Study Of Tribological Behavior And Mechanisms Of Carbon-based Composite Coating With Lunar-dust Damage

Lunar-dust space environment is the special environment that human society must be faced in human exploration of the moon. Nowadays, Space-born systems have a number of moveable mechanical parts, whose reliability is severely limited by the degradation of lubricants and excessive wear from lunar-dust. The diamond-like carbon（DLC） and MoS₂ coatings have received considerable research attention in the space applications owing to their remarkably improved tribological characteristics. However, there are few researches and reports the damage and tribological behavior of those solid lubricating coatings under lunar-dust existing environment. In this thesis, a series of MoS₂/DLC multilayer coatings with different bilayer periods were successfully fabricated by medium frequency magnetron sputtering technique. The design of multilayer coatings is based on the composition and structure of composite coating. The Institute of Aerospace 510 fabricated artificial synthetic lunar dust as the simulating abrasive particles. The tribological performances of MoS₂/DLC multilayer coatings under conditions without and with lunar-dust particles were systematically studied. Based on the effects of applied load, rotating speed and friction time on the tribological performances of selected MoS₂/DLC multilayer coatings, the friction and wear mechanisms of MoS₂/DLC multilayer coating under condition with lunar-dust were analysed and proposed. The results showed that:1. The fabricated MoS₂/DLC multilayer coatings are uniform and smooth, and the cross-sectional have a homogeneous and dense microstructure. By comparing the mechanical performance and vacuum frictional behavior of the MoS₂/DLC multilayer coatings, we find that the optimum bilayer period is 2（MoS₂/DLC-4L）.2. The friction coefficients and wear rates of the MoS₂/DLC-4L coating present a phenomenon that decreased first and increased afterwards with the increase in rotating speed, while do not exhibit the significant variation with the increase in load under vacuum condition without SLD. However, In case of the condition with SLD, the friction coefficients as a function of load show the fluctuation under 3 and 10 N whereas maintain a steady value under 5 and 7 N. Under the applied load of 7 N, the wear rates of the MoS₂/DLC-4L coating decrease by a power law function with an sliding cycle exponent, implying that the initial material loss is significantly higher than the steady-state wear. The material loss is mainly occurred in the soft layer（MoS₂）, whereas the hard layer（DLC） as a bearing layer can effectively slow down the abrasive wear of coating.3. The pure MoS₂ under vacuum condition with lunar-dust only maintain a short lifetime. The main reason is that the soft coating is easy to shear off from the substrate due to the mechanical damage by the angular SLD particles. The poor friction behavior of DLC coating can be attributed to the high adhesive effect in vacuum. However, the Mo S2/DLC-4L coating maintains a steady friction coefficient of 0.02 and a longer wear life under high vacuum condition with lunar-dust.4. With the increase of sliding cycle, the central region of wear track of MoS₂/DLC-4L coating will accumulates some “ridge” layer, and the SLD particles can roll freely between the ridge layer and the steel ball surface thus reducing the friction and wear. Moreover, the counterpart surface forms a composite transfer layer containing nanoparticles and coating materials, which can effectively reduce the friction and wear.