22 Aug Surface Treatment Options for High-Temperature Parts

When parts are to be used in applications where parts will be subject to sustained heat then the coating must be given as much consideration as the substrate material. Not all coatings can withstand high temperatures and in some cases can actually cause more damage than intended.

Thermal Spray Ceramic Coatings

These are the thermal spray ceramic coatings (Alumina and Zirconia) which are sprayed using a plasma gun. These coatings are good for high temperature applications, such as parts in gas turbines, where gas temperatures can exceed 1,000°C. As the thermal conductivity of these coatings is low, they work by insulating the substrate.

High-Phosphorus Electroless Nickel

High-phosphorus electroless nickel coatings will offer good oxidation resistance up to approximately 350°C, after which the coating will begin to crystallise, losing its amorphous structure. Below 350°C a high-phosphorus electroless nickel coating is a good material to use, where there is wear involved and/or mild chemical attack. However, above 350°C the coating will begin to become brittle.

Hard Anodising on Aluminium

Aluminium does not make a good base for high-temperature parts as the anodic layer starts to soften and degrade above temperatures around 130 to 150 °C and the aluminium itself starts to lose strength. While anodic layers provide excellent wear resistance in mildly chemical aggressive environments, they are not suitable for high-temperature use as the primary method of protection.

MCrAlY Alloy Coatings

Metallic coatings of MCrAlY (where M=Co or Ni) have been used for many years to protect surfaces from the effects of oxidation in both static and thermal cycled conditions. When surfaces are heated up, they can form a stable surface layer of alumina by diffusion of aluminium out of the coating. This can form a very effective diffusion barrier to the substrate beneath and is frequently used as a bond coat to support a top coat of ceramic for thermal barrier applications for turbine components etc.

Substrate and Expansion Mismatch

The use of surface treatments to components that heat-cycle needs to consider the difference in thermal expansion between coating and substrate. A surface treatment that is suitable for bonding at ambient temperature will, in some cases, crack or spall as the coating and substrate expand and contract at different rates. If you are looking for Surface Treatments, see www.poeton.co.uk/surface-treatments.

So long as the correct coating is used from the outset, the resulting amount of rework when the part is in service is minimal.