The effect of surface roughness on the transfer of polymer films under unlubricated testing conditions

Highlights

• Tribology of high bearing polymers, ATSP, Vespel and PEEK against gray cast iron.

• ATSP exhibits lower wear under unlubricated unidirectional sliding conditions.

• Smoother cast iron surfaces result in uniform polymer transfer films.

• Rougher cast iron surfaces result in patchy polymer transfer films.

Abstract

Blended Polyetheretherketone (PEEK) and polyimide-based polymers are excellent lighter tribomaterials that make possible the replacement of sliding metallic components in machinery. In this work a quantitative analysis of the effect of surface roughness on the transfer of polymer films was investigated using a specialized tribometer, under air-conditioning and refrigeration specific operating conditions. Specifically, polyimide (Vespel SP-21), polyimide (Vespel SP-211), PEEK with carbon fibers, and Aromatic Thermosetting Polyester (ATSP) polymer pins were tested against gray cast iron disks under unlubricated unidirectional conditions. It was found that a continuous transfer layer of polymer on the surface of the cast iron disks with mirror finish (low surface roughness) was produced in all the combinations of tested blended polymers. In the case of the polymer pins tested against cast iron disks with high surface roughness, a discontinuous transfer layer was observed.

Introduction

The advent of high bearing inexpensive tribomaterials (in bulk and coating formats) has shifted the attention of the air-conditioning and refrigeration industries to polymer/metal bearing sliding pairs. Such materials could circumvent the problem of energy consumption caused by viscous losses (at the full film regime) and asperity interaction at the boundary/mixed lubrication regimes, as well as the adverse thermodynamic effects related to the solubility of refrigerants and liquid lubricants. Transfer films play a significant role in sliding applications due to the self-lubricating ability of some polymers, which decreases the use of lubricants contributing to a greener environment [1], [2]. Several researchers have shown the benefits of solid lubrication in the form of transfer layers to provide a reduction in friction and wear. For instance, several filled (blended) and unfilled polymers were tested under unlubricated conditions against gray cast iron and aluminum samples at a typical automotive air-conditioning compressor sliding speed of 2.4 m/s. It was found that a discontinuous or loosely adhered transfer layer was formed during the testing of unfilled Polytetrafluorethylene (PTFE) and PEEK polymer pins [3]. However, when testing was performed under a lower sliding speed of 0.5 m/s, a thicker and uniform layer of approximately 2 μm was measured.

Researchers also studied the effect of counterface roughness and PEEK material transfer onto metallic sliding surfaces. For instance, two different grades of PEEK were tested against hardened D2 tool steel to investigate the influence of wear path shapes on the wear of PEEK. The authors observed irregular and discontinuous films when the sliding motion overlapped, but thinner and more uniform transfer films when the motion at the interface was reciprocating, resulting in lower wear [4]. Also, it was reported that taller and distant peaks of asperities on the tool steel produced spare films and enhanced polymer plowing. The effect of tool steel surface topography on the mechanisms involved in the wear rate of Polyacetal (POM)-20% PTFE composites and the transfer layer morphology were evaluated using a reciprocating pin-on-plate configuration. A strong correlation on the transfer layer of POM-20% PTFE with tool steel surface roughness was found, as well a correlation of the surface topography on the steady-state wear rate [5]. The role of surface texture and roughness of M40 steel disks on the friction and transfer layer formation of soft polypropylene pins was studied using a pin-on-plate configuration. It was found that surface texture plays an important role on the friction behavior during sliding and the mean slope of the profile was the roughness parameter that influenced friction coefficient the most. It was shown that a higher friction coefficient correlated with a higher mean slope value, inducing higher level of shear traction, and thus resulting in more transfer material [6]. Similar results were observed when a statistical correlation of the coefficient of friction and wear rate of PTFE based composites with 42CrMo4 steel counterfaces, was carried out. It was found that statistical parameters related to the shape of the asperities have a crucial influence on the wear of the composites [7].

A transition from relatively low wear to high wear was measured for counterfaces with surface root-mean-square (rms) roughness in the range of 0.13 (smooth) to 0.76 (rough) μm, respectively [8]. A different morphology of the wear debris was also observed: it changed from sheet like (due to fatigue-delamination) to small irregularly shaped particles (attributed to abrasive cutting or low cycle fatigue), for the smooth and rough surfaces respectively. In another study, the effect of surface roughness on the wear of commercial polymers such as POM, PA6.6, and UHMWPE was investigated [9]. An increase in surface roughness caused an increase in wear, being more noticeable for POM, compared to PA6.6 and UHMWPE. In the same study, the authors suggested abrasive wear as the wear mode for counterfaces with surface roughness above 0.8 μm. The friction and wear behavior of different combinations of ATSP/PTFE pins tested against gray cast iron disks was studied in [10]. It was shown that pure ATSP exhibited low wear and high friction and pure PTFE exhibited high wear and low friction. Their wear and friction behavior improved significantly when blended together in different combinations [10]. In fact, lower friction coefficient was observed when the PTFE content in the blends increased. Lower wear rates were observed when the ATSP content increased in the blends, pointing out the beneficial tribological effect of the ATSP/PTFE blends, compared to pure ATSP and pure PTFE.

Zhang et al. [11] published a quantitative analysis of the geometrical shape of PEEK wear debris particles using fractals. Using a pin-on-disk apparatus, their findings showed that wear debris behaves like fractals and the fractal dimension of the wear particles increased with contact load. Their overall analysis provides evidence that fractal dimension of the wear debris might be used to measure the wear rate of PEEK, which is valuable in the monitoring and status of frictional tribopairs of machinery components during service. Friedrich et al. [12] studied the effect of different fillers on the sliding wear of polymer composites. According to their findings, short fiber thermoplastic polymer composites can be used for sliding applications were metallic materials were originally employed. Their applications include temperatures up to 220 °C, contact pressures up to 10 MPa, and sliding velocities up to 3 m/s. It was also shown that a significant improvement in the friction and wear behavior of thermoplastics and thermoset composites could be achieved by the incorporation of nanoparticles with short carbon fibers. This improvement was attributed to the rolling effect of the nanoparticles. Zhang and Schlarb [13] studied the morphology of the wear debris of PEEK under unlubricated sliding conditions against 100Cr6 steel using a block on rig apparatus. They showed particle-like wear debris formed at contact pressures of 1 MPa owing to the plowing effect of the harder asperities of the counterpart on the PEEK surface. Once contact pressure increased from 1 to 2 MPa the wear debris changed from particle-like to rod-like morphology. In the case of 4 MPa, the wear debris displayed rod-like and bamboo-raft-like morphology with a minority of film-like debris. The authors also claimed that at higher ductility (lower stiffness) of PEEK, the wear debris changes from particle-like (due to micro-cutting) to plume-like wear debris. Yeo and Polycarpou [10] reported that the wear debris of PTFE stayed trapped at the sliding interface acting as a solid lubricant while PEEK coatings displayed a flake-like wear debris morphology being continually removed and replaced during sliding, which explained the higher wear rates compared to PTFE based coatings. The motivation of the current study was to understand the effect of counterpart surface roughness in the transfer of polymer films, specific of air-conditioning and refrigeration compressor conditions. Understanding the role of counterpart surface roughness on the transfer of polymer films is important for practical applications where the absence of lubricant is desirable.