ASTM B568-98 (R2021) pdf free download.Standard Test Method for Measurement of Coating Thickness by X-Ray Spectrometry.
1.1 This test method covers the use of X-ray spectrometry to determine thickness of metallic and some nonmetallic coatings.
1.2 The maximum measurable thickness for a given coating is that thickness beyond which the intensity of the characteristic secondary X radiation from the coating or the substrate is no longer sensitive to small changes in thickness.
1.3 This test method measures the mass of coating per unit area. which can also he expressed in units of linear thickness provided that the density of the coating is known.
1.4 Problems of personnel protection against radiation generated in an X-ray tube or emanating from a radioisotope source are not covered by this test method. For intbrmation on this important aspect, reference should be made to current documents of the National Committee on Radiation Protection and Measurement. Federal Register, Nuclear Regulatory Commission, National Institute of Standards and Technology (formerly the National Bureau of Standards), and to state and local codes if such exist.
3. Terminology
3.1 Definitions of technical terms used in this test method may he found in Terminology E135.
4. Summary of Test Method
4.1 Excitation—The measurement of the thickness of coatings by X-ray spectrometric methods is based on the combined interaction of the coating and substrate with incident radiation of sufficient energy to cause the emission of secondary radiations characteristic of the elements composing the coating and substrate. The exciting radiation may be generated by an X-ray tube or by certain radioisotopes.
4. 1.1 Excitation by an X-Ray Tube—Suitable exciting radiation will he produced by an X-ray tube if sufficient potential is applied to the tube. This is on the order of 35 to 50 kV for most thickness-measurement applications. The chief advantage of X-ray tube excitation is the high intensity provided.
4. 1 .2 Excitation by a Radioisotope —Of the many available radioisotopes. only a few emit gamma radiations in the energy range suitable for coating-thickness measurement. ideally, the exciting radiation is slightly more energetic (shorter in wavelength) than the desired characteristic X rays. The advantages of radioisotope excitation include more compact instrumenta(ion essentially monochromatic radiation, and very low back— ground intensity. The major disadvantage of radioisotope excitation is the niuch lower intensities available as compared with X-ray tube sources. X-ray tubes typically have intensities that are several orders of magnitude greater than radioisotope sources. Due to the low intensity of radiolsotopes. they are unsuitable for measurements on small areas (less than 0.3 mm in diameter). Other disadvantages include the limited number of suitable radioisotopes. their rather short useful lifetimes, and the personnel protection problems associated with high- intensity radioactive sources.ASTM B568 pdf download.