Patent Application
20140324384
Several methods are known for measuring the thicknesses of coatings supported by substrates. Some of these methods make use of:
Nevertheless, those methods have drawbacks. The first above-mentioned method cannot measure accurately the thickness of the coating carried by a cylindrical surface, e.g. when the substrate is in the form of a body of revolution such as a tube for airplane landing gear. Measuring the diameter of the coated cylinder gives an idea about the sum of the thicknesses of the coatings on the opposite sides of the cylinder, but it does not make it possible to know the individual values of each of those thicknesses. With such a method, it is not possible to guarantee that the coating formed on the cylindrical substrate does indeed have a minimum thickness as required by standards.
When coating a material that is electrically conductive, such as tungsten carbide (WC—Co—Cr) placed on a substrate that is conductive and non-magnetic, such as titanium, neither of the last two methods gives satisfaction.
The second method cannot be used when the substrate is non-magnetic, since the magnetic field used in measuring magnetic permeability is not disturbed by titanium. It is therefore not possible to detect any variation in the magnetic field as a function of the distance of the probe from the substrate, so is it not possible to deduce therefrom the thickness of the coating layer.
The use of eddy currents for detecting the thickness of an electrically conductive coating carried on a substrate that is also conductive lacks accuracy and it is desirable to improve such accuracy.
An object of the invention is to provide a method and/or a device making it possible, by performing measurements in non-destructive manner, to estimate the thickness of a coating supported by a substrate, it being understood that the coating and the substrate are both electrically conductive and that the substrate is preferably non-magnetic.
The non-magnetic material forming the substrate is typically an alloy of titanium such as:
The material forming the coating of thickness that is to be measured is an electrically conductive alloy, typically a carbide such as tungsten carbide (WC), cobalt, and chromium.
The invention relates essentially to a method of measuring the thickness of a coating layer of a workpiece formed on a substrate of the workpiece. The coating and the substrate are constituted respectively by first and second mutually distinct electrically conductive materials. The method further comprises the steps of:
The first indicator value is determined from a measurement of said at least one physical characteristic taken when the electrical signal has a first given frequency, and the second indicator value is determined from a measurement of said at least one physical characteristic taken when the electrical signal has a second given frequency distinct from the first frequency.
Thereafter, the method of the invention comprises the following steps:
By taking a plurality of measurements on different workpieces having exactly the same pair of first and second materials, but with layer thicknesses that are different from one to another, and by using the same induction means and the same probe electrical signal, it is found that the variation in the thickness of the coating layer is a parameter that has an influence on the measured physical characteristic, which specifically is an impedance Z that varies as a function of the magnetic field induced in the workpiece. Thus, there is indeed a connection between the thickness of the layer and the characteristic of the magnetic field passing through said layer. Nevertheless, by performing numerous tests, it has been found that the correlation between the variation in the thickness of the coating layer and the variation in the measured physical characteristic of the magnetic field is not easy to define.
In summary, in the method of the invention, first and second measurements are made of the physical characteristic that varies as a function of the magnetic field induced in the workpiece. The first measurement of the physical characteristic is then taken while the induction means are being fed with a probe signal having a first frequency and the second physical characteristic measurement is taken while the induction means are being fed with a probe signal having a second frequency different from the first frequency.
Using the result of the first measurement, a first value taken by an indicator is determined.
Using the result of the second measurement, a second value taken by the indicator is determined.
Specifically, the indicator evaluated from the measured physical characteristic is preferably the electrical conductivity of the portion of the workpiece through which the magnetic field passes. Thus, in the examples below, the first indicator value is the conductivity of the portion of the workpiece through which there passes the magnetic field that is generated while the probe signal has a first frequency F1, and the second indicator value is the conductivity of the portion of the workpiece through which there passes the magnetic field that is generated while the probe signal has a second frequency F2.
An essential aspect of the invention consists in using the difference between these first and second values of the indicator in order to determine the thickness of the coating layer. It has been found that the thickness of the coating layer as determined by using the difference between the first and second values of the indicator as measured with distinct probe frequencies is more accurate than the thickness that would be determined if use was made of values obtained from two measurements of the characteristic of the magnetic field taken with probe signals having a same frequency.
Furthermore, the use of this difference makes it possible to evaluate the thickness of the coating layer relatively accurately, while greatly simplifying calculation, as explained below.
An advantage of this method of evaluating the thickness of a coating layer is that it requires little calculation, specifically possibly no more than a simple subtraction. Thus, the thickness evaluation device for performing the method of the invention may include calculation means that are simple and inexpensive.
