NDI OF INTERFACES IN COATING SYSTEMS USING DIGITAL INTERFEROMETRY

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Letitia Curtis
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1 NDI OF INTERFACES IN COATING SYSTEMS USING DIGITAL INTERFEROMETRY J.F. Silv Gomes (*), J.M. Monteiro nd M.A.P. Vz Experimentl Mechnics nd New Mterils Reserch Unit INEGI/FEUP-University of Porto Ru dos Brgs, Porto Codex, Portugl Tel nd Fx Abstrct This pper describes new ppliction of two lser interferometry techniques into the non-destructive inspection of coted surfces. The purpose is to detect interfcil disbond between the coting nd the substrte. Debonding is detected by properly exciting the surfce of the object under inspection in such wy tht the interference fringe pttern is modified rendering the disbond redily visible. The fringe ptterns resulting from the ssocited imges were cptured using Electronic Speckle Pttern Interferometry (ESPI) nd Sherogrphy. Imge processing techniques re pplied to enhnce the detection nd better definition of the debonded lyer. Some results nd discussions re presented to illustrte the pplicbility of these two opticl techniques to therml brrier cotings. Keywords: Interfces, Cotings, Hologrphy, Sherogrphy, ESPI, Non-destructive, Debonding. 1. Introduction Modern technologies often require the use of new mterils for specil pplictions, which rely upon high performnce components possessing not only high stiffness/weight rtio but lso high therml resistnce nd high surfce hrdness. The bove combintion of such properties is not esily ttinble in polycrystlline mterils. However, surfce cotings cn help to meet such requirements. Mterils composed of substrte providing the optiml mechnicl properties nd cotings with superior frictionl, therml nd electricl chrcteristics re incresingly used in the design of new structures. Different techniques cn be used to pply the coting to the bulk mteril. These include chemicl nd physicl vpour deposition, rective evportion, sputtering, diffusion cotings, plsm nd flme sprying nd sol-gel techniques, s described by Bunshh (1987). Most of these techniques cn led to coting deposition defects. In (*) corresponding uthor sg@fe.up.pt -1-

2 criticl coting pplictions, it is importnt to hve ccess to non-destructive inspection (NDI) techniques to evlute the structurl integrity of the reinforced surfce of component. Lser interferometry by Hologrphy, Electronic Speckle Pttern Interferometry (ESPI) nd Sherogrphy re different opticl techniques tht hve been developed during the pst four decdes, nd they proved to be very useful nd powerful tools in the non-destructive evlution of structures nd mterils (Ettemeyer (2000), Sirohi (1999)). In view of their non-destructive nd sensitive nture, the bove lser interferometry techniques hve unquestionble dvntges when compred with other clssicl methods for the inspection of interfcil debonding, see, e. g., Lee (1991). Of prticulr interest to the current study is ESPI nd Sherogrphy. They re field techniques, which mens tht the informtion concerning the behviour of n re over the surfce is obtined t once, llowing n esy detection of loclised disturbnces in the displcement field due to the presence of defects. The differences between the mesurements obtined using these two techniques re shown schemticlly in Fig. 1. When using ESPI, the bsolute vlue of the displcement is mesured nd fringes due to rigid body motion overly the deformtion fringe pttern. In Sherogrphy, the system is only sensitive to displcement grdients. In this cse, only the deformtion due to the presence of defect (two lobe fringes) is observed. Leendertz nd Butters (1973) first put in evidence the potentil of the sherogrphic set-up, bsed on Michelson interferometer, for the mesurement of surfce displcements in Sherogrphy nd ESPI (Lφkberg, 1987; Chousl 1994; Scle 1998; Gong 1999) seem to be well suited for defect detection of different coting systems. Both techniques llow the electronic recording of 25 hologrms/s (Europen video stndrd) nd cn be used with imge processing routines to postprocess the cptured dt, Ms (1997). The present pper describes the ppliction of ESPI nd Sherogrphy to detect interfcil debonding in coting systems using newly developed interferometric rrngement. Debonding is detected by properly exciting the surfce of the coted specimen in such wy tht the interference fringe pttern is modified rendering the disbond re redily visible. Specilly designed nd implemented imge processing techniques were pplied to enhnce the detection nd improve the definition of the debonded lyer. Prticulr excittion, which leds to better detection of interfcil debonding, is lso exmined nd discussed. 2. Theoreticl Considertions Sherogrphy nd ESPI re hologrphic interferometry techniques tht rely on phse recording of light wve fronts. In these techniques, the interference between two coherent wve fronts converts the phse distribution to n intensity pttern tht cn be recorded on qudrtic detectors. For the ske of completeness, we provide few of the essentil opticl reltions tht describe both interferometric systems. Denoting u nd u b two coherent wve fronts tht form the primry interferogrm, it is possible to obtin the intensity distribution using the following reltion -2-

