ASTM_B_748_-_90_2016.pdf

Designation:B74890(Reapproved 2016)Standard Test Method forMeasurement of Thickness of Metallic Coatings byMeasurement of Cross Section with a Scanning ElectronMicroscope1This standard is issued under the fixed designation B748;the number immediately following the designation indicates the year 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()indicates an editorial change since the last revision or reapproval.1.Scope1.1 This test method covers the measurement of metalliccoating thicknesses by examination of a cross section with ascanning electron microsope(SEM).1.2 The values stated in SI units are to be regarded asstandard.No other units of measurement are included in thisstandard.1.3 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 practices and determine the applica-bility of regulatory limitations prior to use.2.Referenced Documents2.1 ASTM Standards:2E3 Guide for Preparation of Metallographic SpecimensE766 Practice for Calibrating the Magnification of a Scan-ning Electron Microscope3.Summary of Test Method3.1 A test specimen is 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 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 thicknessof 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 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 thickness,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 will 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():t=tm cos().(See X1.3.2.)6.3 Specimen TiltAny tilt of the specimen(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 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).1This test method is under the jurisdiction ofASTM Committee B08 on Metallicand Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 onTest Methods.Current edition approved Nov.1,2016.Published November 2016.Originallyapproved in 1985.Last previous edition approved in 2010 as B748 90(2010).DOI:10.1520/B0748-90R16.2For referenced ASTM standards,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.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 6.5 Deformation of CoatingDetrimental deformation ofthe coating can be caused by excessive temperature or pressureduring 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 maydiffer 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 erroneous measurement.Removal of coating material during surface preparation foroverplating can cause a low thickness measurement.6.8 Etch