GB2300477A - Determining surface coating adhesion - Google Patents
Determining surface coating adhesion Download PDFInfo
- Publication number
- GB2300477A GB2300477A GB9508834A GB9508834A GB2300477A GB 2300477 A GB2300477 A GB 2300477A GB 9508834 A GB9508834 A GB 9508834A GB 9508834 A GB9508834 A GB 9508834A GB 2300477 A GB2300477 A GB 2300477A
- Authority
- GB
- United Kingdom
- Prior art keywords
- protective coating
- laser beam
- adhesion
- laser
- strength
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N25/00—Investigating or analyzing materials by the use of thermal means
- G01N25/72—Investigating presence of flaws
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/84—Systems specially adapted for particular applications
- G01N21/8422—Investigating thin films, e.g. matrix isolation method
Abstract
The upper surface 3 of the protective coating 1 of a surface 2 is heated by a pulsed laser beam 4 such that a difference in temperature between it and its lower surface 7 causes a bending stress at the point of heating. The resultant deformation of the coating is measured via a heterodyne interferometer and gives an indication of the level of adhesion between the coating and the protected surface.
Description
METHOD AND APPARATUS FOR DETERMINING SURFACE COATING ADHESION
This invention relates to the determination of the degree of adhesion of a protective coating, such as paint, to a surface.
It is well known to apply various adhesive coatings to surfaces for the purpose of protecting the surface from corrosion or other environmental deterioration.
Apart from the intrinsic properties of the chosen coating material, the degree of protection is dependent on achieving good adhesion between the coating material and all of the protected surface to be protected.
The degree of adhesion is strongly affected by the quality of surface preparation, prior to the application of the protective coating, and it is well-known that the presence of surface contaminants, such as water or organic materials, will result in poor adhesion between the protective coating and the contaminated portion of the surface.
In known production processes, the strength of adhesion is measured by destructive mechanical techniques which essentially involve measuring the force needed to remote the protective coating from the surface. The destructive nature of such techniques precludes the possibility of inspecting the quality of adhesion for each item, and relies on the destructive testing of occasional samples.
It is an object of the present invention to provide a method of, and apparatus for, non destructive testing of the strength of adhesion between a protective coating and a surface, thereby avoiding the waste implicit in destructive testing, and enabling the introduction of more rigorous .testing.
According to one aspect of the invention a method of non-destructive testing of the strength of adhesion between a protective coating and a surface includes heating a small area of the protective coating, measuring the resultant surface deformation, and using the measurement of resultant surface deformation to determine the strength of adhesion.
The method preferably includes producing a temperature rise of between 1"C and 10oC in the outermost surface of the protective coating.
The method preferably includes using a laser beam to heat the small area of the protective coating. The method preferably includes using a pulsed laser beam. The pulsed laser beam is preferably modulated at a fixed frequency in the range of 10 to 1000 pulses per second.
The method preferably includes using heterodyne optical interferometry to measure the resultant surface deformation. Preferably the resultant surface deformation is measured as the velocity with which the outermost surface of the small area of the protective coating moves away from the surface.
According to another aspect of the invention apparatus for non-destructive testing of the strength of adhesion between a protective coating and a surface includes means for directing radiant energy onto a small area of the protective coating, means for measuring the surface deformation of the small area of the protective coating as it is heated by the radiant energy, and means for assessing the strength of adhesion from the surface deformation. Preferably the means for measuring the surface deformation is arranged to measure the rate of surface deformation.
The means for directing radiant energy may include a laser and a modulator for pulsing the laser beam at a fixed frequency, or alternatively a laser diode and a pulsed current source for driving the laser diode at a fixed frequency. In either case the means for directing radiant energy preferably includes a probe to be placed in contact with the surface of the protective coating, and an optical fibre for transmitting the laser to a position immediately adjacent the area of the protective coating that is to be tested.
The means for measuring the surface deformation of the small area of protective coating is preferably a heterodyne optical interferometer. The heterodyne optical inteferometer may include a laser operable to produce a laser beam, a beam splitter for dividing the laser beam into first and second portions, a first modulator for driving the first portion of the laser beam at a first fixed frequency, a second modulator for driving the second portion of the laser beam at a second fixed frequency, means for directing the first portion of the laser beam onto the small area to be tested, means for directing both the second portion of the laser beam at the second fixed frequency and a reflection of the first portion of the laser beam from the small area of the protective coating into overlapping relationship on a photodetector to produce an interference fringe at a frequency which is the difference between the first and second frequencies, a filter having a centre frequency tuned to the difference between the first and second frequencies and arranged to transmit the filtered output of the photodetector to a frequency demodulator to produce a voltage output proportional to the difference between the first and second frequencies, and an integrator arranged to produce a voltage output proportional to the time interval of the input voltage.
