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 μm were measured using Hertzian indentation. Two surface finishes were studied: well-polished (Syton) and coarse-polished (45 μm 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 (Klc) 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. Klc, values were 3.58, 3.45 and 2.96 MPam1 2 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 Klc values determined from the Hertzian tests were consistent with the trends in Klc with crack size from the Vickers indentation tests. Copyright © 1997 Acta Metallurgica Inc