FR3054312A1 - METHOD AND DEVICE FOR MEASURING THE THICKNESS OF A METAL COATING - Google Patents

Abstract

The invention relates to a method and a device for measuring the thickness of a metal coating of a substrate, said method being of the type comprising the steps of: providing a substrate (10) coated with a metallic coating (12) ), said metal coating having a coating surface; and, ultrasonic waves are emitted to said coating surface at a component normal to said coating surface and a reflected signal is recorded to calculate the thickness of said coating. The reflected signal is converted into a frequency signal to determine a frequency difference representative of said thickness of said coating (12) so as to calculate the thickness of said coating.

Description

054 312

57117 ® FRENCH REPUBLIC

NATIONAL INSTITUTE OF INDUSTRIAL PROPERTY © Publication number:

(to be used only for reproduction orders)

©) National registration number

COURBEVOIE © IntCI ⁸ : G 01 B 17/02 (2017.01)

PATENT INVENTION APPLICATION

A1

©) Date of filing: 07.25.16. © Applicant (s): INDUSTRY TECHNICAL CENTER- (© Priority: MECHANICAL TRIES - FR. @ Inventor (s): RIVENEZ JACQUES. ©) Date of public availability of the request: 26.01.18 Bulletin 18/04. ©) List of documents cited in the report preliminary research: Refer to end of present booklet (© References to other national documents ® Holder (s): INDUSTRIAL TECHNICAL CENTER- related: MECHANICAL SORTING. ©) Extension request (s): © Agent (s): CABINET FEDIT LORIOT.

METHOD AND DEVICE FOR MEASURING THE THICKNESS OF A METAL COATING.

FR 3 054 312 - A1

The invention relates to a method and a device for measuring the thickness of a metallic coating of a substrate, said method being of the type comprising the following steps: a substrate (10) provided with a metallic coating (12) is provided ), said metal coating having a coating surface; and, transmitting ultrasonic waves to said coating surface according to a component normal to said coating surface and recording a reflected signal in order to calculate the thickness of said coating. Said reflected signal is transformed into a frequency signal in order to be able to determine a frequency difference representative of said thickness of said coating (12) so as to calculate the thickness of said coating.

Method and device for measuring the thickness of a metal coating

The present invention relates to a method and a device for measuring the thickness of a metal coating of a substrate.

A field of application envisaged is notably, but not exclusively, that of the control of machining operations after deposition on mechanical parts.

It is known, in different industrial sectors, to cover metallic materials with a hard coating in order to be able to increase, all at least locally, their mechanical characteristics. Also, we seek to determine the thickness of this coating reliably using non-destructive methods.

To do this, different techniques exist and are selected according to the thicknesses to be checked, the types of coating or even the types of substrate to be coated. This is the case with magnetic methods which allow the thickness of metallic coatings to be measured. However, these magnetic methods are also sensitive to the intrinsic electrical and magnetic properties of the coating and substrate materials. Also, the reliability of the thickness measurement is relative to the materials present.

To overcome this interference between the control method and the material itself, it was imagined to implement ultrasonic waves and record their travel time in the coating to determine the thickness. Thus, by knowing the speed of propagation of the ultrasonic waves through the coating and by recording the time offset between the incident wave and the reflected wave, a measurement of the thickness of the coating is obtained. This method is well suited to measuring the thickness of metal parts, because the discontinuity of the medium between the part and the air makes it possible to obtain less noisy signals. On the other hand, the discontinuity between a metallic coating and its substrate is less clear and the reflected signal is more complex to analyze. In addition, the coating thicknesses are generally small, which makes it difficult to resolve the echoes since small time intervals between the multiple reflections are obtained.

Also, a problem which arises and which the present invention aims to solve is to provide a method and a device for measuring the thickness of the metallic coating of a substrate which is both reliable and imprinted with precision.

For this purpose, and according to a first object, a method of measuring the thickness of a metallic coating of a substrate is proposed, said method being of the type comprising the following steps: a substrate coated with a coating is provided metallic, said metallic coating having a coating surface; transmitting ultrasonic waves to said coating surface according to a component normal to said coating surface and recording a reflected signal in order to be able to calculate the thickness of said coating; and transforming said reflected signal into a frequency signal in order to be able to determine a frequency difference representative of said thickness of said coating so as to calculate the thickness of said coating.

