Does a relationship exist between hardness and compression strength for advanced ceramics?

Abstract

The Knoop hardness (HK) and compression strength (σc) of 23 advanced ceramics were measured to determine if an overarching HK/σc relationship could be identified for ceramics, or if one exists for a specific class of ceramics, similar to the hardness/yield strength relationship (H/Y ≈ 3) identified by Tabor for metals. Compression strength was determined using a dumbbell‐shaped specimen that virtually eliminates the end splitting that occurs when cylinders or cuboids are tested and provides a more representative compression strength value. HK values were obtained over a range of indentation loads between 0.98 and 98N. Four HK values, HK2, load‐independent HK, the hardness from the proportional specimen resistance model, and a brittleness parameter, were obtained and plotted against compression strength. An overarching relationship could not be identified for ceramics in general and the only class of ceramics that had a consistent relationship was tungsten carbide/cobalt that had a HK/σc of approximately 2.5. The consistent relationship for the WC/Co materials is due to the cobalt plastically deforming during the loading processes, something that does not occur in the other ceramics evaluated.