Abstract:
A system and method for nondestructive testing of a workpiece having a metallic protective coating utilizing nonlinear harmonics techniques to determine degradation within the metallic protective coating. The invention use a time-varying magnetic field to sense magnetic properties of the protective coating. The odd-numbered harmonic frequencies are detected and their amplitudes are related to the magnetic condition of the material under test to determine coating degradation. When no harmonic signal caused by an induced magnetic field is detected, the coating is not degraded. When a harmonic signal is detected, the coating has degraded. Nonlinear harmonics techniques are used to determine the amount of coating degradation.

Description:
BACKGROUND 
     The invention relates generally to nondestructive methods for measuring service-related degradation in protective coatings such as those used to protect gas turbine blades from high temperature oxidation and corrosion. More particularly, the invention is a method and system that uses nonlinear harmonic detection methods to sense degradation-related changes in the magnetic permeability of the high-temperature protective coatings applied to the surface of a workpiece. The invention uses a time-varying magnetic field at a fundamental frequency to detect changes in the magnetic properties of the coatings. The odd-numbered harmonic frequencies are detected and their amplitudes are related to the magnetic permeability of the coating under test to determine coating degradation. By using different fundamental frequencies, it is possible to profile coating degradation with depth and minimize coating thickness effects. 
     Metallic coatings are commonly used on metal components to protect the surfaces of the components from high-temperature oxidation and corrosion. A common use is on combustion turbines to protect the surfaces of components such as blades. One such class of metallic coatings used for protection typically has the composition MCrAlY, where M may represent either cobalt (Co), nickel (Ni), or a combination of both, Cr represents chromium, Al represents aluminum and Y represents yttrium. Detection of degradation or failure of the metallic coatings is important to prevent damage to the underlying components. During service, the aluminum (AL) in the coating diffuses inward into the base material of the component and outward to form a protective aluminum oxide layer which forms on the outside surface of the coating. Eventually, the aluminum in the coating becomes depleted and can no longer support the aluminum oxide layer. This results in coating failure and lack of protection for the underlying component, such as the blade base metal of combustion turbine components. Coatings can be stripped and replaced, provided this is done before the coating fails. If the protective coating has degraded to the point where it is no longer functional, damage to the underlying component can occur primarily due to high temperature oxidation or corrosion of the material. If the degradation of the coating is not detected prior to coating failure, it may be necessary to replace the entire component. 
     It has been observed that in some classes of coating systems, degradation causes the coating to change from an initial nonferromagnetic condition to a ferromagnetic condition. By measuring the magnetic properties of the coating, it is possible to nondestructively determine the condition of the coating. Techniques using eddy currents and permeability probes have been applied to this problem. The eddy current measurement technique uses a time-varying magnetic field to induce eddy currents into the component. The eddy current sensing device always generates a reading even if the material is in a nonferromagnetic state (and therefore the coating is not degraded) because the component material is electrically conductive and the eddy current method responds to conductivity as well as magnetic condition. Also, if the thickness of the coating changes, the eddy current measurement will change even if the coating is not degraded. As the coating degrades, the eddy current reading changes only slightly and it is difficult to determine that this small change is due to degradation and not to other factors. The permeability probe uses a permanent magnet which supplies a non-time-varying magnetic field and a magnetic sensor, such as a Hall-effect probe, to sense the magnetic field. The presence of ferromagnetic material, such degraded coating, affects the field distribution and the field measured by the magnetic sensor. Both the eddy current and permeability probe measurements are sensitive to the magnetic condition of the coating and to probe liftoff and tilting effects. The eddy current measurements are also sensitive to electrical conductivity and to the thickness of the coating even when the coating is not degraded. Even though the permeability probe is sensitive to coating thickness after the coating has degraded, because a permanent magnet is used, it is not possible to use different frequencies to control penetration into the coating (skin depth) so as to be able to vary penetration depth into the coating, nor is it possible to profile coating degradation with depth. These variations in measurement capability and sensitivity make it difficult to accurately detect and characterize coating degradation. 
     Therefore, a nondestructive evaluation technique is needed which can be used in the field to provide an accurate measurement of coating degradation so that the coating may be replaced before damage to the underlying component occurs. There is also a need for a technique that is not sensitive to probe liftoff and tilting effects, electrical conductivity and thickness of the coating. In addition, a technique is needed that allows a profile of coating degradation as a function of depth. 
