ASTM B567-98(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: B567 − 98 (Reapproved 2021)
Standard Test Method for
Measurement of Coating Thickness by the Beta Backscatter
Method
This standard is issued under the fixed designation B567; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber 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. Terminology
1.1 This test method covers the beta backscatter gages for 2.1 Definitions of Terms Specific to This Standard:
the nondestructive measurement of metallic and nonmetallic 2.1.1 activity—the nuclei of all radioisotopes are unstable
coatings on both metallic and nonmetallic substrate materials.
and tend to change into a stable condition by spontaneously
emitting energy or particles, or both. This process is known as
1.2 The test method measures the mass of coating per unit
radioactive decay.The total number of disintegrations during a
area, which can also be expressed in linear thickness units
suitably small interval of time divided by that interval of time
provided that the density of the coating is known.
is called “activity.” Therefore, in beta backscatter
1.3 Thetestmethodisapplicableonlyiftheatomicnumbers
measurements, a higher activity corresponds to a greater
or equivalent atomic numbers of the coating and substrate
emissionofbetaparticles.Theactivityofaradioactiveelement
differ by an appropriate amount (see 6.2).
used in beta backscatter gages is generally expressed in
1.4 Beta backscatter instruments employ a number of dif- microcuries (1 µCi=3.7×10 disintegrations per second).
ferent radioactive isotopes. Although the activities of these
2.1.2 aperture—the opening of the mask abutting the test
isotopes are normally very low, they can present a hazard if
specimen. It determines the size of the area on which the
handled incorrectly. This standard does not purport to address
coating thickness is measured. This mask is also referred to as
the safety issues and the proper handling of radioactive
a platen, an aperture plate, a specimen support, or a specimen
materials. It is the responsibility of the user to comply with
mask.
applicable State and Federal regulations concerning the han-
2.1.3 backscatter—when beta particles pass through matter,
dling and use of radioactive material. Some States require
they collide with atoms. Among other things, this interaction
licensing and registration of the radioactive isotopes.
will change their direction and reduce their speed. If the
1.5 The values stated in SI units are to be regarded as
deflections are such that the beta particle leaves the body of
standard. No other units of measurement are included in this
matter from the same surface at which it entered, the beta
standard.
particle is said to be backscattered.
1.6 This standard does not purport to address all of the
2.1.4 backscatter coeffıcient—the backscatter coefficient of
safety concerns, if any, associated with its use. It is the
a body, R, is the ratio of the number of beta particles
responsibility of the user of this standard to establish appro-
backscattered to that entering the body. R is independent of the
priate safety, health, and environmental practices and deter-
activity of the isotope and of the measuring time.
mine the applicability of regulatory limitations prior to use.
2.1.5 backscatter count:
1.7 This international standard was developed in accor-
2.1.5.1 absolute backscatter count—the absolute backscat-
dance with internationally recognized principles on standard-
ter count, X, is the number of beta particles that are backscat-
ization established in the Decision on Principles for the
tered during a finite interval of time and displayed by the
Development of International Standards, Guides and Recom-
instrument. X will, therefore, depend on the activity of the
mendations issued by the World Trade Organization Technical
source, the measuring time, the geometric configuration of the
Barriers to Trade (TBT) Committee.
measuringsystem,andthepropertiesofthedetector,aswellas
the coating thickness and the atomic numbers of the coating
and substrate materials. X is the count produced by the
ThistestmethodisunderthejurisdictionofASTMCommitteeB08onMetallic
and Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 on
uncoated substrate, and Xs, that of the coating material. To
Test Methods.
obtainthesevalues,itisnecessarythatboththesematerialsare
Current edition approved April 1, 2021. Published May 2021. Originally
available with a thickness greater than the saturation thickness
approvedin1972.Lastpreviouseditionapprovedin2014asB567–98(2014).DOI:
10.1520/B0567-98R21. (see 2.1.12).
