The present disclosure relates generally to an eddy current based method of measuring the thickness of a coating.
Coating thickness is a variable that plays an important role in product quality, process control, and cost control. Measurement of coating thickness can be done with many different instruments. The issues that determine what method is best for a given coating measurement include the type of coating, the substrate material, the thickness range of the coating, the size and shape of the part, and the cost of the equipment. Nondestructive thickness testing methods such as ultrasonic pulse-echo techniques, magnetic pull-off or electromagnetic induction based techniques using magnetic film gages, and eddy current based techniques are commonly used to measure the thickness of coatings in the industry.
Eddy current based techniques are typically used to measure the thickness of nonconductive coatings on nonmagnetic and conductive substrates. A coil of fine wire conducting an alternating current is used to set up an alternating magnetic field at the surface of the instrument's probe. When the probe is brought in contact with the surface of the coating, the alternating magnetic field will set up eddy currents on the surface of the conductive substrate. The coating acts as a spacer between the probe and the conductive substrate. As the distance between the probe and the conductive base metal increases, the eddy current field strength decreases because less of the probe's magnetic field can interact with the base metal. Electrical impedance, which is the total opposition that a circuit presents to alternating current, is used as a measure of the eddy current field strength. Electrical impedance, which is measured in ohms, includes three components—resistance, inductive reactance, and capacitive reactance. Since typical eddy current probes have very low capacitance, the capacitive reactance component can be ignored. The resistance and the inductive reactance (“reactance”) components of the impedance are out of phase, so the impedance is the vector sum of the resistance and reactance components. Typically, impedance measurements obtained from eddy current probes are displayed as an impedance plane plot, which is a graph with resistance on the x-axis and the reactance on the y-axis.
Specialized eddy current coating thickness gages that operate on this principle and display the thickness of a coating on an LCD screen are available to measure the thickness of nonconductive coatings on nonmagnetic conductive substrates. These gages use internal calibration curves to correlate the measured impedance magnitudes to a thickness value. If the phase information of the measured impedance is also recorded, thickness of conductive coating on ferromagnetic substrates may be obtained as well. A more versatile eddy current flaw detector may also be used to measure coating thickness using calibration specimens. The calibration specimens are used to establish calibration curves that plot the variation of the instruments response to coating thickness. The instruments response to a sample having an unknown coating thickness is then obtained using the calibration curve. Common practices of eddy current based coating thickness measurement are described in ASTM B244 standards for nonconductive coatings on nonmagnetic substrates. Another method utilizing an eddy current flaw detector for coating thickness measurement is described in U.S. Pat. No. 6,762,604 B2 issued to Le (“the '604 patent”). In the method of '604 patent, an eddy current monitoring system is used to measure the thickness of a coating on a semiconductor wafer using calibration curves. While the method of ASTM B244 and the '604 patent may be suitable to measure the thickness of a coating on a substrate having a constant conductivity, it may not be suitable to measure the thickness of a coating when the conductivity of the substrate changes due to the deposition process.
The disclosed method of thickness measurement is directed to overcoming one or more of the problems set forth above.
In one aspect, a method of configuring an eddy current detector to measure a thickness of a coating on a conductive substrate is disclosed. The eddy current detector may be adapted to measure impedance of the coated substrate. The impedance may include an inductive reactance component and a resistance component. The method may include establishing an impedance plane plot using a computer. The impedance plane plot may indicate a variation of the impedance of the coated substrate as a function of working frequency, coating thickness, coating electrical conductivity and substrate electrical conductivity. The method may include determining a rotation angle. The rotation angle may be an angle of rotation of the impedance plane plot that will make the inductive reactance component of the impedance substantially insensitive to substrate electrical conductivity within a coating thickness range. The method may further include establishing a calibration curve that is substantially insensitive to substrate electrical conductivity using the rotation angle. The calibration curve may be a curve that relates the inductive reactance component of the impedance to coating thickness.
