Surface fracture testing of alumina with spherical indentation and recording hardness tester

Two kinds of surface fracture tests, spherical indentation test and recording hardness test, were conducted on alumina ceramics with different grain size. In spherical indentation test, the size of the Hertz crack was measured. It decreased with increasing the grain size, but the degradation tendency was different from that on bending strength. As the size of the Hertz crack is proportional to the surface stress on the cracking, the diameter can indicate the surface fracture strength of the materials. In recording hardness test, we calculated the energy consumption ratio for forming the indents on the specimen. This ratio did not depend on the grain size. The difference between these two tests may be caused by that the spherical indentation test measures the fracture in tension, but the recording hardness test measures the fracture in compression.