     SUMMARY 
     The present invention is a nondestructive system and method for measuring service-related degradation in protective coatings, usually high temperature protective coatings such as those used to protect gas turbine blades from high temperature oxidation and corrosion. The invention uses nonlinear harmonics detection methods to sense degradation-related changes in the magnetic permeability of the coating applied to a component. When the coating is first applied to a component, the coating is initially nonferromagnetic. Degradation causes the coating to change from an initial nonferromagnetic condition to a ferromagnetic condition. The nonlinear harmonics method detects this change in the ferromagnetic properties of the coating of the component. 
     The nonlinear harmonics method is typically implemented by applying a sinusoidal current at a fundamental excitation frequency to the component of interest using an excitation coil. Any resulting magnetic induction is measured with a magnetic field sensor such as a sensing coil. The excitation frequency can range between about 1 kHz to about 10 MHz. The sensor output is amplified and the harmonic frequency content, typically the third harmonic, is determined using a spectrum analyzer or lock-in amplifier referenced to the driving waveform. Simpler configurations such as band-pass filtering the third harmonic frequency and detecting the output may also be used. 
     When the coating applied to the material is not degraded, the coating is nonferromagnetic so no magnetic signal is induced using the nonlinear harmonics method. Since there is no signal, there are no odd-numbered harmonic frequencies generated. As the coating on the component degrades, it becomes ferromagnetic. Because of magnetic hysteresis and nonlinear permeability of ferromagnetic material, the magnetic induction in the material becomes distorted. The distorted magnetic induction waveform contains odd numbered harmonic frequencies of the applied magnetic field. The distorted magnetic induction waveform contains odd harmonic frequencies of the applied magnetic field. Using this nonlinear harmonics method, one or more of these harmonic frequencies are detected and their amplitudes are related to the magnetic properties of the material under test. Coating degradation may then be identified and characterized by the nonlinear harmonics response. Probe liftoff and probe tilt effects can be minimized by utilizing phase information in the signal and by combining measurements of both the fundamental frequency and the harmonic frequency or by using multiple harmonic frequencies. In addition, by adjusting the fundamental frequency, the penetration depth of the magnetic field into the coating can be controlled thus allowing profiling of the coating with depth. By using high excitation frequencies, the penetration depth may be limited to the near surface of the coating and coating thickness effects are minimized. 
     The present invention comprises a system for nondestructive testing utilizing nonlinear harmonics techniques to determine degradation within metallic protective coatings affixed to a surface of a workpiece. The invention has a means for supplying a time varying current at a fundamental frequency to a nonlinear harmonic sensor and for outputting a phase reference signal. The nonlinear harmonic sensor comprises an excitation coil for generating a magnetic field when supplied with the time varying current and a sensing coil for detecting a signal caused by an induced magnetic field in the metallic coating. Both the excitation and sensing coils are positioned in close proximity to the surface of the workpiece having a metallic coating. The present invention also comprises a means for amplifying and selecting a portion of the detected signal that represents the harmonic frequency component of the signal and generating an output signal using the phase reference signal, along with a means for displaying the output signal. The means for supplying the time varying current at the fundamental frequency to a nonlinear harmonic sensor and for outputting the phase reference signal may be a signal generator and power amplifier. The means for displaying the output signal may be a computer controlled display device or a meter that displays a harmonic amplitude of the output signal. In an alternate embodiment, a computer means may be used for displaying and analyzing the harmonic signal to detect coating degradation. 
     When no harmonic signal caused by an induced magnetic field in the metallic coating is detected, the metallic protective coating is not degraded. When a harmonic signal caused by an induced magnetic field in the metallic coating is detected, the metallic protective coating is degraded. The metallic coating may be a high temperature protective coating and may contain elements selected from the group consisting of cobalt, nickel, chromium, yttrium and aluminum. 
     The harmonic signal may be typically at a third harmonic frequency of the fundamental frequency. The harmonic signal stored for analysis comprises in-phase and quadrature signal components. The phase reference signal component is output to an amplifier which generates and in-phase and quadrature reference signal component. The fundamental frequency may be a selected frequency in the range of between about 1 kHz to about 10 MHz. 
