Alumina Fracture toughness

SiC whisker-reinforced alumina is a major advance in tool material development, as it provides a means to increase the fracture toughness of the material via the composite material approach. It is entirely possible that in the next century many new whiskers of refractory, hard materials will be made... 

There are, of course, many more ceramics available than those listed here alumina is available in many densities, silicon carbide in many qualities. As before, the structure-insensitive properties (density, modulus and melting point) depend little on quality -they do not vary by more than 10%. But the structure-sensitive properties (fracture toughness, modulus of rupture and some thermal properties including expansion) are much more variable. For these, it is essential to consult manufacturers data sheets or conduct your own tests. 

The presence of mullite and hence compressive surface stresses appears to improve the hardness and fracture toughness (see Table 5.2). These values are at least two to three times higher than those reported for the mullite/ alumina system described above. Clearly, the presence of mullite is desirable for inducing compressive stresses in the vicinity of the surface region by virtue of the mismatch in thermal expansion between ZTA and mullite. This significant improvement in the observed fracture toughness was attributed to... 

Figure 5.14 shows the depth-profile variation of hardness (Hv) and fracture toughness (KiC) of the hybrid sample. Fracture toughness is evidently higher near the surface due to the abundant presence of both AT and mullite. As the depth increases, the abundance of both phases and thus the fracture toughness decreases. Hardness is lower near the surface due to the abundant presence of much softer AT, but increases with depth towards the alumina region. It should be noted that since measurements were made on a polished cross-sectioned sample, the indentation data could not be collected at distances less than 0.5 mm from the surface. Hence, hardness and fracture toughness values on the surface were measured separately. 

Hoffman, M., Rodel, J., Stemitzke, M. et al., Fracture toughness and subcritical crack growth in alumina/silicon carbide nanocomposites , Fracture Mechanics of Ceramics, 1996, 12 179. 

Xia, Z., Curtin, W.A., Sheldon, B.W., Fracture toughness of highly ordered carbon nanotube/alumina nanocomposites, J. Eng. Mater. Tech., 2004, 126(3) 238. 

It is most unlikely that matrix grain refinement alone can explain the high fracture strengths that have been reported. Using the reported maximum fracture strengths (o = 1100 MPa)12 and the nominal fracture toughness of alumina (A"Ic = 3.5 MPa.m1/2), the critical flaw size, (c), can be estimated from... 

Collin, K.M., Rowcliffe, D.J. (2001), Influence of thermal conductivity and fracture toughness on the thermal shork resistance of alumina-silicon-carbide-whisker compositer Journal of the American Ceramic Society, 84(6), 1334—1340. 

Ellen, Y.S. etal. (1998), Microstructural design of silicon nitride with improved fracture toughness II, Effects of yttria and alumina additives , J. Am. Ceram. Soc., 81(11), 2831-40. 

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