Polymeric Nanocomposites for Tribological Applications

Abstract

Composites in TribologyPolymer composites are well known for offering engineers high strength-to-weight ratios and flexibility in material design. [1,2] The physical properties of a composite can be tuned to satisfy various functional requirements of a target application, including stiffness and strength, thermal and electrical transport, and wear resistance to name a few. Often, composites are designed to fulfill several functions simultaneously.One area of engineering that is particularly invested in the development and design of high performance polymer composites is tribology, the science related to interacting surfaces in relative motion. Bearings are systems that contain sliding interfaces, and are relied upon by nearly all moving mechanical systems. Though rarely recognized, Feature ArticlePolymer nanocomposites operate in applications where fluid and grease lubricants fail, and have superior tribological performance to traditional polymer composites. Nanoparticle fillers have been a part of notable reductions in the wear rate of the polymer matrix at very low loadings. Despite instances of remarkable wear reductions at unprecedented loadings (3 000 times at 0.5% loading in one case), there is a lack of general agreement within the literature on the mechanisms of wear resistance in these nanocomposites. In addition, results appear to vary widely from study to study with only subtle changes of the filler material or blending technique. The apparent wide variation in tribological results is likely a result of processing and experimental differences. Tribology is inherently complex with no governing laws for dry sliding friction or wear, and the state of the art in polymeric nanocomposites tribology includes many qualitative descriptors of important system parameters, such as particle dispersion, bulk mechanical properties, debris morphology, and transfer film adhesion, morphology, composition, and chemistry. The coupling of inherent tribological complexities with the complicated mechanics of poorly characterized nanocomposites makes interpretation of experimental results and the state of the field extremely difficult. This paper reviews the state of the art in polymeric nanocomposites tribology and highlights the need for more quantitative studies. Examples of such quantitative measurements are given from recent studies, which mostly involve investigation of polytetrafluoroethylene matrix nanocomposites.

A Review of Transfer Films and Their Role in Ultra-Low-Wear Sliding of Polymers

“…Burris et al [37] measured the wear resistance of another ultra-low-wear PTFE composite transfer film and reported no signs of wear in post-test analysis after thousands of sliding cycles; in this case, however, films were self-mated. Thus, it appears that wear rates of transfer films depend strongly on the conditions used to make the measurement.…”

“…Interestingly, alumina, an inert filler, has proven to be one of the most successful fillers for reducing PTFE wear to date [18]. This fact is especially interesting because different types of alumina fillers have been shown to yield orders of magnitude variations in wear rates of identically-prepared PTFE nanocomposite [13,37]. Figure 1 illustrates the transfer film of a typical PTFE composite; in this case, the PTFE was filled with 15 wt % 76 µm particles of Ni.…”

“…In addition to producing thinner and more uniform transfer films, low-wear alumina-PTFE transfer films are obviously discolored [13,18,20,[23][24][25][26][27]29,31,32,[37][38][39], which suggests chemical reactions had taken place and possibly enhanced adhesion of the transfer film. In 2013, Ye and Burris used optical microscopy to determine the degree to which transfer films persist during sliding [20].…”

“…As shown in Figure 5b, the free-space length appears to be a relatively good predictor of wear rate; the results also suggest there is a lower bound associated with full coverage and an upper bound associated with no coverage. The free-space length is also closely related to the visual cues that typically motivate adjectives like "uniform", "coherent", "continuous", and "complete" in the transfer film literature [15][16][17]37,44]. To date, however, there remain relatively few quantitative studies of transfer film morphology and the community has yet to agree on a single metric by which transfer film quality should be evaluated.…”

The Effect of Counter-face Roughness on the Tribological Behavior of Filled and Unfilled PTFE

“…two or four orders of value by adding any micro or nano sized fillers such as Al 2 O 3 [4], ZnO, MoS 2 , SiC, SiO 2 , ZrO 2 , Al , TiO 2 , Cu, Pb, Ni , CNF (carbon nano fiber), PEEK (polyetheretherketone), carbon fiber , glass fiber, bronze , and graphite powder into the PTFE [1]. The decrease in wear rate depends on various factors such as aspect ratio, type, size, shape, and crystal structure of the fillers, degree of dispersion of filler, and operating conditions [5,6].…”