Tribological behaviour of goethite (alfa-FeOOH) based nanolubricants

Nanofluids containing lyotropic mineral particles responsive to external magnetic fields are rising large interest, since they combine the fluidity and anisotropy of liquid crystals with the specific magnetic and transport properties of dispersed mineral compounds. Goethite (?-FeOOH) based colloids are mineral liquid crystals exhibiting a peculiar magnetic behaviour, which allows tuning their properties by the application of an external magnetic field. In this work, suspensions in polyalphaolefin of goethite nanorods with different morphology and size have been prepared for anti-friction and anti-wear purposes. The tribological performance of ?-FeOOH has been rarely reported in literature. As goethite is a chemically reactive material, it represents a suitable functional additive in lubricants to promote the formation of a protective tribofilm during the rubbing process, because of tribo-chemical reactions, especially under severe working conditions, such as mixed and boundary lubrication. Nanofluids have been characterized regarding both the viscosity and the stability over time. A ball-on-flat test configuration has been used, in presence of variably oriented magnetic field. The orientation of the external magnetic field and the resulting orientation of nanorods within the lubricant influenced the friction coefficient in the steady state. The anti-wear mechanisms were analysed by comprehensive characterization of tribofilm using Raman spectroscopy, SEM, X-EDS and nanoindentation techniques. This work demonstrates the outstanding anti-wear properties of goethite-based nanofluids, since a decrease of one order of magnitude has been measured for worn volume. The slight decrease of the friction coefficient suggests that nano-additives primarily accomplish an effective anti-wear function rather than simply acting as friction modifiers.