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ASTM B748-90(R2021) pdf download

ASTM B748-90(R2021) pdf download.Standard Test Method for Measurement of Thickness of Metallic Coatings by Measurement of Cross Section with a Scanning Electron Microscope
1.1 This test method covers the measurement of metallic coating thicknesses by examination of a cross section with a scanning electron microsope (SEM).
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3 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 appro- priate safety, health, and environmental practices and deter- mine the applicability ofregulatory limitations prior to use.
1.4 This international standard was developed in accor- dance with internationally recognized principles on standard- ization established in the Decision on Principles for the Development of International Standards, Guides and Recom- mendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
6.1 Surface Roughness—If the coating or its substrate is rough relative to the coating thickness, one or both of the interfaces bounding the coating cross section may be too irregular to permit accurate measurement of the average thickness in the field of view.
6.2 Taper ofCross Section—If the plane of the cross section is not perpendicular to the plane of the coating, the measured thickness will be greater than the true thickness. For example, an inclination of 10° to the perpendicular will contribute a 1.5 % error. True thickness, (t), equals measured thickness, (t m ), multiplied by the cosine of the angle of inclination (θ): t = t m × cos(θ). (See X1.3.2.)
6.3 Specimen Tilt—Any tilt of the specimen (plane of the cross section) with respect to the SEM beam, may result in an erroneous measurement. The instrument should always be set for zero tilt.
6.4 Oblique Measurement—If the thickness measurement is not perpendicular to the plane of the coating, even when there is no taper (6.2) or tilt (6.3), the measured value will be greater than the true thickness. This consideration applies to the conventional micrograph (9.3.1) and to the direction of the single video waveform scans (9.3.2).
6.5 Deformation of Coating—Detrimental deformation of the coating can be caused by excessive temperature or pressure during the mounting and preparation of cross sections of soft coatings.
6.6 Rounding ofEdge ofCoating—Ifthe edge ofthe coating cross section is rounded, that is, if the coating cross section is not completely flat up to its edges, the observed thickness may differ from the true thickness. Edge rounding can be caused by improper mounting, grinding, polishing, or etching.
6.7 Overplating ofSpecimen—Overplating of the test speci- men serves to protect the coating edges during preparation of cross sections and thus to prevent an erroneous measurement. Removal of coating material during surface preparation for overplating can cause a low thickness measurement.
6.8 Etching—Optimum etching will produce a clearly de- fined and narrow dark line at the interface of two metals. A wide or poorly defined line can result in an inaccurate measurement.
6.9 Smearing—Polishing may leave smeared metal that obscures the true boundary between the two metals and results in an inaccurate measurement. This may occur with soft metals like lead, indium, and gold. To help identify whether or not there is smearing, repeat the polishing, etching, and measure- ment several times. Any significant variations in readings indicates possible smearing.
6.10 Poor Contrast—The visual contrast between metals in the SEM is poor when their atomic numbers are close together. For example, bright and semibright nickel layers may not be discriminable unless their common boundary can be brought out 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 (see X1.4.6) may be helpful.

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