Wear of Polytetrafluoroethylene Nanocomposites: Effects of Dispersion, Crystallinity and Toughness

Solid lubricants are commonly found in applications where more traditional lubrication techniques are undesirable or precluded. Polytetrafluoroethylene (PTFE) is an attractive solid lubricant, but high wear rates limit its use. While nanofillers have been found, in some cases, to provide wear resistance without introducing other detrimental effects, the mechanisms of wear resistance remain unclear. These studies examine the effects of dispersion, thermal and mechanical properties on the wear resistance of PTFE nanocomposites. Nanoparticles were found capable of imparting substantially increased crystallinity, toughness and wear resistance, though wear resistance did not correlate to dispersion, crystallinity or toughness. It was found that the phase of the alumina filler dominated the wear resistance of the nanocomposite. INTRODUCTION Polytetrafluoroethylene (PTFE) is a technologically important material that often finds use as a solid lubricant in extreme applications. PTFE has very low friction coefficients but suffers from abnormally high wear rates. Hard micro-scale fillers have been shown to consistently improve wear rates by several orders of magnitude, but often, it is at the expense of other properties (mechanical, chemical and thermal). Nanoparticles have high specific surface areas and number densities, and while some scientists believed that low loadings of nanoparticles may provide wear resistance without affecting other properties, many believed that nanoparticles were too small to offer the necessary load support and crack deflection. Several studies have confirmed that nanoparticles can impart PTFE with improved wear behavior [1-4], but 10-100X reductions in wear required high loadings (15-25 wt%) making them comparable to PTFE microcomposites. In 2005, Burris and Sawyer found unique behavior with one particular aluminaPTFE nanocomposite having over 1,000X improved wear resistance at 1wt% loading [5,6]. To date, the factors responsible for these large experimental differences remain unclear. In the nanocomposite materials literature, dispersion is often cited as a primary factor in determining various physical properties of the system that include toughness and polymer morphology [7]. PTFE suffers delamination wear, and subsurface rather than surface mechanics drive the elevated wear rates. In previous studies of the very wear resistant PTFE nanocomposites, fibrillation under concentrated stresses was repeatedly observed. The hypothesis was that improved nanoparticle dispersion and interaction with the matrix induced an altered crystalline structure and fibrillation induced toughness that led to high wear resistance. In this study, dispersion, crystallinity, toughness and wear resistance of PTFE nanocomposites were examined to probe this hypothesis. EXPERIMENTAL Alumina-PTFE nanocomposites were created following the procedure described in Sawyer et al.[3, 5, 6]. The phase and dispersion of alumina were varied to study their effects on wear; α phase alumina has been found capable of high wear resistance at low loadings [5], while Δ:Γ phase alumina has not [3, 6]. Two mixing techniques, hand mixing and jet-milling, were used to vary nanoparticle dispersion. The jet-milling procedure described in Sawyer et al. [3], results in the decoration of the PTFE particles by disbanded nanoparticles. An intentionally agglomerated dispersion was achieved by gently mixing the powders by hand as described in [8]. A linear reciprocating tribometer was used to quantify wear. The design and uncertainties of this tribometer are discussed in Schmitz et al. [9, 10]; test were performed at 50 mm/s sliding speed and 6.3 MPa normal pressure over a 25 mm