Investigation Of Electroless Plating Ni-P, Ni-P-MoS₂Composite Coating And Their Wear Performance On The Surface Of Titanium Alloy

In this paper, the technology formula of electroless plating Ni-P coating and Ni-P-MoS₂composite coating on titanium alloy surface were obtained, the methods of improving electroless plating adhesive strength were explored, and meanwhile the influences of solution constituents and process parameters on deposition rate, phosphorus content of coating and MoS₂content of composite coating were researched. The morphology, composition, structure of Ni-P and Ni-P-MoS₂composite coatings were analyzed. The co-deposition mechanism of MoS₂particles with Ni-P was discussed. Furthermore, the micro-hardness, the friction and wear performance of Ni-P and Ni-P-MoS₂composite coatings were measured.The wear mechanisms of two kinds of coating were discussed.The results show that reasonable selection of solution constituents and technology parameters of electroless plating can improve deposition rate and control the phosphorus content of the coating. The highest and the lowest phosphorus content obtained in Ni-P coating were respectively11.76wt%and3.25wt%by changing content of NaH₂PO₂H₂O, complexing agent and pH value.The technology formula of electroless plating Ni-P coating was NiSO₄6H₂O0.1mol/L, NaH₂PO₂H₂O0.25mol/L, Na₃C₆H₅O₇2H₂O0.05mol/L, C₃H₆O₃0.2mol/L, C₄H₆O₅0.05mol/L, KIO₃0.5mg/L， CH₄N₂S1.0mg/L, pH5.0, temperature90℃，time60⁹0min. Improved double zincating process operated simply and had good activation effect. In addition, double zincating can increase the adhesive strength in the coating/substrate interface, which was more than20N. After heat treatment at600℃, adhesive strength of coating reached35N.Wear resistance of Ni-P coating was better than that of TC4titanium alloy, the hardnes was665HV, wear loss was2.27mg and the lowest friction coefficient was0.35of the coating as plated, while the hardness, wear loss and friction coefficient of TC4titanium alloy substrate were respectively378HV,9.6mg and0.9. With the increase in phosphorus content of coating, wear resistance decreased. Wear resistance of coating with phosphorus content3.25wt%was the best. Heat treatment can improve the hardness and wear resistance of Ni-P coating. The optimum heat treatment temperature of Ni-P coating with high, medium, low phosphorus content in order to get the best wear resistance were respectively600℃,500℃and500℃. The wear resistance of Ni-P coating decreased with the increase in wear load. The wear mechanisms of Ni-P coating were mainly abrasive, adhesive and oxidative wear, even Ni-P coating with high phosphorus content showed fatigue wear mechanism under high load.On the basis of technology formula of electroless plating Ni-P coating, better self-lubricity composite coating was gained by adding cationic （A1）0.25g/L and non-ionic（C1）3ml/L surfactants into solution. Deposition mechanism of MoS2particles in Ni-P-MoS₂composite coating was coaction of mechanical and electrochemical mechanism, and the main mechanism was mechanical function. MoS₂content in Ni-P-MoS2composite coating was dependent on temperature, pH value, stirring rate and MoS2content in solution. The composite coating with maxium phosphorus content of10.46wt%can be prepared. Electroless plating Ni-P-MoS₂composite coating had lower friction coefficient and better self-lubricity. Friction coefficient of Ni-P-MoS₂composite coating decreased when there was a fall of MoS₂content of coating, indicating that wear resistance of Ni-P-MoS₂composite coating increased with the decrease of MoS₂content of coating. For composite coating with phosphorus content of10.46wt%, friction coefficient is the lowest0.05and wear loss is0.06mg, revealing the best wear resistance. Hardness and wear resistance of composite coating after heat treatment at400℃for1hour were increased, at the same time friction coefficient was further reduced. With the rise of wear load, friction coefficient of Ni-P-MoS₂composite coating increased, that is, wear resistance decreased. The wear mechanism of Ni-P-MoS2composite coating was mainly abrasive wear, but adhesive wear presented in the condition of high load.