3 I = u + u 2 b = I + I b + u u * b + u * u b in the bove expression, I represents the intensity with the subscripts nd b indicting the two coherent wve fronts. In ddition, * stnds for the conjugte complex. An interferometric fringe pttern, lso known s interferogrm, is obtined by correlting two primry interferogrms recorded prior to nd fter excittion of the object. In Sherogrphy, both wve fronts re speckled nd decorrelte with excittion. As result, the intensity of the interferogrm is described by ( I I ) = 2( 1 c )( I + I ) + 4 c I I [ 1 cos( φ + α) ] 2 1 b b (2) where the pointed brckets indicte the verge of the function. For the ESPI set-up, speculr reference is used nd only u is speckled nd decorrelte during object excittion. In this cse, the fringe pttern intensity is given by the following expression ( I ) = 2 I ( 1 c ) + 4 I I 4 c I cos( φ + α) I (3) 2 1 b I b In (2) nd (3), I 1 nd I 2 represent the intensity of ech primry interferogrm, φ is the phse difference induced by object deformtion, α is constnt phse shift introduced in order to llow phse-shift interferometry, nd c is correltion coefficient tht cn be clculted ccording to the Owner-Petersen (1991) reltion: 1 c = P()( f P f f t ) ( 2 if d o )df A exp π (4) where P(f) is the pupil function describing the entrnce pupil in the frequency domin, (P=1 inside the pupil nd P=0 outside) nd d o stnds for the in-plne displcement of ech speckle. The imges resulting from ESPI nd Sherogrphy re then digitlly processed. In this work, temporl phse shift techniques were used to ssess the phse mps corresponding to ech interferometric pttern. A PZT mounted mirror plced in one of the interferometer rms llows the introduction of known phses α, independent of the object excittion. By collecting three or more imges, ccording to the lgorithm used, it is possible to clculte the sptil phse distribution of the interferogrms. In our work, specil four imges phse shift lgorithm ws used to clculte the phse of ech pixel using φ( x, y ) I = rctn I d ( x, y) I ( ) ( ) ( ) b x, y x, y I c x, y In the bove expression, the subscripts to d indicte the four interferogrms. The continuous phse mps were then obtined by using specil unwrpping lgorithm. (1) (5) -3-

