HARDNESS TESTING OF CERAMIC MATERIALS

This paper reviews the applicability of hardness test methods to polycrystalline ceramics. It is concluded that Rockwell Superficial HR45N tests are appropriate high-load tests for dense ceramics, because the indentation depth recorded is not confused by local cracking. In contrast the degree of cracking and spalling around Vickers or Knoop indentations limits loads to 2. 5 kgf (25 N) or less in order to measure indentation diagonals accurately. However, at loads of less than 1 kgf (10 N), indentation dimensions in hard ceramics become small and subject to considerable scatter in size as a result of the locally anisotropic hardness. Operator biases and practical limits of resolution limit the accuracy and repeatability of results for purposes of materials comparison and specification.