Study On Preparation And Properties Of Nanocrystalline Ni-based Alloy Coating

Nanocrystalline Ni-based films on the mild steels were prepared by electricplating with NiSO4 as the main salt. Two kinds of alloy Ni-Fe alloy and Ni-Cr alloy were prepared with different parameters of electrode-position process. The properties of the alloys were tested and analyzed.The result showed that, the optimal processing parameters for Ni-Fe alloy is when the electroplating current density is about 7A/dm2, the bath temperature is about 60?; For Ni-Cr alloy is when the electroplating current density is about 20A/dm2, the bath temperature is about 30?, the pH value is about 2.5. The maximum micro hardness of nanocrystalline Ni-Fe alloy can reach 600 HV. The minimum coefficient of friction is 0.65 under the load of 3N. The samples prepared under the optimum process parameters have good corrosion resistance in 3.5% NaCl solution, and the self-corrosion potential is-0.78 V.The surface morphology(SEM) of electrodeposition Ni-Cr alloys showed a large number of micro cracks appearing on the surface of the alloys. The maximum micro hardness of Ni-Cr alloys can reach 668 HV. The minimum coefficient of friction is 0.37 under the load of 1N. As large numbers of micro cracks on the surface of the alloy coating, the corrosion resistance of the alloy material in 3.5% NaCl solution is poor, and the selfcorrosion potential is-1.0V. Through the TEM test analysis of nanocrystalline Ni-Fe alloy material, the grain size was grown with the heat treatment temperature increased. When the temperature reach 300?, the grain size appears a mutation, and the average grain size is above 200 nm. Uniaxial tensile test on the heat treatment of nanocrystalline Ni-Fe alloy material showed that when the heat treatment temperature below 250?, The tensile strength of the alloy has little change, Maximum tensile strength above 1700MPa; when the heat treatment temperature reach 300?, the tensile strength decreases sharply to1050 MPa. This is mainly due to the rapid growth of the grain of the alloy at high temperature.