Phase transformation and sustained load crack growth in ZrO2 + 3 mol% Y2O3: Experiments and kinetic modeling

To better understand environmentally assisted crack growth (SCG) in yttria stabilized zirconia, experimental studies were undertaken to characterize the kinetics of crack growth and the associated stress/moisture induced phase transformation in ZrO2 + 3 mol% Y2O3 (3Y-TZP) in water, dry nitrogen and toluene from 3 to 70 C. The results showed that crack growth in water depended strongly on stress intensity factor (K1) and temperature (T) and involved the transformation of a thin layer of material near the crack tip from the tetragonal (t) to the monoclinic (m) phase. These results, combined with literature data on moisture-induced phase transformation, suggested that crack growth enhancement by water is controlled by the rate of this transformation and reflects the environmental cracking susceptibility of the transformed m-phase. A model was developed to link subcritical crack growth (SCG) rate to the kinetics of t → m phase transformation. The SCG rate is expressed as an exponential function of stress-free activation energy, a stress-dependent contribution in terms of the mode 1 stress intensity factor KI and actuation volume, and temperature. The stress-free activation energies for water and the inert environments were determined to be 82 ± 3 and 169 ± 4 kJ/mol, respectively, at the 95% confidence level, and the corresponding activation volumes were 14 and 35 unit cells. The decreases in activation energy and activation volume may be attributed to a change in surface energy by water