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B567 − 98 (2021)
2.1.5.2 normalized backscatter—the normalized 3. Summary of Test Method
backscatter, x , is a quantity that is independent of the activity
n
3.1 When beta particles impinge upon a material, a certain
of the source, the measuring time, and the properties of the
portionofthemisbackscattered.Thisbackscatterisessentially
detector. The normalized backscatter is defined by the
a function of the atomic number of the material.
equation:
3.2 If the body has a surface coating and if the atomic
numbers of the substrate and of the coating material are
X 2 X
sufficiently different, the intensity of the backscatter will be
x 5
n
X 2 X
s 0
between two limits: the backscatter intensity of the substrate
and that of the coating. Thus, with proper instrumentation and
where:
if suitably displayed, the intensity of the backscatter can be
X = count from the substrate,
used for the measurement of mass per unit area of the coating,
X = count from the coating material, and
s
which, if the density remains the same, is directly proportional
X = count from the coated specimen, and each count is for
to the thickness.
the same interval of time.
3.3 The curve expressing coating thickness (mass per unit
Because X is always ≥X and ≤ X , x can only take values
0 s n
area)versusbetabackscatterintensityiscontinuousandcanbe
between 0 and 1. (For reasons of simplicity, it is often
subdivided into three distinct regions, as shown in Fig. 1. The
advantageous to express the normalized count as a percentage
normalized count rate, x , is plotted on the X-axis, and the
n
by multiplying x by 100.)
n
logarithm of the coating thickness, on the Y-axis. In the range
2.1.5.3 normalized backscatter curve—the curve obtained
0≤ x ≤0.35, the relationship is essentially linear. In the range
n
by plotting the coating thickness as a function of x .
n 0.35≤x ≤0.85, the curve is nearly logarithmic; this means
n
that, when drawn on semilogarithmic graph paper, as in Fig. 1,
2.1.6 beta particles—beta particles or beta rays are high-
the curve approximates a straight line. In the range
speed electrons that are emitted from the nuclei of materials
0.85≤ x ≤1, the relationship is nearly hyperbolic.
undergoing a nuclear transformation. These materials are n
called beta-emitting isotopes, beta-emitting sources, or beta
3.4 Radiation other than the beta rays are emitted or
emitters.
backscattered by the coating or substrate, and may be included
in the backscatter measurements.Whenever the term backscat-
2.1.7 coating thickness—in this test method, coating thick-
ter is used in this method, it is to be assumed that reference is
ness refers to mass per unit area as well as geometrical
made to the total radiation measured.
thickness.
2.1.8 dead time or resolving time—Geiger-Müller tubes
4. Significance and Use
used for counting beta particles have characteristic recovery
4.1 The thickness or mass per unit area of a coating is often
times that depend on their construction and the count rate.
critical to its performance.
Afterreadingapulse,thecounterisunresponsivetosuccessive
4.2 Forsomecoating-substratecombinations,thebetaback-
pulsesuntilatimeintervalequaltoorgreaterthanitsdeadtime
scatter method is a reliable method for measuring the coating
has elapsed.
nondestructively.
2.1.9 energy—it is possible to classify beta emitters by the
4.3 The test method is suitable for thickness specification
maximum energy of the particles that they release during their
acceptance if the mass per unit area is specified. It is not
disintegration. This energy is generally given in mega-
suitable for specification acceptance if the coating thickness is
electronvolts, MeV.
specified and the density of the coating material can vary or is
2.1.10 equivalent (or apparent) atomic number— the
not known.
equivalent atomic number of an alloy or compound is the
atomic number of an element that has the same backscatter
5. Instrumentation
coefficient as the material.
