NASCAR transmissions: the advantage of coatings

Surface finishing and coating processes continue to make their mark on the motorsport arena, enabling engineers to improve the properties of components that experience direct metal-on-metal contact. NASCAR is no exception to this and, within a very tight regulatory framework, teams are keen to gain any advantage they can. Power levels are very closely matched between different manufacturers' engines, so very small gains in power can be decisive.

This does not necessarily mean increasing engine power though; efficiency gains in the transmission can be just as useful, allowing engine power to be better exploited. While the internals of the gearbox and differential are tightly controlled, engineers have been able to put coating technology to good use here.

There are many different proprietary coatings on the market, but they all are aimed at reducing wear and friction between rotating assemblies. Most manufacturers and teams remain understandably tight-lipped about the exact make-up and performance of various coatings, although one manufacturer has published research to show that its coating technology reduced parasitic power losses in a ring and pinion gear set by between 0.15 and 0.21%, which on a 500 hp engine equates to almost 1 hp. However, to gain a better understanding of the impact coating technology can have on gear longevity it is necessary to look a little further a field - into outer space.

To the casual observer, NASCAR could not be further removed from NASA (apart from similar acronyms) but both industries attract some of the best and brightest engineers and present them with an extremely harsh working environment for their designs. Engineers at the NASA Glenn Research Center undertook a research study* to investigate wear characteristics on both uncoated and coated spur gears.

For the experiments, a lot of spur test gears made from AISI 9310 gear steel were case-carburised and ground to aerospace specifications. The geometries of the 28-tooth, eight-pitch gears were verified as meeting American Gear Manufacturing Association (AGMA) quality class 12, after which one-half of the gears were randomly selected for coating.

The method of coating was selected to achieve the desired adhesion, toughness, hardness and low-friction characteristics. First the gears to be coated were prepared by blasting (vapour honing) with aluminium oxide particles, then they were given a thin adhesion layer of elemental chromium followed by magnetron sputtering of the outer coating consisting of carbon (70%), hydrogen (15%), tungsten (12%) and nickel (3%). In total, the coating thickness was about 2.5-3 µm. As compared with the steel substrate, the coated surface was harder by a factor of about two and had a lower elastic modulus.

All gears were tested using a control synthetic oil, at 10,000 rpm and with a Hertzian contact stress of at least 1.7 GPa (250 ksi). Tests were run until either surface fatigue occurred or 300 million stress cycles were completed, using either a pair of uncoated gears or a pair of coated gears (coated gears mated with uncoated gears were not evaluated). The results showed that the coating extended the surface fatigue lives of the gears by a factor of about five compared to the uncoated gears. When combined with the potential frictional gains, it is clear to see why coatings technology has proved so attractive to transmission developers.

*Krantz, Timothy L., et al, "Increased Surface Fatigue Lives of Spur Gears by Application of a Coating", NASA, 2003

Written by Lawrence Butcher