1、Designation: E 2338 06Standard Practice forCharacterization of Coatings Using Conformable Eddy-Current Sensors without Coating Reference Standards1This standard is issued under the fixed designation E 2338; the number immediately following the designation indicates the year oforiginal adoption or, i
2、n the case of revision, 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.1. Scope1.1 This practice covers the use of conformable eddy-current sensors for nondestructi
3、ve characterization of coatingswithout standardization on coated reference parts. It includesthe following: (1) thickness measurement of a conductivecoating on a conductive substrate, (2) detection and character-ization of local regions of increased porosity of a conductivecoating, and (3) measureme
4、nt of thickness for nonconductivecoatings on a conductive substrate or on a conductive coating.This practice includes only nonmagnetic coatings on eithermagnetic ( fi 0) or nonmagnetic ( = 0) substrates. Thispractice can also be used to measure the effective thickness ofa process-affected zone (for
5、example, shot peened layer foraluminum alloys, alpha case for titanium alloys). For specifictypes of coated parts, the user may need a more specificprocedure tailored to a specific application.1.2 Specific uses of conventional eddy-current sensors arecovered by Practice D 7091 and the following test
6、 methodsissued by ASTM: Test Methods B 244, E 376, E 1004, andG12.1.3 The values stated in SI units are to be regarded asstandard. The inch-pound units are provided for information.1.4 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresp
7、onsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2B 244 Test Method for Measurement of Thickness of An-odic Coatings on Aluminum and of Other
8、 NonconductiveCoatings on Nonmagnetic Basis Metals with Eddy-CurrentInstrumentsD 7091 Practice for Nondestructive Measurement of DryFilm Thickness of Nonmagnetic Coatings Applied toFerrous Metals and Nonmagnetic, Nonconductive Coat-ings Applied to Non-Ferrous MetalsE 376 Practice for Measuring Coati
9、ng Thickness byMagnetic-Field or Eddy-Current (Electromagnetic) Exami-nation MethodsE 543 Specification for Agencies Performing Nondestruc-tive TestingE 1004 Practice for Determining Electrical ConductivityUsing the Electromagnetic (Eddy-Current) MethodE 1316 Terminology for Nondestructive Examinati
10、onsG12 Test Method for Nondestructive Measurement of FilmThickness of Pipeline Coatings on Steel2.2 ASNT Documents:3SNT-TC-1A Recommended Practice for Personnel Qualifi-cation and Certification In Nondestructive TestingANSI/ASNT-CP-189 Standard for Qualification and Certi-fication of NDT Personnel2.
11、3 AIA Standard:NAS 410 Certification and Qualification of NondestructiveTesting Personnel4NOTE 1See Appendix X1.3. Terminology3.1 DefinitionsDefinitions of terms relating to electromag-netic examination are given in Terminology E 1316. Thefollowing definitions are specific to the conformable sensors
12、:3.1.1 conformablerefers to an ability of sensors or sensorarrays to conform to nonplanar surfaces without any significanteffects on the measurement results.3.1.2 lift-offnormal distance from the conformable sensorwinding plane to the top of the first conducting layer of the partunder examination.3.
