Hardness and fracture toughness of moissanite

Disparities prevail among the reported hardness and fracture toughness values for hard and brittle materials. A better understanding of the physical nature of hardness and fracture toughness and a standardized technique for reliable measurements of these quantities is urgently needed. We strongly recommend the use of the measured hardness after the bend in the hardness versus load (H−FLoad) curve, when the hardness approaches its asymptotic value. The present work reports a systematic study of hardness and fracture toughness on moissanite (single crystal hexagonal silicon carbide, 6H-SiC) samples. The measurements were performed over a broad load range from 0.49 to 294 N with the direct indentation method. Asymptotic values of Knoop hardness of HK = 19 GPa and Vickers hardness of HV = 22 GPa were reached at a high load between 50 N and 100 N. A consistent fracture toughness of KIC = 1.8 MPa·m1/2 was obtained across the entire load range. Our study presents experimental results for the hardness and fracture toughness of moissanite in the asymptotic-hardness region, and it raises concern regarding the application of moissanite single crystals as anvil material under shear/fracture conditions.