A study on friction and wear properties of PTFE coatings under vacuum conditions
Introduction
Polytetrafluoroethylene (PTFE) is a popular polymer solid lubricant because of its resistance to chemical attack in a wide variety of solvents and solutions, high melting point, low coefficient of friction, and biocompatibility. It is commonly used in bearing and seal applications [1]. PTFE exhibits poor wear and abrasion resistance, leading to early failure and leakage problem in the machine parts. Gong et al. [2] and Blanchet and Kennedy [3] report unfilled wear-rates of 7.36 × 10−4 and 7.41 × 10−4 mm3/N m, respectively. To minimize this problem, various suitable fillers were added to PTFE. Many investigations [4], [5], [6], [7] report that the coefficient of friction can, generally, be reduced and the wear resistance improved when the polymers are reinforced with glass, carbon and aramid fibers. Zhang et al. [8] examined the friction and wear properties of PTFE composites filled with Pb3O4, Cu2O or PbO (addition quantity is 30%, in volume fraction) sliding against GCr15-bearing steel under dry friction condition by using an MHK2500 ring-block wear tester. The results show that the friction properties of these metal oxides filled PTFE composites are almost the same as that of pure PTFE, but the anti-wear properties of these filled PTFE composites are much better than that of pure PTFE. Gregory Sawyer et al. [1] reported a solid lubricant composite material made by compression molding PTFE and 40 nm alumina particles. The results indicate that the friction coefficient of the composite increased over unfilled samples from roughly μ = 0.15 to μ = 0.2, At filler concentrations of 20 wt.%, the wear resistance improved 600×, and the wear resistance of this composite increased monotonically with increasing filler concentration. Five kinds of polytetrafluoroethylene (PTFE)-based composites, pure PTFE, PTFE + 30(v)% MoS2, PTFE + 30(v)% PbS, PTFE + 30(v)% CuS, and PTFE + 30(v)% graphite (GR) composites, were first prepared by Zhang et al. [9]. The results indicate that filling with MoS2, PbS, CuS, or graphite to PTFE can reduce the wear of the PTFE composites by two orders of magnitude compared to that of pure PTFE under dry friction conditions. The friction and wear properties of polytetrafluoroethylene (PTFE) filled with ultrafine diamond (UFD) were studied in detail on a block-on-ring wear tester under dry sliding conditions. Experimental results showed that there was no significant change in coefficient of friction, but the wear rate of the PTFE composite was orders of magnitude less than that of pure PTFE with increasing purified UFD content [10]. Furthermore, Menzel et al. [11] found that the wear resistance of bulk polytetrafluoroethylene (PTFE) when siding against polished steel counterface is shown to be improved by over two orders of magnitude through the use of gamma irradiation.
To this date, a literature search on polymer coatings used in engineering applications yields a scarce amount of work on PTFE-based coatings and other polymer coatings with thicknesses in the order of 15–30 μm, while to the author's knowledge there is few data in the open literature for tribological performance of PTFE-based coatings under vacuum conditions. Polymeric, PTFE-based coatings were investigated for use in air-conditioning compressors. The PTFE-based coatings showed low friction characteristics and high load carrying capacity and it was found that they were not greatly affected by the testing environment. There was a significant increase in wear of the coatings under high contact pressures, however, it was shown that the wear debris generated acted as a third-body lubricant with a beneficial role in the overall wear performance [12]. Guo et al. [13] studied the effects of heat treatment temperature and time on the hardness and the wear resistance of RE (rare earth)-Ni-W-P-PTFE-SiC and RE-Ni-W-P-SiC composite coatings. The results indicated that the abrasion rate was lowest at 400 °C. The rate of abrasion increased with a further rise in the temperature. The wear resistance increased with a rise in heat treatment time, and reached their peak values after 2 h of heat treatment. The experimental results also showed that the wear rate diminished correspondingly with an increase in PTFE quantity. Balaji