ASTM B567-98(2014)
(Test Method)Standard Test Method for Measurement of Coating Thickness by the Beta Backscatter Method
Standard Test Method for Measurement of Coating Thickness by the Beta Backscatter Method
SIGNIFICANCE AND USE
4.1 The thickness or mass per unit area of a coating is often critical to its performance.
4.2 For some coating-substrate combinations, the beta backscatter method is a reliable method for measuring the coating nondestructively.
4.3 The test method is suitable for thickness specification acceptance if the mass per unit area is specified. It is not suitable for specification acceptance if the coating thickness is specified and the density of the coating material can vary or is not known.
SCOPE
1.1 This test method covers the beta backscatter gages for the nondestructive measurement of metallic and nonmetallic coatings on both metallic and nonmetallic substrate materials.
1.2 The test method measures the mass of coating per unit area, which can also be expressed in linear thickness units provided that the density of the coating is known.
1.3 The test method is applicable only if the atomic numbers or equivalent atomic numbers of the coating and substrate differ by an appropriate amount (see 6.2).
1.4 Beta backscatter instruments employ a number of different radioactive isotopes. Although the activities of these isotopes are normally very low, they can present a hazard if handled incorrectly. This standard does not purport to address the safety issues and the proper handling of radioactive materials. It is the responsibility of the user to comply with applicable State and Federal regulations concerning the handling and use of radioactive material. Some States require licensing and registration of the radioactive isotopes.
1.5 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.6 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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Designation: B567 − 98 (Reapproved 2014)
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 radioactive decay.The total number of disintegrations during a
suitably small interval of time divided by that interval of time
1.1 This test method covers the beta backscatter gages for
is called “activity.” Therefore, in beta backscatter
the nondestructive measurement of metallic and nonmetallic
measurements, a higher activity corresponds to a greater
coatings on both metallic and nonmetallic substrate materials.
emissionofbetaparticles.Theactivityofaradioactiveelement
1.2 The test method measures the mass of coating per unit
used in beta backscatter gages is generally expressed in
area, which can also be expressed in linear thickness units
microcuries (1 µCi=3.7×10 disintegrations per second).
provided that the density of the coating is known.
2.1.2 aperture—the opening of the mask abutting the test
1.3 Thetestmethodisapplicableonlyiftheatomicnumbers
specimen. It determines the size of the area on which the
or equivalent atomic numbers of the coating and substrate
coating thickness is measured. This mask is also referred to as
differ by an appropriate amount (see 6.2).
a platen, an aperture plate, a specimen support, or a specimen
1.4 Beta backscatter instruments employ a number of dif-
mask.
ferent radioactive isotopes. Although the activities of these
2.1.3 backscatter—when beta particles pass through matter,
isotopes are normally very low, they can present a hazard if
they collide with atoms. Among other things, this interaction
handled incorrectly. This standard does not purport to address
will change their direction and reduce their speed. If the
the safety issues and the proper handling of radioactive
materials. It is the responsibility of the user to comply with deflections are such that the beta particle leaves the body of
applicable State and Federal regulations concerning the han- matter from the same surface at which it entered, the beta
dling and use of radioactive material. Some States require
particle is said to be backscattered.
licensing and registration of the radioactive isotopes.
2.1.4 backscatter coeffıcient—the backscatter coefficient of
1.5 The values stated in SI units are to be regarded as
a body, R, is the ratio of the number of beta particles
standard. No other units of measurement are included in this
backscattered to that entering the body. R is independent of the
standard.
activity of the isotope and of the measuring time.
1.6 This standard does not purport to address all of the
2.1.5 backscatter count:
safety concerns, if any, associated with its use. It is the
2.1.5.1 absolute backscatter count—the absolute backscat-
responsibility of the user of this standard to establish appro-
ter count, X, is the number of beta particles that are backscat-
priate safety and health practices and determine the applica-
tered during a finite interval of time and displayed by the
bility of regulatory limitations prior to use.
instrument. X will, therefore, depend on the activity of the
2. Terminology
source, the measuring time, the geometric configuration of the
measuringsystem,andthepropertiesofthedetector,aswellas
2.1 Definitions of Terms Specific to This Standard:
the coating thickness and the atomic numbers of the coating
2.1.1 activity—the nuclei of all radioisotopes are unstable
and substrate materials. X is the count produced by the
and tend to change into a stable condition by spontaneously 0
uncoated substrate, and Xs, that of the coating material. To
emitting energy or particles, or both. This process is known as
obtainthesevalues,itisnecessarythatboththesematerialsare
available with a thickness greater than the saturation thickness
ThistestmethodisunderthejurisdictionofASTMCommitteeB08onMetallic
(see 2.1.12).
and Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 on
Test Methods.
