1、Designation: B 568 98 (Reapproved 2004)Standard Test Method forMeasurement of Coating Thickness by X-Ray Spectrometry1This standard is issued under the fixed designation B 568; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the ye
2、ar of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the Department of Defense.1. Scope1.1 This test method covers the us
3、e of X-ray spectrometry todetermine thickness of metallic and some nonmetallic coatings.1.2 The maximum measurable thickness for a given coatingis that thickness beyond which the intensity of the character-istic secondary X radiation from the coating or the substrate isno longer sensitive to small c
4、hanges in thickness.1.3 This test method measures the mass of coating per unitarea, which can also be expressed in units of linear thicknessprovided that the density of the coating is known.1.4 Problems of personnel protection against radiation gen-erated in an X-ray tube or emanating from a radiois
5、otopesource are not covered by this test method. For information onthis important aspect, reference should be made to currentdocuments of the National Committee on Radiation Protectionand Measurement, Federal Register, Nuclear Regulatory Com-mission, National Institute of Standards and Technology (f
6、or-merly the National Bureau of Standards), and to state and localcodes if such exist.1.5 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 a
7、nd determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E 135 Terminology Relating to Analytical Chemistry forMetals, Ores, and Related Materials2.2 International Standard:ISO 3497 Metallic CoatingsMeasurement of CoatingThicknessX-ray Spectro
8、metric Methods3. Terminology3.1 Definitions of technical terms used in this test methodmay be found in Terminology E 135.4. Summary of Test Method4.1 ExcitationThe measurement of the thickness of coat-ings by X-ray spectrometric methods is based on the combinedinteraction of the coating and substrat
9、e with incident radiationof sufficient energy to cause the emission of secondary radia-tions characteristic of the elements composing the coating andsubstrate. The exciting radiation may be generated by an X-raytube or by certain radioisotopes.4.1.1 Excitation by an X-Ray TubeSuitable exciting radia
10、-tion will be produced by an X-ray tube if sufficient potential isapplied to the tube. This is on the order of 35 to 50 kV for mostthickness-measurement applications. The chief advantage ofX-ray tube excitation is the high intensity provided.4.1.2 Excitation by a RadioisotopeOf the many availablerad
11、ioisotopes, only a few emit gamma radiations in the energyrange suitable for coating-thickness measurement. Ideally, theexciting radiation is slightly more energetic (shorter in wave-length) than the desired characteristic X rays. The advantagesof radioisotope excitation include more compact instrum
12、enta-tion essentially monochromatic radiation, and very low back-ground intensity. The major disadvantage of radioisotopeexcitation is the much lower intensities available as comparedwith X-ray tube sources. X-ray tubes typically have intensitiesthat are several orders of magnitude greater than radi
13、oisotopesources. Due to the low intensity of radioisotopes, they areunsuitable for measurements on small areas (less than 0.3 mmin diameter). Other disadvantages include the limited numberof suitable radioisotopes, their rather short useful lifetimes, andthe personnel protection problems associated
14、with high-intensity radioactive sources.4.2 DispersionThe secondary radiation resulting from theexposure of an electroplated surface to X radiation usuallycontains many components in addition to those characteristicof the coating metal(s) and the substrate. It is necessary,therefore, to have a means
15、 of separating the desired compo-nents so that their intensities can be measured. This can be1This test method is under the jurisdiction of ASTM Committee B08 on Metallicand Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 onTest Methods.Current edition approved June 1, 200
16、4. Published June 2004. Originallyapproved in 1972. Last previous edition approved in 1998 as B 568 98.2For 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
17、 Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.done either by diffraction (wavelength dispersion) or byelectronic discrimination (energy dispersion).4.2.1 Wavelength DispersionBy means of a
18、single-crystalspectrogoniometer, wavelengths characteristic of either thecoating or the substrate may be selected for measurement.Published data in tabular form are available that relate spec-trogoniometer settings to the characteristic emissions of ele-ments for each of the commonly used analyzing
