Influence Of Rare Earth On Microstructure And Frictional Wear Properties Of TiC_x Reinforced Ti-based Laser Cladding Coatings

The Ti6Al4V possesses many remarkable properties such as light weight and high strength,excellent corrosion resistance and superior biocompatibility,which is widely applied to aerospace,marine engineering,biomedical engineering,and other fields.However,the popularization and application of Ti6Al4V under friction conditions is seriously restricted duo to its low surface hardness and poor wear resistance.Laser cladding is an efficient surface modification technology,which has been widely used in surface modification and repair in recent years.In order to improve the surface hardness and wear resistance of Ti6Al4V,the Ti6Al4V/Ni Cr-Cr₃C₂/CeO₂mixed powders were cladding on the its surface by multi-track overlapping and coaxial powder feeding technology.The influence of CeO₂ content on the microstructure and friction and wear properties of the composite coatings were studied synthetically.All these investigations have important scientific research and engineering application values,which provide scientific support for the optimization of laser cladding Ni Cr-Cr₃C₂ material system on Ti6Al4V surface,and laid a theoretical and technical foundation for the application and promotion of the wear-resistant coating.The major work is as follows:（1）The influence rule of CeO₂ amount on the forming quality of composite cladding coating was studied.The addition of CeO₂ successfully inhibits the cracking and reduce porosity of the composite coating.When 2 wt.%CeO₂ is added,the surface cracks of the coating are eliminated,and the porosity of the coating is the least（1.26%）when the addition of CeO₂is 3 wt.%.In addition,CeO₂ can intensify the dilution of substrate to the composite coating,and the dilution effect is most severe when 2 wt.%or 3 wt.%CeO₂ is added,which with a dilution rate of 54%.In particular,the variation of the thickness of the composite coating is basically consistent with the variation of the dilution rate.（2）The influencing mechanism of CeO₂ amount on microstructure of the composite cladding coating was clarified.The main phase of the coating is non-stoichiometric titanium carbide（Ti₂C,reinforcement phase）andβsolid solution（Cr Ti₄,matrix phase）.The addition of CeO₂ does not affect the phase types of the composite cladding coating,but CeO₂ and a small amount of Ce₂O₃ can be introduced.And the rare earth oxides are mainly distributed at the phase boundary between Ti₂C and Cr Ti₄.The Ti₂C morphology of each micro-zone in the coating is significantly different.The cladding zone,transitional and bonding zone are developed dendrite,granular and short needle,respectively.The content of C element in primary dendrite is higher than that in secondary dendrite.Elements Ni and Cr is segregated in the matrix.The relative content of dendritic titanium carbide in the coating reached the maximum（55%）when CeO₂ was not added.With the addition of CeO₂,the content of dendritic titanium carbide in the coating first decreased and then increased,and the minimum is 35%when 3 wt.%CeO₂ was added.（3）The influence law of CeO₂ amount on wear resistance of composite cladding layer was revealed.The surface hardness and wear resistance are significantly improved by Ti₂C reinforced Cr Ti₄ matrix composite coating compared with the Ti6Al4V substrate.The hardness of the coating decreases with the addition of CeO₂,and the hardness of the coating without CeO₂ is the highest(488.2 HV_0.5,1.28 times that of the substrate),The addition of CeO₂ results in a slight decrease in coating hardness.Similarly,the wear resistance of the coating without CeO₂ is also the best,which is 22.3 times than the substrate.Among the composite coatings with different CeO₂ amount,the hardness distribution of 2 wt.%CeO₂ coating is the most uniform,and the friction coefficient is the lowest.Thus,considering the forming quality and wear resistance comprehensively,the high temperature friction and wear test of the coating was carried out the coating.The results show that the composite coating has good tribological properties at high temperature,the wear rate of the coating at 300℃and 600℃is only 27.1%and 0.24%of that at ambient temperature.