Study On Microstructures And Properties Of Nb-Mo Coating By Electrospark Deposition

H13 steel is one of the most widely used hot working die steels in the world,but the die surface is often subjected to wear,corrosion,thermal fatigue and other failure defects because of harsh service conditions.In order to extend the service life of the mould and save the production cost,it is an effective solution to improve the performance of H13 steel surface by using surface engineering technology.In this paper,Nb coating,Mo coating and composite coating?Nb-Mo and Mo-Nb?were prepared on H13 steel substrate by electrospark deposition technology with Nb and Mo as electrodes to improve the properties of the substrate surface.The optimal process parameters were searched and the microstructures and properties of the deposited coating obtained under the optimal process parameters were studied.The optimal process parameters of Nb coating were:output voltage 100V,deposition power 1000W,discharge frequency 700Hz,specific deposition time3min/cm².The Nb coating was about 40⁵5?m thickness with a surface roughness of3.0³.6?m.The coating is mainly composed of Fe₂Nb and Fe_0.2Nb_0.8 phases consisting of a large number of microcrystalline and amorphous structures without defects.The microhardness of the coating was 642.0HV which was about 3.2 times higher than that of H13 steel substrate.The wear mass loss was one third of the H13steel substrate in the equal wear conditions.The electrochemical self-corrosion potential of the coating was improved about 113mV.The optimal process parameters of Mo coating were:output voltage 60V,deposition power 1000W,discharge frequency 700Hz,specific deposition time4min/cm².The Mo coating was about 45⁵0?m thickness with a surface roughness of5.0?m containing Fe₇Mo₃ and Fe₂Mo phases.The microhardness of the coating was1369.5HV which was about 6.7 times higher than that of H13 steel substrate.The wear mass loss was one seventh of the H13 steel substrate and the wear scratches were shallow.The electrochemical self-corrosion potential of the coating was improved about 61mV.The Nb-Mo composite coating and the Mo-Nb composite coating were prepared on the surface of H13 steel.Only the sequence of electrode deposition was different between the two coatings.The preparation of composite coating needed to get through two electrospark deposition processes,during which two elements interdiffusion occurred.The surface roughness of the composite coating was large and existed cracks because of the large thermal stress.The Nb-Mo composite coating was about60?m thickness,mainly composed of Fe₇Mo,Fe_0.54Mo_0.73 and Fe₂Nb phases,with a few cracks.The microhardness of the coating was 1229.0HV which was about 6 times higher than that of H13 steel substrate and the wear resistance was significantly improved.The electrochemical self-corrosion potential of the coating was improved about 31mV.The Mo-Nb composite coating was about 30?m thickness with many cracks and stomatal defects.The coating was mainly composed of a large amount of Fe₇Mo and Fe₂Nb phases and a small amount of Fe₂Mo phases.The microhardness was 1102.1HV which was about 5.5 times higher than that of H13 steel substrate and the wear resistance was improved.The electrochemical self-corrosion potential of the coating was improved about 103mV.The microstructures and properties of Nb coating,Mo coating and composite coating were analyzed.The results showed that the microhardness,wear resistance and corrosion resistance of H13 steel surface after electrospark deposition were significantly improved.