The use of free-energy relationships in the selection of lubricants for high-temperature applications

Abstract

As a part of a program of developing lubricants and lubrication techniques for high-speed ball bearings operating over the temperature range from room temperature to 1,200°F, candidate lubricants were screened in a high-temperature, high-speed rolling-disk apparatus. Finely divided, inorganic solids suspended in a gas carrier were the principal lubricants selected for the screening evaluations. In order to choose a small number of the most promising compounds from this large group of materials for evaluation, selection criteria were established and applied. These criteria included some of the physical properties which have traditionally been considered important lubricant characteristics. In addition, thermodynamic calculations were used to judge the probability of chemical reaction between prospective lubricants and the substrate materials since it was felt that adherent solid lubricant films may be formed and maintained by chemical bonding with the bearing surfaces. On the basis of the rolling disk experimental results, softness and thermodynamic properties are the most important of the criteria used in the selection of candidate lubricants. With a few exceptions, those compounds which are soft and are thermodynamically favorable for chemical reaction with the substrate provided the lowest friction and wear.