Low-temperature ageing degradation of 2.5Y-TZP heat-treated at 1650 °C

Low temperature ageing degradation at 250 °C in air of 2.5 mol% Y-TZP with various grain sizes has been investigated by using specimens in the form of discs of 8 mm diameter and 1 mm in thickness. The increase in tetragonal grain size, as well as some cubic phase, were attained by heat-treating 0.3 μm fine grain size Y-TZP at 1650 °C for different times. XRD and Raman spectroscopy analysis were used to evaluate the amount of tetragonal to monoclinic (t-m) phase transformation after different ageing times at 250 °C in air up to 900 h and its effect on strength was measured by using the biaxial ball-on-three-balls test. It is shown that transformability under ageing conditions at 250 °C increases with tetragonal grain size. Raman analysis was done punctually at various distances form the edge of the aged samples and it is shown that the nucleation of monoclinic sites begins at the edge of the samples and that the depth of the transformation zone varies with the distance from the edge to the centre of the samples. Biaxial strength always decreases with ageing time and is influenced by the presence of micro and macrocracks formed at the surface centre of the aged samples. This depends strongly on the tetragonal grain size and on the depth of the transformed layer at the central part of the discs.