Hardide Coatings’ technical director, Dr Yuri Zhuk discusses the methods for improving component life in the aerospace sector.
In all industries, problem solving, developments in technology and challenging standard practices are key to improving performance. In extreme environments, or when wear is an issue, the challenge is to maintain high performance whilst ensuring economic viability.
In the aerospace industry, leak-tightness of aircraft hydraulic actuators and rotating shafts depends on seals. In abrasive and corrosive environments, metal seal track or piston rod surface finish degradation can accelerate the seal wear rate by an order of magnitude.
Use of hard wear-resistant coatings can help increase the component life, improve dimensional stability and quality of the component surface finish, and as a result prevent the seal’s premature wear. This can help reduce downtime costs and improve overall competitiveness.
Hardide Coatings, an innovator in advanced surface coating technology, has developed a range of surface engineering solutions to help industry solve problems, improve efficiency and reduce downtime. Using Hardide’s low temperature chemical vapour deposition (CVD) advanced nano-structured tungsten carbide coatings, the lifetime of critical components can be increased dramatically.
Engineering a solution
Hard chrome plating (HCP) has been widely used in the aerospace industry for many years. However, its production process is being banned in September 2017 under EU REACh environmental and health and safety regulations, unless otherwise authorised by the EU Commission, as it uses carcinogenic hexavalent chromium salts in its production. Increasingly tight restrictions are also being imposed in the USA by OSHA.
A number of Hardide coating variants are offered to solve various problems, such as wear, corrosion or galling. Coatings are selected based on the individual application, operating environment and can also be tailored to specific requirements. Hardide-A matches the standard thickness (50-100 microns) and hardness (800-1200 Hv) of HCP, simplifying the transition without the need for dimensional changes or drawing re-design. HCP’s intrinsic performance limitations hinder its more demanding wear applications and Hardide-A outperforms it in several key areas including enhanced protection against corrosion, wear and chemically aggressive media, improved fatigue life and a non-porous structure.
Other alternatives to HCP are available including thermal spray, in particular high-velocity oxy-fuel (HVOF) and emerging processes such as electroless-nickel composite plating, explosive bonding, electro-deposited nanocrystalline cobalt-phosphorus alloys and physical vapour deposition (PVD) coatings. To date, HVOF and other spray coatings have been considered the best available alternative to HCP. Although successful in some applications, each coating has limitations.
Thermal spray coatings can build a very thick and durable layer, but the resultant coatings are rough and porous in structure and often require post-coating grinding which is not possible on intricate shapes. PVD coatings can produce an extremely hard layer with accurately controlled thickness but are very thin, typically less than four microns and have limited load-bearing capacity. However, Hardide-A provides several advantages over HVOF, such as the ability to coat complex geometric shapes and internal bores, improved corrosion and fatigue resistance, a smooth as coated low friction surface and ease of finishing.
Hardide coatings are advanced nano-structured tungsten carbide-based coatings applied by low temperature CVD (chemical vapour deposition). Providing exceptional wear and corrosion resistance combined with toughness and ductility, these patented coatings add value to components and reduce operational costs by saving downtime, increasing productivity and improving performance.
The coating process
Chemical Vapour Deposition coatings are crystallised from the gas phase atom-by-atom, producing a conformal coating which can coat internal and external surfaces and complex shapes. The coatings are a metallic tungsten matrix with dispersed nano-particles of tungsten carbide typically between 1 and 10 nanometres in size. Dispersed tungsten carbide nano-particles give the material enhanced hardness which can be controlled and tailored to give a typical range of hardness of between 800 and 1200Hv and, with some types of Hardide coating, up to 3500Hv. Abrasion resistance is up to 12 times better than hard chrome, 500 times better than Inconel and four times better than HVOF tungsten carbide.
The CVD coating is applied by a batch process and can be polished to Ra 0.2 – 0.3 microns (8-12 micro-inches) or super-finished to Ra 0.02 (0.8 micro-inches) without the need for grinding. This finish does not degrade over time and is a very effective and ‘friendly’ counterface to seals as it protects metal shafts or plungers from scratching and scoring that can result from rotation or reciprocation and which can accelerate the seal wear. Unlike HVOF, the Hardide coating is free from cobalt binder which can be leached from the thermal spray coating in a corrosive environment leaving a rough and abrasive surface. As a result, the CVD coated metal counter-surface against which the seal operates retains a good finish in operation for longer - even in an abrasive or corrosive environment - and is less abrasive for the seal.
When dimensional accuracy is required, Hardide coatings can be diamond ground and super-finished for critical bearing surfaces.
Other typical aerospace applications include pins, bushes, bearings, hooks, catches, landing gear, flap tracks and slats, sleeves, rods, valves, pistons, actuators, compressors, shafts, hydraulic and pneumatic cylinders.
Hardide tungsten carbide coatings have been used on Eurofighter Typhoon jet components since 2005 and were recently technically approved by Airbus as a potential alternative to HCP on some specific Airbus aircraft components. It met the Airbus requirements for thick CVD tungsten carbide coatings and is a suitable alternative for hard chrome plating and HVOF applied coatings.
Benefits of CVD coatings
In industries where equipment and tools are pushed to the extreme of their operating capacity, companies are seeking ways to improve performance while delivering reduction in downtime and meeting environmental regulations.
Challenging environments - such as exposure to chemicals, heat, abrasion, friction and corrosion - puts pressure on the equipment, leading to failure of critical components, downtime and loss of productivity. Hardide coatings range of tungsten carbide CVD coatings provide an effective solution for these problems, ensuring extended life of components and less time spent on maintenance.
Using the latest materials technology, Hardide Coating solutions are a game-changing evolution in performance, not just an improvement of existing coatings. The use of this CVD process opens the door to enable a level of engineering flexibility that is not possible with alternative technologies.