DLC and pistons

It is fair to say that DLC (diamond-like carbon) coatings have proven to be something of a revelation to the motor racing and wider automotive sectors. Motorsport, with its relatively big budgets per component and small production quantities, is generally able to investigate, develop and test new ideas, products and processes far more quickly and efficiently than the mainstream automotive industry, and the widespread use of DLC coatings in racing valvetrains has been transferred to the production vehicle models of at least one major passenger car manufacturer for mass-production models.

DLC is not a single coating but a family of coatings based on carbon, which has a proportion of bonds that are the same as that found in diamond. The proportion of diamond-like bonds in the coating depends on the specific process; there are also wide variations in the percentage of hydrogen in the coatings, and there are other elements which are added in order to tailor a coating’s properties.

The application of DLC to aluminium automatically reduces the number of coatings that can be considered, as many of the processes are carried out at temperatures that would cause a significant loss of strength and durability in the aluminium itself. From this smaller pool of coatings applied at low temperatures, there are others that are unsuited for use with automotive lubricants and some that suffer from fundamentally poor adhesion. Furthermore, the ‘pre-processing’ of the parts in terms of surface preparation is critical. There are only a few companies manufacturing pistons who apply a DLC coating to them and, as a consequence of the difficulties in getting the coating to work properly, only a few companies to whom the task of coating the components is entrusted.

There are fundamental differences in the design of a piston where a DLC coating is to be specified, and the machining of the skirt and lands is also markedly different, owing to the hardness of the coating. All DLC coatings are very hard and wear-resistant; they are widely used for coating metal-cutting tools, and if the piston is not to become an effective metal-cutting tool within the engine, its surface topology needs to be considered carefully. Those who have managed to develop DLC coatings on pistons have arrived at their chosen surface topology via a great deal of testing, which probably involved a lot of mechanical damage along the way.

The piston skirt and land profiles need to be better optimised than for an uncoated piston or a piston with a softer coating. In the case of the soft-coated or uncoated piston skirt, there is generally some expectation of minor wear or at least some minor displacement of material, but this process of the skirt ‘wearing in’ during use will not happen with a hard coating such as DLC, so any excessive contact pressure is likely to result in wear of the adjacent part.

For a number of years DLC-coated pistons were used only on very high budget engines, and were not available commercially. That has now changed, however, and at least one piston manufacturer will supply DLC-coated pistons to its customers. There is though a significant cost penalty to the use of such pistons, but with reduced frictional losses and increased durability as the advantages, there are a lot of racing customers who would take the opportunity to use such pistons.

Written by Wayne Ward