May 29, 2020
TMI Staff & Contributors

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Industrial gas turbines rely on specialized coatings to deliver continued performance and reliability. Renewing these protection systems is an important part of the routine maintenance schedule. Quality is dependent on the expertise of the refurbishment team and attention to detail down to the microscopic level.

Industrial gas turbine coatings require an array of application methods that involve specific processes and equipment. High velocity oxygen fuel (HVOF), plasma, arc wire, combustion, air spray and chemical vapor deposition (CVD) are all used in the refurbishment of components.

Coatings also have slightly varied bonding properties with different substrates. It is essential to understand the conditions required to achieve a perfect bond. Furthermore, the remaining range of properties of the finished coating must be sufficient for the application.

For example, the hardness value is an indicator of the proper application of wear coatings while surface roughness will have a major impact on flow efficiency. By inspecting the microstructure and mechanical properties of the coating, it is possible to verify that it was applied to the required specifications and will provide all of the expected benefits in operation.

In every refurbishment project, the right process foundation is essential to long-term success and durability. This involves detailing the equipment and parameters as well as the coating properties required, such as its tensile strength, microstructure characteristics, hardness and surface roughness.

In many cases, coatings are applied as one of the final stages of a larger repair project. It is important to make sure all prerequisite steps have been taken to ensure the substrate is properly prepared for application. A sound substrate is essential for optimum performance.

Pre-coating checks

Most of the superalloys used in gas turbine components develop oxidation and corrosion while in operation. It is essential that any of these contaminants are removed completely, including any remnants of the previous coating. The presence of any intermediate layer between the substrate and the new coating will likely cause bonding issues.

However, care should be taken when grit blasting or blending, to minimize 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 intermediate layers are removed, further processes may be required. In some cases, the component’s microstructure needs to be prepared in terms of applicable heat treatments. These 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. The thickness of the newly overlaid coating will affect the final dimensions of the component. In many situations, it will be necessary to remove some base material or adjust geometric profiles to facilitate 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.

It is also possible that the part came into contact with contaminants, such as oil, machining fluid and non-destructive evaluation (NDE) penetrant fluid. They must be removed via chemical or thermal means in the degrease process. After that, extreme care must be taken to ensure contaminants are not re-introduced to the substrate as this could jeopardize bonding.

Next, grit profiling roughens the target surface, creating an anchor-tooth pattern for the coating to mechanically bond to. Care should be taken to use virgin grit and not re-used grit to prevent contamination.

Industrial robot arms controlled by positioning software provide consistent application. If done properly, they provide a leap forward in quality control and consistency when compared to manual processes. Once applied, the base coat in some cases requires heat treatment.

Following any heat treatment process, it is essential that an NDE is completed to ensure that no voids opened during the heat treatment process. This will typically be a penetrant inspection using red dye or even fluorescent dye to detect even the slightest defect.

When applicable, a top coat, typically a thermal barrier coating (TBC), is applied in a similar quality-controlled manner as the bond coat. After this application, it is important to carefully remove overspray and polish the coating so it meets the specified surface roughness. The final quality inspection should identify any areas that may need minor repairs and confirm all the required specifications have been met.

Following the coating inspection, test fitting or dimensional checks should be performed to ensure the coating has not pushed the dimensions of the component out of specification. If a third party is being used, they should be involved with this process. For components with cooling channels, any change in flow rate can lead to decreased turbine efficiency, overheating of components and even failure.

Therefore, it is critical that flow checks are performed once more to ensure coating, grit or any other foreign matter has not caused the component’s cooling air to flow below its specified rate. During these post-coating processes and any further handling of coated components, ensure that the coating remains protected and in pristine condition until the component is reinstalled. This is particularly important for brittle TBCs. In all cases the performance of gas turbines is dependent on the proper application of specialized coatings.

By Garrett Haegelin

Garret Haegelin is HICoat Division Superintendent at Sulzer. For more information, visit Sulzer.com.