Research On Formation And Effecting Mechanisms Of Friction Layer Of Ni₃Al Matrix Self-lubricating Composites

Solid self-lubrication is one of the most effective way to achieve good friction state under dry friction and harsh conditions such as high temperature,overload,high vacuum and strong radiation.As the direct contact part during the sliding process,the friction layer determines the self-lubricating performance and tribological property of the self-lubricating composites.But in the actual friction process,a variety of processes exist in friction layer,such as plastic deformation,work hardening,grain refinement,oxidation,material transfer,mechanical mixing and phase change,etc.,resulting in the lack of systematic research of the friction layer.In this regard,the paper selects the Ni3 Al matrix self-lubricating composites(NMCs)as the research object,the friction layer with micro/nano structures and friction characteristics are investigated.Meanwhile,the friction layer formation mechanism,the friction reduction and anti-wear mechanisms also are investigated.This thesis seeks to obtain the cognition of tribological properties of NMCs and the micro/nano structure of the friction layer.In order to investigate the formation and effecting mechanisms of the friction layer of NMCs,several kinds of lubricants,such as soft metal(Ag),layered lubricants(Mo S2,Ti3 Si C2,graphene nanoplates(GNPs)),metallic oxide(Zn O),which are commonly used in alloy,are chosen to explore the effects of various types of lubricants on the microstructure of the friction layer.The research shows that the formation of the friction layer plays an important role in the tribological properties of the self lubricating materials,and can improve the lubricating performance and wear resistance of the friction surface.Based on the improvement of the lubricity and the wear resistance of the NMCs after formation of the friction layer in dry sliding process,the microstructural evolution of the friction layer in the friction and wear process is analyzed in view of the transition of the tribological properties of NMCs.Moreover,the formation mechanism and microstructure evolution of friction layer in sliding process is discussed.The research results show that one of the main factors of low friction and wear is the formation of a specific structure of the friction layer,containing the soft lubricating layer(0.6 GPa)supported by the harder grain refinement layer(7.4 GPa).In order to further reveal the formation mechanism of the friction layer which can result in the improvement of the lubricity and the wear resistance,the effect of the friction layer on the equivalent stress distribution of material and internal equivalent stress state of friction layers are investigated by finite element analysis.In addition,the effect of grain refinement on the mechanical properties of the friction layer is analyzed based on the Kocks-Mecking-Estrin model.A model of the relationship between grain refinement and mechanical properties of the friction layer is established.The intrinsic relationship of grain refinement,mechanical properties and tribological properties of the friction layer is analyzed by combining experimental works with theoretical model.The results show that grain refinement has significantly improvement on yield strength of the friction layer,leading to the increase of the wear resistance.Meanwhile,grain refinement leads to the reduction of micro pores,uniform lubrication phase on the worn surface,and good adhesivity on the interface of the lubricating layer and the grain refinement layer,resulting in the increase of overall lubrication performance.In addition,the friction layer has a significant reduction of the equivalent stress in the substrate,which is beneficial to the reduction of the internal stress concentration and the prevention of internal micro cracks,improving the stability of the friction layer and the matrix structures.Finally,in order to investigate the dynamic lubricating performance of the friction layer during dry sliding process,the effects of the important parameters on the friction layer stability and tribological properties of self-lubricating material are analyzed using the method of experiment works combined with mathematical model involving the frictional heat.The local crack propagation of the friction layer and the relationship between the local failure and the lubricating performance are discussed.During sliding contact process,the increase of elastic modulus of the contact surface and the decrease of effective contact area are significant to the reduction of friction coefficient of NMCs.In addition,the crack propagation could occur in the friction layer when the contact surface stress value reaches a critical stress condition,resulting in the increase of friction coefficient.