XPS study of transfer films of MoS2 filled nylon-46 composites sliding against steel counterfaces

Abstract

When polymers and their composites are slid against metal counterfaces, they are worn off, and a part of the wear fragments deposits onto the counteface as a polymer transfer film [1]. In repeated sliding of polymeric materials on the same sliding track, their tribological behavior is affected by the transfer films [1–3]. Accordingly, for a comprehensive understanding about the tribological behavior of polymeric materials, it is very important to clarify the physical and chemical nature of the transfer films, such as their thickness, coherency and chemical composition. The effects of the transfer films on the tribological properties have been investigated in various polymer-metal sliding combinations [2–5]. Nylon is one of the engineering plastics used for many engineering parts and components. Among the various kinds of nylon, such as nylon-6, 66, 610 and 46, nylon-46 is a new engineering plastic which is expected to be superior to the other nylons because of its higher modulus and strength as a result of being highly crystalline. In our preliminary experiments on the tribological properties of nylon-46, it was observed that nylon-46 exhibited a relatively high sliding friction as well as the other nylons. Expecting to reduce the friction of nylon-46, a solid lubricant of MoS2 was examined as a filler. However, the incorporated MoS2 fillers had little effect in lowering the friction. It is well-known that the lubrication performance of MoS2 itself is controlled by many physical and chemical factors, such as crystallographic imperfection, water vapor, temperature and environmental atmosphere [6, 7]. Moreover, it was also reported that MoS2 could oxidize during sliding contact [8]. More recently, with MoS2 filled nylon 6 composites sliding against steel counterfaces, it was reported on the basis of X-ray photoelectron spectroscopy (XPS) that MoS2 fillers in the transfer films oxidized into MoO3, and that such a oxide of MoS2 may deteriorate the wear resistance of nylon 6 composites [9]. In the present study, with nylon-46 filled with various size of MoS2 particles, the characteristics of the transfer films produced in the sliding contact were investigated by means of XPS, especially the effect of filler size on the chemical change of MoS2 filler in the transfer films. As shown in Table I, nylon-46 and four kinds of its composites filled with various size MoS2 fillers of which the mean particle diameters of 0.5, 1.5, 10 and 100 μm were prepared. The filler content was 20wt% for all composite samples. A pin-on-disk type wear testing apparatus was employed in the sliding experiments. In order to investigate the characteristics of the polymer transfer films, the disk specimens for XPS analysis and SEM observation were prepared by rubbing one end surface of a cylindrical pin 3 mm in diameter against a 25 mm square steel disk 2 mm in thickness. The frictional surfaces of the cylindrical pin were finished with a #800 emery paper. The disk surfaces were finished with #1500 and #100 abrasive papers, and their surface roughnesses were 0.03 and 0.5 μm Ra, respectively. After the surface finishing, they were cleaned ultrasonically in acetone for 15 min before sliding testing. The pin was slid repeatedly against a circular track 16 mm in diameter with load of 10 N and velocity of 0.1 m/s, and in the ambient atmosphere at room temperature and at about 60% relative humidity. After 20 000 disk revolutions, the steel disk was immediately introduced into the analysis chamber. XPS spectra were recorded on a Model-255 ESCA/AES spectrometer (Ulvac-Phi Co.) using MgKα radiation. The pass energy of the analyzer was set at 25 eV. The analysis area of the XPS was estimated to be a circular one about 5 mm in diameter. Fig. 1a and b shows typical examples of SEM observations of transferred polymer materials adhered on the disks. Frictional tracks on the steel disks appear to be covered with relatively coherent transfer films, since the original random scratches finished with abrasive papers were not recognizable on the track. Fig. 2a, b and c shows XPS spectra of Mo3d, S2p and N1s for the transfer films of nylon-46 composites filled with MoS2 filler of 1.5 μm particle size in comparison with the spectra for the bulk composite. As shown in Fig. 2a, Mo3d spectrum of bulk composite exhibits a spin doublet composed of Mo3d5/2 and Mo3d3/2, named as Mo(1) and Mo(2). On the other hand, with those of the transfer films, a new additional Mo3d peak, named as Mo(3), at the binding energy of 238 eV that is about 3 eV higher than those of Mo3d3/2 spectrum of the bulk polymer Mo(2). This observation suggests that