Local Chemical Bonding at Grain Boundary of Si3N4 Ceramics

Abstract

We perform a first principles molecular orbital calculation with special interest on the local chemical bonding at the grain boundary of Si3Nin4 The computational result is twofold: 1) On the basis of our calculations using model clusters, the Si-L23 edge ELNES of the intergranular glassy film reported in literature is interpreted; The N/(O+N) ratio of the intergranular glassy film is found to be in the range of 30 to 40%. A model of atomic arrangement at the interface between the prismatic plane of α-Si3N4 and the glassy film is proposed which has no broken bond at the interface. 2) Magnitude of chemical bond- strength around rare-earth ions at an interstitial sites in bulk Si3N4 is examined by the calculation of bond overlap populations. The difference in solubility of rare-earth ions between β- and α-Si3N4 matrix is well explained. Finally the bond-strength around rare-earth ions at the interface between α-Si3N4 and intergranular glassy film is examined. The rare-earth ions are implied to be more stable at this interface than at the interstitial site of the bulk crystal. The grain-boundary bond-strength is suggested to be weakened when large rare-earth ions such as La3+ are present at this interface. This computational result explains why the interfacial bond strength varies with ionic radius of rare-earth ions in doped α-Si3N4 ceramics.