Thermal Spray Materials for Gas Turbine Engines and BeyondMr. Bill Jarosinski
Parxair Inc., Speedway, IN
Three major components of the thermal spray process are consumables, equipment, and service. These are highly interactive and depend on each other. The same material (consumable) sprayed with different equipment (even with identical parameters) may result in different service. Likewise different consumables (even though specified the same) sprayed with the same equipment may result in different service.
Today's gas turbine engines can utilize as many as 200 - 300 different coatings within the assembly. Operating temperatures sometimes exceed the melting point of substrate materials, making the coating critical against catastrophic failure.
The consumables for coatings come from three major material groups: carbides, metal alloys, and ceramics. Carbides as used in wear critical areas, metal alloys for various forms of corrosion, and ceramics as thermal barrier coatings.
Thermal spray materials start as powder. An ideal powder (from a control and reproducibility standpoint) would be a mono-sized, predictably shaped particle. Unfortunately, powders cannot be produced this way. Generally speaking the closer to mono-sized a powder becomes, the more expensive it is. Powder particles all follow a size distribution curve, and the more selective you become, the more off-fall powder exists thus higher cost. Screening and air classification are the main ways powder particle size is controlled.
There are several methods of powder manufacture. By what used to be the order of increasing cost these are: "Cast and crush", "sintering & crush", and "agglomerated particle atomization". Because the first two methods have become much lower volume processes, the three now tend to have similar costs because of economy of scale and the popularity of the agglomerated particle atomization method.
Carbide coatings applied with a HVOF/D-gun is used in gas turbine blades for wear resistance where blade components tend to rub during start up and run down of an engine.
Metal Alloys is a very broad category of coating materials. It includes alloys of Nickel, Cobalt, Iron, Aluminum, and Copper, superalloys such as Inconels, Rene, Hastalloys, Waspalloy, blends, and other specialty alloys. A class of metal alloys commonly used in hot areas of gas turbines is "MCrAlY". Here the "M" stands for any of the metals Nickel, Cobalt, Iron, or alloys of these; "Cr" is chromium, "Al" is aluminum, and "Y" is Yttrium. The major (metal alloy) component imparts high temperature strength. The Chromium is primarily a provider of corrosion resistance (heat corrosion). The Aluminum (usually around 8-12% of the coating) offers its thin tenacious oxide as a barrier against further oxidation. Finally the Yttrium gives the coating an ability to adhere well. The use of such coatings in hot areas of a gas turbine is usually referred to as the "bond coat".
Because of demand for ever increasing operating temperatures in gas turbines it is now required to add a ceramic "top coat" in areas where temperatures approach or exceed the melting point of the engine components. This is usually referred to as the TBC (Thermal Barrier Coating). Usually the top coat is something like Yttria Stabilized Zirconia applied by spray dry technology. Because Zirconia naturally undergoes a phase transformation during heating (with large volumetric change) it is necessary to stabilize the high temperature phase which is accomplished by adding Yttria. Once stabilized, the coating expands in a well behaved manner when heated. Thermal barrier coatings often have considerable porosity which is a good thing because of the insulating effected of dead air.
Blending is a methods of producing a heterogeneous coating which is desirable in some applications.
As for the "beyond", we could consider the future of Thermal Spray, or the future of its application to Gas Turbines. In the area of "beyond thermal spray" there appears to be a future for such processes as Electron Beam Plasma Vapor Deposition, Tribomet (a type of plating process), Pt-aluminize processing, wire spray, and laser cladding. In applications beyond Gas Turbines, we see increasing use for thermal spray on wear tracks, actuators, and landing gear in the aviation field. There is also a government pushed trend to replace chrome plating with thermal spray wherever possible because of the environmental problems associated with the hexavalent chrome byproducts of chrome plating.
Currently development efforts in Thermal Spray materials and technology focus on doing existing things at lower cost rather than developing entirely new materials and processes.