ASTM B748-90(2001)
(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
SCOPE
1.1 The test method covers the measurement of metallic coating thicknesses by examination of a cross section with a scanning electron microscope (SEM).
1.2 This standard may involve hazardous materials, operations, and equipment. This standard does not purport to address all of the safety problems 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:B748–90(Reapproved 2001)
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 B 748; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope 4.4 Accurate measurements by this test method generally
require very careful sample preparation, especially at the
1.1 This test method covers the measurement of metallic
greater magnifications.
coating thicknesses by examination of a cross section with a
4.5 The coating thickness is an important factor in the
scanning electron microsope (SEM).
performance of a coating in service.
1.2 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
5. Equipment
responsibility of the user of this standard to establish appro-
5.1 The scanning electron microscope shall have a resolu-
priate safety and health practices and determine the applica-
tion of at least 50 nm. Suitable instruments are available
bility of regulatory limitations prior to use.
commercially.
2. Referenced Documents
6. Factors Affecting the Measurement Reliability
2.1 ASTM Standards:
6.1 Surface Roughness—If the coating or its substrate is
E 3 Guide for Preparation of Metallographic Specimens
rough relative to the coating thickness, one or both of the
E 766 Practice for Calibrating the Magnification of a Scan-
interfaces bounding the coating cross section may be too
ning Electron Microscope
irregular to permit accurate measurement of the average
3. Summary of Test Method thickness in the field of view.
6.2 Taper of Cross Section—If the plane of the cross section
3.1 A test specimen is cut, ground, and polished for metal-
is not perpendicular to the plane of the coating, the measured
lographic examination by an SEM of a cross section of the
thickness will be greater than the true thickness. For example,
coating. The measurement is made on a conventional micro-
an inclination of 10° to the perpendicular will contribute a
graph or on a photograph of the video waveform signal for a
1.5 %error.Truethickness, (t),equalsmeasuredthickness, (t ),
single scan across the coating.
m
multiplied by the cosine of the angle of inclination (u): t=t
4. Significance and Use
m 3 cos(u). (See X1.3.2.)
6.3 Specimen Tilt—Any tilt of the specimen (plane of the
4.1 This test method is useful for the direct measurement of
cross section) with respect to the SEM beam, may result in an
the thicknesses of metallic coatings and of individual layers of
erroneous measurement. The instrument should always be set
composite coatings, particularly for layers thinner than nor-
for zero tilt.
mally measured with the light microscope.
6.4 Oblique Measurement—If the thickness measurement is
4.2 This test method is suitable for acceptance testing.
not perpendicular to the plane of the coating, even when there
4.3 This test method is for the measurement of the thickness
is no taper (6.2) or tilt (6.3), the measured value will be greater
of the coating over a very small area and not of the average or
than the true thickness. This consideration applies to the
minimum thickness per se.
conventional micrograph (9.3.1) and to the directon of the
single video waveform scans (9.3.2).
This test method is under the jurisdiction ofASTM Committee B08 on Metallic
6.5 Deformation of Coating—Detrimental deformation of
and Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 on
the coating can be caused by excessive temperature or pressure
Test Methods.
during the mounting and preparation of cross sections of soft
Current edition approved Feb. 23, 1990. Published April 1990. Originally
published as B 748 – 85. Last previous edition B 748 – 85. coatings.
Annual Book of ASTM Standards, Vol 03.01.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
B748
6.6 Rounding of Edge of Coating—Iftheedgeofthecoating 6.14 Stability of Micrographs—Dimensional changes of
cross section is rounded, that is, if the coating cross section is micrographscantakeplacewithtimeandwithtemperatureand
not completely flat up to its edges, the observed thickness may humidity changes. If the calibration micrograph of the stage
differ from the true thickness. Edge rounding can be caused by micrometer scale and the micrograph of the test specimen are
improper mounting, grinding, polishing, or etching. kept together and time is allowed for stabilization of the
photographic paper, errors from this source will be minimized.
6.7 Overplating of Specimen—Overplating of the test speci-
men serves to protect the coating edges during preparation of
7. Preparations of Cross Sections
cross sections and thus to prevent an erroneous measurement.
7.1 Prepare,mount,grind,polish,andetchthetestspecimen
Removal of coating material during surface preparation for
so that the following occurs:
overplating can cause a low thickness measurement.
7.1.1 The cross section is perpendicular to the plane of the
6.8 Etching—Optimum etching will produce a clearly de-
coating,
fined and narrow dark line at the interface of two metals. A
7.1.2 The surface is flat and the entire width of the coating
wide or poorly defined line can result in an inaccurate
image is simultaneously in focus at the magnification to be
measurement.
used for the measurement,
6.9 Smearing—Polishing may leave smeared metal that
7.1.3 All material deformed by cutting or cross sectioning is
obscures the true boundary between the two metals and results
removed,
in an inaccurate measurement.This may occur with soft metals
7.1.4 Theboundariesofthecoatingcrosssectionaresharply
like lead, indium, and gold. To help identify whether or not
definedbycontrastingappearance,orbyanarrow,well-defined
there is smearing, repeat the polishing, etching, and measure-
line, and
ment several times. Any significant variations in readings
7.1.5 If the video waveform signal is to be measured, the
indicates possible smearing.
signal trace is flat except across the two boundaries of the
6.10 Poor Contrast—The visual contrast between metals in
coating.
the SEM is poor when their atomic numbers are close together.
7.2 For further guidance see Appendix X1.
For example, bright and semibright nickel layers may not be
discriminable unless their common boundary can be brought 8. Calibration of Magnification
out sufficiently by appropriate etching and SEM techniques.
8.1 Calibrate the SEM with an SEM stage micrometer and
For some metal combinations, energy dispersive X-ray tech-
determine the magnification factor, M, in accordance with
niques (see X1.4.5) or backscatter image techniques (see
Practice E 766 (see X1.4.2). Other calibration methods may be
X1.4.6) may be helpful.
usedifitcanbedemonstratedthattheyaresufficientlyaccurate
6.11 Magnification: for meeting the requirement of Section 12.
8.2 If practical, the stage micrometer and the test specimen
6.11.1 For any given coating thickness, measurement errors
shall be mounted side by side on the SEM stage.
tend to increase with decreasing magnification. If practical, the
magnification should be chosen so that the field of view is
9. Procedure
between 1.5 and 33 the coating thickness.
9.1 Operate the SEM in accordance with the manufacturer’s
6.11.2 The magnification readout of an SEM is often poorer
instructions.
than the 5 % accuracy often quoted and the magnification has
9.2 Take into account the factors listed in Sections 6 and 12.
been found for some instruments to vary by 25 % across the
9.3 Make a micrograph of the test specimen under the same
field. Magnification errors are minimized by appropriate use of
conditions and instrument settings as used for the calibration
an SEM stage micrometer and appropriate experimental pro-
and make an appropriate measurement of the micrograph
cedure. (See Practice E 766.)
image. Carry out this step in accordance with 9.3.1 or 9.3.2.
6.12 Uniformity of Magnification—Because the magnifica-
9.3.1 Conventional Micrograph:
tion may not be uniform over the entire field, errors can occur
9.3.1.1 With the boundaries of the coating clearly and
if both the calibration and the measurement are not made over
sharply defined, make conventional micrographs of the SEM
the same portion of the field. This can be very important.
stage micrometer scale and of the test specimen.
6.13 Stability of Magnification:
9.3.1.
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