Method and sensor assembly for measuring the thickness of at least one coating layer - Patents.com

Abstract

The present invention relates to a method and sensor assembly for non-destructively measuring the thickness of a coating layer formed on a substrate of a workpiece, wherein a first coating layer made of an electrically conductive and either diamagnetic or paramagnetic material is optionally disposed on a second coating layer made of a ferromagnetic material, the method including the steps of: (a) placing an inductance sensor head near the workpiece; (b) measuring inductance at multiple frequencies within a frequency range to obtain an inductance spectrum, and measuring inductance at a predetermined frequency that is in the lower third of the frequency range or below a lower limit of the frequency range; (c) determining a characteristic frequency from the inductance spectrum, the frequency at which the phase angle of inductance is -90°; (d) calculating the thickness of the first coating layer from the characteristic frequency; and (e) calculating the thickness of the second coating layer from the inductance at the predetermined frequency and the calculated thickness of the first coating layer.

Description

[Technical Field]

[0001] The present invention relates to a method and sensor assembly for measuring the thickness of at least one coating layer formed on a substrate of a workpiece, wherein the substrate optionally comprises a first coating layer comprised of an electrically conductive and either diamagnetic or paramagnetic material, the first coating layer being disposed over a second coating layer comprised of a ferromagnetic, ferrimagnetic, or antiferromagnetic material. [Background technology]

[0002] Eddy current detectors have been developed to nondestructively measure the thickness of electrically conductive coating layers on substrates. For example, Patent Document US2013 / 0132012-A1 discloses an eddy current detector configured to measure the thickness of a coating on a substrate, the eddy current detector being configured to measure the impedance of the coated substrate, the impedance including an inductive reactance component and a resistive component. The eddy current detector is configured by using a computer to establish an impedance plane plot showing the variation in impedance of the coated substrate as a function of coating thickness and substrate electrical conductivity, and establishing a calibration curve that is substantially insensitive to the substrate electrical conductivity. Patent Document US2014 / 324384A1 also discloses a method for measuring the thickness of a coating layer formed on a substrate, the coating and substrate being made of first and second different electrically conductive materials. The method includes positioning an induction means relative to the coating layer and supplying an AC electrical signal to the induction means to induce a magnetic field in the workpiece.

[0003] Another known method for measuring the thickness of a coating layer uses Hall sensors, as described for example in patent document EP 028487-A1 and patent document US RE 35,703.

[0004] However, these known methods can only determine the thickness of a single coating layer on a substrate. However, there are important applications in which two coating layers are arranged one on top of the other on a substrate. For example, wear-resistant coatings on copper substrates for continuous casting molds used in steel casting plants typically consist of two layers: the top layer is composed of an electrically conductive ceramic or cermet coating, and the intermediate layer between the top layer and the copper substrate is composed of a ferromagnetic Ni-based alloy. The substrate is composed of copper or a copper-based alloy, which is electrically conductive and diamagnetic. The top coating layer may be thinner than the intermediate layer, and the total thickness of the coating layers can vary from about 100 μm to several mm. After a mold is refurbished, the thickness of the individual layers may vary and may not be consistent at different locations across the substrate. In particular, after a fresh Ni-based coating layer is applied during a mold repair process, the Ni-based coating layer may be thicker than the recommended thickness. However, this can affect the measurement accuracy of eddy current-based mold level sensors used to control the molten steel meniscus level of continuous casting molds during casting operations. This can cause serious instability problems during the casting process. Therefore, it is important to measure the thickness of the mold, especially the Ni-based coating layer, before using it.

[0005] However, the above-mentioned known solutions based on magnetometers or eddy current sensors can only measure the thickness of a single coating layer, i.e., the top coating layer, and cannot be used to measure the thickness of an individual layer of an electrically conductive coating on top of a ferromagnetic coating.

[0006] Patent document CN101532816-A describes an eddy current-based approach for measuring two conductive, non-ferromagnetic layers on a substrate, where the electrical conductivities of the two layers can be distinguished from each other. This method relies on an eddy current detection device based on a giant magnetoresistive sensor and an intelligent algorithm that includes a neural network that needs to be trained in a time-consuming training process and may not converge. This method is not suitable for measuring the thickness of ferromagnetic coatings.

[0007] Patent document CN103852000-A1 also relates to an eddy current detection device, which consists of a drive coil and a Hall sensor. The drive coil is driven by an alternating current (AC) current to generate a magnetic field, and a Hall sensor probe is positioned between the drive coil and a test object to sense changes in the magnetic field level. The changes in the magnetic field level at two frequencies are related to the thicknesses of two conductive coating layers by a quadratic function, which is then solved to determine the thicknesses of the two coating layers. The quadratic function used here is suitable for two electrically conductive (but non-ferromagnetic) coating layers that do not have the same conductivity. However, this prior art is no longer applicable to a conductive coating layer on a ferromagnetic coating layer.

[0008] Therefore, there is no solution available for non-destructively measuring the thickness of a ferromagnetic layer disposed between a substrate and an upper layer composed of an electrically conductive material. Summary of the Invention [Problem to be solved by the invention]

[0009] It is therefore an object of the present invention to provide a measurement method and sensor assembly for measuring the thickness of two or more coating layers on a substrate, which is easy to implement and perform, and which provides reliable thickness measurements.

[0010] It is a further object of the present invention to provide a method and sensor assembly for measuring the thickness of a ferromagnetic coating on a metal substrate. [Means for solving the problem]

[0011] These objects are achieved or exceeded by a method for non-destructively measuring the thickness of at least one coating layer formed on a substrate of a workpiece according to claim 1 and a sensor assembly according to claim 11. Further advantageous features and embodiments are set out in the dependent claims.

