Ca Segregation to Basal Surfaces in α-Alumina

High-resolution transmission electron microscopy (HRTEM) and analytical electron microscopy (AEM) were used to investigate the structure and chemistry of (0001) {alpha}-Al{sub 2}O{sub 3}/Al interfaces in melt-infiltrated polycrystalline alumina composites. HRTEM revealed an interfacial region different from both Al and {alpha}-Al{sub 2}O{sub 3}, with a structural width of 0.8 {plus_minus} 0.2 nm. AEM of the same interfaces revealed a Ca excess of {Lambda} = 2.5 {plus_minus} 0.5 Ca atoms per nm{sup 2} (Ca/nm{sup 2}). AEM of a basal twin boundary in the {alpha}-Al{sub 2}O{sub 3} also revealed a Ca excess ({Lambda} = 1.0 {plus_minus} 0.5 Ca/nm{sup 2}). Since the metal-ceramic interfaces were 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 {plus_minus} 1 basal cation layers and forms an interfacial phase with a nominal composition of CaO{center_dot}6Al{sub 2}O{sub 3}.