Coating Selection Critical to Turbine Performance
By Hans van Esch and Joseph DeBarro, Hickham Industries Inc.
All Gas Turbine Users are Aware That Identical Gas Turbines Can Perform Differently Due to
differences in altitude, ambient temperature, fuel type, load conditions etc. The environment inside the turbine can also play a role, primarily through its impact on internal components and their protective coatings. When a coating fails prematurely, turbine performance can degrade significantly.
Thermal barrier coatings (TBCs) are applied to protect gas turbine components against oxidation/corrosion and excess temperatures. A 10-mil (0.010 inches) thick TBC, for example, typically results in a 100 to 300 F temperature reduction capability to the coated component, limiting its susceptibility to thermal stresses.
There are many properties–such as chemical composition, cleanliness of the interface, porosity, unmelted particle levels, average coating thickness, bond strength, thickness distribution of the coating, surface finish, equipment, application procedures and quality control methods–that determine the quality, and thus affect the performance, of a TBC. Care must be exercised, therefore, in selecting the correct coating process and supplier.
When the base material is not correctly stripped and/or cleaned, any applied coating will have adhesion problems. The old coating can be chemically removed or mechanically stripped through grit cleaning or blending. Base material that demonstrates corrosion or oxidation needs to be cleaned as well. Any remaining corrosion products or other contaminants can attack the coating and/or components during service.
A heat tint should be performed to ensure that all coating and corroded material is removed. Heat tinting detects differences in surface composition by heating the part in air to produce thin oxide scales of varying colors. A clean surface will look evenly dark blue to black, coating will show up as gold, and any other discoloration may indicate contamination. A customer witness is advisable unless the supplier has an established quality control program.
Within a few hours before a coating process actually begins, a thorough cleaning should be performed for both refurbished and new parts. A proper degreasing step that removes all grease, oil and other organic products from the component surface is mandatory; wiping or dipping with a solvent is not sufficient. After degreasing, the parts must be handled and stored with care. The area between the cleaning process and the spraying process should be spotless, and operators should take precautions to avoid contamination.
Grit cleaning, typically using aluminum oxide, is performed to remove any solid contamination as well as to roughen the surface to ensure good coating adhesion. Only new grit and dedicated grit cleaning equipment can ensure a proper interface and avoid adhesion problems. The masking process should be simple but sufficient, providing protection where needed but with minimal handling. A complex or insufficient masking process can cause adhesion problems at the interface and at the edges of the coating.
Air Plasma Sprayed TBC
In large industrial gas turbines, the air plasma spray (APS) method is primarily used for TBC application. An APS MCrAlY bond coat and a 6 to 8 percent yttrium-stabilized zirconia topcoat is considered a standard TBC. The yttrium prevents a phase change in the zirconia at approximately 1,830 F. If the zirconia changes phase, the thermal expansion difference between the TBC and the base material will increase, thereby increasing the risk of coating delamination.
In addition to chemistry, there are other criteria that impact TBC performance. Application procedures, for example, can alter coating properties. Coating powder suppliers such as Sulzer Metco will give starting point parameters for applying a particular coating. Nozzle parameters, delivery gas pressure and flow, arc current and voltage, spray rate and spray distance are some of the variables that can modify coating characteristics. Because every application is different, an intensive program utilizing metallurgical evaluation and engineering support is required.
Coating thickness can have an enormous influence on coating performance. A thin coating deposit will not provide adequate thermal insulation, while an excessively thick TBC may have poor thermal shock resistance. Although a coating thickness between 8 and 10 mils is optimum, thicknesses between 6 and 12 mils can be accepted in many applications.
Since many coating characteristics can not be evaluated on every part, reproducibility is a must. The keys to reproducibility are closed loop thermal spray controllers and robotic manipulation of parts and spray equipment. A process that depends on the operator to manually control spray equipment settings and spray gun path will have difficulty controlling repeatability.
