Observing Interfacial Sliding Processes in Solid–Solid Contacts

Abstract

Directly seeing into a moving contact is a powerful approach to understanding how solid lubricants develop low-friction, long-lived interfaces. In this article, we present optical microscopy and spectroscopy approaches that can be integrated with friction monitoring instrumentation to provide real-time, in situ evaluation of solid lubrication phenomena. Importantly, these tools allow direct correlation of common tribological events (such as variations in friction and wear) with the responsible sliding-induced mechanical and chemical phenomena. We demonstrate the utility of in situ approaches with applications to a variety of thin-film solid lubricants.what is happening within the sliding solid-solid interfaces has limited both the modeling of contact processes and predictive engineering to improve the performance of sliding components.Solid lubrication of an interface requires both low, stable friction and development of low-wear conditions. A common strategy for solid lubrication is surface modification with a lubricant-containing coating. Unfortunately, because coatings are typically worn away during operation, their lifetimes are limited. This has motivated alternative strategies to resupply lubricants during operation (e.g., delivery of solid lubricant powders 1 or in situ formation of solid lubricants from the gas phase 2-7 ).Because sliding occurs in a buried interface, it is challenging to determine what materials processes are actively enabling stable performance. Similarly, unknown interfacial phenomena cause friction instabilities and debris generation to occur, and wear can go undetected. This article highlights recent advances of in situ tribology instrumentation and approaches that allow observation and quantification of the mechanical and chemical processes influencing friction and wear in solidlubricated contacts. These new approaches allow for an understanding of issues such as: What causes friction instabilities? How do chemistry and mechanics combine to provide low friction? How does varying the ambient environment change the interfacial film morphology and rheology? How do crystal structure and size influence lubricant performance?Several benefits can be obtained by performing tribological measurements in situ in the test environment. First, it simplifies the correlation between friction data and interface events. Second, it eliminates the need to remove the sample from the test environment and thus greatly reduces the risk of surface contamination. Perhaps most importantly, much of the highly speculative nature of deriving explanations for friction changes can be eliminated from direct observation of contact events in real time.

In Situ Attenuated Total Reflection (ATR/FT-IR) Tribometry: A Powerful Tool for Investigating Tribochemistry at the Lubricant–Substrate Interface

“…Vibrational spectroscopies, not requiring ultra-high vacuum for carrying out analytical studies, provide valuable insights into the chemical reactions taking place within the rubbing interface [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. Although the literature demonstrated the possibility of investigating the chemical composition [23][24][25] and the conformation of the lubricant film [15,26], the pressure distribution within the contact [27] as well as the third bodies formed by solid lubricants in the contact [4][5][6][7][8][9][10][11][12], the necessity of replacing one of surfaces with an IR-transparent window can have significant implications for the study of the tribochemistry of lubricant additives and solid lubricants.…”

“…Among the analytical techniques used for carrying out in situ tribological investigations, molecular spectroscopies (i.e., infrared and Raman spectroscopy) were found to provide a valuable insight into the chemical reactions taking place under steady-state conditions [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22], but always require one of the rubbing surfaces to be transparent to the wavelength of radiation concerned.…”

Role of the Transfer Film on the Friction and Wear of Metal Carbide Reinforced Amorphous Carbon Coatings During Run-in

“…Tests are performed using in situ tribometry [13,22,31]. The sliding contact interface is videotaped to monitor the buildup of transfer films during friction tests; after tests, the worn coatings are measured by profilometry to establish wear rates during run-in and steady-state sliding.…”

Energy, Adhesion, and the Elastic Foundation

“…Additionally, numerous finite-element solutions have been performed in the process of deriving approximate solutions to the contact of compliant elastic layers [4,[12][13][14][15]. Thin and compliant surface films are ubiquitous in tribology [16,17], and are often a critical part of the lubrication strategy, examples include: self-assembled monolayers [18][19][20], transfer films [21,22], and other tribofilms and anti-wear films that are generated during operation [23][24][25]. This theory is entirely based on the conservation of energy, and the models are developed using an elastic foundation.…”