Impact of Wetting Transparency on Lubricating and Adhesive Properties of Two-Dimensional Materials

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Poorly optimized lubrication properties of oils and greases results in increased friction causing parasitic energy losses in machines (such as automotive engines, wind turbines, bearings), as well as often resulting in the gradual wear of the individual components that are required for the machine to operate. The associated industrial energy losses have a significant impact on both the environment and the economy. Two-dimensional (2D) materials have been used on their own and as additives in oils, greases, and other base lubricants to improve their proeprties beyond what could be achieved through conventional materials. 2D materials are only one atom thick and have a micron or larger dimension in the other two directions. This proposal targets the further improvement of lubrication strategies using these advanced materials through fundamental examination of surfaces covered by 2D materials. This fundamental examination is required to determine the exact mechanisms by which 2D materials reduce friction and wear. This current shortcoming has reduced the effectiveness of 2D materials such that they are currently not suitable for real-world applications, be it in terms of reliability, life-times, or sufficiently improved lubrication properties. In this research, the lubricating mechanism of 2D materials will be achieved through experiments using ultra-high vacuum atomic force microscopy (UHV AFM). These experiments will reveal the true properties of these lubricants in the absence of environmental or other contamination. Further, a comparison of the lubricating properties of 2D materials determined under UHV conditions and real-world conditions will reveal the environmental factors that truly impact the friction and wear reducing properties of 2D materials in industrial applications. The fundamental knowledge gained through this AFM experimentation will be used to improve the analytical and physical models that describe friction, wear, and other energy-loss mechanisms in sliding mechanical contacts. The improvement of these models is critical for predictive control and design of low-energy mechanical systems. The proposed research program will use AFM to specifically measure changes in surface energy at the surface when varying numbers of layers of these lubricants cover the surface. The innate thinness of these materials is much smaller than the distance at which attractive adhesive forces typically extend from surfaces. However, complications with environmental contamination, the difficulty in accessing material properties at this length-scale, and challenges in linking computer atomistic simulations with experimental work has resulted in conflicting findings over the past years. Given the additional importance of surface energy, beyond impacting the lubricating properties of these materials, to mixing of composite materials, adhesive materials, and other fields, significant impact of this work in other fields is anticipated.