Spin doctor

A newly-developed metal-based coating should offer gas turbine blades better protection from the elements as well as extending their life. Stuart Nathan reports.

Gas turbine blades could run hotter, for longer, and with less maintenance thanks to the development of a high-temperature coating.

Developed by Siemens, whose joint venture with Westinghouse is one of the world’s largest producers of gas turbines, the metal-based coating provides an extra layer of protection for the blades, which operate in an extremely hostile atmosphere.

It’s tough for turbine blades. At normal operating conditions, they spin at 3,600rpm, with the tips reaching speeds of 800mph. They are subject to forces equivalent to around 15g. And amid all this, they are constantly bathed with a mixture of gases resulting from the combustion of methane which, at 1,300°C, are also corrosive.

Although the blades are made from extremely strong and durable materials — typically a crystalline form of nickel, which has very few weak points — they need all the help they can get if they are to survive intact for the lifetime of the turbine.

The blade material can withstand temperatures of up to 900°C, so to cope with the higher temperatures inside the turbine, designers have incorporated pneumatic cooling systems inside the blades themselves, which carry the heat away from the outside surfaces. Some are also coated with a ceramic insulator to prevent the heat from reaching the surface.

But even so, corrosion can be severe; some blades oxidise so much that they cannot surpass the 25,000-hour service life typically requested by the customer.

At Siemens, turbine materials specialist Werner Stamm has addressed the problem by developing an extra protective layer that sits between the blade itself and the insulating coating. This coating is an alloy of several metals, which improves the blades’ heat resistance as well as protecting it from oxidative corrosion.

The coating is a mixture of nickel, cobalt and aluminium, doped with two less familiar metals, yttrium and rhenium. It’s the rhenium which is the vital component for the coating.

Better known as a component of chemical industry catalysts than as a structural material, rhenium was the last naturally-occurring element to be discovered and has several properties at the extreme end of the scale: it’s one of the densest metals, and has one of the highest melting points. Because of these properties, adding rhenium to the protective layer increases increases its heat resistance and helps to prevent cracks from forming and propagating.

The alloy coating acts as an adhesive layer between the blade and the ceramic insulator, and at high temperatures, it forms a slow-growing aluminium oxide layer on its surface which adds yet another oxidation barrier. ‘The invention makes it possible to operate the turbines for longer periods and under greater mechanical stress,’ said Stamm. ‘The longer service life of the parts exposed to hot gas makes the turbines more economical to operate.’

Siemens plans to use the rhenium-containing coating on all its gas turbines in future. In the meantime, Stamm is turning his attention to the ceramic coatings. ‘I want to calculate with even greater reliability how long a ceramic coating will last under specific operating conditions and loads,’ he said.

‘Because if you really know the causes of possible failure and can describe them precisely, you can optimise the coatings and do an even better job of avoiding errors.’