Humidity-resistant low-friction dry lubricant metallic-polymeric based coatings on PEEK and PET

Polymeric based materials such as polyether ether ketone (PEEK) or polyethylene terephthalate (PET) are one of the most important thermoplastic synthetic materials in industry with a wide range of mechanical, tribological and thermal applications. In the unlubricated state, unfilled materials suffer from a relatively high friction coefficient with a COF >0.8. Deposition strategies were investigated to obtain low friction and wear-resistant coatings on PEEK and PET surfaces via atmospheric pressure plasma deposition (APPD). Several composite coatings based on molybdenum disulphide (MoS 2 ), graphite (C) and zinc (Zn) were developed to achieve good adhesion and low-friction properties. Given that the MoS 2 -C-Zn composite coating system is affected by humidity under ambient conditions, a polymeric based dry lubricant coating was intensively conceptualised to prevent this issue. The designed polymeric lubricant is based on a polyamide 12 (PA12) and a polyethylene terephthalate (PTFE) feedstock fraction. The strategy is to embed high quantities of PTFE inside the PA12 matrix with excellent anchoring properties within the plasma deposition process by generating a humidity independent low friction coating. Therefore, the PA12 feedstock with a relatively low melting temperature (i.e. 180 °C) was ideal to transfer the solid particles into liquid droplets for inserting the PTFE fraction as solid (i.e. with a significant higher melting point of 327 °C) during the coating process. The final PA12-PTFE coatings on PEEK demonstrated a stable COF <0.1 over 10,000 rotational cycles (i.e. 324 m) with no humidity impact during the ball-on-disc measurements under ambient conditions. The specific wear rates of the metallic (9.4 × 10 −4 mm 3 N −1 m −1 ) and polymeric (9.1 × 10 −5 mm 3 N −1 m −1 ) based coatings were also investigated. • Embedding of PTFE particles in PA12 by an atmospheric pressure plasma jet • Cohesion properties between PA12 and PTFE are tunable by the plasma process. • PA12-PTFE films with a COF <0.1 generated on unfilled PEEK. • MoS 2 -C-Zn films exhibit a COF 0.2 when the relative humidity level is <10 %.