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

•The effect of machining on fatigue strength of CAD/CAM ceramics was evaluated.•Machining reduces the flexural fatigue strength of CAD/CAM ceramics.•Y-TZP showed the greatest reduction in the fatigue strength due to machining.•Either soft machining or hard machining can be deleterious.•Higher values of roughness may be expected after machining.

ObjectivesTo 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.MethodsDisc-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 (α = 5%) to compare roughness between the M and MP conditions.ResultsMachining 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 μm and MP = 0.08 μm; lithium disilicate: M = 1.84 μm and MP = 0.13 μm; Y-TZP: M = 1.79 μm and MP = 0.18 μm).SignificanceMachining negatively affected the flexural fatigue strength of CAD/CAM ceramics, indicating that machining of partially or fully sintered ceramics is deleterious to fatigue strength.