Preparation And Performance Of Nickel-based Composite Coatings On Superalloy

Superalloy is a kind of metal materials which can be used in continuous high temperature. The superalloy has many excellent characteristics, such as high temperature oxidation resistance, thermal fatigue resistance, good thermoplastic, good resistance to stress corrosion, etc. is widely used in the application of aviation engine, gas turbine and gas turbine parts. When used in a long time and in a harsh enviroment, the degradations of the superalloy, such as mechanical performance, oxisation corrosion, fatigue and high temperature creep, can happen.Electrochemical composite plating, based on the electroplating technology, can reinforce the hardness and wear resistance of the metal surface. In order to improve the hardness, wear resistance and thermal properties of the superalloy, the technology of electrochemical composite plating was used to prepare a playing coating. The methods of single factor experiment and orthogonal test were used to character the process conditions of composite plating. From the results, it can be shown that the maximal hardness with a 642.6 HV van be obtained when the process conditions shown as follows, plating temperature 40 ℃, current density 4 a/dm2, stirring speed 200 r/min, particle amount concentration 5 g/L and appropriate time 70 min. Through the analysis of extreme difference and variance, temperature has a significant effect on the test results. High temperature is not conducive to the promotion of the coating hardness. With the analysis of grain amount and time of single factor, deposition is accord with two-step sedimentary theory. The deposition rate of particles slows down when in the late deposition process. From the results of metallographic analysis, it can be shown that the flash plating process is very important to the adhesion strength of the coating. The flash plating can make coating surface be more smooth and shiny.The hardness, corrosion resistance of the composite coating and bright nickel plating are better than that of the superallloy. Comparing with the microstructure morphology on composite plating and nickel plating, it was shown that the composite plating had smaller grain size than nickel plating when in same plating process, and the smaller grain size was one of the most important factors which increased the high hardness. The modes of wear damage of the two electroplating coating were different. For composite coating, the mode of wear damage was adhesive wear, while the mode of wear damage of the nickel plating was extrusion wear. Comparing the corrosion resistance of the nickel plating and the composite plating, it was shown that the two methods to obtain coating were better than bare superalloy. Both of the two coatings had smaller corrosion rate and smaller corrosion area with the salt spraying test.Heat treatment can increase the hardness of the coatings but decreased the corrosion resistance. Through the XRD analysis, it can be found that the grain of the crystal was refining to 15.42 nm and the hardness increasing to 672.4HV when heat treatment at 200℃. When the heat temperature increased to 400℃, the grain size increased to 47.15 nm. Long time’ heat treatment can cause slight oxidation on the coating surface. The coating can obtain the maximum hardness when heat treatment is 2 h. Fracture of coating surface did not occur when thermal fatigue 300 times to show good thermal performance.