1、Designation: B 748 90 (Reapproved 2006)Standard Test Method forMeasurement of Thickness of Metallic Coatings byMeasurement of Cross Section with a Scanning ElectronMicroscope1This standard is issued under the fixed designation B 748; the number immediately following the designation indicates the yea
2、r oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers the measurement of metalliccoa
3、ting thicknesses by examination of a cross section with ascanning electron microsope (SEM).1.2 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practi
4、ces and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E3 Guide for Preparation of Metallographic SpecimensE 766 Practice for Calibrating the Magnification of a Scan-ning Electron Microscope3. Summary of Test Method3.1 A test specimen i
5、s cut, ground, and polished for metal-lographic examination by an SEM of a cross section of thecoating. The measurement is made on a conventional micro-graph or on a photograph of the video waveform signal for asingle scan across the coating.4. Significance and Use4.1 This test method is useful for
6、the direct measurement ofthe thicknesses of metallic coatings and of individual layers ofcomposite coatings, particularly for layers thinner than nor-mally measured with the light microscope.4.2 This test method is suitable for acceptance testing.4.3 This test method is for the measurement of the th
7、icknessof the coating over a very small area and not of the average orminimum thickness per se.4.4 Accurate measurements by this test method generallyrequire very careful sample preparation, especially at thegreater magnifications.4.5 The coating thickness is an important factor in theperformance of
8、 a coating in service.5. Equipment5.1 The scanning electron microscope shall have a resolu-tion of at least 50 nm. Suitable instruments are availablecommercially.6. Factors Affecting the Measurement Reliability6.1 Surface RoughnessIf the coating or its substrate isrough relative to the coating thick
9、ness, one or both of theinterfaces bounding the coating cross section may be tooirregular to permit accurate measurement of the averagethickness in the field of view.6.2 Taper of Cross SectionIf the plane of the cross sectionis not perpendicular to the plane of the coating, the measuredthickness wil
10、l be greater than the true thickness. For example,an inclination of 10 to the perpendicular will contribute a1.5 % error. True thickness, (t), equals measured thickness,(tm), multiplied by the cosine of the angle of inclination (u):t=tm3 cos(u). (See X1.3.2.)6.3 Specimen TiltAny tilt of the specimen
11、 (plane of thecross section) with respect to the SEM beam, may result in anerroneous measurement. The instrument should always be setfor zero tilt.6.4 Oblique MeasurementIf the thickness measurement isnot perpendicular to the plane of the coating, even when thereis no taper (6.2) or tilt (6.3), the
12、measured value will be greaterthan the true thickness. This consideration applies to theconventional micrograph (9.3.1) and to the direction of thesingle video waveform scans (9.3.2).6.5 Deformation of CoatingDetrimental deformation ofthe coating can be caused by excessive temperature or pressuredur
13、ing the mounting and preparation of cross sections of softcoatings.6.6 Rounding of Edge of CoatingIf the edge of the coatingcross section is rounded, that is, if the coating cross section isnot completely flat up to its edges, the observed thickness may1This test method is under the jurisdiction ofA
14、STM Committee B08 on Metallicand Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 onTest Methods.Current edition approved April 1, 2006. Published April 2006. Originallyapproved in 1985. Last previous edition approved in 2001 as B 748 90 (2001).2For referenced ASTM standard
15、s, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 1
16、9428-2959, United States.differ from the true thickness. Edge rounding can be caused byimproper mounting, grinding, polishing, or etching.6.7 Overplating of SpecimenOverplating of the test speci-men serves to protect the coating edges during preparation ofcross sections and thus to prevent an errone
17、ous measurement.Removal of coating material during surface preparation foroverplating can cause a low thickness measurement.6.8 EtchingOptimum etching will produce a clearly de-fined and narrow dark line at the interface of two metals. Awide or poorly defined line can result in an inaccuratemeasurem
18、ent.6.9 SmearingPolishing may leave smeared metal thatobscures the true boundary between the two metals and resultsin an inaccurate measurement. This may occur with soft metalslike lead, indium, and gold. To help identify whether or notthere is smearing, repeat the polishing, etching, and measure-me
