1、Designation: B 567 98 (Reapproved 2003)Standard Test Method forMeasurement of Coating Thickness by the Beta BackscatterMethod1This standard is issued under the fixed designation B 567; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision
2、, the year 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 cover
3、s the beta backscatter gages forthe nondestructive measurement of metallic and nonmetalliccoatings on both metallic and nonmetallic substrate materials.1.2 The test method measures the mass of coating per unitarea, which can also be expressed in linear thickness unitsprovided that the density of the
4、 coating is known.1.3 The test method is applicable only if the atomic numbersor equivalent atomic numbers of the coating and substratediffer by an appropriate amount (see 7.2).1.4 Beta backscatter instruments employ a number of dif-ferent radioactive isotopes. Although the activities of theseisotop
5、es are normally very low, they can present a hazard ifhandled incorrectly. This standard does not purport to addressthe safety issues and the proper handling of radioactivematerials. It is the responsibility of the user to comply withapplicable State and Federal regulations concerning the han-dling
6、and use of radioactive material. Some States requirelicensing and registration of the radioactive isotopes.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 a
7、nd health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 International standard:ISO 3543: Metallic and Nonmetallic CoatingsMeasurement of ThicknessBeta Backscatter Method3. Terminology3.1 Descriptions of Terms:3.1.1 activitythe nuclei of
8、all radioisotopes are unstableand tend to change into a stable condition by spontaneouslyemitting energy or particles, or both. This process is known asradioactive decay. The total number of disintegrations during asuitably small interval of time divided by that interval of timeis called “activity.”
9、 Therefore, in beta backscatter measure-ments, a higher activity corresponds to a greater emission ofbeta particles. The activity of a radioactive element used in betabackscatter gages is generally expressed in microcuries (1Ci = 3.7 3 104disintegrations per second).3.1.2 aperturethe opening of the
10、mask abutting the testspecimen. It determines the size of the area on which thecoating thickness is measured. This mask is also referred to asa platen, an aperture plate, a specimen support, or a specimenmask.3.1.3 backscatterwhen beta particles pass through matter,they collide with atoms. Among oth
11、er things, this interactionwill change their direction and reduce their speed. If thedeflections are such that the beta particle leaves the body ofmatter from the same surface at which it entered, the betaparticle is said to be backscattered.3.1.4 backscatter coeffcientthe backscatter coefficient of
12、a body, R, is the ratio of the number of beta particlesbackscattered to that entering the body. R is independent of theactivity of the isotope and of the measuring time.3.1.5 backscatter count:3.1.5.1 absolute backscatter countthe absolute backscat-ter count, X, is the number of beta particles that
13、are backscat-tered during a finite interval of time and displayed by theinstrument. X will, therefore, depend on the activity of thesource, the measuring time, the geometric configuration of themeasuring system, and the properties of the detector, as well asthe coating thickness and the atomic numbe
14、rs of the coatingand substrate materials. X0is the count produced by theuncoated substrate, and Xs, that of the coating material. Toobtain these values, it is necessary that both these materials areavailable with a thickness greater than the saturation thickness(see 3.1.12).3.1.5.2 normalized backsc
15、atterthe normalized backscat-ter, xn, is a quantity that is independent of the activity of thesource, the measuring time, and the properties of the detector.The normalized backscatter is defined by the equation:xn5X 2 X0Xs2 X01This test method is under the jurisdiction of ASTM Committee B08 on Metal
16、licand Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 onTest Methods.Current edition approved Oct. 1, 2003. Published October 2003. Originallyapproved in 1972. Last previous edition approved in 1998 as B 567 98.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box
17、C700, West Conshohocken, PA 19428-2959, United States.where:X0= count from the substrate,Xs= count from the coating material, andX = count from the coated specimen, and each count is forthe same interval of time.Because X is always $X0and # Xs, xncan only take valuesbetween 0 and 1. (For reasons of
18、simplicity, it is oftenadvantageous to express the normalized count as a percentageby multiplying xnby 100.)3.1.5.3 normalized backscatter curvethe curve obtainedby plotting the coating thickness as a function of xn.3.1.6 beta particlesbeta particles or beta rays are high-speed electrons that are em
19、itted from the nuclei of materialsundergoing a nuclear transformation. These materials arecalled beta-emitting isotopes, beta-emitting sources, or betaemitters.3.1.7 coating thicknessin this test method, coating thick-ness refers to mass per unit area as well as geometricalthickness.3.1.8 dead time
20、or resolving timeGeiger-Mller tubesused for counting beta particles have characteristic recoverytimes that depend on their construction and the count rate.After reading a pulse, the counter is unresponsive to successivepulses until a time interval equal to or greater than its dead timehas elapsed.3.
