Abrasive machining of porcelain and zirconia with a dental handpiece

The machining characteristics and material removal mechanisms of two dental ceramics—feldspathic porcelain and yttria-stabilized tetragonal zirconia—were investigated using a dental handpiece and diamond burs with different grit sizes. The material removal rates were measured as a function of total machining time using a constant load of 2 N on the bur, consistent with clinical cutting conditions. As the diamond grit size was increased from ultrafine (UF) (10 μm) to fine (F) (41 μm) and coarse (C) (172 μm), the removal rate and the resulting surface roughness for each material increased substantially. The mechanisms of material removal determined through microscopic examination of the machined surfaces and the machining debris on the burs were found to consist of a combination of ductile and brittle-type chip formation processes. The occurrence of brittle fracture increased as the diamond grit size was increased. While the material removal process in porcelain was dominated by brittle fracture, zirconia was primarily subjected to ductile cutting. Four wear processes were identified on the burs in prolonged cutting tests: grit microfracture, grit pullout, wear flat generation, and matrix abrasion. The results demonstrated that while the material removal rate for the zirconia evaluated in this study was lower than those for porcelain and many other dental ceramics, the zirconia could be machined under clinical conditions with no edge chipping damage.