CHEMICAL BONDING AT GRAIN-BOUNDARIES - MGO ON BETA-SI3N4

We have studied chemical bonding on the beta-Si3N4(110) surface using extended Huckel tight-binding calculations. Due to the large affinity of the environmental oxygen and surface silicon atoms, crystal faces are invariantly oxygen contaminated. The electronic structure proves the instability of such partly oxidized ceramics. The instability, amplified through the contact of the grain boundaries, provides the electronic mechanism for the segregation forcing electropositive atoms to migrate towards grain boundaries thus settling the instability. The stabilizing mechanism of grain boundaries is discussed stressing the role of additives. The electron configuration of the metal atom seems crucial in achieving the stable state. Two types, ionic and/or covalent metal-to-oxygen interaction are proposed, the latter permitting better mechanical properties of ceramics. Finally, the grain boundary stabilization is demonstrated for MgO additive serving as the widely used sintering aid.