Q. Tai. A. Mocellin/Ceramics International 25(1999)393-408 ties because of its lower flow stress and stronger interfacial cohesion. The apparent activa tion energy was 570 KJ mol-I, which is higher than that of Al,O3(420 KJ mol-)obtained by the authors. The authors suggested that an increase in the boundary dif- usion activation energy due to dopants was correlated to the corresponding decrease in the interfacial energy that is the cohesion at the interface increased with the activation energy. Observation of the influence of grain size on the creep behaviour showed that the addition of TiO, into Al,O3 made the values of p decrease from 3 to 2. This means that the addition of titanium increased the lattice cation vacancy concentration of Al2O3 matrix, so that the lat tice diffusion of aluminium overtook boundary diffu- 100 sion, thus resulting in the change of the values of p STRESS( MPa) It must be pointed out that the Al2O3-TiO2 compo- Fig. 10. Influence of stress on stress exponent in Al2 w, com- site shows a poor high temperature fluxural strength posites(+)SiC content: 0, 10-40 MPa, n=1.3;(A)SiC content because titania forms a nearly continuous network of 20-240MPa,n=18;(■) Sic content:15,30-80MPa,n=0.8,125- aluminium- titanate at the grain boundary in the as-sin 410MPa,n=3.4;(●) SiC content:30,30-165MPa,n=0.9,170-370 tered specimens and this compound deteriorates the MPa,n=5.9[50 fracture strength of the Al2O3-TiO2 composite 3. 2. A2O based non-oxide ceramic particle or whisker rate. De Arellano-Lopez et al. [50] introduced a critical composites stress level ae which depended on the content and impurities of whiskers, as well as the test conditions 3.2.1.A120r-SiC(w)composites When stresses were larger than ac, the stress exponent Since the initial work on the creep behaviour of changed from its lower value to higher value. Higher Al2O SiC(w) composites reported by Chokshi and Por- content, higher purity of whiskers, lower impurity of the ter [41], many studies on such materials have been specimens and an inert atmosphere in tests might lead to made. Generally the SiC whisker diameters were in the a higher value of de range 0 I to I um and their aspect ratio about 10-20. Experiments on the influence of temperature on the Almost all the studies on Al2O3-SiC(w) composites creep behaviours showed that a higher temperature ( Table 2) have shown that their creep resistance was often resulted in a higher value of n [41-48, 51]. The generally far superior to that of the unreinforced matrix stress exponents for Al2O3-33 vol% SiC(w) obtained by [41-52. The creep rate was found to be one or two Lipetzky et al changed from I at 1200C to 3 at 1300- orders of magnitude lower than that of Al2O3 matrix 1400 C[48](Fig. 11). The stress exponents for different [45, 49. Most authors attributed the improvement in content(10-50 vol%)SiC(w) composites obtained by Lin reep resistance to the whiskers which act as hard pin- et al. varied from 2-3 at 1200C to 3. 5-6 at 1300C ning particles on the grain boundary surfaces and as (45, 47, 52 Higher values of n(3.8-6.3) were also hard particles penetrating across the boundary planes, obtained by Chokshi et al., [41] Porter et al. [42]and Xia thus retarding creep deformation by grain boundary et al. [44, 51]. Their experiments were all performed at sliding [45-50, 52 higher temperatures (1400-1600C) using four-point Inspection of the values of the stress exponent of bending creep tests. Although the values of n obtained Al2O-SiCw) composites shown in Table 2 indicates by DeArellano-Lopez et al. [146] and Swan et al. [491 that the values of n vary from I to 7-8, depending upon using compression tests were lower(1-1. 8), their results the method of test, the stress, temperature ranges, the also showed that a higher temperature favoured a SiC(w) content, and so on. The variation of the stress higher value of n. The increase of n at higher tempera- range often resulted in the change of the values of n ture is attributed to two factors: more extensive cavita [43, 45, 50]. From levels of about 1-2 at lower stresses, tion and crack caused by the stress concentrations they increased to 5-7 at higher stresses(Fig. 10). This is resulted from thermal mismatch and more glassy phases attributed to a change in the creep mechanism. Higher at grain boundaries caused by the thermal oxidation of stresses often lead to extensive cavitation occurring SiC whiskers within glass pockets at interfaces and grain boundaries, The effect of the content of SiC whiskers on the creep and crack generation which causes matrix grains to behaviours was studied by some authors. When the separate from the whiskers, thus increasing the creep whisker content was <20 vol%, the creep resistancesuperplastic properties