Tribological Testing of High Velocity Oxy Fuel (HVOF) Sprayed Composite Cermet Coatings Using Ball on Disk Configuration

Abstract

Abstract Thermal sprayed (HVOF) cermet coatings (WC-17Ni) with and without solid lubricant (PTFE) were deposited on steel disks for sliding wear evaluation using the ball on disk configuration. Alumina (Al2O3) balls were slid against coated disks. The coated disks were ground and lapped to roughness of 0.3µm Ra and 0.05µm Ra respectively. A significant reduction in the average coefficient of friction was observed when solid lubricant was incorporated into the cermet coating. There were reductions in average coefficients of friction from 0.84 to 0.55 for ground disks and from 0.65 to 0.48 for lapped disks. These represented reductions in coefficient of friction of 34.5% and 26% by incorporating solid lubricant into the ground disks and lapped disks respectively. The inclusion of solid lubricant in the coating also resulted in significant reduction in the average specific wear rate for both the ball and disk. Abrasive wear was the predominant wear mechanism observed on the worn samples’ surfaces using optical microscopy. A conceptual wear mechanism with the introduction of solid lubricant into the cermet composite coating was proposed. A hypothesis on the formation of lubricating PTFE films on contacting surfaces during sliding wear was supported with elemental mapping by Scanning Electron Microscopy/Energy Dispersion X-ray (SEM/EDX) analysis of worn ball surfaces after sliding against disk counterfaces deposited with composite cermet coating containing solid lubricant. The EDX mapping showed a homogenous distribution of fluorine element across the surfaces analyzed.