5.1 In general, a beta backscatter instrument will comprise:
2.1.11 half-life, radioactive—for a single radioactive decay
(1)aradiationsource(isotope)emittingprimarilybetaparticles
process, the time required for the activity to decrease by half.
having energies appropriate to the coating thickness to be
measured (see Appendix X2), (2) a probe or measuring system
2.1.12 saturation thickness—the minimum thickness of a
witharangeofaperturesthatlimitthebetaparticlestothearea
material that produces a backscatter that is not changed when
of the test specimen on which the coating thickness is to be
the thickness is increased. (See also Appendix X1.)
measured, and containing a detector capable of counting the
2.1.13 sealed source or isotope—aradioactivesourcesealed
number of backscattered particles (for example, a Geiger-
in a container or having a bonded cover, the container or cover
Müller counter (or tube)), and (3) a readout instrument where
being strong enough to prevent contact with and dispersion of
the intensity of the backscatter is displayed.The display, in the
the radioactive material under the conditions of use and wear
form of a meter reading or a digital readout can be: (a)
for which it was designed.
proportional to the count, (b) the normalized count, or (c) the
2.1.14 source geometry—the spatial arrangement of the coatingthicknessexpressedeitherinthicknessormassperunit
source,theaperture,andthedetectorwithrespecttoeachother. area units.
B567 − 98 (2021)
σ5=X; in 95% of all cases, the true count will be within
X 62σ. To judge the significance of the precision, it is often
helpful to express the standard deviation as a percentage of the
count, that is, 100=X/X, or 100/=X. Thus, a count of 100000
willgiveavaluetentimesmoreprecisethanthatobtainedwith
acountof1000.Wheneverpossible,acountingintervalshould
be chosen that will provide a total count of at least 10000,
which corresponds to a statistical error of 1% for the count
rate. It should be noted, however, that a 1% error in the count
rate can correspond to a much larger percentage error in the
thickness measurement, the relative error depending on the
atomic number spread or ratio between coating and substrate
materials.
6.1.3 Direct-reading instruments are also subject to these
statistical random errors. However, if these instruments do not
permit the display of the actual counting rate or the standard
deviation,theonlywaytodeterminethemeasuringprecisionis
to make a large number of measurements at the same coated
location on the same coated specimen, and calculate the
standard deviation by conventional means.
NOTE 1—The accuracy of a thickness measurement by beta backscatter
is generally poorer than the precision described in 5.1, inasmuch as it also
depends on other factors that are described below. Methods to determine
the random errors of thickness measurements before an actual measure-
ment are available from some manufacturers.
6.2 Coating and Substrate Materials—Because the back-
scatter intensity depends on the atomic numbers of the sub-
strateandthecoating,therepeatabilityofthemeasurementwill
depend to a large degree on the difference between these
atomicnumbers;thus,withthesamemeasuringparameters,the
greater this difference, the more precise the measurement will
be.As a rule of thumb, for most applications, the difference in
atomicnumbersshouldbeatleast5.Formaterialswithatomic
numbers below 20, the difference may be reduced to 25% of
the higher atomic number; for materials with atomic numbers
above 50, the difference should be at least 10% of the higher
atomicnumber.Mostplasticsandrelatedorganicmaterials(for
example, photoresists) may be assumed to have an equivalent
atomicnumbercloseto6.(AppendixX3givesatomicnumbers
of commonly used coating and substrate materials.)
FIG. 1 Normalized Backscatter
6.3 Aperture:
6.3.1 Despite the collimated nature of the sources used in
commercial backscatter instruments, the backscatter recorded
by the detector is, nearly always, the sum of the backscatter
6. Factors Affecting the Measuring Accuracy
produced by the test specimen exposed through the aperture
6.1 Counting Statistics:
andthatoftheapertureplate(n).Itis,therefore,desirabletouse
6.1.1 Radioactive disintegration takes place randomly.
amaterialwithalowatomicnumberfortheconstructionofthe
Thus, during a fixed time interval, the number of beta particles
platen and to select the largest aperture possible. Measuring
backscattered will not always be the same. This gives rise to
errorswillbeincreasediftheedgesoftheapertureopeningare
statisticalerrorsinherenttoradiationcounting.Inconsequence,
worn or damaged, or if the test specimen does not properly
an estimate of the counting rate based on a short counting
contact these edges.
interval (for example, 5 s) may be appreciably different from
6.3.2 Because the measuring area on the test specimen has
an estimate based on a longer counting interval, particularly if
to be constant to prevent the introduction of another variable,
the counting rate is low. To reduce the statistical error to an
namely the geometrical dimensions of the test specimen, it is
acceptable level, it is necessary to use a counting interval long
essentialthattheaperturebesmallerthanthecoatedareaofthe
enough to accumulate a sufficient number of counts.
surface on which the measurement is made.
