Thermal barrier coatings: improving thermal protection

Citation

Dorfmann, M. and Dambra, C. (2002), "Thermal barrier coatings: improving thermal protection", Aircraft Engineering and Aerospace Technology, Vol. 74 No. 4. https://doi.org/10.1108/aeat.2002.12774daf.001

Thermal barrier coatings: improving thermal protection

Keywords: Substrate, Bond coat, Topcoat

In the past thirty years, thermal barrier coatings have become essential for increasing the life of industrial gas turbines, aircraft engines and marine diesels. Sulzer Metco is continuously improving material, equipment and manufacturing in order to carry on the success of thermal barrier coatings.

Thermal barrier coatings (TBCs) are being used to insulate gas turbine components that are subjected to excessive temperatures. Since low thermal conductivity allows these coatings to act as thermal barriers, the metal surface temperatures are decreased. Lower metal temperatures give greater component durability by reducing creep stress and fatigue while also reducing oxidation and corrosion rates. Large temperature drops allow engine manufacturers

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  to reduce costs by permitting the use of less exotic materials in the design of engine components and

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  to increase the fuel efficiency of the engine by operating at higher temperatures with reduced cooling flows.

Today, repeatable coating microstructures are being obtained with new powder development concepts, gun designs, controllers and work handling systems. The overall benefits are a wide range of unique TBC microstructures and properties that are then tailored to meet the needs of specific applications in the aerospace and industrial gas turbine industries (Plate 1).

Plate 1 Thermal barrier coatings help to increase the life of industrial gas turbines and aircraft engines. In the figure, a combustion chamber part of an industrial gas turbine sprayed with 1.5 mm thick yttria-stabilized zirconia TBC coating

Plasma-sprayed coatings

Thermal barrier coatings consist of an oxidation-resistant bond coat and a thermal insulating ceramic topcoat, both applied using the plasma spray process. State-of-the- art TBC systems consist of a dense, oxide-free MCrAlY bond coat (M consisting of nickel and/or cobalt) and a porous, finely microcracked 7–8 wt. per cent yttria- stabilized zirconia coating (Plate 2). Previous systems incorporated either NiAl or nickel chromium bond coats along with24 wt. per cent magnesia or 5 wt. per cent calcia-stabilized zirconia ceramic. Materials that fill these requirements include Amdry 995C, 9951 and 9954. Oxides such as calcia, magnesia and yttria act to stabilize the crystallographic structure, preventing large volume changes during heating and cool- down of the coated component. Bond coats are important to the life of TBCs, because elements such as chromium and aluminum form well-adherent oxide scales at high temperatures, allowing for the ceramic to mechanically bond to the MCrAlY coating.

Plate 2 This micrograph shows a MCrAlY bond coat and a yttria-stabilized zirconia topcoat

Ceramic powder characterization

Consistent coating performance and repeatable microstructures cannot be realized without a quality powder. Therefore, it is very important that once an application is established, and the spray parameters are developed, that the powder manufacturers supply powder in a consistent fashion. Customers such as gas turbine engine manufacturers typically write powder specifications which control powder properties. In general, the type of manufacturing method a vendor employs will influence the powder properties.

Ceramic powder properties

Particle size distribution is an important variable that affects coating characteristics. Powder size should be controlled to a specific particle size distribution to assure proper melting in the plasma flame for a given set of spray parameters. If powders are manufactured outside a given specification, the coating quality will start to deteriorate. Typically, powder distributions that are too coarse give low density coating microstructures, while fine particle size distribution gives dense microstructures.

Ceramic manufacturing processes

The type of manufacturing process used by a vendor controls all materials properties. Presently, there are four major manufacturing processes employed:

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  Spray-dried powder

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  Sintered and crushed powder

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  Fused and crushed powder

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  Spray-dried and fused powder

Spray-dried powders that are subsequently fused combine the benefits of prealloying, and the advantages of fused and sintered products together with the free flowing, consistent shapes of spray-dried powders. A good fraction of these powders are also hollow in nature, which assists in the melting of the material. Sulzer Metco's proprietary process for these powders is called HOSP and is an acronym for "hollow oven spherical powder" (Plate 3). Today, these powders, including Metco 204NS, 204NS-1, 204C-NS, 204B-NS, and 204NS-G, are approved to many OEM aircraft and IGT specifications.

