Friction Behaviors And Mechanisms Of Solid Lubrication Films In The CO₂ Environment

The CO₂ environment is the space environment that human society must be faced in human exploration of the Mars.The diamond-like carbon?DLC?and MoS₂ films are widely used in space equipment due to their perfect frictional lubrication properties,and the space environment is an important factor influencing the friction behavior of the films.So far researchers have systematically examined the effects of various space environments on the friction behavior of DLC and MoS₂ films.However,there have been few reports about the friction behavior and mechanism of films in CO₂ environment.Therefore,the friction and behavioral mechanism of hydrogen-free DLC film and MoS₂ film in CO₂ environment are studied,which will provide important experimental data and theoretical basis for the application of two solid lubricant films in Mars environment.In this study,the friction behavior and mechanism of hydrogen-free DLC film and MoS₂ film in vacuum,atmosphere and CO₂ environment and the effects of different dual materials on the friction behavior of hydrogen-free DLC films in CO₂ environment are studied by experimental research and first-principles calculations.The tribological properties of the film in different environments are tested using a CSM high-low temperature vacuum friction tester.The microscopic morphology and compositional changes of wear scars after friction testing are analyzed using SEM,Raman,XPS.The friction mechanism of hydrogen-free DLC film and MoS₂ film in CO₂ environment are studied by the first-principles calculation.The main conclusions are as follows:?1?The hydrogen-free DLC films exhibit lower friction coefficient in CO₂ environment than that in atmosphere and vacuum environment.The experimental tests show that the CO₂molecules could occur tribochemical reactions on the surface of DLC films.The first-principles calculations find that the CO₂ molecules and C atoms of diamond surface can form internal lipid structures and passivate the“danging bonds”of diamond.There is a repulsive force between the friction interfaces after passivation of CO₂ molecule,which prevents the interface distance approaching and eliminates the strong interactions caused by the“danging bonds”.So the DLC films exhibit low friction coefficient.In the atmosphere environment,the friction interfaces after passivation by H₂O molecule have only weak van waals force,and the DLC film exhibits higher coefficient of friction than CO₂ environment.But,in the vacuum environment,the“dangling bonds”of the DLC film can not be passivated,and the friction interfaces have strong attraction,resulting in high the friction coefficient of the film.?2?When using ZrO₂ as the mating material,it is found that the friction performance of the hydrogen-free DLC films in the vacuum and CO₂ environments are better than that of Al as the mating material by the experiment tests.In the vacuum environment,the first-principles calculation results show that the friction interfaces formed by the mating material and diamond can both form a bond across the interface and increase the interaction between the interfaces.And,the C-O bond strength is significantly weaker than the C-Al bond strength.Therefore,the films exhibits lower friction coefficient when ZrO₂ is dual materials.In the CO₂ environment,the interaction between ZrO₂ and Al and the diamond interface is reduced due to the carbon dioxide molecules passivated the“danging bonds”of the diamond surface,which prevents the formation of cross-interface bonds.The separation work between ZrO₂and diamond interface is smaller than the separation work between Al and diamond interface,indicating that the attraction between ZrO₂ and diamond interface is weaker,and the film exhibits a low friction coefficient.?3?For the MoS₂ films,compared with the CO₂ environment,the friction properties of the MoS₂ films are better than that in vacuum and atmosphere environment.The experiments show that the films is not oxidized in CO₂ and vacuum environment,while MoS₂ is oxidized to MoO₃ in the atmosphere,resulting in high friction coefficient.The first-principles calculations show that the S-vacancy can cause MoS₂ and CO₂ molecules to easily product charge exchange,and the S-vacancy can be filled by CO₂ molecules.In addition,there is a stronger repulsive force at the MoS₂ interfaces after filling the S-vacancy than the S-vacancy MoS₂ interfaces,and the layer is more likely to slide between layers.resulting in lower friction coefficient.So the film shows lower friction coefficient in the CO₂ environment than the vacuum environment.?4?The DLC film and MoS₂ film show low friction coefficient in the CO₂ environment.From the results of this study,the DLC films have a long period of high friction coefficient in the CO₂ environment,causing large damage to the film.While the MoS₂ films have low friction coefficient in the CO₂ environment,and the wear of MoS₂ films is more smaller.So MoS₂ film is more suitable for the application of mechanical parts in CO₂ environment.