Polyalkylene Glycol Based Lubricants and Tribological Behaviour: Role of Ionic Liquids and Graphene Oxide as Additives.

The polyalkylene glycol (PAG) lubricants are widely used as gear, bearing and compressor oils in refrigerant systems. They are designed to provide outstanding benefits in terms of efficiency, long oil life, and equipment protection. These fully synthetic lubricants have been developed for use under operating conditions beyond the capabilities of other synthetic lubricants and mineral oils as their characteristic low pour point ensures excellent fluidity at low-temperature. On the other hand, the main drawback of PAG oils is the marked hygroscopicity, i.e., they absorb and bind moisture from the ambient air. In this paper, the results from tribological testing of PAG oil featuring 46 cSt at 40 °C with solid (graphene oxide) and liquid (ionic liquids) additives are discussed. Moreover, in this study the influence of absorbed moisture from ambient air has been investigated. Preliminary experiments with ball-on-flat setup by using reciprocatory tribometer tested the response of base oil PAG 46 blended with 1-Ethyl-3-methylimidazolium acetate or graphene oxide, alternatively. In such a way, the individual influence of each additive was questioned. Based on preliminary analysis, this research focuses on hybrid formulations to substantiate potential improvements of anti-friction and anti-wear PAG 46 performance over broad range of lubrication regimes. Experimental tests have been carried out by using rotational disc tribometer in ball-on-disc configuration. Additional experiments after samples exposure to ambient air and ensuing moisture absorption were carried out. The results highlight that in most cases limited absorbed moisture does not introduce detrimental effects on the tribological performances of the tested oil samples: indeed, water content could reduce wear of the sliding steel surfaces over the whole testing spectrum and in most of the cases the frictional energy dissipation.