Hertzian indentation technique was used to measure surface flaw sizes on polished dense polycrystalline alumina specimens with grain sizes G = 1.2, 3.8 and 14.1 µm. Two surfaces finishes were studied: well-polished (Syton) and coarse-polished (45 µm diamond paste). Flaw sizes depended on the surface finish and increased with increasing grain size. Fracture toughness (K Ic) for each material (relating to the propagation of flaws of a few µm depth) was determined from the minimum fracture load in a series of Hertzian tests. K Ic values were 3.58, 3.45 and 2.96 MPam½ for G=1.2, 3.8 and 14.1 µm, respectively. Fracture toughness values were also determined by Vickers indentation over a range of loads; the K Ic values determined from the Hertzian tests were consistent with the trends in K Ic with crack size from the Vickers indentation tests.
surface; Hertzian indentation; flaw size; alumina
