Wear Analysis of Polytetrafluoroethylene (PTFE) Reinforced with Carbon and Bronze

Abstract

Conventional oil-lubricated reciprocating air compressors use a lubricating oil to minimize the friction and piston ring wear. But this necessitates replacement of oil and oil filters at regular intervals, in addition to oil condensate treatment. Oil-free compressors minimize these operational costs, protect the environment, minimize leaks and energy costs. To achieve this, oil-free compressors use piston rings that are fabricated using dry lubricating materials like Polytetrafluoroethylene (PTFE) composites. For sustained air delivery, it becomes necessary to ensure that the rings have better tribological properties. Different hard filler elements along with solid lubricants are added to PTFE composites to upgrade their tribological properties. However, their suitability and design point particularly for compressor application are not well researched and hence continue to possess design challenge. This research investigates the methods of improving tribological properties of the PTFE composites and hand picks the best suitable composition for further analysis on real application. In this, after analyzing piston rings from different compressor manufacturers and literature survey, PTFE filled with three different filler materials (i) Carbon (ii) Bronze (iii) MoS2, with various respective content in increments is manufactured. Wear analysis is performed on the specimens on a pin-on-disc tribometer and worn out surfaces are studied using scanning electron microscope. Further, a study using design of experiment is performed to determine individual and interactive effect of load and sliding velocity on wear rate and friction coefficient. The investigation reveals that the tribological properties strongly depend on the PTFE composite content and wear disc material. The properties are distinctive with no pattern emerging out from the same material family. The results revealed that 25–35% carbon-filled PTFE has better tribological properties and is more suitable for piston ring application. Adding graphite or MoS2 does not improve both wear rate and friction coefficient simultaneously. The result also revealed that for coefficient of friction to be low, the maximum sliding velocity should be restricted.