Characterizing PTFE transfer film properties to elucidate transfer film's role in ultra-low wear sliding of polymer nanocomposites
Date
2014
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Publisher
University of Delaware
Abstract
Polymers are used as self-lubricating materials in applications where the use of traditional lubricants is undesirable or precluded. Neat polymers often lack sufficient lubricity or wear resistance for bearing applications, so fillers are used to reinforce the polymer and improve its performance in sliding conditions. In the last two decades, the use of nanofillers has been explored to provide reinforcement without the abrasive effects of the more traditional fillers. For the most successful of these tribological nanocomposites, thin uniform films of polymer are found to be strongly adhered to the hard bearing steel countersurfaces. The improvements in tribological properties for these nanocomposites are widely attributed to the quality of these so-called transfer films. The proposed transfer film attributes responsible for the wear reductions include: (1) thickness, (2) area fraction, (3) uniformity and continuity, (4) intrinsic wear resistance, (5) counterface adhesion, (6) chemical composition and (7) mechanical properties of transfer film. However, few of these has been studied quantitatively and it remains unclear which is most closely related to tribological performance. This thesis aims to elucidate how these attributes affect wear performance by 1) developing quantitative metrics for transfer film properties and 2) correlating these properties to wear properties using a particularly effective and well-studied polymer nanocomposite system. The results suggest the transfer film continuity is most strongly related to system's wear and can be quantified using the free-space length ( Lf ), a new metric developed here to access transfer film quality. Other results suggest that effective nanofillers interact with the matrix polymer and possibly lead to the reduction of wear debris size during sliding. Very small debris (<1 μm) inherently adheres to the counterface and initiates a strongly adhered transfer film. Over time, more transferred small wear debris accumulate and form a thin, continuous and complete transfer film. The sliding frictional energy further induces excessive polymer degradation in the transfer film which increases the hardness ( H ), modulus (E ), surface energy (γ) and intrinsic wear resistance (Kfilm ) of the film. Debris size regulation, polymer degradation and improved film-counterface adhesion are identified as the most important factors in the wear reduction mechanisms.