In industries such as aerospace, characterized by extreme operating environments that push materials and components to the edge of their design capabilities, companies are challenged to continually seek economically viable ways to maintain and improve performance.
Corrosion, galling, fretting and wear are of particular importance: the 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. Plus, the high vibrations experienced in helicopter rotor parts including extension sleeves, fold pins and pistons, as well as in fixed wing components such as flight control actuation systems, puts considerable pressure on these components.
The use of hard, wear-resistant coatings can help increase component life, improve dimensional stability and component quality, while reducing downtime costs and enhancing competitiveness.
Hard Chrome Plating
Hard Chrome Plating (HCP), where an electric current deposits a thin layer of chromium on to metal, has been widely used in the aerospace industry to protect steel components against wear, corrosion and galling. However, because its production process uses carcinogenic hexavalent chromium salts, its use has been banned in Europe under regulations that came into force in September 2017.
Additionally, the Occupational Safety and Health Administration (OSHA) is imposing increasingly tight restrictions in the USA. As such, and for some time now, companies using hard chrome-plated components have been migrating away from the technology.
Analyzing the HCP alternatives
HCP alternatives include thermal spray - in particular HVOF - and emerging processes such as electroless-nickel composite plating, explosive bonding, electro-deposited nanocrystalline cobalt-phosphorus alloys, and physical vapor deposition (PVD) coatings. To date, HVOF and other spray coatings have been considered the best available alternative to HCP. Yet, although successful in some applications, each coating has limitations.
Thermal spray coatings can build a very thick and durable layer, but cannot be applied to internal surfaces. They are rough and porous, and often require post-coat 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. Various wet electroplating and electroless coatings are more suitable for internal surfaces and complex shaped parts, but have lower hardness than HCP and thus inferior wear-resistance. Metal plating has other limitations - in some cases it provides insufficient corrosion protection due to the plating porosity, and could have insufficient adhesion to steel substrates.
Engineering a Solution
Extensive testing has proven that low-temperature CVD coatings can be tough and provide a practical, technically and commercially-viable solution, capable of dramatically increasing aircraft component life.
The Hardide CVD coating belongs to a novel family of nano-structured Tungsten/Tungsten Carbide coatings. It is crystallized atom-by-atom from low-pressure gas media in a vacuum reactor — building a dense layer with the Tungsten and Tungsten Carbide constituents bonded together — enabling the uniform, pore-free coating of internal surfaces and complex shapes, in contrast with traditional line-of-sight technologies.
The Hardide-A variant was developed as a replacement for HCP, especially for designs and geometries where spray coatings cannot be used. Facilitating straightforward switching from HCP with only minimal part design changes pre-coating, this CVD coating matches HCP in thickness (50 to 100 microns) and hardness (800 to 1200 Hv), with the upper limit exceeding HCP's maximum hardness. Hardide A has a low friction, smooth ‘as coated’ surface, and for tighter dimensional accuracy it can be diamond ground, honed and superfinished.
The nanostructured CVD coating combines high hardness with enhanced toughness and ductility, increasing wear and erosion resistance and its ability to survive impact and part deformation.
Furthermore, CVD outperforms HCP and HVOF in several other important characteristics, in particular as a barrier against corrosion, as well as enhanced fatigue resistance.
Corrosion and Galling
Other traditionally-used coatings such as HCP, sprayed coatings and electroplating have micro-pores and microcracks that widen when the substrate deforms under load, allowing media to attack the substrate.
Sealing can improve the corrosion protective performance, but there are several limitations, including in some cases, restricting the temperature to which the coatings can be exposed to below 400°F. Plus, as the coating wears, deeper, previously concealed pores that are not sealed will eventually open.
On the other hand, due to its deposition mechanism, CVD tungsten carbide coatings effectively have zero porosity and do not need sealing.
