ASTM B568-98
(Test Method)Standard Test Method for Measurement of Coating Thickness by X-Ray Spectrometry
Standard Test Method for Measurement of Coating Thickness by X-Ray Spectrometry
SCOPE
1.1 This test method covers the use of X-ray spectrometry to determine thickness of metallic and some nonmetallic coatings.
1.2 The maximum measurable thickness for a given coating is that thickness beyond which the intensity of the characteristic secondary X radiation from the coating or the substrate is no longer sensitive to small changes in thickness.
1.3 This test method measures the mass of coating per unit area, which can also be expressed in units of linear thickness provided that the density of the coating is known.
1.4 Problems of personnel protection against radiation generated in an X-ray tube or emanating from a radioisotope source are not covered by this test method. For information on this important aspect, reference should be made to current documents of the National Committee on Radiation Protection and Measurement, Federal Register, Nuclear Regulatory Commission, National Institute of Standards and Technology (formerly the National Bureau of Standards), and to state and local codes if such exist.
1.5 This standard does not purport to address all of the safety problems, if any, associated with its use. 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.
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Designation: B 568 – 98
Standard Test Method for
Measurement of Coating Thickness by X-Ray Spectrometry
This standard is issued under the fixed designation B 568; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. 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.
1. Scope 4. Summary of Test Method
1.1 This test method covers the use of X-ray spectrometry to 4.1 Excitation—The measurement of the thickness of coat-
determine thickness of metallic and some nonmetallic coatings. ings by X-ray spectrometric methods is based on the combined
1.2 The maximum measurable thickness for a given coating interaction of the coating and substrate with incident radiation
is that thickness beyond which the intensity of the character- of sufficient energy to cause the emission of secondary radia-
istic secondary X radiation from the coating or the substrate is tions characteristic of the elements composing the coating and
no longer sensitive to small changes in thickness. substrate. The exciting radiation may be generated by an X-ray
1.3 This test method measures the mass of coating per unit tube or by certain radioisotopes.
area, which can also be expressed in units of linear thickness 4.1.1 Excitation by an X-Ray Tube—Suitable exciting radia-
provided that the density of the coating is known. tion will be produced by an X-ray tube if sufficient potential is
1.4 Problems of personnel protection against radiation gen- applied to the tube. This is on the order of 35 to 50 kV for most
erated in an X-ray tube or emanating from a radioisotope thickness-measurement applications. The chief advantage of
source are not covered by this test method. For information on X-ray tube excitation is the high intensity provided.
this important aspect, reference should be made to current 4.1.2 Excitation by a Radioisotope—Of the many available
documents of the National Committee on Radiation Protection radioisotopes, only a few emit gamma radiations in the energy
and Measurement, Federal Register, Nuclear Regulatory Com- range suitable for coating-thickness measurement. Ideally, the
mission, National Institute of Standards and Technology (for- exciting radiation is slightly more energetic (shorter in wave-
merly the National Bureau of Standards), and to state and local length) than the desired characteristic X rays. The advantages
codes if such exist. of radioisotope excitation include more compact instrumenta-
1.5 This standard does not purport to address all of the tion essentially monochromatic radiation, and very low back-
safety concerns, if any, associated with its use. It is the ground intensity. The major disadvantage of radioisotope
responsibility of the user of this standard to establish appro- excitation is the much lower intensities available as compared
priate safety and health practices and determine the applica- with X-ray tube sources. X-ray tubes typically have intensities
bility of regulatory limitations prior to use. that are several orders of magnitude greater than radioisotope
sources. Due to the low intensity of radioisotopes, they are
2. Referenced Documents
unsuitable for measurements on small areas (less than 0.3 mm
2.1 ASTM Standards: in diameter). Other disadvantages include the limited number
E 135 Terminology Relating to Analytical Chemistry for
of suitable radioisotopes, their rather short useful lifetimes, and
Metals, Ores, and Related Materials the personnel protection problems associated with high-
2.2 International Standard:
intensity radioactive sources.
ISO 3497 Metallic Coatings—Measurement of Coating 4.2 Dispersion—The secondary radiation resulting from the
Thickness—X-ray Spectrometric Methods
exposure of an electroplated surface to X radiation usually
contains many components in addition to those characteristic
3. Terminology
of the coating metal(s) and the substrate. It is necessary,
3.1 Definitions of technical terms used in this test method
therefore, to have a means of separating the desired compo-
may be found in Terminology E 135.
nents so that their intensities can be measured. This can be
done either by diffraction (wavelength dispersion) or by
electronic discrimination (energy dispersion).