[0012] According to a first aspect, the present invention provides a method for measuring the thickness of at least one coating layer formed on a substrate of a workpiece, comprising: the workpiece optionally comprises a first coating layer composed of a material that is electrically conductive and either diamagnetic or paramagnetic; the first coating layer is disposed on a second coating layer made of a ferromagnetic material, a ferrimagnetic material, or an antiferromagnetic material; the second coating layer is disposed between the first coating layer and the substrate; The method comprises the steps of: (a) placing an inductance sensor head in close proximity to the workpiece; (b) measuring the inductance at a plurality of frequencies within a frequency range to obtain an inductance spectrum, and measuring the inductance at a predetermined frequency that is in the lower third of the frequency range or below a lower limit of the frequency range; (c) determining a characteristic frequency from the inductance spectrum, wherein the characteristic frequency is the frequency of a zero crossing of the real part of the inductance, or the frequency at which the imaginary part of the inductance is a minimum, or the frequency at which the phase angle of the inductance is −90°; (d) calculating a thickness of the first coating layer from the characteristic frequency, wherein the thickness may be zero; (e) calculating the thickness of the second coating layer from the inductance at the predetermined frequency and the calculated thickness of the first coating layer. The method includes:

[0013] The measurement method of the present invention uses an inductance sensor head, e.g., an inductance coil sensor, to measure the thickness of individual layers of a single- or double-layer coating on a substrate, where one layer is composed of a ferromagnetic material. It has been found that by analyzing the inductance spectrum at multiple frequencies, the thickness of a single or multiple coating layers can be estimated in a simple and accurate manner. The idea of ​​the present invention can be extended to three or more coating layers.

[0014] The upper coating layer, also referred to as the first coating layer, is not ferromagnetic, but may be diamagnetic or paramagnetic. The first coating layer is electrically conductive, at least to a low degree. The first coating layer has a resistivity of, for example, less than 1 Ω·m, preferably less than 1×10 at 20°C. -3 The electrical resistivity ρ may be less than Ω·m, more preferably less than 1 μΩ·m. The lower limit of the electrical resistivity is 1.59×10 -8The resistivity may be Ω·m (silver resistivity). An example of the material for the first coating layer is tungsten carbide. The method of the present invention can also be applied to workpieces that do not have a first coating layer, or to workpieces where the first coating layer is present in some locations but not others, for example, when it has been removed by abrasion. In these cases, the thickness of the first coating layer is detected as zero. The method of the present invention can also be applied to workpieces where the first coating layer is composed of an insulator, and the thickness is also detected as zero. If present, the first coating layer is disposed on top of a second coating layer composed of a ferromagnetic, ferrimagnetic, or antiferromagnetic material, with the second coating layer disposed between the first coating layer (if present) and the substrate. The measurement method of the present invention can be used when the second coating layer is disposed directly on the substrate, but can also be used when there is an additional layer disposed between the second coating layer and the substrate. The second coating layer, also referred to as the lower layer, is preferably ferromagnetic. Hereinafter, the term "ferromagnetic" is used to include ferrimagnetic and antiferromagnetic materials. The second coating layer is also preferably electrically conductive, at least to a low degree, e.g., less than 1 Ω·m, preferably less than 1×10 at 20° C. -3 The electrical resistivity ρ may be less than Ω·m, more preferably less than 1 μΩ·m. The lower limit of the electrical resistivity is 1.59×10 -8 The resistivity may be Ω·m (resistivity of silver). The substrate may be made of any material, both electrically conductive and non-conductive, i.e., it may be an insulator. In an advantageous application of the measuring method of the present invention, the substrate is made of an electrically conductive material. The substrate is preferably not ferromagnetic. The substrate may be diamagnetic or paramagnetic. The substrate may be made of a non-ferromagnetic metal or metal alloy.

[0015] The measurement method of the present invention includes a step of positioning an inductance sensor head in close proximity to a workpiece, preferably by installing the inductance sensor head near the side or surface of the substrate on which the coating layer is formed. "In close proximity" may mean a distance of less than about 1 mm, preferably less than about 200 μm. Preferably, the inductance sensor head is installed at a specified distance from the top coating layer (i.e., the first coating layer if present, or the second coating layer if not present). The inductance sensor head may be installed directly on the top layer. The inductance sensor head may be held at a specified distance from the top coating layer by a spacer, which may be made of an insulating material such as plastic, ceramic, or rubber, and the specified distance may be less than 1 mm, for example, 1 μm to 200 μm. The inductance sensor head may be configured to measure the thickness of the coating layer at one location on the workpiece and may be moved across the workpiece to measure the thickness at additional locations.

[0016] The workpiece may be any kind of tool, industrial equipment, vehicle part, or jewelry. Therefore, the measurement method of the present invention can be applied in various industries and applications where a coating layer needs to be measured on a substrate, preferably on a metal substrate, for example, a turbine blade, in nuclear power plants, the ship industry, the metal industry, or the jewelry industry. Preferably, the measurement method of the present invention is used to measure the coating thickness on a metal object, in particular on a coating thickness on copper or copper-based alloys that form a substrate as part of a continuous casting machine mold for continuous casting of steel.

[0017] The inductance sensor head may include a driving coil and at least one sensing coil, the driving coil being driven with an AC current to generate magnetic fields that alternate at multiple frequencies, particularly at the frequency of the AC current used to drive the driving coil. The at least one sensing coil senses changes in the magnetic field caused by the presence of a workpiece in the magnetic field. This can generate an inductance spectrum that is sensitive to the thickness of the coating layer.

[0018] The inventors have discovered that the characteristic frequency that can be determined from the inductance spectrum is sensitive almost exclusively to the thickness of the first (top) coating layer. In particular, the characteristic frequency is relatively independent of the thickness of the second ferromagnetic coating layer. The independence of the characteristic frequency on the thickness of the second coating layer can be further improved by downscaling the sensor coil of the inductance sensor head until the dependence is sufficiently low.