because of its lower ¯ow stress and stronger interfacial cohesion. The apparent activa￾tion energy was 570 KJ molÿ1 , which is higher than that of Al2O3 (420 KJ molÿ1 ) obtained by the authors. The authors suggested that an increase in the boundary dif￾fusion activation energy due to dopants was correlated to the corresponding decrease in the interfacial energy, that is the cohesion at the interface increased with the activation energy. Observation of the in¯uence of grain size on the creep behaviour showed that the addition of TiO2 into Al2O3 made the values of p decrease from 3 to 2. This means that the addition of titanium increased the lattice cation vacancy concentration of Al2O3 matrix, so that the lat￾tice di€usion of aluminium overtook boundary di€u￾sion, thus resulting in the change of the values of p. It must be pointed out that the Al2O3±TiO2 compo￾site shows a poor high temperature ¯uxural strength because titania forms a nearly continuous network of aluminium-titanate at the grain boundary in the as-sin￾tered specimens and this compound deteriorates the fracture strength of the Al2O3±TiO2 composite. 3.2. Al2O3±based non-oxide ceramic particle or whisker composites 3.2.1. Al2O3±SiC(w) composites Since the initial work on the creep behaviour of Al2O3±SiC(w) composites reported by Chokshi and Por￾ter [41], many studies on such materials have been made. Generally the SiC whisker diameters were in the range 0.1 to 1m and their aspect ratio about 10±20. Almost all the studies on Al2O3±SiC(w) composites (Table 2) have shown that their creep resistance was generally far superior to that of the unreinforced matrix [41±52]. The creep rate was found to be one or two orders of magnitude lower than that of Al2O3 matrix [45,49]. Most authors attributed the improvement in creep resistance to the whiskers which act as hard pin￾ning particles on the grain boundary surfaces and as hard particles penetrating across the boundary planes, thus retarding creep deformation by grain boundary sliding [45±50,52]. Inspection of the values of the stress exponent of Al2O3±SiC(w) composites shown in Table 2 indicates that the values of n vary from 1 to 7±8, depending upon the method of test, the stress, temperature ranges, the SiC(w) content, and so on. The variation of the stress range often resulted in the change of the values of n [43,45,50]. From levels of about 1±2 at lower stresses, they increased to 5±7 at higher stresses (Fig. 10). This is attributed to a change in the creep mechanism. Higher stresses often lead to extensive cavitation occurring within glass pockets at interfaces and grain boundaries, and crack generation which causes matrix grains to separate from the whiskers, thus increasing the creep rate. DeArellano-Lopez et al. [50] introduced a critical stress level c which depended on the content and impurities of whiskers, as well as the test conditions. When stresses were larger than c, the stress exponent changed from its lower value to higher value. Higher content, higher purity of whiskers, lower impurity of the specimens and an inert atmosphere in tests might lead to a higher value of c. Experiments on the in¯uence of temperature on the creep behaviours showed that a higher temperature often resulted in a higher value of n [41±48,51]. The stress exponents for Al2O3-33 vol% SiC(w) obtained by Lipetzky et al. changed from 1 at 1200C to 3 at 1300± 1400C [48] (Fig. 11). The stress exponents for di€erent content (10±50 vol%) SiC(w) composites obtained by Lin et al. varied from 2±3 at 1200C to 3.5±6 at 1300C. [45,47,52] Higher values of n (3.8±6.3) were also obtained by Chokshi et al., [41] Porter et al. [42] and Xia et al. [44,51]. Their experiments were all performed at higher temperatures (1400±1600C) using four-point bending creep tests. Although the values of n obtained by DeArellano-Lopez et al. [46] and Swan et al. [49] using compression tests were lower (1±1.8), their results also showed that a higher temperature favoured a higher value of n. The increase of n at higher tempera￾ture is attributed to two factors: more extensive cavita￾tion and crack caused by the stress concentrations resulted from thermal mismatch and more glassy phases at grain boundaries caused by the thermal oxidation of SiC whiskers. The e€ect of the content of SiC whiskers on the creep behaviours was studied by some authors. When the whisker content was 420 vol%, the creep resistance Fig. 10. In¯uence of stress on stress exponent in Al2O3-SiC(w) com￾posites (+) SiC content: 0, 10±40 MPa, n=1.3; (~) SiC content: 5, 20±240 MPa, n=1.8; (&) SiC content: 15, 30±80 MPa, n=0.8, 125± 410 MPa, n=3.4; (*) SiC content: 30, 30±165 MPa, n=0.9, 170±370 MPa, n=5.9 [50]. Q. Tai. A. Mocellin / Ceramics International 25 (1999) 395±408 403