ANNEALING EFFECT ON FRACTURE TOUGHNESS MEASUREMENT OF ZrO//2-TOUGHENED Al//2O//3 CERAMICS USING VICKERS INDENTATION.

Surface grinding and polishing ZrO//2 prime s tetragonal-to-monoclinic transformation resulted in residual compressive surface stress in the ZrO//2-toughened Al//2O//3 (ZTA) ceramics. When the surface stress was removed by annealing the specimens at 1350 degree C for 1 h, the fracture toughness (K//i//c) values determined by Vickers indentation technique were independent of the Vickers load. But with the as-polished specimens, the K//i//c appeared larger and strongly dependent on the Vickers load, because the surface stress impeded the crack extension.