Studies Of Damage And Crack Evolutions As Well As Tribological Characteristics Of Solid Lubricant Coatings

The progressive surface engineering as one of the key elements for the development of modern industry has become a frontier technology of the advanced manufacturing,and to be an important forward position for catching up with high-end level in the world.The growth of surface engineering heavily promotes the progress of solid lubricant coating.Forming solid lubricant coatings with excellent performance of anti-friction and anti-wear on surfaces of friction pairs can prolong their service life,improve mechanical efficiencies,and save materials.Currently,solid lubricant coatings have been widely applied in the modern industrial technologies,such as engines,and mechanical transmission,etc.,and can also achieve enormous economic value and social effect.The huge compressive residual stress caused by the mismatch of coating and substrate structures and preparation process is to serve as a catalyst for coating-substrate system failures including coating cracking and interfacial delamination.The theoretical study of this issue is urgently intensified,which has significant guidance for the preparation of solid lubricant coating.Moreover,how to characterize and evaluate problems of interfacial delamination and coating cracking has become a barrier to design solid lubricant coating with perfect durability and reliability.The fundamental theoretical research of this conundrum should be pressingly implemented,which has great significance in prediction and assessment of coating failures.Additionally,in order to avoid failures of the coating-substrate system and enhance the carrying capacity of the friction couple,it is essential to investigate the tribological characteristics of solid lubricant coatings under lubricated conditions and make proper designs of coating parameters and lubrication.It possesses remarkable academic significance and practical value for exploiting the application of coating tribology.To figure out the above scientific issues of damage and crack evolutions as well as tribological characteristics of solid lubricant coatings,the present thesis has a comprehensive study for the system consisting of physical vapor deposition coating and steel substrate.The prime contents and achievements are summarized as follows.(1)In terms of the failure problem of interfacial delamination and coating cracking in the coating-substrate system that may arise under residual stress,the thesis establishes residual stress model taking rough surfaces into account.The risk of failure of residually stressed coating-substrate systems is evaluated by using the extended finite element method,J integral theory,and cohesive zone model.It is found that rough surfaces have great impacts on both coating residual stress and substrate plasticity.One can see that the interfacial failure is more sensitive to the shear traction delamination than to the normal one.For the puzzle of multi-crack,two closely spaced crack tips are vulnerable to coalesce during the propagation process.Additionally,in the light of mechanics,it is demonstrated that selecting an interlayer which has moderate coefficient of thermal expansion is effective in failure protection for the entire system.(2)Regarding the problem of how to evaluate interfacial delamination and coating cracking,the thesis characterizes the system failures by means of nanoindentation.Then,the finite element model including cohesive elements is built.The effects of the cohesive zones,coating elastic modulus,and coating thickness on the indentation response are evaluated.Importantly,the interplay of coating cracking and interfacial delamination is investigated.One can found that increasing the cohesive strength/energy of coating reduces the probabilities of crack generation and propagation,whereas increases the susceptibility to interfacial delamination.At a critical value of the cohesive strength of the bonding layer,the resistance to interfacial delamination reaches its minimum.Nevertheless,the coating cracking is not sensitive to the interface adhesive properties.Moreover,a coating can receive better protection with lower coating elastic modulus.Additionally,increasing the coating thickness generally increases the critical load for coating/interface failures,but opposite effect occurs when the coating is thinner than a critical thickness.(3)The diamond-like carbon(DLC)coating is deposited on the carburizing steel by the composite ion plating technology.The nanoindentation test is performed on the coating surface,and the force-displacement data of indenter are recorded in real time.The phenomenon of pop-ins correlated with the material cracking or interfacial delamination is appeared on the force-displacement curve.After the indentation,circumferential cracks as well as interfacial delaminations in the coating-substrate system are observed by scanning electron microscopy and focused ion beam method.The experimental findings validate the correctness of the numerical solutions.Finally,the coating fracture toughness and interfacial adhesion energy are estimated through indentation results.(4)The micro elastohydrodynamic lubrication(EHL)model of the coating-substrate system is established.Based on the Full-system finite element method,the EHL tribological characteristic is studied.Effects of the coating thickness,coating elastic modulus,operating conditions,interfacial micro-valley,rough coating surface and interface,and multi-layer on the mechanical responses are evaluated.The results can predict the potential failure position of the system under heavy-duty conditions.It is found coating thickness,coating elastic modulus,and operating conditions have significant influences upon EHL characteristics.Compared with that of a stiff coating,the EHL responses of a compliant coating are more sensitive to changes in both the speed and load.Also,it is proved that micro-cracks normally initiate from interfacial micro-valleys in the viewpoint of mechanics.Moreover,the more compliant and thicker coatings are less influenced by the rough coating surface,and the interfacial shear behavior on the rough interface strongly depends on the mismatch of elastic modulus between the coating and substrate.Additionally,the functionally graded multi-layer coating can reduce the stress gradient in the system and mitigate the failures of interfacial delamination and subsurface pitting.(5)The thesis carries out a study to compare the friction and wear performance of TiN,WC/C,and DLC solid lubricant coatings by means of the four-ball test,and reveals their tribological mechanisms under heavy-duty and lubricated conditions.One can see that the TiN coating shows the highest mean steady-state coefficient of friciton,whereas the DLC coating exhibits the lowest one.The oxidation wear and peeling as well as the fatigue pitting are respectively the main failure modes of the TiN and WC/C coatings,while transfer layers around wear scars are gradually formed on the DLC coating.Thus,the WC/C and DLC coatings show outstanding performance of anti-friction,anti-wear,and running-in.