Diz 75 en
Solid lubricants at the nanoscale: Frictional behavior in silico
Author: Victor Claerbout
Friction and wear occur when two surfaces are in contact and relative motion, and are considered responsible for approximately 23% of all energy consumed. The types of materials in contact and environmental conditions, such as temperature and humidity, are highly correlated with friction and wear. Introducing a lubricant into the sliding contact can help reduce (or even eliminate) friction, wear, and associated processes – a technique dating as far back as Ancient Egypt. Common everyday classical wet lubricants are either oil- or water-based. However, increasingly strict sustainability standards and rising demands of our modern industrial society have surpassed the application of these wet lubricants in severe operating conditions, such as extremely high/low contact pressures and working temperatures or when used in nanoscale devices. Starting from the 1940s, solid lubricants, which are layered crystalline materials, have received substantial attention due to their excellent frictional properties. To date, they have mainly been used as additives to impart improved lubrication properties to lubricants. Transition metal dichalcogenides are some of the best-known examples of solid lubricants, of which molybdenum disulfide (MoS 2 ) is considered prototypical. When applied as a coating (as opposed to an additive), they have the potential to overcome a vast number of problems encountered by wet lubricants. Despite the considerable scientific effort, large-scale industrial implementation of MoS 2 , and solid lubricants in general, has not been achieved. A combination of problems lies at the origin of this. We do not possess a fundamental understanding of the actual frictional mechanisms of layered systems, both at the nanoscale and macroscale. Moreover, solid lubricants are highly sensitive to deterioration resulting from interactions with their environments, limiting the exploitation of their promising potential in a wide range of applications. In this thesis, we make an effort towards elucidating the mechanisms that determine the frictional properties of MoS 2 at the nanoscale by employing computational modeling. This is done by investigating the intrinsic frictional aspects of MoS 2 , for example, by studying frictional anisotropy, and considering the effect of external conditions such as water contamination, which is commonly known to deteriorate the frictional properties of MoS 2 . The use of prototypical materials allowed us to provide new understandings in the application and design of solid lubricants in general. Novel insights were obtained on the principles of achieving superlubricity in homostructures, where the relevance of the interlayer potential energy surface topology was once more confirmed. Additionally, the results on water contamination highlight the effect of interlayer intercalation of water on the frictional properties of solid lubricants and thus guide the design of nanoscale devices in ambient conditions. Since 2004 single-layer solid lubricants have been readily available, which has opened the door to a new class of materials, so-called heterostructures. These are layered structures that combine several different types of solid lubricants. Although heterostructures create the potential for tuning the properties of solid lubricants, the actual stability of these structures is often left out of consideration. Therefore, in this work, the twist angle energetic stability of a prototypical heterostructure built from MoS 2 and graphene is extensively explored through epitaxy theory. Our study on heterostructures reveals new insights into the observations made in experiments and the stability of twistronic devices. In conclusion, this thesis has actively contributed to making the next step for large-scale industrial implementation of solid lubricants feasible.