Friction Reduction Mechanism of Aqueous Hydroxyethyl Cellulose Solution Enhanced by Synergistic Effect of APTES

In this paper, the coupling agent of APTES was employed to improve the lubricating properties of hydrated HEC. The synergistic effect between APTES and HEC was analyzed by means of Raman, FT-IR, and optical observations. The lubricating properties of water, HEC, APTES, and APTES–HEC solutions were separately investigated by the tribological tests with Si 3 N 4 ball and textured Ti–6Al–4V. It was found that APTES can promote the interaction between HEC molecules, resulting in the formation of homogeneous films without clustering, while thickening the solutions. The APTES-enhanced HEC exhibits excellent lubrication behavior with minimal coefficient of friction, which decreases from 0.029 to 0.017 (down by 38%) as friction goes on. In addition, by measuring the wear loss of the balls, it was shown that the mixture of APTES and HEC provided the best anti-wear capability. The film thickness calculated by the Dowson–Hamrock theory matches the trend of the friction coefficient. However, when the friction velocity exceeds a threshold value, the theory is not applicable anymore. The SEM images detected the presence of two structures in the wear area, namely plow and asperity, and suggested two corresponding lubrication states. The hydrodynamic lubrication occurs within the plow. XPS results suggest that APTES can form Si–O–Ti via chemical reaction, which induces the adsorption of APTES–HEC chains onto the asperities of worn Ti–6Al–4V. The adsorption of APTES–HEC produces an adsorbed lubrication layer, which prevents direct contact between the asperities where boundary lubrication occurs. Graphical Abstract