Study Of Ultra-Low Friction Behavior And By Mechanism Of Ta-C Film Under Oil Lubrication

Diamond-Like Carbon(DLC)films are widely used in optical windows,magnetic storage disks,automotive engine components,biomedical coatings,and other fields because of their good anti-wear properties,low coefficient of friction,high hardness,and excellent corrosion resistance.Especially in the automotive industry,DLC can reduce surface friction,improve engine performance and transmission components,and increase energy efficiency.With the continuous innovation and development of coating technology technology,hydrogen-free tetrahedral amorphous carbon(ta-C)films have attracted extensive attention from researchers.It is found that ta-C has higher hardness and better frictional properties compared with DLC.Therefore,in the current life production field,ta-C films greatly broaden the application scope of DLC coatings with its superior performance.However,it is inevitable in the automotive industry that there is a certain amount of friction between the mechanical components and the driving mechanism in the process of use,and over time,the surface friction will aggravate the loss between the two,therefore,in order to minimize the friction in the driving process and save power,in addition to the coating on the surface of the components,an effective lubricant needs to be added between the mechanical components and the driving mechanism to the two friction surfaces directly The direct contact between the two friction surfaces is improved and the life of the machinery is prolonged.However,due to chemical differences between carbon and steel,conventional lubricants developed for steel-based materials based on DLC,ta-C solid-liquid synergistic lubrication systems are not ideal and sometimes cause high wear of ta-C films.And the traditional mineral oil-based lubricants contain some non-degradable elements which are harmful to the environment,organic environmental friendly lubricants are gradually becoming the best alternative to mineral oil-based lubricants.On the other hand,the related research on the ultra-low frictional wear behavior between DLC,ta-C film and lubricant has not yet achieved a breakthrough.Therefore,in this paper,DLC films with different sp~3contents were prepared by magnetic filtration cathodic vacuum arc technique(FCVA),and the ultra-low friction behavior of ta-C under glycerol lubrication was compared and analyzed by UME multifunctional friction and wear testing machine to distinguish ta-C from a-C and steel,as well as the friction and wear performance of ta-C under different lubricants.The characterization techniques including scanning electron microscopy,Raman spectroscopy,infrared spectroscopy,X-ray photoelectron spectroscopy,combined with Stribeck curve theory,were used to systematically investigate the frictional behavior of ta-C films under lubricants and to clarify the friction reduction mechanism of lubricants,and the main findings are as follows:(1)The mechanical properties of ta-C and a-C were evaluated,and overall ta-C contains more sp3 hybrid bonding content and therefore has the best overall performance.(2)In the mixed state,only the steel friction sub coated with ultra-smooth ta-C exhibited super-lubricating properties under glycerol lubrication.The friction coefficient is lower than 0.02 in the thin film EHL/mixed state and lower than 0.045 in the boundary state.indicates that the ultra-low friction state of ta-C is related to the change of lubrication state and the nature of lubricant;ta-C/steel contact has the lowest friction coefficient and higher ball wear;steel/steel contact has the lowest ball wear and the highest friction coefficient.a-C/steel contact has slightly higher wear than steel/steel,but coefficient of friction is slightly lower than steel/steel contact.ta-C has the highest hardness and almost all of the wear occurs on the balls due to the relatively soft friction process on the grinding balls,so ta-C has the highest ball wear.(3)The friction test study under different lubricants revealed that ta-C reached ultra-low friction coefficient only under glycerol lubrication,and the highest wear under GMO lubrication,which was due to the oleic acid generated during the decomposition of GMO,resulting in high wear;the formation of friction film on the hydroxyl(-OH)capped end of the glycerol-lubricated ta-C surface was observed by Raman spectroscopy,XPS analysis and infrared spectroscopy to explain its Friction reduction mechanism:Under glycerol lubrication,the sliding surface of ta-C coating surface is adsorbed by hydroxyl(-OH),forming a very thin and low shear strength friction film,and when severe boundary/mixed lubrication conditions occur,friction occurs between the low shear strength friction film,leading to an ultra-low friction state(0.01-0.02).