Coating technologies help drive down cost in the aerospace industry

Commercial aviation in the early decades was expensive and exclusive. Today, however, we have come to expect frequent and relatively low cost air travel. But this market explosion has an environmental cost that continues to proliferate around the world.

Combined with increased fuel and operational costs, the aircraft industry is increasingly looking to reduce fuel burn and the corresponding exhaust emissions such as carbon dioxide.

Much like the automotive industry and the humble plastic bag, commercial air travel has become another face for environmental concern, and consequently a target for immediate action.

Aircraft manufacturers have been moving quickly to respond to rising operating costs combined with growing pressures to reduce environmental impact. The result is that the aviation industry is looking at evermore ways of making aircraft structures lighter and engines more efficient.

Material substitution has seen major uptake in recent years with the primary structure of the latest airliners comprising of more than 50% carbon fibre.

Another consequence is the amount of titanium used has also increased, as have more exotic coatings within engines.

However, while many unconventional materials have offered advantage on paper, the practicality of implementation has introduced many new, and unforeseen, challenges.

Making titanium fit-for-purpose
For example, titanium is strong, lightweight and naturally resistance to corrosion making it a natural choice for use on aircraft. However, it tends to have a poor coefficient of friction due its relatively low hardness.

All this means that without surface treatment, adhesion and galling can become a problem, and this creates a need for more frequent maintenance.

"For parts that experience low loading, these problems can be overcome by coating the alloy using physical vapour deposition (PVD) of titanium nitride or chromium nitride," says Ian Haggan, business development manager for TecVac, the coatings division of the Wallwork Group. "This aids lubricity and hardness. However, in situations where components are subject to frequent heavy loading such as undercarriage bearings, another solution has to be sought."

Heat treatment and hard coating specialists Wallwork solve the problem by applying a duplex coating, where the substrate is first hardened by the production of a hardened nitride diffusion zone. This is then covered in the more lubricious PVD applied surface coating. This duplex process, though applying two coating properties on the substrate, can be achieved in a single process cycle.

Extending engine life
The company has also seen successful application of coating technology on turbine fan blades, in particular the compressor stage. These are affected by the incoming high-speed air that can be contaminated with small amounts of debris. The result is erosion and encrustation on the surface of the compressor blades where even subtle changes to the aerofoil geometry affects fuel consumption, and this can again lead to more frequent and extended maintenance.

Wallwork's Nitron Flight is a family of coatings based on titanium, chromium, chromium/aluminium and carbon/metal carbide that is able to overcome these problems when applied in multiple layers.

This multi-layering creates a low stress surface coating that is more resilient than a single layer coating of the same thickness. This makes the surface closely resemble a highly polished finish.

The chromium based variation of the coating has been extensively proven with major international maintenance overhaul and repair contractors demonstrating its effectiveness in side-by-side engine trials of compressor blades on the same aircraft. And while surfaces on the treated blades were subject to some scale build-up, they were also more easily descaled because of the coating.

In addition the engine using the coating showed improved fuel consumption as the engines were able to run at higher temperatures without causing damage.

Nano coatings offer more advantage?
Advances in turbine engine coatings are not just happening in the UK, however. Researchers at University West in Sweden have started using nano particles as a heat-insulating surface layer to protect fan blades, as well as other key components, against excessive heat more effectively.

The heat insulating layer is a nano particle based ceramic powder that is heated to approximately 7,000 to 8,000°C before being sprayed directly onto the metal surfaces using a plasma stream.

The ceramic particles essentially melt and strike the surface, where they are able to form a protective layer approximately half a millimetre thick.

In tests, the coating has tripled service life and due to its low heat conduction, the turbine can run even hotter, improving fuel consumption while reducing emissions and operating costs.

Nicholas Curry, lead researcher on the project, says: "A conventional surface layer looks like a sandwich, it's layer upon layer. But, the surface layers we produce using this method are better compared to standing columns. This makes the layer more flexible and easier to monitor. And it adheres to the metal, regardless of whether the surface is completely smooth or not.

"The ceramic layer of the coating is subject to great stress with the enormous changes in temperature that make the material expand and contract. So it's been important to make the layers elastic and focus on refining the microstructure. This resulted in thousands of new layer being developed and put through 'thermal shock tests' to simulate the temperature changes in an aircraft engine."

The research has been conducted in close collaboration with GKN Aerospace and Siemens Industrial Turbomachinery, with the aim to incorporate the surface coating technology on to both aircraft as well as larger power generation gas turbines within the next two years.

Justin Cunningham

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