Fracture toughness, surface flaw sizes and flaw densities in Al2O3

Surface flaw sizes and flaw densities on polished dense polycrystalline alumina specimens with grain sizes G = 1.2, 3.8 and 14.1 mu m were measured using Hertzian indentation. Two surface finishes were studied: well-polished (Syton) and coarse-polished (45 mu m diamond paste). Flaw sizes and densities depended on the surface finish and increased with increasing grain size. Fine grained specimens exhibited a high density of small flaws while coarse grained specimens exhibited a higher density of larger flaws. Fracture toughness (K(I)c) for each material (relating to the propagation of flaws of a few mu m depth) was determined from the minimum fracture load in a series of Hertzian tests. K(I)c values were 3.58, 3.45 and 2.96 MPam(1/2) for G = 1.2, 3.8 and 14.1 pm, respectively. Fracture toughness values were also determined by Vickers indentation over a range of loads; the K(I)c values determined from the Hertzian tests were consistent with the trends in K(I)c, with crack size from the Vickers indentation tests. Copyright (C) 1997 Acta Metallurgica Inc.