Perfluorocarboxylic acids as additives for friction reduction in full-film EHD contacts: Influence of additive chain length

Six perfluorinated, saturated and un-branched monocarboxylic acids (CF) with different chain lengths (8–18 C atoms) were used as additives to a polyalphaolefin base oil under full-film elastohydrodynamic (EHD) conditions in steel/steel and steel/sapphire contacts at ambient temperature. In the steel/steel contacts the longer alkyl chain lengths (CF14, CF16 and CF18) provided as much as 37% friction reduction, while the shorter alkyl chain lengths (CF8, CF10 and CF12) resulted in up to 6% friction reduction compared to the base oil. The CF16 and CF18 additives also maintained low friction during prolonged tests with more than 30% and 17% friction reduction, respectively. The steel/sapphire contacts had an up to 60% friction reduction compared to the base oil. The Fourier-transform infrared spectroscopy, wetting, white-light interferometry and scanning electron microscopy confirmed that achieving a complete, coherent and uniform additive CF layer with a low polar surface energy is essential to provide effective boundary slip, which was found to be the responsible mechanism for such a large reduction in the friction. This additive-enabled EHD slip mechanism, confirmed at ambient temperature, is a noticeable improvement for the applicability of the EHL boundary-slip concept, since it enabled additive-based friction reduction also at low temperatures, which was so far obtained only at high temperatures with previously reported simple polar oleophobic additives. Moreover, the amount of friction reduction surpassed previously reported values.