20 Minutes With Dr. Martin Webster
By Karl M. Phipps, Managing Editor | TLT President's Report September 2012
This ExxonMobil researcher and his team are using the latest technology to develop next-generation lubricants.
MARTIN WEBSTER - The Quick File
Martin Webster is a program leader at ExxonMobil’s Corporate Strategic Research Laboratory in Clinton, N.J. he received his bachelor’s and master’s of science degrees in aeronautical engineering, as well as a doctorate in tribology from Imperial College in London. In 1986 he received the Tribology Bronze Medal from the Institute of Mechanical Engineers for his work on rough surface contact mechanics. He also served as a post-doctoral intern at Shell Research and later worked for Taylor Woodrow’s Wind Energy Group as an engineer before moving to the U.S. to join Mobil’s Central Research Laboratory.
Martin joined STLE in 1989 and has participated on several committees, both at the national and local levels. Currently, he’s serving a one-year term as treasurer of the society’s Executive Committee. In addition, he will serve one-year terms as secretary and vice president, assuming the presidency at STLE’s 70th Annual Meeting & Exhibition in Dallas in 2015.
Industry Affiliations and Professional Achievements
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STLE Board of Directors, 2006-Present
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STLE Gears and Gear Lubrication Technical Committee, chairman
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STLE Editorial & Publications Committee, chairman
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Served on the ASME Bearings and AGMA Gear Rating committees.
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Authored several papers and book chapters on contact mechanics, EHL, traction, gear and bearing fatigue and micropitting, gear oil development, lubrication of DLC coatings, mixed lubrication and hydrodynamics.
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Holds patents on new lubricant components, novel lubrication mechanisms and test methods.
Dr. Martin Webster
TLT: What is the most interesting aspect of your job?
Webster: For most of my career, I have been involved in either fundamental research or lubricant product development. In my current role, I lead and participate in several teams working on new materials and lubrication concepts that can be used to develop high-performance lubricants. I consider myself very fortunate to be in a position that allows me the freedom to explore new ideas and apply them to the development of commercial products.
I still get excited about the prospect of discovering or inventing something that has not been seen or understood before. During my graduate work, I finally succeeded in establishing a new method for solving rough surface contact mechanics problems. Since then the field of rough surface contact mechanics has become a very active area of research, and the models are infinitely more complex than those I developed. I think it’s that special feeling that occurs when a new discovery is made that has shaped my career to remain in research.
Of course, new discoveries are not made every day. Most often they result from a concerted effort from a dedicated research team. Also, most discoveries do not, in the end, result in new products. However, the multidisciplinary research teams that I interact with typically include product developers who will, ultimately, turn new discoveries into real products. This often takes many years but over the course of my career I have seen numerous new products evolve out of ideas based on laboratory-scale experiments.
For example, in the world of motorsports, the path from discovery to final product moves at a much faster pace. Along with a small team of colleagues, I’ve been involved in the development of lubricants for use by the Vodafone McLaren Mercedes Formula 1 team. Grand Prix Racing is considered to be the most technically advanced form of motor racing. New products for these highly sophisticated machines result from a close collaboration between the engineers working on the vehicle and the lubricant researchers.
Developing products for these vehicles provides us with a unique opportunity to rapidly try out new materials and concepts beyond the laboratory in full-size equipment operating under very demanding conditions. Thus, I have witnessed discoveries in the lab end up being used at some of the most famous racing circuits around the world. In many ways, I think I have the best of both worlds—the thrill of new discoveries and the opportunity to work on them all the way through to a final product. It really is like having my cake and eating it, too!
Figure 1. Motorsports provides an opportunity to test new ideas under real-world conditions.
TLT: When studying lubricant performance and properties, what are your biggest challenges?
Webster: One of the major challenges in tribology research is that all the action takes place within the complex environment that exists between two surfaces in motion. In some cases, such as the formation of elastohydrodynamic lubrication (EHL) films, there are optical techniques that allow us to both observe and measure the phenomenon of fluid film generation. However, since one of the test specimens needs to be transparent, the technique is not so easily applied to
in situ studies of friction and wear processes occurring in all metal contacts.
As new techniques have been developed, this situation is improving. Optical techniques have now been applied to the study of tribofilm formation by surface-active lubricant components such as antiwear and friction modifiers. Figure 2 compares the growth of surface films generated over time by several different types of antiwear additive technology. In this case, the test is periodically stopped and an optical interference image is automatically collected from the contact surface of the test ball. While this is not truly an in situ technique, it does reveal how tribofilms evolve on surfaces.
Figure 2. Results showing the progression of tribofilm formation from different lubricant additives during a wear test. (Sectioned view of MTM-SLIM test machine courtesy of PCS Instruments Ltd., UK)
Another significant challenge is that in many cases we are trying to reproduce phenomenon that, in practice, evolve over a long period of time. For example, consider trying to evaluate the wear performance of a typical engine lubricant. In the real world, the wear takes place over the lifetime of the vehicle. During this time, the surfaces of the parts can change, both chemically and morphologically. To make matters even more interesting, it is likely that the oil will age in between oil changes such that the composition changes over time. Adding a further layer of complexity is the fact that the lubricant typically interacts with all of the engine components. Along the way, the oil may pick up wear debris and other forms of contamination, which can accumulate in the oil.
