ASTM B487-85(2002)
(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:B487–85 (Reapproved 2002)
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 4.2 This method is suitable for acceptance testing.
1.1 This test method covers measurement of the local
5. Factors Influencing the Measurement Result
thickness of metal and oxide coatings by the microscopical
5.1 Surface Roughness—If the coating or its substrate has a
examination of cross sections using an optical microscope.
rough surface, one or both of the interfaces bounding the
1.2 Under good conditions, when using an optical micro-
coating cross section may be too irregular to permit accurate
scope, the method is capable of giving an absolute measuring
measurement. (See X1.4)
accuracy of 0.8 µm. This will determine the suitability of the
5.2 Taper of Cross Section—If the plane of the cross section
method for measuring the thickness of thin coatings.
is not perpendicular to the plane of the coating, the measured
1.3 This standard does not purport to address all of the
thickness will be greater than the true thickness. For example,
safety concerns, if any, associated with its use. It is the
an inclination of 10° to the perpendicular will contribute a
responsibility of the user of this standard to establish appro-
1.5 % error.
priate safety and health practices and determine the applica-
5.3 Deformation of the Coating—Detrimental deformation
bility of regulatory limitations prior to use. (This is especially
of the coating can be caused by excessive temperature or
applicable to the chemicals cited in Table X2.1.)
pressure during mounting and preparation of cross sections of
2. Referenced Documents soft coatings or coatings melting at low temperatures, and also
by excessive abrasion of brittle materials during preparation of
2.1 ASTM Standards:
cross sections.
E 3 Methods of Preparation of Metallographic Specimens
5.4 Rounding of Edge of Coating—Iftheedgeofthecoating
3. Summary of Test Method cross section is rounded, that is, if the coating cross section is
not completely flat up to its edges, the true thickness cannot be
3.1 This test method consists of cutting out a portion of the
observed microscopically. Edge rounding can be caused by
test specimen, mounting it, and preparing the mounted cross
improper mounting, grinding, polishing, or etching. It is
section by suitable techniques of grinding, polishing, and
usually minimized by overplating the test specimen before
etching. The thickness of the cross section is measured with an
mounting. (See X1.2.)
optical microscope.
5.5 Overplating—Overplatingofthetestspecimenservesto
NOTE 1—These techniques will be familiar to experienced metallogra-
protect the coating edges during preparation of cross sections
phers but some guidance is given in Section 5 and inAppendix X1 for less
and thus to prevent an erroneous measurement. Removal of
experienced operators.
coating material during surface preparation for overplating can
cause a low-thickness measurement.
4. Significance and Use
5.6 Etching—Optimum etching will produce a clearly de-
4.1 Coating thickness is an important factor in the perfor-
fined and narrow dark line at the interface of two metals.
mance of a coating in service and is usually specified in a
Excessive etching produces a poorly defined or wide line
coating specification.
which may result in an erroneous measurement.
5.7 Smearing—Improper polishing may leave one metal
ThistestmethodisunderthejurisdictionofASTMCommitteeB08onMetallic smearedovertheothermetalsoastoobscurethetrueboundary
and Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 on
between the two metals.The apparent boundary may be poorly
General Test Methods.
defined or very irregular instead of straight and well defined.
Current edition approved Feb. 22, 1985. Published May 1985. Originally
Toverifytheabsenceofsmearing,thecoatingthicknessshould
published as B487 – 68. Last previous edition B487 – 79.
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.
B487
be measured and the polishing, etching, and thickness mea- the tube, when the focus of the eyepiece tube is changed, and,
surement repeated. A significant change in apparent thickness for some microscopes, when the fine focus is adjusted or the
indicates that smearing was probably present during one of the interpupillary distance for binoculars is changed.
measurements.
