ASTM B748-90(2010)
(Test Method)Standard Test Method for Measurement of Thickness of Metallic Coatings by Measurement of Cross Section with a Scanning Electron Microscope
Standard Test Method for Measurement of Thickness of Metallic Coatings by Measurement of Cross Section with a Scanning Electron Microscope
SIGNIFICANCE AND USE
This test method is useful for the direct measurement of the thicknesses of metallic coatings and of individual layers of composite coatings, particularly for layers thinner than normally measured with the light microscope.
This test method is suitable for acceptance testing.
This test method is for the measurement of the thickness of the coating over a very small area and not of the average or minimum thickness per se.
Accurate measurements by this test method generally require very careful sample preparation, especially at the greater magnifications.
The coating thickness is an important factor in the performance of a coating in service.
SCOPE
1.1 This test method covers the measurement of metallic coating thicknesses by examination of a cross section with a scanning electron microsope (SEM).
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3 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NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: B748 − 90 (Reapproved2010)
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 4.2 This test method is suitable for acceptance testing.
1.1 This test method covers the measurement of metallic
4.3 Thistestmethodisforthemeasurementofthethickness
coating thicknesses by examination of a cross section with a
of the coating over a very small area and not of the average or
scanning electron microsope (SEM).
minimum thickness per se.
1.2 The values stated in SI units are to be regarded as
4.4 Accurate measurements by this test method generally
standard. No other units of measurement are included in this
require very careful sample preparation, especially at the
standard.
greater magnifications.
1.3 This standard does not purport to address all of the
4.5 The coating thickness is an important factor in the
safety concerns, if any, associated with its use. It is the
performance of a coating in service.
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
5. Equipment
bility of regulatory limitations prior to use.
5.1 The scanning electron microscope shall have a resolu-
2. Referenced Documents
tion of at least 50 nm. Suitable instruments are available
commercially.
2.1 ASTM Standards:
E3Guide for Preparation of Metallographic Specimens
6. Factors Affecting the Measurement Reliability
E766Practice for Calibrating the Magnification of a Scan-
ning Electron Microscope
6.1 Surface Roughness—If the coating or its substrate is
rough relative to the coating thickness, one or both of the
3. Summary of Test Method
interfaces bounding the coating cross section may be too
3.1 A test specimen is cut, ground, and polished for metal-
irregular to permit accurate measurement of the average
lographic examination by an SEM of a cross section of the
thickness in the field of view.
coating. The measurement is made on a conventional micro-
6.2 Taper of Cross Section—If the plane of the cross section
graph or on a photograph of the video waveform signal for a
is not perpendicular to the plane of the coating, the measured
single scan across the coating.
thickness will be greater than the true thickness. For example,
an inclination of 10° to the perpendicular will contribute a
4. Significance and Use
1.5% error. True thickness, (t), equals measured thickness,
4.1 This test method is useful for the direct measurement of
(t ), multiplied by the cosine of the angle of inclination (θ):
m
the thicknesses of metallic coatings and of individual layers of
t=t ×cos(θ). (See X1.3.2.)
m
composite coatings, particularly for layers thinner than nor-
mally measured with the light microscope. 6.3 Specimen Tilt—Any tilt of the specimen (plane of the
cross section) with respect to the SEM beam, may result in an
erroneous measurement. The instrument should always be set
ThistestmethodisunderthejurisdictionofASTMCommitteeB08onMetallic
for zero tilt.
and Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 on
Test Methods.
6.4 Oblique Measurement—If the thickness measurement is
Current edition approved Nov. 1, 2010. Published November 2010. Originally
not perpendicular to the plane of the coating, even when there
approved in 1985. Last previous edition approved in 2006 as B748–90 (2006).
DOI: 10.1520/B0748-90R10.
isnotaper(6.2)ortilt(6.3),themeasuredvaluewillbegreater
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
than the true thickness. This consideration applies to the
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
conventional micrograph (9.3.1) and to the direction of the
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. single video waveform scans (9.3.2).
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B748 − 90 (2010)
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
focal point of the SEM beam.
6.6 Rounding of Edge of Coating—Iftheedgeofthecoating
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
kept together and time is allowed for stabilization of the
6.7 Overplating of Specimen—Overplating of the test speci-
photographic paper, errors from this source will be minimized.
men serves to protect the coating edges during preparation of
cross sections and thus to prevent an erroneous measurement.
7. Preparations of Cross Sections
Removal of coating material during surface preparation for
overplating can cause a low thickness measurement.
7.1 Prepare,mount,grind,polish,andetchthetestspecimen
so that the following occurs:
6.8 Etching—Optimum etching will produce a clearly de-
7.1.1 The cross section is perpendicular to the plane of the
fined and narrow dark line at the interface of two metals. A
coating,
wide or poorly defined line can result in an inaccurate
7.1.2 The surface is flat and the entire width of the coating
measurement.
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
coating.
theSEMispoorwhentheiratomicnumbersareclosetogether.
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-
8.1 Calibrate the SEM with an SEM stage micrometer and
niques (see X1.4.5) or backscatter image techniques (see
determine the magnification factor, M, in accordance with
X1.4.6) may be helpful.
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 instructions.
an SEM stage micrometer and appropriate experimental pro-
9.2 TakeintoaccountthefactorslistedinSections6and12.
cedure (see Practice E766).
9.3 Make a micrograph of the test specimen under the same
6.12 Uniformity of Magnification—Because the magnifica-
conditions and instrument settings as used for the calibration
tion may not be uniform over the entire field, errors can occur
and make an appropriate measurement of the micrograph
if both the calibration and the
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