Boundary Lubrication of Steel Surfaces with Borate, Phosphorus, and Sulfur Containing Lubricants at Relatively Low and Elevated Temperatures

Prolonging the life of engineering components through lubricant formulation to achieve better wear resistance and higher oxidation stability is of paramount importance to many mechanical systems, such as automotive gears and bearings. This can be accomplished with formulated lubricants that limit the generation of wear debris causing severe abrasion and protect the contacting surfaces through the formation of wear-resistant tribofilms. In this study, a ball-on-disk tribometer was used to characterize the friction and wear properties of steel surfaces slid in the boundary lubrication regime. An experimental scheme was developed to allow the statistical screening of various lubricant formulations. Sliding experiments were performed in baths of different lubricants at relatively low and elevated temperatures, approximately 32 and 100°C, respectively, under conditions of constant load and sliding speed. Surface profilometry, optical microscopy, and scanning electron microscopy were used to characterize the dominant friction and wear mechanisms. The tribological properties were found to strongly depend on the temperature and the additives (e.g., borate, phosphorus, and sulfur) present in the blended lubricants. The superior high-temperature wear performance of the lubricant with the higher borate content is indicative of the formation of a durable tribofilm that reduces metal-to-metal adhesion, material transfer, and surface plowing by wear debris. Presented as a Society of Tribologists and Lubrication Engineers Paper at the ASME/STLE Tribology Conference in Seattle, Washington, October 1-4, 2000