This test method is under the jurisdiction of ASTM Committee B-8 on Metallic
4.2.1 Wavelength Dispersion—By means of a single-crystal
and Inorganic Coatingsand is the direct responsibility of Subcommittee B08.10on
spectrogoniometer, wavelengths characteristic of either the
General Test Methods.
Current edition approved Nov. 10, 1998. Published January 1999. Originally
coating or the substrate may be selected for measurement.
published as B 568 – 72. Last previous edition B 568 – 91 (1997).
Annual Book of ASTM Standards, Vol 03.05.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
B 568
Published data in tabular form are available that relate spec- 4.5 Thickness Measurement by X-Ray Emission—In this
trogoniometer settings to the characteristic emissions of ele- technique, the spectrogoniometer is positioned to record the
ments for each of the commonly used analyzing crystals.
intensity of a prominent wavelength characteristic of the
4.2.2 Energy Dispersion—X-ray quanta are usually speci-
coating metal or, in the case of an energy-dispersive system,
fied in terms of their wavelengths, in angstroms (Å), or their
the multichannel analyzer is set to accept the range of energies
equivalent energies in kiloelectron volts (keV). The relation-
comprising the desired characteristic emission. The intensity of
ship between these units is as follows:
the coating’s X-ray emission (coating ROI) will be at a
~keV!~Å!512.396
minimum for a sample of the bare substrate where it will
consist of that portion of the substrate fluorescence which may
where:
overlap the ROI of the coating and a contribution due to
keV 5 the quantum energy in thousands of electron volts
background radiation. This background radiation is due to the
and
-10
portion of the X-ray tube’s output which is the same energy as
Å 5 the equivalent wavelength in angstroms (10 m).
the coating’s X-ray emission. The sample will always scatter
In a suitable detector (see 4.3.2), X rays of different energies
some of these X rays into the detector. If the characteristic
will produce output pulses of different amplitudes. After
emission energies of the coating and substrate are sufficiently
suitable amplification, these pulses can be sorted on the basis
different, the only contribution of the substrate will be due to
of their amplitudes and stored in certain designated channels of
a multichannel analyzer, each adjacent channel representing an background. For a thick sample of the solid coating metal or
increment of energy. Typically, a channel may represent a span
for an electroplated specimen having an “infinitely thick”
of 20 eV for a lithium-drifted silicon detector or 150 to 200 eV
coating, the intensity will have its maximum value for a given
for a proportional counter. From six to sixty adjacent channels
set of conditions. For a sample having a coating of less than
can be used to store the pulses representing a selected
“infinite” thickness, the intensity will have an intermediate
characteristic emission of one element, the number of channels
value. The intensity of the emitted secondary X radiation
depending on the width of the emission peak (usually displayed
depends, in general, upon the excitation energy, the atomic
on the face of a cathode ray tube). The adjacent channels used
numbers of the coating and substrate, the area of the specimen
to store the pulses from the material under analysis are called
exposed to the primary radiation, the power of the X-ray tube,
the “region of interest” or ROI.
and the thickness of the coating. If all of the other variables are
4.3 Detection:
fixed, the intensity of the characteristic secondary radiation is
4.3.1 Wavelength Dispersive Systems—The intensity of a
a function of the thickness or mass per unit area of the coating.
wavelength is measured by means of an appropriate radiation
The exact relationship between the measured intensity and the
detector in conjunction with electronic pulse-counting cir-
coating thickness must be established by the use of standards
cuitry, that is, a scaler. With wavelength dispersive systems, the
having the same coating and substrate compositions as the
types of detectors commonly used as the gas-filled types and
samples to be measured. The maximum thickness that can be
the scintillation detector coupled to a photomultiplier tube.
measured by this method is somewhat less than what is,
4.3.2 Energy-Dispersive Systems—For the highest energy
effectively, infinite thickness. This limiting thickness depends,
resolution with energy dispersive systems, a solid-state device
in general, upon the energy of the characteristic X-ray and the
such as the lithium-drifted silicon detector must be used. This
type of detector is maintained at a very low temperature in a density and absorption properties of the material under analy-
liquid-nitrogen cryostat (77K). Acceptable energy resolution sis. The typical relationship between a coating thickness and
for most thickness measurement requirements can be realized
the intensity of a characteristic emission from the coating metal
with proportional counters, and these detectors are being used
is illustrated by the curve in the Appendix, Fig. 1.
