FIELD OF DISCLOSURE
This disclosure relates to a methodology for thickness measurement of extremely thin optically translucent coatings on steel surface. Particularly the disclosure relates to an optical technique which enables in-situ thickness measurement of extremely thin optically translucent coatings on rough steel or other metallic surfaces.
BACKGROUND OF THE DISCLOSURE
A cold rolling mill in a steel plant produces sheets which may be a few meters wide and several kilometers in length. Depending on the final application, the steel sheets may be either galvanized or kept in bare condition. These sheets are stored in the form of coils for easy handling and transportation. In both cases, it is extremely important to preserve the surface condition of sheets from storage stains, oxidation etc. For this purpose, a protective layer is generally applied in the form of rust preventive oils for bare sheets or a secondary coating in the form of chromate, thin organic coatings etc. for galvanized sheets. It is essential to monitor the uniformity of these coatings along the entire length and width of the coil to ensure that the entire surface is coated uniformly. This will aid both as a quality assurance tool and a feedback for controlling the coating process.
These coatings are generally applied in the form of extremely thin layers of the order of 1-2 µm. Although there are destructive methods for measuring the coating thickness, it is imperative that a non-destructive method be developed which will enable online measurements in the mill. Since the coatings are applied in the form of such thin layers, any measurement tool should be at least able to measure it with an accuracy of 100-200 nm or better as a matter of consistency.

This is a significant challenge due to the inherent high-roughness of the base steel sheets with respect to the coating thicknesses in question. In addition, most non-destructive testing methods are indirect measurements which extract thickness information based on the interaction of some physical phenomenon with the coating. These physical phenomena may be influenced by various factors like properties coating material, roughness of steel surface etc.
Prior Art:
Surface of steel sheets (101) manufactured in a production line usually have a roughness of 400-500 nm (Ra) or greater. Thin coatings or oils (102) of thickness about 1 µm are applied on theses surfaces. It is therefore imperative that any measurement technique be able to measure the thickness of coating (102) under such conditions.
US 3601492 and US 4293224 use the interference fringe period ∆v (in cm1), the film refractive index n and the angle of incidence θ to calculate the film thickness from the expression: d=0.5*∆v(n2-sin2θ)1/2. This approach is however valid only for relatively uniform films whose thickness variations are small.
US 4748329 describes an optical thickness measuring apparatus using a polychromatic light source for projecting an incident light beam onto a surface of a sheet and detecting means for detecting the light reflected from or transmitted through the sheet at least three different wavelengths λ1, λ2 and λ3. λ1 corresponds to an absorption band of a material of the sheet. The second and the third wavelengths are outside the absorption band and are substantially free from absorption by the material. The thickness of the sheet is determined by quantifying the absorption of λ1 as compared to the other

wavelengths. This approach however is only valid for thickness measurements of sheets made of organic materials (containing CH bonds).
Many commercial X-ray and gamma ray based thickness gauges are available for online measurement metallic coatings. These gauges measure the coating thickness by quantifying the absorption or scattering of incident radiation. Scattering and absorption of high energy radiation are proportional to the density. These techniques are therefore applicable for metallic coatings or very thick layers of organic coatings, oils, lacquers etc as the absorption of radiation will be very low and thereby indistinguishable in case of extremely thin coatings. In addition, these techniques require to be operated under positive isolation due to the radiation hazard posed. (G. Kämpf, "A survey of methods for measuring the thickness of paint films," Progress in Organic Coatings, vol. 1, pp. 335-350, 1973.)
This disclosure focuses on a methodology for an optical technique for in-situ thickness measurement of extremely thin optically translucent coatings on steel or other metallic surfaces.
OBJECTS OF THE DISCLOSURE
Objective of the present disclosure is to measure the thickness of extremely thin optically translucent coatings on rough steel or other metallic surfaces. The present invention relates to a non-contact optical sensor consisting of a white light source, optical fiber and related optics for focusing of incident light and carrying the reflected light to a spectrophotometer which enables in-situ thickness measurement of optically translucent coatings on rough metallic surfaces.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
FIG. 1: Schematic depicting thin coating on rough steel surface.

FIG. 2: Reflection and transmission of light for a uniform coating on smooth
steel substrate
FIG. 3: Reflection spectra from a smooth steel sample with 1200nm thick
coating.
FIG. 4: Schematic representation of destructive interference of spectra from
a coating on rough metallic surface.
FIG. 5 illustrates various steps of a method for assessing thickness of thin optically translucent coating on substrate having roughness.
FIG. 6: Schematic representation of multi-core optical fiber and focusing
arrangement for measurements on coating on rough steel or metallic
surfaces.
FIG. 7: Measurement spectra from a 800nm thick coating on rough steel
surface.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
A ray of light travelling from a media with refractive index n1 into another media with refractive index n2 at an angle θ1 obeys Snell’s Law: n1sinθ1=n2sinθ2. Considering the case of a uniform coating with thickness d (201) and refractive index n2 on smooth steel substrate (202): White light incident on the coating from air (203) will partly reflect back into air (204) and partly transmit into the coating (205). Angles of reflection and transmission will be as per Snell’s Law.
In addition, the amplitude of reflection and transmission depends on the nature of polarization of light and is dictated by Fresnel coefficients:


