Old meets new at International Joint Tribology Conference

Dr. Robert M. Gresham, Contributing Editor | TLT Lubrication Fundamentals January 2012

Researchers are unraveling the secrets of materials and processes that have long stumped the lubricants industry.

 

KEY CONCEPTS
• The IJTC gives tribology researchers a platform to unveil the technologies that will lead to tomorrow’s lubricant products.
• Conference highlights included presentations on how to improve the performance of lubricants for wind turbine gearboxes.
• Other papers further developed our understanding of the steady-state and slip-stick frictional responses.

LIKE MANY OF YOU, I attended the International Joint Tribology Conference, co-sponsored by ASME and STLE, last October in Los Angeles. It was a good conference with great presentations and a large and fully engaged audience.

Several themes dominated the conference. First, there was another round of papers related to wind energy, led off by keynote speaker Dr. Mike Robinson, deputy center director of the National Renewable Energy Laboratory’s National Wind Technology Center and chief technology officer for the Dept. of Energy’s Wind and Water Power Program. Among the many issues Robinson highlighted were the daunting technical problems that have yet to be solved with these machines.

As those in STLE and ASME are keenly aware, many of the bearings and gears in a wind turbine gearbox fail prematurely, on the order of 3-7 years, instead of the desired design criteria of 20-30 years. Also, there are problems associated with building and installing megasized wind turbines in the ocean, both on floating and permanent platforms. Finally, there is the problem of the antiquated power grid and the fact that people and the grid are not located where there is sustainable wind. Yet, at this stage we are ahead of our goals for electricity generation from wind sources.

Additionally, the conference highlighted sessions devoted to machine component tribology, biotribology, nanotribology, particle tribology, biometrics and green tribology. I was struck by how rapidly we are now moving forward to characterize and control tribofilms and interacting surfaces at the atomic level.

In my day we knew there were important triboprocesses taking place, but we could not get a handle on them. As the 2011 IJTC showed, we are now learning a lot about these triboprocesses and structures at the molecular level, issues that were only a dream earlier in my career. These advances are possible because of our enhanced ability to make complex modeling calculations and simulations, surface texturing, atomic force microscopes and related testing at essentially the nanoscale. Further, we also can perform much more sophisticated failure analysis of machine components.

I didn’t get to as many technical sessions as I would have liked, but following are some examples of IJTC papers I found particularly interesting.

I’ve always been intrigued by the process of slip-stick. A number of people working with Dr. Mike Lovell with the University of Wisconsin- Milwaukee provided an analysis of steady-state and stick-slip motion as a function of surface texture, surface roughness, normal load, hardness, crystal structure and lubrication. During my era, macrosurface roughness, normal load, hardness and macrolubrication were about it.


A key question examined at the IJTC was how to keep wind turbine gearboxes, which are designed to last 20-30 years, from failing after 3-7 years.

Mike and the other researchers concluded that the coefficient of friction is controlled by the surface texture of the harder mating surfaces. Two kinds of frictional response, steady-state and stick-slip, were observed during sliding. Steady-state frictional response was observed for the face-centered cubic metals, alloys and materials with higher hardness. Stick-slip frictional response was observed for the soft metals having a limited number of slip systems such as body-centered tetragonal and hexagonal close-packed structures.

They further concluded that the stick-slip frictional response was dependent on the normal load, lubrication, hardness and surface texture of the counterpart material and that roughness of the surface affected neither the average coefficient of friction nor the amplitude of stick-slip oscillation significantly. That was all pretty much news to me, or at least a newer and more in-depth way to look at the process of stick-slip.

I was especially intrigued by a NASA paper that gives us an idea of the true merits of a new-old material—nickel-titanium-based superelastic materials, which are emerging as candidates for rolling element bearing applications. When properly prepared (and that’s important) these unique intermetallics are hard, exhibit excellent tribological properties and are intrinsically corrosion immune.

These superelastics can endure much higher levels of recoverable elastic strain during compressive deformation. This behavior enables bearings that are more resilient to load-induced damage such as raceway denting and from the ingestion of hard particles. That’s a pretty significant development. I certainly hope NASA can push this forward for real-world practical applications.

Returning to wind turbines, there were presentations describing in greater detail gearbox failure modes, development of a low-cost, in-line vibrational viscometer for gearboxes and metallic debris sensors, which can go a long way toward resolving the problem of condition monitoring maintenance in the field where the distances are great and locations are remote. I also was intrigued by a paper on the characterization of “white etch” areas in microcracks found in failing bearings, providing further insight into how these failure modes manifest themselves.

Given my history in solid film lubricants, I was keenly interested in a presentation by Dave Burris and Harmandeep Khare, both of the University of Delaware, on work-horse molybdenum disulphide. While in my day we could only look at macrolevel properties, Burris and Khare studied the nano level, describing surface film formation in both dry and humid conditions. These were effects we knew happened but didn’t understand.

Then there was a paper by Emil Sandoz-Rosado and ElonTerrell, both from Columbia University, on graphene, one of the mechanically strongest materials ever measured as a nano-level protective coating and as a lubricant. Dan Dickerell from the University of Florida described nanoscale surface film characterization and engineered plastics for both engineering applications and joint replacements.

I could go on and on with great examples of the shattering of older roadblocks to our understanding, but the point of all this is that the scope and scale of the conference was outstanding. All you had to do was watch the body language of the participants to feel the energy and excitement as both old and new questions are being answered and meaningful discoveries are happening on a regular basis.

Bob Gresham is STLE’s director of professional development. You can reach him at rgresham@stle.org.