LU83767A1 - METHOD AND DEVICE FOR MEASURING THE THICKNESS OF A COATING ON A METAL SURFACE - Google Patents

Description

- 1 - C 2149/8111.

> METALLURGICAL RESEARCH CENTER -CENTRUM VOOR RESEARCH IN DE METALLURGIE,

Non-profit association -Vereniging zonder winstoogmerk in BRUXELLES, (Belgium).

Method and device for measuring the thickness of a coating on a metal surface.

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

t

The metallic surfaces, in particular the surfaces of the sheets, frequently bear a coating which may have been produced intentionally (protection, lubrication, etc.) or not (oxide layer formed at high temperature, layer of elements having segregated near the surface, ...). This coating can have favorable effects (layer of oil, phosphate, ...) or harmful (layer of pollutant, layer of scale, ...) on the possibilities of use of the surface.

4 7L- / I "- 2 -

In the typical case of continuous annealing of steel sheets, an oxide is formed at high temperature on the surface of the sheets; the thickness and the nature of this oxide depend on the annealing conditions (temperature, running speed, etc.) and it is essential that the thickness of the oxidized layer does not exceed a critical value so that subsequent pickling is not not compromised.

To ensure the quality of a surface and therefore of a product, it is necessary to detect and measure the characteristics (nature, thickness, etc.) of the layer present on the metal.

These data also make it possible to act on the manufacturing process to optimize the quality of the surface. For example, in the case of continuous annealing, it is possible, inter alia, to act on the speed of movement of the sheet to keep the thickness of the oxide layer within acceptable limits.

With regard to coatings of relatively large thickness, there are different measurement methods (magnetic, acoustic, etc.), some of which are applicable continuously.

For thinner coatings, on the other hand, the problem is more complex and the measurements are currently carried out by means of superficial microanalyses (electron microprobe, ionic microprobe) which are 'very precise techniques, but relatively slow and inapplicable for control. continued.

The present invention relates to a device making it possible to remedy these drawbacks.

We know that it is possible to detect the presence and to measure certain characteristics (nature, thickness) of the coating of metallic surfaces by measuring the coefficient jl of reflection or absorption of the surfaces concerned with respect to r * - 3 - from the light and especially advantageously, vis-à-vis a light located in the far infrared.

The coefficient of reflection or absorption of a surface can be defined as equal to the value of the fraction of light energy reflected or absorbed by this surface, when it is touched by a light beam of given energy. The reflected energy to be considered is measured by accumulating the reflected energies in all directions, that is to say that it is independent of the direction of reflection.

If we refer to FIG. 1 representing an incident beam 1, a metallic surface 2 and a corresponding reflected beam 3, we have the following references: I = incident light energy R.Iq = reflected light energy R '"reflection coefficient A .Iq = absorbed light energy A = absorption coefficient A + R = 1.

The infrared range (wavelengths between 0.7 and 1000 yU "m) and particularly the far infrared (wavelengths between 3 and 1000 / tm) is well suited for these measurements, because the absorption of metal surfaces are strongly influenced by the presence of a coating.

Indeed, for these wavelengths: - the usual metals (steel, stainless steel, tinned steel, aluminum, copper, brass, ...) have a low absorption coefficient. For example, with respect to a wavelength of 10 / ira, the base metals all have an absorption coefficient I of less than 0.10.

î ~ t - 4 -

If we refer to FIG. 2 representing an emission source 4, an incident beam 5, an uncoated metal surface 6, a corresponding reflected beam 7 and a measuring device 8, we have the following references: I = incident light energy o I (1 -) = reflected light energy o ms I .o (= absorbed light energy 0 ms = absorption coefficient of the “'ms ^ metallic surface (~ 0.1).

- The coatings commonly present on the surface of the metal / (çocyde, phosphate, water, oil, ...) have a high surface absorption coefficient (° ^ og) and a high mass absorption coefficient (0 (^)). This mass absorption coefficient om ^ es ^ PS * relation: I'x "o * e" where I '= light energy, at the entrance of the coating 1' = remaining light energy, after crossing a thickness x of coating.

Referring to FIG. 3 representing an emission source 9, an incident beam 10, a coated metal surface 11, two corresponding reflected beams, respectively one 12 at the entrance to the coating and the other 13 at 1 coating surface - metal surface and a measuring device 14, the following references are available: 1 = incident light energy 1 (1- 0 (Qg) = light energy reflected on the exterior surface of the coating // I -0 ^ . · E. ^ ° ^ om ^. (L -) = light energy reflected at the coating interface - ^ metal surface Γ λ;. ·. · "" · - 5 - R = overall reflection coefficient = (1- 0 (J + (1 - *), <e ~ 2 * ° rnâ os ms ^ os A - overall absorption coefficient

= 0 <Γΐ - (1 - OC) e "2 * omdl os ms. J

The two coefficients R and A are directly related to the distance d traveled by the light in the coating and can therefore be used to measure this thickness.

On the basis of these considerations, the process which is the subject of the present invention is essentially characterized in that a light beam of intensity I is directed in the direction of the coated surface, in that the totality of the light reflected in all directions (Ir) and in that the thickness of the coating is calculated using the known formula I = R. I where R is the reflection coefficient which ro is a function of said thickness .

