Two-dimensional Layered Materials:Microstructure Control by Laser and Their Tribology Property

Author:Luo Zuo

Supervisor:cao bing qiang


Degree Year:2019





Two-dimensional(2D) layered materials not only have unique optical,electrical and thermal properties,but also have good mechanical properties.They can not only be used as lubricants alone or lubricant additives to significantly improve their tribological properties,but also can exhibit novel frictional phenomena such as superlubricity.However,it is very difficult for 2D layered materials used as addtives in the lubricating oil,because of its large specific surface area and high surface activity,which lead to easily aggregation in oil or oxidized failure during the friction process.Now,the most urgent research problems are to design novel 2D composite structures,find a suitable preparation technology,analyze the tribological mechanism in depth for 2D layered materials in this field.In this study,based on the lubrication mechanism of the micro/nano particles as lubricating oil additives,the simple and ultra-fast laser irradiation in liquid wae used to regulate the microstructure of tungsten disulfide(WS2),molybdenum disulfide(MoS2) and graphene.Then,their tribological properties as lubricant additives were explored systematically.(1)A one-step laser-assisted growth strategy for solid WS2 submicrospheres in liquid is proposed.Big WS2 slices were used as targets.At ambient conditions,the fragmentation of big WS2 microslices and the forming of near-closed solid spherical structure were realized by the extreme non-equilibrium environment of ultra-high temperature and ultra-high pressure induced by the instant laser irradiation of solid targets and the rapid cooling of the surrounding liquid,which can effectively simplify the preparation process and reduce the cost.The tribological properties of WS2 submicrospheres as paraffin liquid additives were studied.The results show that WS2 submicrospheres can deposit tribofilm on the surface of friction pairs and play the role of microbearing under the action of shear force,thus exhibiting excellent anti-friction and anti-wear properties.(2)One monodisperse MoS2 nanospheres with fullerene-like structure(IF-MoS2) were developed by simple laser irradiation preparation method.MoS2 nanosheets prepared by hydrothermal method were used as targets.Nanosecond pulse laser irradiated the dispersions of 2D MoS2 nanosheets directly.Monodisperse IF-MoS2 nanospheres with smooth surface were obtained by the effect of laser-induced thermal and rapid cooling of the surrounding liquid.Due to the high spherical degree,small particle size,and stable fullerenes structure with the closing of part dangling bonds,such IF-MoS2 nanospheres as additives in paraffin liquid can effectively enter into the contact area to form transfer film,restrain the oxidation failure caused by the friction-induced high temperature,therefore,exhibit excellent antiwear and extreme pressure properties.(3)Based on the advantages of zero-dimensional(0D) and two-dimensional(2D) nanomaterials,a novel 0D/2D laminated structure was developed by one-step laser irradiation method.The solution composed of 2D MoS2 nanoflakes and GO nanosheets were used as targets.Laser irradiation-induced photothermal effect can reduce graphene oxide,and lead to MoS2 nanosheets melt for the reconstruction of nanospheres to release the high surface energy,and adhesion between neighboring graphene nanosheets.Finally,the laminated composite of MoS2 spheres/graphene nanosheets with 0D/2D combination structure were obtained.The composites with relatively loose structure have good dispersion stability in lubricating oil.Moreover,it has significantly improved anti-wear and anti-friction performance in four-ball tribology tests.(4)Inspired by the good suspension stability of noctiluca scientillans in the ocean,and based on the good synergistic lubrication effect of the combination of different materials with different hardness,a core-shell structure with super-hard SiC spheres as the core,flexible graphene as the shell and the surface decorated by floating graphene nanosheets was designed to improve the dispersion of the material in the liquid phase medium.The composite structure can be obtained by laser irradiating the suspension of super-hard SiC particles using for reference of epitaxial graphene on SiC substrate through vapor growth strategy.SiC nanoparticles with high specific surface area was activated by photothermal effect and melted into spheres under the simple and facile pulsed laser irradiation in liquid.At the same time,the extreme non-equilibrium conditions on the laser-solid targets interface lead to the etching SiC decomposition and graphene forming from C atoms recombination,and then the formation of SiC@G submicrospheres decorated by floating nanosheets.This composite materials showed excellent dispersion in the liquid phase medium.Due to the micro-polishing and micro-bearing effect of super-hard SiC,and good adsorption and self-lubricating properties of graphene,the composite additives exhibit excellent anti-friction and anti-wear performance.(5)Ultrafine SiC@G nanospheres were prepared by ultra-fast and low-temperature laser irradiation growth technology,and the superlubricity(coefficient of friction is less than 0.01) were achieved preliminarily.The target materials were still the suspension of SiC particles.Ice bath was used to limit the growth rate of particles in the process of laser irradiation.Then,ultrafine SiC@G nanospheres less than 10 nm were obtained.When such nanospheres were dispersed in PAO 4 and paraffin liquid,superlubricity can be observed.It was found in the ball-on-disk tribology test that SiC@G nanospheres used in superlubricity systems of PAO 4 and paraffin liquid could effectively reduce running-in time,coefficient of friction and wear,and improve the contact pressure in the process of superlubricity.