Thus, a characteristic of the invention lies in the implementation of an operation aimed at transforming the reflected signal, the representation of which is temporal, into a frequency signal. For example, the time signal is transformed, thanks to a Fourier transform, or FFT, acronym of the English “Fast Fourier Transform”, into a frequency signal. In this way, it is highlighted on the spectrum of the signal then obtained, and in given frequency intervals, "dips", or even minima for which the derivative of the corresponding curve is canceled out, the spacing of which reveals a frequency representative of the thickness of the coating, as will be explained in more detail in the following description. Such a method is relatively easy to implement by means of simple and robust devices and proves to be relatively precise. In addition, by transforming the time signal into a frequency signal, it is easier to highlight the periodic elements of the signal, and it is then freed from the resolution of the echoes, for thin coatings. It will be observed that the measurement method is suitable for the coating surfaces, whether they are rough or rectified.

Also, the metal coating is advantageously not ferromagnetic, in other words, it has the capacity to magnetize under the effect of a magnetic field and to lose this magnetization when the magnetic field disappears. In addition, the substrate is preferably a metallic substrate. It is also preferably non-ferromagnetic.

Preferably, ultrasonic waves are emitted at a high frequency, for example 50 MHz.

According to a particularly advantageous embodiment of the invention, and in particular when the "hollows" do not appear, the uncoated substrate having a substrate surface is also provided; further transmitting ultrasonic waves to said substrate surface and recording another reflected signal; transforming said other reflected signal into another frequency signal; and, the ratio of said frequency signal and said other frequency signal is established in order to be able to determine said frequency difference representative of said thickness of said coating.

In this way, the ratio of the spectra of the reflected signal and the other reflected signal corresponding to the uncoated substrate, which corresponds to a transfer function, makes it possible to obtain a periodic signal on which the differences in frequencies representative of l appear clearly. coating thickness.

However, for certain coatings and / or certain substrates, despite the use of the uncoated substrate to establish a spectrum of the reference signal, and to relate it to the reflected frequency signal of the coated substrate, no identifiable periodic signal appears . This is particularly the case when the coating is raw and has not been rectified. Also, in these circumstances, advantageously, a polynomial regression is also carried out over a sliding frequency interval of said ratio of said frequency signal and said other frequency signal in order to be able to determine said frequency difference. Thus, thanks to this operation, we come to more closely match the shape of the signal and thus reveal its periodicity. Therefore, it is easier to determine the frequency difference representative of the thickness of the coating.

Preferably, the frequency values are smoothed from said ratio of said frequency signal and from said other frequency signal by subsampling. In this way, a more regular signal is obtained. In addition, according to a particularly advantageous, but non-limiting characteristic of the invention, the frequency values are smoothed from said ratio of said frequency signal and from said other frequency signal by sliding average. The determination of the frequency differences is then easier and is reproducible.

In addition, a plurality of substrates coated with the same metallic coating of different known thicknesses are advantageously provided, and ultrasonic waves are emitted on the surface of the coatings of each of the coated substrates and a plurality of signals are recorded which are transformed into a plurality of frequency signals so as to be able to determine a plurality of frequency differences corresponding to said known thicknesses.

Preferably, a function is determined as a function of said frequency differences and of said known thicknesses, relating the thickness of a coating and the frequency difference. Thus, a calibration is carried out in order to be able to determine the frequency differences as a function of the thickness of the coating. It will be observed that the thickness is an inverse function of the frequency differences and which is also a function of the speed of the ultrasonic waves through the coating. Also, the parameters of a function Y = A / X are determined so as to be able to provide a thickness value for each frequency difference value.

According to another object, a device is provided for measuring the thickness of a metal coating of a substrate, said metal coating having a coating surface, said measurement device comprising: a translator for emitting ultrasonic waves to said coating surface according to a component normal to said coating surface; a memory member for recording a reflected signal and a calculation member connected to said memory member so as to be able to calculate the thickness of said coating from said stored reflected signal; and said calculating member further transforms said reflected signal into a frequency signal in order to be able to determine a frequency difference representative of said thickness of said coating so as to calculate the thickness of said coating.

