Research On The Micro-contact Mechanics Behavior Of Coating-substrate System

With the development of surface modification technology and material science, coatings such as Ti C, Ti N and DLC etc., and surface strengthen layers such as ion inplanting layer, duplex hardening layer etc., have been widely employed to tribo-parts, such as rolling bearings and gears etc., especially for those working under high load, high speed and high tempurature severe performing condition or facing short time outage of luricating oil. However in the design and development of coating according to operating condition, the trial and error method based on experience and test has blineness and randomness, and the application and design of coatings and strengthening layers still lacks a widely recognized design criteria. In order to offer an analysis method and theoretical guidance for the application and design of coatings, the effect of coating and micro rough topography of contact surface is investigated in this thesis. At first, a new numerical simulation method based on random switching system is proposed for generating rough surface as an input to the succeeding micro-contact model. Then elastic point contact model of homogeneous coating, elastic point contact model of graded coating and thermoelastic point contact model of homogeneous coating with heat partition considered are established to study the effect of coating and rough topography of contact surface on the contact behavior.A new simulation method of rough surface based on random swithing system is proposed by combining polar method and Johnson translation system. The deviation of statistical parameters between the modeled rough surface and desired ones is under effective control by using iterative algorithm. And present method overcomes the disadvantage of existing methods that the agreement of autocorrelation function between the modeled surface and the dersired one becomes worse with the increase of the autocorrelation length. Thus the simulation precision is improved. Further more, simulation of real engineering rough surface shows that present method can be adopted to simulate rough surface without an exponential-type autocorrelation function. Thus the range of application is widened.The elastic field soluton in frequency domain of solids coated with a homogeneous coating subjected to surface mechanical loading is deduced by employing Papkovich-Neuber potential function, and the influence factor matrix of displacement and stress related to surface pressure and tangential traction is obtained by using a conversion method based on fast Fourier transform method, then an elastic point contact model between a ball and a homogeneous coating-substrate system is established. The model is verified by comparison with the analytical solution of Hertz contact theory and that of finite element model. With this model, the effect of coating elastic modulus, coating thickness and rough surface on contact pressure and stress in the subsurface is futher investigated.The elastic field solution in frequency domain of solids coated with an exponential-type graded coating subjected to surface mechanical loading is further deduced by using Fourier integral transform, and the influence factor matrix of displacement and stress related to surface normal pressure and tangential traction is obtained with the same conversion method. Then the elastic point contact model of homogeneous coating is extended to an elastic contact model of graded coating. The contact model is verified by comparison with the solution of the contact model of homogeneous coating and that of the finite element model. With the elastic contact model of graded coating, the effect of the elastic modulus on the surface of graded coating and the graded coating thickness on the contact pressure and stress in subsurface is also studied and compared with that of homogeneous coating.Lastly, the effect of frictional heat on the contact behavior of solids coated with homogeneous coating is also considered in this thesis. The temperature rise and thermal elastic field solution in frequency domain of solids coated with a homogeneous coating subjected to surface frictional heat flux is further deduced by applying Fourier integral transform, and the influence factor matrix of temperature rise, thermal displacement and thermal stress is obtained by adopting the conversion mehthod of influence factor matrix. Then a thermoelastic point contact model with frictional heat partition considered is further established by introducing a heat partition model which is based on the continuity of contact inte rface temperature within the real contact area. The model is verified by comparison between its solution of a contact problem without coating and the research result of predecessor. With the thermoelastic contact model, the effect of sliding speed, frictio n coefficient, thermal conductivity of coating,thermal expansion coefficient of coating and rough surface on the contact pressure, temperature rise of contact surface, surface heat flux of coating and stress in subsurface is investigated.The model and method presented in this thesis offer a theoretical basis for the design of coating and strengthening layer on the surface of long-life tribo-parts working under complex and severe serving condition, the state analysis of solid-liquid duplex lubrication and the prediction of life and failure mode shift, etc.