In order to accentuate the contrast between the light beam and the environment (the object's own radiation), trains of pulses can be emitted.

According to a mode of the invention, the light beam is directed towards the coated surface with reflection on an auxiliary surface which returns it again to the coated surface and so on by multiple reflections before detection, so as to increase the sensitivity of the device.

In fact, according to FIG. 4, the source 15 emits a light beam 16 which meets at 17 the coated metallic surface 18 with overall reflection coefficient R. This surface 17 reflects the following beam 19, in the direction of a surface / auxiliary 20 of reflection coefficient R ^ which it reaches at point 21. After multiple reflections between these two surfaces * * - 6 - (16, 19, 22, 23, 24, ..., n,) the beam results in the measuring device 25.

L Light intensity arriving on the device 25 is: __n + l_n ",, η + 1 _ n IR R, or I (1 - A) R, O 1 o 1

According to an advantageous variant of the previous modality, the auxiliary surface by which the multiple reflections occur simultaneously acts as a detection of the intensity of the light beam.

FIG. 5 represents such a variant where the surface 17 is replaced by the measuring device 26, the same references representing the same elements.

The light intensity arriving on the apparatus 26 is worth:

IQ R Q + R Rx + (R Rx) 2 + (R R ^ 3 + ... J

and for a sufficiently large number of reflections, it is worth:

I R

° 1 - R R.

* 1

For R and R ^ of the order of 0.90, this intensity is approximately 5.26 I R, that is to say that the multiple reflections make 5.26 times greater the intensity of the reflected light.

The present invention also relates to a device for implementing the method described above.

The device, object of the present invention, is essentially characterized in that it comprises a light source, preferably infrared, a device for measuring the intensity! luminous site capable of integrating the reflected light in all directions and means for guiding the light between these two elements.

According to an advantageous embodiment of the invention, the light source must be powerful enough to allow a rapid response. This mode is necessary if you want to carry out a measurement continuously. A well suited source in such a case is a far infrared emitting laser.

There are lasers with DC> 2 emitting on a wavelength of 10.6 ^ m and capable of powers in very variable ranges going from a fraction of watt to several tens of watts (instrumentation laser).

The detector must obviously be sensitive in the wavelengths implemented by the transmitter. This detector can consist of a photoelectric cell, a calorimeter or a pyro-electric device.,

Figures 6 and 7 relate to practical embodiments of the invention, by way of non-limiting example.

Figure 6 relates to the case of a continuous annealing line and Figure 7, the case of an inspection line after pickling or phosphat at ion.

In FIG. 6, the sheet 27 to be examined moves in the direction of the arrow 28. The device consists of a pulsed source 29, a detector 30 and an electronic coupling system 31. The beam 32 emitted by the source 29 meets the sheet 27 at 33 and is reflected along 34, in the direction of detector 30.

The oxide layer covering the sheet 27 is relatively thick and a single reflection is sufficient to obtain satisfactory results. However, the sheet metal is at a fairly high temperature, of the order of several hundred de-JJ will, with consequent stray radiation. To f # k - 8 - increase the contrast between the measurement light and this stray radiation, the source is pulsed and the detector is electronically coupled to the frequency of the source.

In FIG. 7, the sheet 35 to be examined moves in the direction of the arrow 36. The device comprises a source 37, a lens 38 and a convex detector 39 whose convexity is turned towards the sheet 35. The beam 40 emitted by the source 37 is focused by the lens 38 and passes through the orifice 41 formed in the convex detector 39 to reach the sheet 35. The incident rays 42 and reflected 43 are multiple to obtain the maximum sensitivity necessary because of the small thickness of the coating. The sheet is at room temperature and the background noise is low: it is therefore not necessary to use a pulsed source.

at? / /> '*

Claims (8)

1. Method for measuring the thickness of a coating on a metal surface, characterized in that a light beam of intensity I is directed towards the surface o coated, in that the whole of the light reflected in all directions (Ir) and in that the thickness of the coating is calculated using the known formula I = R. I where R is the reflection coefficient which is a function of said thickness . 2. Method according to claim 1, characterized in that one transmits trains of light pulses and in that one • synchronizes the detection with the emission. 3. Method according to either of claims 1 and 2, characterized in that the light beam is directed towards the coated surface with reflection on an auxiliary surface which returns it again to the coated surface and so on, by multiple reflections, before detection. 4. Method according to claim 3, characterized in that the auxiliary surface by which the multiple reflections occur at the same time function of detecting the intensity of the light beam. " 5. Device for implementing the method according to either of claims 1 to 4, characterized in that it comprises a preferably infrared light source, a device for measuring the light intensity capable of 'integrate the light reflected in all directions and J means to guide the light between these two elements. f «* - 10 - 6. Device according to claim 5, characterized in that the light source is sufficiently powerful to allow a rapid response. 7. Device according to either of Claims 5 and 6, characterized in that the light source is a laser emitting in the far infrared. 8. Device according to claim 7, characterized in that the light source is a CC ^ laser emitting on a wavelength of 10.6 /^.m. / I / '| y »φ