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

摘要

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 degrees C for 2 h and characterized. After sintering, dense Y-TZP samples presenting grain size average of 0.81 +/- 0.23 mu m, 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 mu m. The surface roughness of the samples was influenced by the machining methodology using NCT mills, with R-a = 0.386 +/- 0.149 mu m and R-a = 0.292 +/- 0.105 mu m 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 mu m for samples milled perpendicularly, and R-a = 0.919 +/- 0.164 mu m 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 mu m in the cutting edge.