ASTM B568-98(2021)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: B568 − 98 (Reapproved 2021)
Standard Test Method for
Measurement of Coating Thickness by X-Ray Spectrometry
This standard is issued under the fixed designation B568; 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 (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope 2. Referenced Documents
2.1 ASTM Standards:
1.1 ThistestmethodcoverstheuseofX-rayspectrometryto
E135 Terminology Relating to Analytical Chemistry for
determinethicknessofmetallicandsomenonmetalliccoatings.
Metals, Ores, and Related Materials
1.2 The maximum measurable thickness for a given coating
2.2 International Standard:
is that thickness beyond which the intensity of the character-
ISO 3497 Metallic Coatings—Measurement of Coating
istic secondary X radiation from the coating or the substrate is
Thickness—X-ray Spectrometric Methods
no longer sensitive to small changes in thickness.
3. Terminology
1.3 This test method measures the mass of coating per unit
area, which can also be expressed in units of linear thickness
3.1 Definitions of technical terms used in this test method
provided that the density of the coating is known.
may be found in Terminology E135.
1.4 Problems of personnel protection against radiation gen-
4. Summary of Test Method
erated in an X-ray tube or emanating from a radioisotope
4.1 Excitation—The measurement of the thickness of coat-
source are not covered by this test method. For information on
ings by X-ray spectrometric methods is based on the combined
this important aspect, reference should be made to current
interaction of the coating and substrate with incident radiation
documents of the National Committee on Radiation Protection
of sufficient energy to cause the emission of secondary radia-
and Measurement, Federal Register, Nuclear Regulatory
tions characteristic of the elements composing the coating and
Commission, National Institute of Standards and Technology
substrate.The exciting radiation may be generated by an X-ray
(formerly the National Bureau of Standards), and to state and
tube or by certain radioisotopes.
local codes if such exist.
4.1.1 Excitation by an X-Ray Tube—Suitableexcitingradia-
1.5 This standard does not purport to address all of the
tion will be produced by an X-ray tube if sufficient potential is
safety concerns, if any, associated with its use. It is the
appliedtothetube.Thisisontheorderof35to50kVformost
responsibility of the user of this standard to establish appro-
thickness-measurement applications. The chief advantage of
priate safety, health, and environmental practices and deter-
X-ray tube excitation is the high intensity provided.
mine the applicability of regulatory limitations prior to use.
4.1.2 Excitation by a Radioisotope —Of the many available
1.6 This international standard was developed in accor-
radioisotopes, only a few emit gamma radiations in the energy
dance with internationally recognized principles on standard-
range suitable for coating-thickness measurement. Ideally, the
ization established in the Decision on Principles for the
exciting radiation is slightly more energetic (shorter in wave-
Development of International Standards, Guides and Recom-
length) than the desired characteristic X rays. The advantages
mendations issued by the World Trade Organization Technical
of radioisotope excitation include more compact instrumenta-
Barriers to Trade (TBT) Committee.
tion essentially monochromatic radiation, and very low back-
ground intensity. The major disadvantage of radioisotope
excitation is the much lower intensities available as compared
ThistestmethodisunderthejurisdictionofASTMCommitteeB08onMetallic
and Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 on
Test Methods. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved April 1, 2021. Published May 2021. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approvedin1972.Lastpreviouseditionapprovedin2014asB568 – 98(2014).DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/B0568-98R21. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B568 − 98 (2021)
with X-ray tube sources. X-ray tubes typically have intensities liquid-nitrogen cryostat (77K). Acceptable energy resolution
that are several orders of magnitude greater than radioisotope for most thickness measurement requirements can be realized
sources. Due to the low intensity of radioisotopes, they are with proportional counters, and these detectors are being used
unsuitable for measurements on small areas (less than 0.3 mm on most of the commercially available thickness gages based
in diameter). Other disadvantages include the limited number on X-ray spectrometry. In setting up a procedure for coating-
ofsuitableradioisotopes,theirrathershortusefullifetimes,and thickness measurement using an energy-dispersive system,
the personnel protection problems associated with high- consideration should be given to the fact that the detector
intensity radioactive sources. “sees” and must process not only those pulses of interest but
also those emanating from the substrate and from supporting
4.2 Dispersion—The secondary radiation resulting from the
and masking materials in the excitation enclosure. Therefore,
exposure of an electroplated surface to X radiation usually
consideration should be given to restricting the radiation to the
contains many components in addition to those characteristic
area of interest by masking or collimation at the radiation
of the coating metal(s) and the substrate. It is necessary,
source. Similarly, the detector may also be masked so that it
therefore, to have a means of separating the desired compo-
will see only that area of the specimen on which the coating
nents so that their intensities can be measured. This can be
thickness is to be determined.
