Crystal structure of dental zirconia: surface and bulk phases

Objectives: The structural details Y-TZP especially regarding the surface phases still raise confusion. A sound combination of advanced X-ray-diffraction (XRD) measurement and evaluation techniques should yield a fundamental description of the structure of the bulk material and of the surface phases obtained after mechanical treatment. Methods: Two platelet samples, LAVA zirconia (3M ESPE) as fired (AF) and surface grinded with diamond tool (GD) were subjected to a Bragg-Brentano geometry XRD measurement (BB-XRD, penetration depth 5µm). For GB also a Grazing Incidence measurement was applied (GI-XRD, incidence angle 3°, penetration depth). All experiments were performed on a Bruker D8 diffractometer using Cu-K-alpha radiation and a “Lynxeye” semiconductor detector. Structure refinement and quantitative phase analysis were carried out by the Rietveld fit method (Topas software, Bruker). Results: BB-XRD of AF yields tetragonal (t) and cubic (c) phase in a ratio of ca. t/c 80/20 w/w, for GD in addition the monoclinic (m) and a second cubic phase (c2) become apparent. However, GI-XRD of GD reveals that m and c2 phases are located within the outermost surface layer <1µm. So, for GD the bulk composition remains unchanged, whereas for the surface layer c2/m 90/10 w/w is calculated. For t and c crystallite sizes of 80 respectively 50 nm are determined (not identical to grain sizes!) which is well line with nanostructure investigations by Matsui et al. (J.Am.Ceram.Soc. 86 (2003) 1401-1408). Unlike for the other phases, for c2 a significant distortion of the crystal lattice (strain) has to be assumed to explain the experimental pattern. Conclusion: An overall structural description of Y-TZP including bulk phase and surface phases due to mechanical treatment is obtained. Particularly, a detailed explanation of the c2 surface phase is given. This work is a mandatory basis for further studies on surface phases of dental zirconia.