Margaret A. Whitney
Researchers at the US Department of Energy's Argonne National Laboratory have developed a laser glazing technique that could reduce costs, pollution and danger in railroad systems.
Rails are plagued with subsurface cracks that multiply parallel to the rail surface in the direction of travel, particularly at the gauge corners, according to Ron DiMelfi, a materials scientist at Argonne. Problems are greatest at curves where combined loading from normal forces and steering are the greatest, and severe cracks can even cause trains to derail.
Friction is required on top of the rails for traction, but problematic friction occurs when the train wheels rub the sides of the rails. To eliminate the unwanted friction, oil and grease are commonly used to lubricate the rails, but they wash away in the rain and are environmentally unsound. They also reduce the needed friction if they migrate to the top of the rail.
The laser glazing technique produces a solid-state lubricant that, besides being better than others at ameliorating cracking along the corners, stays where it is applied, can be applied to rails in service and is environmentally benign.
Laser glazing involves a rapid solidification of a thin melt layer produced on the rail by a moving laser beam. The result is a more than 50 percent higher yield strength than the substrate material -- in this case, steel. Laser-treated railroad tracks are also 30 percent more elastically compliant.
The researchers used a pulsed 1.6-kW Nd:YAG laser from Electrox to lubricate a 6-ft length of rail. The 1.06-µm wavelength allows glazing without the need for application of an absorptive coating. About 1 kW was delivered to the sample.
At its test facilities in Pueblo, Colo., the Association of American Railroads conducted friction tests on the laser-glazed sample. After more than 30,000 back-and-forth cycles at typical loads, the treated rails showed reduced friction by as much as 40 percent, which can reduce rail cracking by up to 75 percent.
Another advantage is the portability of the system: The beam can be delivered through flexible fiber optic cable, which is useful for remote applications.
"In addition to rail and wheels, the process can be applied to many steel surfaces, such as bearings and gears," DiMelfi said.