on most of the commercially available thickness gages based
4.6 Thickness Measurements by X-Ray Absorption—In this
on X-ray spectrometry. In setting up a procedure for coating-
technique the spectrometer, in the case of a wavelength-
thickness measurement using an energy-dispersive system,
dispersive system, is set to record the intensity of a selected
consideration should be given to the fact that the detector
emission characteristic of the basis metal. In an energy-
“sees” and must process not only those pulses of interest but
dispersive system, the multichannel analyzer is set to accumu-
also those emanating from the substrate and from supporting
late the pulses comprising the same energy peak. The intensity
and masking materials in the excitation enclosure. Therefore,
will be a maximum for a sample of the uncoated basis metal
consideration should be given to restricting the radiation to the
and will decrease with increasing coating thickness. This is
area of interest by masking or collimation at the radiation
because both the exciting and secondary characteristic radia-
source. Similarly, the detector may also be masked so that it
tions undergo attenuation in passing through the coating.
will see only that area of the specimen on which the coating
Depending upon the atomic number of the coating, when the
thickness is to be determined.
coating thickness is increased to a certain value, the character-
4.4 Basic Principle—A relationship exists between coating
istic radiation from the substrate will disappear, although a
thickness and secondary radiation intensity up to the limiting
certain amount of scattered radiation will still be detected. The
thickness mentioned in 1.2. Both of the techniques described
measurement of a coating thickness by X-ray absorption is not
below are based on the use of primary standards of known
applicable if an intermediate coating is present because of the
coating thicknesses which serve to correlate quantitatively the
radiation intensity and thickness. indeterminate absorption effect of intermediate layer. The
B 568
typical relationship between coating thickness and the intensity statistical error to an acceptable level, it is necessary to use a
of a characteristic emission from the substrate is shown in the counting interval long enough to accumulate a sufficient
Appendix, Fig. 2. number of counts. When an energy-dispersive system is being
4.7 Thickness and Composition Measurement by Simulta- used it should be recognized that a significant portion of an
neous X-ray Emission and Absorption (Ratio Method)—It is intended counting period may be consumed as dead time, that
possible to combine the X-ray absorption and emission tech- is, time during which the count-rate capacity of the system is
niques when coating thicknesses and alloy composition are exceeded. It is possible to correct for dead-time losses. The
determined from the ratio of the respective intensities of manufacturer’s instructions for accomplishing this with his
substrate and coating materials. Measurements by this ratio particular instrumentation should be followed.
method are largely independent of the distance between test
6.1.1 The standard deviation, s, of this random error will
specimen and detector.
closely approximate the square root of the total count; that is,
4.8 Multilayer Measurements—Many products have multi-
s 5 N. The “true” count will lie within N 6 2 s 95 % of the
=
layer coatings in which it is possible to measure each of the
time. To understand the significance of the precision, it is
coating layers by using the multiple-energy-region capability
helpful to express the standard deviation as a percent of the
of the multichannel analyzer of an energy-dispersive system.
count, 100 N/N 5 100/ N. Thus, 100 000 would give a
= =
The measuring methods permit the simultaneous measurement
standard deviation indicating 10 times the precision (one-tenth
of coating systems with up to 3 layers. Or the simultaneous
the standard deviation) obtained from 1000 counts. This is
measurement of thickness and compositions of layers with up
because ~100/ 1000!/~100/ 100 000! 5 10. This does not
= =
to 3 components. Such measurements require unique data
mean that the result would necessarily be ten times as accurate
processing for each multilayer combination to separate the
(see 7.2).
various characteristic emissions involved, to account for the
6.1.2 A counting interval should be chosen that will provide
absorption by intermediate layers, and to allow for any
a net count of at least 10 000. This would correspond to a
secondary excitation which may occur between layers. Typical
statistical error in the count rate of 1 %. The corresponding
examples of such combinations are gold on nickel on copper
standard deviation in the thickness measurement is a function
and nickel on copper on steel.
of the slope of the calibration curve at the point of measure-
4.9 Mathematical Deconvolution—When using a multi-
ment. Most commercially a
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