Friction

February 1, 2012

Friction in the right places can be useful. Without friction, I wouldn't be able to lift my coffee cup without its slipping out of my hand. Automobile travel would be impossible if the tires just spun without grabbing the road surface. Friction also enables the brake pads to stop my automobile when required.

There's a dark side of friction in automobiles, and that's the reason we need lubricating oil as well as gasoline. Not surprisingly, only 21.5% of the energy content of fuel actually moves an automobile powered by an internal combustion engine.

If we look at where all the energy in fuel goes, we find that 33% is lost in exhaust, 29% goes to cooling and only 38% becomes mechanical energy. Not all that mechanical energy is used to move the automobile, since 33% goes to friction losses and 5% to air resistance.[1]

According to a study by the VTT Technical Research Centre of Finland and Argonne National Laboratory, friction losses in an average automobile are about 11,860 MJ per year, which corresponds to 90 US gallons of gasoline.

These losses are partitioned as follows: 35% overcoming rolling resistance in the wheels, 35% in the engine, 15% in the transmission and 15% in braking.[1] Since there are 612 million cars in the world, today, this adds up to 55 billion gallons of fuel wasted on friction per year.

One useful application of friction in material processing. Fig. 1 of US Patent No. 5,460,317, "Friction Welding," by Wayne M. Thomas, et al., October 24, 1995.[3] (Via Google Patents).

VTT and Argonne state that there are technologies available to reduce friction by 10-80%, depending on the component. The benefit of rolling out these technologies would be a reduction in fuel consumption by 18% in 5-10 years, and 61% in 15-25 years. Such technologies include new lubricant additives and low-viscosity lubricants, ionic liquids as lubricants and lubricant additives, surface coating and surface texturing, and tires that can be inflated to high pressures.[1]

Diamond-like carbon and nanocomposites are surface treatments with the potential of reducing friction by 10-50%. Lasers can be used to pattern microchannels on surfaces to facilitate lubricant flow and reduce friction by 25-50%. Since ionic liquids are composed of charged molecules that repel each other, they make excellent lubricants that allow a friction reduction of 25-50%.[1]

Driver attention can help, also. If vehicle speed is reduced by 10%, fuel consumption is reduced by 16%. The report claims that reduced driving speed allows a 25% higher tire pressure, which also translates to a fuel saving; but I would be hesitant to try that for safety reasons. The analysis and recommendations were published in an article in Tribology International.[2]

Scientists continue their quest to discover the origin of friction in particular materials. Graphite and molybdenum disulfide (molybdenite, MoS₂) are two popular solid state lubricants, and their lubricity appears to lie in their layered crystal structure; that is, they possess easy slip planes. I wrote about molybdenum disulfide in a recent article (Molybdenum Disulfide Circuitry, December 9, 2011).

Now that we have graphene, single layers of graphite, to investigate, there are some surprises. Scientists at the National Institute of Standards and Technology (NIST) have used Brownian dynamics simulation to explain the observed friction of atomic force microscope (AFM) tips on single and multi-layer graphene sheets.[4-5]

Experiments have shown that single layers of graphene have considerable friction, but the friction is reduced in graphene stacks. The simple explanation is that the top layer of a graphene stack deforms more when there are fewer layers beneath it.[4]

A NIST simulation of an AFM tip moving left on a four sheet stack of graphene. Friction decreases as more layers are stacked. (Image: A Smolyanitsky/NIST).

The friction arises from the deflection of graphene below and around the AFM tip. This reversible warping creates rolling resistance. When there are fewer layers, the top layer deflects more, so the friction is larger.[4]

References:

1. One third of car fuel consumption is due to friction loss, VTT Press Release, January 12, 2012.
2. Kenneth Holmberg, Peter Andersson and Ali Erdemir, "Global energy consumption due to friction in passenger cars," Tribology International (in press), online version, December 6, 2011.
3. Wayne M. Thomas, Edward D. Nicholas, James C. Needham, Michael G. Murch, Peter Temple-Smith, Christopher J. Dawes, "Friction Welding," US Patent No. 5,460,317, October 24, 1995.
4. Laura Ost, "Slippery When Stacked: NIST Theorists Quantify the Friction of Graphene," NIST Tech Beat, January 10, 2012.
5. A. Smolyanitsky, J. P. Killgore and V. K. Tewary, "Effect of elastic deformation on frictional properties of few-layer graphene," Phys. Rev. B, vol. 85, no. 3 (2012), Document No. 035412 (6 pages).