Trying to reproduce all of these effects in a small bench test is not practical. However, based on an understanding of the application and operating conditions, it is often possible to design a test or a sequence of experiments that captures enough of the physical processes to screen new concepts, prior to testing in a full-size engine or equipment test.
TLT: What instruments do you use to study tribology and lubricant performance?
Webster: Since lubricants are used in such a wide variety of applications, there is no one single instrument that can recreate all the possible combinations of conditions. In order to support lubricant and lubrication research and development, it is necessary to have access to different types of equipment that provides the flexibility to design experiments covering a range of different configurations. Over the years, we have established a tribology laboratory that contains a wide variety of different friction, wear, rheological and analytical tools that enable us to study problems under most conditions found in mechanical systems.
The tools within the laboratory can be divided into two main classes. The first class helps us define the fundamental physical properties of the oil. A good example is viscosity. Even this apparently simple property requires multiple instruments to do the job properly. I don’t think the average person, or even many engineers, fully appreciates the vast range of conditions that modern oils are subjected to. Within the tiny contacts formed between the mating surfaces of gears and rolling element bearings, the contact pressure is huge. In most rolling element bearing contacts, the peak pressures exceed 1 GPa. At these pressures, lubricants have very different rheological properties than those measured at ambient pressures.
Similarly since the films are so thin, the shear rate between moving surfaces is extremely high to the extent that many oils exhibit non-Newtonian effects such as shear thinning (i.e., the viscosity reduces as the shear rate is increased). The response of an oil to pressure and shear is determined by the molecular structure and combination of the components used. In order to characterize these effects, we use an array of instruments that allow us to gather data over a very wide range of conditions. These are either specialized commercially available instruments or customized instruments that have been made to our specifications. Figures 3 and 4 show some typical comparisons between widely different lubricant responses to pressure and shear, respectively. Such data can be used to help identify the most desirable characteristics to improve the performance of next generation lubricants.
Figure 3. Measured viscosity versus pressure data for two different types of lubricant basestock.
Figure 4. Viscosity versus shear results from multiple viscometers for different model engine lubricants.
The second class of instruments is focused on measuring the performance properties of lubricants. In our tribology lab, this most often translates to measuring friction, wear and the ability to generate lubricant films. These properties depend even more on our ability to simulate the conditions that exist in different applications. I often get asked what the friction performance of a particular oil or additive is. It is important to realize that the friction properties of a lubricant depend on many factors such as the materials being rubbed together, the roughness or morphology of the surface and the operating conditions (e.g., load, speed, temperature, etc.). Even when comparing the relative performance of different lubricants and components, it is important that experiments are designed to reproduce as closely as possible the mechanisms that occur in the application of final use.
Most of the equipment we use allows us to select conditions that represent different types of contact. As an example, if we wish to study friction and wear that occurs in a gear contact, we would use a device that is capable of running under EHL conditions. Also, we would prefer a test that combines both rolling and sliding to replicate part of the motion of a gear contact. While it is not possible to replicate the exact conditions within a gear contact, this approach is more likely to generate similar mechanisms than, say, a simply reciprocating sliding test. Figure 5 shows some examples of the main test configurations and operating variables that should be considered when developing a new test under boundary or EHL conditions. A similar approach is necessary for tests under hydrodynamic conditions, with the added complexity of having to use a conforming (i.e., lower contact pressure) contact capable of reproducing a hydrodynamic film.
Figure 5. List of possible test configurations and operating variables that must be considered when designing a new test.
Running bench-scale laboratory tests on a series of lubricants can help screen candidates before they are run in more complex tests. However, it is often more important to understand why certain oils perform well, or even not so well. Therefore, we often perform additional analyses on the test parts to help interpret the results. This can range from some simple measurements to quantify changes in surface topography all the way through to a full chemical and physical characterization of the test surfaces and lubricant. In fact, in some cases, we might design an experiment specifically tailored to generate a surface suitable for post-test analysis. A good example is shown in Figure 6, which compares the morphology of additive films generated by different combinations of additive types. In this case, we designed the experimental conditions to favor the generation of additive tribofilms rather than producing a wear track. A colleague then used an atomic force microscope (AFM) to study the morphology of these films. As discussed above, the selection of a suitable test method requires us to have a reasonable understanding of the conditions we are trying to replicate. It is also important to define what questions you are hoping to answer. We most often find that our test strategy evolves over time as we learn more about the specific problem being studied. It is the variety of tools at our disposal that, ultimately, provides us with the flexibility to tackle a wide range of lubrication- related research topics.
Figure 6. AFM images taken after wear tests designed to generate additive films. The results show the interaction of different additive types on film morphology.
