Thermal barrier coatings (TBCs) are often used to improve the efficiency and performance of metal parts which are exposed to high temperatures. Aircraft engines and land-based turbines are made from such parts. The combustion gas temperatures present in turbines are maintained as high as possible for operating efficiency. Turbine blades and other elements of the engine are usually made of alloys which can resist the high temperature environment, e.g., superalloys, which have an operating temperature limit of about 1000.degree. C.-1150.degree. C. Operation above these temperatures may cause the various turbine elements to fail and damage the engine.
The thermal barrier coatings effectively increase the operating temperature of the turbine by maintaining or reducing the surface temperature of the alloys used to form the various engine components. Most thermal barrier coatings are ceramic-based, e.g., based on a material like zirconia (zirconium oxide), which is usually chemically stabilized with another material such as yttria. For a turbine, the coatings are applied to various surfaces, such as turbine blades and vanes, combustor liners, and combustor nozzles. Usually, the thermal barrier coating ceramics are applied to an intervening bond layer which has been applied directly to the surface of the metal part.
The thermal barrier coatings are often applied to the part by a thermal spray technique, such as a plasma spray process. In this technique, an electric arc is typically used to heat various gasses, such as air, oxygen, nitrogen, argon, helium, or hydrogen, to temperatures of about 8000.degree. C. or greater. (When the process is carried out in an air environment, it is often referred to as air plasma spray or "APS".) The gasses are expelled from an annulus at high velocity, creating a characteristic thermal plume. Powder material (e.g., the zirconia-based composition) is fed into the plume, and the melted particles are accelerated toward the substrate being coated. For some applications, plasma-spray techniques have numerous advantages over other coating techniques, such as electron beam physical vapor deposition (EB-PVD). As an example, plasma spray systems are usually less costly than EB-PVD. Moreover, they are well suited for coating large parts, with maximum control over the thickness and uniformity of the coatings.
Despite the advantages associated with plasma-sprayed thermal barrier coatings, the use of these processes can present some problems under various circumstances. For example, a plasma-sprayed coating often has a relatively rough surface, e.g., an "R.sub.a " or Ra (arithmetic roughness average) value greater than about 600 micro-inches. Much smoother surfaces are required when the coating is to be applied to turbine components like airfoils, so that the convective component of the heat flux delivered to the coating can be reduced. Moreover, the aerodynamic drag losses can also be reduced.
The thermal barrier coating surface can be smoothed by several techniques, such as grinding, tumbling, or heavy-sanding operations. However, these processes can be very time-consuming, adding considerably to the overall cost of fabrication. Moreover, they can sometimes mechanically damage the thermal barrier coating. For example, a sand-tumbling operation can sometimes result in the preferential smoothing/wearing of certain areas of the coating. The decreased thickness in those areas can undesirably lower the thermal resistance of the thermal barrier coating. Grinding, on the other hand, can induce stresses in the coating, thereby reducing its service life.
Surface-smoothing processes for thermal barrier coatings have been practiced in the art. For example, H. L. Tsai et al describe the use of a continuous wave laser to glaze the surface layer of a plasma-sprayed coating based on yttria-stabilized zirconia (Materials Science and Engineering, A161 (1993), 145-155). The process is said to be capable of producing shiny surfaces of low roughness. However, a laser system can be a considerable capital investment, adding to the cost and complexity of the overall thermal barrier coating process. Moreover, it may sometimes be quite difficult to adjust the wavelength of the laser to melt the most appropriate surface-portion of the thermal barrier coating, i.e., a layer thick enough to form a smooth surface, but thin enough to preserve the overall integrity of the protective coating.
From this discussion, it should be apparent that new methods for modifying the surface of a thermal barrier coating would be welcome in the art. The new processes should smooth the surface to a degree suitable for aerodynamic applications, while maintaining all of the beneficial characteristics of the coating. Moreover, the processes should be fully compatible with the application of the thermal barrier coating over a substrate, and should not add excessive cost or time to the overall production operation.