Mechanical strengths of silicon nitrides studied byab initiocalculations

The stress-strain relationships under tensile and shear loads are calculated for hcp(β)-Si3N4 and fcc(NaCl)-SiN by means of ab initio density functional theory. The ideal shear strengths for fcc-SiN are much lower than those for hcp-Si3N4. This is in agreement with experiments which show that the interfacial fcc-SiN can strengthen the TiN∕SiN heterostructures only when its thickness is about 1–2 ML. Based on the calculated electronic density of states, the physical origin of the mechanical strengths is addressed.