Thus, the measuring device implements the method described above and therefore has all the advantages. The translator emits longitudinal waves at high frequency, for example 50 MHz, and at a normal incidence on the surface of the coating. In addition, the translator is broadband and has an acoustic silica relay.

According to a preferred embodiment, said translator further emits ultrasonic waves on the surface of the uncoated substrate, while said memory member records another reflected signal; and said calculating member transforms said other reflected signal into another frequency signal and establishes the ratio of said frequency signal and said other frequency signal so as to be able to determine said frequency difference representative of said thickness of said coating. The device thus implements a characteristic of the method described above and thus adopts all of its attributes.

In addition, said calculating member also performs a polynomial regression on a sliding frequency interval of said ratio of said frequency signal and said other frequency signal in order to be able to determine said frequency difference.

Other particularities and advantages of the invention will emerge on reading the description given below of a particular embodiment of the invention, given by way of indication but not limitation, with reference to the appended drawings in which:

- Figure 1 is a block diagram of the device for implementing the method according to the invention for measuring the thickness of a coating;

- Figure 2 is a schematic view of an element of the invention shown in Figure 1 in a preparatory phase;

- Figure 3 is a graph of the module of a first signal reflected as a function of time, recorded using the device illustrated in Figure 1, according to an exemplary embodiment;

- Figure 4 is a graph of the frequency spectrum module of the first reflected signal shown in Figure 3;

- Figure 5 is a graph of the module of a second signal reflected as a function of time, recorded using the device illustrated in Figure 2;

- Figure 6 is a graph of the frequency spectrum module of the second reflected signal shown in Figure 5;

FIG. 7 is a graph resulting from the ratio of the spectra represented in FIGS. 4 and 6;

- Figure 8 is a graph showing the values measured according to the invention as a function of the thickness of the coating;

- Figure 9 is a graph of the module of a third signal reflected as a function of time, recorded using the device illustrated in Figure 1, according to a second embodiment;

- Figure 10 is a graph of the frequency spectrum module of the third reflected signal shown in Figure 9;

- Figure 11 is a graph resulting from the ratio of the frequency spectrum 20 shown in Figure 10 and the frequency spectrum of a fourth signal obtained using the device illustrated in Figure 2;

- Figure 12 is a graph resulting from a first processing of the ratio of the spectra illustrated in Figure 11; and,

- Figure 13 is a graph resulting from a second processing of the graph 25 illustrated in Figure 12.

Figure 1 illustrates a metal substrate 10 and a metal coating

12. In this case it is a metallic substrate made of titanium alloy comprising 6% aluminum and 4% vanadium. As for the metallic coating, it is tungsten carbide with a thickness of 200 μm. Obviously, other types of substrate and metallic coating can be used.

FIG. 1 also illustrates a high frequency translator 14 making it possible to emit ultrasonic waves, here 50 MHz, longitudinally and at wide band. The translator 14 is equipped with an acoustic silica relay. The translator 14 is adjusted relative to the surface of the metal coating 12 so as to be able to emit the ultrasonic waves under normal incidence. In addition, the translator 14 is connected to an electronic card 16 which controls it and also collects the signals reflected by the metal coating 12 and the metal substrate 10. To do this, the electronic card 16 includes a control component 18 and a component. storage unit 20 as well as a calculation component 22 and a display terminal 24.

Thus, the translator 14 is intended to emit ultrasonic waves on the surface of the coating 12 at normal incidence, under the control of the control component 18, and to receive in return an echo signal, which signal is then recorded in the component. storage 20 so that it can then be processed in the calculation component 22.

Figure 2 illustrates the same high frequency translator 14 applied directly to the metal substrate 10 without coating. The electronic card 16 is not shown here for the sake of clarity. The advantage of using the high frequency translator 14 directly on the substrate 10 will be explained below.

