Boundary lubrication of bearing steel in water-based lubricants with functional additives

This thesis focuses on the effect of additives on boundary lubrication of bearing steel for water-based lubrication systems. The oil-in-water (O/W) emulsion and the water-glycol based liquid were selected as the base fluids for research. Sulfur compounds, nitrogen heterocycles and graphene derivatives were synthesized and used as additives. The tribological properties and corrosion resistance of the bearing steel friction couple in water-based lubricants are investigated. Based on the results of tribological tests and the analysis of friction surfaces, various tribochemical mechanisms are suggested to explain the friction processes, as well as the formation of a boundary lubricating film. In addition, the design of water-based additives and the effects of additives and working conditions on the tribological properties of water-based lubricants are described. Firstly, the effect of the interfacial film on the lubrication performance of O/W emulsion was researched. For that, graphene oxide (GO) underwent asymmetric chemical modification with myristyltrimethylammonium bromide (TTAB) to get modified graphene oxide (MGO). This MGO was used as an emulsifier and additive in oil-in-water emulsion. The emulsifying tests showed that MGO greatly improved the stability of base emulsion and decreased its droplet size. Tribological test results showed that under the boundary lubrication condition with MGO emulsion, the friction coefficient (COF) and the wear rate of the steel ball decreased by about 18% and 48% respectively in comparison with base emulsion lubrication. The tribological mechanism of MGO emulsion could be explained by the strong film-forming ability on the metal surface, the high lubricity of its small droplets, and the formed adsorption film, transfer film and tribofilm between the contact surfaces. Secondly, the effect of the oil phase on lubrication performance of O/W emulsion was researched. For that, graphene oxide (GO) was single and double-sided modified with octadecylamine (ODA) to get MGO1 and MGO2 respectively. MGO1 and MGO2 were incorporated into the O/W base emulsion to change the composition of the interface and the oil phase. It was found that the tribological properties of MGO1-containing emulsion were better than those of MGO2-containing emulsion. Chemical composition analysis of the worn surfaces suggests that MGO1 nanosheets might adsorb more easily onto the metal surface and react better with metal surfaces to produce C-N-O-Fe-containing compounds than do MGO2 nanosheets, which means MGO1 nanosheets are more actively involved in the shear process that occurs in the contact area. Nanoindentation indicated that there was a non-uniform softer MGO1 tribofilm present on the metal substrate. Thirdly, the effect of the water phase on lubrication performance of O/W emulsion was researched. For that, two nitrogen-containing water-soluble additives are synthesized and used as an additive in O/W emulsion. The friction tests results showed that the two water-soluble additives might be more rapidly adsorbed to the metal surface than the emulsion droplets could be “plated out” on the metal surface, thereby changing the friction-reducing and anti-wear performance of base emulsion. In addition, it was found that the tribological properties of these two additives were related not only to nitrogen content but also to the functional groups formed by the nitrogen atoms. Fourthly, the effects of sulfur element and applied load on the tribological performance of water-glycol fluid were studied. For that, two triazine derivatives, STB and STC were synthesized and used as additives in water-glycol base fluid (mass ratio 1:1). Water solubility tests indicated that STB and STC met the solubility prerequisite for a water-soluble additive. The tribological tests using a four-ball tribometer showed that STB and STC both improved greatly the PB value, anti-wear and friction-reducing capacities of the base fluid. Based on friction test results and wear scar analysis, it was concluded that the differences in tribological performances of STB and STC samples were due mainly to the differences of the sulfur content and the sulfur activity in additives. XPS found that STB-tribofilm mainly contained sulfate, but STC tribofilm contained sulfide as well. Finally, the effect of molecular chain length, additive concentration and sliding velocity on the tribological performance of water-glycol fluid was investigated. For that, three novel xanthate-containing water-soluble triazine derivatives, EXT, BXT and HXT, were synthesized and used as additives in water-glycol base fluid. It was found that the additive-containing samples had no corrosive effect on the cast iron and copper surface within 2.5 wt.% additive. The tribological test results using a four-ball tribometer showed that the three additives greatly improved the PB value, friction-reducing and anti-wear performances of water-glycol. XPS results indicated that the additives had reacted with the contact metal surfaces and formed complex tribofilms composed of iron oxide, iron sulfide and iron sulfate, which may contribute to the reduction of friction coefficient and wear rate of the friction system. In summary, for O/W emulsion lubrication, if the droplets can be quickly adsorbed onto metal surface, forming an oily layer between the rubbing surfaces, the COF will be reduced. If coupled with the formation of an effective adsorption film and tribofilm by additives, the COF will be further reduced. For water-glycol lubrication, the differences in the tribological performances are due mainly to the differences of the formed adsorption film and the tribofilm on the rubbing surface, including the forming velocity, strength and thickness and the composition. These change not only with the additives but also with the working conditions