Effect of Different PTFE Coatings Applied to 18CrNiMo7-6 Steel on the Coefficient of Friction and Wear Under Dry Sliding Contact Using the Ball-on-Disk Method at Different Loads

This study investigates the tribological performance of three commercial PTFE-based Xylan® coatings—Xylan® 1425, Xylan® 1052, and Xylan® 1010—applied to 18CrNiMo7-6 steel under dry sliding conditions. Ball-on-Disk tests were conducted at normal loads of 5, 7.5, and 10 N, wear-track radii of 12, 16, and 20 mm, and corresponding sliding velocities of 0.31–0.52 m·s−1. The tribological evaluation was complemented by measurements of coating thickness, surface roughness, nanoindentation, wear-track profilometry, scanning electron microscopy, EDS mapping, and post-test cross-sectional microscopy. All coatings reduced the coefficient of friction from approximately 0.49–0.64 for the uncoated steel to 0.09–0.13, corresponding to an average reduction of 78–80%. Xylan® 1425 exhibited the highest nanohardness of 57.02 MPa, the highest reduced elastic modulus of 3.33 GPa, and the most favorable elastoplastic indices. It also achieved the lowest wear, with a volumetric loss of approximately 0.03 mm3 under the most severe conditions, representing a reduction of more than 99% compared with the substrate. Xylan® 1010 provided the lowest friction but showed pronounced plastic deformation, whereas Xylan® 1052 exhibited fragmentation and increased wear. Post-test cross-sectional microscopy confirmed local exposure of the steel substrate in both coatings. Overall, Xylan® 1425 provided the best balance of low friction, mechanical stability, coating continuity, and wear resistance.