| The space tribology faces a great challenge as the development of aerospace technology.In the space environment,the mechanical mechanisms of spacecraft are faced special working conditions,such as ultra-high vacuum,atomic oxygen,solar radiation,and temperature cycling,and how to ensure the reliability of aerospace devices under these special conditions is a key problem in space tribology.The vacuum and microgravity of the space environment are the main factors affecting the performance of lubricants.In a vacuum environment,the adhesion between atoms is greatly enhanced and aerospace devices are always failure due to cold welding.The microgravity induces irregular collisions when components are slightly disturbed.Although the molybdenum disulfide（Mo S2）has an excellent lubrication performance in the space environment,the lubrication properties of Mo S2 under the collision friction environment is still unknown.Therefore,the studies on the collision friction properties of Mo S2 films in space environment play an important role in the application of Mo S2 in space environment.In this paper,a molecular dynamic model is proposed to describe the collision friction of Mo S2 films under different working conditions,and the lubrication properties of Mo S2 films during collision friction process are studied.Besides,the lubrication performances of different lubrication combinations are discussed.Firstly,a molecular dynamics model is proposed to describe the collision friction process under different lubrication conditions.The results show that under soft metal lubrication,the average friction forces are divided into three stages.When collision frequencies are lower than the natural frequency of the substrate,the average friction forces do not change compared with the situation when the indenter does not vibrate.When the frequencies are higher than the natural frequency,the friction forces decrease with the increase of the frequency.When the frequency is increased to 27.8GHz,there is no obvious change in the average friction force as the frequency increases.Under the natural frequency,the average friction forces are not changed when the amplitude is increased,while increasing the amplitude at the other two frequencies leads to the reduction of the average friction forces.Secondly,the collision friction process under the Mo S2 lubrication condition is studied.The results show that the friction forces in the collision friction process are reduced by the Mo S2.The variation of the average friction forces with frequencies under the Mo S2 lubrication condition is completely different from the situation under the soft metal lubrication condition.When the frequencies of the indenter are lower than the natural frequency of the substrate,the average friction forces do not change with the increase of frequencies.When the frequencies are higher than the natural frequency,the average friction forces increase with the increase of the frequencies.When the frequencies exceed 21.7GHz,the average friction forces decrease with the increase of frequencies,and the average friction forces no longer change with frequencies when the frequency reaches 27.8 GHz.The puckering effect of the two-dimensional material has a great influence on the friction force,and the phase transition of the Mo S2 films is not observed during the collision friction process.The lubrication performances of different lubrication combinations are compared,and the results show that the lubrication combination of the Mo S2 and soft metals and textured surfaces shows the best lubrication performance.Then,the molecular dynamics models of collision friction with different collision clearance is proposed,and the effects of gaps on the collision friction process are studied.The results show that under the soft metal lubrication condition,the larger the gaps,the smaller the average friction force during collision friction.In addition,the influence of the gaps on the collision friction process is not obvious under the Mo S2 lubrication condition.Changing the gaps will only change the position where the maximum average friction force occurs,and it will not reduce the value of the maximum average friction force.Finally,the paper is summarized and the shortcomings is pointed.Besides,the research direction in future is suggested. |
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