The invention will now be described by way of example only, with reference to the accompanying drawings, in which:
Figure 1 is a diagram illustrating the effect of heating a small area of a protective coating;
Figure 2 is a diagram of one form of apparatus for non-destructive testing of the strength of adhesion between a coating of paint and a surface, and
Figure 3 is a diagram illustrating an alternative form of apparatus.
Paint coatings may consist of single or multiple layers with overall thickness typically in the range 0.01 - 1 .0mum depending on the application. Painted metal articles for domestic use may have paint thicknesses of 0.01 - 0.05mm, whereas coatings for industrial tanks containing corrosive liquids may be have thicknesses of about 1 mm.
The principle of the invention, which is now described with reference to Figure 1, involves illuminating a small area of a paint layer 1 deposited on a surface 2 which may for example be a metal surface. The upper surface 3 of the paint layer 1 is heated, preferably by a focused laser beam 4 from a laser 5. The output from laser 5 is pulse modulated typically at a fixed frequency in the range 10 - 1000 pulses per second, by a modulator 6, which could be a mechanical chopper. Alternatively, if the laser 5 is a laser diode or laser diode array, the laser output may be modulated by current pulses applied to the laser diode, or a laser diode array, by a pulsed current drive circuit (not shown).The laser power and pulse length are made sufficient to produce a periodic temperature rise in the upper surface 3 of the paint layer 1 (typically in the range 1 - 10 C) in the small area where the laser beam 4 is applied. The resulting temperature difference between the upper surface 3 and the lower surface 7 of the paint layer 1 causes a bending stress in the layer at the point where the laser beam 4 is applied. If the strength of adhesion between the paint layer 1 and the underlying surface 2 is poor, then the paint will momentarily lift away from the underlying surface as a result of the force exerted by the bending stress in paint layer 1. Measurement of the displacement of paint layer 1 at the point where it is illuminated by laser beam 4 provides a means for determining the strength of the bond between paint layer 1 and the surface 2.
The periodic displacement of the paint surface resulting from pulsed illumination by laser beam 4 may have a peak amplitude varying from a few nanometres for thick well adhered coatings to a few micrometers for thin poorly adhered coatings. A sensitive means is therefore required for measuring the periodic surface displacement, such as heterodyne optical interferoiiietry.
A suitable type of heterodyne interferometer capable of measuring the surface displacement with the required sensitivity is now described with reference to Figure 2. A second laser 8 produces a stable single frequency beam, at a wavelength substantially different from laser 5, which passes through an optical isolator 9 and a beamsplitter 10. The light passing through beamsplitter 10 is diffracted into the first diffraction order by acousto-optic modulator 11 which is driven at a fixed frequency Fl provided by a signal generator 12. The light emerging from acousto-optic modulator 11 is shifted in frequency by the drive frequency F1.
This beam passes through a second beamsplitter 13 and is then focused by a lens 14 onto the paint surface at the point where the laser beam 4 strikes the surface. Light scattered by the paint surface 3 returns through lens 14 forming a collimated beam which is partially reflected from the beamsplitter 13. The reflected beam is reflected from a plane mirror 15 through a third beamsplitter 16 onto the surface of photodetector 18 via a narrow-band optical filter 17 tuned to the frequency of the laser 8. The function of the filter 17 is greatly to attenuate light at the wavelength of the pulsed laser 5 and to prevent light from the laser 5 from reaching a photodetector 18. Light from the laser 8 is partially reflected from beamsplitter 10 to pass through a second acousto-optic modulator 19 driven by a second signal generator 20 operating at a fixed frequency F2 different from F1.Light diffracted into the first order of diffraction by the acoustosptic modulator 19 is transmitted through the beamsplitter 16 onto the photodetector 18 via the filter 17. The two beams incident on the photodetector 18 are arranged to be parallel and to substantially overlap. The frequency stability of the single frequency laser 8 is chosen to be such that the coherence length of its laser beam is substantially greater than the path difference between the two beams incident on the photodetector 18, as measured between the laser 8 and the photodetector 18. Under these conditions a single interference fringe is formed on the surface of photodetector 18, modulated in intensity at a frequency (F1-F2). This produces an alternating component of photocurrent at frequency (F1-F2) in the output from photodetector 18, which is amplified by an amplifier 21.The amplified signal passes through a narrow band filter 22 having a centre frequency tuned to (F1-F2). The output from the narrow band filter 22 passes into a frequency demodulator 23 which produces a voltage output proportional to the frequency (F1-F2). The voltage output from the frequency demodulator 22 passes into an integrator 24 which produces a voltage output proportional to the time integral of the input voltage. The output is displayed on an oscilloscope 25 triggered from laser modulator 6.
