The present invention relates to processes for depositing protective coatings. More particularly, this invention relates to a process for forming a diffusion aluminide bond coat of a thermal barrier coating system, such as of the type used to protect gas turbine engine components.
The operating environment within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature alloys have been achieved through the formulation of iron, nickel and cobalt-base superalloys, though components formed from such alloys often cannot withstand long service exposures if located in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor. A common solution is to provide turbine, combustor and augmentor components with an environmental coating that inhibits oxidation and hot corrosion, or a thermal barrier coating (TBC) system that, in addition to inhibiting oxidation and hot corrosion, also thermally insulates the component surface from its operating environment.
Coating materials that have found wide use as environmental coatings include diffusion aluminide coatings, which are generally single-layer oxidation-resistant layers formed by a diffusion process, such as pack cementation. Diffusion processes generally entail reacting the surface of a component with an aluminum-containing gas composition to form two distinct zones, the outermost of which is an additive layer containing an environmentally-resistant intermetallic represented by MAl, where M is iron, nickel or cobalt, depending on the substrate material. Beneath the additive layer is a diffusion zone comprising various intermetallic and metastable phases that form during the coating reaction as a result of diffusional gradients and changes in elemental solubility in the local region of the substrate. During high temperature exposure in air, the MAl intermetallic forms a protective aluminum oxide (alumina) scale or layer that inhibits oxidation of the diffusion coating and the underlying substrate.
For particularly high temperature applications, a thermal barrier coating (TBC) can be deposited on a diffusion coating, then termed a bond coat, to form a thermal barrier coating system. Various ceramic materials have been employed as the TBC, particularly zirconia (ZrO2) fully or partially stabilized by yttria (Y2O3), magnesia (MgO), ceria (CeO2), scandia (Sc2O3), or other oxides. These particular materials are widely employed in the art because they exhibit desirable thermal cycle fatigue properties, and also because they can be readily deposited by plasma spray, flame spray and vapor deposition techniques.
A bond coat is critical to the service life of the thermal barrier coating system in which it is employed, and is therefore also critical to the service life of the component protected by the coating system. The oxide scale formed by a diffusion aluminide bond coat is adherent and continuous, and therefore not only protects the bond coat and its underlying superalloy substrate by serving as an oxidation barrier, but also chemically bonds the ceramic layer. Nonetheless, aluminide bond coats inherently continue to oxidize over time at elevated temperatures, which gradually depletes aluminum from the bond coat and increases the thickness of the oxide scale. Eventually, the scale reaches a critical thickness that leads to spallation of the ceramic layer at the interface between the bond coat and the aluminum oxide scale. Once spallation has occurred, the component will deteriorate rapidly, and therefore must be refurbished or scrapped at considerable cost.
Improved TBC life has been achieved with the addition of platinum group metals in diffusion aluminide bond coats. Typically, platinum or palladium is introduced by plating the substrate prior to the diffusion aluminizing process, such that upon aluminizing the additive layer includes PtAl intermetallic phases, usually PtAl2 or platinum in solution in the MAl phase. The presence of a platinum group metal is believed to inhibit the diffusion of refractory metals into the oxide scale surface, where they would otherwise form phases containing little aluminum and therefore would oxidize rapidly. It would be desirable if the oxide scale growth rate of an aluminide bond coat could be further reduced to yield a thermal barrier coating system, and therefore the component protected by the coating system, that exhibits improved service life.
The present invention generally provides a thermal barrier coating system and a method for forming the coating system on a component designed for use in a hostile thermal environment, such as superalloy turbine, combustor and augmentor components of a gas turbine engine. The method is particularly directed to a thermal barrier coating system that includes an oxidation-resistant diffusion aluminide bond coat on which an aluminum oxide scale is grown to protect the underlying surface of the component and adhere an overlying thermal-insulating ceramic layer.
According to this invention, the oxide growth rate on the diffusion aluminide bond coat can be significantly reduced to improve spallation resistance for the ceramic layer by forming the bond coat to include a dispersion of aluminum, chromium, nickel, cobalt and/or platinum group metal oxides. The oxides preferably constitute about five to about twenty volume percent of the bond coat, with a preferred level being about seven to about fifteen volume percent oxides. While applicable to any diffusion aluminide bond coat, a preferred bond coat is a platinum aluminide. The bond coat may optionally overlie or underlie a layer formed of one or more of the same oxides as for the oxide dispersion, e.g., aluminum, chromium, nickel, cobalt and platinum group metal oxides.
According to the invention, a preferred method for forming the bond coat is to initiate a diffusion aluminizing process in the absence of oxygen to deposit a base layer of diffusion aluminide, and then intermittently introduce an oxygen-containing gas into the diffusion aluminizing process to form within the bond coat the desired dispersion of oxides. Thereafter, a ceramic layer is deposited on the bond coat to form a thermal barrier coating.
According to this invention, the process described above yields finely distributed primary and complex (i.e., compound) oxides of aluminum, nickel, chromium and, if present, platinum group metals, yielding a bond coat that exhibits enhanced cyclic oxidation resistance and a reduced oxide growth rate. The result is a thermal barrier coating system that can exhibit an improved thermal cycle fatigue life of three-times lordlier than an otherwise identical coating system without the fine oxide dispersion in the bond coat.
Other objects and advantages of this invention will be better appreciated from the following detailed description.