Thermoelectric Experiments with Extreme-Pressure Lubricants
Authors: Schnurmann Robert 
Published: January 2005

Reference is made to the conjectural nature of our present views on the mechanism of extreme-pressure lubrication. A differentiation is made between sliding and tearing so that sliding is defined as the relative motion of two solid bodies in frictional contact under the action of a tangential force with the frictional force increasing in direct proportion with the normal load. On the other hand, when the surfaces of the friction elements suffer macroscopic damage during relative motion, the frictional force increases at a more than proportionate rate with the normal load and a case of tearing is established.
The general requirement made of a lubricant is the reduction of the frictional force. The requirement made of an extreme-pressure lubricant is that it should reduce the frictional force at a sufficiently high value of the normal load for a pressure of at least 1.5×104 kg-wt./cm2 to apply throughout the run, and in addition, the extreme-pressure lubricant should prevent tearing or scoring of the surfaces at these comparatively high bearing pressures.
A loaded revolving steel ball in frictional contact with a hardened steel block is a model of a bearing requiring extreme-pressure lubrication. The temperature elevation at the rubbing contact should be a measure of the energy dissipated by friction, and this can be determined by connecting the rubbing contact to a galvanometer which records the thermal electromotive force generated at the contact. The record of the electromotive force represents the entire frictional history of the contact in every detail. Evidence was found that extreme-pressure addition agents reduced the frictional force and extended the region of normal loads over which the frictional force was a direct proportion of the normal load.
Various types of extreme-pressure addition agents gave results which suggested the existence of an optimum dope concentration. In general, over the region of normal loads over which Amontons' law of the direct proportion between the frictional force and the normal load applied, lubricants which reduced the thermal electromotive forces generated at the rubbing contact also reduced the wear diameter. On the other hand, when tearing took place no such parallelism between friction and wear became apparent since the final wear diameter is the integrated result of the wear processes occurring during one run, whereas the record of the electromotive forces gives the details of the change of the frictional force in the course of such a run.
It is suggested that the optimum dope concentration may be a function of two counteracting effects. On the one hand, the extreme-pressure addition agent is likely to decrease the dielectric breakdown field strength of the thin layer of lubricant and to reduce thereby the electrostatic component of the frictional force. On the other hand, the lubricant becomes more corrosive with increasing concentration of the extreme-pressure addition agents. As soon as appreciable corrosion takes place, tearing sets in and causes the frictional force to increase. Thus a balance will be struck between the reduction of the frictional force by diminishing its electrostatic component, and an increase of the frictional force owing to the corrosive action of the extreme-pressure addition agents.
Evidence was found to support this latter view when metallic films were wiped out by hand on metal blocks. It was then observed that under ``dry'' conditions the metallic films tended to reduce the thermal electromotive forces and that the films were more effective in reducing these forces in the order tin, copper, lead. On the other hand, in the presence of an extreme-pressure lubricant of optimum dope concentration similar films increased the thermal electromotive force, the increase being now in the order lead, tin, and copper, in which the corrosive action might be expected to be promoted by these films.
An attempt was made to calibrate the thermal electromotive forces in terms of temperature.