Patent Publication Number: US-7896726-B1

Title: Method and apparatus for removing coatings from a substrate using multiple sequential steps

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of and thus is entitled to the benefit of, and claims priority to, U.S. patent application Ser. No. 11/999,667, filed Dec. 6, 2007, and entitled “Method And Apparatus For Removing Coatings From A Substrate Using Multiple Sequential Steps,” which claims priority of provisional U.S. Patent Application Ser. No. 60/874,571, filed on Dec. 13, 2006, and entitled “Method And Apparatus For Removing Coatings From A Substrate Using Multiple Sequential Steps,” the disclosures of which are incorporated herein by reference as if set forth fully herein. 
    
    
     BACKGROUND OF THE PRESENT INVENTION 
     The present invention relates generally to a method and apparatus for removing coatings from a workpiece or substrate, and more particularly to a method and apparatus for removing such coatings in a controlled sequence of steps. 
     Some types of workpieces and parts require that one or more layers of coatings be applied to the workpiece to protect the underlying base metal from heat, abrasion, and other elements encountered by the workpiece in its normal usage. For example, turbine blades and other turbine parts may be formed of a base metal, a bond coating that is often diffusion bonded metallurgically to the base metal, and an outer thermal barrier coating (TBC) applied to the bond coat. As is well known, the composition of the TBC (e.g. ceramic) makes it relatively easy to remove using manual grinding, dry grit blasting, chemical stripping and water jet stripping. However, as set forth in greater detail in U.S. Pat. No. 6,905,396, which is incorporated herein by reference, all of these known manually applied removal methods have various drawbacks in that they are not particularly effective in removing the more difficult bond coat, and, in the case of acid or chemical stripping, environmental problems are encountered and there is a tendency to damage the underlying base metal as part of the removal process. 
     The invention described in the aforesaid &#39;396 patent provides for a controlled removal method that can be carried out using an abrasive water jet that is mechanically, rather than manually, operated to move relative to the workpiece and thereby remove coatings from the base metal as the abrasive water jet is passed relative to the surface of the workpiece. However, even though the relative movement of the abrasive water jet across the workpiece surface is controlled, the abrasive water jet itself generally removes substantially equal amounts of the coatings during each pass of the abrasive water jet, and the abrasive water jet is not controlled in a way that would properly compensate for variations in the thickness of the TBC caused by the use of the workpiece, such as a turbine blade. Additionally, because of variations in the overall thickness of the coatings to be removed, the relatively constant flow of the abrasive jet would sometimes remove more or less of the base metal than was desired. 
     Accordingly, a need exists for a method of removing layers of coatings in a controlled manner that compensates for variations in the overall thickness of the coatings. A need also exists for such a method that is controlled in a way that it removes a desired small amount of the base metal, such as the diffusion layer formed when a bond coat is diffusion bonded to the base metal, to provide a clean, pristine surface on the base metal after the coatings have been removed so that new coatings can be applied to the base metal. 
     SUMMARY OF THE INVENTION 
     The present invention provides an abrasive water jet apparatus for removing a coating from the surface of a workpiece in which the coating has an irregular exterior surface configuration with high and low points. The apparatus includes a workpiece holding system for holding and the workpiece; a movable abrasive water jet that discharges a high pressure jet of water having an entrained abrasive material against and along the coating of the workpiece held by the work holding system to remove the coating therefrom at variable material removal rates; and a sensor mounted generally adjacent the work holding system that senses the topography of the irregular exterior surface of the coating at a plurality of points and transmits electrical signals that are a function of the thickness of the coating at the plurality of sensed points. The apparatus also includes a control system that receives the signals from the sensor and controls the movement and operation of an abrasive water jet to make multiple passes relative to the surface of the workpiece in which a small layer of the coating is removed during each pass and to even out the irregular surface configuration of the coating by operating the abrasive water jet at higher material removal rates when the abrasive water jet passes the higher points in the surface configuration and at lower material removal rates when the abrasive water jet passes the lower points in the surface configuration. 
     The workpiece may be a turbine component formed of a base metal, a bond coat diffusion bonded to the base metal and overcoated with a TBC having the irregular surface configuration, and the control system can control the abrasive water jet to remove the entire TBC down to the bond coat. 
     The sensor may be a touch probe than engages the surface of the coating to be removed, or an Eddy current probe that senses the thickness of the coating. 
