CA SEGREGATION TO BASAL SURFACES IN ALPHA-ALUMINA

High-resolution transmission electron microscopy (HRTEM) and analytical electron microscopy (AEM) mere used to investigate the structure and chemistry of (0001) alpha-Al2O3/Al interfaces in melt-infiltrated polycrystalline alumina composites, HRTEM revealed an interfacial region different from both Al and alpha-Al2O3, with a structural width of 0.8 +/- 0.2 nm, AEM of the same interfaces revealed a Ca excess of Gamma = 2.5 +/- 0.5 Ca atoms per nm(2) (Ca/nm(2)). AEM of a basal twin boundary in the alpha-Al2O3 also revealed a Ca excess (Gamma = 1.0 +/- 0.5 Ca/nm(2)). Since the metal-ceramic interfaces mere the free surfaces of pores before melt infiltration, it can be concluded that Ca segregates to the basal surface of alumina, as well as to basal twin boundaries. Furthermore, the Ca at the free surfaces does not reside on only one cation plane, but is spread over 4 +/- 1 basal cation layers and forms an interfacial phase with a nominal composition of CaO . 6Al(2)O(3).