Design And Preparation Of Thermal Stable DLC Films And Its Synergistic Lubrication Mechanism With Nano Additives

Solid-liquid compound lubrication refers to a composite lubrication system formed by a solid lubricant film and a liquid lubricant medium.Due to the high load-bearing,low-friction wear advantages of the carbon film,a solid-liquid composite lubrication system composed of a carbon-based film has a broad application prospects in automotive and aerospace field.At present,DLC-based solid-liquid composite lubrication system can show excellent lubrication performance at room temperature,but in some high temperature environment,it is prone to lubrication failure due to the large internal stress of the non-thermodynamically balanced carbon film,the strength and hardness are significantly reduced and graphitization occurs in a high temperature environment,resulting in deterioration of the thermal stability.On the other hand,because the traditional small molecule additive is mainly designed for the metal friction pair design,there is a poor matching with the diamond-like carbon（DLC）film and the phenomenon that is easily decomposed in the high temperature.Unlike small-molecule additives,nano-additives have the advantages of easy film formation,good stability,and high heat resistance.In order to meet the stability of the DLC solid-liquid compound lubrication system at service temperature and improve the lubrication efficiency of the system,it is necessary to study in depth the structural design of the heat-stabilized solid lubricant film and the synergy between the additive and its lubrication in order to provide theoretical and practical guidance for the design and preparation of the thermally stable DLC film in the solid-liquid compound lubrication system.In this paper,the structural optimization design of silicon-doped DLC films for thermal stability was studied.According to the mismatch between the existing small-molecule additives and DLC,the cooperative lubrication mechanisms of various thermally stable nano-additives and Si-doped DLC films were discussed.The specific research contents are as follows:1.Effect of structural design of Si doped DLC thin films on its thermal stability and tribological propertiesThe composition and structure of silicon-doped carbon film were optimized for the service temperature of solid-liquid compound lubrication system.A series of single-layer carbon films with different Si content and multilayer carbon films with different modulus ratio and different thickness ratio were prepared.The effects of Si content,modulus ratio and thickness ratio on the thermal stability and tribological properties of carbon films were investigated.The tribological properties of the film in poly alpha olefin（PAO）base oil at different service temperatures were studied by the reciprocating cycle friction test machine,and the mechanical properties and internal valence bond structure were analyzed.The results show that the single-layer carbon film with Si content of 11.92%has the best tribological properties at 150°C.This is because the high temperature promotes the silicon in the film to tend to combine with oxygen,and the deconstruction from the silicon-carbon bond promotes more the formation of multiple carbon-carbon single bonds,which makes the hardness of the silicon-doped film improved.At the same time,in the solid-liquid compound lubrication system,the friction-induced formation of silicon oxide forms a friction film on the friction surface.The wear resistance of the multilayer carbon film with the modulus ratio of 1.267 is the best,which may be because the high sublayer modulus ratio is more beneficial to prevent the crack extension,and the thickness of each sublayer is the lowest when the relative thickness of the sublayer is equal,which limits the further expansion of the crack in the sublayer,making the multi-layer film best antiwear at this time.2.Synergistic lubrication mechanism of nano additives and silicon doped DLC in solid liquid composite lubrication systemThe oil soluble metal copper,two tungsten sulfide and Zinc Oxide nano additive are used as additives for the solid-liquid composite lubrication system of silicon carbon film.The co-lubrication mechanism of different types of additives（metal,metal sulfide,metal oxide）and silicon carbon film under different service temperatures are systematically studied.At the same time,it is compared with the existing small molecule additive ZDDP.The tribological performance of the film was tested by a reciprocating cycle friction tester.The elemental surface distribution of the wear scar on the steel ball and the chemical state and the bond structure of the element at the wear trace were analyzed.It was found that the Cu nano additives had the best antifriction and antiwear properties at 25°C,and the friction coefficient and wear rate were reduced by about 25%and 99%,respectively.This was because it formed a lubricating transfer film on the surface of the friction pair at room temperature.At 150°C,although the nano additives did not provide anti-friction effect,they effectively improved the wear resistance of the carbon film and the wear rate decreased by about 70%.This is related to the formation of a derivative film of WS₂ and ZnO nano additives on the surface of the friction pair at high temperature.It is also found that the Cu nano additive has the best comprehensive performance at 25 and 150°C,and the nano additive can improve the anti-wear performance of the carbon film effectively at 150°C compared with the ZDDP small molecule additive.3.Tribological properties of nano additives in solid liquid composite lubrication system of silicon doped DLC/product oilThe tribological properties of DLC films were studied in two kinds of finished oil（Guang qi,5W40and Beijing modern,5W20）with different additives（ZnO,WS₂ and Cu）at different service temperatures.The tribological properties of the films were tested by reciprocating cycle friction tester,and the element surface distribution of the steel balls was analyzed.It was found that although the nano-additives could not improve the anti-friction performance of the carbon film in the finished oil,at 25°C,the nano-additives effectively improved the anti-wear performance of the carbon film,especially the addition of Cu nano-additives.The wear rate is reduced by 60%-70%.At 150°C,the wear resistance of the carbon film in Guang qi refined oil has been further optimized.Because the wear trace is not easily detected,the wear rate cannot be measured.In Xian dai product oils,the carbon film wears out.