ASTM B – Designation: B – 98 (Reapproved ) UNCONTROLLED (if printed out) Standar. Basic Description: ASTM B X-Ray Fluorescence (XRF) is a non-destructive method of measuring plating thickness by using an X-ray beam to excite the. Designation: B − 98 (Reapproved ) Standard Test Method for This test method is under the jurisdiction of ASTM Committee B08 on Metallic.
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A number in parentheses indicates the year of last reapproval. A superscript epsilon e indicates an editorial change since the last revision or reapproval. This standard has been approved for use by agencies of the Department of Defense. For information on this important aspect, reference should be made to current documents of the National Committee on Radiation Aztm and Measurement, Federal Register, Nuclear Regulatory Commission, National Institute of Standards and Technology formerly the National Bureau of Standardsand to state and local codes if such exist.
It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Current edition approved June 1, Originally approved in Last aztm edition approved in as B — Summary of Test Method 4. The exciting radiation may be generated by an X-ray tube or by certain radioisotopes.
This is on the order of 35 to 50 kV for most thickness-measurement applications. The chief advantage of X-ray tube excitation is the high intensity provided. Ideally, the exciting radiation is slightly more energetic shorter in wavelength than the desired characteristic X rays.
The advantages of radioisotope excitation include more compact instrumentation essentially monochromatic radiation, and very low background intensity.
ASTM B – PLATING THICKNESS BY XRF TESTING SERVICES
The major disadvantage of radioisotope excitation is the much lower intensities available as compared with X-ray tube sources. X-ray tubes typically have intensities that are several orders of magnitude greater than radioisotope sources.
Due to the low intensity of b5668, they are unsuitable for measurements on small areas less than 0. Other disadvantages include the limited number of suitable radioisotopes, their rather short useful lifetimes, and the personnel protection problems associated with highintensity radioactive sources.
It is necessary, therefore, to have a means of separating the desired components so that their intensities can be measured. This can be Copyright? Published data in tabular form are available that relate spectrogoniometer settings to the characteristic emissions of elements for each of the commonly used analyzing crystals. The relationship between these units is as follows: In a suitable detector see 4.
Typically, a channel may represent a span of 20 eV for a lithium-drifted silicon detector or to eV for a proportional counter. From six to sixty adjacent channels can be used to store the pulses representing h568 selected characteristic b5568 of one element, the number of channels depending on 5b68 width of the emission peak usually displayed on the face of a cathode ray tube.
With wavelength dispersive systems, the types of detectors commonly used as the gas-? This type of detector is maintained at a very low temperature in a liquid-nitrogen cryostat 77K. Acceptable energy resolution for most aatm measurement requirements can be realized with proportional counters, and these detectors are being used on most of the commercially available thickness gages based on X-ray spectrometry.
Therefore, consideration should be given to restricting the radiation to the area of interest by masking or collimation at the radiation source. G568, the detector may also be masked so that it will see only that area of the specimen on which the coating thickness is to be determined.
Both of the techniques described below are based on the ast of primary standards of known coating thicknesses which serve to correlate quantitatively the radiation intensity and thickness. The sample will always scatter some of these X rays into the detector. If the characteristic emission energies of the coating and substrate are sufficiently different, the only contribution of the substrate will be due to background. The intensity of the emitted secondary X radiation depends, in general, upon the excitation energy, the atomic numbers of the coating and aztm, the area of the specimen exposed to the primary radiation, the power of the X-ray tube, and the thickness of the coating.
If all of the other variables are? The exact relationship between the measured intensity and the coating thickness must be established by the use of standards having the same coating and substrate compositions as b56 samples to be measured. The maximum thickness that can be measured by this method is somewhat less than what is, effectively, in?
This limiting thickness depends, in general, upon the energy of the characteristic X-ray and the density zstm absorption properties of the material under analysis.
The typical relationship between a coating thickness and the intensity of a characteristic emission from the coating metal is illustrated by the curve in the Appendix, Fig.
In an energydispersive system, the multichannel analyzer is set to accumulate the pulses comprising the same energy peak.
The intensity will be a maximum for a sample of the uncoated basis metal and will decrease with increasing coating thickness. This is because both the exciting and secondary characteristic radiations undergo attenuation h568 passing through the coating. Depending upon the atomic number of the coating, when the coating thickness is increased to a certain value, the characteristic radiation from the substrate will g568, although a 2 Copyright by ASTM Int’l all rights reserved ; Reproduction authorized per License Agreement with Victoria Boyer ; Wed Mar 23 The measurement of a coating thickness by X-ray absorption is not applicable if an intermediate coating is v568 because of the indeterminate absorption effect of intermediate layer.
The typical relationship between coating thickness and the intensity of a characteristic emission from the substrate is shown in the Appendix, see Fig.