[0019] On the other hand, the inductance at a single predetermined frequency in the lower third of the frequency range or below the lower limit of the frequency range is sensitive to both the thickness of the first coating layer and the thickness of the second coating layer. This predetermined frequency is relatively low, for example, 10 Hz to 1000 Hz, preferably 50 Hz to 600 Hz, and more preferably 100 Hz to 300 Hz. Therefore, the predetermined frequency is also referred to as the "low frequency" hereinafter. A suitable low frequency can be determined depending on the electromagnetic properties of the ferromagnetic layer. In particular, the magnitude and real part of the inductance at low frequencies are highly sensitive to the thickness of the second coating layer, but the effect of the first coating layer needs to be corrected.

[0020] Therefore, the thicknesses of the two coating layers are estimated in at least two steps. In a first step (d), the thickness of the first coating layer is determined from the characteristic frequency of the inductance spectrum. This first layer thickness is then applied to compensate for the effect of the upper layer on the inductance at low frequencies. After this compensation, in a second step (e), the inductance at low frequencies (also called the "low frequency signal") can be used to determine the thickness of the second coating layer. These two steps involving data analysis can be performed by a data processing unit.

[0021] Thus, the present invention provides a measurement method capable of measuring the thickness of two coating layers on a substrate, preferably a metal substrate. The thicknesses of the two coating layers can be measured simultaneously using the same sensor. The sensor head is simple to construct and can be assembled for continuous scanning over a large coating area. In one embodiment, a measurement model requiring only four to six, preferably five, parameters can be used to estimate the coating layer thickness from the measured inductance value. Multiple reference samples with known coating thicknesses can be used in a calibration procedure to calibrate the measurement method and sensor assembly prior to measurement.

[0022] Specifically, the inductance sensor head is used to measure inductance at multiple frequencies within a frequency range to obtain an inductance spectrum, as well as to measure inductance at a predetermined frequency. The inductance may be the mutual inductance of the drive coil and the sense coil. The inductance is preferably measured as a complex value, i.e., having a magnitude and a phase angle, as well as real and imaginary parts. Inductance may be measured by measuring impedance. The inductance sensor head essentially functions like a metal detector, in which an alternating current is used to excite the drive coil, creating an oscillating magnetic field. This magnetic field induces eddy currents as it penetrates a workpiece near the sensor head, creating an opposing magnetic field. This secondary magnetic field causes an induced voltage in the sense coil, which can be recorded as an induction signal or induction. In an advantageous embodiment of the present invention, there may be two sense coils, and the recorded inductance is the difference in inductance between the two sense coils.

[0023] The inductance spectrum may be measured by the following steps: Before starting the actual measurement, preferably the (complex) inductance spectrum L0 may be measured at multiple frequencies when no workpiece is present. This step may be performed only once for the sensor head or once for each series of measurements, and the results may be stored, for example, in a computer or storage medium connected to a data processing unit that performs the data analysis steps.

[0024] After the sensor head is placed in close proximity to the workpiece to be measured, the (complex) inductance spectrum L1 at the same frequency point as L0 can be measured. The change in inductance spectrum ΔL caused by the workpiece can be calculated as ΔL = L1 - L0. This inductance change ΔL, or alternatively L1, can be used as the (usually complex) inductance spectrum to be used for further processing.

[0025] In the next step (c), the characteristic frequency is estimated from the inductance spectrum. The characteristic frequency can be determined as the frequency of the zero crossing of the real part of the inductance, i.e., the frequency point where the real inductance is zero, or the frequency where the imaginary part of the inductance is minimum, or the frequency where the phase angle of the inductance is -90°. In most embodiments, all three analysis methods result in the same frequency, i.e., they are alternative approaches to determining the characteristic frequency. Because the inductance spectrum can only be measured at a limited number of frequency points, the spectrum can be interpolated to determine the characteristic frequency. For example, interpolation can be performed on the inductance phase spectrum to determine the characteristic frequency point where the phase angle is -90°. Similarly, the real part of the inductance can also be interpolated to find the frequency where the real part is zero (zero crossing). In an alternative embodiment, a suitable parametric function can be fitted to the inductance spectrum to determine the characteristic frequency point.

[0026] The inventors have found that the characteristic frequency is highly independent of the properties and thickness of the second coating layer and can be used to determine the thickness of the first coating layer. This can be done in the next step (d) by simple calculation using a parametric model, the parameters of which may have been determined in a calibration procedure. The parametric model can be described by a parametric function, in particular a polynomial function, an exponential function, a logarithmic function, a power function, a root function, or a trigonometric function, or a combination thereof. The parametric model can preferably have one to six parameters, more preferably two to three parameters. If the first coating layer is not present or is composed of an insulating material, the resulting thickness of the first coating layer is zero.

[0027] In the next step (e), the thickness of the second coating layer is calculated from the inductance at a predetermined frequency and the calculated thickness of the first coating layer. This step also preferably uses a parametric model, the parameters of which can be determined in a calibration procedure. The parametric model can be described by a parametric function, in particular a polynomial function, an exponential function, a logarithmic function, a power function, a root function, or a trigonometric function, or a combination thereof. The parametric model preferably has one to six, more preferably two to four, parameters. This step (e) can be divided into several, in particular two, substeps. In the first substep, the inductance signal at a predetermined low frequency is processed to compensate for the effect of the first coating layer using the previously calculated thickness of the first coating layer. In the second substep, the thickness of the second coating layer is determined from the compensated inductance at low frequency obtained in the previous step.

[0028] Prior to the measurement method, a calibration procedure may be performed using a reference sample to obtain the parameters of the parametric function / model used in the measurement method.

