X-ray or gamma ray systems or devices – Specific application – Diffraction – reflection – or scattering analysis
Reexamination Certificate
1999-01-19
2001-06-26
Church, Craig E. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Diffraction, reflection, or scattering analysis
Reexamination Certificate
active
06252930
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an in situ and non-destructive method and device for measuring the thickness of coatings comprising low-Z components, and more particularly the present invention relates to a method and device for measuring thickness of paint layers on substrates using backscattering of x-rays.
BACKGROUND OF THE INVENTION
The ability to measure, nondestructively and in situ the thickness of growing thin films is very advantageous in many industrial applications. For example, it is important to be able to monitor the thickness of paint being sprayed on cars, trucks or aircraft during production. The costs associated with painting vehicles, particularly in assembly line production is quite significant so that applying too thick a paint layer has serious economic repercussions. Alternatively, if there is too thin a paint layer this may result in the vehicle having to be repainted.
Post production painting of vehicles usually involves applying three distinct layers comprising a primer coating or layer applied directly to the metal substrate, a base coating containing the pigment applied on top of the primer coating and a clear coating applied on top of the base coating. The total thickness of these layers is about 0.05 mm to 0.10 mm with about half of the total thickness being due to the top clear coat. It is preferable that each layer be of a uniform thickness and manufacturers are particularly concerned about controlling the thickness of the base coat; however the base coat is the thinnest layer (about 0.01 mm) which makes it very difficult to control its thickness.
There are several known ways of estimating the average thickness of the paint layers. One is to simply weigh the paint used to cover a certain area and, knowing the mean density of the paint, calculate the average thickness which is generally expressed in units of mg/cm
2
, known as the “areal density.” Disadvantages of this and similar techniques is it is not an in situ technique, it is very labour intensive and does not give any information about the uniformity of the layers.
Another method and device for measuring paint thickness is disclosed in EP-A-0 380 226. This method relies upon irradiating a coating with x-rays and measuring fluorescent x-rays, Compton peaks and Rayleigh peaks from the spectrum of scattered x-rays. There are several drawbacks to the method and device. The device relies upon use of an expensive LO-AX hyperpure germanium detector. The method only works by adding a non-radioactive label (that fluoresces upon activation by x-rays or gamma rays) of atomic number greater than 20 and the label must be compatible with the radiation source due to correlation between fluorescence efficiency and energy of the x-rays. The method involves measuring the ratio of fluorescent x-rays and Compton rays and the ratio of Compton and Rayleigh yields. The fact that this method of measuring paint thickness is dependent upon adding a label is a major drawback since compatibility of the paint and label material must be taken into account. In addition, it is not practicable or economic to add labels to paints in large scale paint applications such as in painting of automobiles. In order to measure paint thickness with useful precision by using the fluorescent x-rays of additives, it is necessary to add several parts per thousand of label material. Such concentration is difficult to maintain as a homogeneous mixture and it can also degrade the weather resistance of the paint. Therefore it would be very advantageous to provide a method of measuring paint thickness which avoids the need for adding labels to the paint.
At present there is no single, reliable, economic method for accurate, in-situ and nondestructive monitoring of paint thickness as it is being applied to substrates. X-ray backscattering is one method which shows promise as a technique for estimating film thicknesses; however, this technique has severe limitations. A simplified model used in considering backscattering of x-rays from a paint layer on a metal backing is based on two assumptions: 1) the x-rays interact with the paint layer only by the mechanism of Compton scattering and because the total Compton scattering cross-section is almost exactly proportional to the mass, the backscattered x-ray intensity should be proportional to the mass/unit area of the paint layer over a broad range of x-ray energies; and 2) that x-rays penetrating through to the steel backing or substrate are fully attenuated or absorbed in the substrate and not back scattered.
The assumption that the intensity of the backscattered x-rays from the paint is almost exactly proportional to the paint thickness over a broad range of x-ray energies usually holds because the paint layer is so thin and comprised of elements of low atomic number. The model breaks down generally because of the assumption that the metal panel is a perfect absorber over the same broad range of energies. This will be more fully discussed below but this drawback has severely limited the application of x-ray backscattering as a viable in situ technique because the backscattered intensity from the substrate exceeds that from the paint by a large factor unless special precautions are observed.
SUMMARY OF THE INVENTION
The present invention provides a non-destructive, in-situ method of measuring thickness of layers comprising elements with low atomic numbers coated on a substrate during or after deposition on the substrate.
The present invention provides a method for measuring thickness of a coating comprising elements of low atomic numbers such as, but not limited to paint on a surface of a metal substrate such as aluminum, titanium, iron, galvanized iron, copper, alloys thereof and equivalents thereof such as metals and alloys falling on the albedo plot in
FIG. 3
between Al and Ge. The method comprises the steps of providing an x-ray source in a source holder, the x-ray source producing primary photons having energies in the range from about 14 keV to about 25 keV sufficiently high to provide sufficient enhancement of Compton scattering from the paint coating over Compton and Rayleigh scattering from the substrate but low enough to provide sufficient contrast between photoelectric absorption in the paint coating and the metal substrate. The method includes positioning the source holder in opposing relation to the surface of the paint-coated substrate and measuring a total intensity of backscattered secondary photons from the paint-coated metal substrate and thereafter determining the thickness of the paint coating from the total intensity of backscattered photons.
In this aspect of the invention the step of providing a radioactive source in a source holder includes providing a cylindrically symmetric source holder to provide an axially symmetric beam of primary photons emerging from the source holder. The x-ray source is a radioactive source and may be selected from the group consisting of
57
Co,
241
Am,
109
Cd, and
244
Cm sources.
The present invention also provides a method for controlled application of paint to a surface of a metal substrate such as aluminum and its alloys, titanium and its alloys, iron and iron alloys including galvanized iron, copper and its alloys, and equivalents thereof. The method comprises the steps of providing an x-ray source in a source holder, the x-ray source being selected to produce primary photons having energies in the range from about 14 keV to about 25 keV sufficiently high to provide sufficient enhancement of Compton scattering from the paint coating over Compton and Rayleigh scattering from the substrate but low enough to provide sufficient contrast between photoelectric absorption in the paint coating and the metal substrate. The method includes applying paint to the surface and positioning the holder in opposing relation to the surface of the metal substrate being painted and measuring a total intensity of backscattered secondary photons from the paint coated metal substrate and processing the intensity of backscattered secon
Church Craig E.
Hill & Schumacher
Schumacher Lynn C.
University of Guelph
LandOfFree
Method and apparatus for measuring thickness of coating... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for measuring thickness of coating..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for measuring thickness of coating... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2528909