GB2220065A

GB2220065A - Coating inspection

Info

Publication number: GB2220065A
Application number: GB8912131A
Authority: GB
Inventors: James Morris Milne
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1988-06-07
Filing date: 1989-05-26
Publication date: 1989-12-28
Also published as: GB8813423D0; GB8912131D0

Abstract

A method and an apparatus are provided for inspecting a coating (12) on a substrate (10) both to enable any disbonds to be located and to measure the thickness of the coating (12). The coated surface (13) is illuminated by a single brief pulse of radiation from a flash lamp (15) synchronlsed with the frames of an infra-red camera (20). The camera produces a TV- compatible signal which is recorded by a video recorder (22). Slow replay of the recording on a TV monitor (24) enables disbonds and thickness variations to be located, while averaging a stripped line of signal at a predetermined time interval after the pulse enables the thickness of that portion of the coating (12) to be determined. The relationship between surface temperature and coating thickness can be found by a calibration experiment. <IMAGE>

Description

Coating Inspection This invention relates to a method and an apparatus for inspecting a coasting on a substrate, to determine the thickness of the coating.

A technique is known from EP 0 089 760 B for testing an object for sub-surface defects and for obtaining an image indicating any such defects, the technique involving subjecting the surface of the object to a brief pulse of radiant energy, and observing the subsequent distribution of thermal energy over the surface with a TV-compatible infra-red camera. The signal from the camera is recorded and then played back, a field at a time, to find the field on which the defects are shown with best contrast.

A thermal wave technique is described in an article by P.M. Patel and D.P. Almond in Journal of Materials Science 20 (1985) pages 955-966 which is said to enable both the integrity (i.e. lack of coating delaminations or adhesion defects) and the thickness of a thermally sprayed coating on a substrate to be determined. This involves subjecting a spot on the surface of the coasting to a modulated or pulsed laser beam to generate thermal waves in the coating, and observing the temperature of that spot with an infra-red detector. From the phase of the observed temperature oscillations the thickness of the coating can be determined. A defect of adhesion can be detected similarly. This technique is however applicable only to a very small area at any one time.

According to the present invention there is provided a method for inspecting a coating on a substrate to determine the thickness of the coating, the method comprising subjecting the surface of the coating to a single brief pulse of radiant energy, observing the surface with an infra-red scanning camera which produces a TV-compatible signal and whose fields are synchronised with the pulse, recording the signal from the camera, and then determining from the signal the temperature of the surface of at least part of the coating at a predetermined time after the pulse, from which the layer thickness of that part of the coating is then determined.

Preferably the temperature is measured by stripping a line from the TV signal which corresponds to the image of the said part of the surface, and calculating the average signal in the stripped line.

The method can readily be combined with checking for any disbonds within the coating or between the coating and the substrate, as these will give an anomalously high temperature. The location of such disbonds can be checked by viewing complete frames of the recorded image on a monitor screen.

Initially, immediately after the pulse of radiant energy is received, all parts of the surface of the coating will be raised to the same temperature above ambient. Heat then diffuses through the coating and into the substrate, and eventually all parts of the surface will again be at ambient temperature. Where the thermal diffusivity of the coating is less than that of the substrate, then areas of thick coating will, at a certain time interval after the pulse, be at a higher temperature than areas of thin coating. The relationship between temperature and thickness may be determined by a calibration measurement using coatings of known thickness, and such a calibration is also desirably done to determine that time at which the differences between the temperatures of different thicknesses of coating are greatest.

The invention'also provides an apparatus for performing the above described method.

The invention will now he further and more particularly described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 shows a diagrammatic perspective view of an apparatus for inspecting a coating on a substrate; and Figure 2 shows graphically the variation of surface temperature with coating thickness at a particular time.

Referring to Fiqure 1, a sheet of steel 10 with a coating 12 of a nickel-chromium alloy on one surface 13 is set up with the coated surface 13 facing a 2000 joule photographic flash tube 15, which is connected to a power supply unit 16. An infrared camera 20 which produces a TV compatible output signal is set up facing the surface 13 of the sheet 10, and is connected to a video recorder 22 and a television monitor set 24. The video recorder 22 is also connected to a line-stripper and computer 26. The camera 20 produces a complete frame in 40 milliseconds, consisting of two interlineated fields, each taking 20 milliseconds.

It is sensitive to radiation of wavelength between 8 and 13 micrometres, and so is sensitive to the peak radiation intensity from a black-body radiator at about 300K, i.e.

near room temperature, and it is sensitive to temperature differences of as little as about 0.1K near room temperature. A suitable camera is available from Rank Taylor Hobson of Leicester, England.

