Hardness, elastic modulus and their correlations in the transparent silicon nitrides

Abstract

Transparent silicon nitrides exhibit rich crystal structures, higher hardness, high elastic modulus and are suitable to investigate the relationship between elastic modulus and hardness. In this work, due to the difficulty of synthesizing single crystal, transparent polycrystalline silicon nitrides were prepared by high pressure and high temperature method because the transparent polycrystalline Si3N4 exhibit nearly intrinsic hardness of single crystals. By using in-situ high-pressure X-ray diffraction, Vickers hardness and nano-indentation measurement, the hardness and elastic modulus of transparent polycrystalline silicon nitrides were measured. The Birch-Murnaghan equation of state (BM-EOS) fitting of high-pressure X-ray diffraction give bulk modulus of K0α = 266.7 ± 9 GPa, K0β = 240.6 ± 7 GPa, K0γ = 335.5 ± 10 GPa for α-Si3N4, β-Si3N4 and γ-Si3N4. This is consistent with nanoindentation results and demonstrates that the elastic modulus of transparent Si3N4 is close to that of single crystal. The nanoindentations measurement of α-Si3N4, β-Si3N4, γ-Si3N4 give hardness of HNα = 30.3 ± 1.3 GPa, HNβ = 27.7 ± 1.7 GPa, HNγ = 51.2 ± 1.5 GPa. The different stacking order of [SiN4] tetrahedrons result in different bonding configurations and hardness. According to the elastic modulus and hardness value, hardness is linearly proportional to the elastic modulus of silicon nitrides, and can be used to predict hardness directly.