Tribochemical Interactions between a Polymer Composite and Metals

Abstract

The high specific strength, chemical resistance and processability of polymer composites have made them an attractive alternative to traditional metals and ceramics in many industries. For tribological applications, polymer composites also have the ability to eliminate the need of lubricants and lower maintenance costs. The use of carbon fiber, carbon black and polytetrefluoroethylene (PTFE) are well established in the literature as effective reinforcement agents and solid lubricants respectively but not many studies have explored the tribochemical interactions that occurs during sliding. This study investigates the tribochemical interactions between a polyphenylene ether (PPE) and high impact polystyrene (HIPS) blend based composite and different metal surfaces. Four common metals used in industry were chosen for this study: carbon steel C1018, naval brass 485, Inconel 625 and stainless steel 316. In order to isolate the effect of tribochemical interactions between the polymer composite and counterface metals, consistent pressure and velocity (PV) settings were used for all tests. Frictional forces and temperature data were recorded during testing and the wear rates were determined by weighing samples before and after testing. The polymer sand metal washer surfaces were then examined under scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) for their PTFE surface morphology and transfer film composition, respectively. The surface roughness of both polymer and metal samples were also measured. It was observed that tribological performance of the polymer composite was affected by the composition of the metal counterface, and each metal had a different tendency to operating in a stable and unstable state. The surface morphology of the PTFE phase and the transfer film composition on the metal washers also differed between each polymer-metal system. SEM micrographs reveal agglomeration of PTFE domains on the polymer surface and each system had a different domain size distribution and PTFE surface coverage. The polymer-brass system was found to be the most consistent and give the most stable operations with the highest PTFE coverage on the polymer sample’s surface due to brass’ relatively high reactivity. This was explained by tribochemical reaction that occurs at the interface and the reactivity of each metal alloy. Adhesion must be high in order to enable a thicker and more uniform transfer film to adhere, which provides a smooth asperity-free surface for the polymer to slide against, resulting in a stable and low wear operation. A reactive interface allows the introduction of carboxyl groups on both the surfaces and increase electrostatic adhesion between the polymer transfer film and metal surface. Overall, the reactivity of each metal alloy correlated well with the number of stable tests that each polymer-metal system demonstrated as well as the resulting surface coverage of PTFE. This was taken as evidence of the tribochemical interactions.