6. Preparation of Cross Sections
5.8 Magnification—For any given coating thickness, mea-
6.1 Prepare, mount, polish, and etch the specimen so that:
surement errors generally increase with decreasing magnifica-
6.1.1 The cross section is perpendicular to the coating;
tion. If possible, the magnification should be chosen so that the
6.1.2 The surface is flat and the entire width of the coating
field of view is between 1.5 and 3 3 the coating thickness.
image is simultaneously in focus at the magnification used for
5.9 Calibration of Stage Micrometer—Any error in calibra-
the measurement;
tion of the stage micrometer will be reflected in the measure-
6.1.3 All material deformed by cutting or cross sectioning is
ment of the specimen. Errors of several percent are not
removed.
unrealistic unless the scale has been calibrated or has been
6.1.4 Theboundariesofthecoatingcrosssectionaresharply
certified by a responsible supplier. The distance between two
definedbynomorethancontrastingappearanceorbyanarrow,
lines of a stage micrometer used for the calibration shall be
well-defined line.
known to within 0.2 µm or 0.1 %, whichever is the greater. If
a stage micrometer is not certified for accuracy, it should be
NOTE 2—Further guidance is given in Appendix X1. Some typical
calibrated. A generally satisfactory means of calibration is to etchants are described in Appendix X2.
assume that the stated length of the full scale is correct, to
7. Procedure
measure each subdivision with a filar micrometer, and to
7.1 GiveappropriateattentiontothefactorslistedinSection
calculate the length of each subdivision by simple proportion.
5 and Appendix X1.
5.10 Calibration of Micrometer Eyepiece:
7.2 Calibrate the microscope and its measuring device with
5.10.1 A filar micrometer eyepiece generally provides the
a certified or calibrated stage micrometer.
most satisfactory means of making the measurement of the
7.3 Measure the width of the image of the coating cross
specimen. The measurement will be no more accurate than the
section at no less than five points distributed along a length of
calibration of the eyepiece. As calibration is operator depen-
the microsection, and calculate the arithmetic mean of the
dent, the eyepiece shall be calibrated by the person making the
measurements (see 8.1.5 and 8.1.6).
measurement.
5.10.2 Repeated calibrations of the micrometer eyepiece
8. Test Report
can be reasonably expected to have a spread of less than 1 %.
8.1 The test report shall include the following information:
5.10.3 Some image-splitting micrometer eyepieces have a
8.1.1 The date of test;
nonlinearity that introduces an error of up to 1 % for short
8.1.2 The number and title of this test method;
measurement distances.
8.1.3 The identification of the test specimens;
5.11 Alignment—Errors can be introduced by backlash in
8.1.4 The location on the coated item at which the cross
the movement of the micrometer eyepiece. If the final motion
section was made;
during alignment of the hairline is always made in the same
8.1.5 The measured thickness, in micrometres (millimetres
direction, this error will be eliminated.
if greater than 1 mm) at each point (7.3), and the length of
5.12 Uniformity of Magnification—Because the magnifica-
section over which the measurements were distributed;
tion may not be uniform over the entire field, errors can occur
8.1.6 The local thickness, that is, the arithmetic mean of the
if both the calibration and the measurement are not made over
measured thicknesses;
the same portion of the field with the measured boundaries
8.1.7 Any deviations from this test method;
centered about the optical axis.
8.1.8 Any factors that might influence interpretation of the
5.13 Lens Quality—Lack of sharpness of the image contrib-
reported results; and
utes to the uncertainty of the measurement. Poor quality lenses
8.1.9 The name of the operator and testing laboratory.
could preclude accurate measurements. Sometimes image
9. Precision and Bias
sharpness can be improved by using monochromatic light.
5.14 Orientation of Eyepiece—The movement of the hair- 9.1 The microscope and associated equipment, its use, its
lineoftheeyepieceforalignmenthastobeperpendiculartothe calibration, and the method of preparation of the cross section
boundaries of the coating cross section. For example, 10° shall be chosen so as to allow the coating thickness to be
misalignment will contribute a 1.5 % error. determinedtowithin1µmor10 %,whicheveristhegreater,of
5.15 Tube Length—A change in the tube length of the the actual coating thickness. Under good conditions, when
microscope
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