A small addition of CuO to 2-mol%-Yz03-stabilized tetragonal zirconia polycrystals significantly enhances superplasticity by forming an amor hous grain-boundary phase containing primarily C u ' , Y", Zr", and 0'-. This phase apparently melts at around 1130°C, but it already provides a fast diffusion path even below the melting temperature. There are abrupt changes in stress exponent, activation energy, and grain size exponent across the melting temperature. Superplasticity is diffusion-controlled below the melting temperature and is interfaced-controlled above that. [