Molecular Science of Lubricant Additives

Abstract

This review aims at introducing an engineering field of lubrication to researchers who are not familiar with tribology, thereby emphasizing the importance of lubricant chemistry in applied science. It provides initial guidance regarding additive chemistry in lubrication systems for researchers with different backgrounds. The readers will be introduced to molecular sciences underlying lubrication engineering. Currently, lubricant chemistry, especially "additive technology", looks like a very complicated field. It seems that scientific information is not always shared by researchers. The cause of this is that lubrication engineering is based on empirical methods and focuses on market requirements. In this regard, engineering knowhow is held by individuals and is not being disclosed to scientific communities. Under these circumstances, a bird's-eye view of lubricant chemistry in scientific words is necessary. The novelty of this review is to concisely explain the whole picture of additive technology in chemical terms. The roles and functions of additives as the leading actors in lubrication systems are highlighted within the scope of molecular science. First, I give an overview of the fundamental lubrication model and the role of lubricants in machine operations. The existing additives are categorized by the role and work mechanism in lubrication system. Examples of additives are shown with representative molecular structure. The second half of this review explains the scientific background of the lubrication engineering. It includes interactions of different components in lubrication systems. Finally, this review predicts the technical trends in lubricant chemistry and requirements in molecular science. This review does not aim to be a comprehensive chart or present manufacturing knowhow in lubrication engineering. References were carefully selected and cited to extract "the most common opinion" in lubricant chemistry and therefore many engineering articles were omitted for conciseness.

Tribochemistry: A Review of Reactive Molecular Dynamics Simulations

“…The term tribochemistry has been used relatively flexibly in the literature, but can generally be thought of to encompass chemical reactions that are driven by conditions arising in tribological contacts [1][2][3][4]. Tribochemical reactions play a critical role in most tribological applications [5]. As such, tremendous efforts have been and are being made in research and development to optimize friction and reduce wear using approaches based on tribochemical mechanisms.…”

rGO/GO Nanosheets in Tribology: From the State of the Art to the Future Prospective

“…Therefore, the utilization of dispersing agents is not a novel technique in the attempt to stabilize rGO/GO nanosheet suspensions. Dispersants are mainly surfactants with multiple polar groups that enable a better interaction with suspended contaminants [73].…”

Metal-containing nanomaterials as lubricant additives: State-of-the-art and future development

“…Recently, the use of nanomaterials as lubricant additives (also known as nanolubricants) has become an important research area [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. The nanolubricant approach is used to overcome the drawbacks of conventional anti-wear and anti-friction additives associated with the need for chemical reactions with substrates, and hence the induction period for obtaining a tribo-film on the friction surface.…”

“…It is well known that lubrication can be divided into three different regimes: boundary lubrication, mixed lubrication, and elastohydrodynamic/hydrodynamic lubrication ( Fig. 1) [9,46,47].…”

“…No exhaustive analysis of the entire array of current experimental data will be attempted; rather, the focus will primarily be on the composition, the factors influencing the tribological characteristics, Fig. 1 Stribeck curve and lubrication regime [9]. H m (minimum thickness of liquid film developed from the base fluid between the surfaces, clearance of the surfaces) could be calculated from the operating parameters (load and velocity) and material parameters (elastic modulus and pressure-viscosity relations).…”