     The abrasive water jet apparatus of the present invention may be used to remove a bond coat that is diffusion bonded to a base metal of a workpiece in which a diffusion layer having at least one unique element is formed between the bond coat and the base metal. The apparatus includes the workpiece holding system and abrasive water jet as described above, and a detection device mounted generally adjacent the work holding system that detects the presence or absence of the unique element in the diffusion layer and transmits electrical signals to the control system that indicate whether the unique element is present or absent, and the control system receives the signals from the detection device and controls to the movement and operation of an abrasive water jet to make multiple passes relative to the surface of the workpiece in which a small layer of the bond coat and then the diffusion layer is removed, and to stop the removal of material by the abrasive water jet when the control system receives a signal from the detection device indicating the absence of the unique element. 
     Preferably, the detection device is a XFR analyzer and the unique element in the diffusion layer is Yttrium. 
     The present invention also provides a method of removing a coating from the surface of a substrate in which the coating has an irregular exterior surface configuration with high and low points, such method comprising the steps of (a) mapping the topography of the irregular surface configuration of the coating to be removed; (b) using the topography information determined by the mapping step to create an input into a control system that controls the operation and movement of an abrasive water jet apparatus; and (c) utilizing the control system to control the movement and operation of an abrasive water jet to make multiple passes relative to the surface of the substrate in which a small layer of the coating is removed during each pass and to even out the irregular surface configuration of the substrate by operating the abrasive water jet at higher material removal rates when the abrasive water jet passes the higher points in the surface configuration and at lower material removal rates when the abrasive water jet passes the lower points in the surface configuration. 
     In one preferred embodiment of this method, the mapping step includes moving the substrate relative to a probe with the probe engaging the irregular surface configuration of the substrate at predetermined probe points to generate electrical signals that are a function of the irregular surface configuration at the probe points, and wherein the electrical signals are transmitted to a programmed computer in the control system. The mapping step may include using an Eddy current probe for determining the thickness of the irregular surface configuration, and generating electrical signals that are a function of the thickness of the irregular surface configuration. 
     In this method, the abrasive water jet apparatus may include a workpiece holder for holding the substrate relative to the movable abrasive water jet, and the method includes the step of positioning the abrasive water jet generally adjacent the probe so that the probe can move relative to the workpiece during the mapping step and the abrasive water jet and the abrasive water jet can move relative to the workpiece during the material removal step. 
     The present invention also includes a method of removing a bond coat diffusion bonded to a base metal and removing the diffusion layer between the bond coat and the base metal, the diffusion layer having an element that is unique to the diffusion layer. This method includes the steps of removing the bond coat material by moving an abrasive water jet relative to the bond coat and the base metal in multiple passes and in sequential steps; detecting the presence or absence of the unique element in the diffusion layer after predetermined passes of the abrasive water jet; and stopping the movement and operation of the abrasive water jet when the presence of the unique element is no longer detected. 
     Additionally, the detection devices may detect the amount of the unique element that is present at the surface of the diffusion layer during passes of the workpiece relative to the detection device, and the signals transmitted to the control system by the detection device can indicate the amount of the unique element that present at the surface of the diffusion layer. The control system determines the amount of the unique element that is removed during predetermined passes of the workpiece and the differential between the amount of the unique element that is removed during such passes, and the control system controls the operation of the abrasive water jet and the movable workpiece holder to increase the material removal rate when the differential is less than a predetermined amount and to decrease the material removal rate when the differential is greater than a predetermined amount. 
     In this method, the detection step may include using an x-ray fluorescence detector, and moving the detector relative to the surface in which the presence or absence of the unique element is to be detected. The unique element of the diffusion layer may be Yttrium. 
     This method may used with an abrasive water jet apparatus that includes a movable workpiece holder for holding the surface to be detected relative to the movable abrasive water jet, and the method includes the step of positioning the abrasive water jet generally adjacent the detector so that the abrasive water jet can move relative to the workpiece relative to the abrasive water jet during the material removal step and the detection device can move relative to the workpiece during the element detection step. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a typical substrate having coatings that can be removed by the present invention; 
         FIG. 2  is a general perspective view of the preferred abrasive jet apparatus of the present invention; and 
         FIG. 3  is a diagrammatic view of the abrasive jet acting against a substrate, and related elements of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In describing the invention below, the substrate will sometimes be identified as a turbine blade, which is a typical machine part having coatings that are removable using the method of the present invention, but it is to be understood that the present invention could also be used in removing coatings from a variety of other substrates and machine parts, such as vanes, shrouds, liners and transition pieces. 
