Kinetic Friction in or Near the Boundary Region. II. The Influence of Sliding Velocity and Other Variables on Kinetic Friction in or Near the Boundary Region

An investigation has been made of the causes underlying change of friction with sliding velocity. The method adopted was to measure the friction of several different combinations of materials under three different conditions: dry, with excess of various lubricants, and with thin films of various lubricants applied by two different methods. The experiments made show that changes in friction with velocity may be derived from at least three sources. First, such a change may be derived from the properties of the clean metal surfaces. Certain combinations of materials show, when unlubricated, a decrease in friction with increasing velocity, and this tendency may remain in a modified form even when a lubricant is added. Various hypotheses to explain this tendency are discussed. Secondly, with a boundary film of lubricant present, friction may increase with increasing velocity, and the evidence suggests that this change is due to partial destruction of the boundary film, the rate of which destruction rises with increasing sliding speed. Thirdly, when there is excess of lubricant, fluid film effects may occur even with restricted areas of contact and sliding speeds as low as 0∙5 cm./sec.; such fluid film effects bring about a decrease in friction with in creasing velocity. This tendency is most marked when one component is a soft metal, and may well explain the good ‘antifriction’ properties of the ‘Babbitt’ and similar alloys. A brief investigation was also made of the effects of surface finish and of continual sliding or ‘running-in’, and the results obtained are discussed in the light of in formation gained from the experiments on sliding velocity.