Extraction of Anisotropic Mechanical Properties From Nanoindentation of SiC-6H Single Crystals

Because brittle solids fail catastrophically during normal tension and compression testing, nanoindentation is often a useful alternative technique for measuring their mechanical properties and assessing their deformation characteristics. One practical question to be addressed in such studies is the relationship between the anisotropy in the uniaxial mechanical behavior to that in the indentation response. To this end, a systematic study of the mechanical behavior the 6H polytype of a hexagonal silicon carbide single crystal (SiC-6H) was performed using standard nanoindentation methods. The indentation elastic modulus and hardness measured using a Berkovich indenter at a peak load of 500 mN varied over a wide range of crystal orientation by only a few percent. The variation in modulus is shown to be consistent with an anisotropic elastic contact analysis based on the known single crystal elastic constants of the material. The variation in hardness is examined using a single crystal plasticity model that considers the anisotropy of slip in hexagonal crystals. When compared to experimental measurements, the analysis confirms that plasticity in SiC-6H is dominated by basal slip. An anisotropic elastic contact analysis provides insights into the relationship between the pop-in load, which characterizes the transition from elasticity to plasticity during nanoindentation testing, and the theoretical strength of the material. The observations and analyses lay the foundations for further examination of the deformation and failure mechanisms in anisotropic materials by nanoindentation techniques.