     As seen in  FIG. 1 , the turbine blade  18  consists of a base metal  10  to which a bond coat  12  is diffusion bonded metallurgically to the base metal  10 . The bond coat  12  is then overcoated with a ceramic material to form a TBC  14 . It will be noted that the TBC  14  has a very irregular thickness and surface configuration which is a result of heat and other elements applied to the turbine blade in use, and this irregular surface configuration is exaggerated in  FIG. 1  for purposes of illustration. Additionally, it will be noted that when the bond coat  12  is diffusion bonded to the base metal  10 , there is a relatively thin diffusion layer  16  that is inherently formed as part of the diffusion bonding process. This diffusion layer  16  may, and usually does, include foreign matter such as dirt and the like and often includes surface cracking  20 , sometimes referred to as “craze cracking”, all of which is undesirable as a surface upon which a new coating can be applied with a very secure bond after the old coatings have been removed. Accordingly, one feature of the method and apparatus of the present invention is that it can be utilized to not only remove the TBC and the bond coat, it can be effectively used to also remove just enough of the base metal to remove the diffusion layer  16  and thereby leave the cleaned surface of the base metal  10  as a pristine surface without cracking or the presence of foreign matter that could adversely effect the new bond coat when it is applied to the base metal. 
     In one preferred embodiment of the present invention, removal of the coatings is carried out using a known abrasive jet apparatus of the type disclosed in more detail in the aforesaid &#39;396 patent, which is enhanced in accordance with the present invention as described in greater detail below. The details of the abrasive water jet apparatus itself, as disclosed in the &#39;396 patent, form no part of the present invention, and these basic components of the abrasive water jet apparatus  22  are illustrated in  FIGS. 2 and 3 . They include a distribution head  24  having a mixing chamber  26  from which a pressurized liquid (e.g. water) is provided to the mixing chamber  26  from a water source  28 . The apparatus  22  includes a source  30  of an abrasive material which is also delivered to the mixing chamber  26 , and the combined water and abrasive is delivered from a delivery nozzle  32  as a jetted fluid stream or abrasive water jet  34 , usually in the range of 5,000 psi to 55,000 psi. As best seen in  FIG. 2 , the delivery nozzle  32  is manipulated relative to the workpiece about a plurality of axes (indicated by arrows) by a workpiece holding system  35  that includes by a plurality of motors  36 , only one of which is shown diagrammatically in  FIG. 2 , and these motors are controlled though a conventional control system  38  that includes a conventional programmable computer to position and move the workpiece so that the delivery nozzle  32  has relative movement along a desired path across workpiece, and to properly control the various parameters associated with the apparatus  22  (e.g. the amount of the abrasive delivered to the mixing chamber  26  and the pressure of the abrasive water jet  34  exiting the delivery nozzle  32 ) to vary the material removal rate of the abrasive water jet  34 . 
     The conventional abrasive water jet apparatus  22  described above is modified in accordance with the present invention to include a tool  40  for mapping the surface configuration of a substrate and generating signals that are a function of the irregular surface configuration and transmitting these signals to the control system  38 , and a detection device  42  for detecting the presence or absence of an element that is unique to an element in the substrate and generating and transmitting signals to the control system  38 , all as will be described in greater detail below. 
