Research On Design,Preparation And Self-lubricating Mechanisms Of Friction Interface Structure Of Ni₃Al Matrix Self-lubricating Composites

Most mechanical systems have friction and wear problems during contact.Solid self-lubrication is one of the effective methods to achieve good friction state under extreme service conditions such as high temperature,high load and high vacuum.As the contact part of solid self-lubricating material during sliding process,the friction interface layer has a direct effect on the friction and wear resistance of the material.However,during the actual friction process,the formation and evolution of the friction interface layer include many complicated processes,such as plastic deformation,work hardening,grain refinement,oxidation reaction,etc.The formation of friction interface layer with excellent antifriction and wear resistance is limited by many factors,such as loading condition and friction time.Based on the formation,friction-reducing and wear resistance mechanisms of friction interface layer of Ni₃Al based self-lubricating material containing graphene nanoplatelets?Ni₃Al-GNPs?,a self-lubricating material with multilayer friction interface structure is prepared by laser melt deposition in order to achieve effective control of the composition,structure of each layer and properties of the friction interface structure of Ni₃Al matrix self-lubricating materials?Ni₃Al-FIS?.The systematic understandings of design and preparation methods,as well as self-lubrication mechanism of multilayer friction interface structure are obtained.The finding are as follows:?1?The laser scanning rate is chosen to study the effects of laser forming technology on the microstructure and tribological properties of Ni₃Al based self-lubricating materials.The microstructure,tribological performances and wear mechanism of Ni₃Al based self-lubricating materials prepared by laser melt deposition at different laser scanning velocities are systematically analyzed.The study shows that laser scanning rate has an important effect on the microstructure and tribological properties of Ni₃Al-based self-lubricating materials.The microstructure,mechanical properties and tribological performances of the samples can be effectively controlled by optimizing the laser scanning velocity.?2?The relationships between the evolutions of microstructure,properties and composition of Ni₃Al-GNPs and its friction and wear behavior are systematically studied.The evolution laws of the friction interface layer microstructure at different wear stages,as well as the systematic and time-varying characteristics of friction and wear behavior are analyzed.The study shows that Ni₃Al-GNPs exhibits excellent tribological performance along with the formation of 5-10?m friction interface layer after sliding friction for a certain time under the optimal working condition.The friction and wear behaviors of Ni₃Al-GNPs show a certain dependence of working condition and friction time.The material subsurface layer with high hardness of nanocrystalline structure can effectively support the lubricating layer rich in graphene and oxides.This kind of friction interface structure characteristic is helpful for Ni₃Al-GNPs to exhibit excellent antifriction and wear resistance during friction and wear process.?3?In order to further reveal the enhancing mechanism of antifriction and wear resistance of Ni₃Al-GNPs from the initial wear stage to stable wear stage,the enhancement effect and mechanism of grain boundary strengthening and dislocation strengthening in improving the mechanical and tribological properties of Ni₃Al-GNPs are systematically analyzed.The results show that grain boundary strengthening is the main strengthening mechanism of friction interface layer of Ni₃Al-GNPs.Dislocation enhancement is secondary reinforcement mechanism.In addition,aiming at the inner grain refinement of friction interfacial layer,the relationship between the grain refinement in friction interface layer,as well as the improvement of mechanical properties,friction reduction and wear resistance are analyzed based on Kocks-Mecking-Estrin model.The results of experimental analysis and theoretical modeling show that the mechanical properties of the friction interfacial layer increase with the decrease of grain size,and the wear resistance of whole material is obviously improved,which is helpful to improving the stability of the matrix and the friction interface layer structure.?4?Based on the formation mechanism,as well as friction reduction and wear resistance mechanism of friction interface layer,Ni₃Al based self-lubricating materials with multilayer friction interface structure?Ni₃Al-FIS?are prepared by laser melt deposition.The thickness of each component layers of Ni₃Al-FIS are optimized using orthogonal tests.Based on the finite element analysis,the influence of thickness difference on the distribution of equivalent stress at the joint of each layer is analyzed.The results show that the equivalent stress at the interface between the lubricating layer and the middle layer decreases with the increase of the thickness of the lubrication layer.The rationality of structural parameter selection of the friction interface is verified by the finite element modeling and friction experiments.Compared with Ni3Al-GNPs,Ni3Al-FIS can enter the stable wear stage with better lubricating performance in a short time under different loading conditions,and its tribological performance is less affected by the change of working conditions.The tribological behaviors of Ni3Al-FIS and Ni3Al-GNPs are similar under the optimal working conditions.The tribological performance of Ni₃Al-FIS compared with Ni₃Al-GNPs is better under other test conditions.It indicates that the multilayer interface structure of Ni₃Al-FIS can reduce the dependence of friction and wear behavior on friction time and test condition to a certain extent,which better exerts the antifriction and wear resistance of the material under different test conditions.?5?In order to monitor the lubrication performance of Ni₃Al-FIS under different test conditions and the wear state during sliding friction,the variation laws of wear debris size and worn surface characteristic parameters of friction pair with wear process of Ni₃Al-FIS,as well as the the mapping relationship between wear rates and characteristic parameters in different wear stages are analyzed.The wear rate prediction model of Ni₃Al-FIS is established by pattern recognition of wear state characteristic parameters based on Elman neural network.The results show that the wear rates of Ni₃Al-FIS after sliding friction for a certain time can be effectively evaluated by predicting the wear rates of Ni₃Al-FIS under different test conditions.In this thesis,the evolution laws of microstructure,antifriction and wear resistance characteristics of friction interface layer are systematically analyzed,and the systematic and time-varying characteristics of friction and wear behaviors are deeply expounded.The friction and wear behaviors of self-lubricating materials are effectively controlled by the design and preparation of friction interface structure,realizing the excellent antifriction and wear resistance characteristics under different working conditions.The research results have important reference value and guiding significance for tribological design and reliability improvement of metal-based self-lubricating materials.