Diamond-like carbon, the initials of which are used to describe the ubiquitous DLC coating, has found very wide acceptance in motorsport, especially in engines. Initially, it was suitable only for steels with relatively high tempering temperatures – high process temperatures would have led to many steels being ‘over-tempered’, losing strength and durability. However, in recent years much work has been done to reduce process temperatures, and this has meant that DLC coatings have been used not only for low-tempering temperature steels but for other materials too, including aluminium.
Given the contribution of the piston assembly to overall engine friction, it should come as no surprise that engineers soon tried to apply DLC to pistons, often with poor results. There are peer-reviewed technical papers, such as that by Demas et al*, which find that DLC shows little advantage. However, in the pages of Race Engine Technology we have seen that Formula One engines definitely use DLC. The magazine has run very detailed articles on Toyota and Cosworth Formula One engines, and both show DLC-coated pistons in use. Be under no illusion, if the Formula One engine companies use it, there is an advantage to be had, either in terms of performance, reliability or both. So why do the likes of Cosworth find a gain from DLC coating where researchers like Demas et al find none?
The answer lies in the fine detail. The Demas paper notes that, for their experiment, sections of an existing piston were used. Speak to someone who knows how to get a DLC-coated piston to work and they will tell you that simply applying the coating to an existing piston is most definitely not the way to go. One only has to look at a DLC-coated Formula One piston and compare it to a conventional piston, and there are some obvious differences.
The visual difference is the machining. The skirt of a conventional piston has some very coarse machining, almost like a soft single-cut file. An optimised DLC-coated piston though has a very smooth skirt. If we coat a conventionally machined piston, we create something akin to a file with hard teeth. It can easily wear its counterpart – the cylinder bore – working as a very efficient broaching tool.
Uncoated aluminium pistons wear in a very benign way, with the piston skirt ‘bedding-in’ over time, and the very small amount of material displaced does not cause any detriment. In fact, it was common not so long ago for Formula One pistons to have a resin-bonded polymer skirt coating, precisely to accommodate some skirt wear as the piston bedded into the cylinder bore. DLC coatings are not apt to wear, so the piston doesn’t wear in; rather, it can tend to wear out its counterpart, or the coating fails. It is therefore very important that the DLC-coated piston skirt profile is more highly optimised than its uncoated equivalent.
* Demas, N.G., Erck, R.A., Ajayi, O.O., and Fenske, G.R., “Tribological studies of coated pistons sliding against cylinder liners under laboratory test conditions”, Lubrication Science
Written by Wayne Ward