Understanding friction mechanisms of Si-DLC/steel interfaces under aqueous lubrication

A key driver in current research on lubricant formulation is the need to move away from older technology that is highly reliant on resources derived from industries associated with high carbon dioxide emissions. In this paper, the adoption of water based (or aqueous) lubrication is explored. This is in direct contrast with most lubricated systems that rely on oil or other petroleum products. In nature, most known biological systems employ aqueous lubrication for tribological contacts, such as those found in cartilage and more widely in mammalian joints including hips and knees giving friction coefficients as low as μ < 0.002. This is achieved very effectively without the presence of an oil or grease as a base lubricant. In most engineering applications, however, oils and greases are used to achieve desired low friction levels. While effective, this comes with the associated higher costs and carbon footprint of using petroleum derived products. In recent years, certain engineering applications have shifted to aqueous lubrication, a notable example of which is stern tube bearings in maritime applications. These are typically low pressure contacts though. Depending on speed of travel this can range from 100-400 MPa. The research detailed in this paper explores the viability of high pressure contacts lubricated with novel aqueous packages and what effects this shift may have on friction and wear profiles of the system. The work reported herein demonstrates that with some modifications, effective lubrication can be achieved using aqueous lubricant packages.