Characterisation and properties of alumina-matrix LGMs

Electron microprobe analysis of concentration profiles across sections of the sintered samples revealed the existence of concentration gradients, the mullite content decreasing with increasing distance from the surface of the bodies [Marple Green, 1993], SEM examination also revealed a microstructural effect the alumina grain size tended to increase from the surface of the 

The abundance of mullite in the mullite/ZTA system increased with increasing infiltration time [Low et al., 1993]. The density (p) and mullite content of the sintered sample as a function of infiltration time are shown in Table 5.2. The results suggest that the infiltration process was time (t) dependent and diffusion-controlled with the infiltration front travelled as a function of t112. The content of mullite was greatest near the surface and decreased sharply towards the core of the sample. 

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 

Infiltration time (h) Density (g/cm3) Mullite content (vol%) Hardness (GPa) Toughness (MPa.m1/2) 

2 Alumina/aluminium titanate and alumina-zirconia/ aluminium titanate systems 

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