Many surfaces on aerospace components can be subjected to extreme heat during service. The effects of such high temperatures on an untreated metal surface can be dramatic and very short lived, with the part being destroyed in a matter of hours. A well chosen coating can extend the life of a part subjected to such conditions dramatically. A poor choice can result in a quiet and very expensive failure.
The Temperatures Parts Actually Face
Temperature of aerospace parts in service. For hot-section parts such as turbine blades, the surface temperature can reach as high as 1,000°C. Other parts such as exhaust components, and airframe parts, can experience large temperature cycles, alternating between ambient temperature and high temperatures. Some parts such as airframe fasteners and landing gear components can be at constant high temperatures of around 200°C to 300°C. Each of these temperature ranges requires a specific coating approach.
How Thermal Barrier Coatings Work
There are thermal barrier coatings. These are primarily made of yttria-stabilised zirconia (YSZ) that is applied by plasma spray. These are typically applied on top of a metallic bond coat (which is typically a MCrAlY alloy) to prevent oxidation from occurring between the ceramic surface coating and the metal substrate.
Tungsten Carbide and Ceramic Sprays at High Temperature
Spray coating of wear resistant surfaces such as tungsten carbide on components such as exhaust nozzles, wearing parts in oil lubrication systems, wear plates, scraper blades and other similar parts is typically carried out using high-velocity oxy-fuel (HVOF) spray. The coating provides excellent wear resistance up to 500°C, depending on substrate material. Higher temperature applications, up to 800°C and above where normal metallic coating materials would soften or oxidize are carried out using a range of advanced ceramic materials including chromium oxide.
Where Hard Chrome and Electroless Nickel Fall Short
Hard chrome can provide a good wear life up to around 400oC but then it will start to soften. Similarly, Electroless nickel is used for wear resistance up to around 180oC to 200oC and again then it starts to fail. It is often found that alternative thermal spray coatings can provide a good alternative for high temperature applications.
Aerospace Surface Treatments is covered in more detail at https://www.poeton.co.uk/about/industries/aerospace-defence/.
Oxidation and Coating Adhesion at Elevated Temperatures
As temperature increases, the inter-diffusion between coating and substrate increases which can lead to a decrease in coating to substrate adhesion and an increase in oxidation at the interface between coating and substrate. The coatings selected for use at high temperature must have a coefficient of thermal expansion close to that of the substrate to which they will be attached to prevent spalling. Guidance for safe handling of high-temperature industrial processes should be referenced by operators using these processes to manage risk for handling coated parts.
Getting the coating specification correct prior to processing through thermal treatment, prior to service and through service, can save a lot of expense caused by rework of components already processed through thermal treatment.