     In the present invention, the workpiece  18 , such as a turbine blade, from which coatings  12  and  14  (see  FIG. 1 ) are to be removed is probed by the tool  40  to map the topography of the irregular surface configuration of the outer coating  14 . The tool  40  is a conventional digitizing device such as touch probes, x-rays, eddy-current probes, laser probes and scanners, structured light scanners, and the like. It is moved relative to the irregular surface configuration and generates electrical signals at predetermined probe points during such movement that are a function of the topography of the surface configuration of the outer coating  14  of the turbine blade  18 . These signals are then delivered through electrical lines  44  to the computer  38  which has software that is programmed to store the signals and to utilize the signals as part of the control the operation of an abrasive jet apparatus  22 . More specifically, since the topography of the irregular surface configuration (e.g. higher and lower points along the surface configuration of the turbine blade  18 ) is programmed in the computer  38  of the abrasive jet apparatus  22 , the abrasive jet apparatus  22  can be controlled by the computer  38  to control the various parameters of the system (e.g. the amount of the water pressure generated by the jet, the amount of abrasive material entrained in the water jet, and other variables) and thereby vary the material removal rate of the abrasive water jet  34  so that as the abrasive water jet  34  is moved relative to the irregular surface configuration of the turbine blade  18 , more of the TBC coating  14  will be removed at the higher points of the surface configuration as determined by the mapping, and less coating material is removed at the lower spots. As a result, after this first step is carried out, the surface configuration of the coating will be approximately smooth, but it will be understood that this first step could include one or multiple passes of the abrasive water jet  34  relative to the surface of the turbine blade  18 . In most cases, this first step removes the TBC, which is relatively easy to remove, and creates a generally smooth surface configuration along the remaining bond coat  12 . It will be understood that this step of mapping the topography of one layer of coating and then controlling the abrasive water jet  34  to smooth out the uneven surface can be used not only where multiple layers of coatings are present in the workpiece, but also on a unitary substrate that has a uneven surface configuration like that shown for the TBC  14  in  FIG. 1 . 
     Next, the abrasive water jet apparatus  22  is controlled so that it moves relative to the surface of the bond coat  12  at a generally uniform speed and at a generally uniform material removal rate so that a relatively thin but uniform portion of the bond coat is removed during each pass of the abrasive water jet  34 . During this step, in most cases there will be multiple passes of the abrasive water jet  34 , with only a small amount of the bond coat  12  being removed during each pass, whereby it will be easier and more accurate to determine when the undesirable diffusion layer  16  has been completely removed without removing any more of the base metal  10  than is necessary to remove the diffusion layer  16 . 
     In accordance with a further feature of the present invention, the removal process described above can be significantly enhanced in many applications by utilizing the detection device  42  to detect the presence or absence of an element that is unique to the coating to be removed. For example, in one bond coat  12  that is applied to a turbine blade  18 , Yttrium is a unique element of the bond coat, and Yttrium can be detected by the detection device  42 . Accordingly, at the conclusion of each pass of the abrasive water jet  34  relative to the surface of the bond coat  12  that removes a thin layer thereof, detection device  42  is used to test the exposed surface of the bond coat  12  and generate an electrical signal that indicates whether the unique element is present or absent, and this electrical signal is transmitted to the control system  38  through electrical lines  44 . If Yttrium is detected, this indicates that some of the diffusion layer  16  still remains on the base metal  10 , and it is therefore necessary for the control system  38  to control the apparatus so the abrasive water jet  34  makes another pass or passes relative to the surface of the turbine blade  18 . At some point, after a pass of the abrasive water jet  34 , the detection device  42  will detect that no Yttrium is present and transmit an electrical signal to the computer. At that point, it can be safely determined that all of the diffusion layer  16  has been removed from the base metal  10 , leaving a pure and pristine base metal surface upon which a new coating can be securely applied, and the control system  38  is programmed to stop further operation of the abrasive water jet  34  and thereby stop the further removal of material from the base metal  10 . It will be appreciated that the diffusion layer  16  is normally relatively small in thickness (e.g. perhaps 0.001 to 0.002 inches), and the combination offered by the present invention of removing only a thin layer of the bond coat  12  on each pass, and then detecting the presence or absence of an element that is unique to the diffusion layer (e.g. Yttrium) makes it possible to effectively remove just the diffusion layer without removing, unnecessarily, any additional base metal material  10 . While Yttrium is an element that is unique to most bond coats used in turbine components, it will be appreciated that other types of coatings can be removed utilizing the present invention, and the element that is unique to such coatings may be different from Yttrium, and the detection device  42  will have to be selected based on its ability to detect that unique element. 
     In addition to using the signals from the detection device  42  to determine just the presence and absence of the unique element, the signals from the detection device  42  can also be used to control the rate of material removal by the abrasive jet apparatus  22 . More specifically, the signal transmitted to the control system  38  by the detection device  42  can, in addition to indicating the presence or absence of the unique element, indicate the amount of the unique element that is present. The control system  38  can be programmed to analyze the signals and determine the amount of the unique element that is removed during each of a sequence of passes based on the signals it receives from the detection device  42  and determine the differential between the amounts removed during successive passes, and if the differential of the amount of the unique element that is removed from one pass to the next is smaller than a predetermined level, which would indicate that smaller amounts of the coating material are being removed than desired, the apparatus  22  can be controlled to vary the operating parameters of the apparatus  22  to increase the material removal rate of the abrasive water jet  34  applied on subsequent passes and thereby increase the amount of the coating removed during each pass. Similarly, if the differential is larger than the predetermined level, the apparatus  22  can be controlled to decrease the material removal rate of the abrasive water jet  34  on subsequent passes. 