done either by diffraction (wavelength dispersion) or by
4.4 Basic Principle—A relationship exists between coating
electronic discrimination (energy dispersion).
thickness and secondary radiation intensity up to the limiting
4.2.1 Wavelength Dispersion—By means of a single-crystal
thickness mentioned in 1.2. Both of the techniques described
spectrogoniometer, wavelengths characteristic of either the
below are based on the use of primary standards of known
coating or the substrate may be selected for measurement.
coating thicknesses which serve to correlate quantitatively the
Published data in tabular form are available that relate spec-
radiation intensity and thickness.
trogoniometer settings to the characteristic emissions of ele-
ments for each of the commonly used analyzing crystals.
4.5 Thickness Measurement by X-Ray Emission—In this
4.2.2 Energy Dispersion—X-ray quanta are usually speci-
technique, the spectrogoniometer is positioned to record the
fied in terms of their wavelengths, in angstroms (Å), or their
intensity of a prominent wavelength characteristic of the
equivalent energies in kiloelectron volts (keV). The relation-
coating metal or, in the case of an energy-dispersive system,
ship between these units is as follows:
the multichannel analyzer is set to accept the range of energies
comprisingthedesiredcharacteristicemission.Theintensityof
the coating’s X-ray emission (coating ROI) will be at a
˚
keV ~A! 5 12.396
~ !
minimum for a sample of the bare substrate where it will
where:
consist of that portion of the substrate fluorescence which may
overlap the ROI of the coating and a contribution due to
keV = thequantumenergyinthousandsofelectronvolts,and
-10
Å = the equivalent wavelength in angstroms (10 m).
background radiation. This background radiation is due to the
portion of the X-ray tube’s output which is the same energy as
In a suitable detector (see 4.3.2), X rays of different energies
the coating’s X-ray emission. The sample will always scatter
will produce output pulses of different amplitudes. After
some of these X rays into the detector. If the characteristic
suitable amplification, these pulses can be sorted on the basis
emission energies of the coating and substrate are sufficiently
oftheiramplitudesandstoredincertaindesignatedchannelsof
different, the only contribution of the substrate will be due to
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 eVfor 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
dependingonthewidthoftheemissionpeak(usuallydisplayed
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
coating thickness must be established by the use of standards
circuitry, that is, a scaler. With wavelength dispersive systems,
having the same coating and substrate compositions as the
the types of detectors commonly used as the gas-filled types
samples to be measured. The maximum thickness that can be
and 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 density and absorption properties of the material under analy-
type of detector is maintained at a very low temperature in a sis. The typical relationship between a coating thickness and
B568 − 98 (2021)
theintensityofacharacteristicemissionfromthecoatingmetal cases, coating composition) of metallic and some nonmetallic
is illustrated by the curve in the Appendix, Fig. X1.1. coatings over a range of thicknesses from as little as 0.01 µm
to as much as 75 µm depending on the coating and substrate
4.6 Thickness Measurements by X-Ray Absorption—In this
materials. It can be used to measure coating and base combi-
technique the spectrometer, in the case of a wavelength-
nations that are not readily measured by other techniques.
dispersive system, is set to record the intensity of a selected
emission characteristic of the basis metal. In an energy-
5.2 The coating thickness is an important factor in the
dispersive system, the multichannel analyzer is set to accumu-
performance of a coating in service.
late the pulses comprising the same energy peak. The intensity
will be a maximum for a sample of the uncoated basis metal
6. Factors Affecting Accuracy
and will decrease with increasing coating thickness. This is
6.1 Counting Statistics—The production of X-ray quanta is
because both the exciting and secondary characteristic radia-
random with respect to time. This means that during a fixed
tions undergo attenuation in passing through the coating.