6.1.2 At large total counts, the standard deviation (σ) will
closely approximate the square root of the total count, that is 6.4 Coating Thickness:
B567 − 98 (2021)
6.4.1 In the logarithmic range, the relative measuring error 6.8 Substrate Thickness:
is nearly constant and has its smallest value.
6.8.1 Test Specimens with Single-Layer Coatings:
6.4.2 In the linear range, the absolute measuring error,
6.8.1.1 This test method is sensitive to the thickness of thin
expressedinmassperunitareaorthickness,isnearlyconstant,
substrates, but for each isotope and material there is a critical
which means that as the coating thickness decreases, the
thickness, called “saturation thickness,” beyond which the
relative measuring error increases. At or near x =0.35, the
measurement will no longer be affected by an increase of the
n
relativeerrorsofthelinearandlogarithmicrangesareaboutthe
substrate thickness. This thickness depends on the energy of
same. Thus, the relative error at this point may, for most
the isotope and on the density of material. If the saturation
practical purposes, be used to calculate the absolute error over
thickness is not supplied by the manufacturer, it should be
the linear range.
determined experimentally.
6.4.3 In the hyperbolic range, the measuring error is always
6.8.1.2 If the substrate thickness is less than the saturation
large because a small variation in the intensity of the beta
thickness, effective saturation thickness can sometimes be
backscatter will produce a large variation in the measured
obtained by backing up the substrate with more of the same
coating thickness.
material, but only if the substrate is not coated on both sides.
6.4.4 For instruments that indicate only backscatter count
If the substrate is of constant thickness, the instrument may be
rate and not thickness directly, the count rate is normally
calibrated for that thickness of substrate. However, if the
converted to a thickness by means of an appropriate graph.
substratethicknessislessthanthesaturationthicknessandalso
Such graphs are generally valid only within a specific range of
variesinthickness,thismethodwillnotyieldasinglevaluefor
coating thicknesses so that extrapolation of a linear range
the coating thickness, but a range of values with an upper and
calibration curve (straight line on rectangular coordinates) into
lower limit.
the logarithmic thickness range will result in measurement
6.8.2 Test Specimens with Multiple-Layer Coatings:
errors. Similarly, extrapolation of a logarithmic range calibra-
6.8.2.1 If the intermediate layer adjacent to the coating is
tionintothelinearthicknessrangewillalsoproducesignificant
thicker than the saturation thickness, this test method will not
errors. Many instruments that indicate coating thickness di-
be affected by any variations in the substrate thickness as long
rectly are limited to the combined linear and logarithmic
as the instrument is calibrated with standards having the
coating thickness ranges but will be in error if measurements
intermediate coating material as the basis material.
are attempted in the hyperbolic thickness range. The instru-
6.8.2.2 If the thickness of the intermediate layer is less than
ment manufacturer’s instructions must be followed relative to
saturation thickness, but constant in thickness, the instrument
the limiting coating thicknesses beyond which the particular
may be calibrated for that particular combination of materials.
instrument being used may give substantial errors.
However, if the thickness of this intermediate layer is less than
6.5 Resolving Time of the Detector—Because of the dead
saturation thickness and varies in thickness, this method will
time of Geiger-Müller tubes (see 2.1.8), the number of pulses
notyieldasinglevalueforthecoatingthickness,butarangeof
displayed by the readout instrument is always less than the
values with an upper and lo
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