where, rs and rp are the coefficient of reflection for s-polarized light and p-polarized light respectively. n1 and n2 are the refractive indices of the air and coating respectively and θ1 and θ2 are the angles of incidence and transmission respectively. Reflection of light transmitted into the coating will occur at the coating-metal interface (206). The light emerging from the coating (207) will have a phase difference δ with respect to the light directly reflected from the air-coating interface. The waves 204 and 207 will interfere with each other due to their relative phase difference as per Eqn. 4.

where, λ1 and λ2 are wavelengths of light in air and coating respectively. Phase difference condition for constructive interference being 2nπ and for destructive interference being (2n - 1)π; where n is an integer. Based on various trials, it was established that normal incidence of light (θ=0) on the samples give best signals for measurement. White light by its nature consists of a range of spectra with different wavelengths in the range of 390 to 700 nm. As per Eqn. 4 each wavelength component of white light will interfere with different intensities for different coating thicknesses. Thus, by obtaining the reflection spectra (wavelength vs amplitude) of the interfered white light (301) (obtained spectrograph) using a spectrophotometer and optical arrangement and curve fitting the obtained spectra iteratively with Eqn. 1-4 (302); we obtain the reference spectrograph which is used to calculate the coating thickness.

The interference of light is extremely sensitive to the thickness of the coating. Thus, for a particular beam of light having finite diameter (401), any rough metallic surface (402) will cause the beam to interact with a large variety of coating thicknesses. Due to these large variations in the coating thicknesses at various points of interaction of the beam, the spectra at every point of interaction will be different from the other (403). Thus, the net result will be the superposition of all the reflection spectra at all these points of interaction: resulting in destructive interference of the spectra (404). This disallows the measurement of coating thickness at rough surfaces.
A process (500) for assessing thickness of thin optically translucent coating on substrate having roughness has been proposed as shown in FIG. 5 The method comprises steps of:
step (504)creating a plurality of reference spectrograph between wavelength and reflectance of white light for each coating thickness using Fresnel’s equations.
At step (508) a spectrograph between wavelength and reflectance of the translucent coating at a point over substrate is obtained. This is done by means of spectrophotometer having other optical arrangement.
At step (512) the said spectrograph is curve fitted with the plurality of reference spectrograph and subsequently the thickness of the coating is determined.
At step (516) the steps 2 and 3 are repeated to identify thickness of various points over the substrate. Hence the thickness of the coating over the substrate is determined. In one of the embodiment the substrate is galvanized steel.

The optically translucent coating is thin organic coating or rust preventive oil, or chromate or similar.
A suitable arrangement is proposed for addressing this issue of destructive interference from rough surfaces. A multi-core optical fiber (601) having a central optical fiber for carrying the incident light (602) from a white light source (603). The surrounding optical fibers (604) are used for carrying the reflected light from the coating (605) to a spectrophotometer (606). The spectrophotometer is the device used for extracting the wavelength vs intensity spectra from reflected light. An optical focusing mechanism (607) in the form of a sequence of lenses is used for focusing the incident light (608) onto a very small area (609) on the test surface. This focusing ensures that the spectrum from a very small region of the coating is taken for analysis. Due to this focusing, the problem of large variation in coating thicknesses due to roughness (500nm Ra or less) is reduced to a great extent thereby giving reliable measurements.
A spectrum from thin organic coating on rough galvanized steel surface using a focal spot size of 7 μm is shown in Fig 7 (blue colored spectrogram). The method is applicable for any optically translucent coatings like rust preventive oils, chromate etc.

WE CLAIM :
1. A process for assessing thickness of thin optically translucent coating on
substrate having roughness, the process comprising steps of:
step-1 creating a plurality of reference spectrograph between
wavelength and reflectance of white light for each coating thickness
using Fresnel’s equations;
step-2 obtaining a spectrograph between wavelength and reflectance
of the translucent coating at a point over substrate by means of
spectrophotometer and optical arrangement;
step-3 curve fitting the said spectrograph with the plurality of
reference spectrograph for determining the thickness of the coating;
and
step-4 repeating the steps 2 and 3 to identify thickness of various
points over the substrate.
2. The process for assessing coating thickness of thin optically translucent coating on substrate, the substrate being galvanized steel.
3. The process for assessing coating thickness of thin optically translucent coating on substrate, the optically translucent coating being thin organic coating.
4. The process for assessing coating thickness of thin optically translucent coating on substrate, the optically translucent coating being rust preventive oil.

5. The process for assessing coating thickness of thin optically translucent coating on substrate, the surface roughness of the substrate being of the order 500nm (RA) or less.