PTFE for Lubricating Greases Mary Moon, Ph.D. marymoonphd@gmail.com Consultant to Shamrock Technologies, Inc. Newark, NJ 07114 USA Abstract
The purpose of this study was to investigate the performance of seven experimental PTFE powders in grease and evaluate effects of particle size and dispersion on consistency, stability, friction and wear. Laboratoryscale batches of simple lithium greases were prepared and milled with a high-pressure homogenizer. All PTFE greases were comparable to controls in appearance (offwhite), NLGI grade (2), texture (smooth, buttery) and storage stability (<5% bleed after 4-5 months storage). Four-ball friction and wear tests demonstrated the value of specific PTFE powders as additives. Submicron PTFE powders, formulated in grease without other extreme pressure (EP) or anti-wear additives, reduced wear, friction and temperature during mixed/ boundary lubrication. These PTFE powders prevented adhesive wear due to oil film failure at applied loads up to 180 kg. Test results suggested that PTFE was entrained in contacts and helped support loads. These results depended upon PTFE dispersion in specific formulations.
Introduction
The present paper takes a direct and pragmatic approach to investigate effects of experimental PTFE powders in simple lithium greases formulated without other additives. This Introduction provides background information about PTFE powders. In prior studies of PTFE in greases and liquid lubricants, results were situational and depended upon particle size, dispersion and tribology test conditions such as speed, load and roll-to-slide ratio. This study compares seven powders with primary (fully dispersed) particles that range from
sub-micron to larger sizes. This Introduction compares these particle sizes to nanoparticles, grease fibers and wear scars. [1] Grease preparation is the subject of the next section. Each lab-scale batch of grease was blended and milled with the same procedure to model PTFE dispersion in manufacturing processes. Results for grease consistency and storage stability are presented next. Then, the heart of this paper focuses on four-ball friction and wear testing. Data collected during tests with ASTM D2266 [2] indicated that start-up and run-in affected test results. A four-ball technique was customized by gradually applying loads to lessen start-up and run-in effects. Friction and temperature data and wear scars were analyzed to characterize effects of PTFE powders on lubrication. Polytetrafluoroethylene or PTFE is a synthetic fluoropolymer of tetrafluoroethylene (-CF2-CF2-)n with a chemical formula that resembles ethylene (-CH2CH2-)n but with fluorine instead of hydrogen atoms. In contrast to hydrocarbons, PTFE chains are stiffer with a greater tendency to pack in semi-crystalline arrangements. PTFE films and solid surfaces tend to be white, waxy and smooth. (The most well-known application of PTFE is non-stick coatings on cookware.) PTFE is extremely slippery with coefficient of friction, COF, between 0.04 (sliding friction, D1894 [3]) and 0.1. Other useful properties include resistance to chemicals, solvents and temperature extremes (highs and lows). [4] Lubricants are formulated with PTFE particles that are either synthesized or produced by milling solids. PTFE
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