     The means for amplifying and selecting the portion of the signal that represents the harmonic frequency component of the signal and generating a harmonic signal using the phase reference signal may further comprise a filter means for filtering the output signal to remove frequencies other than harmonic frequencies. It may also comprise a means for frequency multiplying the phase reference signal and passing the multiplied phase reference signal to a lock-in amplifier that uses the multiplied phase reference signal and filtered harmonic signal to generate a complex harmonic signal with in-phase and quadrature signal components. 
     The invention also comprises a method for nondestructive testing utilizing nonlinear harmonics techniques to determine degradation within metallic protective coatings affixed to a workpiece comprising the steps of: supplying a time varying current at a fundamental frequency to an excitation coil, positioned in close proximity to a surface of a workpiece having a metallic coating, for generating a magnetic field and outputting a fundamental phase reference signal, and detecting a signal caused by an induced magnetic field using a sensing coil positioned in close proximity to the surface of the workpiece. The portion of the signal that represents a harmonic frequency component of the signal is amplified and output. Both the harmonic signal and phase reference signals are stored and the amplitude of the harmonic signal is analyzed to detect coating degradation using a computer program. The method may further comprise analyzing the amplitude of the in-phase and quadrature harmonic signal components which contain coating degradation components and probe liftoff signal components and determining the amount to phase shift the in-phase and quadrature harmonic signal components to remove the liftoff signal components. The in-phase and quadrature signal components are phase shifted by a selected number of degrees to remove the liftoff signal components, with the resulting signal indicating the coating degradation components. The number of degrees of phase shifting may be determined by using an optimization software program which phase shifts the signal in incremental amounts to determine a largest amplitude for a signal of interest and a smallest amplitude for a signal not of interest. 
     Analyzing the amplitude of the harmonic signal to detect coating degradation may further comprise comparing the amplitude of the in-phase and quadrature harmonic signal components to an amplitude of a threshold signal level and characterizing the amount of coating degradation using a calibration curve, if the amplitude of the in-phase and/or quadrature harmonic signal is greater than the amplitude of a threshold level. The threshold level signal may be determined by using a specimen that is the same material as the workpiece having the same coating on the surface of the workpiece, placing a nonlinear harmonics sensing device on the specimen, supplying a time varying current at a given frequency to the nonlinear harmonics sensing device and outputting a fundamental phase reference signal, detecting a signal caused by induced magnetic field in the specimen, amplifying and selecting a portion of the signal of that represents a harmonic frequency component of the signal and generating a harmonic signal. The coating of the specimen may be degraded by a known amount and the steps above repeated. A calibration curve may then be constructed by plotting the harmonic signal corresponding to each degradation of the coating to generate a threshold signal level. The calibration curve can also be used to characterize coating degradation. Alternatively, the threshold level signal may be determined by using a specimen that is the same material as the workpiece with the same coating and having a known amount of coating degradation. 
     The fundamental frequency may be a high fundamental frequency. The high fundamental frequency may correspond to a skin penetration depth that is less than or equal to the smallest coating thickness, so a coating thickness change outside of the smallest coating thickness does not affect the harmonic frequency components. The method can be repeated for a plurality of selected high fundamental frequencies, with the selected frequencies ranging from between a frequency with a skin penetration depth that is near the surface of the coating to between a frequency with a skin penetration depth equal to or greater than the entire coating thickness. The amplitudes of the harmonic signals at the selected frequencies may then be compared to profile coating degradation as a function of depth. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
     FIGS. 1A,  1 B and  1 C show the distortion of magnetic induction in a ferromagnetic material caused by hysteresis and nonlinearity. 
     FIG. 2 shows a block diagram of a nonlinear harmonics sensing system in accordance with the present inventive concept. 
     FIG. 3 shows a block diagram of an alternative embodiment of a nonlinear harmonics sensing system with the present inventive concept. 
     FIG. 4 shows a plot of nonlinear harmonics measurement data from a metallic coated turbine blade with coating degradation in accordance with the present inventive concept. 
     FIG. 5 a  shows a plot of nonlinear harmonics response to probe liftoff from a coated surface with varying amounts of coating degradation in accordance with the present inventive concept. 
