ASTM B588-88(1994)
(Test Method)Standard Test Method for Measurement of Thickness of Transparent or Opaque Coatings by Double-Beam Interference Microscope Technique
Standard Test Method for Measurement of Thickness of Transparent or Opaque Coatings by Double-Beam Interference Microscope Technique
SCOPE
1.1 This test method covers the measurement of the thickness of transparent metal oxide and metallic coatings by utilizing a double-beam interference microscope.
1.2 The test method requires that the specimen surface or surfaces be sufficiently mirrorlike to form recognizable fringes.
1.3 This test method can be used nondestructively to measure 1 to 10μ m thick transparent coatings, such as anodic coatings on aluminum. The test method is used destructively for 0.1 to 10 μm thick opaque coatings by stripping a portion of the coating and measuring the step height between the coating and the exposed substrate. The stripping method can also be used to measure 0.2 to 10 μm thick anodic coatings on aluminum.
1.4 The test method is usable as a reference method for the measurement of the thickness of the anodic film on aluminum or of metallic coatings when the technique includes complete stripping of a portion of the coating without attack of the substrate. For anodic films on aluminum, the thickness must be greater than 0.4 μm; the uncertainty can be as great as 0.2 μm. For metallic coatings, the thickness must be greater than 0.25 μm; the uncertainty can be as great as 0.1 μm.
1.5 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.
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NOTICE: This standard has either been superseded and replaced by a new version or discontinued.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 588 – 88 (Reapproved 1994)
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 Transparent or Opaque
Coatings by Double-Beam Interference Microscope
Technique
This standard is issued under the fixed designation B 588; 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 3. Summary of Test Method
1.1 This test method covers the measurement of the thick- 3.1 While observing the specimen surface through the
ness of transparent metal oxide and metallic coatings by interference microscope, the top surface of the coating and the
utilizing a double-beam interference microscope. substrate surface are located with white light interference
1.2 The test method requires that the specimen surface or fringe group(s). Then the elevation difference between the two
surfaces be sufficiently mirrorlike to form recognizable fringes. surfaces is ascertained by counting the number of monochro-
1.3 This test method can be used nondestructively to mea- matic fringes by which the white light fringes are displaced.
sure 1 to 10μ m thick transparent coatings, such as anodic The number of fringes, multiplied by one half of the light
coatings on aluminum. The test method is used destructively wavelength, is the film thickness.
for 0.1 to 10 μm thick opaque coatings by stripping a portion 3.2 When light is reflected, it undergoes a phase shift, the
of the coating and measuring the step height between the magnitude of which depends on the material and on its
coating and the exposed substrate. The stripping method can structure. The uncertainty of the thickness measurement due to
also be used to measure 0.2 to 10 μm thick anodic coatings on this phenomenon is, theoretically, less than ⁄8 the wavelength
aluminum. of the light for metals and ⁄4 wavelength for nonmetallic
1.4 The test method is usable as a reference method for the coatings on metal. Those uncertainties are included in those
measurement of the thickness of the anodic film on aluminum given in 1.4. They can be eliminated for measurements made in
or of metallic coatings when the technique includes complete accordance with 1.3 and 7.1.2 by coating the specimen after the
stripping of a portion of the coating without attack of the stripping operation with a thin but uniform reflective layer of a
substrate. For anodic films on aluminum, the thickness must be metal by evaporation. The two reflecting surfaces will then be
greater than 0.4 μm; the uncertainty can be as great as 0.2 μm. of the same material and the phase shifts will be the same.
For metallic coatings, the thickness must be greater than 0.25 3.3 The aperture of the microscope objective contributes to
μm; the uncertainty can be as great as 0.1 μm. the fringe displacement by an amount determined by the
1.5 This standard does not purport to address all of the aperture size. Therefore, a correction is added equal to a /4
safety concerns, if any, associated with its use. It is the where a, expressed in radians, is the arc sine of the numerical
responsibility of the user of this standard to establish appro- aperture of the microscope objective.
priate safety and health practices and determine the applica-
NOTE 1—When the angle is given in radians and is less than 0.6, the
bility of regulatory limitations prior to use.
angle is approximately equal to its sine.