Plate 3 This scanning electron microscope image shows the spherical morphology of standard HOSP (hollow oven spherical powder) yttria-stabilized zirconia powder

New equipment designs

Regardless of new material and manufacturing development activities, the performance success of TBCs has increased in recent years with the development of new equipment designs. New equipment technology has helped to control and reproduce the established coating microstructures. The overall result is less down time, reduced application cost, and improved process reliability, with the net result being longer times between repair and overhaul.

Some of the newer equipment designs developed over recent years include:

^.Advanced plasma guns: The Sulzer Metco Triplex II gun (Plate 4) improves process stability, repeatability and the economics of thermal barrier coatings. Unlike a traditional plasma gun, the Triplex II has a more uniform plasma temperature within the plume and lower noise levels. Consumables last significantly longer than with traditional guns, allowing for large parts to be sprayed with minimal down time. Typical nozzle and electrode life is approximately 200 hours, compared to 40 hours of life for standard equipment. Deposition rates are typically higher due to the relatively slow, uniform plasma flame.

Plate 4 The Triplex plasma gun has three cathodes that are supplied by independent power sources. This leads to more uniform plasma temperatures and therefore to better coating results

Another gun introduced for internal diameter spraying with ceramics is the SM-F100 Connex gun. This gun combines maximum productivity for internal and external applications at a recommendation of 20 kW, with a minimum internal diameter of four inches.

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  Closed-loop controller and monitoring of the spray process: Sulzer Metco has sophisticated integrated thermal spray systems such as MultiCoat® Vision and UniCoat™ to monitor and control the process. This PC-based software records the entire data logging history of the spray process for quality control purposes, allows process visualization, has spray parameter storage capability, and permits for closed- loop control of spray parameters such as gases and kW.

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  New powder feeder with closed loop feeding capability: Sulzer Metco has powder feeders that feed materials in a consistent and reproducible manner. The SM 9MP-CL powder feeder is closed-loop controlled for carrier gas and air vibrator pressure with a powder control accuracy of ±1 per cent or ±1 g/m of set point.

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  Advanced work handling equipment and thermal spray robots: The use of robots maintains precise gun spray distances with the correct gun orientation, resulting in optimum spray efficiency and coating microstructure. Application temperatures and gun speeds are controlled. Today, Sulzer Metco has a wide number of multi- axis robots that can be integrated into the customer's application needs.

Better processing methods

Other areas of growth to improve the process are:

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  On-line particle velocity/temperature diagnostics

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  Improved non-destructive evaluation methods

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  Better and/or less expensive pre- and post- processing methods. An example of such a method is Sulzer Metco's Protal, a major innovation in thermal barrier coatings that combines the preparation of the substrate and the thermal spraying in one operation. A high-energy laser pulse removes surface oxygen, improving adhesion of the bond coat to the substrate, and prolonging surface life. The TBCs are therefore cleaner, denser and more repeatable. The Protal process cuts coating time and is environmentally compatible because it eliminates solvent and blasting material disposal.

Robust equipment decreases down time

There is no doubt that TBC materials and systems are growing with the transfer of TBC design ideas from the aerospace market to the industrial gas turbine sector. IGT parts are typically larger and sometimes more complex than aerospace components. Coating thickness in some cases is 1–2 mm and may take 9–12 hours to spray, compared to thinner coatings for aerospace applications. It is for this reason, today's designers and customers require robust equipment and materials that can spray consistently for long periods of time, without any down time.

Sulzer Metco recognizes its customers' needs and is up to the challenge of working to help advance their applications and the technology of the future.

For more details contact: Mitchell Dorfman, Sulzer Metco (US) Inc., 1101 Prospect Avenue, Westbury, NY 11590, USA. Tel: +1 (1)516-338 22 51; Fax: +1 (1)516-338 24 88; E-mail mitch.dorfman@sulzer.com

Mitchell Dorfman and Chris DambraSulzer Metco