Other characteristics and advantages of the invention appear clearly from the following description given by way of non-limiting indication and made with reference to the accompanying drawings, in which:
In
In order to perform the method of measuring the thickness δ1im or er of the coating layer Rev, use is made of induction means Mind that are connected specifically to a generator G for generating a probe alternating electrical signal S. At each instant n, this signal S has given voltage Un, current In, and frequency Fn. In each of
It should be observed that these measurement means Mmes may also be made using measurement electronics M connected directly to the coil L1 in order to measure variation in the impedance of the coil L1 as a function of the signal S delivered by the generator G and as a function of the magnetic field Bn induced in the workpiece P. Under such circumstances, there is no need for the coil L2, since the coil L1 suffices.
The measurement means Mmes deliver a signal Szn to a calculation unit Uc, which signal is representative of the physical characteristic measured by the measurement means Mmes constituted in this example by the impedance Zn of the coil L2. As can be seen in
The devices Disp of
The calculation unit Uc stores the position R of the induction means Mind relative to the workpiece P, and it associates at least some of the stored positions R with the corresponding measured thickness values er, δ1im. The device of the invention Disp can thus be used to generate files that show variations in the thickness er of the layer Rev, by specifying for a series of positions R the corresponding measured thicknesses of the layer.
The calculation unit Uc is also connected to induction means, in particular to the generator G for generating the signal S in order to transmit a control signal ScdeF for controlling the signal, e.g. by determining the frequency Fn of the signal S that the generator is to deliver to the coil L1.
The device Disp may also have a nozzle for projecting the coating Rev onto the coating substrate Sub, this nozzle B being connected to means Mot for controlling movement of the nozzle B relative to the workpiece P. In the embodiment of
The movement control means Mot may also include means for measuring the position of the nozzle relative to the workpiece.
In the embodiment of
Ideally, while measuring thickness using a method of the invention, it is ensured that the relative movement between the workpiece P and the induction means Mind is limited in order to obtain an accurate indication of the thickness and of the position R of the measurement.
Thus, in an implementation of the method of the invention, the positioning means Mot prevent the induction means Mind from moving relative to the workpiece P, at least for the time taken to measure conductivity. In this implementation, if it is desired to measure thickness during machining, arrangements can be made to move the tool B independently of the induction means and to prevent the induction means Mind from moving relative to the workpiece, at least during the time required for measurement, but without that stopping movement of the tool relative to the workpiece. Specifically, if the tool is a spray nozzle B, then spraying is performed continuously and while taking measurements, and the induction means are held stationary for an instant relative to the workpiece.
Alternatively, it can be accepted that the measurement is taken while the induction means Mind are moving relative to the workpiece P, which is acceptable for the method of the invention since the calculations it requires are simple and relatively few, thereby reducing the time required to take thickness measurements. This implementation is used for example when it is desired to perform measurements during machining while the workpiece P is moving relative to the induction means Mind.
In the embodiment of
While the layer is being made, the device of the invention makes it possible to ensure that the layer complies with the setpoint.
There follows a description of how the thickness measurement method of the invention is performed.
With reference to
In this given frequency range, there is a frequency F1im that is the theoretical frequency from which the field B passes through the entire thickness of the layer Rev and stops at the interface between the layer Rev and the substrate Sub. Over a first frequency range F1im−Fmax the penetration depth of the magnetic field into the workpiece is such that the magnetic field B passes into the first material of the coating layer Rev but does not penetrate into the second material of the substrate Sub. Thus, over this first range F1im−Fmax, the measured physical characteristic that varies as a function of the magnetic field B does not depend on the conductivity σSub of the material of the substrate, but depends solely on the conductivity σRev of the material of the layer. In contrast, over a second frequency range Fmin−F1im, the penetration depth δn of the magnetic field Bn into the workpiece P is such that the magnetic field passes through the first material of the coating layer Rev and penetrates into the second material of the substrate Sub. Thus, over this range Fmin−F1im, the measured physical characteristic Zn that varies as a function of the magnetic field depends not only on the conductivity σRev of the coating material, but also on the conductivity σSub of the substrate material.
In other words, in the range Fmin−F1im, the lower the frequency of the probe signal S, the greater the influence of the conductivity σsub on the measured characteristic Zn and the smaller the influence of the conductivity σRev on the measured characteristic Zn. In order to measure the thickness of the coating δ1im using the method of the invention, it is desired to characterize the penetration of the magnetic field B into the workpiece P, and in order to do that a physical characteristic is measured that varies as a function of the magnetic field.