19、nt several times. Any significant variations in readingsindicates possible smearing.6.10 Poor ContrastThe visual contrast between metals inthe SEM is poor when their atomic numbers are close together.For example, bright and semibright nickel layers may not bediscriminable unless their common boundar
20、y can be broughtout sufficiently by appropriate etching and SEM techniques.For some metal combinations, energy dispersive X-ray tech-niques (see X1.4.5) or backscatter image techniques (seeX1.4.6) may be helpful.6.11 Magnification:6.11.1 For any given coating thickness, measurement errorstend to inc
21、rease with decreasing magnification. If practical, themagnification should be chosen so that the field of view isbetween 1.5 and 33 the coating thickness.6.11.2 The magnification readout of an SEM is often poorerthan the 5 % accuracy often quoted and the magnification hasbeen found for some instrume
22、nts to vary by 25 % across thefield. Magnification errors are minimized by appropriate use ofan SEM stage micrometer and appropriate experimental pro-cedure (see Practice E 766).6.12 Uniformity of MagnificationBecause the magnifica-tion may not be uniform over the entire field, errors can occurif bo
23、th the calibration and the measurement are not made overthe same portion of the field. This can be very important.6.13 Stability of Magnification:6.13.1 The magnification of an SEM often changes or driftswith time. This effect is minimized by mounting the stagemicrometer and test specimen side by si
24、de on the SEM stage soas to keep the transfer time short.6.13.2 A change in magnification can occur when adjust-ments are made with the focusing and other electronic SEMcontrols. Such a change is prevented by not using the electronicfocus controls or other electronic SEM controls after photo-graphin
25、g the stage micrometer scale except to focus with themechanical X, Y, and Z controls. Appropriate manipulation ofthe X, Y, and Z controls will bring the specimen surface to thefocal point of the SEM beam.6.14 Stability of MicrographsDimensional changes ofmicrographs can take place with time and with
26、 temperature andhumidity changes. If the calibration micrograph of the stagemicrometer scale and the micrograph of the test specimen arekept together and time is allowed for stabilization of thephotographic paper, errors from this source will be minimized.7. Preparations of Cross Sections7.1 Prepare
27、, mount, grind, polish, and etch the test specimenso that the following occurs:7.1.1 The cross section is perpendicular to the plane of thecoating,7.1.2 The surface is flat and the entire width of the coatingimage is simultaneously in focus at the magnification to beused for the measurement,7.1.3 Al
28、l material deformed by cutting or cross sectioning isremoved,7.1.4 The boundaries of the coating cross section are sharplydefined by contrasting appearance, or by a narrow, well-definedline, and7.1.5 If the video waveform signal is to be measured, thesignal trace is flat except across the two bounda
29、ries of thecoating.7.2 For further guidance see Appendix X1.8. Calibration of Magnification8.1 Calibrate the SEM with an SEM stage micrometer anddetermine the magnification factor, M, in accordance withPractice E 766 (see X1.4.2). Other calibration methods may beused if it can be demonstrated that t
30、hey are sufficiently accuratefor meeting the requirement of Section 12.8.2 If practical, the stage micrometer and the test specimenshall be mounted side by side on the SEM stage.9. Procedure9.1 Operate the SEM in accordance with the manufacturersinstructions.9.2 Take into account the factors listed
31、in Sections 6 and 12.9.3 Make a micrograph of the test specimen under the sameconditions and instrument settings as used for the calibrationand make an appropriate measurement of the micrographimage. Carry out this step in accordance with 9.3.1 or 9.3.2.9.3.1 Conventional Micrograph:9.3.1.1 With the
32、 boundaries of the coating clearly andsharply defined, make conventional micrographs of the SEMstage micrometer scale and of the test specimen.9.3.1.2 Measure the micrographs to at least the nearest 0.1mm using a diffraction plate reader or equivalent device. If thisis not practical, it may be becau
33、se poor sample preparation iscausing the boundaries of the coating to be poorly defined.9.3.2 Video Waveform Signal:9.3.2.1 Photograph the video waveform signal for a singlescan across the coating cross section and across the SEM stagemicrometer scale.9.3.2.2 To measure the coating, measure the hori
34、zontaldistance between the inflection points of the vertical portions ofthe scan at the boundaries of the coating. Make the measure-ments to the nearest 0.1 mm using a diffraction plate reader orequivalent device.9.3.3 For further guidance see Appendix X1.10. Calculation and Expression of Results10.
