Abstract:
A method and apparatus are provided for inspecting a coated substrate such as a multi-layer coating on a substrate of a turbine airfoil. At each of a number of locations along the airfoil a number of frequencies of alternating current are passed through the airfoil. At least one impedance parameter is measured. The measured impedance parameters are utilized to determine a condition of the coating.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    (1) Field of the Invention  
           [0002]    This invention relates to inspection of thermal barrier coatings, and more particularly to inspection of coatings on turbine components.  
           [0003]    (2) Description of the Related Art  
           [0004]    Gas turbine engine components (e.g., blades, vanes, seals, combustor panels, and the like) are commonly formed of nickel- or cobalt-based superalloys. Desired operating temperatures often exceed that possible for the alloys alone. Thermal barrier coatings (TBCs) are in common use on such components to permit use at elevated temperatures. Various coating compositions (e.g., ceramics) and various coating methods (e.g., electron beam physical vapor deposition (EBPVD) and plasma spray deposition) are known.  
           [0005]    An exemplary modern coating system is applied to the superalloy substrate by an EB-PVD technique. An exemplary coating system includes a metallic bondcoat layer (e.g., an overlay of NiCoCrAlY alloy or diffusion aluminide) atop the substrate. A thermally insulating ceramic top coat layer (e.g., zirconia stablized with yttria) is deposited atop the bondcoat. During this deposition, a thermally grown oxide layer (TGO), e.g., alumina, forms on the bondcoat and intervenes between the remaining underlying portion of the bondcoat and the top coat.  
           [0006]    The coatings are subject to potential defects. For example, the TGO to bondcoat interface tends to suffer from separations/delaminations. Such defects tend to be inherent, so threshold degrees of defect may determine the utility of a given component. Defects may also form during use.  
           [0007]    Much of existing inspection involves destructive testing used to approve or reject batches of components. Exemplary destructive testing involves epoxy-mounting and sectioning a component followed by microscopic examination. The TGO is a critical element. This may be viewed via scanning electron microscope (SEM) at 1,000× or higher. Quality standards are used to approve or reject the batch based upon visual interpretation of the SEM images.  
           [0008]    Destructive testing suffers from many general drawbacks as do its various particular techniques. The former includes the cost of destroyed components, the inaccuracy inherent in batch sampling, and the cost of time. U.S. Pat. No. 6,352,406 discloses an alternate system involving coating of a pre-couponed turbine blade facsimile in lieu of cutting an actual blade. This may slightly reduce the time spent, but does not address the fundamental problems of destructive testing.  
           [0009]    Laser fluorescence has been used for nondestructive evaluation of limited coating parameters. In one example, the beam of a ruby laser is shined on the ceramic top coat and passes therethrough to reach the TGO. The TGO fluoresces and the emitted light passes through the top coat to a sensor. Characteristics of the flurorecence indicate stress in the TGO. Separation/delamination voids are associated with reduced stress and can thus be detected. U.S. Pat. No. 6,072,568 discloses such inspection.  
           [0010]    There remains a substantial need for improvement in testing techniques.  
         BRIEF SUMMARY OF THE INVENTION  
         [0011]    Accordingly, one aspect of the invention is a method for inspecting a multi-layer coating on a substrate of a turbine element airfoil. At each of a number of locations along the airfoil, a number of frequencies of alternating current are passed through the airfoil. At least one impedance parameter is measured. Based upon the measured impedance parameters, a condition of the coating is determined.  
           [0012]    The current may pass through an electrolyte wetting the airfoil. The method may determine thicknesses of one or more layers of the coating and may identify or characterize voids within the coating or between the substrate and the coating. The method may advantageously be performed in situ with the turbine element installed on a turbomachine. The method may be performed seriatim on a number of turbine elements on the turbonmachine.  
           [0013]    Another aspect of the invention is an inspection apparatus. The apparatus may have a source of the current and electrodes for passing the current through the airfoil. The apparatus may have means for measuring the impedance parameter and means for determining the coating condition.  
           [0014]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a view of a coating test/inspection system.  
         [0016]    [0016]FIG. 2 is a sectional view of a coated item.  
         [0017]    [0017]FIG. 3 is an alternate view of a test/inspection system.  
         [0018]    [0018]FIG. 4 is a circuit equivalent model of a coating.  
         [0019]    [0019]FIG. 5 is a graph of impedance and phase angle against frequency for an exemplary coating. 