[0029] According to one embodiment, the thickness of the first coating layer is calculated from the characteristic frequency by means of a first parametric function, the parameters of which have been determined in a calibration procedure, the first parametric function being in particular:

number

[0030] A "parametric function" can be any equation that defines a quantity (i.e., coating thickness) as a function of one or several independent variables called parameters. The parametric function [1] shown above is fitted to numerically simulated data, but may also be obtained from measured data. The invention is not limited to equation [1]; for example, other functions may provide a better fit for other coating materials.

[0031] In this embodiment, the thickness of the first coating layer is determined as a function of the characteristic frequency and only two parameters p1 and p2, which in this case can be obtained from a calibration procedure, which will be described in more detail below. The parametric model may be based on the results of a numerical simulation of the complex inductance, for example, using a first coating layer of a ceramic or cermet coating (e.g., tungsten carbide) and a second coating layer of a Ni-based alloy (e.g., a NiCo-based alloy). The parametric model presented herein was obtained by analyzing the results of a simulation using numerical calculations based on analytical eddy current modeling (Dodd-Deeds model) of a multilayer conductive structure. The simulation resulted in a hyperbolic dependence of the characteristic frequency on the thickness of the first coating layer, as expressed by the following equation:

[0032]

number

[0033] According to one embodiment, the thickness of the second coating layer is calculated from the magnitude or real part of the inductance at a predetermined frequency by a second parametric function, the parameters of which have been determined in a calibration procedure, and the second parametric function is in particular

number

[0034] Therefore, it was found that the inductance at relatively low frequencies is determined by the thickness of both the first and second coating layers. Knowing the thickness of the first coating layer allows the thickness of the second coating layer to be determined from a single parametric function. It was found that the thickness of the second coating layer is primarily indicative of the magnitude or real part of the inductance at a given frequency. Thus, Equation [2] above uses the magnitude of the inductance. An alternative approach to determining the thickness of the second coating layer is to use the real part of the inductance at low frequencies instead of the magnitude of the inductance. Similar models and approaches can be applied to measurements.

[0035] According to one embodiment, the thickness t2 of the second coating layer is: (e1) (first substep) calculating a predetermined frequency (f) using the thickness of the first coating layer as an input to a third parametric function, the parameters of which have been determined in a calibration procedure. low ) compensating for the effect of the first coating layer on the inductance in the (e2) (Second substep) determining the thickness of the second coating layer from the compensated inductance at a predetermined frequency using a fourth parametric function, the parameters of which have been determined in a calibration procedure. It is calculated as follows.

[0036] In a first sub-step, the effect of the first coating layer on the inductance can be compensated for by inputting t1 into the third parametric equation:

[0037]

number

[0038] where p3 and p4 are parameters obtained from the calibration procedure, and L comp is the compensated inductance. In one example, Equation 3 can be:

[0039]

number

[0040] In a second substep, the thickness of the first coating layer can be determined from the compensated inductance at a predetermined frequency using a fourth function: where p5 is a parameter obtained from the calibration procedure.

[0041]

number

[0042] In one example, Equation 4 can be:

[0043]

number

[0044] Therefore, for a given frequency f low The effect of the top coating layer on the inductance at a given frequency (which may be, for example, 200 Hz) is compensated for using Equation 3. In a next step, the thickness of the second coating layer can be determined from the compensated inductance at a given frequency using Equation 4. The calculations of Equations 3 and 4 may be performed in one step using Equation 2.

[0045] According to one embodiment, the inductance is measured at multiple frequencies within a frequency range, with the lower limit of the frequency range being 10 Hz to 1000 Hz, preferably 100 Hz to 500 Hz, and the upper limit of the frequency range being 5 kHz to 800 kHz, preferably 10 kHz to 400 kHz, more preferably 20 kHz to 100 kHz. The upper and lower limits of the frequency range can be adapted depending on the electromagnetic properties of the coating and substrate, as well as the expected thicknesses of the first and second coating layers. For example, in the case of a first coating made of a cermet coating and having a thickness of approximately 0.1 mm to 0.6 mm, and a second coating layer made of a Ni-based alloy and having a thickness of 0.3 mm to 1.3 mm, the frequency range can cover, for example, approximately 300 Hz to 20 kHz, since the characteristic frequency may lie within this range.

[0046] The predetermined frequency is preferably a low frequency, i.e., it is in the lower third of the frequency range or below the lower limit of the frequency range, preferably the lower quarter or fifth of the frequency range or below the lower limit of the frequency range. The predetermined frequency may be one of multiple frequencies within the frequency range. Thus, this inductance measurement can be used both to determine the characteristic frequency and to calculate the thickness of the second coating layer. The predetermined frequency may be 10 Hz to 1000 Hz, preferably 50 Hz to 600 Hz, more preferably 100 Hz to 300 Hz, e.g., 200 Hz. The sensor diameter may be configured so that the characteristic frequency is sensitive only to the upper layer (i.e., the first coating layer), while the low-frequency inductance is sensitive to both the first and second layers.

[0047] According to one embodiment, the inductance is measured at 4 to 32 frequencies within the frequency range, for example, 6 to 16 frequencies, for example, 8 frequency points. Therefore, only a limited number of measurement points are required to obtain an inductance spectrum. Alternatively, the inductance can also be obtained in a sweep across all frequencies within the frequency range. The frequency sweep can, for example, cover a frequency range from 1 Hz to 1 MHz within 10 seconds.

