Tribological Behavior Of TiN Films Under Different Atmospheres And Study On The Mechanism Of Low Friction In Vacuum

The development of space technology not only greatly broadens the scope of people’s understanding of nature,but also brings many special tribological problems,such as the lubrication problem of space machinery exposed to ultra-high vacuum,irradiation,atomic oxygen,cosmic dust,extreme high and low temperature and microgravity environment.Under the action of harsh environmental factors in space,grease lubricants are prone to evaporation,decomposition or cross-linking,which makes them hard to be qualified for the lubrication of space machinery.The solid lubrication film has a wide operating temperature range,radiation resistance,corrosion resistance and other advantages.In recent years,MoS₂ and diamond-like（DLC）solid lubricating films have extremely low friction coefficient in vacuum environment,showing great application prospects.However,in the outer space environment,the two-dimensional thin film material due to the low hardness,resulting in serious mechanical failure in service.In view of the influence of vacuum,temperature difference between day and night,atomic oxygen erosion and irradiation effect,it is urgent to seek other alternative coatings for the wear resistance of components.Transition group metal TiN film has high hardness,high wear resistance,good corrosion resistance,high temperature stability and other advantages,has been widely used as protective coatings for working molds,decorative coatings for materials,diffusion barrier layer in the field of microelectronics and biological fields.However,the research on tribological behavior of TiN films mainly focuses on the atmospheric environment,and the tribological behavior in vacuum environment is rarely reported systematically.In this paper,TiN film was deposited on the surface of 304 stainless steel by multi-arc ion plating.The tribological behaviors of TiN films under atmospheric and vacuum conditions were compared,and the tribological characteristics and wear mechanism of TiN films under environmental media（oxygen and nitrogen）were discussed.The results show that:（1）TiN films prepared by multi-arc ion plating have excellent properties.The film surface is dense without obvious defects.The microstructure of the columnar crystal can be clearly observed by cross-section.The hardness is up to 28GPa,and the bonding strength with the matrix is 56N.（2）The tribological behavior of TiN films is very different in atmosphere and vacuum.In the atmospheric environment,due to the existence of a large number of active media at the friction interface,a large number of abrasive particles are generated at the sliding interface.In the reciprocating sliding process,abrasive particles are included in the friction interface,resulting in the increasing friction coefficient.However,surprisingly,under the vacuum condition,there is almost no adhesion at the friction interface.The friction coefficient of TiN film under the vacuum environment is relatively low,about half of that in the atmospheric environment（0.3）,and the friction coefficient is stable,showing a small range of fluctuations.（3）In consideration of the influence of friction and oxidation reaction under atmospheric environment,friction experiments under different partial pressure of oxygen were constructed.The results showed that the friction coefficient of the film increased with the increase of partial pressure of oxygen.In the oxygen atmosphere,mechanical load and friction heat can promote the steel ball,which is easy to shear,to produce a large number of grinding debris and adhere to the friction interface.In addition,oxygen can also induce the oxidation reaction of friction products,so that the abrasive particles mainly exist as Fe₃O₄ components under high temperature and high pressure conditions.These hard particles constantly scrape the surface of the friction pair,resulting in an increase in the friction coefficient.（4）The influence of nitrogen in the atmosphere cannot be ignored.Compared with oxygen atmosphere,the friction experiments under different nitrogen partial pressures also show the same change trend,that is,the friction coefficient of the film increases with the increase of nitrogen partial pressure.In addition,in the nitrogen-vacuum alternating atmosphere,the friction coefficient of the film presents a periodic change,that is,in the nitrogen atmosphere friction stage,the friction coefficient is relatively low（0.45±0.1）.In the stage of vacuum friction,the friction coefficient is relatively high（0.65±0.1）.During the process of reciprocating friction,nitrogen molecules can be adsorbed at the sliding interface,and under the action of mechanical load,physical and chemical changes occur on the surface of the film.The ionic potential difference between oxides at the friction interface and the electrostatic repulsion between nitrogen atoms can lead to periodic changes in the friction coefficient.（5）Examining the tribological behavior of TiN films and different duals in harsh environments is instructive to the actual working environment.The TiN film has a low friction coefficient（0.2）with the SiC friction pair in a vacuum environment,and the film wear rate is as low as 8.8×10^-7mm³/mN,which is mainly due to the carbonaceous transfer of the lubricant phase forming the sliding interface Film;the friction coefficient of the TiN film with the WC friction pair in a nitrogen atmosphere is 0.3,and the film wear rate is 4.5×10^-7mm³/m N,which may be due to the nitrogen molecules inhibiting the direct contact of the friction pair.The large Hertzian contact promotes mechanical polishing and thus exhibits low friction.