Effect of nitrogen plasma immersion ion implantation of polyamide-6 on its sliding properties against steel surface

The sliding behaviour of polyamide-6 (PA6) treated by nitrogen plasma immersion ion implantation (N PIII) has not been studied yet, even if PIII is a promising technique for polymer surface modification. The effect of N PIII of PA6 on its sliding properties against low carbon structural steel S235 was studied. The surface changes were characterised by XPS, contact angle measurements and optical microscopy, while alterations in the sliding properties including friction coefficient, wear and temperature close to contact were studied by a pin-on-disc tribometer under dry and variously lubricated conditions. The N-content of the surface layer increased, while those of C and O decreased upon N PIII treatment. The contact angles decreased and the total surface energy and its polar and dispersive components increased significantly. Under dry sliding in the low Pv regime, the friction coefficient of the treated sample started from a value lower than that of the untreated version and the corresponding specific wear rate was also smaller. Under dry sliding, the increased surface energy caused increased adhesion of the treated PA6, associated with the formation of a transfer layer on the steel disc. Water lubrication was more efficient – in accordance with the increased polar component of surface energy – for the treated PA6 than found earlier for PETP. For the treated PA6, a decreased friction was observed upon run-out type lubrication test with gearbox oil, due to increased retention of oil on the treated surface characterised by an increased dispersive component of the surface free energy.

► Nitrogen-enrichment and increase of surface energy for N PIII-treated PA6.
► Decreased starting friction with subsequent rise due to increased adhesion and formation of transfer layer.
► Agreement of increased surface energy components with enhanced lubrication by water and oil.