ASTM B568-98(2004)
(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
SIGNIFICANCE AND USE
This is a sensitive, noncontact, and nondestructive method for measuring the coating thickness (and in some cases, coating composition) of metallic and some nonmetallic 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 materials. It can be used to measure coating and base combinations that are not readily measured by other techniques.
The coating thickness is an important factor in the performance of a coating in service.
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 concerns, 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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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information.
Designation:B568–98(Reapproved 2004)
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 Department of Defense.
1. Scope ISO 3497 Metallic Coatings—Measurement of Coating
Thickness—X-ray Spectrometric Methods
1.1 ThistestmethodcoverstheuseofX-rayspectrometryto
determinethicknessofmetallicandsomenonmetalliccoatings.
3. Terminology
1.2 The maximum measurable thickness for a given coating
3.1 Definitions of technical terms used in this test method
is that thickness beyond which the intensity of the character-
may be found in Terminology E135.
istic secondary X radiation from the coating or the substrate is
no longer sensitive to small changes in thickness.
4. Summary of Test Method
1.3 This test method measures the mass of coating per unit
4.1 Excitation—The measurement of the thickness of coat-
area, which can also be expressed in units of linear thickness
ings by X-ray spectrometric methods is based on the combined
provided that the density of the coating is known.
interaction of the coating and substrate with incident radiation
1.4 Problems of personnel protection against radiation gen-
of sufficient energy to cause the emission of secondary radia-
erated in an X-ray tube or emanating from a radioisotope
tions characteristic of the elements composing the coating and
source are not covered by this test method. For information on
substrate.The exciting radiation may be generated by an X-ray
this important aspect, reference should be made to current
tube or by certain radioisotopes.
documents of the National Committee on Radiation Protection
4.1.1 Excitation by an X-Ray Tube—Suitableexcitingradia-
and Measurement, Federal Register, Nuclear Regulatory Com-
tion will be produced by an X-ray tube if sufficient potential is
mission, National Institute of Standards and Technology (for-
appliedtothetube.Thisisontheorderof35to50kVformost
merly the National Bureau of Standards), and to state and local
thickness-measurement applications. The chief advantage of
codes if such exist.
X-ray tube excitation is the high intensity provided.
1.5 This standard does not purport to address all of the
4.1.2 Excitation by a Radioisotope—Of the many available
safety concerns, if any, associated with its use. It is the
radioisotopes, only a few emit gamma radiations in the energy
responsibility of the user of this standard to establish appro-
range suitable for coating-thickness measurement. Ideally, the
priate safety and health practices and determine the applica-
exciting radiation is slightly more energetic (shorter in wave-
bility of regulatory limitations prior to use.
length) than the desired characteristic X rays. The advantages
2. Referenced Documents of radioisotope excitation include more compact instrumenta-
2 tion essentially monochromatic radiation, and very low back-
2.1 ASTM Standards:
ground intensity. The major disadvantage of radioisotope
E135 Terminology Relating to Analytical Chemistry for
excitation is the much lower intensities available as compared
Metals, Ores, and Related Materials
with X-ray tube sources. X-ray tubes typically have intensities
2.2 International Standard:
that are several orders of magnitude greater than radioisotope
sources. Due to the low intensity of radioisotopes, they are
ThistestmethodisunderthejurisdictionofASTMCommitteeB08onMetallic unsuitable for measurements on small areas (less than 0.3 mm
and Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 on
in diameter). Other disadvantages include the limited number
Test Methods.
ofsuitableradioisotopes,theirrathershortusefullifetimes,and
Current edition approved June 1, 2004. Published June 2004. Originally
the personnel protection problems associated with high-
approved in 1972. Last previous edition approved in 1998 as B568 – 98. DOI:
10.1520/B0568-98R04.
intensity radioactive sources.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
4.2 Dispersion—The secondary radiation resulting from the
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
exposure of an electroplated surface to X radiation usually
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
contains many components in addition to those characteristic
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
B568–98 (2004)
of the coating metal(s) and the substrate. It is necessary, will see only that area of the specimen on which the coating
therefore, to have a means of separating the desired compo- thickness is to be determined.
nents so that their intensities can be measured. This can be
4.4 Basic Principle—A relationship exists between coating
done either by diffraction (wavelength dispersion) or by
thickness and secondary radiation intensity up to the limiting
electronic discrimination (energy dispersion).
thickness mentioned in 1.2. Both of the techniques described
4.2.1 Wavelength Dispersion—By means of a single-crystal
below are based on the use of primary standards of known
spectrogoniometer, wavelengths characteristic of either the
coating thicknesses which serve to correlate quantitatively the
coating or the substrate may be selected for measurement.
radiation intensity and thickness.
