Study On Ion Nitriding Of Ti-Cu-Al Coating On Cold Deformed TC4 Surface

TC4 titanium alloy is widely used in marine,shipbuilding,aerospace and other fields because of its high specific strength,low density,and excellent corrosion resistance.However,the applications of TC4 alloy are limited by the low surface hardness（290～375HV）,poor wear resistance and high temperature oxidation resistance,etc.Therefore,domestic and foreign scholars have conducted a lot of academic research on the surface strengthening of TC4 titanium alloy,and plasma nitriding is currently the most widely used surface strengthening method.However,the traditional plasma nitriding temperature is high（700°C～1000°C）,and the formed nitride layer is brittle;the bonding strength is low.Reducing the nitriding temperature has become an urgent problem to be solved.In the nitriding process,the diffusion activation energy required for nitrogen atoms at grain boundaries and dislocations is smaller than that in the lattice,and they tend to diffuse along grain boundaries and dislocation channels.Therefore,rolling TC4 titanium alloy can lower the nitriding temperature and realize low-temperature nitriding.However,at the same time,due to the low nitriding temperature,the nitrided layer obtained by low-temperature nitriding is thin and the hardness change gradient is large.The multi-layer structure design on the surface can effectively improve the above problems.The soft layer can improve the adhesion between the nitride layer and the substrate and decrease the brittleness of the nitride layer.Meanwhile,the hard layer can increase the strength of the nitride layer.The combination of soft film and hard film can balance the relationship between the thickness,strength and plasticity of the nitride layer.Therefore,the preparation of nitrided layers with gradient changes in hardness has important theoretical research significance.In this paper,theoretical calculations and experiments are used to study the hardness gradient of composite low-temperature nitriding with Al-Cu-Ti multilayer coating on the surface of rolled TC4 titanium alloy.First,through thermodynamic calculations and first-principles calculations,predicting all possible phases in the Al-Cu-Ti-N composite modified layer,and then using magnetron sputtering to prepare Al-Cu-Ti multilayer film on the surface of the rolled TC4 titanium alloy.Finally,low-temperature plasma nitriding is performed to obtain an Al-Cu-Ti-N composite modified layer.The detailed microstructure characterization and mechanical performance characterization of the composite modified layer were carried out.After nitriding at 450℃,the surface hardness of the composite modified can reach948HV_0.1,which is 2.7 times that of the rolled TC4 titanium alloy matrix.Thermodynamic calculations and first-principles calculations show that the possible compounds in the Al-Cu-Ti-N composite modified layer include Ti N,Ti₂N,Cu₃Ti₂,Cu₄Ti₃,Cu₄Ti,Cu₂Ti,Cu₃Ti,Cu Ti,Cu Ti₂ andθ（Al₂Cu）,η2(Al_0.48Cu_0.52),ζ(Al₉Cu₁₁),γ1（Al₄Cu₉）,δ1（Al₅Cu₈）,α2(Al_0.23Cu_0.77),and the experimental results show that the Al-Cu-Ti-N composite modified layer.The stable compounds are mainly Ti₂N,Al₄Cu₉,Cu Ti₃.The phase compositions of Ti₂N and Al₄Cu₉ are consistent with the theoretical calculation.Ti-N phase hardness>Cu-Ti phase hardness>Al-Cu phase hardness.Therefore,the coating sequence of the Al-Cu-Ti multilayer film is:Al→Cu→Ti.The deposition rates of magnetron sputtering Al,Cu,and Ti single-layer films are3.17μm/h,6.41μm/h,and 2.33μm/h,respectively.In the Al-Cu-Ti multilayer film,the layers are tightly combined,the thickness is uniform,and the film quality is good.The phase composition of the modified layer after plasma nitridation is Ti₂N,Al₄Cu₉,Cu Ti₃,β-Ti and Cu.As the nitriding temperature increases,the surface N content and the interdiffusion between Al,Cu,Ti,and N increases.The mechanical performance results show that the formation of Ti₂N in the composite modified layer can significantly increase the surface hardness.After nitriding at 450°C for 4h,the surface hardness reached the maximum,948HV_0.1,which was 2.7 times that of the rolled TC4 titanium alloy matrix.The section hardness has a certain gradient distribution,which is consistent with the hardness gradient change trend of the theoretical calculation design.The wear rate is lower than that of the rolled TC4 titanium alloy matrix,wear resistance is improved.The wear rate of the composite modified layer obtained by nitriding at 500°C for 4hours is the lowest,which is 0.25×10^-7gr^-1N^-1,which is 90%lower than that of the rolled TC4titanium alloy matrix.The wear mechanism gradually changes from abrasive wear to adhesive wear.