The yield strength and elastic anisotropy of c-Si3N4 to 70 GPa

Boris Kiefer, Sean R. Shieh, Thomas. S. Duffy (Princeton University) and T. Sekine (National Institute for Materials Science)

facility: NSLS-X17C (DAC)

An area of active research in materials sciences is the synthesis and characterization of novel materials. Recently a new superhard form of silicon nitride compound (c-Si₃N₄) has been synthesized in high pressure/high temperature experiments. The excellent metastability of this material and its hardness make this material technologically interesting. Numerous experimental and theoretical studies have been undertaken to study the equation of state, the hardness and the electronic properties of c-Si₃N₄. However, the elastic properties of c-Si₃N₄ have only been predicted at ambient pressure and the mechanical properties of this phase remain largely unknown.

We performed radial x-ray diffraction experiments at pressures up to 70 GPa to access the yield strength and elastic anisotropy of c-Si3N4 for the first time at pressure. The sample consisted of a fine powder that was recovered from shock wave experiments by one of us (T. Sekine). We compressed the sample in a DAC without a pressure medium in order to overcome the yield strength of c-Si₃N₄. Energy-dispersive X-ray diffraction patterns were collected at beamline X17C at the NSLS for different orientations of the DAC axis with respect to the X-ray scattering plane. Lattice strain theory provides the conceptual framework for the analysis of our measurements and allows us to gain insights into the elastic and plastic properties of c-Si3N4. However, the determination of the yield strength from our measurements requires a partial knowledge of the elasticity of c-Si₃N₄. Since this information is not available at elevated pressures, we augmented our study with first-principle calculations of the full elastic constant tensor at pressure. The hydrostatic equation of state as derived from our measurements is in good agreement with previous measurements. Our results show a large and weakly pressure dependent elastic anisotropy (A=2*C₄₄/(C₁₁-C₁₂)) of 1.5(1), a trend which is consistent with our first-principle calculations. We find that the yield strength of c-Si₃N₄ increases rapidly from 9.1(1.5) to 18.7(3.1) GPa between 20 and 45 GPa and increases at a much lower rate to 22 GPa at 70 GPa. Thus c-Si₃N₄ shows one of the highest yield strength observed to date but is still not as strong as diamond.