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-
comprisingthedesiredcharacteristicemission.Theintensityof
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 = 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 = 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
oftheiramplitudesandstoredincertaindesignatedchannelsof
background. For a thick sample of the solid coating metal or
a multichannel analyzer, each adjacent channel representing an
for an electroplated specimen having an “infinitely thick”
increment of energy.Typically, a channel may represent a span
coating, the intensity will have its maximum value for a given
of 20 eVfor a lithium-drifted silicon detector or 150 to 200 eV
set of conditions. For a sample having a coating of less than
for a proportional counter. From six to sixty adjacent channels
“infinite” thickness, the intensity will have an intermediate
can be used to store the pulses representing a selected
value. The intensity of the emitted secondary X radiation
characteristic emission of one element, the number of channels
depends, in general, upon the excitation energy, the atomic
dependingonthewidthoftheemissionpeak(usuallydisplayed
numbers of the coating and substrate, the area of the specimen
on the face of a cathode ray tube). The adjacent channels used
exposed to the primary radiation, the power of the X-ray tube,
to store the pulses from the material under analysis are called
and the thickness of the coating. If all of the other variables are
the “region of interest” or ROI.
fixed, the intensity of the characteristic secondary radiation is
4.3 Detection:
a function of the thickness or mass per unit area of the coating.
4.3.1 Wavelength Dispersive Systems—The intensity of a
The exact relationship between the measured intensity and the
wavelength is measured by means of an appropriate radiation
coating thickness must be established by the use of standards
detector in conjunction with electronic pulse-counting cir-
having the same coating and substrate compositions as the
cuitry,thatis,ascaler.Withwavelengthdispersivesystems,the
samples to be measured. The maximum thickness that can be
types of detectors commonly used as the gas-filled types and
measured by this method is somewhat less than what is,
the scintillation detector coupled to a photomultiplier tube.
effectively, infinite thickness. This limiting thickness depends,
4.3.2 Energy-Dispersive Systems—For the highest energy
in general, upon the energy of the characteristic X-ray and the
resolution with energy dispersive systems, a solid-state device
density and absorption properties of the material under analy-
such as the lithium-drifted silicon detector must be used. This
sis. The typical relationship between a coating thickness and
type of detector is maintained at a very low temperature in a
theintensityofacharacteristicemissionfromthecoatingmetal
liquid-nitrogen cryostat (77K). Acceptable energy resolution
is illustrated by the curve in the Appendix, Fig. X1.1.
for most thickness measurement requirements can be realized
with proportional counters, and these detectors are being used 4.6 Thickness Measurements by X-Ray Absorption—In this
on most of the commercially available thickness gages based technique the spectrometer, in the case of a wavelength-
on X-ray spectrometry. In setting up a procedure for coating- dispersive system, is set to record the intensity of a selected
thickness measurement using an energy-dispersive system, emission characteristic of the basis metal. In an energy-
consideration should be given to the fact that the detector dispersive system, the multichannel analyzer is set to accumu-
“sees” and must process not only those pulses of interest but late the pulses comprising the same energy peak. The intensity
also those emanating from the substrate and from supporting will be a maximum for a sample of the uncoated basis metal
and masking materials in the excitation enclosure. Therefore, and will decrease with increasing coating thickness. This is
consideration should be given to restricting the radiation to the because both the exciting and secondary characteristic radia-
area of interest by masking or collimation at the radiation tions undergo attenuation in passing through the coating.
source. Similarly, the detector may also be masked so that it Depending upon the atomic number of the coating, when the
B568–98 (2004)
coating thickness is increased to a certain value, the character- estimate of the counting rate based on a short counting interval
istic radiation from the substrate will disappear, although a (for example, 1 or 2 s) may be appreciably different from an
certain amount of scattered radiation will still be detected. The estimate based on a longer counting period, particularly if the
measurement of a coating thickness by X-ray absorption is not counting rate is low. This error is independent of other sources
applicable if an intermediate coating is present because of the of error such as those arising from mistakes on the part of the
indeterminate absorption effect of intermediate layer. The operator or from the use of inaccurate standards. To reduce the
typicalrelationshipbetweencoatingthicknessandtheintensity 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, see Fig. X1.2. 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”countwillliewithinN 6 2s95 %ofthe
=
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 three 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! = 10. This does not
= =
to three 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 Acounting interval should be chosen that will p
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