can be brie¯y summarized as follows: The introduction of SiC into Al2O3 increases the creep resistance of composites to about one or two orders of magnitude superior to that of Al2O3 matrix, resulting from the pinning or penetrating role of the SiC whiskers or particles at grain boundaries. The creep resistance of the composites increase with increasing SiC content, but too high a SiC content is unfavourable to the creep resistance. Higher temperatures and higher stresses often result in both a higher value of n and a higher creep rate due to more extensive cavitation and microcracks caused by the stress concentration. The SiC whiskers or particles are easily oxidised in air at elevated temperatures, forming grain boundary glassy phase Fig. 12. Variation in strain rate with stress for Al2O3 and Al2O3± 17 vol% SiC (p) nanocomposite (®lled symbol: in the tension test; open symbol: in the ¯exture test) [53]. Q. Tai. A. Mocellin / Ceramics International 25 (1999) 395±408 405