Influence of CAD-CAM milling on the flexural strength of Y-TZP dental ceramics

Abstract

In this work, the influence of milling strategy and cutter wear on the flexural strength of sintered Y-TZP ceramics machined in a CAD-CAM system, was investigated. Pre-sintered Y-TZP blocks were machined with tungsten carbide (WC) milling cutters and divided into three groups: a) samples machined with New Cutting Tools (NCT); b) samples machined with End-of-life cutting tools (ELCT); and c) samples machined and subsequently polished. The samples were machined parallel and perpendicularly to the diameter of pre-sintered Y-TZP blocks aiming to evaluate the effect of the machining orientation on roughness. After cutting, the specimens were sintered at 1530 °C for 2 h and characterized. After sintering, dense Y-TZP samples presenting grain size average of 0.81 ± 0.23 μm, hardness of 1205 ± 12 HV, K C of 7.7 ± 0.4 MPam 1/2 and flexural strength of 1207 ± 199 MPa were obtained. Furthermore, the control roughness (polished surfaces) presented R a = 0.058 ± 0.011 μm. The surface roughness of the samples was influenced by the machining methodology using NCT mills, with R a = 0.386 ± 0.149 μm and R a = 0.292 ± 0.105 μm for samples machined parallel and perpendicularly to the pre-sintered Y-TZP blocks, respectively. The use of ELCT led to a roughness increasing (R a = 0.582 ± 0.183 μm for samples milled perpendicularly, and R a = 0.919 ± 0.164 μm for samples cut parallel to the pre-sintered blocks. The flexural strength of the sintered Y-TZP decreases with use of ELCT as consequence of surface roughness increasing, resulting from wear of WC mills, which presented chippings with sizes ranging from 30 to 340 μm in the cutting edge.