detailing the equipment and parameters as well as the properties required for the coating, such as its tensile strength, microstructure characteristics, hardness and surface roughness values. Together with a revision-controlled shop process scope, this information forms the basis of a high- quality application. Qualifying and freezing all influencing parameters of the process for each layer and each component helps ensure the quality and consistency that is provided by the vendor.

PREPARATION In many cases, coatings are applied as one of the final stages of a larger repair project. It is therefore important to first make sure all prerequisite steps have been taken to ensure the substrate is properly prepared for application. Te criticality of preparation for coating is magnified when the repair and coating suppliers are not integrated. Without a mutual understanding of the importance of surface preparation, repair projects can be protracted and offer less than optimum results.

A sound substrate is essential for optimum performance of the protection

systems. Terefore, the component repair process is critical to the quality of the coating. Once the coating is applied, minimal process can be done without removing or damaging the coating.

PRE-COATING CHECKS Most of the superalloys that are used in gas turbine components develop oxidation and corrosion while in operation. It is essential that any of these contaminants are removed completely, including remnants of the previous coating. Te presence of any intermediate layer between the substrate and the new coating will likely cause issues with the bond between the two. However, care should be taken when grit blasting or blending, to minimise any removal of the original substrate. To identify any remaining areas of oxidation or residual coating, components are heat tinted. If contaminants remain, the process repeats until suitable results are achieved. Once any intermediate layers are

removed, further processes may be required. In some cases, the component’s microstructure needs to be prepared in terms of any applicable heat treatments.

Tese processes should be performed prior to application to ensure the coating is not subjected to anything outside of its previously qualified specifications. Similarly, the component may need to be dimensionally altered prior to coating. Te thickness of the newly overlaid coating will affect the final dimensions of the component, so in many situations it will be necessary to remove some base material or adjust geometric profiles to accept the additional thickness. Final pre-coat quality control checks should be completed, including dimensions, flow checks and inspections for defects, using penetrant if necessary. Coatings will only bond properly if there are no gaps or cracks in the substrate; any such flaws will cause rapid deterioration of a new coating.

APPLICATION PROCESS Having removed any debris, determined the part is crack free, dimensionally ready to accept the coating and all other repairs completed, the component is nearly ready for the coating application. Up to this point, the part likely came into contact with various contaminants, such as oil, machining

Extensive preparation is

required to ensure specialised coatings bond properly with substrate materials 19

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