21、1.9 energyit is possible to classify beta emitters by themaximum energy of the particles that they release during theirdisintegration. This energy is generally given in mega-electronvolts, MeV.3.1.10 equivalent (or apparent) atomic number theequivalent atomic number of an alloy or compound is theato
22、mic number of an element that has the same backscattercoefficient as the material.3.1.11 half-life, radioactivefor a single radioactive decayprocess, the time required for the activity to decrease by half.3.1.12 saturation thicknessthe minimum thickness of amaterial that produces a backscatter that
23、is not changed whenthe thickness is increased. (See also Appendix X1.)3.1.13 sealed source or isotopea radioactive sourcesealed in a container or having a bonded cover, the container orcover being strong enough to prevent contact with and disper-sion of the radioactive material under the conditions
24、of use andwear for which it was designed.3.1.14 source geometrythe spatial arrangement of thesource, the aperture, and the detector with respect to each other.4. Summary of Test Method4.1 When beta particles impinge upon a material, a certainportion of them is backscattered. This backscatter is esse
25、ntiallya function of the atomic number of the material.4.2 If the body has a surface coating and if the atomicnumbers of the substrate and of the coating material aresufficiently different, the intensity of the backscatter will bebetween two limits: the backscatter intensity of the substrateand that
26、 of the coating. Thus, with proper instrumentation andif suitably displayed, the intensity of the backscatter can beused for the measurement of mass per unit area of the coating,which, if the density remains the same, is directly proportionalto the thickness.4.3 The curve expressing coating thicknes
27、s (mass per unitarea) versus beta backscatter intensity is continuous and can besubdivided into three distinct regions, as shown in Fig. 1. Thenormalized count rate, xn, is plotted on the X-axis, and thelogarithm of the coating thickness, on the Y-axis. In the range0 # xn# 0.35, the relationship is
28、essentially linear. In therange 0.35 #xn# 0.85, the curve is nearly logarithmic; thismeans that, when drawn on semilogarithmic graph paper, as inFig. 1, the curve approximates a straight line. In the range0.85 # xn#1, the relationship is nearly hyperbolic.4.4 Radiation other than the beta rays are e
29、mitted orbackscattered by the coating or substrate, and may be includedin the backscatter measurements. Whenever the term backscat-ter is used in this method, it is to be assumed that reference ismade to the total radiation measured.FIG. 1 Normalized BackscatterB 567 98 (2003)25. Significance and Us
30、e5.1 The thickness or mass per unit area of a coating is oftencritical to its performance.5.2 For some coating-substrate combinations, the beta back-scatter method is a reliable method for measuring the coatingnondestructively.5.3 The test method is suitable for thickness specificationacceptance if
31、the mass per unit area is specified. It is notsuitable for specification acceptance if the coating thickness isspecified and the density of the coating material can vary or isnot known.6. Instrumentation6.1 In general, a beta backscatter instrument will comprise:(1) a radiation source (isotope) emit
32、ting primarily beta particleshaving energies appropriate to the coating thickness to bemeasured (see Appendix X2), (2) a probe or measuring systemwith a range of apertures that limit the beta particles to the areaof the test specimen on which the coating thickness is to bemeasured, and containing a
33、detector capable of counting thenumber of backscattered particles (for example, a Geiger-Mller counter (or tube), and (3) a readout instrument wherethe intensity of the backscatter is displayed. The display, in theform of a meter reading or a digital readout can be: (a)proportional to the count, ( b
34、) the normalized count, or (c) thecoating thickness expressed either in thickness or mass per unitarea units.7. Factors Affecting the Measuring Accuracy7.1 Counting Statistics:7.1.1 Radioactive disintegration takes place randomly.Thus, during a fixed time interval, the number of beta particlesbacksc
35、attered will not always be the same. This gives rise tostatistical errors inherent to radiation counting. In consequence,an estimate of the counting rate based on a short countinginterval (for example, 5 s) may be appreciably different froman estimate based on a longer counting interval, particularl
