The effect of rare earth dopants on grain boundary cohesion in alumina

The effect of rare earth (RE) grain boundary segregation on mode of fracture in alumina has been investigated. In order to isolate the effects of microstructure (i.e. grain size and residual porosity) from those due to grain boundary chemistry, the fracture behaviour of virtually pore-free (i.e. nearly transparent) undoped alumina has also been studied. This showed that mode of fracture becomes increasingly transgranular as grain size is reduced, a trend which has been explained by thermal expansion anisotropy effects.The addition of RE (i.e. Yb, Gd or La) dopants to alumina resulted in a substantial increase in the proportion of intergranular fracture relative to the undoped material of similar grain size. This can be explained by the significant reduction in the free surface energy that results from RE segregation at grain boundaries, which reduces the work of fracture for intergranular failure. This is expected to lead to a reduction in strength compared to undoped aluminas with equivalent microstructures, although this is often more than offset by the improved microstructures that using a RE dopant can provide.