Tough coating for aircraft

An Oxfordshire-based company has signed an agreement with Airbus to help expand the market for its 'ultra-hard' coating technology into the aerospace industry.

Hardide Coatings' tungsten carbide-based technology will undergo a three-year approval test programme with the aircraft manufacturer to assess its suitability for applications where the less environmentally friendly chrome is normally used.

The coating, Hardide, is said to be very tough, with a high level of abrasion and chemical resistance.

'Hardide is corrosion and wear resistant and extremely hard wearing,' said Hardide's Neill Ricketts. 'Its main use is in the oil and gas industry, where there are some of the harshest environments you can imagine.'

The coating, which consists of nanoparticles of tungsten carbide integrated into a dense and pore-free alloyed tungsten matrix, is produced using a low-temperature chemical vapour deposition process, which has a processing temperature of 500ºC.

This means the materials that can be coated are limited to metals such as stainless steel and nickel, cobalt and copper-based alloys.

Hardide is now being used on the canopy of the Eurofighter Typhoon, but the company hopes to work towards commercial applications in aerospace, essentially using it as a chrome-replacement product.

'Airbus is looking to characterise the coating and assess the performance. It could be used to replace chrome, which is used extensively on engines, landing gear and avionics. It has also been used in applications of internal coating, where high velocity oxygen-fuelled spray (HVOF) cannot reach.' said Ricketts.

'It also has quite good corrosion resistance, which means it can be used in applications which previously used cadmium, now banned under recent legislation,' he added.

So the coating could be used instead and HVOF in corrosion-prone areas such as an aircraft's undercarriage, engines and wings.

The technology usually deposits 50 to 70 microns of the coating on to a surface. The process works by filling in micro-pores and defects in the coating layer as it is being grown at the nano-level, which results in a dense and 'extremely uniform' coating.

'Ninety per cent of our coatings are in the range of 1200 to 1300 Hv [Hv is the measure of hardness on the Vickers scale],' said Ricketts.

To illustrate what this level of hardness means, one of the company's published research documents reports that in an instant coffee manufacturing application, hard, chrome-plated ball valves experienced intensive abrasion and erosion and had to be replaced every few days. After being coated with Hardide, however, the valves were in continuous service for more than 18 months.

In addition to developing its products in the oil and gas industry, and entering the aerospace industry, Hardide's future plans include steps into the food processing and pharmaceutical sectors.

'Most other coatings have cobalt in their matrices which means they are unsuitable for food processing,' said Ricketts.

'We have FDA approval for food applications, and this will be the first type of coating available for food processing — homogenisers for milk products for example,' he added.

Anh Nguyen