Impact of machining on the flexural fatigue strength of glass and polycrystalline CAD/CAM ceramics

Abstract

Objectives. To assess the effect of machining on the flexural fatigue strength and on the surface roughness of different computer-aided design, computer-aided manufacturing (CAD/CAM) ceramics by comparing machined and polished after machining specimens. Methods. Disc-shaped specimens of yttria-stabilized polycrystalline tetragonal zirconia (Y-TZP), leucite-, and lithium disilicate-based glass ceramics were prepared by CAD/CAM machining, and divided into two groups: machining (M) and machining followed by polishing (MP). The surface roughness was measured and the flexural fatigue strength was evaluated by the step-test method (n = 20). The initial load and the load increment for each ceramic material were based on a monotonic test (n = 5). A maximum of 10,000 cycles was applied in each load step, at 1.4 Hz. Weibull probability statistics was used for the analysis of the flexural fatigue strength, and Mann-Whitney test (alpha = 5%) to compare roughness between the M and MP conditions. Results. Machining resulted in lower values of characteristic flexural fatigue strength than machining followed by polishing. The greatest reduction in flexural fatigue strength from MP to M was observed for Y-TZP (40%; M = 536.48 MPa; MP = 894.50 MPa), followed by lithium disilicate (33%; M = 187.71 MPa; MP = 278.93 MPa) and leucite (29%; M = 72.61 MPa; MP = 102.55 MPa). Significantly higher values of roughness (Ra) were observed for M compared to MP (leucite: M = 1.59 mu m and MP = 0.08 mu m; lithium disilicate: M = 1.84 mu m and MP = 0.13 mu m; Y-TZP: M = 1.79 mu m and MP = 0.18 mu m). Significance. Machining negatively affected the flexural fatigue strength of CAD/CAM ceramics, indicating that machining of partially or fully sintered ceramics is deleterious to fatigue strength. (C) 2017 The Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.