20 Minutes With Dr. Robert Carpick
Karl M. Phipps, Managing Editor | TLT 20 Minutes January 2015
This University of Pennsylvania researcher discusses the potential—and challenges—of nanotechnology for tribological applications.
ROB CARPICK - The Quick File
Robert W. Carpick is the John Henry Towne Professor of Engineering and Applied Science at the University of Pennsylvania in Philadelphia. Since July 2011, he has served as chair of the department of mechanical engineering and applied mechanics. He holds a secondary appointment in the department of materials science and engineering and is a member of the Physics Graduate Group. Prior to joining UPenn in January 2007, Rob served on the faculty in the department of engineering physics at the University of Wisconsin-Madison for seven years.
Rob works at the intersection of mechanics, materials and physics to conduct research into nanotribology (the atomic-scale origins of friction, adhesion, lubrication and wear), nanomechanics, nanostructured materials and scanning probe microscopy (SPM). His primary focus is on using SPM and other surface science and material characterization techniques to probe the fundamental nature of materials in contact and to apply the results to nanotechnology applications. Recently, he has focused extensively on the science and technology of ultra-hard, carbon-based thin films, including nanocrystalline diamond, ultra-thin materials such as graphene and materials under extreme conditions.
Rob was named fellow of the American Physical Society and American Vacuum Society in 2012 and 2014, respectively. He currently serves on the journal editorial boards of Tribology Letters and Advanced Materials Interfaces. He was the recipient of a CAREER Award from the National Science Foundation in 2001 and was named Outstanding New Mechanics Educator by the American Society for Engineering Education in 2003. In 2009, he was the recipient of the Burt L. Newkirk Award of the American Society of Mechanical Engineers. He is a co-recipient of an R&D 100 Award for the co-development of ultrananocrystalline diamond AFM probes in 2009. In addition, he served as the UPenn director of the Nanotechnology Institute (2007-2011), a multi-university, state-funded consortium that supports the commercialization of university research in nanotechnology. He has taught several invited short courses on nanomechanics and scanning probe microscopy. He also holds three patents and is the author of more than 100 peer-reviewed archival papers.
Rob received his bachelor’s of science degree with high distinction in physics from the University of Toronto in 1991 and his doctorate in physics from the University of California at Berkeley in 1997, under the supervision of Dr. Miquel Salmeron. He spent two years as a post-doctoral appointee at Sandia National Laboratory in the surface and interface science department.
Dr. Robert Carpick
TLT: How did you become interested in nanotribology?
Carpick: I was searching for a research advisor during the first year working on my doctorate at the University of California at Berkeley when I had my heart set on superconductivity because of the fun I’d had studying superconductors through undergraduate research. I was drawn to that because the physics was novel, but also the potential applications (especially saving energy) were beneficial.
During my search for faculty who had research openings in superconductors, I heard that a researcher, Dr. Miquel Salmeron, at Lawrence Berkeley Laboratory had openings in a different area—surface science. Upon meeting him, I was impressed with his drive and intellect. He had just received funding from the U.S. Department of Energy to conduct research in nanotribology using atomic force microscopy. When he explained that, in fact, very little was understood fundamentally about the origins of friction but that there was potential to learn things using AFM, I was immediately intrigued.
The value of the work was obvious. All the energy being wasted by friction seemed something worthy to go after. With that, I was hooked on learning more. Later that summer, I started my thesis research with Dr. Salmeron, and my career in nanotribology got started.
TLT: Why do you think nanotribology is an important and useful field?
Carpick: Nanotribology provides insights that can help put macroscopic tribology on a fundamental, predictive basis. We all know that tribology in general is crucial for saving energy, conserving materials, making applications run reliably and safely, as well as human benefits, such as orthopedics and other surgical and medical device applications. So nanotribology is useful because it provides one important avenue for providing fundamental support to get at the challenging buried interface that renders tribology problems so difficult. This includes everything from orthopedic implants to car engines and earthquakes. How many fields of research can claim to touch on so many different and important topics?
Nanotribology is also important because we have many nanoscale applications like nanoelectromechanical systems, nano- and microswitches, scanning probe microscopy, hard disks and many others where we need nanoscale knowledge because friction, adhesion and wear are so dominant at small scales.
Furthermore, it just offers all sorts of great scientific challenges to address, leading to excellent and rich fundamental science. It’s exciting to look at what happens when materials come to contact and react, are subjected to stresses and harsh environments and high temperatures and electric fields, where they are far from equilibrium, with a lot of dynamics and motion. It’s a fascinating area to be engaged in with never-ending challenges and new ideas that touch on so many areas of science and engineering.
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TLT: Can nanotribology be connected to macroscopic properties like friction coefficients and wear rates?
Carpick: While we can’t use nanotribology results to predict a friction coefficient or wear rate yet, there are now several examples where insight from nanotribology has advanced our understanding of macroscopic friction and wear. For example, we did some studies with Dr. Greg Sawyer of the University of Florida that showed that we could really understand why friction between diamond surfaces has such a strong dependence on the humidity. We got at that by looking at the top few nanometers of the worn surfaces slid in different levels of water vapor using X-ray absorption spectroscopy.
In low-wear situations, once a surface runs in, you can have rather steady-state behavior. There is still wear, and there are still variations in friction. But if this is a good, low-friction, low-wear tribopair, then the changes are gradual. This situation is important to understand because it’s the situation we want, but we need to know the limits. How gradual? How long until the coating wears out or fresh lubricant is needed? Nanotribology studies of single asperities have great potential to address this because we can look at the gradual wear that occurs. Quantifying and understanding gradual wear has been a significant advance in nanotribology recently. Overall, the potential to directly feed into macroscopic tribology predictions is real, and we are getting closer.