TLT: What are the limitations of the instruments available today to study lubricants?
Webster: Evaluating the wear performance of lubricants has always represented one of the most difficult challenges we face. Tests for catastrophic wear processes such as extreme adhesive wear (e.g., scuffing or scoring) can be based on a strategy where the test conditions are increased in severity until a predetermined failure criteria is reached. However, milder wear phenomenon are more difficult to reproduce and measure.
Most often we need to make post-test measurements of wear tracks or material loss. The low wear rates typical of this regime can require us to increase the severity of the test in order to obtain measurable wear in a reasonable length of test. Some care is required to select conditions that generate measurable wear, which are not so severe to change the underlying wear mechanisms and response to different lubricants.
Modern surface measurement tools, such as mechanical and optical profilometers, and, more recently, AFMs allow us to resolve very small amounts of material removed from a surface. This improvement in resolution over simple weight loss measurements provides us with some flexibility to design wear tests operating under more realistic mild wear conditions. However, even these have their limitations, and care is required to ensure that the results are not misunderstood. A good example is shown in Figure 7. During a program to evaluate the wear performance of a wide range of additives, we observed that the wear tracks all showed significant and similar amounts of wear. In this case, we had used an optical profilometer that uses interferometry to map the surface heights.
Figure 7. Comparison between optical and mechanical profilometry results for a wear track containing a reacted tribofilm.
When we checked the measurements using a mechanical stylus instrument, we observed what we assumed to be a surface film that was actually formed above the original surface height. Suspecting that the interferometer response was being corrupted by the presence of a partially transparent film formed during the wear tests, we repeated the optical measurements after applying an ultrathin reflective gold coating to the test specimens. The new results confirmed the presence of a surface film and compared well with the mechanical stylus instrument profiles.
This is a simple example of the need to be aware of the limitations of the test and analytical techniques used. There are a large number of very sophisticated methods that can be used to study the chemical and physical properties of the films formed by lubricant additives. Each has its own merits and limitations, and we try to take great care in making sure that we interpret the output appropriately.
A more fundamental limitation is not related to individual test configurations or accuracy. A typical modern lubricant comprises of a combination of basestocks and a range of different performance additives. The exact types and combinations vary, depending on the lubricant class (e.g., engine oil, gear oil, hydraulic oil, etc.). When coupled with the desire to map response over a range of conditions, the resulting degrees of freedom rapidly result in an impossibly large test matrix. Even when we reduce the number of variables, the number of tests remains high. This results in a constant tension between trying to complete a given program or study within a reasonable amount of time and the need to develop meaningful and robust data, which allows the program to move to the next stage. Improved automation helps, but anything that improves throughput provides clear advantages.
TLT: If you could have one new instrument that does not exist today, what would it be?
Webster: As I discussed previously, the ability to gather detailed real-time information from within an operating contact has remained a challenge. Significant progress has been made, and there are examples of techniques that move us ever closer to this ideal. However, if I could wave a magic wand to remove all the limitations, I would like to be able to observe and measure directly the complex physical and chemical processes that occur deep within a heavily loaded contact between two moving components as the lubricant passes through a contact.
TLT: Where do you see tribology research going in the next five years?
Webster: Given the highly interdisciplinary nature of tribology, I believe that we will be impacted by new developments in many diverse fields. One area that interests me is the growth in so-called engineered surfaces. During the last century, we have moved from simply manufacturing surfaces to meet a vaguely defined goal or roughness standard to a point where it is possible to design and make surfaces with amazing micro- and even nano-scaled structures.
Coupled with the development of new materials and lubricants, I think there will be opportunities to develop systems in which the lubricant and surfaces are designed to truly work together to optimize a whole range of desirable properties. We have seen the beginnings of this in the pioneering work using laser dimpling on bearing surfaces, as just one example. I think the future possibilities are almost endless, perhaps even taking cues from the natural world. The range of applications may also be extraordinarily wide, spanning nanotechnology all the way to more conventional mechanical systems.
One of the benefits I have gained from my involvement in STLE is that it has allowed me to remain aware of developments in adjacent fields not necessarily directly connected to lubricant development. Both the STLE annual meeting and the International Joint Tribology Conference provide a diverse mixture of research topics presented by experts in their respective fields. Over the course of time, methods or concepts developed in one field can feed into other arenas. One example is the growth of so-called nanotools, which, at first, were most closely associated with areas such as magnetic- recording media and microelectromechanical systems. However, these tools are now almost routinely used to study a much broader class of problems, including the understanding of lubricant tribofilm formation.
I suspect that many of the future innovations in lubricant development will result from interaction with or developments between different disciplines. Some of these might be driven by the development and use of alternate equipment designs, manufacturing processes and materials. Others may be due to the unanticipated influence from adjacent technology areas. I believe that STLE provides the ideal forums for these vital interactions to occur. I’m just hoping that I will be smart enough to spot the best opportunities first!
You can reach Martin Webster at martin.n.webster@exxonmobil.com.