35、1 Calculate the thickness according to the expression:B 748 90 (2006)2T 5 1000 3 d/M (1)where:T = coating thickness, in m,d = linear distance on micrograph, in mm, andM = magnification factor as defined in Practice E 766.11. Report11.1 The report of the measurements shall give the follow-ing informa
36、tion:11.1.1 Date measurements were made,11.1.2 The title, number, and year of issue of this testmethod,11.1.3 Identification of the test specimen(s),11.1.4 Location of measurement on test specimen(s),11.1.5 The measured values and their arithmetic mean,11.1.6 The calibrated magnification as measured
37、 with anSEM micrometer scale immediately before the test specimenmeasurements,11.1.7 Type of measurement: conventional micrograph orvideo waveform signal,11.1.8 Any unusual feature of the measurement that mightaffect the results, and11.1.9 Name of individual responsible for the measure-ments.12. Pre
38、cision and Bias12.1 The instrument, its operation, and its calibration shallbe such that the uncertainty of the measurements shall be lessthan 0.1 m or 10 %, whichever is larger.12.2 For a thin gold coating, one laboratory reported mea-surement uncertainty of 0.039 m for the SEM stage microme-ter sc
39、ale, 0.02 m for the measurement of the calibrationmicrographs, and 0.02 m for measurement of the videowaveform signal scan. Based on practical experience, a repeat-ability of 0.1 m or better may be assumed.APPENDIX(Nonmandatory Information)X1. TECHNIQUES OF SPECIMEN PREPARATION AND USE OF THE SEMX1.
40、1 IntroductionX1.1.1 The preparation of specimens and measurement ofcoating thickness are greatly dependent on individual tech-niques and there is a variety of suitable techniques available(see Guide E3). It is not reasonable to specify only one set oftechniques, and it is impractical to include all
41、 suitable tech-niques. The techniques described in this appendix are intendedas guidance.X1.2 MountingX1.2.1 To prevent rounding of the edge of the coating crosssection, the free surface of the coating should be supported sothat there is no space between the coating and its support. Thisis usually a
42、chieved by overplating the coating with a coating atleast 10 m thick of a metal of similar hardness to the coating.The overplate should also give an electron signal intensitydifferent from that of the coating. The mounting material orsample surface must be electrically conducting and groundedto prev
43、ent a surface charge buildup in the SEM.X1.3 Grinding and PolishingX1.3.1 It is essential to keep the cross section surface of themount perpendicular to the coating. This is facilitated byincorporating additional pieces of a similar metal in the plasticmounting, near the outer edges, by periodically
44、 changing thedirection of grinding (rotating through 90 and by keeping thegrind time and pressure to a minimum). If, before grinding,reference marks are inscribed on the sides of the mount, anyinclination from horizontal is easily measurable. Grind themounted specimens on suitable abrasive paper, us
45、ing anacceptable lubricant, such as water, and apply minimumpressure to avoid bevelling the surface. Initial grinding shouldemploy 100 or 180 grade abrasive to reveal the true specimenprofile and to remove any deformed metal. Subsequently, useGrades 240, 320, 500, and 600 without exceeding grindingt
46、imes of 30 to 40 s on each paper; alter the direction ofscratches by 90 for each change of paper. Then polishsuccessively with 6 to 9, 1, and 0.5-m diamond on microcloth.Some metallographers prefer the use of 0.3- and 0.05-malumina.X1.3.2 A convenient way to check for tapering of the crosssection is
47、 to mount a small diameter rod or wire with thespecimen so that the perpendicular cross section of the rod isparallel to that of the coating. If a taper is present, the crosssection of the rod will be elliptical.X1.3.3 If the video waveform signal scan technique is used,it is important that scratche
48、s be completely removed and thatoverpolishing does not selectively remove one of the metalsmore than the other so that the signal scan is distorted. Withcareful polishing, it is often unnecessary to use chemicaletches.X1.4 Use of SEMX1.4.1 If the image of the cross section, as revealed in aconventio
49、nal micrograph, is measured; and if the boundaries ofthe coating cross section are revealed solely by the photo-graphed contrast between the two materials; the apparent widthof the coating cross section can vary, depending on the contrastand brightness settings. The variation can be as great as 10 %without any change in instrument magnification. To minimizethe resulting uncertainty, adjust the contrast and brightness soB 748 90 (2006)3that the image contains surface detail of the materials on eitherside of each boundary.X1.4.2 Because the magnification of a
