The application of a coating to a substrate of a prescribed geometry is required in a variety of engineering applications, including thermal or environmental protection of the substrate, improved wear resistance, altered optical properties or creation of devices on the substrate such as thin film batteries. In all such cases, the ability to deposit compositionally controlled coatings efficiently, uniformly, at a high rate, with high part throughput, and in a cost-effective manner is desired. Some illustrative examples of deposition systems are provided in the following applications and patents and are co-assigned to the present assignee 1) U.S. Pat. No. 5,534,314, filed Aug. 31, 1994, entitled “Directed Vapor Deposition of Electron Beam Evaporant”, 2) U.S. application Ser. No. 09/634,457, filed Aug. 7, 2000, entitled “Apparatus and Method for Intra-layer Modulation of the Material Deposition and Assist Beam and the Multilayer Structure Produced Therefrom”, and 3) PCT International Application No. PCT/US01/16693, filed May 23, 2001, entitled “Process and Apparatus for the Plasma Activated Deposition in a Vacuum”. These applications are hereby incorporated by reference herein in their entirety. The present invention discloses, among other things an apparatus and a method for applying a coating(s) on a substrate(s) in an improved and more efficient manner. The present invention method can be applied to coatings of any type onto any substrate geometry. In this document, the application of a thermal barrier coating to a turbine blade is used as a non-limiting example.
Vapor phase processes are widely used for applying thermal and environmental protection coating systems to components. They are widely used to protect the hot structural components of many gas turbine engines that must operate at temperatures approaching their melting point [1]. As gas inlet temperatures continue to rise, failure by thermally-induced mechanisms has been avoided by making airfoil components with internal cooling conduits, and injecting compressor discharge air to decrease the component temperature [2]. To maximize engine efficiency, however, it is desirable to minimize the use of this air for cooling purposes. Traditionally, this has been accomplished by designing more efficient cooling geometries within the component and by film cooling of the component surface using drilled holes. These approaches have now matured and alternate strategies that exploit the insulating abilities of thermal barrier coatings (TBC's) are being investigated for the thermal protection of engine components.
The TBC systems currently in use are multilayer systems consisting of an yttria partially-stabilized zirconia (YSZ) top layer that thermally protects the superalloy component, and an underlying MCrAlY (M=Ni, Co) or nickel aluminide bond coat which improves the YSZ adhesion. The YSZ layer has a relatively high thermal expansion coefficient to limit thermally induced strains and a low thermal conductivity resulting in surface temperature reductions of up to 170° C. [3]. This layer is well bonded to a thin (approximately 1 μm) thermally grown (aluminum) oxide (TGO) layer which impedes oxidation and hot corrosion of the underlying component [4]. This TGO layer is formed on the surface of the aluminum-rich alloy layer (bond coat). Either a low pressure plasma spray [5] (LPPS) or pack cementation [6] approach is used to apply the bond coat layer. The high temperature oxidization environment present prior to and during deposition leads to growth of a thin TGO layer at the interface between the TBC and the bond coat layer [7]. The generated YSZ layer consists of a “nontransformable” tetragonal (t′) phase having a complex microstructure consisting of twins and anti-phase boundaries. This microstructure yields a thermomechanically tough coating which has been shown to improve TBC performance by limiting crack propagation in the YSZ layer [8].
To date, the lowest cost TBC's have been applied using the plasma spray (PS) process, such as an air plasma spray (APS) process. The approach employs a plasma or combustion torch to melt and spray deposit YSZ droplets onto airfoil substrates. These deposits contain disc-like pores in the plane of the coating resulting in a YSZ top layer that has an extremely low thermal conductivity. This is due to the high thermal resistance of the pores oriented normal to the heat flow direction. Unfortunately, these layers also have poor spallation resistance, resulting from a combination of the disc-like coating defects and the large thermal expansion mismatch between the YSZ layer and the bond coat [9]. This lack of reliability limits these coatings to component life extension at current operating temperatures (i.e., they cannot be used to increase engine temperature).
