Glycerol aqueous solutions for the enhanced tribological behaviour of polymer composites sliding against steel

In this study the potential benefits of using glycerol solutions containing 20%, 40%, 60%, or 80% water as environmentally adapted lubricants (EALs) on the tribological properties of polyoxymethylene (POM) and polyetheretherketone (PEEK) polymer composites was examined. For comparative purposes, pure glycerol and water were used. The friction and wear mechanisms of three different polymer composites were analysed: PEEK CF30 (PEEK reinforced with 30% carbon fibre), PEEK MOD (PEEK reinforced with 10% carbon fibre, 10% graphite and 10% polytetrafluoroethylene), and POM CF30 (POM with 30% carbon fibre). The tribological measurements were performed under reciprocating sliding conditions with a steel ball on a polymer plate. For all the composites in glycerol and in glycerol with up to 40% water the coefficient of friction was low (∼ 0.02). The specific wear rate was also low and about the same for all polymers, of the order of 10−8 mm3/(Nm). However, at higher water contents (80% of water and pure water) both the coefficient of friction and the specific wear rate increased and a transfer film formed on the steel counterparts. In the lubricant with a higher water content the PEEK CF30 showed the lowest specific wear-rate increase (24 times in the lubricant with 80% water), followed by the POM CF30 (52 times in the lubricant with 80% water). At higher water contents the POM CF30 showed the smallest increase in the coefficient of friction (3.6 times in the lubricant with 80% water), an increase up to two times less than for the PEEK polymers.