ASTM B748-90(1997)
(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 microsope (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.
General Information
Relations
Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: B 748 – 90 (Reapproved 1997)
AMERICAN SOCIETY FOR TESTING AND MATERIALS
100 Barr Harbor Dr., West Conshohocken, PA 19428
Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
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 5. Equipment
1.1 The test method covers the measurement of metallic 5.1 The scanning electron microscope shall have a resolu-
coating thicknesses by examination of a cross section with a tion of at least 50 nm. Suitable instruments are available
scanning electron microsope (SEM). commercially.
1.2 This standard does not purport to address all of the
6. Factors Affecting the Measurement Reliability
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro- 6.1 Surface Roughness—If the coating or its substrate is
priate safety and health practices and determine the applica- rough relative to the coating thickness, one or both of the
interfaces bounding the coating cross section may be too
bility of regulatory limitations prior to use.
irregular to permit accurate measurement of the average
2. Referenced Documents
thickness in the field of view.
2.1 ASTM Standards: 6.2 Taper of Cross Section—If the plane of the cross section
E 3 Methods of Preparation of Metallographic Specimens is not perpendicular to the plane of the coating, the measured
E 766 Practice for Calibrating the Magnification of Scan- thickness will be greater than the true thickness. For example,
ning Electron Microscope an inclination of 10° to the perpendicular will contribute a
1.5 % error. True thickness, (t), equals measured thickness,
3. Summary of Test Method
(t ), multiplied by the cosine of the angle of inclination (u):
m
3.1 A test specimen is cut, ground, and polished for metal-
t 5 t 3 cos(u). (See X1.3.2.)
m
lographic examination by an SEM of a cross section of the 6.3 Specimen Tilt—Any tilt of the specimen (plane of the
coating. The measurement is made on a conventional micro-
cross section) with respect to the SEM beam, may result in an
graph or on a photograph of the video waveform signal for a erroneous measurement. The instrument should always be set
single scan across the coating.
for zero tilt.
6.4 Oblique Measurement—If the thickness measurement is
4. Significance and Use
not perpendicular to the plane of the coating, even when there
4.1 This test method is useful for the direct measurement of
is no taper (6.2) or tilt (6.3), the measured value will be greater
the thicknesses of metallic coatings and of individual layers of
than the true thickness. This consideration applies to the
composite coatings, particularly for layers thinner than nor-
conventional micrograph (9.3.1) and to the directon of the
mally measured with the light microscope.
single video waveform scans (9.3.2).
4.2 This test method is suitable for acceptance testing.
6.5 Deformation of Coating—Detrimental deformation of
4.3 This test method is for the measurement of the thickness
the coating can be caused by excessive temperature or pressure
of the coating over a very small area and not of the average or
during the mounting and preparation of cross sections of soft
minimum thickness per se.
coatings.
4.4 Accurate measurements by this test method generally
6.6 Rounding of Edge of Coating—If the edge of the coating
require very careful sample preparation, especially at the
cross section is rounded, that is, if the coating cross section is
greater magnifications.
not completely flat up to its edges, the observed thickness may
4.5 The coating thickness is an important factor in the
differ from the true thickness. Edge rounding can be caused by
performance of a coating in service.
improper mounting, grinding, polishing, or etching.
6.7 Overplating of Specimen—Overplating of the test speci-
men serves to protect the coating edges during preparation of
This test method is under the jurisdiction of ASTM Committee B-8 on Metallic
and Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 on
cross sections and thus to prevent an erroneous measurement.
General Test Methods.
Removal of coating material during surface preparation for
Current edition approved Feb. 23, 1990. Published April 1990. Originally
overplating can cause a low thickness measurement.
published as B 748 – 85. Last previous edition B 748 – 85.
Annual Book of ASTM Standards, Vol 03.01.
B 748
6.8 Etching—Optimum etching will produce a clearly de- 7.1.2 The surface is flat and the entire width of the coating
fined and narrow dark line at the interface of two metals. A image is simultaneously in focus at the magnification to be
wide or poorly defined line can result in an inaccurate used for the measurement,
measurement.
7.1.3 All material deformed by cutting or cross sectioning is
6.9 Smearing—Polishing may leave smeared metal that
removed,
obscures the true boundary between the two metals and results
7.1.4 The boundaries of the coating cross section are sharply
in an inaccurate measurement. This may occur with soft metals
defined by contrasting appearance, or by a narrow, well-defined
like lead, indium, and gold. To help identify whether or not
line, and
there is smearing, repeat the polishing, etching, and measure-
7.1.5 If the video waveform signal is to be measured, the
ment several times. Any significant variations in readings
signal trace is flat except across the two boundaries of the
indicates possible smearing.
coating.
6.10 Poor Contrast—The visual contrast between metals in
7.2 For further guidance see Appendix X1.
the SEM is poor when their atomic numbers are close together.
For example, bright and semibright nickel layers may not be
8. Calibration of Magnification
discriminable unless their common boundary can be brought
8.1 Calibrate the SEM with an SEM stage micrometer and
out sufficiently by appropriate etching and SEM techniques.
determine the magnification factor, M, in accordance with
For some metal combinations, energy dispersive X-ray tech-
Practice E 766 (see X1.4.2). Other calibration methods may be
niques (see X1.4.5) or backscatter image techniques (see
used if it can be demonstrated that they are sufficiently accurate
X1.4.6) may be helpful.
for meeting the requirement of Section 12.
6.11 Magnification:
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
field. Magnification errors are minimized by appropriate use of 9.3 Make a micrograph of the test specimen under the same
conditions and instrument settings as used for the calibration
an SEM stage micrometer and appropriate experimental pro-
cedure. (See Practice E 766.) and make an appropriate measurement of the micrograph
image. Carry out this step in accordance with 9.3.1 or 9.3.2.
6.12 Uniformity of Magnification—Because the magnifica-
tion may not be uniform over the entire field, errors can occur
9.3.1 Conventional Micrograph:
if both the calibration and the measurement are not made over
9.3.1.1 With the boundaries of the coating clearly and
the same portion of the field. This can be very important.
sharply defined, make conventional micrographs of the SEM
6.13 Stability of Magnification:
stage micrometer scale and of the test specimen.
6.13.1 The magnification of an SEM often changes or drifts
9.3.1.2 Measure the micrographs to at least the nearest 0.1
with time. This effect is minimized by mounting the stage
mm using a diffraction plate reader or equivalent device. If this
micrometer and test specimen side by side on the SEM stage so
is not practical, it may be because poor sample preparation is
as to keep the tr
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.