In another aspect, a method of configuring an eddy current flaw detector to determine if a thickness of a coating on a substrate is within an upper limit and a lower limit is disclosed. The electrical conductivity of the substrate may vary as a function of the coating thickness. The method may include measuring an impedance of a coated substrate using the detector. The impedance may include an inductive reactance component and a resistance component. The method may include determining a rotation angle. The rotation angle may be an angle of rotation of the impedance that makes the inductive reactance component of the impedance substantially insensitive to the electrical conductivity of the substrate within the upper limit and the lower limit of coating thickness. The method may also include inputting the rotation angle into the eddy current flaw detector to adjust the measured impedance. The method may also include establishing a window on the eddy current flaw detector using the measured impedance. An upper end of the window may be representative of the upper limit of thickness and the lower end of the window may be representative of the lower limit of thickness. The method may further include monitoring the measured impedance using the window.
In yet another aspect, a method of measuring a thickness of a coating on a substrate using an eddy current detector is disclosed. The eddy current detector may be configured to measure an impedance of the coated substrate. The impedance may include an inductive reactance component and a resistance component. The electrical conductivity of the substrate may varying as a function of the coating thickness. The method may include determining a rotation angle using a computer modeling approach or use a limited number of calibration blocks with known coating thickness and substrate conductivity. The rotation angle may be an angle of rotation of the impedance that will make the inductive reactance component of the impedance substantially insensitive to the electrical conductivity of the substrate within a range of coating thickness. The method may also include establishing a calibration curve using the computer or calibration blocks with the determined rotation angle to relate coating thickness to the inductive reactance component of the impedance. The method may further include determining the thickness of the coating on the coated substrate by comparing the inductive reactance component of a measured impedance of the coated substrate with the calibration curve.
In a further embodiment, a system to measure a thickness of a coating on a substrate, is disclosed. The system includes an eddy current detector adapted to measure impedance of the coating on the substrate. The impedance includes an inductive reactance component and a resistance component. The system also includes a computer. The computer may be configured to establish an impedance plane plot. The impedance plane plot may indicate a variation of the impedance of the coating on the substrate as a function of coating thickness and substrate electrical conductivity. The computer may also be configured to determine a rotation angle. The rotation angle may be an angle of rotation of the impedance plane plot that will make the inductive reactance component of the impedance substantially insensitive to substrate electrical conductivity within a coating thickness range. The computer may further be configured to establish a calibration curve that is substantially insensitive to substrate electrical conductivity using the rotation angle. The calibration curve may be a curve that relates the inductive reactance component of the impedance to coating thickness.
After depositing coating 14 on substrate 12, an eddy current probe may be used to non-destructively measure the impedance of the coated substrate. Any commercially available eddy current probe may be used to measure the impedance of the coated substrate. The measured impedance value changes with the thickness of coating 14. Previously established calibration curves may be used to determine the thickness of the deposited coating 14 from the measured impedance value. Calibration curves are curves that plot the variation of impedance with coating thickness. In the current disclosure, the reference to curves that plot data describe both figures that graphically represent the relationship between two variables, and a formula that describe a mathematical relationship between the two variables. From a previously established calibration curve, the thickness of a deposited coating 14 may be determined from a measured impedance value using known techniques (extrapolation, interpolation, curve fitting, etc.). The calibration curves may be established either experimentally or by numerical modeling. To establish a calibration curve experimentally, coatings 14 having different thicknesses are deposited (typically using the same coating process) on substantially similar substrates 12, and their impedance measurements obtained using the eddy current probe. A calibration curve may also be obtained by numerical simulations techniques, such as, for example, finite element based techniques. While such calibration curves (discussed above) provide relatively error free thickness measurements when the substrate 12 and the coating 14 electrical conductivity remains a constant, for the reasons discussed previously, errors may be introduced when the substrate 12 and/or coating 14 electrical conductivity changes during the deposition of the coating 14.
As discussed in the background section, an impedance value measured by an eddy current probe includes two main components, resistance and reactance, that may be represented in an impedance plane plot.
In the impedance plane plot of
Due to the change in substrate conductivity as a result of the high temperature deposition process, the conductivity of the substrate 12 may change from K1 when the coating thickness is tA, to K2 when the coating thickness is tB, and to K3 when the coating thickness is tC. Therefore, as a result of the change in substrate conductivity, the measured impedance values at coating thickness tA, tB, and tC may be A, B′, C″, respectively. As discussed above, changing coating 14 conductivity during the deposition process may also affect the measured impedance values in a manner similar to that discussed above. Since the change in substrate conductivity with coating thicknesses is not known, determining coating thickness using constant conductivity calibration curves (such as, curves C1, C2, and C3) may be error-prone. The method described in the instant application minimizes this error by accounting for the variation in substrate and/or coating conductivity in the calibration curves.
