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Coating superalloys using CVD

In various RET-Monitor articles on materials, the use of superalloys in race engines has been covered. Their originally intended use was in gas turbine engines for the aerospace industry, and they were developed specifically for their capability at elevated temperatures. They are directly responsible for significant steps forward in gas turbine engine development, with limiting component surface temperature having increased well over 150 C since the 1970s due to materials alone.

In motorsport engines, superalloys have found a great many uses, both for elevated temperature use and many applications at much more modest temperatures, where their ability to be processed during manufacture at increased temperature can give them an advantage over more conventional high-strength steels.

However, superalloys on their own have not been wholly responsible for the increase in surface temperatures in aero engines. The increase in the maximum surface temperature has been much greater than that allowed by improved materials, and the remainder has been possible due to some very clever internal cooling and coatings.

The three high-temperature applications of superalloys in race engines are exhaust valves, exhaust systems and turbocharger components. For most of these race engine components, it would not possible to use the same cooling strategies that the aero engine makers use.

However, we could use the coatings that find success on gas turbines, especially thermal barrier coatings. These are plasma sprayed, but the longevity of the components lies in the oxidation-resistant CVD bond coat applied to the component before the actual thermal barrier. Thermal barriers are generally composed of zirconium oxide, normally called zirconia, but zirconia is almost ‘transparent’ as far as oxygen is concerned, and at elevated temperatures oxidation of superalloys can be a serious problem.

The CVD bond coat is impervious to and does not react with oxygen. The bond coats are generally aluminium-based coatings, where the aim is to produce an intermetallic compound with the substrate; most nickel alloys contain a lot of nickel, and NiAl (nickel aluminide) is highly resistant to oxidation. It may be necessary to apply further thermal treatments to fully form the aluminide coating, as it relies on diffusion (a process whose rate is controlled by temperature) of aluminium and nickel.

The actual composition of the coating, namely other elements added to the aluminium, controls how reactive it is. ‘Low-activity’ coatings rely on the outward diffusion of nickel into the coating, while high-activity coatings rely on the inward diffusion of aluminium into the substrate. After the diffusion stage the coating is extremely tenacious; it is no longer a discrete layer on the component, but is now an integral part of it.

The only problem with the CVD coating of superalloys is that the composition of the coating is difficult to control, as the various elements deposit at different rates.

A successful thermal barrier coating applied to components such as turbine wheels could allow them to run cooler for a given gas temperature, improving reliability or allowing the designer to produce a component that is lighter/has lower inertia. A lower-mass exhaust valve might be very much appreciated by development engineers in Formula One wishing to explore new valve lift curves.

Written by Wayne Ward

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