     FIG. 5 b  shows the data from FIG. 5 b  after phase shifting in accordance with the present inventive concept. 
     FIG. 6 shows a typical nonlinear harmonics sensor probe configuration in accordance with the present inventive concept. 
     FIG. 7 is a flowchart of a method of analyzing the harmonic signal to detect coating degradation in accordance with the present inventive concept. 
     FIG. 8 is a flowchart of an alternative method of analyzing the harmonic signal to detect coating degradation in accordance with the present inventive concept. 
     FIG. 9 is a flowchart of a method of determining the threshold level signal and constructing a calibration curve in accordance with the present inventive concept. 
     FIG. 10 is a flowchart of an alternative method of analyzing the amplitude of the harmonic signal to detect coating degradation in accordance with the present inventive concept. 
     FIG. 11 is a flowchart of a method of using a high fundamental frequency to detect coating degradation in accordance with the present inventive concept. 
    
    
     DETAILED DESCRIPTION 
     Turning now to FIGS. 1A,  1 B and  1 C, the distortion of magnetic induction in a ferromagnetic material caused by hysteresis and nonlinearity is shown. When a sinusoidal external magnetic field H,  102  in FIG. 1A is applied to a ferromagnetic material, the resulting magnetic induction, B  102  in FIG. 1B is not sinusoidal but distorted because of the magnetic hysteresis and nonlinear permeability of the material which is shown in FIG.  1 C. This distorted waveform  102  of the magnetic induction, or equivalently the voltage induced in a coil by the magnetic induction, contains odd harmonic frequencies of the applied magnetic field. The amount of harmonic component depends on the shape of the hysteresis loop  103  (FIG.  1 C). 
     The nonlinear harmonics techniques may be implemented with the arrangement shown in FIG.  2 . FIG. 2 is a block diagram of a nonlinear harmonics sensing system used on a workpiece  201  having a protective coating  214 . The magnetic field is applied to the coating  214  by an excitation coil  202 . The resulting magnetic induction is measured with a magnetic field sensor such as a sensing coil  203 . A sinusoidal current of a given frequency is supplied to the excitation coil  202  using a signal generator and power amplifier  204 . If the coating  214  is not degraded it remains nonferromagnetic, so no voltage at harmonic frequencies is induced in the sensing coil  203 . Therefore, there is no signal detected and there is no harmonic frequency content. When the coating  214  has degraded, it becomes ferromagnetic and voltages at harmonic frequencies are induced in the sensing coil  203 , which is amplified and its harmonic frequency component, typically the third harmonic, is determined using a spectrum analyzer or harmonic lock-in amplifier  205  referenced to the driving waveform. The phase reference signal  207  is passed through a frequency multiplier  213  that multiplies the fundamental or excitation frequency (in this case by three) and passes it to the harmonic lock-in amplifier  205 . The induced voltage in the sensing coil  203  is also output to the fundamental lock-in amplifier  210 . A phase reference signal  207  is sent from the signal generator and power amplifier  204  to the fundamental lock-in amplifier  210 . Prior to being passed to the harmonic lock-in amplifier  205 , the sensor output may be passed through a high pass or bandpass filter  206  to reduce the fundamental component. Simpler configurations such as bandpass filtering the harmonic frequency and detecting the output could also be used. Two orthogonal signal outputs, an in-phase output  208  and a quadrature output  209  are output from the harmonic lock-in amplifier  205 . A fundamental in-phase signal output  211  and a fundamental quadrature signal output  212  are output from the fundamental lock-in amplifier  210 . The induced voltage in the sensing coil  203  contains odd numbered harmonics, with the third harmonic frequency having the highest amplitude among the harmonics. In general, the amplitude of harmonics higher than the third are quite small. Therefore, generally the amplitude of the third harmonic frequency is used to determine coating degradation and to characterize the amount of degradation, however other harmonic frequencies may also be used. The arrangement in FIG. 2, may also consist of a plurality of excitation coils and sensing coils that may be placed in different locations on the workpiece  201  to characterize coating  214  degradation at different locations along the surface of the workpiece  201 . The harmonic and fundamental in-phase and quadrature signal outputs  208 ,  209 ,  211  and  212  are fed to a display device  215  which may be a meter or other type device which displays the amplitude of any signal output. 
