Dental abrasive finishing of a fine-grained feldspathic porcelain was performed on a computer-assisted apparatus for simulation of a 2-degrees-of-freedom restorative operation with a dental handpiece and a coarse diamond bur of grit size of 106-125 mum. Finishing forces, surface roughness, and morphology were investigated as functions of finishing conditions. The tangential and normal forces were measured using a piezoelectric dynamometer and a data processing system. The results indicated that these forces increased with either the depth of cut or with the feed rate, in the ranges of 0.12-0.31 N and 0.45-1.09 N, respectively. However, an increase in either depth of cut or feed rate affected neither the surface roughness measured using a stylus profilometer nor the morphology observed under a scanning electron microscope. The finished porcelain surfaces were found to consist of the microfracture and chipping areas, ductile removal areas, smeared areas, and debris. Irregular fracture and chipping resulted from the extension of lateral/median cracks; ductile micromachining was attributed to the plastic deformation accompanied by distributed microcracks. It was determined that a combination of the microfracture and ductile micromachining was the primary mechanism for material removal.

Finite element analysis (FEA) was used to investigate the stress fields and the degrees of subsurface damage of ceramic prostheses in simulated intraoral dental resurfacing operations using clinical diamond burs. A two-dimensional finite element model was established with the dental operational parameters and the material properties as input variables. This model enabled to predict the stress fields and to evaluate the depths of subsurface damage in ceramic prostheses as functions of the dental resurfacing operational conditions. The results indicate that the tensile, shear, compressive, and equivalent von Mises stresses were all centered under the diamond bur-specimen contact zone. The maximum values of these stresses were concentrated at the diamond grit exit point, decreasing with an increase in depth of cut. The predicted depths of subsurface damage increased with an increase in both the depth of cut and the maximum chip thickness, in the range of 30-140 microm. Also, the depths of subsurface damage were experimentally measured using scanning electron microscopy (SEM). The FEA predictions were found to be in agreement with the SEM experimental observations.

Intraoral adjustment of ceramic prostheses involving micro-finishing using diamond burs is a critical procedure in restorative dentistry because the durability of a restoration depends on the finishing process and quality. Force, energy and surface integrity in micro-fine finishing of a feldspar porcelain versus operational parameters were investigated using a 2-DOF (two-degrees-of-freedom) high-speed dental handpiece and a fine diamond bur of 20-30 microm grits. The tangential and normal forces were measured as being significantly small in the ranges 0.18-0.35 N and 0.22-0.59 N, respectively. High specific finishing energy of 110-2523J/mm(3) was observed in material removal, particularly when decreasing either the depth of cut or the feed rate. Scanning electron microscopy observations indicated that the surfaces generated were mainly due to ductile flow; however, microfractures also occurred in porcelain. Surface roughness was measured as 0.43-0.74 microm in terms of arithmetic mean value (R(a)), decreasing with the depth of cut, but insignificantly changing with the feed rate (ANOVA, P>0.05). Recommendations for clinical practice are made on the basis of our testing results.

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This work reports on the effect of diamond burs with coarse, medium and fine grit sizes and nickel or chromium coatings on in vitro dental resurfacing of a restorative porcelain. Process parameters such as tangential and normal forces, surface roughness, surface damage and morphology were studied as a function of removal rate using the different burs. At the lower removal rate, the differences for both the tangential and the normal forces were not significant among the coarse, medium and fine burs. However, when the porcelain was removed at the higher removal rate, both the tangential and the normal forces were markedly higher using the fine bur than those using the medium and coarse burs. Surface roughness values in terms of arithmetic mean and maximum roughness decreased significantly with a decrease in diamond grit size. The scale of surface damage in the form of brittle fracture decreased, and more transitions from brittle removal to ductile flow were observed when using finer grit diamond burs. In a comparison of the diamond bur topographies before and after dental finishing, it was found that minimal wear occurred on the nickel-coated coarse diamond bur, while minor abrasive wear occurred on the nickel-coated medium and chromium-coated fine burs.