36、y ifthe counting rate is low. To reduce the statistical error to anacceptable level, it is necessary to use a counting interval longenough to accumulate a sufficient number of counts.7.1.2 At large total counts, the standard deviation (s) willclosely approximate the square root of the total count, t
37、hat iss5=X; in 95 % of all cases, the true count will be withinX 6 2s. To judge the significance of the precision, it is oftenhelpful to express the standard deviation as a percentage of thecount, that is, 100=X/X, or 100/=X. Thus, a count of100 000 will give a value ten times more precise than that
38、obtained with a count of 1000. Whenever possible, a countinginterval should be chosen that will provide a total count of atleast 10 000, which corresponds to a statistical error of 1 % forthe count rate. It should be noted, however, that a 1 % error inthe count rate can correspond to a much larger p
39、ercentage errorin the thickness measurement, the relative error depending onthe atomic number spread or ratio between coating andsubstrate materials.7.1.3 Direct-reading instruments are also subject to thesestatistical random errors. However, if these instruments do notpermit the display of the actu
40、al counting rate or the standarddeviation, the only way to determine the measuring precision isto make a large number of measurements at the same coatedlocation on the same coated specimen, and calculate thestandard deviation by conventional means.NOTE 1The accuracy of a thickness measurement by bet
41、a backscatteris generally poorer than the precision described in 6.1, inasmuch as it alsodepends on other factors that are described below. Methods to determinethe random errors of thickness measurements before an actual measure-ment are available from some manufacturers.7.2 Coating and Substrate Ma
42、terialsBecause the back-scatter intensity depends on the atomic numbers of the sub-strate and the coating, the repeatability of the measurement willdepend to a large degree on the difference between theseatomic numbers; thus, with the same measuring parameters, thegreater this difference, the more p
43、recise the measurement willbe. As a rule of thumb, for most applications, the difference inatomic numbers should be at least 5. For materials with atomicnumbers below 20, the difference may be reduced to 25 % ofthe higher atomic number; for materials with atomic numbersabove 50, the difference shoul
44、d be at least 10 % of the higheratomic number. Most plastics and related organic materials (forexample, photoresists) may be assumed to have an equivalentatomic number close to 6. (Appendix X3 gives atomic numbersof commonly used coating and substrate materials.)7.3 Aperture:7.3.1 Despite the collim
45、ated nature of the sources used incommercial backscatter instruments, the backscatter recordedby the detector is, nearly always, the sum of the backscatterproduced by the test specimen exposed through the apertureand that of the aperture plate(n). It is, therefore, desirable to usea material with a
46、low atomic number for the construction of theplaten and to select the largest aperture possible. Measuringerrors will be increased if the edges of the aperture opening areworn or damaged, or if the test specimen does not properlycontact these edges.7.3.2 Because the measuring area on the test specim
47、en hasto be constant to prevent the introduction of another variable,namely the geometrical dimensions of the test specimen, it isessential that the aperture be smaller than the coated area of thesurface on which the measurement is made.7.4 Coating Thickness:7.4.1 In the logarithmic range, the relat
48、ive measuring erroris nearly constant and has its smallest value.7.4.2 In the linear range, the absolute measuring error,expressed in mass per unit area or thickness, is nearly constant,which means that as the coating thickness decreases, therelative measuring error increases. At or near xn= 0.35, t
49、herelative errors of the linear and logarithmic ranges are about thesame. Thus, the relative error at this point may, for mostpractical purposes, be used to calculate the absolute error overthe linear range.7.4.3 In the hyperbolic range, the measuring error is alwayslarge because a small variation in the intensity of the betabackscatter will produce a large variation in the measuredcoating thickness.7.4.4 For instruments that indicate only backscatter countrate and not thickness directly, the count rate is normallyconverted to a thickness by means of an appropr
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