VICKERS AND KNOOP INDENTATION BEHAVIOR OF CUBIC AND PARTIALLY-STABILIZED ZIRCONIA CRYSTALS

Hardness and fracture toughness of cubic zirconia crystals (CZC) doped with yttria and partially stabilized zirconia crystals (PSZC) doped with yttria, terbia and ceria were studied at indentation loads of 0.1-300 N. MOR and K-1c of these materials were measured under bending conditions. A maximum hardness value of H-K = 21.2 GPa was obtained for CZC at a 0.1 N indentation load when a longer diagonal of the Knoop indenter being directed along [110]. Starting from a 2 N load, PSZC exhibited lateral cracks and phase transition zones. Radial cracks in Vickers indentation tests of Y-Tb-PSZC and Y-Ce-PSZC appeared above 40 N and 50 N respectively. Such cracks in Knoop indentation tests were observed above 300 N. Orientation anisotropy of lateral cracks was noticed in all crystals, radial ones were revealed only in CZC. After grinding off the specimen surfaces, lateral cracks were found to be present under the impressions. The calculations of fracture toughness K-1c(v) using the equation proposed by the authors gave a value corresponding to those materials determined on bending.