2.1.5.2 normalized backscatter—the normalized
Current edition approved May 1, 2014. Published May 2014. Originally
backscatter, x , is a quantity that is independent of the activity
approved in 1972. Last previous edition approved in 2009 as B567 – 98 (2009). n
DOI: 10.1520/B0567-98R14. of the source, the measuring time, and the properties of the
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B567 − 98 (2014)
detector. The normalized backscatter is defined by the 3.2 If the body has a surface coating and if the atomic
equation: numbers of the substrate and of the coating material are
sufficiently different, the intensity of the backscatter will be
between two limits: the backscatter intensity of the substrate
X 2 X
x 5
and that of the coating. Thus, with proper instrumentation and
n
X 2 X
s 0
if suitably displayed, the intensity of the backscatter can be
where:
used for the measurement of mass per unit area of the coating,
which, if the density remains the same, is directly proportional
X = count from the substrate,
X = count from the coating material, and to the thickness.
s
X = count from the coated specimen, and each count is for
3.3 The curve expressing coating thickness (mass per unit
the same interval of time.
area)versusbetabackscatterintensityiscontinuousandcanbe
Because X is always ≥X and ≤ X , x can only take values
subdivided into three distinct regions, as shown in Fig. 1. The
0 s n
between 0 and 1. (For reasons of simplicity, it is often
normalized count rate, x , is plotted on the X-axis, and the
n
advantageous to express the normalized count as a percentage
logarithm of the coating thickness, on the Y-axis. In the range
by multiplying x by 100.)
0≤ x ≤0.35, the relationship is essentially linear. In the range
n
n
2.1.5.3 normalized backscatter curve—the curve obtained
by plotting the coating thickness as a function of x .
n
2.1.6 beta particles—beta particles or beta rays are high-
speed electrons that are emitted from the nuclei of materials
undergoing a nuclear transformation. These materials are
called beta-emitting isotopes, beta-emitting sources, or beta
emitters.
2.1.7 coating thickness—in this test method, coating thick-
ness refers to mass per unit area as well as geometrical
thickness.
2.1.8 dead time or resolving time—Geiger-Müller tubes
used for counting beta particles have characteristic recovery
times that depend on their construction and the count rate.
Afterreadingapulse,thecounterisunresponsivetosuccessive
pulsesuntilatimeintervalequaltoorgreaterthanitsdeadtime
has elapsed.
2.1.9 energy—it is possible to classify beta emitters by the
maximum energy of the particles that they release during their
disintegration. This energy is generally given in mega-
electronvolts, MeV.
2.1.10 equivalent (or apparent) atomic number— the
equivalent atomic number of an alloy or compound is the
atomic number of an element that has the same backscatter
coefficient as the material.
2.1.11 half-life, radioactive—for a single radioactive decay
process, the time required for the activity to decrease by half.
2.1.12 saturation thickness—the minimum thickness of a
material that produces a backscatter that is not changed when
the thickness is increased. (See also Appendix X1.)
2.1.13 sealed source or isotope—aradioactivesourcesealed
in a container or having a bonded cover, the container or cover
being strong enough to prevent contact with and dispersion of
the radioactive material under the conditions of use and wear
for which it was designed.
2.1.14 source geometry—the spatial arrangement of the
source,theaperture,andthedetectorwithrespecttoeachother.
