Dr Yuri Zhuk explores CVD tungsten/tungsten carbide as a replacement for hard chrome and thermal spray coatings.
A tungsten/tungsten carbide metal matrix coating developed in the UK is being adopted by Airbus and other global users of critical aerospace, energy and flow control components as a replacement for hard chrome plating (HCP) and the thermal spray coating, HVOF
EU and UK REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) regulations will ban the use of the carcinogenic hexavalent chromium salts used in HCP production by April 2024 due to health and environmental risks. The search for a suitable replacement has been a key supply chain priority for aircraft manufacturers and other users of HCP for several years.
HVOF is one of the most widely used thermal spray coatings and is often considered as an alternative to HCP. However, it has several limitations; thermal spray coatings can build a thick (up to several mm) layer on external surfaces but cannot be applied on out-of-sight surfaces or on complex geometries. They do not form a strong chemical or metallurgical bond to the coated part and depend on the substrate’s roughness to form the key for its mechanical adhesion. As a result, the deposited coating is very rough and, as with HCP, requires grinding to achieve an acceptable finish.
Hardide Coatings, a UK-based developer of advanced surface coatings, has developed a family of nanostructured, low temperature chemical vapour deposition (CVD) tungsten/tungsten carbide metal matrix coatings designed as alternatives to HCP and HVOF outperforming both in many critical properties.
The CVD process means the Hardide CVD coating can reach internal as well as external surfaces, and coat complex geometries unlike HCP and HVOF. HCP and HVOF have to be ground after coating to achieve tolerances and a uniform surface finish. This adds cost and production time, and restricts the shape of the parts to be coated to simple flat or cylindrical faces. Hardide CVD coatings as applied typically have a surface finish ~0.5 - 0.6 microns Ra, which can be polished to 0.2 - 0.3 microns Ra without the need for grinding.
Hardide-A was developed specifically to meet the needs of the aerospace industry and matches the thickness (50 – 100 microns) and hardness (800 – 1200 Hv) of HCP which simplifies the switchover by eliminating the need to change pre-coating drawings and dimensions.
Due to Hardide-A’s low thermal expansion the coating as deposited is normally under compressive residual stress and is free from the micro-cracks typical of HCP. As a result, the CVD tungsten carbide coating is an excellent barrier against corrosion, outperforming HCP in this important characteristic. The coating has enhanced ductility and toughness, minimum fatigue debit – much better high cycle fatigue performance than HCP or HVOF, and provides enhanced protection against galling.
After a lengthy and extensive test programme, Hardide Coatings, was selected by Airbus to provide its Hardide-A coating as a replacement for HCP and is now coating wing components for the Airbus A320, A330, A380 and A400M aircraft. One of the parts made from thin steel plate had HVOF coating specified, but in production most parts buckled after HVOF coating when the thermal spray layer cooled. The Hardide CVD coating proved to be a better alternative with no distortion of the parts. A European helicopter manufacturer approved the Hardide CVD coating on a gearbox shaft replacing HVOF coating, as the smooth Hardide coating was shown to be far less abrasive for seals, reducing the risk of oil leakage and cutting maintenance. A major US-based producer of positive displacement pumps is increasingly using the CVD coating to replace HCP on both pistons and cylinders and has reported 3x increase in pump life when handling abrasive fluids.
Every alternative to hard chrome and HVOF has different characteristics and none will be an exact replacement, so evaluation of the application and its performance requirements is essential to determine the best solution. That approach followed up by extensive testing and evidence-based evaluation to ensure that performance criteria are met, will identify the most suitable alternative for hard chrome and HVOF replacement on a component-by-component basis.
Dr Yuri Zhuk is the Technical Director at Hardide Coatings