By way of example, this measured physical characteristic that varies as a function of the magnetic field may be:
It should be observed that the conductivity σn or the resistivity of the portion of the workpiece through which the magnetic field passes depends on the penetration depth of the field into the workpiece, which in turn depends on the frequency Fn of the probe signal S. Conductivity σn or resistivity is generally measured by measuring impedance Zn.
After measuring the physical characteristic Zn, the value of an indicator that depends at least on the electrical conductivity σRev of the first material is deduced therefrom, where this indicator may for example be the conductivity σn of the portion of the workpiece through which the magnetic field passes, it being understood that this conductivity varies:
In summary, in this particular implementation of the invention, for a given probe frequency Fn, a value is measured for a physical characteristic that is an impedance Zn, and the value of an indicator is deduced therefrom, specifically a conductivity σn and/or a resistivity of a portion of the workpiece P through which the magnetic field Bn passes.
In the method of the invention, the alternating electrical signal S fed to the induction means Mind is selected to adopt a plurality of predetermined given frequencies F1 and F2 over time. How these frequencies F1 and F2 are selected is explained below under the heading of pre-calibrating measurements.
When the signal S is under steady conditions (i.e. when the frequency of the electrical signal is equal to the predetermined frequency F1 or F2 to within ±5%), a measurement is made of the physical characteristic that varies as a function of the frequency Fn, of the conductivity σRev of the material of the coating layer, and consequently as a function of the magnetic field B induced in the workpiece P.
Two respective measurements Z1 and Z2 are thus obtained for the physical parameter Zn, one being made when the frequency is equal to F1 and the other when the frequency is equal to F2.
From these respective impedance measurements Z1 and Z2, values σ1 and σ2 are calculated for an indicator σn that is constituted by the conductivity of a portion of the workpiece through which the magnetic field Bn passes.
For this purpose, a predetermined calculation formula is used having the general form σn=f(Zn, Fn, In, Un) in which Zn is the measured impedance that is representative of the eddy current induced in the workpiece P by the induction means Mind fed with the probe signal S of frequency Fn, current In, and voltage Un.
After calculating the respective values σ1 and σ2 taken by the indicator, the difference between σ1 and σ2 is taken using the formula:
Difference=|σ1−σ2|.
Finally, by using data predetermined during calibration of the measurement means (see below under the heading calibration), the thickness of the coating layer situated in register with the induction means is calculated as a function of the measured difference between the indicator values σn measured at the respective frequencies F1 and F2.
This predetermined data correlates possible differences between the indicator values σn and corresponding values for the thickness er of the coating layer. The data may be recorded in a database associating predetermined difference values with predetermined layer thicknesses. Alternatively, the prerecorded data may be in the form of a function having the form:
layer thickness value=f(difference between the indicator values).
Calibration: Determining Data Correlating Indicator Value Differences with Thickness Values
In order to obtain this predetermined data, use is made of a series of calibration testpieces A, B, C, D, E, F, G, H, and I, each comprising a substrate made of said second material and a layer made of said first material, each of the calibration testpieces having a coating layer of known thickness.
Thereafter, for each calibration testpiece, at least one series of measurements is taken consisting in positioning the electromagnetic field induction means, or possibly other similar induction means, relative to the calibration testpiece layer and in evaluating first and second calibration values that are values taken by said indicator σn, and specifically in this example conductivity values σn. The first calibration value is determined for at least one measurement of a physical characteristic Z, specifically an impedance Zet1 taken while the calibration induction means are fed with a first calibration signal Seta of frequency F1 equal to said first frequency. In the same manner, the second calibration value is determined from at least one measurement of the physical characteristic, specifically an impedance Zet2 taken while the calibration induction means are being fed with a second calibration signal Seta of frequency F2 equal to said second frequency. Arrangements are preferably made to store the characteristics of the calibration signal in such a manner that the probe signal S generated at the moment of measuring the thickness of the layer on the workpiece P has the same characteristics as the calibration signal Seta Thus, for every workpiece P made of the same materials as the calibration testpiece and having substantially the same layer thickness, the following applies:
Still for the purpose of ensuring that calibration conditions are as close as possible to measurement conditions performed on a workpiece P, it is ensured that the measurement of the physical characteristic, i.e. the impedance, is taken during calibration using the same measurement means Mmes as are used for taking measurements on a workpiece P. In a degraded mode, if the measurement means Mmes used while taking a measurement on a workpiece are different from the measurement means used while taking a measurement on a calibration testpiece, then precautions should be taken to ensure that they have the same electrical characteristics (same resistance, same inductance).