19、crystals.4.2.2 Energy DispersionX-ray quanta are usually speci-fied in terms of their wavelengths, in angstroms (), or theirequivalent energies in kiloelectron volts (keV). The relation-ship between these units is as follows:keV!512.396where:keV = the quantum energy in thousands of electron volts,an
20、d = the equivalent wavelength in angstroms (10-10m).In a suitable detector (see 4.3.2), X rays of different energieswill produce output pulses of different amplitudes. Aftersuitable amplification, these pulses can be sorted on the basisof their amplitudes and stored in certain designated channels of
21、a multichannel analyzer, each adjacent channel representing anincrement of energy. Typically, a channel may represent a spanof 20 eV for a lithium-drifted silicon detector or 150 to 200 eVfor a proportional counter. From six to sixty adjacent channelscan be used to store the pulses representing a se
22、lectedcharacteristic emission of one element, the number of channelsdepending on the width of the emission peak (usually displayedon the face of a cathode ray tube). The adjacent channels usedto store the pulses from the material under analysis are calledthe “region of interest” or ROI.4.3 Detection
23、:4.3.1 Wavelength Dispersive SystemsThe intensity of awavelength is measured by means of an appropriate radiationdetector in conjunction with electronic pulse-counting cir-cuitry, that is, a scaler. With wavelength dispersive systems, thetypes of detectors commonly used as the gas-filled types andth
24、e scintillation detector coupled to a photomultiplier tube.4.3.2 Energy-Dispersive SystemsFor the highest energyresolution with energy dispersive systems, a solid-state devicesuch as the lithium-drifted silicon detector must be used. Thistype of detector is maintained at a very low temperature in al
25、iquid-nitrogen cryostat (77K). Acceptable energy resolutionfor most thickness measurement requirements can be realizedwith proportional counters, and these detectors are being usedon most of the commercially available thickness gages basedon X-ray spectrometry. In setting up a procedure for coating-
26、thickness measurement using an energy-dispersive system,consideration should be given to the fact that the detector“sees” and must process not only those pulses of interest butalso those emanating from the substrate and from supportingand masking materials in the excitation enclosure. Therefore,cons
27、ideration should be given to restricting the radiation to thearea of interest by masking or collimation at the radiationsource. Similarly, the detector may also be masked so that itwill see only that area of the specimen on which the coatingthickness is to be determined.4.4 Basic PrincipleA relation
28、ship exists between coatingthickness and secondary radiation intensity up to the limitingthickness mentioned in 1.2. Both of the techniques describedbelow are based on the use of primary standards of knowncoating thicknesses which serve to correlate quantitatively theradiation intensity and thicknes
29、s.4.5 Thickness Measurement by X-Ray EmissionIn thistechnique, the spectrogoniometer is positioned to record theintensity of a prominent wavelength characteristic of thecoating metal or, in the case of an energy-dispersive system,the multichannel analyzer is set to accept the range of energiescompri
30、sing the desired characteristic emission. The intensity ofthe coatings X-ray emission (coating ROI) will be at aminimum for a sample of the bare substrate where it willconsist of that portion of the substrate fluorescence which mayoverlap the ROI of the coating and a contribution due tobackground ra
31、diation. This background radiation is due to theportion of the X-ray tubes output which is the same energy asthe coatings X-ray emission. The sample will always scattersome of these X rays into the detector. If the characteristicemission energies of the coating and substrate are sufficientlydifferen
32、t, the only contribution of the substrate will be due tobackground. For a thick sample of the solid coating metal orfor an electroplated specimen having an “infinitely thick”coating, the intensity will have its maximum value for a givenset of conditions. For a sample having a coating of less than“in
33、finite” thickness, the intensity will have an intermediatevalue. The intensity of the emitted secondary X radiationdepends, in general, upon the excitation energy, the atomicnumbers of the coating and substrate, the area of the specimenexposed to the primary radiation, the power of the X-ray tube,an
34、d the thickness of the coating. If all of the other variables arefixed, the intensity of the characteristic secondary radiation isa function of the thickness or mass per unit area of the coating.The exact relationship between the measured intensity and thecoating thickness must be established by the
