Practical use of coatings

It is difficult to draw general rules as to which coating is used in which application. Even for a given application and a given engine, different commercial strategies may imply different choice of coatings: opting for a higher efficiency and therefore higher temperatures at the expense of long life means that the operating conditions are changed, and the dominant degradation mechanisms may be different.

Jet engines and land based power generation turbines provide different environments. In general, jet engines high pressure turbine blades (HPT blades) are expected to last for about 30,000 h. For land-based power generation, this figure can vary between 50,000 and 75,000 h (about 9 years).
HPT blades in jet engine will typically undergo one refurbishment (strip coating and re-coat) throughout their life; in power generation applications, one or two refurbishments depending on the target life.

At the time of writing (2004), rough estimates of costs provided by RWE Innogy are (power generation):

• set of blades for HPT: 1.5 million gbp
• cost of refurbishment: variable: 0.3 to 1 million gbp.

The HPT blades in jet engines mainly suffer from oxidation; Pt-aluminide coatings are preferred in these conditions and are commonly used to coat the main surface.

A jet engine HPT blade. Photo courtesy S. Tin, Rolls-Royce UTC.	Alumimised jet engine HPT blade. Photo courtesy S. Tin, Rolls-Royce UTC.

They are also used to coat the internal cooling chanels in power generation applications, this choice being mostly dictated by the process (as discussed earlier, aluminide coatings are typically obtained by pack cementation, which is a non-line-of-sight process allowing the coating of complex geometry).
In power generation applications, the surface of the HPT blades typically receives an MCrAlY coating, which is best suited to provide the corrosion resistance required in these environments. The composition of the bond coat can be adapted depending on the operating conditions (e.g. Cr content, addition of Si etc.)

Other parts of the HPT blades in jet engines may however receive a different coating: the base and tips often receive an MCrAlY coating as corrosion can be more important at these locations.

The conditions in the low pressure turbine are considerably less severe than in the HPT. Blades are simply aluminised; they are not cooled and do not receive a TBC coatings. In the case of land-based gas turbine, they are not necessarily made of superalloys, but the coating may contain additions to enhance corrosion resistance.

Abradable coatings

This subject is slightly off-topic and is only covered very very briefly. Abradable coatings are used to reduce the clearance beetween the tip of the blade and the casing, to an extent that precision machining cannot achieve. MCrAlY or TBC coatings are used depending on the component temperature. For this purpose, a high porosity is usually desirable and thermal spraying is therefore ideal. The use of abradable coatings can result in up to 1% improvment in efficiency.

References/acknowledgements:

1. Much of the above comes from discussions with J. Wells, RWE Innogy and S. Tin, Rolls-Royce UTC.
2. Abradable Coatings Increase Gas Turbine Engine Efficiency.