3.4 With a reticle such as shown in the figures, the fringe
2. Referenced Documents
1 1
count is likely to have an uncertainty of ⁄10 wavelength ( ⁄5
2.1 ASTM Standards:
fringe interval). More precise measurements can be made with
B 504 Test Method for Measurement of Thickness of Me-
the aid of a filar micrometer eyepiece.
tallic Coatings by the Coulometric Method
4. Significance and Use
4.1 The thickness of a coating is often critical to its
performance.
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
4.2 For some coating-substrate combinations, the interfer-
General Test Methods.
ence microscope method is a reliable method for measuring
Current edition approved Feb. 26, 1988. Published April 1988. Originally
2 coating thickness.
published as B 588 – 73. Last previous edition B 588 – 75 (1981)e .
Saur, R. L., “New Interference Microscope Techniques for Microtopographic 4.3 This test method is suitable for specification acceptance.
Measurements in the Electroplating Laboratory,” Plating, PLATA, Vol 52, July
1965, pp. 663–666.
Annual Book of ASTM Standards, Vol 02.05. 4
Bruce, C. F., and Thornton, B. S., Journal of Scientific Instruments, JSINA, Vol
34, 1957, p. 203.
B 588
5. Apparatus
where:
n 5 number of fringes,
5.1 Interference Microscope equipped with a reticle or filar
l5 wavelength of monochromatic light, μm,
micrometer eyepiece for linear measurements.
μ 5 refractive index of coating for light of wave length, l,
5.2 Incandescent and Monochromatic Light Sources.
and
a5 arc sine (numerical aperture of objective) in radians.
6. Sample Preparation for Destructive Technique
Thus for the thickness of the anodic coating on aluminum
6.1 Anodic Coating on Aluminum—After masking (Note 2),
represented in Fig. 1,
the coating is stripped by immersion in a solution containing 33
g/L chromic acid (CrO ) and 0.5 cm /L phosphoric acid
T 5 @~24 3 0.546!/~2 3 1.62!#@1 1 ~0.78 /4!# 5 4.66 μm (2)
(H PO ) (85%). Operating temperature is 85 to 95°C.
3 4
where the monochromatic source is a mercury green light
NOTE 2—Masking for both transparent and opaque coatings can be
with a wavelength of 0.546 μm, where the refractive index of
accomplished by applying an adhesive tape such as 3M #470 or equivalent
the anodic coating is 1.62, and where alpha is equal to 0.78.
with its edge at a location where the thickness measurement is desired.
7.1.2 Destructive Technique:
The tape must be sufficiently adherent and impervious to protect the
coating beneath from subsequent stripping action.
7.1.2.1 Position the boundary between the stripped and
NOTE 3—In certain cases, this method causes attack of the basis metal.
unstripped portion of the specimen in the field of view of the
The attack is usually accompanied by pitting, which is easily observable
microscope.
in the interference microscope by comparing the general contour exhibited
7.1.2.2 As the surface of the specimen is viewed through the
by the fringes on the unstripped portion with the general contour on the
interference microscope using the white light, adjust the
stripped portion. If such attack occurs, the method is not valid.
microscope fine-focus knob and the reference mirror controls
6.2 Metallic Coatings on Metallic Substrates—After mask-
so that the group of fringes arising from the bare substrate and
ing (Note 2), the coating is stripped without attack of the
the weak fringes arising from the coating-air interface are both
substrate (see Appendix X1).
in view, as illustrated in Fig. 2A.
7. Thickness Measurement
7.1.2.3 Determine the number of monochromatic fringes
between the centers of the white light fringe groups. Appendix
NOTE 4—Many surfaces have microscopical ridges or valleys produced
X2 indicates alternative ways of performing this procedure.
by a previous operation (such as rolling or polishing). Measurements of
film thickness are made best with the fringes oriented in a direction
7.1.2.4 Calculate thickness T as follows:
perpendicular to the directional surface roughness.
T 5 nl/2! 1 1 a /4! (3)
~ @ ~ #
7.1 Transparent Coatings:
7.1.1 Nondestructive Technique:
where:
7.1.1.1 As the surface of a specimen is viewed through the n 5 number of fringes,
interference microscope using the incandescent illuminator l5 wavelength of monochromatic light, μm, and
a5 arc sine (numerical aperture of objective) in radians.
(white light), adjust the microscope fine-focus knob and the
reference mirror controls so that a group of strong fringes
7.2 Opaque Coatings—Destructive Technique:
(arising from the coating-substrate interface) and a group of
7.2.1 Position the boundary between the stripped and un-
weak fringes (arising from the c
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.