Water invasion on film formation and grease degradation under humidity conditions

Water invasion is a primary cause of grease lubrication failure, the mechanisms of which can be effectively investigated using the ball-on-disc test rig. However, traditional experimental methods, such as pre-mixing grease with water or directly adding water into the contact, fail to accurately replicate the water distribution on the rolling track in humid environments. Here, a conventional ball-on-disc apparatus was modified by coupling it with a unit for precise relative humidity control. This paper investigates the influence of water on film formation, lubricant distribution, and replenishment for both oil and grease lubrication. The results showed that water plays an important role by influencing the lubricant flow and rheological properties of grease on film formation. For oil lubrication, higher relative humidity extended cavitation length in the outlet region by altering local pressure distribution, reducing oil-water surface tension. For grease lubrication, direct water mixing rapidly degrades performance through immediate thickener-water interaction, emulsification, and structural softening. In contrast, humidity-driven water invasion causes slower but progressive damage by gradually weakening the thickener network. These findings provide valuable insights into lubrication failure under humid conditions, support the development of water-resistant greases, and present a novel test method.