Lubricating Behavior of a Superior PTFE Powder in Lithium Grease Mary Moon, PhD, MBA
Consultant to Shamrock Technologies, Inc., Newark, NJ 07114 USA Abstract
Anti-friction and anti-wear effects of polytetrafluoroethylene (PTFE) additives in lubricating greases depend on characteristics of specific grades of PTFE powders. In a recent study, seven experimental PTFE powders were formulated and milled to prepare models of simple lithium greases. One of seven grades of PTFE gave superior performance in four-ball tests where loads up to 1 765 N (180 kgf) were applied gradually to decrease run-in effects. This grease is a good starting point for formulations with benefits from PTFE. [1-3] The present paper analyzes friction data from these experiments with simple statistics. At low applied loads (< 500-700 N or 50-70 kgf), statistics are similar for polyalphaolefin (PAO) base oil and lithium and PTFE-lithium greases. At higher loads, there are abrupt changes or spikes in friction. For PAO and some PTFElithium greases, these friction spikes are associated with lubrication failure and adhesive wear, which trigger significant increases in friction, temperature and wear scar diameter. A superior grade of PTFE provides antifriction and anti-wear protection against failure and adhesive wear in these tests, however. Analysis of friction data and wear scars indicates that this superior PTFE may be entrained in contacts (and/or at their inlets), compressed and have a resilient protective effect. Results from the literature in support of this mechanism are discussed.
modification significantly reduced run-in effects. Fourball tests with load ramps were used to measure friction, temperature and wear scars for PTFE-lithium greases, control grease (no PTFE) and base oil. [1-3] In lithium grease, one grade of PTFE was superior to the other six grades. As the applied load was increased, abrasive wear was observed at low loads until adhesive wear occurred for base oil, control grease, and six of seven PTFE-lithium greases. Adhesive wear released heat, softened or melted contact surfaces, and significantly increased wear scar diameters. For the seventh grade of PTFE, adhesive wear did not occur as the load was increased to 1 765 N (180 kgf), the limit of the test machine. As a result, friction and temperature were lower and wear scars were smaller, than for other cases where adhesive wear occurred. This superior model PTFE-lithium grease is a good starting point for formulation studies. It could be enhanced with other additives such as anti-oxidants. [1-3] The present paper addresses several intriguing questions associated with lubricating behavior of this superior grade of PTFE. Are results for this PTFE powder anomalous, or are they reasonable in the context of results for other greases and PAO base oil? What are some similarities and differences for lubrication with PAO base oil, control lithium grease and PTFE greases? Is there a possible explanation for effects of superior PTFE on lubrication? Do results from the literature support these findings?
Introduction
Background about PTFE
Previous papers discussed the use of polytetrafluoroethylene (PTFE) as an anti-friction and anti-wear additive, grease formulation guidelines, and a modified four-ball test for friction and wear. Greases were prepared from PTFE, lithium thickener and PAO base oil. Standard four-ball friction and wear tests were performed to evaluate the performance of seven grades of PTFE in greases. However, run-in effects interfered with this comparison. A standard test procedure was modified by gradually applying the load in a ramp. This
Polytetrafluoroethylene or PTFE is a synthetic fluoropolymer of tetrafluoroethylene (-CF2- CF2-)n that resembles ethylene (-CH2-CH2-)n but with fluorine instead of hydrogen atoms. High molecular weight PTFE is a thermoplastic solid that can be ground into powders and dispersed in oil or water. The surface of PTFE is extremely slippery with coefficients of friction, COF, between 0.04 (sliding friction) and 0.1 (ASTM D1894). Other useful properties include resistance to chemicals, solvents and temperature extremes (highs and lows). [4]
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