More recently, TBC's have been produced by electron beam-physical vapor deposition (EB-PVD). Using this technique the YSZ layer has a columnar microstructure with elongated inter-columnar voids aligned perpendicular to the substrate surface. This structure results in a low in-plane stiffness that limits thermomechanical stresses on heating/cooling and improved spallation resistance compared to the LPPS layers [10]. The columns exhibit a tapered shape, growing wider with increased thickness, a faceted surface and a strong {200} crystallographic texture [11]. Failure in these coatings no longer occurs within the YSZ layer but at the TGO/bond coat interface. This failure path appears to result from large stresses within the TGO layer, which increase with oxidation induced layer growth in service [12, 13]. For turbine blade applications, EB-PVD TBC's have the further advantages of limiting the undesirable blocking of air cooling holes during deposition and generating a smoother, more aerodynamic surface [14]. However, EB-PVD coatings have a higher thermal conductivity than their LPPS counterparts [15] and are more costly to apply (due to high equipment costs, deposition efficiencies of about 2-5 percent of the evaporated flux, and relatively slow (approximately 5 micrometers (μm) min−1) deposition rates) [16]. To make vapor phase deposited TBC's a viable means for increasing engine performance, improved deposition techniques/strategies are needed.
The cost of the EB-PVD coatings can be as much as ten times that of PS coatings. The higher equipment costs of EB-PVD are a result of the high vacuum environment that is necessary during deposition (e.g., typically below 10−6 Torr), high cost of high power electron beam guns, and sophisticated component manipulation needed to achieve acceptable coatings. The operating pressure defines the vacuum pump requirements with lower pressures generally needing more expensive pumps. The low deposition rate and low materials utilization efficiency (MUE) of EB-PVD is related to the distribution of the vapor flux as it leaves the evaporated source. Generally, the vapor flux spreads out from the source with a distribution described by a cosnθ function (where n=2,3,4 or more, and θ is the angle to the normal axis). The general alignment of the normal axis is referred to herein as the main direction. When relatively long source-to-substrate distances are required (e.g., as in YSZ deposition using EB-PVD where this distance often approaches 50 cm to avoid substrate overheating) deposition efficiency is dramatically decreased to 1-5 percent of the evaporated flux and the deposition rate is proportionally reduced. To overcome the low deposition rate, the evaporation rate from the source materials is raised by increasing the electron beam power. However, this is costly and during YSZ evaporation, increased beam power leads to the production of molten droplets of material rather than atomistic vapor. This produces coating defects, and as a result, other approaches must be used to increase deposition rates. The high cost of deposition also impedes the use of physical vapor deposition methods for the deposition of bond coats.
The low deposition efficiency results from flux spreading beyond the periphery of the sample. One approach to reduce the spread of the flux exploits entrainment of the vapor in a controllable inert (e.g. helium or argon) carrier gas flow [17]. Such an approach is used in electron beam directed vapor deposition (EB-DVD). In this approach, the combination of a continuously operating 60 kV/10 kW axial e-beam gun (modified to function in a low vacuum environment) and an inert carrier gas jet is used. As shown in FIG. 1, in this system the vaporized material is entrained in the carrier gas jet 5 created using a converging/diverging nozzle 30 configuration and deposited onto the substrate or target 20 at high rate and with a high materials utilization efficiency [18]. Preliminary results have shown that YSZ layers having a columnar structure, a low thermal conductivity, the t′ phase structure and a (200) texture can be produced using this technique [19]. Therefore the use of EB-DVD to produce low cost TBC's appears feasible. However, current versions of the EB-DVD process make inefficient use of the gas jet, which is a significant contribution to the process cost. They are also limited in their ability to spatially manipulate the flux.
There exists a need in the art for a cost-effective method to apply coatings to surfaces. The present invention addresses this need and provides, among other things, how to further manipulate the spatial distribution of the atomic flux and more efficiently utilize the carrier gas.