In. practical applications, the frequencies F1 and F2 may typically be 80Mhz and 80.03Mhz respectively, giving a nominal frequency difference (F1-F2) of 30kHz. The laser 5 may typically be a high power laser diode array giving an overall output power in the range 1 3W at a wavelength in the range of 800 to 1000nm. Laser 8 may typically be a single frequency stabilised helium-neon laser, or a single frequency laser diode operating in the wavelength range 800 to 1 600nm.
The apparatus described with reference to Figure 2 is a heterodyne optical interferometer.
Its function is to produce a voltage output proportional to the displacement of the paint surface 3. It operates in the following way. The component of movement of the paint surface 3 in a direction parallel to the laser beam from the laser 8 causes the scattered light to be shifted in frequency from the frequency of the incident light by the Doppler effect. Movement of the paint surface 3 therefore causes the frequency (F1-F2) produced in the output from the photodetector 18 to be shifted from its nominal value by an amount proportional to the velocity of the paint surface 3. The voltage emerging from the frequency demodulator 23 is therefore proportional to the velocity of the paint surface 3. The output from the integrator 24 is therefore proportional to the displacement of the paint surface 3.The sensitivity of heterodyne interferometers of this type is such that surface displacements very much smaller than a wavelength of light can be measured.
The operation of a paint adhesion measuring apparatus incorporating such a heterodyne interferometer is as follows. The laser 5 is modulated at a fixed frequency, typically in the range 10 to 1000Hz, by the modulator 6. A time profile of the resulting displacement of the paint surface 3 is displayed repetitively on oscilloscope 25. The amplitude and shape of the time profile of the displacement of the paint surface 3 is compared with results obtained from calibrating paint samples of known characteristics. Significant differences in displacement pulse amplitude or in displacement pulse shape indicate a difference in the strength of paint adhesion to the underlying surface 2. The measurement may be repeated at several points on a paint surface to obtain an overall assessment of the coating quality.
Large differences in the amplitude of surface displacement signals may be obtained from coatings of different thicknesses and different values of optical absorption at the laser wavelength of the laser 5. In assessing the quality of coating adhesion it is therefore necessary to compare the displacement signal from a paint coating with that from a reference coating of similar thickness made from the same paint material.
The apparatus described with reference to Figure 2 can be simplified, by the use of fibreoptics, to provide a hand-held instrument such as that now described with reference to Figure 3 in which a diode array 26 is driven from a pulsed current source 27 at a fixed frequency which may be in the range 10 to 1000 pulses per second. The output from the laser diode array 26 is collected by a compound lens 28 and a cylindrical lens 29 which together focus the output of the laser diode array 26 onto the end of a multimode optical fibre 30. The function of the anamorphic lens system, comprising lenses 28 and 29, is to compensate for the highly elliptical output beam shape typically produced by laser diode arrays in order to focus the laser output efficiently, into the circular aperture of the optical fibre 30.The output end of the optical fibre 30 is incorporated in a hand held probe 31 which is placed in contact with the paint surface 32 to be examined. The output end of the fibre 30 is positioned very close to the paint surface 32 (preferably within lmm), so that only a small area of the paint surface is illuminated.
Alternatively the output of the fibre 30 may be focused onto the paint surface by an auxiliary lens (not shown).
A second laser diode 33 having a single frequency near infrared type operates in a continuous mode. This laser is provided, as shown, with a pigtail connection coupling to a second optical fibre 34 of a single mode polarisation-maintaining type. This fibre passes through an optical isolator section 35 which prevents light returning along the fibre from passing into the laser diode 33 and from causing instabilities in its frequency. The optical fibre 34 passes through an optical fibre coupler 36 which couples 50% of the light into a single mode optical fibre 37. The output end of the optical fibre 34 passes into the hand-held probe 31 and is positioned such that the fibre end is held very close to the end of the optical fibre 30, and within lmm of the paint surface 32. Light emerging from the end of the fibre 34 illuminates the paint surface, and a small percentage is scattered back into the core of the fibre 34 to return along the fibre, and is partially coupled into the photodetector 38 via an optical fibre coupler 36 and a narrow band optical filter 39 which prevents light at the wavelength of laser diode array 26 from reaching the detector 38, but allows light from the laser diode 33 to pass through to the detector 38. Light passing along the fibre 37 passes through a piezoelectric phase modulator 40 and is reflected back along the fibre from its reflective end 41. Light passing back along the fibre 37 passes through the optical fibre coupler 36 and the photodetector 38 where it interferes with light reflected from the paint surface.
