Multi-scale Mechanical Properties Of Mo Gradient Modified Layer Of Titanium Alloy

Titanium and titanium alloys have the advantages of low density,strong corrosion resistance,high strength and good biocompatibility.They are widely used in the manufacturing fields of automobiles,ships and biology.However,due to the strong chemical affinity,surface tribological adhesion failure often occurs in the application of titanium alloy as parts.The surface mechanical properties of titanium and titanium alloy can be improved by surface modification.However,due to the low hardness of titanium and titanium alloy,the conventional wear resistant and anti-friction hard coating is not applicable,and due to the problem of soft and hard bearing coordination,it is easy to cause the surface modification layer cracking and peeling off.The composite gradient modification layer can be formed on the surface of titanium and titanium alloy by double-glow plasma surface alloying technology,and the alloy elements can be combined with matrix metallurgy,which can satisfy the surface tribological behavior modification and the surface-based coordination deformation requirements under the service condition of high stress load.The mechanical properties of the gradient modified layer vary with the depth,which is different from that of the homogeneous coating.It is difficult to study the mechanical properties of the gradient modified layer quantitatively,so the mechanical properties of the gradient modified layer are mostly analyzed qualitatively at macro perspective.As a common calculation and analysis method,the finite element method can be used to simplify the complex model into units,so as to realize the simulation of static and dynamic mechanics test.It is of great significance for the development and practical application of thin film materials to quantitatively study the contact mechanics behavior of the gradient modified layer so as to realize the quantitative design of the gradient modified layer and to improve the process.In this paper,the Mo gradient modified layer was prepared on the surface of titanium and titanium alloy by double glow plasma surface alloying technology.The failure behavior of the gradient modified layer at multiple scales was analyzed,and the mechanical properties of the Mo gradient modified layer were quantitatively studied.The main research contents and results are as follows:(1)Conduct micro/nano indentation test on Mo gradient modified layer on the surface of pure titanium.The results show that good connection can be achieved when the depth of indentation is small,which indicates the accuracy of the micro indentation results.Conduct small energy multi impact test on Mo modified layer.The annular crack appeared first,then the radial crack,until the modified layer was peeled off.Combing nano indentation test with finite element inverse analysis.A linear simplified model is proposed to solve the plastic performance parameters of the gradient modified layer.(2)The finite element model is used to quantitative analysis the process of micro indentation and small energy multi impact test.The results show that the failure model of multi impact experiment is cohesion failure.During the loading process,there is no obvious stress steo between each sublayer.It indicated that the Mo diffusion layer can effectively coordinate the deformation of Mo deposition layer and Ti substrate.(3)Combing nano indentation test on the surface of Mo gradient modified layer of TC4 and the linear simplified model,the elastoplastic performance parameters of the modified layer were obtained.The multi impact test on the surface Mo gradient modified layer of TC4 shows that the main reason of multi-impact test failure is fatigue.The stress/strain distribution of Mo modified TC4 during crack generation and further crack propagation was determined by finite element simulation.Based on the above results and analysis,a set of systematic quantitative methods for studying the static and dynamic mechanical properties of the modified layer are summarized.(4)The dynamic mechanical properties of the Mo gradient modified layer at the micro angle were further studied by Nano Test technology.The results show that,due to the high hardness of the Mo deposit layer,when the indentation depth is larger than the Mo deposit layer but smaller than the Mo diffusion layer,the fatigue failure of the modified layer does not occur.It indicated that the Mo diffusion layer plays a coordinating deformation role in the bearing process.