[0048] According to one embodiment, the method of the present invention comprises: (i) placing an inductance sensor head in close proximity to a plurality of calibration samples, the calibration samples having a plurality of different known thicknesses of first and second coating layers; (ii) for each calibration sample, measuring the inductance at multiple frequencies within the frequency range to obtain an inductance spectrum, and measuring the inductance at a predetermined frequency that is in the lower third of the frequency range or below the lower limit of the frequency range; (iii) determining a characteristic frequency from the inductance spectrum; (iv) determining parameters of the first parametric function from the characteristic frequencies of the plurality of calibration samples using the known thickness of the first coating layer; (v) determining parameters of a second parametric function from the inductances at the predetermined frequency of the plurality of calibration samples and the known thicknesses of the first and second coating layers; A calibration procedure including:

[0049] A set of calibration samples may be required to calibrate the measurement method of the present invention, particularly the data analysis steps (d) and (e), and / or a corresponding sensor assembly may be required to implement the measurement method of the present invention. The calibration samples, particularly their coatings and substrates, preferably have the same electrical conductivity and magnetic permeability as the target workpiece, and the coatings are preferably made of the same material. At least four calibration samples may be required, with at least two samples having different first coating layer thicknesses and at least two samples having different second coating layer thicknesses. From the characteristic frequencies of the calibration samples, the parameters of the first parametric function can be determined. For example, in Equation 1, p1 and p2 can be determined simply by solving two equations for two unknowns, i.e., by solving Equation [1] for different f0 values ​​at different first layer thicknesses. In one embodiment in which four calibration samples are used, the average f0 of all samples having the same first layer thickness can be used.

[0050] To find the parametric model of the second (or third and fourth) parametric function, first calculate the normalized magnitude of inductance |L at a given frequency for different thicknesses of the first coating layer. comp (f low )| can be calculated. This allows the parameter required to calculate the thickness of the second coating layer, i.e., parameter p5 in the exemplary equations described herein, to be determined. In the next step, the magnitude of the inductance |L(f low )|The magnitude of the compensated inductance |L comp (f low )|, the parameters p3 and p4 needed to convert || can be determined by fitting the referenced equation to the measured data using the known coating layer thickness.

[0051] One or more of the above parametric functions were discovered through numerical simulation using a cermet coating, specifically tungsten carbide, and a nickel-based alloy, specifically a nickel-cobalt alloy, as the first and second coating layers, respectively. Therefore, other parametric functions may be better models for other materials. However, as long as the first coating layer is non-ferromagnetic and the second coating layer is ferromagnetic, functions 1 through 4 are expected to provide reasonably good models for many different materials.

[0052] The calibration procedure may also include measuring the inductance at a plurality of frequencies within the frequency range and at a predetermined frequency when the inductance sensor head is not affected by any workpiece.

[0053] The measurement method of the present invention can be advantageously applied to workpieces in which the first coating layer has a thickness in the range of 0 μm to 3 mm, preferably 50 μm to 1.5 mm, and more preferably 0.1 mm to 0.6 mm, and the second coating layer has a thickness in the range of 50 μm to 3 mm, preferably 0.1 mm to 1.5 mm, and most preferably 0.3 mm to 1.3 mm. The measurement method has been found to be particularly sensitive within this thickness range. However, by adapting the sensor diameter, other thickness ranges can also be covered. The sensor diameter may be, for example, 5 mm to 50 mm, more preferably 10 mm to 30 mm. In the example described herein, the sensor diameter was 18 mm.

[0054] According to one embodiment, the first coating layer may be a ceramic or cermet coating. Cermets are composite materials composed of ceramic and metallic materials, such as tungsten carbide or zirconium carbide. The second coating layer may be composed of a nickel-based alloy, such as nickel-cobalt. However, the first coating layer may also be composed of other electrically conductive diamagnetic materials, and the second coating layer may also be composed of other ferromagnetic materials. The substrate may be composed of any material, such as copper, and may have a thickness of, for example, 1 mm to 20 mm. Of course, there is no upper limit to the substrate thickness, since the measurement method only requires access from one side of the workpiece. However, the method of the present invention is applicable to any material, particularly metals, in which the first coating layer is electrically conductive and non-ferromagnetic, and the second coating layer is ferromagnetic and preferably electrically conductive at the same time.

[0055] According to one embodiment, the method of the present invention can be used to measure the thickness of a coating layer on a steel continuous casting mold. In such an embodiment, the substrate is typically made of copper or a copper-based alloy, and the thickness of the cermet coating, e.g., a wear-resistant coating made of tungsten carbide, and the adhesive layer made of a Ni-based alloy need to be determined after the mold is refurbished. This is useful because a too thick Ni-based alloy layer can interfere with eddy current sensors used to monitor the meniscus level of molten steel in the mold during the casting process.

[0056] According to a second aspect, the present invention provides a sensor assembly for non-destructively measuring the thickness of a coating layer formed on a substrate of a workpiece, comprising: the substrate optionally having a first coating layer composed of a material that is electrically conductive and either diamagnetic or paramagnetic; the first coating layer is disposed on a second coating layer made of a ferromagnetic material, a ferrimagnetic material, or an antiferromagnetic material; the second coating layer is disposed between the first coating layer and the substrate; The sensor assembly includes: - at least one sensor head comprising at least one coil configured to generate an alternating magnetic field at a plurality of frequencies within a frequency range and to sense changes in the magnetic field caused by the presence of the workpiece near the sensor head; - an impedance analyzer configured to determine the inductance at the plurality of frequencies to obtain an inductance spectrum and to determine the inductance at a predetermined frequency that is in the lower third of the frequency range or below a lower limit of the frequency range; - a data processing unit configured to determine a characteristic frequency from the inductance spectrum, the characteristic frequency being a frequency of a zero crossing of the real part of the inductance, or a frequency at which the imaginary part of the inductance is a minimum, or a frequency at which the phase angle of the inductance is -90°; to calculate a thickness of the first coating layer from the characteristic frequency, the thickness being zero; and to calculate a thickness of the second coating layer from the inductance at the predetermined frequency and the calculated thickness of the first coating layer. The present invention relates to a sensor assembly comprising:

[0057] All embodiments and advantages described in relation to the measurement method may also apply to the sensor assembly, and vice versa. In particular, the sensor head may be as described in relation to the method. The data processing unit may be part of a computer, laptop, cloud computer, mobile device, or any other processing device. The data processing unit may be connected to a digital storage unit, such as a hard disk, memory stick, or cloud database.