The fibre phase modulator 40 consists of a cylinder of piezoelectric material with several turns of the optical fibre wound round it, securely stuck to the surface by a suitable adhesive.
When a voltage is applied between surface electrodes on the inside and outside of the piezoelectric cylinder, its circumference expands, and the length of the optical fibre attached to it increases. The relative phase of an optical wave passing along the fibre can therefore be modulated by applying a voltage to the piezoelectric material. The optical frequency of light passing through the phase modulator can be altered by applying a ramp voltage to the piezoelectric material which increases linearly with time.
The phase modulator 40 is driven by a sawtooth waveform from a signal generator 42 which is locked in frequency to the drive pulses from the laser driver 27. The sawtooth waveform from the signal generator 42 provides a linearly increasing voltage to a piezoelectric phase modulator 40 which alters the frequency of light reflected from the end of the fibre 37.
The signal produced by the photodetector 38 therefore contains an alternating current at the difference frequency generated by the phase modulator 40. The output from the photodetector 38 is amplified by an amplifier 43 and then passed into a frequency demodulator 44 which produces an output voltage proportional to the frequency of the input. The output from the demodulator 44 is passed to an integrator 45 and from there to an oscilloscope dlsplay 46 which is triggered from pulses from the laser driver 27.
Displacements of the paint surface 32 caused by periodic heating from the pulsed laser beam generate Doppler frequency shifts in the return light passing along the fibre 37 which appear as frequency modulation of the signal produced by the photodetector 38. The integrated output appearing on the oscilloscope 46 is a voltage representing the periodic displacement of the paint surface 32, which is compared with corresponding signals from standard paint samples to determine the quality of paint adhesion. The hand held probe would typically be moved to a number of different positions on the paint surface, displacement measurements being made at each point to obtain an overall estimate of the quality of the adhesion between the paint coating 1 and the surface 2.
Claims (18)
1. A.method of non-destructive testing of the strength of adhesion between a protective
coating and a surface, including heating a small area of the protective coating, measuring
the resultant surface deformation, and using the measurement of resultant surface
deformation to determine the strength of adhesion.
2. A method, as in claim 1, including producing a temperature rise of between r C and 10"C in the outermost surface of the protective coating.
3. A method, as in Claim 1 or 2, including using a laser beam to heat the small area of the
protective coating.
4. A method, as in claim 3, including using a pulsed laser beam.
5. A method, as in claim 4, including modulating the pulsed laser beam at a fixed frequency
in the range of 10 to 1000 pulses per second.
6. A method, as in any preceding claim, including using heterodyne optical interferometry
to measure the resultant surface deformation.
7. A method, as in claim 6, including measuring the resultant surface deformation as the
velocity with which the outermost surface of the small area of the protective coating
moves away from the surface.
8. A method of non-destructive testing of the strength of adhesion between a protective
coating and a surface as substantially described herein with reference to any of the
accompanying drawings.
9. Apparatus, for non-destructive testing of the strength of adhesion between a protective
coating and a surface, constructed to operate in accordance with the method of any of
claims 1 to 8.
10. Apparatus for non-destructive testing of the strength of adhesion between a protective
coating and a surface, including means for directing radiant energy onto a small area of
the protective coating, means for measuring the surface deformation of the small area of
the protective coating as it is heated by the radiant energy, and means for assessing the
strength of adhesion from the surface deformation.
11. Apparatus, as in claim 10, in which the means for measuring the surface deformation is
arranged to measure the rate of surface deformation.
12. Apparatus, as in claim 10 or 11, in which the means for directing radiant energy includes
a laser and a modulator for pulsing the laser beam at a fixed frequency.
13. Apparatus, as in claim 10 or 11, in which the means for directing radiant energy includes
a laser diode and a pulsed current source for driving the laser diode at a fixed frequency.
14. Apparatus, as in claims 12 or 13, in which the means for directing radiant energy
includes a probe to be placed in contact with the surface of the protective coating, and
an optical fibre for transmitting the laser to a position immediately adjacent the area of
the protective coating that is to be tested.
15. Apparatus, as in any of claims 10 to 14, in which the means for measuring the surface
deformation of the small area of protective coating is a heterodyne optical interferometer.
