Preparation And Anti-friction Performance Of Self-lubricating Films On The Surface Of Ni-based Cermet

Due to the good wear and corrosion resistance of Ni-based cermet and the excellent anti-friction property of sulfides,complementing their performance is an important scheme for the development of advanced marine equipment by in situ sulphurizing of sulfide film on Ni-based cermet.In this paper,FeS and Cu₂S composite films were prepared by laser cladding and low temperature ion sulfurizing.The nucleation and growth mechanisms of the composite sulfide films on Ni-based cermet were discussed.The self-lubricating mechanism of Ni-based cermet sulfide composite layers under artificial seawater was also studied.The main conclusions in this paper focuse on the following aspects:Ni-based cermet coatings without porosity and micro-cracks were prepared on the surface of 45 steel by laser cladding.The microstructure,the wear resistance and corrosion resistance of Ni-based cermet were investigated.In Ni-based cermet,the major existing form of Fe and Ni is（Fe,Ni）solid solution,and Cr and W exist in the form of carbides.Ni-based cermet shows good corrosion resistance in 3.5%NaCl solution.Owing to the high hardness,Ni-based cermet reduced the adhesion wear and massive spalling of 45 steel,improved the wearability of 45 steel under dry friction by decreasing the weight loss to 43%.The wearing mechanism of Ni-based cermet is plastic deformation and furrowing wear for the substrate,and fatigue flaking of the carbides.The wearability of Ni-based cermet is improved with the lubrication of artificial seawater by decreasing the weight loss to 18%,and the wearing mechanism of Ni-based cermet is uniformity furrowing wear.The reduction of wear of Ni-based cermet is due to the lubrication of artificial seawater,which comes from the formation of water film on friction interface,the reduction of surface contact stresses on Ni-based cermet,taken away of wear debris,and the auxiliary lubrication of the strong adsorptive salts.The effects of working gas,surface nanocrystallization of the substrate,the auxiliary cathodes and sulfurizing temperature on the composition and growth morphology of sulphide films were studied.Because the sputtering effect of Ar on the cathode surface is greater than that of H₂.With the reduction of Ar content in work gases,the content of sputtering metals in the film decreased,the content of S increased,and the growth rate of the film decreased.Ultrasonic peening treatment on Ni-based cermet is helpful for the increasing of the density of sulphide films and the reduction the particle sizes of the sulphide.In the meantime,it has a detrimental influence on the thickning of sulfide film.With the reduction of cavity number,the nucleation centers of sulfides on the surface reduced.Increases in the sulfurizing temperatures could lead to increases in the particle size and S content of sulfide,and the thickness,the surface roughness and the density of sulfide film.Furthermore,aided cathodes made of 45 steel could help to increase the growth rate of sulfide particles,the thickness and density of the film.By optimizing the manufacture technology,FeS composite film with 4μm thickness was prepared by ion sulfurizing at 280 ^oC.The composite films were composed of sulfides（FeS,NiS,FeS₂）and oxides（such as Cr₂O₃）.The nucleation and growth mechanisms of FeS composite film on Ni-based cermet were investigated by phase analysis and calculation of lattice mismatch.The results indicate that Fe atoms could escape from the surface of Ni-based cermet due to the lower sputtering threshold.Then Fe atoms could react with active S atoms to prduce FeS,which could deposite on（Fe,Ni）solid solution owing to a coupled interface between FeS and the solid solution.To reduce the lattice mismatch,the other sulfides could deposit on the surface of FeS to form sulfides islands.Some islands were formed on the surface of（Fe,Ni）solid solution by the adsorption of new particles.The voids were formed on the dendrites because of the incoherent interface with sulfides.The islands were gorwing by absorbing new sulfides and merging each other,and the voids between the islands were filled with sulfides particles based on the volmer-weber mode.Due to the high surface roughness,the surfaces of voids became new nucleation sites.As the sulfides deposited on the sites and continue to grow to sulfide islands,FeS composite films could continue to grow based on the volmer-weber mode.The effects of the auxiliary cathodes on sulfides film were discussed.A novel Cu₂S composite film with 12μm thickness was prepared on Ni-basde cermet at 230 ^oC with the help of the copper cathodes,which was composed of Cu₂S,FeS,NiS,FeS₂ and Cr₂O₃.Both the nucleation and the growth mechanisms of Cu₂S composite film were discussed by the calculations of sputtering yield and thermodynamic computations of the synthetic reaction.The results indicate that a large number of Cu atoms even Cu⁺produced by cathode sputtering because of the highest sputtering yield during the ion sulfurzing.Cu particles adsorbed on the surfaces of Ni-basde cermet,could react with S to form Cu₂S film.Cu₂S granule patterns turned into column and rod at high temperature from sphere at low temperature.The film absorbed the new Cu₂S due to their same lattice parameter.Therefore,the new forming Cu₂S particles were deposited constantly on the former ones to grow into column.With an increase of temperature,the dimensions of Cu₂S particles increased and the density of the film decreased.Cu₂S particles continued to deposit,grew and aggregated into sulfide islands on the space between the columns,and the composite film grew by the compound mode of columnar-island.The wear-resistance of Ni-based cermet sulfides composite coatings,and the anti-friction mechanisms of FeS and Cu₂S composite films were discussed under dry friction,artificial water and artificial seawater.The results indicate that the composite films show good anti-friction performance on Ni-based cermet on these conditions.In contrast with the experiment results of Ni-based cermet under artificial seawater,the wear loss of FeS composite coating dropped by 60%,68%and 80%under the working conditions,the friction coefficient decreased by 5%,22%and 36%respectively.Meanwhile,the wear loss of Cu₂S composite coating dropped by 85%,78%and 82%under the working conditions,the friction coefficient decreased by 76%,30%and 65%respectively.FeS composite film shows good anti-friction effect on Ni-based cermet due to the isolation role between the wear pairs.The carry capacity and anti-friction performance of FeS composite film under seawater were improved,which was benefited from water film lubrication and the isolating of the salts and oxides adsorbed in the film.The protective film composed of paste sediments repaired the cracks in the film resulted from water soak.Because of the auxiliary lubrication of water film,oxides and copper particles formed by cuprous sulfide and oxygen during the process of friction,the harmful impact of water on Cu₂S composite film was alleviated.As Cu₂S composite film is easiest to form transfer film on the surface of the mate parts,the film shows better self-lubrication peformace than FeS composite film under the work conditions.