Research On The Machinability And Analysis Methods Of Dental Ceramic

On account of the favorable biocompatibility, chemical stability, comparable thermal conductivity and abrasion rate to the nature teeth, dental ceramic materials are widely used in the prosthodontics as one kind of the most important members in biomaterial family to resume the function, shape and beauty of the teeth. Because of their outstanding properties, such as high hardness and brittleness of the ceramics, surface damages and sub-surface damages are reviewed during machining, thus reducing their intensity and restricting their application area. Mechanical processing, such as cutting, drilling, grinding, lapping and polishing, is the main method for machining of ceramics, and grinding is the primary choice. Material removal mechanisms are discussed as brittle fracture, powdered removal and plastic removal. Machinability evaluation, the connection between material design and mechanical processing, is the key of machining technology. Mechanical properties are the leading influence factors of machinability, including hardness, fracture toughness, elastic modulus etc.In this paper, the grinding and cutting tests were carried out respectively in different conditions for the dental ceramic materials, such as alumina, zirconia, Empress feldspar ceramics, Empress toughening leucite and glass ceramics (LDG), also the surface of the processed samples were reviewed, analysed and discussed by surface roughness, optical microscope and SEM. Obviously, the samples have smoother surface if they are processed by diamond grinding wheel, higher speed of the wheel and longer time. Four kinds of dental all-ceramic materials, including CS-11, CA-12,I-12, TRI-12, were processed by grinding, cutting and drilling, and the abrasive characteristics of the samples were studied by using MM200 wear testing machine with silica sands of 40 mesh,80 mesh as well as 120 mesh as the abrasion medium. The weight losses of dental ceramic samples after abrasive testing of certain times were measured and surfaces were then observed under SEM. The results show that the presintering ceramics of CA-12 and CS-11 produced by In-Cream technology have the smoother surface quality after grinding, while the traditional feldspar ceramics,I-12 and TRI-12, have better surface feature by cutting test. In the wear test, while abraded by 80 mesh and 120 mesh silica sands, the samples have smoother surfaces than by 40 mesh sand. The biggest weight loss is found for LDG, and obviously, the amount grows by time. The least material removal amount goes to CS-11 under the same testing conditions. More materials are removed from both I-12 and TRI-12 ground by 120 mesh silica sand than by other two kinds of coarser abrasive particles. For the sample ground in the same conditions, the higher hardness, the less weight loss. And the higher toughness, the higher material removal amount. As presintering ceramics, CA-12 and CS-11 have smoother surfaces comparing to 1-12 and TRI-12. According to the mechanism, the materials are removed from the I-12 and TRI-12 samples by brittle fracture, the surfaces of the samples are scraped and collided by the silica sands. And the materials are removed from the CS-11 and CA-12 samples by plastic removal, as the mean of shear chip formation. The surface damages, as cracks are observed on the surface of the LDG In analysis, the surface roughness is proposed for machinability evaluation of dental ceramics as a new method. The testing data and the mechanical properties were taken as sets of available alternatives. Fuzzy comprehensive evaluation principle was applied to establish a model for machinability evaluation of dental ceramics. As an example, three mechanical parameters of dental ceramics were selected to be the alternatives for machinability evaluation. Machinability of several typical dental ceramics was evaluated, and ceramic materials were ranked in order of comprehensive evaluation index. The results show that the fuzzy evaluation is rational, and calculating method is simple and easy. The machinability evaluation method will be an important way for determination of machining parameters.