Polyetheretherketone,hexagonal boron nitride,and tungsten carbide cobalt chromium composite coatings: Mechanical and tribological properties

Composite powder coatings consisting of polyetheretherketone (PEEK), hexagonal boron nitride (hBN), and tungsten carbide cobalt chromium (WC-CoCr) particles were prepared by mechanical grinding and applied on steel substrates by thermal fusion of the thermoplastic polymer. The coatings contained about 20–60 vol% of hBN and WC-CoCr, and were designed to maximize modulus and hardness and minimize friction coefficient and wear rate. The mechanical and tribological properties of single- and double-layered coatings were characterized using nanoindentation and sliding friction and wear measurements. When the hBN concentration was about 30 vol%, the PEEK–hBN composite modulus was lower than that of neat PEEK, which is attributed to the disruption of PEEK crystallization by the filler particles. Upon the inclusion of WC-CoCr particles, the composite's modulus, and hardness showed a substantial increase beyond PEEK values. Elastic moduli of the mixed-filler systems were closer to the Reuss bound than the Voigt bound and could be correlated well with the coating composition using volume-fraction-weighted powers of component properties. Fitted values of the exponent (called the microstructural coefficient) were consistent with the expected continuity and connectivity of the composite's hard and soft phases. Viscoplastic energy dissipation increased with an increase in the polymer-filler interfacial area but decreased with the soft-phase volume fraction. The plasticity index was found to increase logarithmically with the coating modulus. The specific wear rate increased sharply beyond a composition-dependent critical value of the plasticity index. Mechanical polishing of the coating surfaces using abrasive slurries lowered the friction coefficient but increased the wear rate.