Grinding damage assessment on four high-strength ceramics

Abstract

ObjectivesThe purpose of this study was to assess surface and subsurface damage on 4 CAD-CAM high-strength ceramics after grinding with diamond disks of 75μm, 54μm and 18μm and to estimate strength losses based on damage crack sizes. MethodsThe materials tested were: 3Y-TZP (Lava), dense Al2O3 (In-Ceram AL), alumina glass-infiltrated (In-Ceram ALUMINA) and alumina–zirconia glass-infiltrated (In-Ceram ZIRCONIA). Rectangular specimens with 2 mirror polished orthogonal sides were bonded pairwise together prior to degrading the top polished surface with diamond disks of either 75μm, 54μm or 18μm. The induced chip damage was evaluated on the bonded interface using SEM for chip depth measurements. Fracture mechanics were used to estimate fracture stresses based on average and maximum chip depths considering these as critical flaws subjected to tension and to calculate possible losses in strength compared to manufacturer's data. Results3Y-TZP was hardly affected by grinding chip damage viewed on the bonded interface. Average chip depths were of 12.7±5.2μm when grinding with 75μm diamond inducing an estimated loss of 12% in strength compared to manufacturer's reported flexural strength values of 1100MPa. Dense alumina showed elongated chip cracks and was suffering damage of an average chip depth of 48.2±16.3μm after 75μm grinding, representing an estimated loss in strength of 49%. Grinding with 54μm was creating chips of 32.2±9.1μm in average, representing a loss in strength of 23%. Alumina glass-infiltrated ceramic was exposed to chipping after 75μm (mean chip size=62.4±19.3μm) and 54μm grinding (mean chip size=42.8±16.6μm), with respectively 38% and 25% estimated loss in strength. Alumina–zirconia glass-infiltrated ceramic was mainly affected by 75μm grinding damage with a chip average size of 56.8±15.1μm, representing an estimated loss in strength of 34%. All four ceramics were not exposed to critical chipping at 18μm diamond grinding. ConclusionsReshaping a ceramic framework post sintering should be avoided with final diamond grits of 75μm as a general rule. For alumina and the glass-infiltrated alumina, using a 54μm diamond still induces chip damage which may affect strength. Removal of such damage from a reshaped framework is mandatory by using sequentially finer diamonds prior to the application of veneering ceramics especially in critical areas such as margins, connectors and inner surfaces.