Preparation And Properties Of TaC/Ta Composite Coatings On C17200 Beryllium Copper Alloy

C17200 beryllium copper alloy is a alloy with high-strength,high elasticity,good electrical conductivity as well as certain wear and corrosion resistances under normal conditions.However,its inherent tribological and corrosive properties are not enough to meet the performance requirements in the repeated friction and chemical corrosion environment,and the service life is shortened,which limits applications of this alloy.In this paper,a Ta coating was first prepared on the C17200 surface by double glow plasma surface alloying?DGPSA?technique.The effects of alloying temperature and time on microstructure,composition,bonding strength and tribological properties of Ta coating were studied,and the formation mechanism of Ta coating was explored as well.On the basis of the optimization process of Ta alloying,the typical Ta coating was carburized by microwave plasma chemical vapor deposition?MPCVD?,causing a TaC/Ta composite coating formed on the substrate after Ta-C duplex treatments.The formation mechanism of the typical TaC/Ta composite coating selected by analyzing microstructure and bonding strength of TaC/Ta composite coatings formed under different CH₄ concentrations.The influence of loads on the tribological property and wear mechanism were analyzed.The electrochemical corrosion performances of the typical Ta coating and TaC/Ta composite coating in H₂SO₄ medium were also discussed.Results of Ta alloying show that Ta coating with Ta/Ta-Cu-Be diffusion layer is formed on C17200 alloy surface,and mainly contains?-Ta,Ta₂Be and Cu?Be?solid solution.The coating grows through island-like tissue fusion,and the thickness of Ta coating and Ta-Cu-Be layer increase with alloying time.Ta coatings prepared at 750°C,800°C and 850°C have good bonding strengths with substrate,and the surface hardness are 410.5 HV,442.7 HV and 565.7 HV,respectively,which are higher than substrate hardness?190.60 HV?.The wear resistance of Ta coatings increase due to their lower friction coefficient and specific wear rate than that of the substrate.Especially the Ta coating formed at800°C/2 h,about 8.56?m in thickness,has more compact microstructure and the highest bonding strength with substrate,showing a good comprehensive performance.Results of carburizing show that a continuous TaC/Ta composite coating is formed on C17200 surface after Ta-C duplex treatment.At lower CH₄concentration?8-10%?,the composite coatings consist of TaC,?-Ta,Ta₂Be,Cu?Be?.And with increasing CH₄ concentration?12-16%?,Ta₂C was fomed besides above-mentioned phases.Diamond nucleus is found in the composite coating surface when the CH₄ concentration exceeds 10%,and the diamond grains grow and increase with increasing the CH₄ concentration.The surface TaC/Ta₂C layer and diamond grains cause a high surface hardness?466.09-827.40 HV?which varies in a nonlinear relationship with CH₄concentrations.The TaC/Ta composite coating carburized at 10%CH₄ exhibits the highest combination of surface hardness and bonding strength.Results of tribological study show that under 2-5 N loads,the average friction coefficient of C17200 substrate is higher?0.6-0.64?,and the heavy wear occurs due to severe adhesion and furrow effects.TaC/Ta composite coatings have friction coefficients of 0.19,0.37 and 0.38,as well as specific wear rates of6.46×10^-66 mm^-3·N^-1·m^-1,1.01×10^-55 mm^-3·N^-1·m^-11 and 4.34×10^-55 mm^-3·N^-1·m^-1under loads of 2 N,3 N and 5 N.The composite coating surface is slightly worn with abrasive wear,and the tribological properties are further improved compared with Ta coating.The excellent antifriction and wear resistance of TaC/Ta composite coatings are mainly related to the hard TaC surface and diamond grains.And the Ta coating beneath the TaC layer can decrease the coating stress between the hard top layer and soft substrate,and also supports loads to reduce wear.Results of electrochemical corrosion study show that C17200 substrate is seriously corroded in 10%and 20%H₂SO₄ solutions to produce copper oxide in corrode surface.Compared with the substrate,both Ta coating and TaC/Ta coating have higher corrosion potential and lower corrosion current density?1-3orders of magnitude?,indicating the smaller corrosion tendency and corrosion rate.The larger electrode resistance values for both coatings also mean the slower electrode process and lower corrosion rate.When with the concentration of corrosive medium increasing,the passivation phenomenon of substrate,Ta coating and TaC/Ta composite coating become obvious,and the passivation film is thickened.With respect to Ta coatings and TaC/Ta composite coatings,the stable and dense surface Ta₂O₅ film protects the coatings from eroding,leading an improved corrosion resistance.