High temperature parts such as turbine blades, exhaust parts, burner parts and heat shields need to be coated with a surface coating that can withstand prolonged high temperatures without cracking, spalling or allowing substrate material to oxidise.
Choose the Right Spray Material
Yttria-stabilised zirconia (YSZ) coatings are now widely used as thermal barrier coatings on hot-section components and can handle surface temperatures in excess of 1000°C and keep the underlying metal cooler. For short-term exposure to high temperature and where oxidation resistance is only required for short periods of time, MCrAlY type alloys can be very effective. Alumina coatings also offer good heat resistance and in addition provide good electrical insulation.
Use a Bond Coat on Thermally Cycled Parts
This coating must resist thermal shock. It consists of a ceramic top coat applied to metal parts. The ceramic alone would spall on thermal cycling. A thin layer of a MCrAlY alloy, called a bond coat, is applied to the part first. This layer accommodates the difference in thermal expansion of the part and the coating, and a stable oxide layer is formed which the ceramic coating can adhere to.
Account for Thermal Expansion When Specifying Thickness
There is a considerable difference in the thermal expansion of most ceramic coatings and their metal substrates. For thermal barrier coatings, a typical thickness for YSZ thermal barrier coatings could be between 100 and 500 microns (depending on the temperature difference expected during operation). If specified, agreed tolerances must be discussed with the applicator prior to coating, to allow for thermal expansion differential. See also Thermal Sprays.
Understand the Surface Finish You Will Get
Surface finish: a point to consider: The rougher surface produced by thermal spray coatings can be of benefit on heat shields as it enhances their emissivity but on components such as turbine nozzle components where gas flow is of critical importance, it may be necessary to carry out some post coating grinding to smooth out the surface to the required specification as shown on the drawing.
Inspect After Thermal Cycling in Service
Even coatings that pass the initial test will often develop delamination, local cracking or even sudden local spallation after having cycled through several full service temperature cycles. This must be detected and arrested before it spreads and becomes very expensive to repair as substrate damage.
Getting the specification right for thermal spray coatings in demanding service is crucial to get a lasting solution. This starts with choosing the right material and specifying a bond coat where required, and continues with the coating thickness, surface finish and a proper inspection regime.
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