Yttrium segregation and YAG precipitation at surfaces of yttrium-doped α-Al/sub 2/O/sub 3/

Auger electron spectroscopy and low-energy ion scattering were used to assess surface composition of yttrium-doped polycrystalline and single-crystal α-aluminas which were heated under vacuum to temperatures above 15000C. Extensive surface and intergranular precipitation of yttrium aluminum garnet was observed in a compact of nominal Y content of 210 wt ppm, while apparent Y segregation to surface cation sites predominated in a second compact with a Y content of 160 wt ppm. In the latter case, the segregation enthalpy of yttrium to the alumina surface in the temperature range 17000 to 19000C was estimated to be of the order -44 kJ/mol with a maximum surface cation site occupancy of ≅13%