Synthesis and tribological behavior of metakaolinite-based geopolymer composites

Metakaolinite-based geopolymer composites containing 5%–30% (volume fraction) scalelike graphite, polytetrafluoroethylene (PTFE), and molybdenum disulfide (MoS2), respectively, were synthesized in the presence of a compound activator composed of aqueous NaOH and sodium silicate at room temperature. The tribological behaviors of the resulting composites sliding against AISI-1045 steel were investigated on an MM-200 friction and wear tester, and the bending strength and compressive strength of the composites were determined on a universal materials tester. Moreover, the worn surfaces of the composites were analyzed by means of scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and X-ray photoelectron spectroscopy (XPS), which was aimed to reveal the wear mechanisms of the composites. It was found that all the three kinds of the tested solid lubricants contributed to greatly decrease the friction and wear of the composites, and they were also able to remarkably reduce the fluctuation of the friction coefficient. This was attributed to the formation of a lubricating film containing higher content of the detached solid lubricant particulates and the oxidized product Fe2O3 of the counterpart steel wear debris. The metakaolinite-based geopolymer was dominated by severe adhesion wear, while its composites filled with the solid lubricant particulates were characteristic of mild adhesion, scuffing, and delamination.