ICS 25.220.20; 25.220.40
CCS A 29
National Standard of the People’s Republic of China
GB/T 6462-2025
Replaces GB/T 6462-2005
Metallic and oxide coatings -Measurement of coating thinckness - Microscopical method
金属和氧化物覆盖层 厚度测量 显微镜法
Issue date: 2025-10-31 Implementation date: 2026-05-01
Issued by the State Administration for Market Regulation of the People's Republic of China
the Standardization Administration of the People's Republic of China
Metallic and oxide coatings - Measurement of coating thickness - Microscopical method
WARNING: The use of this document may involve the use of hazardous materials, operations and equipment. This document does not purport to address all of the safety problems associated with its use. It is the responsibility of users of this document to take appropriate measures to ensure the safety and health of personnel prior to the application of the document.
1 Scope
This document specifies a method for the measurement of the local thickness of metallic coatings, oxide layers, and porcelain or vitreous enamel coatings, by the microscopical examination of cross-sections using an optical microscope.
This document applies to the measurement of metallic and oxide coatings.
2 Normative references
No normative reference is listed in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
local thickness
mean of the thickness measurements, of which a specified number is made within a reference area
[Source: GB/T 12334-2021, 3.4]
4 Principle
A portion of the test specimen is cut out and mounted. The mounted cross-section is prepared by suitable techniques of grinding, polishing and etching. The thickness of the coating cross-section is measured by means of a calibrated scale.
Note: These techniques will be familiar to experienced metallographers, but some guidance is given in Clause 5 and in Annex A for less experienced operators.
5 Factors relating to measurement uncertainty
5.1 Surface roughness
If the coating or its substrate has a rough surface, one or both of the interfaces bounding the coating cross-section could be too irregular to permit accurate measurement (see A.6).
5.2 Taper of cross-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, e.g. an inclination of 10° to the perpendicular will contribute a 1.5 % uncertainty.
Note: B.1 provides guidance on the taper of a cross-section.
5.3 Deformation of coating
Detrimental deformation of the coating can be caused by excessive temperature or pressure during mounting and preparation of cross-sections of soft coatings or coatings that melt at a low temperature, and also by excessive abrasion of brittle materials during preparation of cross-sections.
5.4 Rounding of edge of coating
If the edge of the coating cross-section is rounded, i.e. if the coating cross-section is not completely flat up to its edges, the true thickness cannot be observed microscopically. Edge rounding can be caused by improper mounting, grinding, polishing or etching. It is usually minimized by overplating the test specimen before mounting (see A.2).
5.5 Overplating
Overplating of the test specimen protects the coating edges during preparation of cross-sections and thus prevents erroneous measurement. Removal of coating material during surface preparation for overplating can result in a low thickness measurement.
5.6 Etching
Optimum etching produces a clearly defined and narrow dark line at the interface of two metals. Excessive etching produces a poorly defined or wide line that can result in erroneous measurement.
5.7 Smearing
Improper polishing or overplating with a softer metal can cause smearing of one metal over the other metal, obscuring the boundary between the coating and the substrate. This problem can be alleviated by repeating the preparation of the cross-section of the coated metal until repeatability of the thickness measurement (see A.3 and A.5) is obtained or also by overplating with a harder metal.
5.8 Magnification
For any given coating thickness, measurement uncertainty generally increases with decreasing magnification. The magnification shall be chosen so that the field of view is between 1,5 × and 3 × the coating thickness.
5.9 Calibration of stage micrometer
Any uncertainty in calibration of the stage micrometer will be reflected in the measurement of the specimen. Therefore, a suitable, traceable length standard shall be used during calibration.
5.10 Calibration of the microscope’s length measuring device
5.10.1 Micrometer eyepiece
The measurement will be no more accurate than the calibration of the eyepiece. The eyepiece, after calibration, will be more accurate. As calibration is operator dependent, the eyepiece shall be calibrated by the person making the measurement.
Repeated calibrations of the micrometer eyepiece can be reasonably expected to have a spread of less than 1 %. The distance between the two lines of a stage micrometer used for the calibration shall be known to within 0.2 µm or 0.1 %, whichever is the greater.
Some image splitting micrometer eyepieces have a nonlinearity that introduces an uncertainty of up to 1 % for short measurement distances.
Uncertainties can be introduced by backlash in the movement of the micrometer eyepiece. To avoid this uncertainty, ensure that the final motion during alignment of the hairline is always made in the same direction.
5.10.2 Digital image processing
Microscopes in the professional standard are equipped with a triocular tube, camera adapters with projecting lens and digital cameras connected to a computer with software for image capturing and processing. 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.
5.10.3 Calibration frequency
The calibration can be on a daily, weekly, monthly or quarterly based according to the actual situation.
5.11 Uniformity of magnification
Uncertainties can occur if the magnification is not uniform over the entire field of view. Thus, ensure that both the calibration and the measurement are made over the same portion of the field of view with the measured boundaries centred about the optical axis.
5.12 Lens quality
As lack of sharpness of the image contributes to the uncertainty of the measurement, ensure that good quality lenses are used.
Note: Sometimes, image sharpness can be improved by using monochromatic light.
5.13 Orientation of measuring lines
Ensure that the movement of the hairline of the eyepiece for alignment or the measuring line of a digital image processing software is perpendicular to the boundaries of the coating cross-section, e.g. 10° misalignment will contribute a 1.5 % uncertainty.
5.14 Tube length
A change in tube length causes a change in magnification and, if this change occurs between the time of calibration and the time of measurement, the measurement will be in uncertainty. Take care to avoid a change in tube length, which can occur when the eyepiece is repositioned within the tube, when the focus of the eyepiece tube is changed, when the camera adapter is repositioned or changed and, for some microscopes, when the fine focus is adjusted.
