Sniegowski: What got you interested in tribology?
Hanrahan: I started as a graduate student in Materials Science and Engineering at the University of Maryland in 2006. I joined the MEMS Sensors and Actuators Laboratory that Fall and my first project was to create a retainer ring (a restraining system for the balls that goes inside the device) for micro-scale ball bearing devices. The senior students in the group were developing micro-turbines, micro-pumps, and micro-motors at the time. While all of these devices were functional, they were all also limited by the micro-scale ball bearings, which were not really well understood. We got together and determined that one of us would need to focus on understanding the tribology of the microball bearings to improve the platform for all future devices. Being a Materials Scientist in a group of device engineers, the fundamental study fell into my lap. From that point, I designed experiments help us better understand the sources of friction, the different wear regimes, and the applicability of lubrication for micro-scale ball bearings.
Sniegowski: What are you currently working on?
Hanrahan: My current research at the U.S. Army Research Lab is focused on micro-scale ball bearing systems, designed to support power and energy applications, including small generators, fuel pumps, sensor platforms, and an array of other applications. The biggest part of this research has been focused pinpointing the sources of rolling friction when you utilize micro-scale geometries, silicon materials systems, and microfabrication technologies. My Ph.D. hypothesis is centered around the influence of adhesion on rolling friction. To address adhesion I have designed experiments that independently address various aspects of adhesion, such as real contact area and adhesive energy. These results, plus continued studies on wear regimes and new microfabrication geometries, will provide a reference point for the engineering of future microfabricated ball bearing systems.
Sniegowski: What recommendations would you give to other students in the field?
Hanrahan: First, teach your parents, colleagues, and teachers what “tribology” means! Second, you may find yourself at the business end of a number of lubrication (lube), friction, or ball-related jokes. Don’t be fazed! Just ask them if they appreciate the fact their car stops, windmills turn, or their engines last a long time. Third, I would guess that with few exceptions, most undergraduates haven’t been introduced to tribology as field of study, so most graduate students don’t seek out tribology, but rather tribology finds them. This comes in the form of a part wearing out, a slow motor, or some other issue that needs solving. My advice to them is to dive-deep into some of the older, seminal works of tribology. You will find that every problem in tribology is specific to the geometry, materials, and operating parameters of your system. It’s best to go back and learn the general formulas, test methods, and models which are universally true before looking into something more specific.
Some of the papers Brendan recommends:
- "Surface Energy and Contact of Elastic Solids" - Johnson, Kendall, Roberts, 1971
- "The Mechanism of Rolling Friction 1: The Plastic Range" - Eldredge and Tabor, 1955
- "The Mechanism of Rolling Friction 2: The Elastic Range" - Tabor, 1955
- "Elastic Deformation and the Laws of Friction" - Archard, 1957