Friction and wear scar analysis of carbon nanofiber-reinforced polymeric composite coatings on alumina/aluminum composite

Alumina/aluminum based composites with excellent physical and mechanical properties offer great potential for lightweight, wear resistant, and high temperature applications. The objective of the present research was to investigate a suitable coating material to provide a low coefficient of friction (COF) during sliding contact. The friction behavior of carbon nanofiber-reinforced aerospace polymer coatings prepared by the spin coating technique were investigated. Polymethylmethacrylate (PMMA), bis A polycarbonate, and two biphenyl endcapped poly(arylene ether phosphine oxide) compositions, namely BPETPP-E and 6FETPP-E, were used as the matrices. Pin-on-disc experiments were performed between 440C stainless steel balls and disc samples of coated alumina/aluminum interpenetrating phase composites at 0.2 m/s sliding velocity, in air, at room temperature under 0.25 and 0.74 N normal load. In all cases, formation of a lubricious carbon layer and its transfer to the steel counterface was observed to result in lower COF (∼0.2–0.3). Higher levels of fiber content (40 and 60 wt.% fibers) contributed to a faster formation of this layer. Wear scar analysis showed the dual roles of the carbon nanofibers, serving as solid lubricants and as reinforcement in the coatings. The amount of debris generated and the coverage of the lubricious carbon-rich film on the scar surface was dependent on the matrix material used. Adherent and uniform coverage of a lubricious carbon-rich film at the wear contact with the least amount of debris fragments was obtained only for composite coatings using BPETPP-E and 6FETPP-E matrices.