True stability of lubricants can be determined when there is minimum change in the contact area and also the intervention of wear debris in the contact zone. Here, we have used the ball-on-disk instrument with the migrating point contact, that is, relative motion between the ball and disk condition to fix the contact area and minimize the wear debris at the contact zone. The jump in the friction coefficient indicates the film failure, which appeared earlier for the motor oil 5W30 compared to 5W40. Such profile was not recorded in absence of relative motion. Therefore, 5W40 was considered to have a better lubricant stability than 5W30. Applying the same test condition to the natural lubricants shows that glycerol has better lubricant stability than glycerol-water mixture. Superior true lubricant stability by glycerol and 5W40 can be related to its high viscosity. However, they were less wear resistant compared to low viscosity lubricants like 5W30 and glycerol-water. We suspect the role of microscopic wear debris at the contact zone for this behavior although it should have been avoided in the migrating point contact condition. Overall, ball-on-disk instrument with a migrating point contact condition is an effective technique to determine the stability of lubricants.
Tortora, A. M., & Veeregowda, D. H. (2016). True Stability of Lubricants Determined Using the Ball-on-Disk Test. Advances in Tribology, 2016. https://doi.org/10.1155/2016/4020537