MRSinsitu2008 - Observing Interfacial Sliding Processes in...

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MRS BULLETIN • VOLUME 33 • DECEMBER 2008 • www.mrs.org/bulletin 1159 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 approaches with applications to a variety of thin-film solid lubricants. O bserving Interfacial Sliding Processes in Solid–Solid Contacts Kathryn J. Wahl and W. Gregory Sawyer what is happening within the sliding solid–solid interfaces has limited both the modeling of contact processes and predic- tive engineering to improve the perform- ance of sliding components. Solid lubrication of an interface requires both low, stable friction and development of low-wear conditions. A common strat- egy for solid lubrication is surface modifi- cation with a lubricant-containing coating. Unfortunately, because coatings are typi- cally worn away during operation, their lifetimes are limited. This has motivated alternative strategies to resupply lubri- cants during operation (e.g., delivery of solid lubricant powders 1 or in situ forma- tion of solid lubricants from the gas phase 2–7 ). Because sliding occurs in a buried inter- face, it is challenging to determine what materials processes are actively enabling stable performance. Similarly, unknown interfacial phenomena cause friction insta- bilities and debris generation to occur, and wear can go undetected. This article high- lights 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 solid- lubricated 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 inter- facial film morphology and rheology? How do crystal structure and size influ- ence lubricant performance? Several benefits can be obtained by per- forming 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 explana- tions for friction changes can be elimi- nated from direct observation of contact events in real time.
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MRSinsitu2008 - Observing Interfacial Sliding Processes in...

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