CAD/CAM-machining effects on Y-TZP zirconia

Objectives. Computer-Aided-Design/Computer-Aided-Manufacturing (CAD/CAM) techniques are gaining importance in fabricating crowns and fixed partial dentures (FPDs) made of yttria stabilized tetragonal zirconia polycrystals (Y-TZP). The specific aims of this study were (1) to test the hypothesis that surface flaws and microcracks are induced by the grinding of crowns (2) to analyze the material removal and the grinding-induced surface layer as well as to estimate the crack size caused by machining Y- TZP under conditions simulating the grinding of crowns. Methods. Y-TZP disks and sectioned cylinders with polished separation planes were used for the analysis of the grinding procedure. While simulating the inner surface grinding of crowns, feed and cutting depth were varied (vw=100, 75, 50 mm/min; ap=0.1, 0.06, 0.02 mm). SEM was used for the quantitative assessment of the machined surface. Results. While the crack length is not significantly influenced by the grinding parameters, the type of material removed varied with the cutting depth as well as with the feed. Grinding induced surface flaws and microcracks were detected at the internal top surface of the crowns. Half-cylinders machined under conditions simulating the inner surface grinding of crowns showed crack lengths between 2 and 15 μm. Significance. Sectioned specimens with polished section planes are suitable for the analysis of the grinding process using the face and peripheral grinding procedure. The inner surface grinding of fixed restorations is the most challenging step of CAM of crowns and FPDs. Most important appeared to be the diamond tool, especially the number and shape of the active diamond grains.