Enhanced Lubrication Behaviors of Gallium-matrix Liquid Metals


Student thesis: Doctoral Thesis

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Awarding Institution
  • Guangneng Dong (External person) (External Supervisor)
  • Zuankai WANG (Supervisor)
Award date13 Jan 2022


With the increasing of aeroengine’s thrust-to-weight ratio, the applied load and speed of aeroengine’s main bearing become higher and higher. Traditional lubricants are hard to meet such severe requirements of the main bearing, therefore it is necessary to develop high load-bearing lubrication materials for the main bearing. Gallium-matrix liquid metal (GLM) is an amorphous and flowable liquid metal at room temperature. It is a kind of potential novel lubricant due to its specific advantages including good fluidity, high load bearing capacity, thermal conductivity and stability. The purposes of this paper are to investigate and enhance the lubrication performance of GLM. Incorporating self-lubricated particles, surface texturing and nanoscale GLM droplets are carried out to improve the lubrication abilities of GLM. Main contributions are as follows:

The lubrication behaviors of GLM under boundary lubrication condition are investigated by tribological tests. The lubrication performance of GLM is closely related to the experimental conditions including applied load, speed and materials type of frictional pairs. When the applied load and speed are set as 10 N and 24 mm/s, the friction coefficient and wear rate of AISI52100/ AISI52100 pair can be reduced by 29% and 75% respectively. While under the same applied load and speed, the friction coefficient and wear rate of C95200/ AISI52100 pair can be reduced by 30% and 93%. Moreover, GLM exhibits limited anti-friction and anti-wear abilities under the conditions of low applied load and sliding speed. Increasing the load and speed is conducive to its lubrication performance.

In order to enhance the lubrication abilities of GLM, molybdenum disulfide (MoS2), hexagonal boron nitride (h-BN), and graphene nanosheets with good lubricity are synthesized and then incorporated with GLM. Sticky oxidized layers around GLM surfaces play a critical role in the incorporation process. Both the types of nanosheets and the addition concentration would affect the lubrication and thermal abilities of GLM. Taking the tribological properties and thermal ability into consideration, fuzzy comprehensive evaluation method based on fuzzy mathematics is implemented to select the best incorporation scheme. The results indicate that GLM with the addition of 12 wt% MoS2 nanosheets exhibits the optimal comprehensive performance with a thermal conductivity coefficient of 13.41 W/m.K, a friction coefficient of 0.26 and a volume wear rate of 2.9×10-7 mm3(N.m)-1.

Surface texture is constructed on frictional surfaces to improve the lubrication efficiency of GLM under boundary lubrication condition, especially for low applied load and sliding speed. Using laser surface texturing, dimple textured surfaces with area ratio from 0.15 to 0.35, maximum depth of 12 μm and dimple diameter of 60 μm are fabricated. The tribological tests demonstrate that the dimple textured surfaces are able to assist the lubrication of GLM and when the area ratio is 0.15, GLM hold the best anti-friction and anti-wear abilities. Under the condition of low load and speed, the average friction coefficient and the wear rate of textured surface with GLM lubrication are respectively reduced by 52% and 59%, compared with untextured surface.

Nanoscale GLM droplets (LMNP-C12) with excellent compatibility in aeroengine oil are achieved. LMNP-C12 is obtained by virtue of self-assembling of 1-dodecanethiol. The diameters of LMNP-C12 are ranging from 150 to 200 nm and it is attested to be able to stably dispersed in Mobil jet oil 387, which is a kind of commonly used aeroengine oil. 0.1 wt% LMNP-C12 in Mobil jet oil 387 can effectively improve its lubrication efficacy. Furthermore, nanoscale GLM droplet compatible with polar-based lubricants, which is denoted as LMNP-CS, can also be obtained by this strategy. The addition of 0.3 wt% LMNP-CS in based lubricant can reduce the friction coefficient and wear rate by 40% and 69% respectively.

The atomic adsorption models for GLM on the surfaces of frictional pairs are established by CASTEP based on first principles, illuminating interfacial interaction processes between GLM and frictional pairs. The adsorption energy of gallium atom on the iron surface is calculated to be -5.7 eV while the adsorption energy of gallium atom on the copper surface is -1.68 eV. Negative values of adsorption energies indicate that gallium atoms are capable of being stably adsorbed on the surfaces of friction pairs. Based on the calculation results of GLM adsorption, an in-situ gallium-contained film which hold good lubricity is deposited on the substrate surfaces of friction pairs, verifying the lubrication mechanisms of GLM.

    Research areas

  • Gallium-matrix liquid metal, Friction and lubrication, Nanoparticle, Lubricant additive, Surface texture