Thus, according to a first example of implementation, the substrate is made of titanium alloy and the coating 12 is made of tungsten carbide with a thickness of 200 μm. The latter has been rectified to make its surface more uniform. Thanks to the translator 14, an ultrasonic wave of 50 MHz is emitted and a time signal as shown in FIG. 3 is recorded in the storage component 20. This gives the modulus of the response on the ordinate as a function of time in microseconds according to the abscissas. To be used, this signal is transformed into a frequency signal thanks to the implementation of a Fourier transform through the computation component 22. The graph illustrated in FIG. 4 is then obtained.

Thus, the graph in FIG. 4 shows a curve 24 of the frequency spectrum module of the signal represented in FIG. 3. This curve has two "valleys" 26, 28, that is to say two frequency values corresponding to a minimum. These two characteristic points are separated from each other by approximately AF = 11.25 MHz. It will be observed that the detection of the two points corresponding to the two "valleys" 26, 28, can easily be carried out automatically by the calculation component 22 by calculating the first derivative of the corresponding curve and by identifying the points where it is canceled .

In addition, it is known that the thickness e of the coating is not only a function of the frequency difference AF, but also of the propagation speed V of the ultrasonic waves in the coating according to the formula e = V / 2AF. The propagation speed V being in this case 4500 m / s, we find the values of frequency difference AF and thickness e.

However, there are circumstances where the identification of the characteristic points is less easy and therefore, a reflected signal is applied without coating, as illustrated in Figure 2.

Consequently, thanks to the translator 14, an ultrasonic wave of 50 MHz is emitted and a time signal which is found illustrated in FIG. 5 is recorded in the storage component 20. In the same way as before, the signal is transformed into a frequency signal thanks to the implementation of a Fourier transform through the computation component 22 and one then obtains the graph illustrated in FIG. 6.

Thanks to its calculation component 22, we then proceed to calculate the ratios of the spectrum modules illustrated in Figure 4 and those obtained in the last graph of Figure 6. And we then obtain the periodic curve 30 shown in Figure 7 , after having carried out on the one hand, a first smoothing by subsampling and on the other hand, a complementary smoothing by the technique known as "sliding averages". Thus, by carrying out the ratio of the spectra, of the signal reflected in the presence of the coating 12 and in the absence of coating, a better definition of the signal is obtained. We thus identify more clearly the frequency difference AF between the extremes 32, 34, 36, which is representative of the thickness of the coating.

By definition of the thickness e as a function of the frequency difference AF, the curve representative of the frequency differences as a function of the thickness is a hyperbola. Thus, by implementing a plurality of coatings of different thickness, for example between 100 μm and 300 μm, the parameters of a hyperbolic curve representative of the thickness of a given coating on a substrate given by compared to the frequency differences obtained according to the invention. Figure 8 illustrates a graph showing such a curve 38.

Of course, it is envisaged to use coatings whose thickness is less than 100 μm in order to be able to measure them according to the method which is the subject of the invention.

Thus, in a process of continuous control of rectified parts, the types of substrate and coating of which are determined, the thickness of the coating can then be determined by simple implementation of the translator 14 via the electronic card 16.

According to a second example of implementation, a substrate of titanium alloy identical to that of the first example is provided, and a coating of tungsten carbide with a thickness of 200 μm is deposited there without rectification. As a result, the surface of the coating is less uniform than that of the rectified coating.

The first example of implementation is then carried out in the same way, and with the translator 14, an ultrasonic wave of 50 MHz is emitted and a time signal shown in FIG. 9 is recorded in the storage component 20. Then, this signal is transformed into a frequency signal thanks to the implementation of a Fourier transform through the calculation component 22. We then obtain the graph illustrated in Figure 4 showing a curve 40. However, unlike curve 24 illustrated on the

In Figure 4, this curve 40 does not present any remarkable singular points. Also, as indicated above, a measurement of a reflected signal without coating is carried out and the ratio of the two spectra with coating and without coating is then carried out. We then obtain the curve 42 illustrated on the graph of FIG. 11. Certain singular points 44, 46 seem to take shape without however appearing clearly.

