ASTM B568-98(2009)
(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NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.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 (Reapproved2009)
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 3. Terminology
1.1 ThistestmethodcoverstheuseofX-rayspectrometryto 3.1 Definitions of technical terms used in this test method
determinethicknessofmetallicandsomenonmetalliccoatings. may be found in Terminology E135.
1.2 The maximum measurable thickness for a given coating
4. Summary of Test Method
is that thickness beyond which the intensity of the character-
istic secondary X radiation from the coating or the substrate is 4.1 Excitation—The measurement of the thickness of coat-
no longer sensitive to small changes in thickness. ings by X-ray spectrometric methods is based on the combined
interaction of the coating and substrate with incident radiation
1.3 This test method measures the mass of coating per unit
of sufficient energy to cause the emission of secondary radia-
area, which can also be expressed in units of linear thickness
tions characteristic of the elements composing the coating and
provided that the density of the coating is known.
substrate.The exciting radiation may be generated by an X-ray
1.4 Problems of personnel protection against radiation gen-
tube or by certain radioisotopes.
erated in an X-ray tube or emanating from a radioisotope
4.1.1 Excitation by an X-Ray Tube—Suitableexcitingradia-
source are not covered by this test method. For information on
tion will be produced by an X-ray tube if sufficient potential is
this important aspect, reference should be made to current
appliedtothetube.Thisisontheorderof35to50kVformost
documents of the National Committee on Radiation Protection
thickness-measurement applications. The chief advantage of
and Measurement, Federal Register, Nuclear Regulatory
X-ray tube excitation is the high intensity provided.
Commission, National Institute of Standards and Technology
4.1.2 Excitation by a Radioisotope —Of the many available
(formerly the National Bureau of Standards), and to state and
radioisotopes, only a few emit gamma radiations in the energy
local codes if such exist.
range suitable for coating-thickness measurement. Ideally, the
exciting radiation is slightly more energetic (shorter in wave-
1.5 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the length) than the desired characteristic X rays. The advantages
of radioisotope excitation include more compact instrumenta-
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica- tion essentially monochromatic radiation, and very low back-
ground intensity. The major disadvantage of radioisotope
bility of regulatory limitations prior to use.
excitation is the much lower intensities available as compared
2. Referenced Documents
with X-ray tube sources. X-ray tubes typically have intensities
that are several orders of magnitude greater than radioisotope
2.1 ASTM Standards:
sources. Due to the low intensity of radioisotopes, they are
E135 Terminology Relating to Analytical Chemistry for
unsuitable for measurements on small areas (less than 0.3 mm
Metals, Ores, and Related Materials
in diameter). Other disadvantages include the limited number
2.2 International Standard:
ofsuitableradioisotopes,theirrathershortusefullifetimes,and
ISO 3497 Metallic Coatings—Measurement of Coating
the personnel protection problems associated with high-
Thickness—X-ray Spectrometric Methods
intensity radioactive sources.
ThistestmethodisunderthejurisdictionofASTMCommitteeB08onMetallic
4.2 Dispersion—The secondary radiation resulting from the
and Inorganic Coatingsand is the direct responsibility of Subcommittee B08.10 on
exposure of an electroplated surface to X radiation usually
Test Methods.
contains many components in addition to those characteristic
Current edition approved Sept. 1, 2009. Published December 2009. Originally
of the coating metal(s) and the substrate. It is necessary,
approved in 1972. Last previous edition approved in 2004 as B568 – 04. DOI:
10.1520/B0568-98R09.
therefore, to have a means of separating the desired compo-
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
nents so that their intensities can be measured. This can be
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
done either by diffraction (wavelength dispersion) or by
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. electronic discrimination (energy dispersion).
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B568 − 98 (2009)
4.2.1 Wavelength Dispersion—By means of a single-crystal 4.4 Basic Principle—A relationship exists between coating
spectrogoniometer, wavelengths characteristic of either the thickness and secondary radiation intensity up to the limiting
thickness mentioned in 1.2. Both of the techniques described
coating or the substrate may be selected for measurement.
below are based on the use of primary standards of known
Published data in tabular form are available that relate spec-
coating thicknesses which serve to correlate quantitatively the
trogoniometer settings to the characteristic emissions of ele-
radiation intensity and thickness.
ments for each of the commonly used analyzing crystals.
4.2.2 Energy Dispersion—X-ray quanta are usually speci-
4.5 Thickness Measurement by X-Ray Emission—In this
fied in terms of their wavelengths, in angstroms (Å), or their technique, the spectrogoniometer is positioned to record the
equivalent energies in kiloelectron volts (keV). The relation-
intensity of a prominent wavelength characteristic of the
ship between these units is as follows: coating metal or, in the case of an energy-dispersive system,
the multichannel analyzer is set to accept the range of energies
˚
~ !
~keV! A 5 12.396
comprisingthedesiredcharacteristicemission.Theintensityof
the coating’s X-ray emission (coating ROI) will be at a
where:
minimum for a sample of the bare substrate where it will
keV = the quantum energy in thousands of electron volts,
consist of that portion of the substrate fluorescence which may
and
-10
overlap the ROI of the coating and a contribution due to
Å = the equivalent wavelength in angstroms (10 m).
background radiation. This background radiation is due to the
In a suitable detector (see 4.3.2), X rays of different energies
portion of the X-ray tube’s output which is the same energy as
will produce output pulses of different amplitudes. After
the coating’s X-ray emission. The sample will always scatter
suitable amplification, these pulses can be sorted on the basis
some of these X rays into the detector. If the characteristic
oftheiramplitudesandstoredincertaindesignatedchannelsof
emission energies of the coating and substrate are sufficiently
a multichannel analyzer, each adjacent channel representing an
different, the only contribution of the substrate will be due to
increment of energy.Typically, a channel may represent a span
background. For a thick sample of the solid coating metal or
of 20 eVfor a lithium-drifted silicon detector or 150 to 200 eV
for an electroplated specimen having an “infinitely thick”
for a proportional counter. From six to sixty adjacent channels
coating, the intensity will have its maximum value for a given
can be used to store the pulses representing a selected
set of conditions. For a sample having a coating of less than
characteristic emission of one element, the number of channels
“infinite” thickness, the intensity will have an intermediate
dependingonthewidthoftheemissionpeak(usuallydisplayed
value. The intensity of the emitted secondary X radiation
on the face of a cathode ray tube). The adjacent channels used depends, in general, upon the excitation energy, the atomic
to store the pulses from the material under analysis are called numbers of the coating and substrate, the area of the specimen
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,thatis,ascaler.Withwavelengthdispersivesystems,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,
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
will see only that area of the specimen on which the coating
coating thickness is increased to a certain value, the character-
thickness is to be determined. istic radiation from the substrate will disappear, although a
B568 − 98 (2009)
certain amount of scattered radiation will still be detected. The estimate of the counting rate based on a short counting interval
measurement of a coating thickness by X-ray absorption is not (for example, 1 or 2 s) may be appreciably different from an
applicable if an intermediate coating is present because of the estimate based on a longer counting period, particularly if the
indeterminate absorption effect of intermediate layer. The counting rate is low. This error is independent of other sources
typicalrelationshipbetweencoatingthicknessandtheintensity of error such as those arising from mistakes on the part of the
of a characteristic emission from the substrate is shown in the operator or from the use of inaccurate standards. To reduce the
Appendix, see Fig. X1.2. statistical error to an acceptable level, it is necessary to use a
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
4.8 Multilayer Measurements—Many products have multi-
closely approximate the square root of the total count; that is,
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
absorption by intermediate layers, and to allow fo
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