16. Apparatus, as in claim 15, in which the heterodyne optical interferometer includes a laser
operable to produce a laser beam, a beam splitter for dividing the laser beam into first
and second portions, a first modulator for driving the first portion of the laser beam at
a first fixed frequency, a second modulator for driving the second portion of the laser
beam at a second fixed frequency, means for directing the first portion of the laser beam
onto the small area to be tested, means for directing both the second portion of the laser
beam at the second fixed frequency and a reflection of the first portion of the laser beam
from the small area of the protective coating into overlapping relationship on a
photodetector to produce an interference fringe at a frequency which is the difference
between the first and second trequencies, a filter having a centre frequency tuned to the
difference between the first and second frequencies and arranged to transmit the filtered
output of the photodetector to a frequency demodulator to produce a voltage output
proportional to the difference between the first and second frequencies, and an integrator
arranged to produce a voltage output proportional to the time interval of the input
voltage.
17. Apparatus, for non-destructive testing of the strength of adhesion between a protective
coating and a surface, substantially as described with reference to Figures 1 and 2 of the
accompanying drawings.
18. Apparatus, for non-destructive testing of the strength of adhesion between a protective
coating and a surface, substantially as described with reference to Figures 1 and 3 of the
accompanying drawings.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9508834A GB2300477B (en) | 1995-05-01 | 1995-05-01 | Method and apparatus for determining surface coating adhesion |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9508834A GB2300477B (en) | 1995-05-01 | 1995-05-01 | Method and apparatus for determining surface coating adhesion |
Publications (3)
| Publication Number | Publication Date |
|---|---|
| GB9508834D0 GB9508834D0 (en) | 1995-06-21 |
| GB2300477A true GB2300477A (en) | 1996-11-06 |
| GB2300477B GB2300477B (en) | 1999-05-26 |
Family
ID=10773810
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB9508834A Expired - Fee Related GB2300477B (en) | 1995-05-01 | 1995-05-01 | Method and apparatus for determining surface coating adhesion |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2300477B (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE10207692A1 (en) * | 2001-12-07 | 2003-06-26 | Fraunhofer Ges Forschung | Method for testing the quality of adhesive/glued bonds in a multilayer structure stiffens a first layer or a covering layer on it by bonding the first layer to a surface via a bonding layer |
| WO2004042373A1 (en) * | 2002-11-06 | 2004-05-21 | Koninklijke Philips Electronics N.V. | Method of adhesion measurement at the interface between layers |
| RU2481569C2 (en) * | 2011-02-02 | 2013-05-10 | Государственное образовательное учреждение высшего профессионального образования Самарский государственный технический университет | Method of evaluating gas content in materials with coatings |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN114858755B (en) * | 2022-07-05 | 2022-10-21 | 中国航发四川燃气涡轮研究院 | Aero-engine coating frequency conversion in-situ laser detection system |
Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0492559A2 (en) * | 1990-12-21 | 1992-07-01 | Laser Technology, Inc.(A Corporation Of Pennsylvania) | Nondestructive testing using air-coupled acoustic excitation |
-
1995
- 1995-05-01 GB GB9508834A patent/GB2300477B/en not_active Expired - Fee Related
Patent Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0492559A2 (en) * | 1990-12-21 | 1992-07-01 | Laser Technology, Inc.(A Corporation Of Pennsylvania) | Nondestructive testing using air-coupled acoustic excitation |
Non-Patent Citations (1)
| Title |
|---|
| Physics Bulletin No.38, 1987, page 291 * |
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE10207692A1 (en) * | 2001-12-07 | 2003-06-26 | Fraunhofer Ges Forschung | Method for testing the quality of adhesive/glued bonds in a multilayer structure stiffens a first layer or a covering layer on it by bonding the first layer to a surface via a bonding layer |
| DE10207692B4 (en) * | 2001-12-07 | 2004-08-12 | Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. | Method for checking the quality of an adhesive or adhesive connection in a multilayer arrangement |
| WO2004042373A1 (en) * | 2002-11-06 | 2004-05-21 | Koninklijke Philips Electronics N.V. | Method of adhesion measurement at the interface between layers |
| RU2481569C2 (en) * | 2011-02-02 | 2013-05-10 | Государственное образовательное учреждение высшего профессионального образования Самарский государственный технический университет | Method of evaluating gas content in materials with coatings |
Also Published As
| Publication number | Publication date |
|---|---|
| GB9508834D0 (en) | 1995-06-21 |
| GB2300477B (en) | 1999-05-26 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| 732E | Amendments to the register in respect of changes of name or changes affecting rights (sect. 32/1977) | ||
| PCNP | Patent ceased through non-payment of renewal fee |
Effective date: 20100501 |