Both Tungsten and Tungsten Carbide have high chemical resistance. Unlike sprayed HVOF coatings, CVD does not use cobalt, which can be affected by acids. As a result, the porefree CVD Tungsten Carbide coating is resistant to many aggressive chemicals and can be used as a very effective anti-corrosion barrier for critical parts.
Testing the corrosion protective properties of the CVD coating involved subjecting mild steel plates — coated with HCP, HVOF, and CVD coatings — to 480-hour neutral salt spray tests. The HCP samples were badly corroded and were consequently removed from test after just 288 hours. HVOF-coated samples showed heavy rust stains and the coating blistered. The CVD samples showed only light staining.
The CVD Tungsten Carbide coating's galling resistance was tested using a Phoenix TE77 high-frequency reciprocating test rig. In comparison with a baseline control test using a stainless steel pin against a stainless steel plate, a CVD-coated pin was tested against a coated plate. The baseline test was stopped due to the sample seizure quickly reaching critical 1.0 coefficient of friction (CoF) with just a 65N load — indicating severe galling.
With the CVD-coated pin, the dry friction coefficient remained low, stablilizing at around 0.2. Importantly, no galling was observed even under the test rig's maximum load of 800 N, equivalent to contact pressure of 810.2 MPa. The steady state dry friction coefficient of HCP was reported at 0.70±0.1 * , more than three times higher, with spray coatings’ dry friction range stated as 0.56 to 0.61.
Hardness and Wear Resistance
The CVD material's hardness has a big effect on its wear-resistance, and G65 wear tests — assessing material loss after 6,000 counter body rotation cycles — showed Hardide-A outperforms HCP by a factor of 13, and various grades of HVOF by up to three times. An additional advantage of CVD Tungsten Carbide coatings is their non-abrasive quality for seals, bearings and other counter-body parts. Their uniform nanostructure means the coatings wear uniformly and retain — even improve — surface finish even in abrasive or corrosive environments. This is important for use on hydraulic actuators, rotating shafts and bearings, as retaining a good surface reduces elastomeric and PTFE seal wear.
HVOF and other thermal spray coatings can suffer from selective wear or leaching of the cobalt binder leaving hard Tungsten Carbide (WC) grains as sharp asperities, which are highly abrasive for seals.
Toughness, Resistance to Impact and Deformations
Toughness, resistance to impact and deformations without spalling or cracking are properties of significant practical importance for applications such as aircraft landing gear, or wing flap actuators. HCP in general has satisfactory toughness; however, it has a network of micro-cracks. Brittleness and poor impact resistance are among its main drawbacks. HVOF coatings, too, are known to crack and spall under high load and high cyclic fatigue conditions.
In contrast, the CVD coating's structure and composition is optimized to maximize toughness. The coating did not exhibit brittle behavior in micro or nano-indenter tests, thus its fracture toughness exceeded the level that can be measured using methods commonly used on hard coatings.
The CVD coating material's enhanced toughness and ductility, together with its uniform, pore-free micro structure and compressive residual stresses, give it enhanced fatigue performance. In tests, it showed minimum fatigue debit ranging from +10% to -10%, compared with HCP's typical fatigue debit of -20%. HVOF has an even higher debit of -60% due to tensile stresses and porous multiphase structure where inclusions and defects can become stress concentrators and crack initiation sites.
Real World Capability
Airbus Group approved the CVD coating in March 2017. Because of its anti-galling properties, the coating is also used by BAE Systems on parts for the Typhoon fighter jet, and test plans are progressing with European helicopter manufacturer Leonardo for safety-critical main rotor components and transmission components.
The CVD coatings offer a unique combination of protective properties, including wear and erosion resistance, protection against aggressive chemicals and corrosion, as well as toughness, impact and crack resistance. This adds value to components, tools and equipment; reduces operational costs; saves downtime; and increases productivity.
This article was written by Dr. Yuri Zhuk, Technical Director, Hardide Coatings (Oxfordshire, UK). For more information, visit here .