Also, in such circumstances, the signal is processed by the Savitzky-Golay algorithm. Thus, a polynomial regression is performed on a sliding frequency interval of the curve 42, that is to say of the ratio of the frequency signals with coating and without coating. And one then obtains the periodic curve 48 shown in the graph of Figure 12. It then reveals the extremes 50, 52, 54 spaced from each other by a value of the frequency substantially equal to 9 MHz. In this way, the thickness is deduced therefrom using the formula e = V / 2AF, as indicated above, after having performed a calibration with different known thicknesses of coating on the substrate considered. In this case, the speed of propagation V through the coating is substantially 3600 m.s' ¹ , and therefore the thickness e of the coating is itself substantially 200 µm.

It is also possible, for a better identification of the frequency interval, to apply to the curve 48 a complementary treatment, ie a second order derivation. We then obtain the curve 56 shown in the graph of Figure 13. The periodic nature of the curve 56 thus appears even more clearly and the frequency interval there is more directly readable.

Claims (10)

1. Method for measuring the thickness of a metallic coating of a substrate, said method being of the type comprising the following steps: - providing a substrate (10) coated with a metal coating (12), said metal coating having a coating surface; - ultrasonic waves are emitted on said coating surface according to a component normal to said coating surface and a reflected signal is recorded in order to be able to calculate the thickness of said coating; characterized in that said reflected signal is transformed into a frequency signal in order to be able to determine a frequency difference representative of said thickness of said coating (12) so as to calculate the thickness of said coating. 2. Measuring method according to claim 1, characterized in that it further comprises the following steps: - the uncoated substrate (10) further having a substrate surface is further provided; - Furthermore, ultrasonic waves are emitted on said substrate surface (10) and another reflected signal is recorded; - transforming said other reflected signal into another frequency signal; and, - Establishing the ratio of said frequency signal and said other frequency signal to be able to determine said frequency difference representative of said thickness of said coating (12). 3. Measuring method according to claim 2, characterized in that a polynomial regression is also carried out on a sliding frequency interval of said ratio of said frequency signal and said other frequency signal in order to be able to determine said frequency difference. 4. Measuring method according to any one of claims 1 to 3, characterized in that one proceeds to smoothing the frequency values of said ratio of said frequency signal and of said other frequency signal by subsampling. 5. Measuring method according to any one of claims 1 to 4, characterized in that one proceeds to smoothing the frequency values of said ratio of said frequency signal and said other frequency signal by sliding average. 6. Measuring method according to any one of claims 1 to 5, characterized in that a plurality of substrates coated with the same metallic coating of different known thicknesses are also provided, and in that waves are emitted ultrasonic to the surface coatings of each of the coated substrates and recording a plurality of ^J that is converted signals into a plurality of frequency signals so as to determine a plurality of frequency differences corresponding to said known thicknesses. 7. A measurement method according to claim 6, characterized in that, as a function of the frequency differences of said plurality of frequency differences and of said known thicknesses, a function relating the thickness of a coating and the difference of frequency. 8. Device for measuring the thickness of a metal coating (12) of a substrate (10), said metal coating having a coating surface, said measurement device comprising: - a translator (14) for emitting ultrasonic waves to said coating surface (12) according to a component normal to said coating surface; - a memory device (20) for recording a reflected signal and a calculation device (22) connected to said memory device in order to be able to calculate the thickness of said coating from said stored reflected signal; characterized in that said calculating member (22) further transforms said reflected signal into a frequency signal in order to be able to determine a frequency difference representative of said thickness of said coating (12) so as to calculate the thickness of said coating. 9. Measuring device according to claim 8, characterized in that said translator (14) further emits ultrasonic waves on the surface of the uncoated substrate, while said memory member (20) records another reflected signal; çt in that said calculating member (22) transforms said other reflected signal into another frequency signal and establishes the ratio of said frequency signal and said other frequency signal in order to be able to determine said difference in frequencies representative of said thickness of said coating. 10. Measuring device according to claim 8, characterized in that said calculating member (22) further performs a polynomial regression on a 5 sliding frequency interval of said ratio of said frequency signal and said other frequency signal to be able to determine said frequency difference. 1/6