Stress-induced phase transformation on zirconia surfaces

Objective: The effect of surface-treatment on the crystal phase composition of Y-TZP and its reversal by application of sufficient heat is to be examined by X-ray diffraction (XRD). Methods: Disc-shaped samples of LAVA zirconia (Y-TZP, 3M ESPE) were surface-treated by either sandblasting or grinding. Sandblasting was carried out by blasting the samples with alumina (particle size 50 µm) at 2 bar. For grinding, a diamond coated instrument (Rotring) from Komet was used at 20.000 rpm with water cooling. Firing of the samples after surface-treatment was carried out at 1000 °C. X-ray measurements were performed with a Bruker D8 Discover in Bragg-Brentano geometry. Quantitative phase analysis was done by the Rietveld method (TOPAS software, Bruker). Given the high electron density of zirconium, the X-ray beam penetrates the samples no more than 5 µm. Results: The control group of untreated zirconia material (AF) displayed mainly a tetragonal (t) crystal structure and, to a minor extend, cubic (c) phase zirconia. It was found that stress applied to the surface lead to a phase transformation towards monoclinic (m) and distorted cubic (c2) phase on the treated surface. Interestingly, the character of the surface treatment had an influence on which new crystalline phase was preferred. Sandblasting (SB) lead to the formation of a higher fraction of monoclinic phase, while grinding (GD) yielded more distorted cubic phase (Table 1). Firing of the surface-treated samples could almost completely restore the original phase composition of the control group, leaving only a slightly increased amount of cubic phase (Table 1). Conclusion: It could be shown that different types of surface-treatment of tetragonal zirconia induce different ways of phase transformation to monoclinic and distorted cubic on the sample surface. Firing of those samples could reverse this transformation and yield a phase composition close to that of untreated zirconia.