Residual stress measurements of computer aided design/computer aided manufacturing (CAD CAM) machined dental ceramics

Residual stress analysis is becoming more important in terms of understanding the strength and fatigue behaviour of ceramic materials. The residual stresses after computer aided design/computer aided manufacturing (CAD/CAM) machining according to dental practice were analysed for two different kinds of dental ceramics, a feldspathic porcelain and a glass-ceramic. A mechanical strain gauge element was used to measure the deformation of dental test inlays during material removal by etching the surface of the sample. From these data the residual stress depth profile could be calculated for crystalline as well as amorphous materials. The strain gauge results were compared to X-ray diffraction data. The depth profile of the residual stress for both ceramics showed compressive stress at the surface of the machined ceramics, changing towards tensile stress at a depth of 10 to 15 mu m from the surface. Ceramics with pronounced plastic deformation behaviour in CAD/CAM machining revealed higher residual stresses as well as a more distinct stress anisotropy in terms of grinding direction.