13、1.3 model for sensor responsea relation between theresponse of the sensor (for example, transimpedance magni-tude and phase or real and imaginary parts) to properties of1This practice is under the jurisdiction of ASTM Committee E07 on Nonde-structive Testing and is the direct responsibility of Subco
14、mmittee E07.07 onElectromagnetic Method.Current edition approved Dec. 1, 2006. Published January 2007. Originallyapproved in 2004. Last previous edition approved in 2004 as E 2338 - 04.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceast
15、m.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3Available fromAmerican Society for Nondestructive Testing (ASNT), P.O. Box28518, 1711 Arlingate Ln., Columbus, OH 43228-0518, http:/www.asnt.org.4Available from Aerospace Indu
16、stries Association of America, Inc. (AIA), 1000Wilson Blvd., Suite 1700,Arlington, VA22209-3928, http:/www.aia-aerospace.org.(Replacement standard for MIL-STD-410.)1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.interest, for example
17、, electrical conductivity, magnetic perme-ability, lift-off, and conductive coating thickness, etc. Thesemodel responses may be obtained from database tables andmay be analysis-based or empirical.3.1.4 depth of sensitivitydepth to which sensor responseto features or properties of interest, for examp
18、le, coatingthickness variations, exceeds a noise threshold.3.1.5 spatial half-wavelengthspacing between the centerof adjacent primary (drive) winding segments with current flowin opposite directions; this spacing affects the depth of sensi-tivity. Spatial wavelength equals two times this spacing. As
19、ingle turn conformable circular coil has an approximatespatial wavelength of twice the coil diameter.3.1.6 insulating shimsconformable insulating foils usedto measure effects of small lift-off excursions on sensorresponse.3.1.7 air standardizationan adjustment of the instrumentwith the sensor in air
20、, that is, at least one spatial wavelengthaway from any conductive or magnetic objects, to match themodel for the sensor response. Measurements on conductivematerials after air standardization should provide absoluteelectrical properties and lift-off values. The performance can beverified on certifi
21、ed reference standards over the frequencyrange of interest.3.1.8 reference substrate standardizationan adjustment ofthe instrument to an appropriate reference substrate standard.The adjustment is to remove offsets between the model for thesensor response and at least two reference substrate measure-
22、ments (for example, two measurements with different lift-offsat the same position on the standard). These standards shouldhave a known electrical conductivity that is essentially uniformwith depth and should have essentially the same electricalconductivity and magnetic permeability as the substrate
23、in thecomponents being characterized.3.1.9 performance verification, uncoated parta measure-ment of electrical conductivity performed on a reference partwith known properties to confirm that the electrical conduc-tivity variation with frequency is within specified tolerances forthe application. When
24、 a reference standardization is performed,reference parts used for standardization should not be used forperformance verification. These variations should be docu-mented in the report (see Section 9). Performance verificationis a quality control procedure recommended prior to or duringmeasurements a
25、fter standardization.3.1.10 performance verification, coated parta measure-ment of coating electrical conductivity and/or thickness on acoated reference part with known properties to confirm that thecoating electrical conductivity and/or thickness are withinspecified tolerances for the application.
26、Performance verifica-tion is a quality control procedure that does not representstandardization and should be documented in the report (seeSection 9).3.1.11 process-affected zonea region near the surface withdepth less than the half wavelength that can be represented bya conductivity that is differe
27、nt than that of the base material,that is, substrate.3.1.12 sensor footprintarea of the sensor face placedagainst the material under examination.4. Significance and Use4.1 Conformable Eddy-Current SensorsConformable,eddy-current sensors can be used on both flat and curvedsurfaces, including fillets,
28、 cylindrical surfaces, etc. When usedwith models for predicting the sensor response and appropriatealgorithms, these sensors can measure variations in physicalproperties, such as electrical conductivity and/or magneticpermeability, as well as thickness of conductive coatings onany substrate and nonc
29、onductive coatings on conductive sub-strates or on a conducting coating. These property variationscan be used to detect and characterize heterogeneous regionswithin the conductive coatings, for example, regions of locallyhigher porosity.4.2 Sensors and Sensor ArraysDepending on the applica-tion, eit
30、her a single-sensing element sensor or a sensor arraycan be used for coating characterization. A sensor array wouldprovide a better capability to map spatial variations in coatingthickness and/or conductivity (reflecting, for example, porosityvariations) and provide better throughput for scanning la
31、rgeareas. The size of the sensor footprint and the size and numberof sensing elements within an array depend on the applicationrequirements and constraints, and the nonconductive (forexample, ceramic) coating thickness.4.3 Coating Thickness RangeThe conductive coatingthickness range over which a sen