[0058] According to one embodiment, the sensor head includes a drive coil configured to generate an alternating magnetic field at multiple frequencies within a frequency range and two sense coils configured to sense changes in the magnetic field caused by the presence of a workpiece near the sensor head, the drive coil being interposed between the two sense coils, and the magnetic field being measured as the difference in output between the two sense coils. In this embodiment, the drive coil may be disposed between the two sense coils, and all three coils may be disposed parallel to the surface of the coated workpiece. The coils may be cylindrical coils with one or several turns.

[0059] The coils may be connected to an impedance analyzer configured to measure impedance, from which the mutual inductance between the coils may be calculated.

[0060] According to one embodiment, the sensor assembly comprises multiple sensor heads arranged in an array and configured to simultaneously measure the thickness of one or more coating layers at multiple locations on the workpiece, and a data processing unit or computer may perform parallel processing of the acquired signals, thereby enabling, for example, rapid inspection of a casting mold at multiple locations.

[0061] The sensor assembly, especially if it includes multiple sensor heads, can be mounted on an automated robotic scanning system for scanning over large surface areas, thereby efficiently inspecting coating thickness on, for example, rolled steel products or rolled or extruded aluminum alloy products.

[0062] According to one embodiment, the sensor assembly includes at least one displacement sensor for measuring the surface profile of the workpiece during the process of measuring the thickness of the coating layer(s). The displacement sensor may be an optical profilometer sensor. The measurement data may thereby be used to reveal comprehensive data regarding the quality of the workpiece and / or its coating, in particular information regarding any thickness reduction and structural integrity of the coating layer(s).

[0063] According to one embodiment, the sensor assembly comprises at least one temperature sensor for measuring the temperature of the workpiece, and the data processing unit is configured to compensate for temperature effects on the sensor output. In this embodiment, additional calibration samples at various temperatures may be used in the calibration procedure, and the parametric model is extended to include the effect of temperature on material properties reflected in the change in inductance.

[0064] In one embodiment, the measurement data, in particular the measured thicknesses of the first and second coating layers, can be visualized in three dimensions (3D) in real time for quick investigation of coating layer quality, in particular variations in coating layer thickness, or defects in the coating layer, which may be cracks in the coating layer or reductions in the coating layer thickness that may occur during use of the workpiece.

[0065] In a further aspect, the present invention relates to the use of the non-destructive measurement method and sensor assembly according to the present invention as described and claimed herein for measuring the thickness of a coating layer present on a copper or copper-based substrate forming a continuous casting mold for casting steel.

[0066] Non-limiting embodiments of the present invention will now be described with reference to the accompanying drawings. [Brief explanation of the drawings]

[0067] [Figure 1]FIG. 1 is a schematic side view of a sensor head according to an embodiment of the present invention. [Figure 2] FIG. 2 is a schematic perspective view of a sensor head according to an embodiment of the present invention. [Figure 3] FIG. 3 shows an example of eight inductance values ​​measured at different frequencies shown in the complex plane. [Figure 4] FIG. 4 is a schematic cross-sectional view of a workpiece that may be tested using the sensor assembly of the present invention. [Figure 5] FIG. 5 is a graph of the phase angle of the inductance phase spectrum for various thicknesses of the first coating layer. [Figure 6] FIG. 6 is a diagram illustrating a parametric function describing the characteristic frequency versus the thickness of the first coating layer. [Figure 7] FIG. 7 is a graph of the magnitude of inductance at 200 Hz for various first and second coating layer thicknesses. [Figure 8] FIG. 8 is a graph of the magnitude of the compensated inductance at 200 Hz versus the thickness of the second coating layer. DETAILED DESCRIPTION OF THE INVENTION

[0068] Similar elements are designated with the same reference numerals.

[0069] FIG. 1 shows a sensor head 1 used in a method and sensor assembly according to one embodiment of the present invention. The sensor head 1 is installed near a workpiece 2 having a coating layer to be measured. The sensor head 1 is disposed within a housing 10. The sensor head 1 or its housing 10 can be maintained a short distance 3 away from the workpiece 2, for example, by using a spacer (not shown). In this embodiment, the sensor head 1 includes three coils 4, 6, and 8 arranged along the axis of a cylinder with an air core. Thus, the coils 4, 6, and 8 may each have one or several turns and may each be elliptical or circular. In this embodiment, the coils 4, 6, and 8 are each substantially flat and disposed parallel to the coated surface of the workpiece 2. However, the shape and size of the coils can be adapted to the geometry of the workpiece 2 to be measured and the thickness and material of the coating layers 16 and 18. In this embodiment, the drive coil 4 is sandwiched between the two sense coils 6 and 8. The drive coil 4 may be disposed between two sense coils 6, 8, where coil 6 can be referred to as the effective coil and coil 8 can be referred to as the dummy coil. This allows for measuring the signal difference between the two sense coils 6, 8 and thereby canceling out any environmental side reactions. The drive coil 4 is excited from an alternating current (AC) generator at multiple frequencies, and the effective and dummy coils 6, 8 deliver complex inductance signals. The magnetic field generated by the AC current is indicated at 12. The drive coil 4 and sense coils 6, 8 are connected to an impedance analyzer 5 for measurement of the mutual impedance Z. The mutual inductance L can be determined from the impedance by using Z = R + jΩL. The impedance analyzer 5 is further connected to a data processing unit 7 for data storage, processing, and analysis of the acquired signal data.

[0070] FIG. 2 further illustrates the arrangement of cylindrical or circular coils 6, 4, 8 along the axis of the air-filled cylinder.

[0071] By driving the drive coil 4 at different frequencies, the complex inductance L can be measured, which is shown on the complex plane in Figure 3, where the axes indicate the real part Re(L) = and imaginary part Im(L) of the complex inductance. Circles 14 represent each complex inductance measured at different frequencies f from 0 to infinity. It can be seen that each inductance 14 has a different magnitude and phase angle φ.

