Study On Superlubricity Failure Mechanism Of Hydrogenated Diamond-Like Carbon Film

In order to accelerate the human's space exploration and the development of space resources,it is urgent to develop long-life and high-reliability spacecraft.Traditional lubrication and surface treatment technology can not meet the growing demand for high technology,and the development of superlubricity technology is very important for the development of long-life and high-reliability spacecraft.Hydrogenated diamond-like carbon(H-DLC)films are considered as one of the most promising superlubricity materials in aviation industry because of their high hardness,low friction coefficient,and high wear resistance.However,the mechanism of superlubricity failure of H-DLC films is not fully understood,and there is still a big gap from the actual engineering application.With the continuous development of H-DLC film superlubricity technology,the lifetime of the spacecraft can grow exponentially.The traditional accelerated life test method can not meet the prediction of the spacecraft's superlubricity life.Therefore,in order to understand the mechanism of superlubricity failure of H-DLC films and establish the prediction model of superlubricity life,it is urgent to study the mechanism of superlubricity failure of H-DLC films under multi-factors.This study not only enriches the basic theory of superlubricity technology,but also promotes the practical application of H-DLC film superlubricity technology.The key conclusions and innovations are as follows:(1)The mechanism of the effect of load on the superlubricity failure of H-DLC films was revealed.It was found that low load in vacuum was beneficial to the stable formation of transfer layer on alumina pairs,while high shear stress caused by high load would lead to the rapid removal of transfer layer and superlubricity failure of H-DLC films.In vacuum environment,H-DLC films can achieve long-term and stable superlubricity state under low load,but high load will accelerate the superlubricity failure.In atmospheric environment,the friction coefficient of H-DLC films decreases with the increase of load.By means of SEM,Raman and AFM measurements,it is found that insufficient hydrogen passivation in vacuum environment may cause the activation of ? bonds in the friction interface of H-DLC films,resulting in the reduction of the wear resistance of H-DLC films,which results in the generation of abrasive debris and the formation of a transfer layer on the wron surface of alumina balls.Under low load,the transfer layer can be formed stably to achieve the superlubricity state,while under high load,high shear stress will lead to the rapid drop of the transfer layer on the wron surface of alumina pairs,which will lead to the superlubricity failure of H-DLC films.(2)The influence mechanism of velocity on the superlubricity failure of H-DLC films is clarified.It is found that low speed in vacuum is beneficial to the stable realization of superlubricity of H-DLC films,and the load fluctuation of friction interface on pairs caused by high speed will lead to the removal of transfer layer,resulting in superlubricity failure.In vacuum environment,the superlubricity stability of H-DLC films becomes worse and worse with the increase of velocity until the superlubricity state of H-DLC films fails rapidly at high velocity.By means of SEM,Raman and AFM measurements,it is found that in vacuum environment,low speed is conducive to forming a continuous and dense transfer layer on the surface of the pair,while at high speed,the vibration of the friction interface will produce instantaneous impact load,which will lead to the shedding of the transfer layer and the superlubricity failure of H-DLC films.In atmospheric environment,the friction coefficient of H-DLC films decreases first and then increases with the increase of velocity,which may be attributed to the coupling effect of transfer layer and gas passivation.(3)It is found that the high contact frequency under reciprocating and rotating motion will lead to the high friction coefficient of H-DLC films,and the alternating stress under reciprocating motion is more likely to lead to the superlubricity failure of H-DLC films.Under reciprocating or rotating motion,the friction coefficient of H-DLC films increases with high contact frequency.The reason is that high contact frequency will lead to higher friction heat generation at the friction interface.On one hand,the friction heat will generate more oxides,on the other hand,it will lead to the dissociation of water molecules adsorbed on the friction interface,which will expose the ? bonds on the surface,increase the shear force of the friction interface,and ultimately lead to the increase of the friction coefficient of H-DLC films.In vacuum environment,H-DLC films can achieve long-term and stable overshoot under rotational motion,but it is easy to cause the superlubricity failure of H-DLC films under reciprocating motion.The results of Raman and AFM show that the high stress points and the defective points of H-DLC films will gradually form very fine cracks under bidirectional alternating stress.With the serious stress concentration at the crack tip,the cracks will gradually expand,resulting in more serious wear of H-DLC film in reciprocating motion.(4)Based on the basic principle of traditional accelerated life test,a criterion of H-DLC film superlubrucity failure is proposed,in which the wear depth is greater than the thickness of the H-DLC film,and a set of H-DLC film accelerated life test method is proposed preliminarily.Because of the reversibility of the superlubricity state of H-DLC films,the superlubricity state of H-DLC films will eventually fail when they are worn through.Therefore,a criterion for the superlubricity failure of H-DLC films is proposed that the wear depth is greater than the thickness of the H-DLC films.A set of accelerated life test method for H-DLC films is preliminarily established.Firstly,The accelerated stress of the ultimate velocity is selected based on the same failure mode and failure mechanism of H-DLC films.Secondly,the wear depth and life curves of H-DLC films under the actual working velocity and the accelerated stress of the ultimate velocity are compared and verified.Finally,an accelerated superlubricity life prediction model is preliminarily established.