TLT: What kinds of research are you currently working on now?
Carpick: I’m very fortunate to have been able to secure funding for several exciting projects, which involve working with some amazing collaborators. These projects have all been successful thanks to the terrific, hardworking students and post-docs in our groups. We work across a lot of length scales. There are two new interdisciplinary projects I’d like to highlight: earthquake friction and lubricant additives for automotive engines.
On the earthquake project, I’m collaborating with earth scientists David Goldsby (UPenn) and Terry Tullis (Brown University). During this work, we showed that nanoscale contacts between silica surfaces (the most common ingredient of rock) exhibit an aging effect, which is similar to what is seen macroscopically for rocks and earthquake faults. With aging, the static friction force is larger if the materials are in contact without sliding for longer periods of time. Our experiments showed this had a chemical origin. As time passes, chemical bonds form across the interface between silicon and oxygen atoms. Previously, this chemical mechanism had only been hypothesized. Now we’re trying to see how this effect depends on the applied load. Some theories suggest that more pressure leads to more bonding. We’d like to see if that occurs for these nanoscale single asperities. If so, it could ultimately lead to better modeling of geological phenomena like earthquakes, although certainly a lot more research is needed.
We also have projects on automotive additives. Tribologists have long known that zinc dialkyldithiophosphates (ZDDP) functions very effectively in motor oil as an antiwear additive. However, as many readers of TLT know, there is a need to reduce or eliminate ZDDP due to the detrimental effects of phosphorus, sulfur and zinc on the performance of the catalytic converter and due to its tendency to increase friction (thus lowering mechanical efficiency).
Despite several decades of research, our current understanding of ZDDP’s antiwear functionality is derived almost entirely from macroscale tests, and there has yet to be a suitable replacement found for ZDDP. The macroscopic tribology tests show that ZDDP decomposes under the combined action of sliding and heating to form thin protective films. There’s been excellent tribometry and characterization of these films over the years and their composition and structure are now well documented. However, it’s challenging to determine the molecular scale mechanisms that lead to these films, since the experiments necessarily involve complex multi-asperity interactions at the buried sliding interface. This inhibits developing a physically based, molecular level understanding of the tribofilm growth, and this, in turn, makes it very difficult to rationally design replacements for ZDDP or for designing better additives in general.
We were able to study the formation of ZDDP antiwear films formed
in situ between two sliding surfaces in a single-asperity contact in the AFM, allowing us to image the tribofilm’s growth. This has been exciting because we saw that the patchy film morphology is intrinsic to the film formation mechanism and we could back out the kinetics of the film growth. We also could see that the film exhibits a self-limiting thickness. All of this helps address some of the multiple conflicting hypotheses in the literature surrounding the nature of these films. We are very excited to keep going with this novel approach, including a new project applying the same method to evaluate other antiwear additives.
All of this has been made possible because of having excellent group members and collaborators to interact with, as well as support from the National Science Foundation, U.S Department of Defense and industry, which have been the crucial enablers.
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TLT: What are the biggest opportunities and challenges facing the tribology community?
Carpick: There are a lot of opportunities and challenges that we need to address as tribologists. The biggest challenge is how to build excellence in the face of declining and uncertain funding, reduced industry support of research and more competition internationally. It’s a concern that the U.S. is not investing enough in tribology research, while other places, including China, Japan and the European Union, are doing so.
Tribology is intrinsically difficult because we’re trying to access a buried interface, and the phenomena are multidisciplinary such as fluid mechanics, solid mechanics, heat transfer, kinetics, multicomponent phase equilibria, diffusion, phase transformations, phonons, transport—you name it, it’s there. That means doing a good job studying from many areas of science that aren’t conventionally covered in one place. Tribology is fun, exciting and always interesting but also difficult to get others to understand how important it is. Sharing knowledge, communicating with other tribologists who come from different backgrounds and supporting each other’s work and progress are what we need to do to succeed as a community.
More important, we need to work hard to educate the next generation of tribologists by offering courses and tutorials, publishing accessible, well-referenced research, as well as encouraging them to give talks at industry events such as the STLE annual meeting and the Tribology Frontiers Conference.
Beyond these very general points, there are some key technologies we need to work on as well. For example, developing better lubricants and materials for use in automobiles is an ongoing issue. It would be great if we could not just make progress incrementally but do something that makes a step-function improvement in engine efficiency through tribology. I’m not sure what that will be yet.
Tribological advances, which enable higher-temperature operation of engines, turbines and other components, would have a big impact because you would get higher thermodynamic efficiencies. At small scales, there is a lot of potential for devices like NEMS switches. These could save tremendous amounts of energy in computers compared to solid-state transistors. Though among other challenges, they suffer from tribological reliability problems. Materials for NEMS electrodes that can survive a quadrillion (that’s 10
15) cycles of contact are all that’s needed. Doesn’t that sound easy?
Finally, the ongoing challenges with orthopedic implants remains a crucial one. The recent recalls, spurred by tribological failure issues among others, is a reminder to us that our lives depend on tribology. How to properly design and fabricate an implant with exceptional performance and reliability is a big challenge, especially considering population demographics.
We aren’t getting any younger, and the tribology problems aren’t getting any easier, but we have great new tools at our disposal and a lot of excellent research. If we keep working together and dedicating ourselves to excellence and innovation in tribology and focus on educating and inspiring new, young tribologists, we’ll make great progress.
You can reach Rob Carpick at carpick@seas.upenn.edu.