Abstract
Abstract A method for the testing of solid lubricants for cold metal forming is proposed: the method includes the evaluation of the coefficient of friction under the condition of small surface enlargement and then the determination of the rate of increase of the coefficient of friction with further enlargement of the surface area. The rate of increase of the coefficient of friction with surface enlargement is taken as an index of the ability for covering the newly developed surface — “coverability”. A low rate is desirable. Several solid lubricants were tested, using a tool steel and an aluminium workpiece. The powder lubricants MoS 2 , graphite and PTFE when sprayed onto the workpiece showed low coefficients of friction and an excellently low rate of increase. Johnson's wax provided an extremely low coefficient of friction and a moderately low rate of increase. The powder lubricants boron nitride, graphite and MoS 2 , mixed with a polymer to cement them to the workpiece also presented low coefficients of friction and fairly low rates of increase. The powder lubricants themselves, however, when simply painted onto the workpiece, gave high coefficients of friction and a high rate of increase which was ascribed to their being easily rubbed from the workpiece surface. In addition to the evaluation of solid lubricants, the effect of the testing conditions — sliding speed, surface roughness and film thickness — on coverability was investigated for sprayed graphite powder.
Published Version
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