The variation of indentation hardness with contact load i.e., the indentation size effect (ISE), was investigated for five ceramic materials: (i) AlON, (ii) AD995 CAP3 Al_2O_3, (iii) pressureless-sintered SiC (Hexoloy SA), (iv) SiC-N, and (v) SiC-B. A Knoop diamond indenter was used to make indentations in the load range from 0.49 N (0.05 Kg) to 137.3 N (14 Kg). The Knoop hardness decreased approximately 26 percent, 38 percent, 36 percent, 32 percent, and 31 percent for the five materials, respectively, over the entire load range. The Knoop hardness of all five materials continued to decrease as the load was increased beyond 2 Kg. In addition, severe cracking around indentation sites generally correlated with a lower Knoop hardness compared to less-cracked indentation sites for the silicon carbide and AD995 CAP3 materials. Accurate hardness measurements are necessary in order to detect variability in hardness not attributable to operator or instrument differences, and which may yield insight into ballistic performance.
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