Papers by Keyword: Transformation Toughening

Abstract: Possible increase in fracture toughness of ceramics can be derived from stress induced martensite transformation from tetragonal to monoclinic polymorph of ZrO₂ particles embedded into a bulk ceramic material. The incidence of transformations depends on zirconia particle size: too small particles remain overstabilized and do not experience transformation while too large particle may spontaneously transform at the technological stage of cooling. The critical particle size is, therefore, of primary concern for toughening of intrinsically brittle materials. We give a brief review of the previous results obtained. Then basing on the Gibbs energy expression and taking into account interface surface energy as well as thermal stresses, external loading and elastic interaction of the inclusions we estimate the proper range of particle sizes needed for considerable increase in fracture toughness. We specify general results obtained for the case of yttria stabilized ZrO₂ particles in Al₂O₃- and WC-based ceramics.

Abstract: Fully dense ZrO2-TiN composites containing 1.75-2 mol %Y2O3, 1 mol% Y2O3 and 1 mol% Nd2O3 stabilizers, small amounts of Al2O3, and electrical conductive TiN particles (40-70 vol%) have been produced by hot pressing and spark plasma sintering at 1550°C. Although the intrinsic hardness of TiN (1400 kg/mm²) is higher than that of t-ZrO2 (1200 kg/mm²), the decreasing hardness trend can be attributed to the larger TiN grain size with the higher TiN content. Since TiN is more brittle, the fracture toughness decreases with increasing TiN content. Transformation toughening has been attributed as the main toughening mechanism as a result of fracture toughness decreasing with the transformability. Spark Plasma sintering temperature was too high for mechanical properties and hydrothermal stability of the mixed stabilized composites. The transformability decreases so hydrothermal stability increases linearly with increasing TiN content as a result of smaller volume fraction of t-ZrO2 grains becoming susceptible to hydrothermal transformation due to the shielding effect of the present TiN grains.