| To meet the higher performance requirements of today's air-conditioning and refrigeration (ACR) compressors, their operating conditions have been getting harsher under higher speed and load, thus making tribological characteristics of interacting surfaces playing a significant role in compressor's reliability. However, the capabilities of conventional fluid lubricants are limited such that the state of lubrication is usually unknown, and, at best, in the boundary/mixed lubrication regimes. Therefore, it becomes necessary to implement some type of advanced protective coatings on the interacting surfaces to withstand stringent contact conditions. Due to favorable tribological performance, polytetrafluoroethylene (PTFE)- and polyetheretherketone (PEEK)-based polymeric coatings have received interest in ACR compressor applications, as a potential solution to supplement and potentially replace conventional oil lubricants. However, compared to a great amount of research and experiments done so far for bulk of polymers, there is limited literature on the tribological performance of PTFE- and PEEK-based polymeric coatings.;In this work, several PTFE-, PEEK-, resin- and fluorocarbon-based polymeric coatings, coated on gray cast iron were tribologically evaluated using a specialized tribometer under compressor specific conditions. The coatings showed good to excellent tribological performance, and in general PTFE-based coatings exhibited better friction and wear behavior than the rest of the coatings, including PEEK-based coatings.;The micromechanical properties of polymeric coatings were examined using instrumented microindentation. The load-unload responses were used to measure the load-bearing properties of the coatings, as well as to extract their elastic modulus and hardness values. Induced structural differences between the PTFE- and PEEK-based coatings were confirmed using scanning electron microscopy. These measurements were used to explain the difference in the tribological performance between PTFE- and PEEK-based coatings.;Additionally, the polymeric coatings were tested under elevated (aggressive) temperature conditions to investigate the effect of increasing temperature on their tribological behavior. The friction coefficient of the polymeric coatings usually increased with temperature, reaching a maximum value in the vicinity of their glass transition temperature, and then dropped significantly with further increase of temperature. A measured property called "recovery" was investigated as a key factor affecting the frictional behavior of these coating surfaces using scratch testing, showing that surfaces with higher recovery exhibited lower friction coefficient.;Finally, the tribological performance of two representative PTFE- and PEEK-based polymeric coatings was evaluated under fretting motion testing. The effect of oil on the friction and wear behavior of the coatings was also studied under fretting test conditions. It was found that the eventual tribological behavior of a polymeric coating depended greatly on the transfer film formed on the counterface. Coating tested with oil showed worse performance than dry condition because the oil prevented the formation of transfer film on the counterface. The morphology of the transfer films on the counterface was observed using SEM and profilometer measurements along with detailed discussion of mechanism of transfer film development and its effect on polymer tribology. |