3. Summary of Test Method
3.1 When beta particles impinge upon a material, a certain
portionofthemisbackscattered.Thisbackscatterisessentially
a function of the atomic number of the material. FIG. 1 Normalized Backscatter
B567 − 98 (2014)
0.35≤x ≤0.85, the curve is nearly logarithmic; this means acountof1000.Wheneverpossible,acountingintervalshould
n
that, when drawn on semilogarithmic graph paper, as in Fig. 1, be chosen that will provide a total count of at least 10000,
the curve approximates a straight line. In the range which corresponds to a statistical error of 1% for the count
0.85≤ x ≤1, the relationship is nearly hyperbolic. rate. It should be noted, however, that a 1% error in the count
n
rate can correspond to a much larger percentage error in the
3.4 Radiation other than the beta rays are emitted or
thickness measurement, the relative error depending on the
backscattered by the coating or substrate, and may be included
atomic number spread or ratio between coating and substrate
in the backscatter measurements.Whenever the term backscat-
materials.
ter is used in this method, it is to be assumed that reference is
made to the total radiation measured.
6.1.3 Direct-reading instruments are also subject to these
statistical random errors. However, if these instruments do not
4. Significance and Use
permit the display of the actual counting rate or the standard
deviation,theonlywaytodeterminethemeasuringprecisionis
4.1 The thickness or mass per unit area of a coating is often
to make a large number of measurements at the same coated
critical to its performance.
location on the same coated specimen, and calculate the
4.2 Forsomecoating-substratecombinations,thebetaback-
standard deviation by conventional means.
scatter method is a reliable method for measuring the coating
nondestructively.
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
4.3 The test method is suitable for thickness specification
depends on other factors that are described below. Methods to determine
acceptance if the mass per unit area is specified. It is not
the random errors of thickness measurements before an actual measure-
suitable for specification acceptance if the coating thickness is ment are available from some manufacturers.
specified and the density of the coating material can vary or is
6.2 Coating and Substrate Materials—Because the back-
not known.
scatter intensity depends on the atomic numbers of the sub-
strateandthecoating,therepeatabilityofthemeasurementwill
5. Instrumentation
depend to a large degree on the difference between these
5.1 In general, a beta backscatter instrument will comprise:
atomicnumbers;thus,withthesamemeasuringparameters,the
(1)aradiationsource(isotope)emittingprimarilybetaparticles
greater this difference, the more precise the measurement will
having energies appropriate to the coating thickness to be
be.As a rule of thumb, for most applications, the difference in
measured (see Appendix X2), (2) a probe or measuring system
atomicnumbersshouldbeatleast5.Formaterialswithatomic
witharangeofaperturesthatlimitthebetaparticlestothearea
numbers below 20, the difference may be reduced to 25% of
of the test specimen on which the coating thickness is to be
the higher atomic number; for materials with atomic numbers
measured, and containing a detector capable of counting the
above 50, the difference should be at least 10% of the higher
number of backscattered particles (for example, a Geiger-
atomicnumber.Mostplasticsandrelatedorganicmaterials(for
Müller counter (or tube)), and (3) a readout instrument where
example, photoresists) may be assumed to have an equivalent
the intensity of the backscatter is displayed.The display, in the
atomicnumbercloseto6.(AppendixX3givesatomicnumbers
form of a meter reading or a digital readout can be: (a)
of commonly used coating and substrate materials.)
proportional to the count, (b) the normalized count, or (c) the
6.3 Aperture:
coatingthicknessexpressedeitherinthicknessormassperunit
6.3.1 Despite the collimated nature of the sources used in
area units.
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:
=
σ5 X ; in 95% of all cases, the true count will be within 6.4.1 In the logarithmic range, the relative measuring error
X 62σ. To judge the significance of the precision, it is often
is nearly constant and has its smallest value.
helpful to express the standard deviation as a percentage of the
6.4.2 In the linear range, the absolute measuring error,
= =
count, that is, 100 X/X, or 100/ X. Thus, a count of 100000 expressedinmassperunitareaorthickness,isnearlyconstant,
willgiveavaluetentimesmoreprecisethanthatobtainedwith which means that as the coating thickness decreases, the
B567 − 98 (2014)
relative measuring error increases. At or near x =0.35, the thickness, called “saturation thickness,” beyond which the
n
relativeerrorsofthelinearandlogarithmicrangesareaboutthe measurement will no longer be affected by an
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