Which statement correctly describes the low-temperature degradation that became a problem for yttria partially stabilized zirconia (Y-TZP) femoral heads in artificial hip joints?

Answer: A. In a moist environment, tetragonal grains transform to monoclinic from the surface with a volume expansion, roughening the surface and lowering the strength

Y-TZP gets its high toughness from transformation toughening, in which metastable tetragonal grains transform to monoclinic and expand under the stress at a crack tip. In a moist environment, however, the tetragonal → monoclinic transformation proceeds slowly from the surface even at temperatures as low as body temperature. The volume expansion of about 4% creates microcracks and grain pull-out at the surface, roughening it and lowering the strength and wear resistance. This is low-temperature degradation (hydrothermal degradation); manufacturing conditions accelerated the transformation and fractures of femoral heads became a problem. For this reason, alumina–zirconia composites and similar materials have been developed. Transformation to the cubic phase does not occur at low temperature and does not strengthen transformation toughening. Pitting is corrosion in which the passive film of a metal breaks down locally; it is not a degradation mechanism of oxide ceramics.