     FIG. 3 shows a block diagram of an alternative embodiment of the nonlinear harmonics sensing system. FIG. 3 is similar to the embodiment shown in FIG. 2 with the exception that the harmonic and fundamental in-phase and quadrature signal outputs  208 ,  209 ,  211  and  212  are fed first to a multi-channel analog digital converter  314 . After conversion, the date may be recorded on a data storage device  315 , such as a solid state, disk or tape data storage device. The data may also be displayed on a computer controlled display device  316 . The computer  316  may also be used for further analysis of the harmonic and fundamental signal data to detect and characterize coating degradation, display the results and store the results within the computer  316  or on a data storage device  315 . and then display. 
     FIG. 4 shows a plot of nonlinear harmonics measurement data from a metallic coated turbine blade with coating degradation. The blade has a CoCrAlY coating applied. The plot of the measurements shows the magnitude of the third harmonic  404  obtained with 10 kHz excitation versus coating degradation  405 . The measurements were taken at numerous locations along a line on the surface of the blade. The blade was sectioned metallurgically and its condition was found to range from little or no degradation  401  to 100-percent degradation  402 . As the coating begins to degrade  403  until it is 100-percent degraded  402 , the nonlinear harmonics response increased dramatically in response to the degradation. 
     FIG. 5 a  shows a plot of nonlinear harmonics response to probe liftoff from a coated surface with varying amounts of coating degradation  501 - 504 . The nonlinear harmonics response is shown as the quadrature component  505  plotted versus the in-phase component  506 . The liftoff lines  507  on the graph are at an angle of approximately 45 degrees from either the quadrature  505  or in-phase axis  506 . A changed in liftoff  507 , as well as a change in coating degradation, will affect both the quadrature  505  and in-phase components  506 . 
     FIG. 5 b  shows the data from FIG. 5 b  after phase shifting in accordance with the present inventive concept. The nonlinear harmonics response is shown as the quadrature component  508  plotted versus the in-phase component  509 . In FIG. 5 b , the data has been rotated by 45 degrees in the clockwise direction by shifting the phase. The liftoff lines  510  are now approximately horizontal. This means that a change in liftoff  510  will only affect the in-phase component  509 . Using this rotation, the quadrature component  508  can then be monitored for changes in coating degradation, independently of liftoff  510  variations will occur primarily in the in-phase component  509 . 
     FIG. 6 shows a typical nonlinear harmonics sensor probe configuration. The sensing coil  601  and excitation coil  602  are wrapped around a ferrite core  603 , and are placed in close proximity to the workpiece  604 . 
     FIG. 7 is a flowchart of a method of analyzing the harmonic signal for coating degradation. The amplitude of the in-phase and quadrature harmonic signal components is analyzed  701 . The amount to phase shift the in-phase and quadrature harmonic signal components is determined  702  and the in-phase and quadrature harmonic signal components are shifted by a selected number of degrees  703  to remove the liftoff signal components with the resulting signal indicating the areas of coating degradation. The amount to phase shift the signal is determined such that the result after phase shifting is similar to that shown in FIG. 5 b , where the liftoff components are removed from the quadrature harmonic signal component. 
     FIG. 8 is a flowchart of an alternative method of analyzing the harmonic signal to detect coating degradation. The amount to phase shift the in-phase and quadrature harmonic signal components is determined  801 . The in-phase and quadrature harmonic signal components are shifted by a selected number of degrees  802  to remove the liftoff signal components with the resulting signal indicating the areas of coating degradation. The amplitude of the in-phase and quadrature harmonic signal components is compared to a threshold level  803 . If the components are greater than the threshold level  804 , the amount of coating degradation is characterized using a calibration curve  805 . If the components are less than the threshold level  804 , the signal is not of interest and processing ends. 
     FIG. 9 is a flowchart of a method of determining the threshold level signal and constructing a calibration curve. Using a specimen of the same material as the workpiece with a known amount of coating degradation  901 , a nonlinear harmonics sensing device is placed on the specimen  902 . A time varying current is supplied to the sensing device  903 . A signal caused by an induced magnetic field in the specimen is detected  904 . The harmonic component of the signal is selected and amplified  905  and a harmonic signal is output  906 . The process of steps  901  through  906  is then repeated for a plurality of specimens with known amounts of coating degradation. After all of the specimens have been measured, a calibration curve is constructed from the known coating degradation of the specimens  907  and a threshold signal level is constructed  908 . 