     When the detection device  42  is used to detect the presence or absence of Yttrium or some other unique element, it can be manually moved relative to the surface that is being detected, but, in accordance with the present invention, it is preferred that the detection device  42  be carried on the abrasive jet apparatus  22  generally adjacent the delivery nozzle  32  as illustrated in  FIGS. 2 and 3  so that it is an integral part of the abrasive water jet apparatus  22  which can be operated by the control system  38  for the apparatus  22  to move with the abrasive water jet  34  relative to workpiece  18  during the detection step. 
     The mapping step for determining the topography of the coating to be removed can be carried out in different ways, depending on the particular application of the removal method. For example, if it is determined that it is only necessary to ultimately even out the high and low points of a TBC, probing of the TBC can be done using a conventional Eddy current probe  40 ′ to determine the topography or thickness of the TBC that is to be removed, and the Eddy current probe  40 ′ can then be used to generate the electrical signals that are transmitted to the computer  38  to control the movement of the abrasive water jet  34  relative to the surface of the TBC as described above. On the other hand, since an Eddy current probe  40 ′ is not suitable for mapping the high and low variations in the TBC, a coordinate measuring machine (CMM) can be used as the mapping tool  40  to map the topography of the coating, and then generating electrical signals which are a function of the higher and lower points along the surface of the coating that are transmitted to the computer  38  for controlling the operation of the abrasive water jet apparatus  22 . The Eddy current probe  40 ′ effectively measures the actual thickness of the TBC, and therefore can determine the high points and low points in the TBC by determining the actual thickness of the coating at the probe point. The CMM  40  does not measure the thickness of a coating, but, rather, engages the surface and directly detects higher and lower points in the topography of the coating. Therefore, in some applications of the coating removal method of the present invention which utilizes the mapping step, the removal process can be carried out using just the Eddy current probe, or just the CMM, or a combination of both. It will be understood also that the abrasive water jet apparatus  22  can be operated so that it removes only a TBC coating  14  having an irregular surface configuration from a particular workpiece using the mapping step described above, or it can be operated so that it removes only the bond coat  12  and diffusion layer  16  from a particular workpiece using the detection step, or it can be operated to remove the TBC coating, the bond coat  12  and the diffusion layer  16  from a particular workpiece. 
     In the preferred embodiment of the present invention, the mapping tool  40  may be a CMM that utilizes a Model LP2 touch probe manufactured and sold by Renishaw Company in Schaumberg, Ill., but other equivalent CMMs can also be used. This touch probe  40  engages the surface being mapped and generates electronic signals representing the point of contact with the surface, which is a function of the height of the coating at the point of contact, and these signals are transmitted to the control system  38  which is programmed to store the signals and then use the signals in determining the operation of the abrasive jet apparatus  22  to apply a greater abrasive force for removing the higher spots on the surface and less abrasive force at the lower spots, all as described above. 
     It is also preferred that the detection device  42  be a x-ray fluorescence device such as an XFR analyzer, Model Nitron XLP, sold by Thermo Electric in Billerica, Mass., which is suited for detecting Yttrium and some other elements. If the unique element in the coating or diffusion layer being detected is other than Yttrium as discussed above, it may be necessary to use other equivalent analyzers designed to detect that unique element. 
     In view of the aforesaid written description of the present invention, it will be readily understood by those persons skilled in the art that the present invention is susceptible of broad utility and application. Many embodiments and adaptations of the present invention other than those herein described, as well as many variations, modifications, and equivalent arrangements, will be apparent from or reasonably suggested by the present invention and the foregoing description thereof, without departing from the substance or scope of the present invention. Accordingly, while the present invention has been described herein in detail in relation to preferred embodiments, it is to be understood that this disclosure is only illustrative and exemplary of the present invention and is made merely for purposes of providing a full and enabling disclosure of the invention. The foregoing disclosure is not intended nor is to be construed to limit the present invention or otherwise to exclude any such other embodiments, adaptations, variations, modifications and equivalent arrangements, the present invention being limited only by the claims appended hereto and the equivalents thereof.