Developing a DLC coating for Formula One transmissions
Transmission durability in Formula One is of utmost importance. The transmission must be as lightweight as possible, yet still provide sufficient durability to ensure reliable race performance, particularly as it now has to last for five races by regulation. In recent years the use of DLC (diamond-like carbon) coatings in transmission applications in Formula One has become widespread.
The impact of such coatings on transmission efficiency and durability can be considerable, netting useful gains in both areas. For the purposes of this article, we will look at the Honda team’s transmission development during its previous foray into Formula One, which ended in 2008.
The main losses found in a transmission relate to the sliding motion of the gear teeth as they rotate. It is well known that DLC coatings have good low-friction characteristics, so coating the gears should help reduce overall losses. When Honda first looked at doing this it realised that existing coatings, used for components such as valvetrain parts, would not have sufficient durability for a transmission because of the high surface pressure loads experienced between gears. To counter this, it therefore set about developing a new coating formulation that could better handle these loads.
There were two key requirements for the coating:
- It must be able to withstand surface pressures of 2.2 GPa for a duration of four races (the then current regulation life of a transmission)
- The application method must allow for a uniform coating over the complex gear tooth shape.
There are some clear differences in requirements between gear and valvetrain coatings. As already mentioned, the surface pressures are higher, but also the lubrication regime differs considerably. In valvetrain applications, the oil film breaks down as valve motion is reversed, which can lead to scuffing. To this end, the coating Honda developed for its valvetrain featured a hard bonding layer under the DLC top coat to prevent scuffing. In a gear train though the oil film does not break down, so Honda decided that the hard coating could be removed, with the development focus being on a top coat composition that had improved wear resistance.
The higher surface pressure led Honda to revise the surface finish on the gears to aid adhesion of the coating film, settling on a roughness of 0.1 Ra combined with enhanced cleaning of the parts to remove contaminants that could potentially compromise the coating. Honda then developed a coating composition to match the requirements of the gears. This consisted of a chromium adhesion layer, on top of which was a layer of metal-impregnated carbon coating, the combined thickness of these layers being 0.6 µm. This was followed by a DLC top coat 1 µm thick, with all the layers being applied using a sputtering process.
The results obtained after coating were impressive. The coating itself, when compared to Honda’s standard DLC as applied to valvetrain parts, showed an improvement in seizing pressure of 40%, as well as a considerable reduction in frictional losses. With the coating applied to all ratios in the box as well as the final drive and bevel gears, overall frictional losses were reduced by 3.3 kW – a very useful gain.
This work shows just how effective the use of coatings can be in increasing transmission efficiency. Better still, with commercial suppliers of such coatings now offering products at ever-decreasing prices, it has even become feasible for teams and constructors outside of the rarefied atmosphere of Formula One to take advantage of such benefits.
Written by Lawrence Butcher