ISO 1463:2021 pdf – Metallic and oxide coatings — Measurement of coating thickness — Microscopical method.
5.10.2 Digital image processing Microscopes with a triocular tube, camera adapters with projecting lens and digital cameras connected to a computer with software for image capturing and processing are nowadays state of the art. Similar to 5.10.1, the measurement will be no more accurate than the adjustment and calibration of the length measurement function (combination of hardware and software). For adjustment, digital images from the stage micrometer (in both directions parallel to the x- and y-axis of the image) are recorded for every combination of objective, if applicable intermediate magnification changer, and resolution setting of the camera (full resolution and typical settings of pixel binning). The length in object space represented by a pixel of the digital image is calculated by measuring a known distance on the stage micrometer with the respective function of the software and is then saved in the software. Usually after such an adjustment, the images are recorded “calibrated”, i.e. with the µm/pixel factor assigned to the image, by selecting the objective, if applicable the intermediate magnification changer, and the pixel setting of the camera in the software at the time of capturing the image. The adjustment and/or calibration are usually stable for long time. Furthermore, they are not operator dependent as long as no changes are applied to the tube, if applicable an intermediate magnification changer, the camera adapter or the camera itself, and as long as the same resolution of the camera (number of pixels in x and y direction) is used for adjustment and/or calibration and for measurement. Normally, it is sufficient to record in regular time intervals images from the stage micrometer and measure known distances. When the deviation between the measured length and the certified length is less than a reasonably defined uncertainty limit for length measurements, which the laboratory wants to achieve, e.g. 1 %, the calibration is still valid and no re-adjustment is necessary.
6 Preparation of cross-sections Prepare, mount, grind, polish and etch the specimen so that a) the cross-section is perpendicular to the coating, b) the surface is flat and the entire width of the coating image is simultaneously in focus at the magnification to be used for the measurement, c) all material deformed by cutting or cross-sectioning is removed, and d) the boundaries of the coating cross-section are sharply defined by no more than contrasting appearance or by a narrow, well-defined line. NOTE Further guidance is given in Clause 5 and in Annex A. Some typical etchants are described in Annex C. 7 Measurement 7.1 Give appropriate attention to the factors listed in Clause 5 and Annex A. 7.2 Calibrate the microscope and its measuring device with a certified or calibrated stage micrometer. 7.3 Measure the width of the image of the coating cross-section on at least five points distributed along a length of the cross-section. NOTE Guidance on the measurement of taper of cross-section and of tooth-constructed coatings is given in Annex B. 8 Measurement uncertainty The microscope and associated equipment, its use, its calibration and the method of preparation of the cross-section shall be chosen so as to allow the coating thickness to be determined to within 1 µm or 10 %, whichever is the greater, of the actual coating thickness. The method is capable of giving an absolute measurement uncertainty of 0,8 µm, and, for thicknesses greater than 25 µm, a reasonable measurement uncertainty of 5 % or better (see also B.3). However, with careful preparation of the specimen and the application of suitable instruments, this method is capable of providing a measurement uncertainty of 0,4 µm under reproducible conditions.