Effect of heat-treatment on grain size, phase assemblage, and mechanical properties of 3 mol% Y-TZP

The effect of heat treatment on the grain size, phase assemblage, and mechanical properties of a 3 mol% Y-TZP ceramic was investigated, Specimens were initially sintered for 2 h at 1450 degrees C to near theoretical density; some specimens were then heat-treated at 1550 degrees, 1650 degrees, 1750 degrees, or 1850 degrees C to coarsen the microstructure. The average grain size increased with heat treatment from <0.5 to similar to 10 mu m. Phase analyses revealed predominantly tetragonal and cubic phases below 1750 degrees C, with a significant decrease in tetragonal content and increase in monoclinic content for temperatures >1750 degrees C, The maximum fraction of tetragonal phase that transformed during fracture corresponded with the largest tetragonal grain size of similar to 5-6 mu m. Strength was on the order of 1 GPa, and was surprisingly insensitive to heat-treatment temperature and grain size, contrary to previous studies, The fracture toughness increased from 4 to 10 MPa . m(1/2) with increasing grain size, owing to an increasing transformation zone size, Grain sizes larger than 5-6 mu m spontaneously transformed to monoclinic phase during cooling, Such critical grain sizes are much larger than those found in past investigations, and may be due to the greater fraction of cubic phase present which decreases the strain energy arising from crystallographic thermal expansion anisotropy of the tetragonal phase.