By rotating the impedance curves by an angle −θ1, the constant thickness lines in segment Y1 (that is, between ta and tb) may be made parallel to the x-axis.
The disclosed eddy current based method of measuring the thickness of a coating may be applicable to measure the coating thickness in any application. The disclosed technique may be especially useful to measure the coating thickness in an application where the substrate and/or coating electrical conductivity changes between thickness measurements. The disclosed methods may minimize the effect of the substrate and/or coating conductivity on one of the components of the measured impedance by rotation of the impedance plane plot by a suitable rotation angle.
The rotation angle for any coating thickness range (such as, for example, θ1 for a thickness range between ta and tb, and θ2 for a thickness range between tc and td) may be obtained by experimentation or by numerical simulation (such as, for example, on a computer).
In embodiments where a computer program is used, the measured impedance values may be input into the program to determine the rotation angle θ1 to make the reactance component substantially insensitive to substrate conductivity. In these embodiments, the computer program may also be configured to rotate the impedance plane plot by the rotation angle θ1, and translate a measured impedance value to a coating thickness after the rotation. In some embodiments, instead of experimentally measuring impedance values on calibration samples (step 110), numerical simulations may be used to determine the impedance values of coatings of different thicknesses on different conductivity substrates. These numerically obtained impedance values may be used to obtain the desired rotation angle θ1. The computer program may also be configured to use this rotation angle θ1 to correct a measured impedance value, and translate the corrected impedance value to a coating thickness. The testing frequency of instrument/probe may be carefully selected to minimize thickness measurement errors and/or to simplify the calibration process.
In some embodiments, the determined rotation angle θ1 may be used with a commercially available eddy current flaw detector to convert the flaw detector into a go/no-go coating thickness monitor. Such a thickness monitor may be used to quickly verify whether the thickness of a coating on a substrate is within acceptable limits (such as, for example, within limits t′a and t′b). An exemplary process of using a commercially available eddy current flaw detector as a go/no-go thickness monitor is described below with reference to
Minimizing the error induced in the coating thickness measurement due to substrate conductivity enables an eddy current probe to be used to measure the coating thickness on a variable conductivity substrate. The error is minimized by determining a rotation angle that makes one of the components of the measured impedance substantially insensitive to substrate conductivity, and rotating the calibration curve of the eddy current probe by the determined angle.
It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed eddy current based method of measuring the thickness of a coating. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed method. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.
Number | Name | Date | Kind |
---|---|---|---|
5963031 | de Halleux et al. | Oct 1999 | A |
6602724 | Redeker et al. | Aug 2003 | B2 |
6715640 | Tapphorn et al. | Apr 2004 | B2 |
6762604 | Le | Jul 2004 | B2 |
6806703 | Le Bihan et al. | Oct 2004 | B2 |
6815947 | Scheiner et al. | Nov 2004 | B2 |
6878036 | Hanawa et al. | Apr 2005 | B2 |
7019519 | Le | Mar 2006 | B2 |
7714572 | Tada et al. | May 2010 | B2 |
8078419 | Kobayashi et al. | Dec 2011 | B2 |
20040138838 | Scheiner et al. | Jul 2004 | A1 |
20050017712 | Le | Jan 2005 | A1 |
20070077362 | Ruzzo et al. | Apr 2007 | A1 |
Number | Date | Country |
---|---|---|
2007-263981 | Oct 2007 | JP |
10-2011-0079862 | Jul 2011 | KR |
Entry |
---|
Hagemaier, D.J.; “Eddy current impedance plane analysis, Materials Evaluation,” 41 (1983) 211-218—abstract. |
Moulder, J., Uzal, E., Rose, J.; “Thickness and conductivity of metallic layers from eddy current measurements,” Rev. Sci. Instrum., 63 (1992) 3455-3465. |
Number | Date | Country | |
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20130132012 A1 | May 2013 | US |