1. Wear
   
   1. Wear of Mechanical Systems
      
   2. Wear of nanoComposites
      
2. Friction and Lubrication
   
   1. Granular Lubrication and Wear
      
   2. Gas Surface Interactions
      
3. Biomaterials Tribology
   
   1. Wear of Total Knee Replacements
      
   2. Hydrogels
      

HERE ARE STRONG indications that polymers filled with hard nanoparticles will exhibit significant improvements in tribological performance as compared to traditional filled polymers. Polymers are widely used in bearing applications because they provide quiet continuous operation, have a low coefficient of friction, absorb vibrations, are compliant and non-abrasive to the counterface, can be easily manufactured, are inexpensive, non-corrosive, and are generally biocompatible. The challenge in designing bearings with homogeneous polymeric materials, is their low wear resistance (high wear rates). Hard filler particles are frequently added to improve the wear resistance, however these hard filler particles increase the abrasive wear to the counterface and increase the sliding coefficient of friction. Lubricious fillers are also added to polymers and under certain operating conditions can reduce the wear rate and the coefficient of friction, however a constant supply of lubricous filler must be available at the wear surface and these materials are frequently sensitive to the environment. 

The ideal filler for polymers would be inert, reinforcing, non-abrasive, and reduce the coefficient of friction. There is good evidence that nanoparticle filled polymers may be this 'ideal' composite. For example, recent studies have shown that the wear resistance can increase in polymer composites filled with hard nanoparticles, while at the same time the wear of the counterbody decreases and the sliding coefficient of friction decreases (Wang et al, 1996-1997, Petrovicova et al., 2000). This type of tribological behavior will have an impact in polymeric bearings covering the spectrum from industrial applications needing dry sliding bearings, to orthopaedic implant materials, to self-lubricating bearings for space environments. 

In order to design composites with the optimum properties and predict performance, however, there are some severe limitations that must be overcome. First, our understanding of the mechanisms contributing to wear performance in filled polymers is poor. For example, the role of the filler / matrix interface and the effect of particle size has not been well studied nor are there appropriate models that consider the interface or size of the filler. Secondly, it is unclear for non-lubricious nanoparticles what the mechanism is that lowers the coefficient of friction. Finally, nanoparticle filled polymers have not comprehensively explored for wear applications despite the strong evidence suggesting large improvements in performance. 

The overall scientific goal of the project is to gain a fundamental understanding of the wear mechanisms in filled polymers by a) experimentally isolating the effects of particle size (10 nm to 10 mm), particle aspect ratio (1 to 1000), dispersion, filler / matrix interface, and matrix properties on performance, and 2) obtaining a parametric understanding of the correlation between wear behavior and other mechanical properties, 3) modeling of the wear properties.

Oxford English Dictionary

Where observation is concerned, chance favors only the prepared mind.

Louis Pasteur

Micrographs of nanotubes dispersed in polyvinyl alcohol, and nanoparticles in epoxy

A comparison of the scratch behavior of nanofilled and neat epoxy. The scratch depth decreases by 50% and the damage surrounding the scratch decreases.