Mechanical strengths of silicon nitrides studied by ab initio calculations

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.