The Restriction of Hazardous Substances Directive took effect in 2006 and has, on the whole, banned six substances deemed dangerous for consumer goods. While the legislation has drawn criticism in certain cases for leading to reduced product quality and reliability, it has forced those that were using the 'hazardous' materials to look elsewhere for serviceable alternatives.
The Registration, Evaluation, Authorisation and Restriction of Chemicals, better known as the REACH Regulation, also came to in to effect in 2006 with restrictions being increasingly tightened up to 2018. These have arguably had a more prolific impact on industry, dictating a need to change away from 'restricted' materials and substances to compliant alternatives.
Under REACH the use of hard chrome plating (HCP), which has been widely used in the aerospace and defence industries for decades, is soon to be heavily restricted. And it is not just the European directive that is taking a hard line on hard chroming with the US Occupational Safety and Health Administration (OSHA) passing legislation to similar effect.
The aerospace industry, globally, uses the process to reduce friction and corrosion of parts while simultaneously improving durability and wear resistance. However, application of hard chrome uses highly toxic and carcinogenic hexavalent chromium salts. These are essentially what is targeted by the EU and now US, with both industrial waste effluent and carcinogens in the air cited as major environmental concerns. The consequence is the aerospace industry has had to find an alternative.
And this has been good news for one materials innovator. Bicester based Hardide Coatings has been able to seize on the opportunity presented by showing exactly what its coatings can do to some of the biggest players in industry.
"We have been working with Airbus for six years on a development programme," says Philip Kirkham, chief executive officer of Hardide Coatings. "We are currently in the final phases of a testing programme that will see them completely move away from using hard chroming processes and the subsequent use of chromium salts.
"We have also been working with AgustaWestland for similar hard chrome replacement applications and we are now doing component testing."
Hardide Coatings produces a range of binder and porous-free Tungsten Carbide / Tungsten nano matrix coatings that are perhaps best known by the oil and gas industry. It's Hardide-T has a reputation for providing very hard coating, rated between 1100-1600Hv, on parts such as pumps and valves to give them the longevity necessary for operating in an environment routinely punished by the challenges of North Sea drilling operations.
"Historically, the company has grown supplying coatings to the oil and gas industry, particularly for down hole tooling applications," says Dr Yuri Zhuk, technical director at Hardide Coatings. "The target originally was to make the coating abrasion resistant to sand, which is really a mixture of minerals with the hardest being quartz, around 1000Hv. Sand particles hitting a metal surface at high velocity can cause serious erosion to the part. This is why we needed to make the coatings harder than the sand, so any abrasion happens on the sand grains and not on the coating."
Unique to the company is not just its capability to produce very hard coatings, but its ability to combine this property with toughness: a measurement of how well a material absorbs energy and plastically/elastically deforms without fracturing. While harder materials might have a reputation for being brittle, Hardide has been able to mix these two seemingly incompatible properties together. It offers exceptional flexibility with no peeling or even particulates coming lose when a small thin test piece, coated with Hardide-T and used for a demonstration, was bent back and forth over a piece paper.
Hardide wants to make engineers beyond the oil and gas industry aware of its coatings potential. Its work with Airbus is a step in the right direction that has seen it develop a variant coating, Hardide-A, specifically for aerospace applications.
Hardide-A is actually less hard than other variants in its product line, due to the requirement to match the standard thickness (50-100µm) and hardness (800-1200Hv) of hard chrome. The driving need was that Airbus wanted no dimensional changes on effected parts.
"They really do not want to have to change those drawings," says Kirkham. "If they change, any effected parts will have to be re-qualified and that is a long and expensive process to go through.
"REACH and OSHA have tried repeatedly to restrict the use of hexavalent chromium salts, but the industry has extended the deadline a number of times as they have been unable to find a viable alternative. With Hardide, they don't get that excuse."
The development of Hardide-A not only seems to offer a solution that satisfies demands from regulatory pressures, but it also outperforms hard chrome in other areas such as corrosion tests. A salt spray corrosion test on a mild steel plate saw it develop no signs of corrosion after 2000hours. This was compared to a high velocity oxy fuel (HVOF) applied Tungsten Carbide coating that showed considerable corrosion after 400hours, and hard chrome which failed after 288 hours.
The company applies Hardide using a variant process based on Chemical Vapour Deposition (CVD). Notably the process uses a lower pressure and temperature, making it suitable for most grades of stainless and low carbon steels.
CVD offers some unique advantages over processes such as plating and spraying, and allows complex shapes to­ be evenly coated throughout, and not just areas an operator's plasma gun can access. It is also a conformal coating, meaning it follows the surface profile of the substrate precisely and does not produce a thicker coating on sharp edges like other processing methods. For the aerospace industry, and indeed anywhere where surface profile is important, this is another distinct advantage.
"We can coat complex shapes and keep that surface complexity," says Dr Zhuk. "By keeping the surface profile, exactly, we can coat turbo drilling stators working in abrasive media and keep the profile of the aerofoil exactly the same. Hardide has the potential to offer advantage in so many applications, we just need to find them."

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