ASTM B748-90(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: B748 − 90 (Reapproved 2021)
Standard Test Method for
Measurement of Thickness of Metallic Coatings by
Measurement of Cross Section with a Scanning Electron
Microscope
This standard is issued under the fixed designation B748; 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.
1. Scope 4. Significance and Use
1.1 This test method covers the measurement of metallic 4.1 This test method is useful for the direct measurement of
coating thicknesses by examination of a cross section with a the thicknesses of metallic coatings and of individual layers of
scanning electron microsope (SEM). composite coatings, particularly for layers thinner than nor-
mally measured with the light microscope.
1.2 The values stated in SI units are to be regarded as
standard. No other units of measurement are included in this 4.2 This test method is suitable for acceptance testing.
standard.
4.3 Thistestmethodisforthemeasurementofthethickness
1.3 This standard does not purport to address all of the
of the coating over a very small area and not of the average or
safety concerns, if any, associated with its use. It is the
minimum thickness per se.
responsibility of the user of this standard to establish appro-
4.4 Accurate measurements by this test method generally
priate safety, health, and environmental practices and deter-
require very careful sample preparation, especially at the
mine the applicability of regulatory limitations prior to use.
greater magnifications.
1.4 This international standard was developed in accor-
4.5 The coating thickness is an important factor in the
dance with internationally recognized principles on standard-
performance of a coating in service.
ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom-
5. Equipment
mendations issued by the World Trade Organization Technical
5.1 The scanning electron microscope shall have a resolu-
Barriers to Trade (TBT) Committee.
tion of at least 50 nm. Suitable instruments are available
commercially.
2. Referenced Documents
2.1 ASTM Standards:
6. Factors Affecting the Measurement Reliability
E3Guide for Preparation of Metallographic Specimens
6.1 Surface Roughness—If the coating or its substrate is
E766Practice for Calibrating the Magnification of a Scan-
rough relative to the coating thickness, one or both of the
ning Electron Microscope
interfaces bounding the coating cross section may be too
irregular to permit accurate measurement of the average
3. Summary of Test Method
thickness in the field of view.
3.1 A test specimen is cut, ground, and polished for metal-
6.2 Taper of Cross Section—If the plane of the cross section
lographic examination by an SEM of a cross section of the
is not perpendicular to the plane of the coating, the measured
coating. The measurement is made on a conventional micro-
thickness will be greater than the true thickness. For example,
graph or on a photograph of the video waveform signal for a
an inclination of 10° to the perpendicular will contribute a
single scan across the coating.
1.5% error. True thickness, (t), equals measured thickness,
(t ), multiplied by the cosine of the angle of inclination (θ):
m
ThistestmethodisunderthejurisdictionofASTMCommitteeB08onMetallic
t=t ×cos(θ). (See X1.3.2.)
m
and Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 on
Test Methods.
6.3 Specimen Tilt—Any tilt of the specimen (plane of the
Current edition approved Oct. 1, 2021. Published November 2021. Originally
cross section) with respect to the SEM beam, may result in an
approvedin1985.Lastpreviouseditionapprovedin2016asB748–90(2016).DOI:
erroneous measurement. The instrument should always be set
10.1520/B0748-90R21.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or for zero tilt.
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
6.4 Oblique Measurement—If the thickness measurement is
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. not perpendicular to the plane of the coating, even when there
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B748 − 90 (2021)
isnotaper(6.2)ortilt(6.3),themeasuredvaluewillbegreater micrometerandtestspecimensidebysideontheSEMstageso
than the true thickness. This consideration applies to the as to keep the transfer time short.
conventional micrograph (9.3.1) and to the direction of the 6.13.2 A change in magnification can occur when adjust-
single video waveform scans (9.3.2). ments are made with the focusing and other electronic SEM
controls.Suchachangeispreventedbynotusingtheelectronic
6.5 Deformation of Coating—Detrimental deformation of
focus controls or other electronic SEM controls after photo-
thecoatingcanbecausedbyexcessivetemperatureorpressure
graphing the stage micrometer scale except to focus with the
during the mounting and preparation of cross sections of soft
mechanical X, Y, and Z controls. Appropriate manipulation of
coatings.
the X, Y, and Z controls will bring the specimen surface to the
6.6 Rounding of Edge of Coating—Iftheedgeofthecoating
focal point of the SEM beam.
cross section is rounded, that is, if the coating cross section is
6.14 Stability of Micrographs—Dimensional changes of
not completely flat up to its edges, the observed thickness may
micrographscantakeplacewithtimeandwithtemperatureand
differ from the true thickness. Edge rounding can be caused by
humidity changes. If the calibration micrograph of the stage
improper mounting, grinding, polishing, or etching.
micrometer scale and the micrograph of the test specimen are
6.7 Overplating of Specimen—Overplating of the test speci-
kept together and time is allowed for stabilization of the
men serves to protect the coating edges during preparation of
photographic paper, errors from this source will be minimized.
cross sections and thus to prevent an erroneous measurement.
Removal of coating material during surface preparation for
7. Preparations of Cross Sections
overplating can cause a low thickness measurement.
7.1 Prepare,mount,grind,polish,andetchthetestspecimen
6.8 Etching—Optimum etching will produce a clearly de-
so that the following occurs:
fined and narrow dark line at the interface of two metals. A
7.1.1 The cross section is perpendicular to the plane of the
wide or poorly defined line can result in an inaccurate
coating,
measurement.
7.1.2 The surface is flat and the entire width of the coating
image is simultaneously in focus at the magnification to be
6.9 Smearing—Polishing may leave smeared metal that
used for the measurement,
obscures the true boundary between the two metals and results
7.1.3 Allmaterialdeformedbycuttingorcrosssectioningis
inaninaccuratemeasurement.Thismayoccurwithsoftmetals
removed,
like lead, indium, and gold. To help identify whether or not
7.1.4 Theboundariesofthecoatingcrosssectionaresharply
there is smearing, repeat the polishing, etching, and measure-
definedbycontrastingappearance,orbyanarrow,well-defined
ment several times. Any significant variations in readings
line, and
indicates possible smearing.
7.1.5 If the video waveform signal is to be measured, the
6.10 Poor Contrast—The visual contrast between metals in
signal trace is flat except across the two boundaries of the
theSEMispoorwhentheiratomicnumbersareclosetogether.
coating.
For example, bright and semibright nickel layers may not be
7.2 For further guidance see Appendix X1.
discriminable unless their common boundary can be brought
out sufficiently by appropriate etching and SEM techniques.
8. Calibration of Magnification
For some metal combinations, energy dispersive X-ray tech-
niques (see X1.4.5) or backscatter image techniques (see 8.1 Calibrate the SEM with an SEM stage micrometer and
X1.4.6) may be helpful. determine the magnification factor, M, in accordance with
Practice E766 (see X1.4.2). Other calibration methods may be
6.11 Magnification:
usedifitcanbedemonstratedthattheyaresufficientlyaccurate
6.11.1 For any given coating thickness, measurement errors
for meeting the requirement of Section 12.
tend to increase with decreasing magnification. If practical, the
8.2 If practical, the stage micrometer and the test specimen
magnification should be chosen so that the field of view is
shall be mounted side by side on the SEM stage.
between 1.5 and 3× the coating thickness.
6.11.2 The magnification readout of an SEM is often poorer
9. Procedure
than the 5% accuracy often quoted and the magnification has
been found for some instruments to vary by 25% across the
9.1 OperatetheSEMinaccordancewiththemanufacturer’s
field.Magnificationerrorsareminimizedbyappropriateuseof
instruction
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