Engineering ultra-stable polymer surfaces by a synergy of traditional and nano-scale fillers
Date
2021
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Tribological polymers are necessary when more traditional forms of lubrication are undesirable or precluded. Tribological synergy, where a composite presents substantially reduced friction, adhesion, and wear than either constituent, is both practically necessary and possible as demonstrated by previous literature. However, the cause of this unexpected synergy remains uncertain. The accepted explanation is that the filler promotes the formation of high-quality tribofilms, which reduce friction while protecting the contacting surfaces from damage. This conclusion, while valid, neglects the question that matters most: how does one promote the formation of a high-quality tribofilm, the result of a wear process, using controllable aspects of materials design? Leveraging material properties to engineer interfacial properties remains a grand challenge in tribology and is the cornerstone of this dissertation. This project aims to establish a new transfer wear framework, elucidate the extent to which tribological improvements can be attributed to tribofilms and associated tribochemical accumulation, and clarify how filler particles affect transfer film formation, tenacity (strength), stability, replacement, and destruction using a unique combination of experimental approaches. ☐ First, a new indexed reciprocation method was developed to isolate the wear-reducing effects of transfer films. Quasi in-situ observation of the contact during tribological testing demonstrated that fillers regulate debris size independently of the transfer film and that variations in filler size and morphology had significant effects on debris size regulation, abrasion, and filler accumulation. The results of chemical and compositional surface analyses were consistent with the hypothesis that stable interfaces are needed to support the accumulation of protective wear-reducing tribochemical products - i.e. that low wear enables the tribochemical accumulation, which reduces wear in a virtuous cycle. ☐ Second, a novel nanocomposite Polyether ether ketone-Polytetrafluoroethylene (PEEK-PTFE) blend was used to isolate the effects of trace nanofillers on surface stability. Contrary to previous studies showing optimal wear resistance at high nanofiller loadings (5-20 wt%), the nanofillers optimized wear reduction at 0.1 wt% when the subsurface was first stabilized by the secondary PEEK filler. These studies are the first to demonstrate that trace nanofillers are capable of >10-fold wear reductions, even in an already low wear polymer blend. Dry environment studies designed to disrupt tribochemical reinforcement increased wear rates by ~2-fold at trace nanofiller loadings and by ~100-fold at the more typical loading of 5 wt%. The results support the hypotheses that nanofillers reinforce surfaces primarily through tribochemical means, that those benefits are optimized at trace loadings, and that excessive nanofillers severely damage surfaces, particularly in the absence of tribochemical reinforcement. ☐ The final study addressed the connections between wear reduction, tribofilm formation, and materials design. This study proposed that composite microstructure can be used to control the contact size, which is known to govern transfer film and debris formation. A novel interferometry method was developed to study the material removal process and track real areas of contact in-situ, the first of its kind. The results showed direct evidence of contact discretization based on composite microstructure. Additionally, wear rates correlated strongly to the characteristic size of individual contact areas. The results provide the first experimental evidence that microstructure can be used to manipulate contact length scales, which in turn dictate the balance between transfer film and debris formation. ☐ This study presents and experimentally tests the first comprehensive framework connecting transfer film formation and ultra-low wear rates with controllable aspects of materials design. It shows that fillers can be used to discretize contacts into a collection of much smaller contacts that reduce debris size and promote the formation of thin stable tribofilm. These tribofilms protect wear surfaces and lower wear rates further. Over time and in the presence of metal oxide nanoparticles, low wear rates allow for the accumulation of tribochemical reaction products that reinforce tribofilms and reduce wear rates further in a virtuous cycle.
Description
Keywords
Nanofiller, Polymer nanocomposites, PTFE, Solid lubricants, Transfer films, Ultra-low wear