     FIG. 10 is a method of analyzing the amplitude of the harmonic signal to detect coating degradation. It is a method of distinguishing coating degradation components from liftoff signal components by scaling the harmonic value to the fundamental value. This method accounts for variations in the fundamental frequency amplitude (also called the excitation frequency) that affect the amplitude of the measured harmonic signal. Using the components of the harmonic signal with coating degradation and probe liftoff signal components  1001  and the fundamental signal components  1002 , the harmonic signal is scaled to the fundamental signal for each area of interest  1003  with the resulting signal having substantially all the liftoff signal components removed and substantially all the coating degradation signal components retained. This method may be accomplished by using the fundamental frequency where liftoff has occurred to scale the harmonic signal so as to correct for liftoff changes. If the fundamental frequency is reduced in amplitude, the amplitude of the resulting harmonic signal in that same position is correspondingly reduced. The result is a scaled signal having most of the liftoff signal components removed and the coating degradation components retained. Other methods of signal scaling may be used to distinguish liftoff signal components from coating degradation signal components. 
     FIG. 11 is a flowchart of a method of using a high fundamental frequency to detect coating degradation. The magnetic field produced by the nonlinear harmonics probe penetrates to different depths at different frequencies. Because penetration depth of the coating is inversely proportional to the square root of the frequency and is calculated using a known skin depth equation, high frequencies can be chosen to limit the skin penetration depth from near the surface of the coating through the entire coating thickness. A time varying current is supplied at a high fundamental frequency to an excitation coil  1100 , positioned in close proximity to a surface of a workpiece having a metallic coating. If a voltage at harmonic frequencies signal is induced in a sensing coil  1101 , it is amplified and its harmonic frequency components are output  1102  and converted to a harmonic signal with an in-phase signal component and a quadrature signal component  1103 . A phase reference signal component is output to an amplifier which generates in-phase and quadrature fundamental signal components  1104 . The harmonic signal and phase reference signal are stored  1105  and the harmonic signal amplitude is analyzed to detect coating degradation  1106 . In one method of use, a single high fundamental frequency may be used (step  1100 ) which may correspond to a skin penetration depth that is less than the smallest thickness of the coating, so a coating thickness change in the coating outside of the smallest coating thickness does not affect the harmonic frequency components. By selecting the high fundamental frequency to be used, the depth of the sensing can be changed and sensitivity to variations in the thickness of the coating can be eliminated. 
     Alternatively, by performing nonlinear harmonics measurements at a number of selected high fundamental frequencies and comparing the amplitudes of the harmonic signals at those selected frequencies, coating degradation as a function of depth may be profiled. Therefore, in this embodiment, steps  1100  to  1105  may be repeated for a selected number of high fundamental frequencies, with the frequencies ranging between a frequency with a skin penetration depth that is near the surface of the coating to a frequency with a skin penetration depth equal to or greater than the entire coating thickness. If multiple high frequencies are used, the amplitudes of the resulting harmonic signals  1106  of each selected frequency can then be compared and used to profile coating degradation as a function of depth. 
     Another way of analyzing the data is to determine coating degradation while ignoring other interfering signals from other parameters such as those from liftoff and coating thickness. This may be done by performing a calibration where the coating degradation signals are measured for different degrees of degradation and the interfering signals are also measured. A least squares fit is then made to the data at different frequencies, but is fit only to the coating degradation response without regard to the interfering signals. This same approach can be taken using the fundamental as one of the frequencies. The equation is typically a polynomial of a given degree. An example of a second degree fit using two frequencies is: 
     
       
           D=C 1+ C   2   R   1   +C   3   R   1   2   +C   4   X   1   +C   5   X   1   2   +C   6   R   2   +C   7   R   2   2   +C   8   X   2   +C   9   X   2   2   
       
     
     where D is the amount of coating degradation, R1 and R2 are the in-phase components and X1 and X2 are the quadrature components, respectively of the two frequencies and C1 through C9 are weighting constants for the least squares fit.