Abstract Cobalt-chromium-molybdenum alloys are widely used in medicine, especially in dentistry, due to their wear resistance, corrosion resistance, high strength and biocompatibility, as well as reasonable prices. In the present study, the effect of cooling rate of alloy during casting by changing of the mold material on the mechanical and corrosion properties of casted alloy were eva‎luated. Three different of molds with different materials and different thermal conduction coefficients were selected and eva‎luated in a precision casting by a centrifugal casting machine. In this method, all the casting parameters including mold thickness, casting temperature, molding temperature, required equipment, time and casting rate, room temperature and alloy composition used for all samples, were considered the same, and the only variable parameter was the different material of the molding. After casting, all samples were stored in a centrifugal machine for five minutes and then cooled to room temperature in the air. eva‎luation and characterization of the samples by optical microscopy, scanning electron microscopy, X-ray diffraction, elemental analysis and mechanical experiments including macro Vickers hardness testing, four-point bending test, and also for corrosion testing of samples from corrosion test Electrochemical (Potentiostatic) were performed. The results showed that by changing the mold material, grain structure, grain size, dendritic arms spacing, size, type, volume fraction and distribution of carbides, changed significantly. There was not much change in the hardness of the alloy. In microscopic observations, it was found that the dendritic arms intervals changed from 17 to 3μm for samples casted in gypsum plaster to samples casted in graphite molds. By increasing the cooling rate by graphite molding, part of the structure was transformed into a hexagonal crystalline structure (HCP) (phase ԑ). Also, samples with a smaller grain size and a small size in the microstructure increased corrosion due to the increase in the grain boundary number, and samples with a more coarse and regular microstructure showed less corrosion rate. The flexural behavior of the samples showed that the structures with more and more regular carbides and more coarse microstructures showed more tolerance to the number of beads less than the applied load pressure. The results showed that dental alloys of cobalt-chromium molybdenum have a coarse-grained microstructure and have regular carbides with appropriate distribution and shape and size, exhibiting better corrosion behavior and mechanical properties.

رضا اميني نجف آبادي

محمد خدايي