time interval, the number of quanta emitted will not always be
Depending upon the atomic number of the coating, when the
the same. This gives rise to the statistical error which is
coating thickness is increased to a certain value, the character-
inherent in all radiation measurements. In consequence, an
istic radiation from the substrate will disappear, although a
estimate of the counting rate based on a short counting interval
certain amount of scattered radiation will still be detected. The
(for example, 1 or 2 s) may be appreciably different from an
measurement of a coating thickness by X-ray absorption is not
estimate based on a longer counting period, particularly if the
applicable if an intermediate coating is present because of the
counting rate is low. This error is independent of other sources
indeterminate absorption effect of intermediate layer. The
of error such as those arising from mistakes on the part of the
typicalrelationshipbetweencoatingthicknessandtheintensity
operator or from the use of inaccurate standards. To reduce the
of a characteristic emission from the substrate is shown in the
statistical error to an acceptable level, it is necessary to use a
Appendix, see Fig. X1.2.
counting interval long enough to accumulate a sufficient
4.7 Thickness and Composition Measurement by Simultane-
number of counts. When an energy-dispersive system is being
ous X-ray Emission and Absorption (Ratio Method)—It is
used it should be recognized that a significant portion of an
possible to combine the X-ray absorption and emission tech-
intended counting period may be consumed as dead time, that
niques when coating thicknesses and alloy composition are
is, time during which the count-rate capacity of the system is
determined from the ratio of the respective intensities of
exceeded. It is possible to correct for dead-time losses. The
substrate and coating materials. Measurements by this ratio
manufacturer’s instructions for accomplishing this with his
method are largely independent of the distance between test
particular instrumentation should be followed.
specimen and detector.
6.1.1 The standard deviation, s, of this random error will
closely approximate the square root of the total count; that is,
4.8 Multilayer Measurements—Many products have multi-
layer coatings in which it is possible to measure each of the
s5=N. The “true” count will lie within N 6 2 s 95 % of the
coating layers by using the multiple-energy-region capability time. To understand the significance of the precision, it is
of the multichannel analyzer of an energy-dispersive system.
helpful to express the standard deviation as a percent of the
The measuring methods permit the simultaneous measurement
count, 100 =N/N5100/=N. Thus, 100 000 would give a stan-
of coating systems with up to three layers. Or the simultaneous
dard deviation indicating 10 times the precision (one-tenth the
measurement of thickness and compositions of layers with up
standarddeviation)obtainedfrom1000counts.Thisisbecause
to three components. Such measurements require unique data
~100/=1000!/~100/=100000!510. This does not mean that the
processing for each multilayer combination to separate the
result would necessarily be ten times as accurate (see 7.2).
various characteristic emissions involved, to account for the
6.1.2 Acounting 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 available instruments display the
channel analyzer a mathematical deconvolution of the second-
standard deviation directly in units of thickness.
aryradiationspectracanbeusedtoextracttheintensitiesofthe
6.2 Coating Thickness—The precision of the measurement
characteristic radiation. This method can be used when the
will be affected by the thickness range being measured. In the
energies of the detected characteristic radiations do not differ
curve shown in theAppendix, see Fig. X1.1, the precision will
sufficiently (for example, characteristic radiation from Au and
be best in the portion of the curve from approximately 0.25 to
Br).This method sometimes is described as numerical filtering
7.5 µm.The precision rapidly becomes poorer in the portion of
in order to distinguish from the technique of setting fixed
the curve above approximately 10 µm. The situation is similar
Region of Interest (ROI) channel limits in the multichannel
for the absorption curve shown in theAppendix, see Fig. X1.2.
analyzer.
At coating thicknesses greater than approximately 10 µm, the
5. Significance and Use
intensity changes very little with the coating thickness and,
5.1 This is a sensitive, noncontact, and nondestructive therefore, the precision in that region is poor. These limiting
method for measuring the coating thickness (and in some thicknesses are, in general, different for each coating material.
B568 − 98 (2021)
6.3 Size of Measuring Area—To obtain satisfactory count- 6.8 Surface Cleanliness—Foreign materials such as dirt,
ingstatistics(see6.1)inareasonablyshortcountingperiod,the grease, or corrosion products will lead to inaccurate thickness
determinations. Protective coatings such as lacquer or chro-
exposed area of the significant surface should be as large as
practicably consistent with the size and shape of the specimen. mate conversion coatings over the coating to be measured will
also affect the results.
Caution must be exercised, however, to see that the use of a
large sample area in conjunction with high power input to the
6.9 Specimen Curvature—Thickness measurements should
X-ray tube does not result in a signal so large as to exceed the
be made on flat surfaces if practical. In those cases where the
count-rate capacity of the detection system.
measurement of thickness on curved surfaces cannot be
avoided, a collimator should be used on the excitation beam,
6.4 Coating Composition—Thickness determinations by
reducing the measurement
...