ASTM B487-85(1997)
(Test Method)Standard Test Method for Measurement of Metal and Oxide Coating Thickness by Microscopical Examination of a Cross Section
Standard Test Method for Measurement of Metal and Oxide Coating Thickness by Microscopical Examination of a Cross Section
SCOPE
1.1 This test method covers measurement of the local thickness of metal and oxide coatings by the microscopical examination of cross sections using an optical microscope.
1.2 Under good conditions, when using an optical microscope, the method is capable of giving an absolute measuring accuracy of 0.8 m. This will determine the suitability of the method for measuring the thickness of thin coatings.
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. (This is especially applicable to the chemicals cited in X2.1.)
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Designation: B 487 – 85 (Reapproved 1997)
AMERICAN SOCIETY FOR TESTING AND MATERIALS
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Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
Standard Test Method for
Measurement of Metal and Oxide Coating Thickness by
Microscopical Examination of a Cross Section
This standard is issued under the fixed designation B 487; 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.
This standard has been approved for use by agencies of the Department of Defense.
1. Scope rough surface, one or both of the interfaces bounding the
coating cross section may be too irregular to permit accurate
1.1 This test method covers measurement of the local
measurement. (See X1.4)
thickness of metal and oxide coatings by the microscopical
5.2 Taper of Cross Section—If the plane of the cross section
examination of cross sections using an optical microscope.
is not perpendicular to the plane of the coating, the measured
1.2 Under good conditions, when using an optical micro-
thickness will be greater than the true thickness. For example,
scope, the method is capable of giving an absolute measuring
an inclination of 10° to the perpendicular will contribute a
accuracy of 0.8 μm. This will determine the suitability of the
1.5 % error.
method for measuring the thickness of thin coatings.
5.3 Deformation of the Coating—Detrimental deformation
1.3 This standard does not purport to address all of the
of the coating can be caused by excessive temperature or
safety concerns, if any, associated with its use. It is the
pressure during mounting and preparation of cross sections of
responsibility of the user of this standard to establish appro-
soft coatings or coatings melting at low temperatures, and also
priate safety and health practices and determine the applica-
by excessive abrasion of brittle materials during preparation of
bility of regulatory limitations prior to use. (This is especially
cross sections.
applicable to the chemicals cited in Table X2.1.)
5.4 Rounding of Edge of Coating—If the edge of the coating
2. Referenced Documents cross section is rounded, that is, if the coating cross section is
not completely flat up to its edges, the true thickness cannot be
2.1 ASTM Standards:
observed microscopically. Edge rounding can be caused by
E 3 Methods of Preparation of Metallographic Specimens
improper mounting, grinding, polishing, or etching. It is
3. Summary of Test Method
usually minimized by overplating the test specimen before
mounting. (See X1.2.)
3.1 This test method consists of cutting out a portion of the
5.5 Overplating—Overplating of the test specimen serves to
test specimen, mounting it, and preparing the mounted cross
protect the coating edges during preparation of cross sections
section by suitable techniques of grinding, polishing, and
and thus to prevent an erroneous measurement. Removal of
etching. The thickness of the cross section is measured with an
coating material during surface preparation for overplating can
optical microscope.
cause a low-thickness measurement.
NOTE 1—These techniques will be familiar to experienced metallogra-
5.6 Etching—Optimum etching will produce a clearly de-
phers but some guidance is given in Section 5 and in Appendix X1 for less
fined and narrow dark line at the interface of two metals.
experienced operators.
Excessive etching produces a poorly defined or wide line
4. Significance and Use which may result in an erroneous measurement.
5.7 Smearing—Improper polishing may leave one metal
4.1 Coating thickness is an important factor in the perfor-
smeared over the other metal so as to obscure the true boundary
mance of a coating in service and is usually specified in a
between the two metals. The apparent boundary may be poorly
coating specification.
defined or very irregular instead of straight and well defined.
4.2 This method is suitable for acceptance testing.
To verify the absence of smearing, the coating thickness should
5. Factors Influencing the Measurement Result
be measured and the polishing, etching, and thickness mea-
surement repeated. A significant change in apparent thickness
5.1 Surface Roughness—If the coating or its substrate has a
indicates that smearing was probably present during one of the
measurements.
