Effects of metal element doping on the lubrication behaviors and mechanisms of gallium-based liquid metals

Gallium-based liquid metals (GLMs) are a novel high-end lubricant with low friction coefficient and excellent load-carrying capacity. This paper aims to improve the lubrication properties of GLMs by regulating the frictional interfaces. Six types of GLMs doped with Zn, Mg, Bi, Cu, Ti, or Sc elements are prepared respectively, and the effects of metal element doping on the lubrication behaviors and mechanisms for AISI 440C steel self-mated pairs are investigated in the mixed lubrication regime with a ball-on-three-plate setup. Doping 1 wt% Zn or Cu elements can improve the lubrication properties, with the friction coefficient reduced by 20.3 % and the wear rate reduced by 55.1 % compared with those under eutectic Ga/In/Sn lubrication. However, doping Ti, Sc, Mg, or Bi all reduce the lubrication properties. Doping Zn improves the lubrication properties of GLMs by promoting the adsorption of gallium on the frictional interfaces. Conversely, doping Bi inhibits the adsorption of gallium. The improvement in the lubrication properties of Cu-doped GLM is attributed to the formation of low-hardness CuGa2 particles. The doping of Ti or Sc generates high-hardness TiGa3 or ScGa3 particles, causing three-body abrasive wear. The doping of Mg generates a large number of Mg2Ga5 particles, seriously disrupting the stable lubrication.