After calculating the first and second values taken by the indicator σn at the probe frequencies F1 and F2, the difference is calculated for each testpiece between the first and second values that are associated therewith.
By way of example, the differences in the values taken by the conductivity indicators σ1 and σ2 are given for the series of calibration testpieces A-I in the table below.
These readings are plotted on
Given the increase in the series of thicknesses of the testpieces A-I and the corresponding variations in σ1 and σ2, it can be seen that the respective testpieces C and I ought to have values σ2 greater than the values for σ2 of the respective testpieces B and H, but that this is not so because of the conditions under which the coating layers were made on the testpieces B and H.
However, surprisingly, the fact of using the difference between first and second values σ1 and σ2 makes it possible, as can be seen from the curve in
Thus, the method of the invention using the difference between the indicator values σ1 and σ2 as correlated with the thickness er makes it possible to measure thickness with improved accuracy. It should be observed that in a preferred implementation, the thickness values σ1 and σ2 of each testpiece are averages obtained from a plurality of successive measurements, thereby limiting any risk of error in the predetermined data correlating differences and thicknesses. It has been found that the method of the invention enables the thickness of the layer to be measured with tolerance of ±10 micrometers (μm) for carbide layers lying in the range 25 μm to 150 μm.
On the basis of the calibration data measured on the testpieces A-I, a polynomial regression is used to define a curve or a bijective function that, over a given thickness range, associates each thickness value with a difference between the values σ1 and σ2 of the conductivity indicator. This correlation relationship by means of a curve and/or by means of a mathematical function is stored in a memory and is subsequently used for taking measurements on workpieces P.
As explained below, the frequencies F1 and F2 are selected as a function of the first and second materials of the workpieces on which layer thickness measurements are to be made. Thus, for conductivity measurements taken on calibration testpieces having a correlation with the layer thicknesses is substantially equal to the correlation between the conductivity measurements taken on workpieces, it is desirable for the substrate thickness of the testpieces to be such that the field penetrates by less than half and preferably by less than one-third into the thickness of the testpiece substrate. The formula for calculating the penetration depths of the field at a given frequency of the probe signal is given below under the heading pre-calibration of the measurements.
Under this heading it is explained how the first and second frequencies F1 and F2 are selected for use in taking measurements on workpieces P and on calibration testpieces in accordance with the method of the invention.
This pair of frequencies F1 and F2 for the alternating probe electrical signal must be suitable for inducing fields B in the workpieces P such as to make it possible to obtain a correlation between the indicator value difference and the thickness of the coating layer on the substrate that is to be measured.
For this purpose, a series of pre-calibration testpieces A-I is prepared in which the material of the substrates and the material of the coating layers are identical respectively with the material of the substrates and the material of the coating layers of the workpieces on which the coating thicknesses are to be measured. In the example below, the pre-calibration testpieces are the same as the calibration testpieces, however it would also be possible to use a series of pre-calibration testpieces that is different from the series of calibration testpieces.
After preparing the series of pre-calibration testpieces A-I, the thicknesses of their respective layers are measured and recorded (these thicknesses being given in the above table).
The frequencies Fn of the probe electrical signal S fed to the induction means Mind are selected so that the magnetic field B generated by the induction means in the workpiece does not pass right through the substrate Sub. This reduces disturbances to measurements that might otherwise occur as a result of the magnetic field Bn being reflected against an outside face Fext of the substrate Sub.
Limiting the penetration depth of the electromagnetic field Bn in this way serves to avoid any need to take account of the thickness es of the substrate Sub when measuring impedance. It should be observed that the substrate thickness of a workpiece is generally well known since it is determined while fabricating the substrate, prior to applying the coating.
The standard penetration depth δ of an induced field is given by the formula:
δ=503/(√σ×μr×Fn)
where:
In order to determine the high frequency, F2, it is desired for the magnetic field B as generated at the frequency F2 to pass through at least one-third of the thickness of the layer that is to be measured. In this way, the measured physical characteristic Z is indeed a function of the thickness of the layer.
In summary, having approximate knowledge of the thickness es of the substrate and of the conductivity σn of the material constituting the substrate Sub, it is ensured that each of the fields induced by the signals at the selected frequencies F1 and F2 do indeed have respective penetration depths δ1 and δ2 that are less than one-third of the thickness es of the substrate Sub. The lower frequency F1 is selected to have a field penetration depth that is greater than the thickness of the layer to be measured. The higher frequency F2 is selected so that the field penetrates into at least one-third of the thickness of the coating layer.
