ASTM_B_588_-_88_2010.pdf

Designation:B58888(Reapproved 2010)Standard Test Method forMeasurement of Thickness of Transparent or OpaqueCoatings by Double-Beam Interference MicroscopeTechnique1This standard is issued under the fixed designation B588;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 the thick-ness of transparent metal oxide and metallic coatings byutilizing a double-beam interference microscope.21.2 The test method requires that the specimen surface orsurfaces be sufficiently mirrorlike to form recognizable fringes.1.3 This test method can be used nondestructively to mea-sure 1 to 10 m thick transparent coatings,such as anodiccoatings on aluminum.The test method is used destructivelyfor 0.1 to 10 m thick opaque coatings by stripping a portionof the coating and measuring the step height between thecoating and the exposed substrate.The stripping method canalso be used to measure 0.2 to 10 m thick anodic coatings onaluminum.1.4 The test method is usable as a reference method for themeasurement of the thickness of the anodic film on aluminumor of metallic coatings when the technique includes completestripping of a portion of the coating without attack of thesubstrate.For anodic films on aluminum,the thickness must begreater than 0.4 m;the uncertainty can be as great as 0.2 m.For metallic coatings,the thickness must be greater than 0.25m;the uncertainty can be as great as 0.1 m.1.5 The values stated in SI units are to be regarded asstandard.No other units of measurement are included in thisstandard.1.6 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:3B504 Test Method for Measurement of Thickness of Metal-lic Coatings by the Coulometric Method3.Summary of Test Method3.1 While observing the specimen surface through theinterference microscope,the top surface of the coating and thesubstrate surface are located with white light interferencefringe group(s).Then the elevation difference between the twosurfaces is ascertained by counting the number of monochro-matic fringes by which the white light fringes are displaced.The number of fringes,multiplied by one half of the lightwavelength,is the film thickness.3.2 When light is reflected,it undergoes a phase shift,themagnitude of which depends on the material and on itsstructure.The uncertainty of the thickness measurement due tothis phenomenon is,theoretically,less than18the wavelengthof the light for metals and14wavelength for nonmetalliccoatings on metal.Those uncertainties are included in thosegiven in 1.4.They can be eliminated for measurements made inaccordance with 1.3 and 7.1.2 by coating the specimen after thestripping operation with a thin but uniform reflective layer of ametal by evaporation.The two reflecting surfaces will then beof the same material and the phase shifts will be the same.3.3 The aperture of the microscope objective contributes tothe fringe displacement by an amount determined by theaperture size.Therefore,a correction4is added equal to 2/4where,expressed in radians,is the arc sine of the numericalaperture of the microscope objective.NOTE1When the angle is given in radians and is less than 0.6,theangle is approximately equal to its sine.3.4 With a reticle such as shown in the figures,the fringecount is likely to have an uncertainty of110wavelength(151This test method is under the jurisdiction ofASTM Committee B08 on Metallicand Inorganic Coatingsand is the direct responsibility of Subcommittee B08.10 onTest Methods.Current edition approved Nov.1,2010.Published November 2010.Originallyapproved in 1973.Last previous edition approved in 2006 as B588 88(2006).DOI:10.1520/B0588-88R10.2Saur,R.L.,“New Interference Microscope Techniques for MicrotopographicMeasurements in the Electroplating Laboratory,”Plating,PLATA,Vol 52,July1965,pp.663666.3For 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.4Bruce,C.F.,and Thornton,B.S.,Journal of Scientific Instruments,JSINA,Vol34,1957,p.203.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 fringe interval).More precise measurements can be made withthe aid of a filar micrometer eyepiece.4.Significance and Use4.1 The thickness of a coating is often critical to itsperformance.4.2 For some coating-substrate combinations,the interfer-enc