Measurements by this ratio method are largely independent of the distance between test specimen and detector. The measuring methods permit the simultaneous measurement of coating systems with up to awtm layers.
Or the simultaneous measurement of thickness and compositions of layers with up to three components. Such measurements require unique data processing for each multilayer combination to separate the various characteristic emissions involved, to account for the qstm by intermediate layers, and to allow for any secondary excitation which may occur between layers.
Typical examples of such combinations are gold on nickel on copper and nickel on copper on steel. This method astj be used when the energies of the detected characteristic radiations do not differ sufficiently for example, characteristic radiation from Au and Br. This method sometimes is described as numerical? It can be used to measure coating and base combinations that are not readily measured by other techniques. Factors Affecting Accuracy 6.
This means that during a? This b58 rise to the statistical error which is inherent in all radiation measurements. In consequence, an estimate of the counting rate based on a short counting interval for example, 1 or 2 s may be appreciably different from an estimate based on a longer counting period, particularly if the counting rate is low. This error is independent of other sources of error such as those arising from mistakes on the part of the operator or from the use of inaccurate standards.
To reduce the statistical error to an acceptable level, it is aetm to use a counting 5b68 long enough to accumulate a sufficient number of counts. When an energy-dispersive system is being used it should be recognized that a signi? It is possible to correct for dead-time losses.
ASTM B (measurement of coating thickness by XRF) | Eastern Applied Research, Inc.
To understand the signi? Thus, would give a standard deviation indicating 10 times the precision one-tenth the standard deviation obtained from counts. This does not mean that the result would necessarily be ten times as accurate see 7. The corresponding standard deviation in the thickness measurement is a function of the slope of the calibration curve at the point of measurement.
Most commercially available instruments display the standard deviation directly in units of thickness. In the curve shown in the Appendix, see Fig.
The precision rapidly becomes poorer in the portion of the curve above approximately 10? The situation is similar for the absorption curve shown in the Appendix, see Fig. At coating thicknesses greater than approximately 10? These limiting thicknesses are, in general, different for each coating material.
Caution must be exercised, however, to see that the use of a large sample area in conjunction with high power input to the X-ray tube does not result in a signal so large as to exceed the count-rate capacity of the detection system.
The sources of error will be eliminated by the use of calibration standards electroplated in the same type of solution under the same conditions as those used in the production of the coatings to be 3 Copyright by ASTM Int’l all rights reserved ; Reproduction authorized per License Agreement with Victoria Boyer ; Wed Mar 23 If pores or voids are present, X-ray methods will give an indication of coating mass per unit area but not of thickness.
This procedure is commonly used for the measurement of hard gold coatings having a density of Variations in density can result either from variations in composition or from variations in plating conditions see 6. However, when thickness measurements are made by the X-ray absorption method the substrate composition of the test specimens must be the same as that of the standards. Care must be taken that the coating and substrate are thick enough to prevent the primary X-ray beam from reaching and?
However, when thickness is to be determined by the X-ray absorption technique, the thickness of the substrate must exceed a certain minimum or critical thickness. It must be established experimentally that the minimum thickness requirements have been met for a particular substrate-source combination, although it is sometimes possible to back up the test specimen substrates with a sufficient thickness of materials of the same composition.
The X-ray absorption method cannot be used when one or more intermediate coating layers are present.
ASTM B568 (measurement of coating thickness by XRF)
Protective coatings such as lacquer or chromate conversion coatings over the coating to be measured will also affect the results. In those cases where the measurement of thickness on curved surfaces cannot be avoided, a collimator should be used on the excitation beam, reducing the measurement area to a size that will minimize the effects of curvature. Spatial relationships between the curved surface, the excitation beam, and the detector are particularly important, and variations in these relationships can introduce errors in measurement.
Calibration standards having the same radius of curvature as that of the test specimens can also be used to eliminate curvature effects. In general, the radiation intensity varies directly with the current and the square of the potential. Therefore, in any method based on a simple relationship between intensity and thickness, the? However, if the method is based on intensity ratios rather than absolute intensities, minor variations in excitation energy are compensated for.
If instability is suspected, a series of twenty or more count measurements should be made on the same specimen without moving the specimen and the standard deviation of the series calculated. Most modern industrial X-ray instrumentation will perform this calculation automatically. The value should not be signi? Some forms of instability become evident if the same specimen is measured periodically. Operation of the gas-? Instrument measurement precision will rapidly decrease with increasing thickness in the hyperbolic range.
For this reason, signi? Standards representing various thickness ranges of a number of coatings on different substrates are generally available from thickness gage manufacturers.
Those that are certi? Calibration standards for gold coatings, certi?
If the standards are visibly scratched or abraded they should be replaced.