Impurity effects on superplasticity in tetragonal zirconia polycrystals

The present study investigated the role of small amounts of impurities on the superplastic deformation behavior of yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) in an effort to determine the physical mechanism by which this process occurs. For pure Y-TZP it was found that deformation was defined by a strain sensitivity value (m) of 0.33 for temperatures between 1150{dollar}spcirc{dollar}C and 1300{dollar}spcirc{dollar}C. A small temperature dependence of this value was diminished by correcting the raw stress-strain data for grain growth which occurred during the test. After this correction, the activation energy for deformation (Q{dollar}rmsb{lcub}def{rcub}){dollar} was determined to be {dollar}565pm15{dollar} kJ/mol. The grain size exponent (p) had a value of 2.; The effect of transition metals on superplastic behavior was determined through small additions (0 to 0.3 mol%) of CuO to 3Y-TZP. At the same time, the effect of silicate-based glasses was studied by adding pure silica to 3Y-TZP (up to 1wt%). It was found that the presence of CuO increases the strain rates in an exponential manner by up to two orders of magnitude, while not significantly altering the value of m from that of pure 3Y-TZP. A nearly linear decrease in Q{dollar}rmsb{lcub}def{rcub}{dollar} with CuO content was found, which explained the strain rate increase phenomenon. In the case of silica, a gradually increase in the m value from 0.33 to 0.65 was noticed. As a consequence, the strain rate at a given stress decreased with increasing amounts of glass for stresses greater than 50 MPa. An increase in the Q{dollar}rmsb{lcub}def{rcub}{dollar} was also observed--contrary to the behavior reported in similar studies.; A physical model for superplastic deformation incorporating the results of the present study has been proposed. This theory--which fundamentally links dynamic grain growth and grain boundary sliding--explains many previous observations of superplasticity. It is also consistent with this study's observation that the relationship between dynamic grain growth and superplastic strain rates is the one factor which is invariable across all superplastic materials--metals and ceramics--even when other characteristics of superplasticity (e.g., m, Q{dollar}rmsb{lcub}def{rcub},{dollar} p values) are changing radically.