Preparation Of Ni-Al Gradient Coating On The Surface Of Pure Cu

In this paper,CuNi solid solution/Ni/NiAl intermetallic compounds gradient coatings are prepared on the surface of pure copper by combining Ni plating with Al powder pack cementation method. Previous studies found that there are some problems to be further explored:many pores appears at Cu-Ni interface; what is the critical condition under which Ni2Al3 phase change to NiAl phase; what is the formation mechanism of Al films; how to design coating structure and achieve it,such as the total thickness of the coating and the thickness of each layer. Based on preliminary studies, we are trying to solve the problem.The surface morphology, phase structure, concentration distribution in cross section and the friction coefficient of the coatings are measured and analyzed by field emission scanning electron microcopy(SEM), energy dispersion spectrum(EDS), X-ray diffraction (XRD). The hardness, high temperature oxidation resistance, wear resistance and thermal shock resistance are measured. Further explore the preparation process,we have made some results.Nickel coatings are electrodeposited on the surface of copper. The effects of current density, pH value and additive saccharin on the deposition rates, the surface morphologies, the hardness,wear resistance of nickel coatings and Cu-Ni diffusion,Ni-Al diffusion in subsequent aluminizing process are studied. The results show that the enhancement of current density, decreases of pH value and the adding of saccharin can accelerate the deposition rates,but only the enhancement of current density can make the grains finer. The preferred orientation (111) of nickel coatings are conducive to form at high current density and low pH value and the preferred orientation (200)of nickel coatings are conducive to form when adding saccharin. The small grain sizes and the preferred orientation (200) is conducive to Cu-Ni diffusion and Ni-Al diffusion,but the loose structure of nickel coatings are not good to Cu-Ni diffusion.The temperature of heat treatment and the treating time on the rate of Cu-Ni diffusion, the distribution of component and reason for formation of pore at Cu-Ni interface are studied. The results show that the increase of temperature of heat treatment can increase the rate of Cu-Ni diffusion, however, the rate of Cu-Ni diffusion decreases with time. The pores are caused by kirkendall effect(the rate of Cu diffusion is greater than Ni), To reduce the formation of Cu-Ni interface pores, the Ni coatings with high density, less coating defects should be prepared to reduce the rate of Cu diffusion.The effects of aluminizing process on the rate of Ni-Al diffusion and the distribution of component are studied, and analyze the process and mechanism of the formation of aluminized layer. The results show that the increase of temperature of heat treatment(600℃-750℃) can increase the rate of Ni-Al diffusion, however, the rate of Cu-Ni diffusion decreases with time(1-3h), similar to Cu-Ni diffusion.Adding of CeO2 can make the grains finer and the surface of aluminized layer smooth,but there are large voids in aluminized layer.And the adding of CeO2 decrease the rate of Cu-Ni diffusion. The effect of CeO2 is complex. The phase of aluminized layer (Ni2Al3 did not change with different aluminizing parameters.Phase change of Ni2Al3 in the oxidation process(850℃-1000℃) is studied. The results show that the Ni2Al3priority change to Ni3Al at low temperature, gradually transformed to NiAl with time, the higher the temperature, The shorter the time required.NiAl is more stable than Ni3Al at high temperature. Ni2Al3 will eventually transformed into NiAl.The infiltration process and mechanism of the formation of the Al layer are analyzed. The total thickness of the coating≈the thickness of nickel plating layer+ the thickness of aluminized layer.Based on the previous studies, the total thickness about 50um of the Cu/Ni solid solution/Ni/Ni2Al3 gradient coatings are prepared at a current density of 0.6A/dm2, bath pH of 5.5, without any chemical additives, and aluminized diffusion lh under temperature 650℃,And comprehensive performance of the coatings are tested. The results show that the hardness is 900 HV0.05, weight gain of Ni2Al3 coating at 800℃ for 60h is 1.156mg·cm-2, average coefficient of friction is 0.477, the thermal shock at 500℃ and 700℃ all reached more than 20 times.