ASTM B588-88(2001)
(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.
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Designation:B588–88(Reapproved 2001)
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 formrecognizable 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 givenin1.4.Theycanbeeliminatedformeasurementsmadein
or of metallic coatings when the technique includes complete accordancewith1.3and7.1.2bycoatingthespecimenafterthe
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
count is likely to have an uncertainty of ⁄10 wavelength
2.1 ASTM Standards:
( ⁄5fringe interval). More precise measurements can be made
B 504 Test Method for Measurement of Thickness of Me-
3 with 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
This test method is under the jurisdiction ofASTM Committee B08 on Metallic
and Inorganic Coatingsand is the direct responsibility of Subcommittee B08.10on
performance.
Test Methods.
Current edition approved Feb. 26, 1988. Published April 1988. Originally
published as B 588 – 73. Last previous edition B 588 – 75 (1981)e .
Saur, R. L., “New Interference Microscope Techniques for Microtopographic
Measurements in the Electroplating Laboratory,” Plating, PLATA, Vol 52, July
1965, pp. 663–666. Bruce, C. F., andThornton, B. S., Journal of Scientific Instruments, JSINA,Vol
Annual Book of ASTM Standards, Vol 02.05. 34, 1957, p. 203.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
B588–88 (2001)
4.2 For some coating-substrate combinations, the interfer- 7.1.1.2 Determine the number of monochromatic fringes
ence microscope method is a reliable method for measuring between the centers of the white light fringe groups.Appendix
coating thickness. X2 indicates alternative ways of doing this.
4.3 This test method is suitable for specification acceptance. 7.1.1.3 Calculate thickness T as follows:
T 5 ~nl/2µ! @1 1 ~a /4!# (1)
5. Apparatus
5.1 Interference Microscope equipped with a reticle or filar
where:
n = number of fringes,
micrometer eyepiece for linear measurements.
l = wavelength of monochromatic light, µm,
5.2 Incandescent and Monochromatic Light Sources.
µ = refractive index of coating for light of wave length, l,
6. Sample Preparation for Destructive Technique and
a = arc sine (numerical aperture of objective) in radians.
6.1 Anodic Coating on Aluminum—After masking (Note 2),
Thus for the thickness of the anodic coating on aluminum
thecoatingisstrippedbyimmersioninasolutioncontaining33
represented in Fig. 1,
g/L chromic acid (CrO ) and 0.5 cm /L phosphoric acid
(H PO ) (85%). Operating temperature is 85 to 95°C.
3 4 T 5 @~24 3 0.546!/~2 3 1.62!#@1 1 ~0.78 /4!# 5 4.66 µm (2)
NOTE 2—Masking for both transparent and opaque coatings can be where the monochromatic source is a mercury green light
accomplishedbyapplyinganadhesivetapesuchas3M#470orequivalent
with a wavelength of 0.546 µm, where the refractive index of
with its edge at a location where the thickness measurement is desired.
the anodic coating is 1.62, and where alpha is equal to 0.78.
The tape must be sufficiently adherent and impervious to protect the
7.1.2 Destructive Technique:
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
intheinterferencemicroscopebycomparingthegeneralcontourexhibited
microscope.
by the fringes on the unstripped portion with the general contour on the
7.1.2.2 Asthesurfaceofthespecimenisviewedthroughthe
stripped portion. If such attack occurs, the method is not valid.
interference microscope using the white light, adjust the
6.2 Metallic Coatings on Metallic Substrates—After mask-
microscope fine-focus knob and the reference mirror controls
ing (Note 2), the coating is stripped without attack of the so that the group of fringes arising from the bare substrate and
substrate (see Appendix X1).
the weak fringes arising from the coating-air interface are both
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 = number of fringes,
interference microscope using the incandescent illuminator l = wavelength of monochromatic light, µm, and
a = 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-
w
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