In an article posted in February 2011 on the subject of coatings, there was a brief discussion of the physical vapour deposition (PVD) method, some of the coatings used in motorsport that are commonly associated with this method, and some of the pitfalls of blindly specifying the latest coating without taking account of the exact type and grade of material being coated.
In this article, I'll look into the method in more depth. The method lends itself to a very wide variety of coating materials, providing that the material can be furnished in a suitable solid or liquid form to be placed into the vacuum chamber as a solid target. PVD coatings lend themselves to use on many different substrate materials and are used not only for high-end technology; cutting tools and even our aluminised crisp packets are often PVD coated.
There are a number of methods of turning our solid or liquid 'target' material into a vapour:
- Sputter deposition (also known as sputter ion plating) is used for many motorsport components. In this method a plasma discharge is focused around the target by a magnetic field, often generated by one of more magnetrons. The plasma vaporises the solid target.
- Electron beam deposition relies on bombarding the target material with a beam of electrons in a vacuum chamber.
- Liquid or solid targets can be evaporated by a number of conventional heating methods - resistive heating or inductive heating, depending on the nature of the material and so on - thus producing a vapour in a vacuum chamber.
- Lasers are used to evaporate material from the surface of a target by a process of ablation. The laser ablation process is also used to good effect in laser peening, which has been covered previously in a RET-Monitor article.
- Electric arc evaporation of electrically conductive targets is also used, and this employs the same principles as the spark erosion methods used in industrial manufacture.
Given the number of methods of vaporising the target, we can use PVD to apply everything from ceramics to metals and carbon coatings such as DLC.
Of special interest to the designer of a race engine, where parts are often subject to very severe cyclic stresses, is the fact that metallic coatings can be applied without the danger of hydrogen embrittlement that comes from electroplating and other coating processes, owing to the fact that the coatings are deposited in near-vacuum conditions. Of course, in order to prevent embrittlement completely, pre-treatment processes need to be carefully selected so that they don't cause hydrogen to be diffused into the surface of metallic components. High-strength steels are well known for suffering hydrogen embrittlement. Although there are processes to mitigate the effect of hydrogen embrittlement, some people are adamant that it can't be fully reversed. PVD coatings appear to be one way to preclude it.
There are a number of ways of depositing coatings that differ from the target material. For instance, the same titanium target material can be used to deposit not only a titanium coating, but also titanium nitride (TiN) and titanium carbonitride (TiCN), simply by introducing a carrier gas to the vacuum chamber which contains ionised titanium vapour. For instance, introducing nitrogen gives a TiN coating, and the use of acetylene (ethyne) gas produces a TiCN coating.
Fig. 1 - Coated valves are only one application of PVD coatings in race engines and transmissions
Written by Wayne Ward