Sliding wear and friction characteristics of polymer nanocomposite PAEK-PDMS with nano-hydroxyapatite and nano-carbon fibres as fillers

The development of a suitable polymeric bioactive composite with hydroxyapatite as a filler is one of the very actively pursued areas in bioapplications. This report concerns development of such a novel polymeric biocomposite viz. poly (aryl ether) ketone-poly (dimethylsiloxane) with a small percentage of nano carbon fibres and varying percentages of nanohydroxyapatite particulates as fillers. The earlier characterization of this material involving mechanical, thermal and bio-compatibility studies showed optimum improved behaviour at about 7% nanohydroxyapatite loading as reported elsewhere. In this study, the wear and friction response of this biocomposite was tested in air under dry sliding conditions against hard steel using a pin-on-disc apparatus. Interestingly, the adhesive wear characteristics of this nanocomposite with varying nanohydroxyapatite percentages showed a trend similar to that in other characteristics with lowest wear occurring around the same nanohydroxyapatite percentage. It was observed that the specific wear rate in this novel nanocomposite was exceptionally low [~ 10^-8 (mm³/N-m)] compared to that in other similar polymer composites. The origin of this very low wear rate can be associated with the multiple strategies used in the preparation of this nanocomposite such as the use of poly (dimethylsiloxane) which is known to provide a cushioning effect in the matrix. In addition, the phosphate grafting of poly (dimethylsiloxane), the nanonature of both the fillers and their specific surface treatments using aminosilane for enhancing the matrix- filler interfacial bonding all of them seem to have played their expected beneficial roles resulting in the above very low wear rate. The earlier studies on this nanocomposite have shown improvement of the mechanical compressive strength with the addition of carbon nanofibres. Interestingly, here the friction coefficient of the nanocomposite with carbon nanofibres is consistently higher than that without carbon nano fibres for different nanohydroxyapatite percentages samples, for both low (5 N) as well as high (30 N) applied load. It could possibly be due to dislodged carbon nano fibres acting as a third body abrasive or fibres acting as weak links in the matrix filler network affecting the friction response. These wear and friction measurements have clearly brought out the various interesting aspects of the tribological response of the nanocomposite material and the intricate roles played by its matrix component poly (dimethylsiloxane) and the surface treated nano fillers nanohydroxyapatite and nano carbon fibre.