Hard wearing, hard work
Cylinders have to ensure high load conditions and high pressure, as well as high temperatures. Even though the cylinder wall is lubricated, the cylinder still has to be constructed from a good bearing material. There are to main types of cylinders, sleeved and in block.
In the block cylinders are bored straight into the block, if the block is cast iron then the pistons can run directly in the block. If the block is Aluminium then the cylinder surface need a protective hard wearing surface. Some cast iron blocks also have coatings for that added protection and to increase the lifetime of the block.
In-block bored cylinders - the block is cast iron an excellent bearing material
As in-block cylinders are self explanatory, we’ll look closer at coatings.
Aluminium isn’t a hard surface, and galls up quite quickly. One of the benefits is that Aluminium is a very good heat conductor, and with a cooler running engine the life of the cylinder is increased. However, the Aluminium wear issue is still a problem. This is where coating come to the rescue.
There are many coatings out there but there are 2 used more than any other. One coating is quite common, especially in the 2 stroke application. This is called Nikasil and is a ceramic coating that is very hard wearing and durable.
Nikasil coating on the inside of a cylinder,
notice the change in colour against the Aluminium.
The other is a very expensive coating primarily used in high end motor racing like Formula 1. This is called DLC which stands for Diamond-like carbon. Most modern razor blades use some form of DLC coating, usually Hydrogen free.
DLC coating on a cylinder sleeve
Both DLC and Nikasil coatings are usually applied to the cylinder wall through a process called physical vapour deposition (PVD), we won’t go into detail but basically the coating is evaporated and deposited onto the cylinder surface in a even smooth coating. These coatings protect the Aluminium underneath, increasing the longevity of the cylinder life.
One of the drawbacks to using these coatings is that wear will eventually prevail and the cylinder will slowly become oval in shape. The coating has to be removed before the cylinder can be re-bored and recoated. This is a difficult and expensive as the coating is very hard.
DLC coatings are becoming cheaper and so more popluar, they are used on both pistons and wrist pins.
DLC coating on the underside of a race piston.
DLC coating on Titanium wrist pins.
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