32、sor performs best depends onthe difference between the electrical conductivity of the sub-strate and conductive coating and available frequency range.For example, a specific sensor geometry with a specificfrequency range for impedance measurements may provideacceptable performance for an MCrAlY coat
33、ing over a nickel-alloy substrate for a relatively wide range of conductivecoating thickness, for example, from 75 to 400 m 0.003 to0.016 in. Yet, for another conductive coating-substrate com-bination, this range may be 10 to 100 m 0.0004 to 0.004 in.The coating characterization performance may also
34、 depend onthe thickness of a nonconductive topcoat. For any coatingsystem, performance verification on representative coatedspecimens is critical to establishing the range of optimumperformance. For nonconductive, for example, ceramic, coat-ings the thickness measurement range increases with anincre
35、ase of the spatial wavelength of the sensor (for example,thicker coatings can be measured with larger sensor windingspatial wavelength). For nonconductive coatings, when rough-ness of the coating may have a significant effect on thethickness measurement, independent measurements of thenonconductive
36、coating roughness, for example, by profilom-etry may provide a correction for the roughness effects.4.4 Process-Affected ZoneFor some processes, for ex-ample, shot peening, the process-affected zone can be repre-sented by an effective layer thickness and conductivity. Thesevalues can in turn be used
37、 to assess process quality. A strongcorrelation must be demonstrated between these “effectivecoating” properties and process quality.4.5 Three-Unknown AlgorithmUse of multi-frequencyimpedance measurements and a three-unknown algorithmpermits independent determination of three unknowns: (1)thickness
38、of conductive nonmagnetic coatings, (2) conductivityE2338062of conductive nonmagnetic coatings, and (3) lift-off thatprovides a measure of the nonconductive coating thickness.5. Interferences5.1 Thickness of CoatingThe precision of a measurementcan change with coating thickness. The thickness of a c
39、oatingshould be less than the maximum depth of sensitivity. Ideally,the depth of sensitivity at the highest frequency should be lessthan the conductive coating thickness, while the depth ofsensitivity at the lowest frequency should be significantlygreater than the conductive coating thickness. The n
40、umber offrequencies used in the selected frequency range should besufficient to provide a reliable representation of the frequency-response shape.5.2 Thickness of SubstrateThe thickness of the substrateshould be larger than the depth of sensitivity at the lowestfrequency. Otherwise, this thickness m
41、ust be known andaccounted for in the model for the sensor response.5.3 Magnetic Permeability and Electrical Conductivity ofBase Metal (Substrate)The magnetic permeability and elec-trical conductivity of the substrate can affect the measurementand must be known prior to coating characterization unles
42、sthey can be determined independently on a coated part. Whenthe substrate properties vary spatially, this variation must bedetermined as part of the coating characterization on a non-coated part that preferably has the same thermal history as thecoated parts. Original uncoated parts may have signifi
43、cantlydifferent microstructure than heat treated coated substrates.Uncoated colder regions of otherwise coated parts may havedifferent properties than the coated substrate due to changesduring coating and heat treatment, and, thus, may or may notbe reasonably representative of the substrate under th
44、e coating.In the case these variations are consistent from component tocomponent, a reference standard essentially equivalent to theactual substrate must be used. Differences between the actualsubstrate values at any coating measurement location and thevalues assumed for property estimation, for exa
45、mple, in thesensor response model, may produce errors in coating propertyestimates.5.4 Electrical Conductivity of CoatingThe precision of ameasurement can change with the electrical conductivity of thecoating. The electrical conductivity of the coating should besubstantially different from the condu
46、ctivity of the substrate.For a nonmagnetic coating on a nonmagnetic substrate, if theelectrical conductivities are essentially the same, reliablecoating thickness measurements cannot be obtained since thecoating and substrate are electromagnetically indistinguish-able. The electrical conductivity of
47、 the coating should also belarge enough for sufficient eddy currents to be induced to affectthe sensor response.5.5 Edge EffectExamination methods may be sensitive toabrupt surface changes of specimens or parts. Therefore,measurements made too near an edge (see 8.5.1) or insidecorner may not be vali
48、d or may be insufficiently accurateunless the instrument is used with a procedure that specificallyaddresses such a measurement. Edge-effect correction proce-dures must either account for edge effects in the propertyestimation algorithm (for example, in the sensor responsemodel) or incorporate caref
49、ul standardization on referenceparts with fixtures to control sensor position relative to theedge.5.6 Curvature of Examination SurfaceFor surfaces with asingle radius of curvature (for example, cylindrical or conical),the radius of curvature should be large compared to the sensorhalf-wavelength. In the case of a double curvature, at least oneof the radii should significantly exceed the sensor footprint andthe other radius should be at least comparable to the sensorfootprint, unless customized sensors are designed to match thedouble curvature. Perfo