et al. [14] prepared the composite coatings of bronze (copper with 10–15% tin) with PTFE (polytetrafluoroethylene) particles by means of CECD (conventional electrodeposition) and SCD (sediment co-deposition) techniques. Results showed that the wear resistance of the bronze–PTFE composite coatings can be improved by the presence of dry lubricant PTFE particles and the wear resistance increases by increasing the PTFE particles in the deposit. Ramalho and Miranda [15] investigated the friction and wear behavior of several electroless NiP and NiP composite coatings with PTFE particles. The role of heat treatment of the coating is discussed. Concerning the wear resistance, the heat treatment was very effective on the NiP coatings, while the NiP + PTFE coating reveals the same wear resistance in both cases: as-deposited and heat treated. Yamane et al. [16] investigated the influence of the counter materials on wear and friction performance of polytetrafluoroethylene (PTFE) reservoirs arranged in distinct patterns on coated surfaces. Binary metal–PTFE coatings that have low friction coefficient and high wear resistance were developed and their tribological performance reported [17], [18]. Bodas et al. [19] deposited poly (tetrafluoro ethylene) (PTFE) films by RF sputtering technique on mirror polished silicon 〈1 0 0〉 substrates. Fu et al. [20] electrodeposited Ag–polytetrafluoroethylene (PTFE) composite film with silver-gilt solution of nicotinic acid by a bi-pulse electroplating power supply on 316 L stainless steel bipolar plate of PEMFC. Surface topography, contact angle, interfacial conductivity and corrosion resistance of the bipolar plate samples were investigated. Xu et al. [21] researched corrosion rate and anode polarization curves of electrodeposited RE-Ni-W-P-Sic-PTFE composite coating in various concentrations of phosphoric and ferric chloride. Electroless Ag-polytetrafluoroethylene (PTFE) composite coatings were prepared on stainless steel sheets. The existence and distribution of PTFE in the coatings were analyzed with an energy dispersive X-ray microanalysis (EDX) [22]. Ni–polytetrafluoroethylene (PTFE) composite coating was successfully prepared by brush electroplating. The microstructure of Ni–PTFE composite coating was observed by S-2700 SEM and Sirion 200 field emission SEM [23].
In this paper, the friction and wear behaviors of PTFE coatings sliding against GCr15-bearing steel ball under vacuum conditions were investigated, and the friction and wear mechanisms of PTFE coatings under vacuum conditions were also studied. It is expected that this study may be helpful to the application of the PTFE coatings in aerospace fields.
Section snippets
Material
The PTFE coatings supplied by China Academy of Space Technology (CAST) were fabricated on LY12 substrates with the diameter of 70 mm and the depth of 10 mm by PVD (physical vapor deposition) method on both sides. The thickness and the hardness of coatings were 20 μm, and 24HV respectively.
Experimental approach
The friction and wear behavior of PTFE coatings was performed on a ball-on-disk wear tester (Model TB-1000) under vacuum conditions (10−4 Pa). An illustration of the ball-on-disk wear tester used in this study was
The wear process of PTFE coatings
Fig. 2 shows the wear process of PTFE coatings sliding at the velocity of 0.8 m/s and different loads under vacuum conditions. Obviously, the wear process of PTFE coatings consists of three stages. This is in agreement with the conclusion obtained by [24]. It is expected that there are self-lubricating films formed on the surface of GCr15-bearing steel ball, which is due to the excellent adhesiveness of PTFE coatings. The formation and damage of the self-lubricating film occurred alternately,
Conclusions
- (1)
The friction coefficients of the PTFE coatings first increase with the increase of sliding velocity and then decrease with the increase of sliding velocity when the sliding velocity is higher than 1.2 m/s under vacuum conditions. The wear of the PTFE coating first decreases with the increase of sliding speed and then increases as the sliding speed increases under vacuum conditions.
- (2)
The friction coefficient decreases with the increase of load under vacuum conditions. The wear of PTFE coating first
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