CVD Coating to Replace Hard Chrome Plating on Internal Surfaces and Complex Shaped Parts

Hard Chrome plating (HCP) is widely used by aircraft manufacturers as a wear-resistant and anti-galling coating with some degree of corrosion protection. But the hexavalent Chrome solutions used to produce HCP are known carcinogens which represent a major hazard. Legislation, including the US OSHA and the EU REACH regulations, is applying increasing pressure on the plating companies which is increasing the cost and reducing the availability of HCP. Aerospace companies wanting Chromate-free design often consider HVOF thermal spray coatings as the best available alternative.  However, spray coatings cannot be applied to internal surfaces and they require post-coat grinding which is not possible on complex geometries.

Hardide Chemical Vapour Deposition (CVD) Tungsten/Tungsten Carbide coating (Hardide-A) is an emerging technology which provides an advantageous replacement for HCP. The CVD coating is crystallised atom-by-atom from low pressure gas media, this enables the uniform pore-free coating of internal surfaces and complex shapes.

Hardide-A matches HCP in both thickness (50 to 100 microns) and hardness (800 to 1200 Hv) and can be polished to a good finish without the need for grinding. This facilitates switching from HCP to Hardide-A with only minimal design changes.

The coating is applied by a low-temperature CVD technology at a temperature below 500^oC. This enables the coating of a wide range of materials: stainless steel, Titanium, Inconel, other steels and alloys stable at 500^oC. The coating has a strong metallurgical adhesion to these substrates, with the bond strength exceeding 70 MPa.

The coating is free from micro-cracks typical of HCP and, as a result, provides much better protection against corrosion.  The coating has enhanced ductility and toughness, protecting against wear and erosion.

Hardide coating has been used on components of the Eurofighter Typhoon jet since 2005, and is being tested by a major civil aircraft manufacturer and several aircraft component producers as an advantageous replacement for HCP. Other applications of Hardide coatings include oil drilling tools, pumps and valves operating in an abrasive, erosive and corrosive environment, where the coating typically triples the life of critical parts.