Tribology of Polymer Nanocomposites

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0099649SawyerThis SGER award will support a feasibility study of the effect of nanoparticle strenghthening of polymers for tribological applications. There are strong indications that polymers filled with hard nanoparticleswill exhibit significant improvements in tribological performance ascompared to traditional filled polymers. Polymers are widely used inbearing applications because they provide quiet continuous operation, have alow coefficient of friction, absorb vibrations, are compliant andnon-abrasive to the counterface, can be easily manufactured, areinexpensive, non-corrosive, and are generally biocompatible. The challengein designing bearings with homogeneous polymeric materials is their low wearresistance (high wear rates). Hard filler particles are frequently added toimprove the wear resistance, however these hard filler particles increasethe abrasive wear to the counterface and increase the sliding coefficient offriction. Lubricious fillers are also added to polymers and under certainoperating conditions can reduce the wear rate and the coefficient offriction, however a constant supply of lubricous filler must be available atthe wear surface and these materials are frequently sensitive to theenvironment. The ideal filler for polymers would be inert, reinforcing,non-abrasive, and reduce the coefficient of friction. There is goodevidence that nanoparticle filled polymers may be this 'ideal' composite.For example, recent studies have shown that the wear resistance can increasein polymer composites filled with hard nanoparticles, while at the same timethe wear of the counterbody decreases and the sliding coefficient offriction decreases. This type of tribological behavior will have an impactin polymeric bearings covering the spectrum from industrial applicationsneeding dry sliding bearings, to orthopaedic implant materials, toself-lubricating bearings for space environments.In order to design composites with the optimum properties and predictperformance, however, some severe limitations must be overcome. First, ourunderstanding of the mechanisms contributing to wear performance in filledpolymers is poor. For example, the role of the filler / matrix interfaceand the effect of particle size has not been well studied nor are thereappropriate models that consider the interface or size of the filler.Secondly, it is unclear for non-lubricious nanoparticles what the mechanismis that lowers the coefficient of friction. Finally, nanoparticle filledpolymers have not comprehensively explored for wear applications despite thestrong evidence suggesting large improvements in performance.The overall scientific goal of the project is to gain a fundamentalunderstanding of the wear mechanisms in filled polymers by a) experimentallyisolating the effects of particle size (10 nm to 10 micrometers), particleaspect ratio (1 to 1000), dispersion, filler / matrix interface, and matrixproperties on performance, and 2) obtaining a parametric understanding ofthe correlation between wear behavior and other mechanical properties, 3)modeling of the wear properties.***