     
    
       [0020]    Like reference numbers and designations in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0021]    [0021]FIG. 1 shows an apparatus  20  for testing/inspecting a coated item such as a turbine element (e.g., a turbine engine blade  22 ). The exemplary blade  22  includes an airfoil  24  extending from a root  26  at a platform  28  to a tip  30 . The airfoil has leading and trailing edges  32  and  34  separating pressure and suction sides  36  and  38 . The platform has an outboard portion  40  for forming an inboard boundary/wall of a core flowpath through the turbine engine. A mounting portion or blade root  42  depends centrally from the underside of the platform  40  for fixing the blade in a disk of the turbine engine. In an exemplary embodiment, the portion  40  and airfoil  24  are coated.  
         [0022]    The exemplary system  20  includes an impedance analyzer  50  coupled by conductors  51  and  52  to a pair of electrodes  53  and  54 . The first electrode  53  may be a standard reference electrode contacted with an uncoated portion of the platform. The second electrode  54  is contacted with a coated portion of the blade and, therefore, is advantageously provided as a wetting electrode. The wetting electrode  54  includes a standard reference electrode  56  mounted in a proximal end of a tubular vessel  58  and contacting an electrolyte  60  within the vessel. A check valve  62  is mounted in a distal end of the vessel  58 . When the check valve  62  is contacted with the coating, it establishes fluid communication between the contact site and the interior of the vessel providing a small wetting of the coated surface with the electrolyte and providing an electrical path through the electrolyte from the coating to the reference electrode  56 .  
         [0023]    [0023]FIG. 2 shows further details of the coating  70  on a metallic substrate  72  of the blade. The blade has an outer surface  74  atop which the coating layers are deposited. The layers include a metallic bondcoat  76  atop the substrate surface  74 , an in situ formed TGO layer  78  atop the bondcoat, and a ceramic topcoat  80  atop the TGO and having an external surface  82 . Contact is made between the electrode  54  and the surface  82  via the wetting electrolyte  84 . Direct electrical contact is made between the electrode  53  and an exposed uncoated surface  86  of the substrate.  
         [0024]    In a laboratory setting, the system  20  of FIG. 1. may include an environmental control chamber  100  (FIG. 3) for containing the blade  22  during testing and that controls various properties of temperature, humidity, pressure, and the like. The current is provided by a current amplifier  102  coupled to an impedance analyzer  104  for measuring impedance parameters. The impedance analyzer  104  is coupled to analysis equipment such as a computer  106 . The computer may display results of the measured parameters and perform analyses to determine quantitative and qualitative properties of the coating based on the received parameters.  
         [0025]    Various theoretical, empirical or hybrid models may be used to determine coating properties. Such properties may include the layer thicknesses and the presence, size, and quantity of imperfections (e.g., voids within layers or between layers (e.g., separations and delaminations)). FIG. 4 shows a basic electric circuit model. From one end of the circuit to the other, the resistance of the electrode  54  (FIG. 1) is shown as R p  in series with an electrolyte solution resistance R s . This, in turn, is in series with the parallel combination of a topcoat resistance R c  and a topcoat capacitance C c . This, in turn, is in series with the parallel combination of a TGO resistance R o  multiplied by a Warburg coefficient W o  and a TGO capacitance C o . This is, in turn, in series with the parallel combination of a resistance R T  of the interface between the superalloy and bondcoat and an interface capacitance C T .  
         [0026]    [0026]FIG. 5 shows an exemplary graph  120  of impedance Ω against frequency w. An exemplary impedance scale is 0-1500 Kohm/cm 2 . FIG. 5 further shows an exemplary graph  122  of phase angle θ against frequency. An exemplary phase angle scale is 0 to 80°. In this model, roughly the location of the impedance peak  130  is indicative of R T . The location of the impedance tail  132  is indicative of R p . In the location of the transition  134  is indicative of R c  and R o . The location of a low frequency phase angle peak  140  is indicative of C o  and the location of a high frequency phase angle peak  142  is indicative of C c . The location of a tail  144  is indicative of C T . The wetting electrode may be moved seriatim to a plurality of positions on the blade and impedance measurements taken. Analysis of data from such multiple positions may be used to even better determine coating properties.  
         [0027]    Less environmentally controlled tests may be performed in situ on an assembled engine such as performing periodic tests on an aircraft engine. Such testing may be used to determine wear and other degradation parameters and determine remaining life of the turbine element. Alternative tests may involve contacting two probes with the coating. This may be appropriate where convenient access to uncoated portions is difficult. Relatively complex models could be used for such a situation.  
         [0028]    One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, details of the particular turbine elements, coatings, test conditions, and examination criteria may influence the structure of the inspection apparatus and implementation of the inspection methods. Accordingly, other embodiments are within the scope of the following claims.