EFFECTS OF HETEROFLOCCULATION OF POWDERS ON MECHANICAL-PROPERTIES OF ZIRCONIA-ALUMINA COMPOSITES

Zirconia-toughened alumina (ZTA) composites colloidally processed from dense aqueous suspensions (> 50 vol% solids) had ZrO2 content varying from 5 to 30 vol%. Tetragonal zirconia (TZ) was used in the unstabili zed, transformable form (0Y-TZ), in the partially transformable form, partially stabilized with 2 mol% yttria (2Y-TZ), and in the nontransfo rmable form stabilized with 3 mol% yttria (3Y-TZ). After sintering in air to similar to 99% theoretical density, the elastic properties, fle xure strength and fracture toughness were examined at room temperature . Dynamic moduli of elasticity of fully deagglomerated compositions di d not show the effects of microcrack formation during sintering, even for materials with unstabilized zirconia. In all compositions made fro m submicron powders and with low content of dispersed phase (less than 10 to 20 vol%), the strength increased with increasing ZrO2 content t o a maxim um of similar to 1 GPa, irrespective of the degree of stabil ization of t-ZrO2. With increasing content of the dispersed phase (> 2 0 vol%), heteroflocculation of powder mixtures during wet-processing l ed to the formation of ZrO2 grain clusters of increasing size. Residua l tensile stresses built within cluster/matrix interfaces upon cooling not only facilitated the t-m ZrO2 phase transformation in final compo sites with transformable t-ZrO2, but also led to lateral microcracking of ZrO2/Al2O3 interfaces. This enhanced fracture toughness, but at la rger ZrO2 contents the flexure strength always decreased due to intens ive microcracking, both radial and lateral. The important microstructu ral aspects of strengthening and toughening mechanisms in ZTA composit es are related in discussion to the effects of heteroflocculation of p owder mixtures during wet-processing.