The effect of surface roughness on the transfer of polymer films under unlubricated testing conditions Emerson Escobar Nunez a , Andreas A. Polycarpou b,n a Grupo de Investigación en Tecnologías para la Manufactura (GITEM), Departamento de Energética y Mecánica, km 2 Vía Cali-Jamundí, Universidad Autónoma de Occidente, Cali-Valle, Colombia b Department of Mechanical Engineering, 3123 TAMU, University of Texas A&M, College Station, TX 77840, USA article info Article history: Received 3 September 2014 Received in revised form 27 December 2014 Accepted 31 December 2014 Available online 8 January 2015 Keywords: Solid lubrication Blended polymers Transfer films Surface roughness Compressor PEEK Polyimide ATSP 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. & 2015 Elsevier B.V. All rights reserved. 1. 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 bear- ing 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 lubrica- tion 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 com- pressor 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 har- dened 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 rough- ness 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 Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/wear Wear http://dx.doi.org/10.1016/j.wear.2014.12.049 0043-1648/& 2015 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ1 979 458 4061; fax: þ1 979 845 3081. E-mail address: apolycarpou@tamu.edu (A.A. Polycarpou). Wear 326-327 (2015) 74–83