Investigating the tribology of poly-ether-ether-ketone at high temperatures
Understanding the temperature-resistance characteristics of polymers is important for characterizing the tribological behavior of these materials in various applications.1 For instance, poly-ether-ether-ketone (PEEK)—compared with other polymers—exhibits good thermal–mechanical properties, as well as low flammability and chemical resistance. PEEK is therefore frequently used as a replacement for metals in a variety of high-performance end-use applications.2–6 With this increasing use of PEEK-matrix materials in various fretting conditions, it is necessary to study the material's response in specific applications and situations.
To date, however, only a few studies have focused on the fretting behavior of PEEK. Moreover, among these studies, the focus was mainly on improving the fretting wear resistance of PEEK with the use of different fibers and solid lubrications.7, 8 Indeed, it has been found that fillers improve the tribological properties of pure PEEK, but at the expense of a degradation in other properties.9, 10 To identify a clear relationship between the microstructure, physical properties, and fretting wear during high-temperature testing, pure PEEK formulations must therefore be chosen.
In this work,11 we used an oscillating reciprocating ball-on-disk tribometer to investigate the fretting wear behavior of PEEK under high-temperature conditions. In particular, we determined the friction coefficients and wear values of our samples. We also examined their worn surfaces, microetching morphologies, and thermophysical properties. In this way, we were able to analyze in detail the wear mechanisms of PEEK at different temperatures. In addition, we used x-ray diffraction (XRD) and the permanganate etching technique to investigate changes in the crystalline structure of PEEK during the fretting tests.
The variation in the coefficient of friction and the specific wear rate of our PEEK samples under different temperatures (25–250°C) is illustrated in Figure 1. We find that a substantial transition in the characteristics of both parameters occurs at a temperature of 150°C. This result thus implies that the transition of PEEK wear mechanisms during the fretting tests occurs at this temperature. This transition in the PEEK wear mechanism is also evident in the observed morphologies of the worn PEEK surfaces (see Figure 2). The scanning electron microscope images reveal observable differences in the worn regions of the samples at different temperatures, and confirm that the wear mechanism transition occurs when the test temperature is higher than the material's glass transition temperature.
Our XRD results (see Figure 3) show that the wear contributes to the formation of more crystalline PEEK. Furthermore, this change in crystallinity affects the tribological behavior of the polymer material. We also note that the plastic deformation on the worn surfaces—which results from adhesive wear—disappears after etching (see Figure 4). This indicates that the amorphous phase of PEEK is the main component of the plastic deformation.
From our various results, we can develop an overall understanding of the PEEK wear characteristics. It is thought that frictional heat can be generated, but not released, during the fretting process.12 The contacting surface of the PEEK specimens therefore reaches a high temperature, softens, and causes material to be transferred. A uniform and continuous transfer film forms on the counter steel during the room temperature test. At higher temperatures, however, the transfer films become thin and discontinuous. Although these transfer films play an important role in wear reduction, the average Shore hardness values of the PEEK samples decrease with increasing temperature and thus result in a decrease in wear resistance. Indeed, this is the reason for the lower wear rate of the room-temperature PEEK sample compared with the specimens tested under hotter conditions.
In summary, we have investigated the fretting wear behavior of PEEK samples at high temperatures. In our experiments we examined the tribological, morphological, and crystallinity characteristics of the specimens both before and after etching, and at a range of temperatures (up to 250°C). Our results indicate that a change in the wear mechanism of PEEK occurs above 150°C (i.e., above the glass transition temperature). In addition, the wear of the samples affects the crystallinity of PEEK, which in turn affects its tribological behavior. Our results provide some guidance in the use and design of PEEK components. In our future work we will evaluate the affect of load, frequency, stroke, and fillers on the fretting wear behavior of PEEK under high temperatures. We will also analyze in detail the corresponding wear mechanisms.
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