Chemical vapour deposition

Friday, April 05, 2013

Tags :  coatings

In the past 15 years or so there has been a huge change in the range of coatings for metallic components in motorsport, with many more suppliers and a much wider choice. Before then, and in terms of hard-wearing coatings, these often implied adding a different material such as a hard metal or ceramic to the component’s surface to a significant thickness before grinding back to finished size. Various spraying techniques were commonly used to apply such materials, and they are still in common use. For low-friction applications there were a number of alternatives, such as polymer coatings, soft metal plating or substances such as molybdenum disulphide. While many of these performed well in terms of friction though, they were often not very wear-resistant.

Two groups of coatings then came along that offered a good combination of low friction and wear resistance – PVD (physical vapour deposition) and CVD (chemical vapour deposition). CVD coatings first became industrially available about 40 years ago, but their high process temperatures (in the region of 800-1200 C) limited their applications to substrates whose strength was unaffected by such temperatures and to components that would not suffer excessive distortion; their application to precision components is also impractical.

There are several variations on the basic CVD process, although the one used to deposit the kind of hard surface coatings we would like for our engine and transmission components requires those components to be hot. The heat promotes chemical reactions between the vapour and the component surface, and the process is essentially to pump reactants into a chamber holding the heated components. The combination of temperature and pressure promotes a reaction at the surface of the component, with one of the products of the reaction being deposited onto the surface.

The process can be used to deposit various substances onto surfaces, including metals and ceramics, in thin layers. The gases used to transport the reagents to the surface need to be specially selected, and often more than one gas is required to produce the correct reactions. For example, tungsten can be deposited at quite low temperatures by using a mixture of a tungsten hexafluoride and hydrogen gas, with a reduction reaction taking place, while nickel can be deposited using a single gaseous reagent through thermal decomposition. To produce such metallic coatings by processes such as PVD, where the source coating material has to be evaporated, might prove very difficult.

Where the process has traditionally required high substrate temperatures, for example titanium carbide (TiC) coatings, recent advances in CVD have reduced processing temperatures significantly. Plasma-assisted CVD (PACVD) and plasma-enhanced CVD (PECVD) are techniques that increase the energy of the gas from which the coatings are deposited. This means that comparatively less heat energy is needed at the component’s surface, allowing lower substrate temperatures. For a coating such as TiC, typical PACVD process temperatures are around 700 C, compared to 1200 C for the process without the assistance of plasma.

The low-temperature CVD processes in particular make the technique attractive for depositing metals onto materials such as aluminium, titanium or steels with low tempering temperatures.

Written by Wayne Ward

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