Many of us who have been involved in designing components for race engines will be familiar with the nitriding process, and even if the process isn't very familiar in its detail, we probably know what its benefits are. It is also likely that we will have only ever specified the treatment on steel components. Any steel containing strong nitride-forming elements such as aluminium and chromium will nitride well, and the process is used on everything from medium alloy steels, through stainless steels to tool steels. Its applications in an engine are commonly crankshafts, piston pins and valve springs, among others.
What is less well known though is that nitriding can be used as a surface treatment for other materials, especially the more modern plasma nitriding processes which can be controlled more easily.
After steels, titanium is the most commonly nitrided material. Titanium is often coated with titanium nitride, but it is also possible to form the nitride directly at the surface, and for the nitride layer to be diffused into the surface as per conventional nitriding of steels. The advantage of forming titanium nitride directly from the substrate rather than applying a discrete layer is that the nitride formed in the process is an integral part of the component and does not suffer from the adhesion problems sometimes associated with coatings. One example from motorsport that compares the methods of nitride coating of titanium versus directly forming the nitride layer is of a steering rack, as cited by Huchel and Strämke*. The PVD-coated rack lasted one race, whereas the nitrided component lasted between up to ten races.
The nitriding of titanium was for a time restricted to quite simple geometry, because the surface could easily be damaged by high heat input, but now pulsed plasma nitriding methods are available that reduce local heat input and therefore allow more complex titanium components to be processed.
The benefits of nitriding titanium cannot be translated directly from experience of using the process on steel. It does not, for example, confer on titanium components any degree of improvement in fatigue life in the same way it does for steel. It does impart some wear-resistance to titanium, although only against adhesive wear. The layer thickness achieved at 1-3 microns is sufficient to prevent micro-welding, but is not sufficient to prevent abrasive wear which is caused by a ‘ploughing’ effect to a greater depth than nitriding is able to protect against.
Nitriding titanium poppet valves is a common application, as the stem and valve seat faces can commonly wear if not coated.
In the past, attempts have been made to coat titanium components with steel, which was then ground to size and subsequently nitrided in order to improve the wear characteristics of titanium. However, with improvements in plasma-nitriding methods, such elaborate processes are not required.
It is also possible to plasma-nitride aluminium, and there has been research in this area in order to provide wear-resistant surfaces on aluminium components.
* U. Huchel, S., and Strämke, “Pulsed Plasma Nitriding of Titanium and Titanium Alloys”, paper presented at the 10th World Conference on Titanium, 2003
Written by Wayne Ward