Hardness and Hardness Determination in Silicon Carbide Materials

The hardness of SiC ceramics plays a significant role in their performance as armor materials. Different hardness measurement techniques and loads, however, substantially influence the hardness values of ceramics, making it difficult to make material comparisons and relate these to performance. In this study hardness tests were performed with: (i) monocrystalline alpha- and beta-SiC crystals, (ii) polycrystalline, sintered (pressureless/gas-pressure sintered) alpha-SiC materials, and (iii) hot-pressed (uniaxially/isostatically) alpha-SiC materials. Hardness numbers were measured by indentation with Knoop and Vickers diamonds in the load range from 0.245 N to 4.9 N on a Leitz Miniload-2 tester. At loads over 0.98 N hardness decreased with increasing load. Highest values for Knoop and Vickers hardness at constant load were obtained with the SiC single crystals. In most cases it was found that Vickers hardness is higher than Knoop hardness. In the hot isostatically pressed SiC (encapsulated HIP-SiC) the Vickers and the Knoop numbers were found to be identical. Furthermore, it was observed that doping elements like Al and B have an effect on hardness. Al-doped SiC ceramics exhibited lower hardness numbers than pure or B-doped materials and, additionally, in these materials the Knoop hardness is higher than the Vickers hardness. Liquid phase sintered SiC showed lowest hardness numbers due to the secondary phase and solubility of Al in the SiC-lattice.