Preparation And Structure And Properties Of Rhodium Coatings On The Surface Of NiTi Alloy

The Rh coatings of low reflectivity was prepared on the surface of NiTi alloy by electroplating in order to avoid the "metal smile" in orthodontics, inhibit the dissolution of nickel ion, improve the corrosion and wear resistance. The preparation, properties, crystal structure and interface structure of Rh coatings were investigated by scanning electron microscope, atomic force microscope, laser scanning confocal microscope, X-ray diffraction analysis, transmission electron microscope, nanoindentation, scratch test, friction test, tension test and electrochemical technology.The results show that, with the pH value of nickel plating solution decreases, the adhesion of the coatings increases. the adhesion of the coatings achieve the highest level of ISO and ASTM when pH value is lower than 0.1. The grain size of nickel increases with the raise of the temperature of the plating solution and the extension of plating time; with the raise of the current density, the grain size increases first and then decreases. The grain of rhodium grow up slowly with the bath temperature increase. With the increase of the current density, the surface of the coatings tends to be smooth and the grain size increases first and then decreases. When electroplating time is prolonged, the thickness of the coatings and internal stress increase, and the crack occurs after the 4min.Analysis show that, the bath temperature had little effect on the roughness and wear resistance of the rhodium coatings; With current density increase, the surface roughness of the coatings decrease. When the current density reachs 3A/dm2, the roughness decrease to 0.95 μm, after which no significant changes occur, the wear resistance increase first and then decrease. When the current density reachs 4A/dm2, the wear resistance is best. With plating time prolong, the surface roughness of the coatings decrease. When the plating time reachs 15 min, the roughness decrease to 0.9 μm, after which no significant changes occur, and the wear resistant changes little. The corrosion resistance of the coatings increased first and then decreased with the increase of bath temperature, current density and plating time.Based on the comprehensive analysis of the influence of the preparation process on the structure and properties of the rhodium coatings, the optimum process parameters were determined. The results as follows, Pickling process: the volume ratio of HF:HNO3:H2O is 1:10:50, the temperature is 80℃,pickling time is 2min; nickel plating process: the pH value is 0.1, the bath temperature is 20℃, the current density is 5A/dm2, the plating time is 3min; rhodium plating process: the bath temperature is 25℃, the current density is 2-4A/dm2, the plating time is 5min.