Preparation And Tribological Properities Of Space Lubricating Grease Containing MoS₂Nanoparticles

The fast development in the exploration to space needs the spacecrafts withlong service periods recent years. The former research results have indicated thatthe friction between moving component is one of the main reasons to shorten theservice life. It is significant to study the high performance space lubricating greasein order to reduce the friction in which the suitable solid lubricating additives isessential to prolong the service life of the spacecrafts. Molybdenum disulphide（MoS₂） is an ideal solid lubricating additive for the lubricating grease because of itsgood lubricity, low volatile rate in vacuum and high friction reduction under highvacuum, strong radiation and varied temperatures. It is found that the tribologicalproperties of nano-MoS₂particles are better than those of micro-MoS₂particles.Firstly, the space lubricating grease containing nano-MoS₂particles wasprepared by one-step thickening method in this thesis. Polyalphaolefin was selectedas the feedstock and polytetrafluoroethylene （PTFE） as thickener. MoS₂particleswith varied sizes were dispersed into the feedstock by ultrasonication, producingMoS₂-containing space lubricating greases with high dropping point and very lowvolatilization.Secondly, the tribological properties of space lubricating greases containingdifferent MoS₂particles （micro-sized MoS₂, MoS₂nano-slices and MoS₂nano-balls）were evaluated using a MQ-800four-ball tribometer, respectively. Themorphologies of the worn surfaces of steel balls were observed using scanningelectron microscope. The results showed that MoS₂nano-slices as additive in spacelubricating grease had the best effects in the aspects of friction reduction and wearresistance. Moreover, the MoS₂nano-slices can also improve the lubricity of spacelubricating grease even at high loads.Subsequently, the tribological properties of the prepared space lubricatinggrease containing nano-MoS₂particles were evaluated using a space tribologicaltest system which simulated space environment. The simulated conditions includedvacuum, thermal cycling, atomic oxygen and ultraviolet irradiation. Themorphologies of the worn surfaces were observed by a3D profile scanner. Theresults showed that the MoS₂nano-balls were not proper to act as a solid lubricating additive in the space lubricating grease. However, the MoS₂nano-slices presentedbetter tribological properties than the micro-MoS₂in the space lubricating grease.Finally it was investigated that the lubricating mechanism of space greasecontaining MoS₂. The elements on the worn surface were analyzed using energydispersive spectrometer （EDS） and X-ray photoelectron spectroscopy （XPS）. Theresults showed that the lubrication of micro-MoS₂was ascribed to the shearing andsliding roles on the rubbing surface in the atmospheric environment. Their bettertribological properties resulted from MoS₂nano-slices and MoS₂nano-balls weremainly attributed to the formation of film produced by the tribo-chemical reactionon the rubbing surface. The lubricating mechanism of micro-MoS₂in the simulatedspace conditions was similar with that in atmospheric environment. The MoS₂nano-balls could be broken easily, and the impulsive force from the nano-ballsbreaking led to the serious wear on the rubbing surface. The MoS₂nano-slicescould be oxidized by the atomic oxygen and produced lubricating film bytribo-chemical reaction.