Thermal stability in air of surface nitrided 3Y-TZP

Surface nitriding can be optimised to protect 3Y-TZP against hydrothermal degradation and at the same time to keep the mechanical properties of the surface and the bulk unchanged. However, thermal stability is a concern that can limit its use above room temperature. In this work, surface nitrided 3Y-TZP was heat treated in air at 200, 400, 600 and 800 °C for 8 h. Treatments up to 600 °C did not change microstructure and Vickers hardness, but heating at 800 °C produced surface defects, inner cracks and grain decohesion, conducing to a dramatic decrease in mechanical strength. Heat treatments above 400 °C led to an increase of indentation fracture toughness and to a complete loss of the hydrothermal degradation resistance achieved by nitriding.