New TBC Developments
For critical components such as blades and vanes, vacuum plasma and low pressure plasma spraying (LPPS) have been used for applying the bond coat. The bond coat is applied to improve adhesion of the TBC to the base metal as well as to increase the oxidation and corrosion resistance of the coating. The development of high-pressure/high-velocity oxygen-fueled spray equipment, such as TAFA Inc.`s JP 5000 and Sulzer Metco`s DJ2600 hybrid, allow thermal spraying in air to achieve TBC quality similar to that of LPPS. A Praxair-patented plating process (electroplating within a solution) also has been developed that can apply bond coats with the same properties as LPPS.
In aircraft gas turbines, the electron beam-physical vapor deposition (EB-PVD) process is used for applying TBC coatings. EB-PVD TBCs are columnar in structure, allowing the TBC to compensate for thermal expansion differences between it and the base material. Also, EB-PVD TBCs have a very smooth surface, which may lead to an increase in the gas turbine`s efficiency.
Although routinely applied in aircraft turbines, EB-PVD TBCs are not yet cost-effective for large land-based gas turbine components. There are lower-cost coatings being developed, however, that copy the EB-PVD structure by spraying in such a way as to produce controlled, vertical cracking, thereby avoiding TBC delamination while in service. There are also new developments to reduce surface roughness. Suppliers are using post-treatments such as polishing and tumbling to reduce surface roughness. Finer TBC powders and redefined settings can reduce roughness from 0.5 mils to 0.1 mils.
Maintaining coating quality requires independent inspectors, thermal spray and metallurgical engineering support, and a well equipped laboratory. ISO 9001 also requires a design and process control program to document and qualify process changes. Changes of a proven application should not be performed on components without extensive tests and engineering reviews.
To ensure that a given part is service able, non-destructive testing and dimensional checking (thic kness measurements, thickness uniformity and roughness) should be performed. For critical parts, destructive metallurgical evaluation and even life assessment may be necessary to guard against premature failure. It is important that the samples taken represent the coating that will be sprayed on actual components. The optimum solution is that one component per batch be chosen at random and sacrificed for evaluation. When this is not possible, samples sprayed during production should be mounted on or next to components. Placement is important since coating a flat sample is significantly different than coating complex geometries.
Metallurgical evaluation reveals unmelted particles, porosity, oxidation, other contamination and interface problems. Sample preparation is critical for consistent and reliable evaluation. Although there is not an industry standard available, the supplier should have a proven and documented procedure for sample evaluation and also should have automated grinding and polishing equipment in place. An image analyzer for determining the porosity and oxidation level of coatings will improve the reliability of the metallurgical evaluation. An experienced metallurgical engineer is required to develop and maintain this program, as well as to offer evaluations and recommendations.
To demonstrate TBC adhesion to the base material, a bond strength test is required. Test samples need to be of the same material as the component and should to be sprayed with that part if possible. Bond strength should exceed 2,500 psi.
A checklist can be helpful in selecting a TBC supplier. To use the checklist in the accompanying table, score one point for each “C” answer, three points for each “B” answer and five points for each “A” answer, and then total the results. Do not consider suppliers that score less than 60 points. If a supplier scores between 60 and 80 points, only use for less critical applications. A supplier should score at least 80 points to qualify for applying TBC on critical parts such as blades and vanes. p
Robotic manipulation and spray equipment improve the reproducibility of coating application.
Bond strength should exceed 2,500 psi for acceptable coating performance.
A coating quality control program should include dimensional checking of thickness, uniformity and roughness.
Hans van Esch is an operations manager for Hickham Industries, responsible for both the Component and HICoat Divisions. He has held several other positions with Elbar, Hickham Elbar and Chromalloy Holland. Van Esch holds a bachelor`s degree in chemical engineering from Enschede, Netherlands and a bachelor`s degreein business administration from Eindhoven, Netherlands.
Joseph DeBarro is the coatings engineer and HICoat supervisor for Hickham Industries. He has six years` previous experience at Sulzer Metco as a materials and applications engineer. DeBarro received a B.gif. in engineering science from SUNY Stony Brook.