The sheet 10 and the coating 12 are initially at ambient temperature. When the power supply unit 16 is energized, the flash tube 15 emits an optical pulse, the duration of which is about six milliseconds. Light from the flash tube 15 incident on the coated surface 13 of the sheet 10 is absorbed and converted to thermal energy within the coating 12 providing about 1 joule/cm2 of thermal energy, and raising the surface temperature by about 10K.

Heat flow then takes place through the coating 12 and into the sheet 10, distributing the thermal energy to cooler regions of the sheet 10. The rate at which the temperature of the surface 13 changes depends upon the thermal diffusivity of the material (i.e. thermal conductivity divided by the product of density and specific heat capacity) of the coating 12 and on its thickness; the surface of thicker regions of the coating cools more slowly than that of thinner regions. In addition if there are any regions of the coating 12 which are not effectively bonded to the sheet 10, the surface of those regions will cool down still less slowly.

The camera 20 and the video recorder 22 record the temperature distribution over the surface 13. The power supply 16 is arranged to be energised simultaneously with the commencement of one frame of the camera 20; and the camera 20 and the video recorder 22 can be left on until the temperature of the surface 13 of the coating 12 on the sheet 10 has become uniform. The temperature distribution following irradiation can subsequently be determined by playing back the recording taken by the video recorder 22 on the monitor 24, and individual frames selected for examination. This enables any regions of disbond, or of above- or below-average coating thickness to be located.

Best contrast will be seen after a certain time interval has elapsed since the optical pulse, and for some materials slow replay of the video recording may be advantageous as that time interval may be less than a tenth of a second.

Where it is desired to measure the thickness of a region of the coating 12 one or more TV lines of the TV image signal corresponding to that region at a particular time are stripped off by the line stripper 26 and averaged.

This signal is a function of the temperature of that region of the surface 13, at that time. By performing this procedure with a calibrating sheet 10 on which different regions of the coating 12 had different known thicknesses of between 50 and 400 micrometres it has been found that the initial rate of decrease of the temperature of the surface 13 is greatest where the coating 12 is thinnest, and that after a certain time has elapsed there is a simple relationship between the surface temperature and the coating thickness.Referring to Figure 2 there is shown graphically the relationship between the average amount by which the video signal exceeds the video signal corresponding to a surface at ambient temperature (which is a function of surface temperature) and the thickness of the coating 12, at a time 0.04 seconds after the pulse, as found by experiment, the squares indicating the experimentally determined points. Such a calibration graph can then be used to relate surface temperature to coating thickness when the thickness is unknown, if the other factors are unchanged (i.e. the time interval since the pulse, the camera sensitivity, and of course the thermal energy density, the surface emissivity, and the coating and substrate materials).

It will be appreciated that the accuracy of the surface temperature measurement (and hence of the coating thickness determination) can readily be increased by repeating the procedure several times, stripping the same line portion each time, and averaging all the signals thereby obtained. It will also be appreciated that the appropriate time interval must also be found by such a calibration experiment for different coatinq materials; if the time interval is too short there will have been no measurable decrease in temperature since the pulse, at least in thicker portions of the coating 12, whereas if the time interval is too long at least the thinner portions of the coating 12 will have already cooled to ambient temperature and so cannot be distinguished. Where the range of thicknesses is very wide it may be necessary to carry out the procedure at two different time intervals, distinguishing thinner portions by measurements taken after a short time interval, and distinguishing between the thicker portions by measurements taken after a longer time interval.

If the thermal diffusivity D of the coating material is known, and the layer thickness d is known approximately, then the time t after the flash at which the contrast is best seen can be estimated from: t = 2 x d2 D Since the stripped line is preferably a line of the camera scan, that is to say a TV line, it will be understood that where the coating thickness variation is to be determined along a particular line on the surface it is preferable to orient the camera 10 so that the scanning lines (or TV lines) are parallel to that line on the surface.

Claims

1. A method for inspecting a coating on a substrate to determine the thickness of the coating, the method comprising subjecting the surface of the coating to a single brief pulse of radiant energy, observing the surface with an infra-red scanning camera which produces a TV-compatible signal and whose fields are synchronised with the pulse, recording the signal from the camera, and then determining from the signal the temperature of the surface of at least part of the coating at a predetermined time after the pulse, from which the layer thickness of that part of the coating is then determined.

2. A method as claimed in Claim 1 wherein the predetermined time t is given approximately by t = 2 x d2 D where D is the thermal diffusivity of the coating material and d is an estimate of the thickness of the coating.

3. A method as claimed in Claim 1 or Claim 2 wherein the temperature is measured by stripping a line from the TV signal which corresponds to the image of the said part of the surface, and calculating the average signal in the stripped line.

4. A method for inspecting a coating on a substrate to determine the thickness of the coating substantially as hereinbefore described with reference to, and as shown in, the accompanyinq drawings.

5. A coating-thickness determining apparatus substantially as hereinbefore described with reference to, and as shown in, Figure 1 of the accompanying drawings.