[0072] 4 shows a workpiece 2 on which a sensor head 1 can be installed to measure the thickness of a first coating layer 16 and a second coating layer 18 disposed on a substrate 20. If the workpiece is a continuous casting mold in a steel casting plant, the wear-resistant coating may comprise a cermet coating, in particular a top or first coating layer 16 made of tungsten carbide. An intermediate or second coating layer 18 interposed between the cermet coating layer and the copper substrate 20 may be made of a ferromagnetic Ni-based alloy, in particular a NiCo-based alloy.

[0073] One embodiment of the measurement method will now be described with reference to the graphs of Figures 5 to 8. These data have been obtained by numerical data analysis, but are expected to correspond well with actual measurement data.

[0074] In step 1, in the absence of a workpiece, the complex inductance spectrum L0 is measured at multiple frequencies, for example, eight frequencies between 1 kHz and 15 kHz. For a series of measurements, step 1 only needs to be performed once and the results are stored in the computer.

[0075] In step 2, the inductance spectrum L is measured at the same frequency point as in step 1 when the sensor head 1 is placed above the workpiece 2. In step 3, the change ΔL in the inductance spectrum caused by the workpiece can be calculated as ΔL=L1−L0. In step 4, interpolation of the inductance phase spectrum can be performed to obtain the characteristic frequency point where the phase angle is −90°.

[0076] This process is performed at 1Hz to 10 6 Figure 5 shows a graph of the inductance phase angle φ versus the frequency spectrum in Hz. Inductance spectra were obtained for samples with different first coating layer thicknesses t1. Thus, several simulated curves 22 for thicknesses ranging from 0.1 μm to 1.0 μm are shown in Figure 5, with the minimum t1 of 0 indicated by the graph on the right, and the arrows indicating the direction of increasing t1. Circles 24 indicate the characteristic frequency of each sample, i.e., the frequency at which the inductance phase angle is -90°. Numerical simulations were performed assuming a cermet coating, specifically a first coating layer composed of tungsten carbide, and a second coating layer composed of a Ni-based alloy, specifically nickel-cobalt, with a thickness of 0.2 mm, disposed on a copper substrate with a thickness of 70 μm.

[0077] In step 5, the thickness t1 can be determined by using each characteristic frequency as an input to a first parametric function. This function is shown in Figure 6, where the stars indicate numerical data and the lines between them are fitted using Equation 1a above. It can be seen that the characteristic frequency f0 is a very good indicator of the thickness of the top layer, at least up to a thickness of 1.2 mm.

[0078] In the next step 6, the effect of the first layer on inductance at a given low frequency is compensated for using the thickness of the first coating layer determined from step 5 as an input to another parametric function, e.g., Equation 3 above.

[0079] In step 7, the thickness of the lower layer can be determined from the compensated inductance at a given low frequency using another parametric function, for example, Equation 4 above.

[0080] The dependence of the inductance magnitude on both the thickness of the first and second coating layers is shown in Figure 7. In particular, Figure 7 shows that the simulated data, indicated by the stars, can be fitted to, for example, the following equation:

[0081]

number

[0082] Therefore, it depends on both the thickness of the first and second coating layers. The curve of increasing t1 is indicated by the arrow. Once the magnitude of inductance |L| is compensated for the effect of the first coating layer, the compensated magnitude of inductance |L comp can be described by a simple exponential function shown in FIG.

[0083] Steps 4 through 7 are preferably performed sequentially so that the thickness of the first coating layer can be determined first and used to compensate for its effect on low frequency inductance before the thickness of the second coating layer can be estimated. The inductance data measured in steps 1 and 2 can be stored and used in step 3 to calculate the inductance change caused by the workpiece.

[0084] Having fully described the present invention, it will be apparent to those skilled in the art that many changes and modifications may be made thereto without departing from the spirit or scope of the invention as set forth herein. Any signs in the claims should not be construed as limiting the scope of the appended claims.

Claims

1. 1. A method for non-destructively measuring the thickness (t1, t2) of at least one coating layer (16, 18) formed on a substrate (20) of a workpiece (2), comprising: the workpiece (2) optionally comprises a first coating layer (16) made of a material that is electrically conductive and either diamagnetic or paramagnetic; the first coating layer (16) is disposed on a second coating layer (18) made of a ferromagnetic, ferrimagnetic or antiferromagnetic material; the second coating layer (18) is disposed between the first coating layer (16) and the substrate (20); The method comprises the following steps: (a) placing an inductance sensor head (1) in close proximity to the workpiece (2); (b) measuring the inductance (14) at a plurality of frequencies within a frequency range to obtain an inductance spectrum (22), and measuring the inductance at a predetermined frequency that is in the lower third of the frequency range or below a lower limit of the frequency range; (c) The characteristic frequency f ０ (24), where the characteristic frequency f ０ (24) is (c-i) the frequency of the zero crossing of the real part of the inductance, or (c-ii) the frequency at which the imaginary part of the inductance is minimum, or (c-iii) the frequency at which the phase angle of the inductance is −90°; (d) the thickness t of the first coating layer (16) from the characteristic frequency (24) １ where the thickness may be zero; (e) the inductance at the predetermined frequency and the calculated thickness t of the first coating layer (16). １ from the thickness t of the second coating layer (18) ２ The process of calculating The method comprising:

2. a thickness t1 of the first coating layer (16) is calculated from the characteristic frequency (24) by a first parametric function, the parameters of which have been determined in a calibration procedure; The first parametric function is, in particular, [Equation 1] [In the formula, t １ is the thickness of the first coating layer (16), and p １ and p ２ are the parameters of the first parametric function, and f ０ is the characteristic frequency (24). The method of claim 1, wherein

