High-speed Laser Cladding Ni-coated c-BN Particles Reinforced Wear-resistant Composite Coating and its Wear Behavior
Titanium alloys are widely used in aerospace and marine industry because of their low density and high specific strengths.However,their low hardness and poor wear resistance limit their application under various working conditions.Coating is an effective and rapid surface modification method that can improve the wear resistance of titanium alloys under extreme service conditions.In this study,a complete and dense c-BN(cubic boron nitride)composite wear-resistant coating was successfully applied on the surface of titanium alloy using a novel ultra-high-speed laser cladding method.To enhance the mobility of the c-BN ceramic powder,it was coated with Ni via electroless plating,and the effects of Ni coating on the microstructure and wear performance of the coating were comparatively studied.The microstructure,phase composition,and mechanical properties of the coatings were analyzed using an XRD/SEM/EDS microhardness tester and a friction wear tester.The results showed that the c-BN particles partially decomposed under laser irradiation and reacted with Ti in the molten pool to generate TiN,TiB,and other reinforced phases in-situ.In addition,in c-BN(Ni)coatings,the Ni coating on the c-BN surface melted under laser irradiation and entered the Ti lattice or partially reacted with Ti to generate Ti-Ni intermetallic compounds in the molten pool during solidification.This strengthened the solid solution and dispersion of the coating.Thus,the microhardness of the c-BN(Ni)coatings was significantly improved compared to that of the titanium alloy and c-BN coatings without Ni coating.Furthermore,the average microhardness of this coating was more than 1 200 HV0.3,which is more than three times that of the titanium alloy.After etching,for both the c-BN coatings,the top of the coating was mainly TiN,and the bottom of the coating was fine TiB;furthermore,the middle of the coating contained mix of TiN and TiB.The results of friction wear tests using Si3N4 as a counter-abrasive pair showed that these reinforced phases produced an intense counter-abrasive wear,resulting in a high coefficient of friction for the c-BN(Ni)coating.However,a higher microhardness significantly improves the wear resistance.Under the same conditions,the width and thickness of the wear profile of the c-BN(Ni)coating were significantly reduced,and its wear volume was reduced by 72％and 22％compared with those of the titanium alloy and uncoated Ni c-BN coating,respectively.The SEM morphology of the wear surfaces showed an obvious adhesive wear on the titanium alloy,and the c-BN coating without an Ni coating exhibited abrasive wear;furthermore,the c-BN(Ni)coating showed a large number of fish-scale like wear scars,owing to the large number of debris generated by the intense grinding of Si3N4 and hard phase in the coating.The morphology of the titanium alloy showed mainly small-and large-sized debris,whereas those of the two c-BN coatings showed small-sized debris.In this study,the c-BN particles were coated by Ni electroless plating before cladding;thus,the mobility of the c-BN ceramic powder improved during the deposition process,and the microhardness of the composite coatings increased owing to solid solution strengthening and dispersion strengthening of Ni,which led to composite coatings with an excellent wear resistance.
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