A coating Rev made of tungsten carbide WC deposited by high velocity spraying (using a technique of the type known as high velocity hydroxy oxygen fuel (HVOF) coating) has conductivity σWC≈0.8 megasiemens per meter (MS/m), a WC—Cr—Co coating has conductivity of 0.8 MS/m, and titanium has conductivity σTi lying in the range 0.6 MS/m<σTi<0.8 MS/m. The conductivity of the Ti10.2.3 titanium alloy is σTi10.2.3≈0.74 MS/m, and the conductivity of the Ti5.5.5.3 alloy is σTi5.5.5.3=0.61 MS/m.
Using the above-mentioned formula and conductivity values, the penetration depths δn of the field Bn in the materials are determined as a function of frequency Fn.
The following values are thus obtained:
Thus, the low frequency Fn≈10 kHz cannot be used unless the thickness of the titanium substrate Sub is greater than 3×δn=5070 μm.
The low frequency Fn≈60 kHz can only be used for taking measurements if the thickness of the titanium substrate Sub is greater than 3×δn=2070 μm.
The low frequency Fn≈120 kHz can only be used for taking measurements if the thickness of the titanium substrate Sub is greater than 3×δn=1400 μm.
In order to determine the values of the first and second frequencies F1 and F2, a series of pre-calibration measurements are performed on each of the pre-calibration testpieces for which the coating layer thicknesses are known, these measurements being taken at the various frequencies Fn of the predefined series of pre-calibration frequencies. The predefined series of pre-calibration frequencies in the presently-described example lies in the range 60 kHz to 960 kHz.
Each pre-calibration measurement in the series of measurements is performed:
After taking all of the measurements at the various frequencies for all of the pre-calibration testpieces, all of the values taken by the conductivity indicator are recorded and this data is analyzed in order to identify the pair of frequencies F1 and F2 that makes it possible to achieve the best correlation between the indicator value difference and the corresponding thickness values for the pre-calibration testpieces.
The analysis for determining the pair F1 and F2 consists:
Thereafter, a series of correlation coefficients is calculated in order to evaluate the suitability of each of these potential pairs of frequencies for the purpose of measuring thickness.
In the above example, the potential pairs of frequencies are a first pair 60 kHz & 120 kHz, a second pair 60 kHz & 480 kHz, a third pair 60 kHz & 960 kHz, a fourth pair 120 kHz & 480 kHz, a fifth pair 120 kHz & 960 kHz, and a sixth pair 480 kHz & 960 kHz.
By using the data obtained for each of the pairs, it is possible for each pair to calculate a corresponding correlation coefficient R2 representative of the correlation between:
For example, by taking measurements at the above-mentioned frequencies on a series of titanium testpieces, each having a layer of tungsten carbide, the smallest correlation coefficient that is calculated is the coefficient that was obtained with the pair of frequencies 120 kHz & 960 kHz, for which R2=0.7162, and the greatest correlation coefficient is the coefficient that was obtained with the pair of frequencies 60 kHz & 480 kHz, for which R2=0.9595.
In order to measure thicknesses on workpieces P having the same substrate and layer materials, the pair of frequencies selected from the potential pairs of frequencies is the frequency pair 60 kHz & 480 kHz, since it presents the best correlation.
In other words, said first and second frequencies F1 and F2 are determined by selecting from the various pairs of frequencies at least one pair that enables at least a predetermined minimum level of correlation to be obtained between:
It should be observed that the predetermined minimum level of correlation is selected to be not less than 0.90, since below that, it is considered that the correlation between the differences and the thicknesses is insufficient to be able to obtain a layer thickness measurement with satisfactory tolerance.
The invention is not limited to the above-described examples and it may include other implementations that are not described.
In particular, the invention may also relate to a method of fabricating a workpiece in which the coating is abraded and/or trued and in which the movement of the abrasion and/or truing tool is controlled as a function of measurements of the thickness of the coating layer performed using the measurement method of the invention. The thickness is preferably measured at the same time as the abrasion and/or truing operation. Controlling the tool by using the measured thickness of the coating layer makes it possible to move the tool so as to cause the thickness of the coating layer to tend towards a predetermined thickness. In this implementation, the nozzle B in
| Number | Date | Country | Kind |
|---|---|---|---|
| 11 59513 | Oct 2011 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2012/070599 | 10/17/2012 | WO | 00 | 4/8/2014 |
