SECTION TITLE
COATING TECHNOLOGY
CARBIDE COATINGS
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. Te search for a suitable replacement has been a key supply chain priority for aircraft manufacturers and other users of HCP for several years.
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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. Tey 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. Te 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. Tis 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
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