35、 use of standardshaving the same coating and substrate compositions as thesamples to be measured. The maximum thickness that can bemeasured by this method is somewhat less than what is,effectively, infinite thickness. This limiting thickness depends,in general, upon the energy of the characteristic
36、X-ray and thedensity and absorption properties of the material under analy-sis. The typical relationship between a coating thickness andthe intensity of a characteristic emission from the coating metalis illustrated by the curve in the Appendix, Fig. X1.1.4.6 Thickness Measurements by X-Ray Absorpti
37、onIn thistechnique the spectrometer, in the case of a wavelength-dispersive system, is set to record the intensity of a selectedemission characteristic of the basis metal. In an energy-dispersive system, the multichannel analyzer is set to accumu-late the pulses comprising the same energy peak. The
38、intensitywill be a maximum for a sample of the uncoated basis metaland will decrease with increasing coating thickness. This isbecause both the exciting and secondary characteristic radia-tions undergo attenuation in passing through the coating.Depending upon the atomic number of the coating, when t
39、hecoating thickness is increased to a certain value, the character-istic radiation from the substrate will disappear, although aB 568 98 (2004)2certain amount of scattered radiation will still be detected. Themeasurement of a coating thickness by X-ray absorption is notapplicable if an intermediate
40、coating is present because of theindeterminate absorption effect of intermediate layer. Thetypical relationship between coating thickness and the intensityof a characteristic emission from the substrate is shown in theAppendix, see Fig. X1.2.4.7 Thickness and Composition Measurement by Simulta-neous
41、 X-ray Emission and Absorption (Ratio Method)It ispossible to combine the X-ray absorption and emission tech-niques when coating thicknesses and alloy composition aredetermined from the ratio of the respective intensities ofsubstrate and coating materials. Measurements by this ratiomethod are largel
42、y independent of the distance between testspecimen and detector.4.8 Multilayer MeasurementsMany products have multi-layer coatings in which it is possible to measure each of thecoating layers by using the multiple-energy-region capabilityof the multichannel analyzer of an energy-dispersive system.Th
43、e measuring methods permit the simultaneous measurementof coating systems with up to three layers. Or the simultaneousmeasurement of thickness and compositions of layers with upto three components. Such measurements require unique dataprocessing for each multilayer combination to separate thevarious
44、 characteristic emissions involved, to account for theabsorption by intermediate layers, and to allow for anysecondary excitation which may occur between layers. Typicalexamples of such combinations are gold on nickel on copperand nickel on copper on steel.4.9 Mathematical DeconvolutionWhen using a
45、multi-channel analyzer a mathematical deconvolution of the second-ary radiation spectra can be used to extract the intensities of thecharacteristic radiation. This method can be used when theenergies of the detected characteristic radiations do not differsufficiently (for example, characteristic rad
46、iation from Au andBr). This method sometimes is described as numerical filteringin order to distinguish from the technique of setting fixedRegion of Interest (ROI) channel limits in the multichannelanalyzer.5. Significance and Use5.1 This is a sensitive, noncontact, and nondestructivemethod for meas
47、uring the coating thickness (and in somecases, coating composition) of metallic and some nonmetalliccoatings over a range of thicknesses from as little as 0.01 mto as much as 75 m depending on the coating and substratematerials. It can be used to measure coating and base combi-nations that are not r
48、eadily measured by other techniques.5.2 The coating thickness is an important factor in theperformance of a coating in service.6. Factors Affecting Accuracy6.1 Counting StatisticsThe production of X-ray quanta israndom with respect to time. This means that during a fixedtime interval, the number of
49、quanta emitted will not always bethe same. This gives rise to the statistical error which isinherent in all radiation measurements. In consequence, anestimate of the counting rate based on a short counting interval(for example, 1 or 2 s) may be appreciably different from anestimate based on a longer counting period, particularly if thecounting rate is low. This error is independent of other sourcesof error such as those arising from mistakes on the part of theoperator or from the use of inaccurate standards. To reduce thestatistical error to an acceptable
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