This test method is under the jurisdiction of ASTM Committee B-8 on Metallic
5.8 Magnification—For any given coating thickness, mea-
and Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 on
General Test Methods.
surement errors generally increase with decreasing magnifica-
Current edition approved Feb. 22, 1985. Published May 1985. Originally
tion. If possible, the magnification should be chosen so that the
published as B487 – 68. Last previous edition B487 – 79.
2 field of view is between 1.5 and 3 3 the coating thickness.
Annual Book of ASTM Standards, Vol 03.01.
B 487
5.9 Calibration of Stage Micrometer—Any error in calibra- 6.1.1 The cross section is perpendicular to the coating;
tion of the stage micrometer will be reflected in the measure-
6.1.2 The surface is flat and the entire width of the coating
ment of the specimen. Errors of several percent are not
image is simultaneously in focus at the magnification used for
unrealistic unless the scale has been calibrated or has been
the measurement;
certified by a responsible supplier. The distance between two
6.1.3 All material deformed by cutting or cross sectioning is
lines of a stage micrometer used for the calibration shall be
removed.
known to within 0.2 μm or 0.1 %, whichever is the greater. If
6.1.4 The boundaries of the coating cross section are sharply
a stage micrometer is not certified for accuracy, it should be
defined by no more than contrasting appearance or by a narrow,
calibrated. A generally satisfactory means of calibration is to
well-defined line.
assume that the stated length of the full scale is correct, to
NOTE 2—Further guidance is given in Appendix X1. Some typical
measure each subdivision with a filar micrometer, and to
etchants are described in Appendix X2.
calculate the length of each subdivision by simple proportion.
5.10 Calibration of Micrometer Eyepiece—
7. Procedure
5.10.1 A filar micrometer eyepiece generally provides the
7.1 Give appropriate attention to the factors listed in Section
most satisfactory means of making the measurement of the
5 and Appendix X1.
specimen. The measurement will be no more accurate than the
calibration of the eyepiece. As calibration is operator depen-
7.2 Calibrate the microscope and its measuring device with
dent, the eyepiece shall be calibrated by the person making the a certified or calibrated stage micrometer.
measurement.
7.3 Measure the width of the image of the coating cross
5.10.2 Repeated calibrations of the micrometer eyepiece
section at no less than five points distributed along a length of
can be reasonably expected to have a spread of less than 1 %.
the microsection, and calculate the arithmetic mean of the
5.10.3 Some image-splitting micrometer eyepieces have a
measurements (see 8.1.5 and 8.1.6).
nonlinearity that introduces an error of up to 1 % for short
measurement distances.
8. Test Report
5.11 Alignment—Errors can be introduced by backlash in
8.1 The test report shall include the following information:
the movement of the micrometer eyepiece. If the final motion
8.1.1 The date of test;
during alignment of the hairline is always made in the same
8.1.2 The number and title of this test method;
direction, this error will be eliminated.
8.1.3 The identification of the test specimens;
5.12 Uniformity of Magnification—Because the magnifica-
tion may not be uniform over the entire field, errors can occur 8.1.4 The location on the coated item at which the cross
if both the calibration and the measurement are not made over section was made;
the same portion of the field with the measured boundaries
8.1.5 The measured thickness, in micrometres (millimetres
centered about the optical axis.
if greater than 1 mm) at each point (7.3), and the length of
5.13 Lens Quality—Lack of sharpness of the image contrib-
section over which the measurements were distributed;
utes to the uncertainty of the measurement. Poor quality lenses
8.1.6 The local thickness, that is, the arithmetic mean of the
could preclude accurate measurements. Sometimes image
measured thicknesses;
sharpness can be improved by using monochromatic light.
8.1.7 Any deviations from this test method;
5.14 Orientation of Eyepiece—The movement of the hair-
8.1.8 Any factors that might influence interpretation of the
line of the eyepiece for alignment has to be perpendicular to the
reported results; and
boundaries of the coating cross section. For example, 10°
8.1.9 The name of the operator and testing laboratory.
misalignment will contribute a 1.5 % error.
5.15 Tube Length—A change in the tube length of the
9. Precision and Bias
microscope causes a change in magnification and if this change
9.1 The microscope and associated equipment, its use, its
occurs between the time of calibration and the time of
measurement, the measurement will be in error. A change in calibration, and the method of preparation of the cross section
shall be chosen so as to allow the coating thickness to be
tube length may occur when the eyepiece is rep
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