Al-Si-Polyester

For small bores with an Al-Si-Polyester abradable coating on the inside, achieving efficient part cooling without disturbing the spray plume can be challenging.

If the base metal is maintained at elevated temperatures for too long, the polyester in the coating may degrade. While this degradation might not be immediately visible after coating, it can lead to premature failure of the abradable coating during service.

Coating color can serve as an indicator of cooling effectiveness:

A light grey coating suggests the base metal was kept at a low temperature.
Insufficient cooling results in a darker grey, sligthly blueish, coating color.

The whiteness intensity of zirconia TBC varies depending on the cooling conditions during spraying.

ZrO2-8Y2O3

Three test pieces were coated with the same TBC thickness and sprayed under identical conditions, except for variations in cooling intensity.

While the color differences are purely visual, if all test pieces are exposed to temperatures above 500°C for several hours, they will eventually turn a similar white color.
In this case, the color does not indicate microstructural defects in the coating, it is actually related to oxygen vacancies in the zirconia. However, it still serves as a visual indicator of the part temperature during the coating process.

It is common for people to mistakenly assume that a grey TBC coating means the top layer of zirconia is not thick enough.