3. The thickness t of the second coating layer (18) ２ is calculated from the magnitude or real part of the inductance (L) at the predetermined frequency by a second parametric function, the parameters of which have been determined in a calibration procedure; The second parametric function is, in particular, [Equation 2] [In the formula, t ２ is the thickness of the second coating layer (18), and p ３ , p ４ and p ５ are the parameters of the second parametric function, and L ｌｏｗ is the inductance measured at the given frequency. The method according to claim 1 or 2, wherein

4. The thickness t of the second coating layer (18) ２ but, (e1) a third parametric function, the parameters of which have been determined in a calibration procedure, the third parametric function including the thickness t of the first coating layer (16) as an input to the third parametric function; １ using the predetermined frequency (f ｌｏｗ ) compensating for the effect of said first coating layer (16) on the inductance in the (e2) calculating the thickness t of the second coating layer (18) from the compensated inductance at the predetermined frequency using a fourth parametric function, the parameters of which have been determined in a calibration procedure; ２ To determine The method according to any one of claims 1 to 3, wherein the calculated value is:

5. 5. The method according to any one of claims 1 to 4, wherein the lower end of the frequency range is from 10 Hz to 1000 Hz, preferably from 100 Hz to 500 Hz, and the upper end of the frequency range is from 5 kHz to 800 kHz, preferably from 10 kHz to 400 kHz, more preferably from 20 kHz to 100 kHz.

6. The method according to any one of claims 1 to 5, wherein the predetermined frequency is between 10 Hz and 1000 Hz, preferably between 50 Hz and 600 Hz, more preferably between 100 Hz and 300 Hz.

7. (i) placing the inductance sensor head (1) in close proximity to a plurality of calibration samples having a plurality of different known thicknesses of the first and second coating layers (16, 18); (ii) for each calibration sample, measuring the inductance at a plurality of frequencies within a frequency range to obtain an inductance spectrum, and measuring the inductance at a predetermined frequency that is in the lower third of the frequency range or below a lower limit of the frequency range; (iii) determining the characteristic frequency from the inductance spectrum; (iv) determining parameters of a first parametric function from characteristic frequencies of the plurality of calibration samples using the known thickness of the first coating layer (16); (v) determining parameters of a second parametric function from the inductances at the predetermined frequency of the plurality of calibration samples and the known thicknesses of the first and second coating layers (16, 18). The method of any one of claims 1 to 6, comprising a calibration procedure comprising:

8. 8. The method according to any one of claims 1 to 7, wherein the first coating layer (16) has a thickness in the range of 0 μm to 3 mm, preferably 50 μm to 1.5 mm, more preferably 0.1 mm to 0.6 mm, and the second coating layer (18) has a thickness in the range of 50 μm to 3 mm, preferably 0.1 mm to 1.5 mm, most preferably 0.3 mm to 1.3 mm.

9. The method according to any one of the preceding claims, wherein the first coating layer (16) consists of a ceramic or cermet coating and / or the second coating layer (18) consists of a nickel-based alloy.

10. 10. A method according to any one of claims 1 to 9 for measuring the thickness of a coating layer (16, 18) on a continuous casting mould of steel (2), preferably formed by a substrate (20) made of copper from a copper-based alloy.

11. 1. A sensor assembly for non-destructively measuring the thickness of a coating layer (16, 18) formed on a substrate (20) of a workpiece (2), comprising: the substrate optionally having a first coating layer (16) composed of a material that is electrically conductive and either diamagnetic or paramagnetic; the first coating layer (16) is disposed on a second coating layer (18) made of a ferromagnetic, ferrimagnetic or antiferromagnetic material; the second coating layer (18) is disposed between the first coating layer (16) and the substrate; The sensor assembly includes: at least one sensor head (1) comprising at least one coil (4, 6, 8), the at least one coil configured to generate an alternating magnetic field (12) at a plurality of frequencies within a frequency range and to sense changes in the magnetic field caused by the presence of the workpiece (2) close to the sensor head; an impedance analyzer (5) configured to determine the inductance at said plurality of frequencies to obtain an inductance spectrum and to determine the inductance at a predetermined frequency in the lower third of said frequency range or below the lower limit of said frequency range; a data processing unit (7) configured to determine a characteristic frequency (24) from the inductance spectrum, the characteristic frequency (24) being the frequency of a zero crossing of the real part of the inductance, or the frequency at which the imaginary part of the inductance is a minimum, or the frequency at which the phase angle of the inductance is −90°; to calculate a thickness of the first coating layer (16) from the characteristic frequency (24), which thickness may be zero; and to calculate a thickness of the second coating layer (18) from the inductance at the predetermined frequency and the calculated thickness of the first coating layer (16). A sensor assembly comprising:

12. 12. The sensor assembly of claim 11, wherein the sensor head (1) comprises a drive coil (4) configured to generate an alternating magnetic field at a plurality of frequencies within a frequency range, and two sense coils (6, 8) configured to sense changes in the magnetic field caused by the presence of the workpiece (2) near the sensor head, the drive coil (4) being interposed between the two sense coils (6, 8), and the changes in the magnetic field being measured as a difference in the outputs of the two sense coils.

13. 13. The sensor assembly of claim 11 or 12, comprising a plurality of sensor heads (1) arranged in an array and configured to simultaneously measure thicknesses of single or multiple coating layers (16, 18) at multiple locations on the workpiece (2).

14. 14. The sensor assembly according to claim 11, comprising at least one displacement sensor for measuring a surface profile of a workpiece (2) during a process of measuring the thickness of one or more coating layers (16, 18) formed on a substrate (20) of the workpiece (2).

15. 15. The sensor assembly according to any one of claims 11 to 14, wherein the sensor assembly comprises at least one temperature sensor for measuring the temperature of the workpiece (2), and wherein the data processing unit is configured to compensate for temperature effects on the sensor output.

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