4 3. Experimentl Investigtions A propriety therml brrier coting system of the detils shown in Fig. 2 ws exmined. This system, which initilly experienced interfcil disbonds, ws developed using mould in which the therml brrier coting s well s the binding lyer were thermlly spryed first, followed by the substrte. The constituents of the coting system nd their corresponding thicknesses re provided in tble 1. Further detils cn be obtined in Durte (1992). The opticl set-up used for ESPI nd Sherogrphy cn be found in erlier publictions by the uthors, see Chousl J.C. et l., (1994). In order to detect the presence of interfcil defects, loclised displcement field ws induced. Different methods of excittion cn be used, e.g. Hung (1998), depending upon the type of mteril nd the defects being investigted, to develop this field. In view of its simplicity nd effectiveness, therml excittion ws crried out using 500W tungsten lmp. By controlling the distnce nd the heting time the temperture rise ws kept round 10ºC. Imges of the speckle ptterns were then cptured by the CCD cmer nd stored in the computer memory redy for processing. Our dt collection system llows us to control the phse of the interferogrms independent of the deformtion pttern resulting from the loclised excittion of the coting system. This llows us to improve the resolution of the mesurements of the displcement grdients nd ultimtely the contrst nd the visibility of the resulting imges. 4. Anlysis Of Results And Discussion This prt is divided into three sections. The first dels with the nlysis of the cptured imges using the two interferometric techniques dopted in the study. The second ddresses their cpbility to detect interfcil defects in the current therml brrier coting system. The third focuses on the sources of errors ssocited with these types of mesurements nd detection. Fig. 3 () shows the rw fringe pttern resulting from the excited therml brrier coting system described bove using the ESPI rrngement. Fig. 3 (b) depicts the phse mp resulting from the cptured rw fringe pttern. In both figures, the high frequency noise, due to the speckle nture of the imges, cn be seen. In Fig. 3 (c), we show the filtered phse mp contours, while in Fig. 3 (d) we show the unwrpped phse mp of the excited re. Four distinct fetures cn be clerly identified from the cptured imges. The first is chrcterised by the presence of loclised closed fringes identifying the debonded region. The second is chrcterised by the thick verticl fringe, which corresponds to the rigid body displcement in the direction of the sensitivity vector ssocited with the imposed therml excittion. The third clerly identifies the boundry of the debonded region (Fig. 3(d)), while the fourth is mesure of the curvture resulting from the loclised deformtion of the debonded region. Figs. 4 ()-4(d) show the respective fringe pttern, phse mp, filtered phse mp nd phse unwrpping of the sme therml brrier coting system investigted using Sherogrphy. The similrity between both sets of imges, resulting from ESPI nd Sherogrphy (Figs. 3 nd 4), is good. The results of Sherogrphy show the sme -4-

5 fetures s those depicted in Fig. 3. However, distinct difference between the two sets of imges is the fct tht the fringe contrst is better in the pttern obtined using ESPI. This is due to the fct tht with ESPI speculr reference is used, whilst with Sherogrphy the fringe pttern results from the correltion between two speckle ptterns. In order to vlidte the bove opticl mesurements, post-mortem exmintion of the coting system investigted ws crried out by sectioning the suspect res. The results, shown in Fig. 5 of typicl cse indicte the presence of debonded lyer in the region predicted by both techniques. Creful mesurement of the debonded re revel tht the current NDI opticl systems re cpble of identifying the debonded region quit ccurtely. It is cler from the resulting imges tht both ESPI nd Sherogrphy re cpble of mesuring interfcil disbond. However, in the cse of ESPI interferometry, errors my result from the presence of rigid body displcements resulting from excittion nd sensitivity to externl disturbnces such s vibrtions. Unlike ESPI, Sherogrphy relies upon the displcement grdients nd the fringe pttern is, in this cse, reflection of the derivtives of the displcements in the direction of verticl sher. Sherogrphy is therefore less sensitive to disturbnces leding to rigid body displcements. In view of its insensitivity to globl disturbnces, simplicity nd robustness, Sherogrphy cn be used to predict interfcil defects on site. 5. Conclusions Our results indicte tht ESPI nd Sherogrphy techniques re well dpted to revel disbond res in the coting system investigted. The study lso revels tht therml excittion led to very stble interferometric ptterns in therml brrier cotings tht cn be esily mesured. Interestingly, both ESPI nd Sherogrphy were cpble of predicting the shpe of the debonded re. Unlike ESPI, Sherogrphy relies upon the displcement grdients nd the fringe pttern is in effect reflection of the derivtives of the displcements in the direction of verticl sher. Therefore, Sherogrphy is less sensitive to globl disturbnces, which my led to rigid body displcements. In view of its insensitivity to globl disturbnces, simplicity nd robustness, Sherogrphy cn be relibly used to predict interfcil disbond in coted systems on site. References Bunshh R.F., (1987). Deposition technologies for films nd cotings. Noyes Dt Corp., New Jersey. Chousl J.C. et l., (1994). Non-destructive testing of composite structures using Sherogrphy. In: Silv Gomes, F. J. et l (Eds), 10th Interntionl Conference in Experimentl Mechnics, Lisbon, Portugl, Durte T., (1992). Estudo de um processo não convencionl pr o fbrico de coquilhs (Study of non-conventionl process to produ ce moulds), MSc thesis, University of Oporto, Portugl. -5-

6 Ettemeyer, A., (2000). Combintion of 3-D deformtion nd shpe mesurements by electronic speckle pttern interferometry for quntittive strin-stress nlysis. Opt. Eng. 39(01), Gong, X. L., Toyook, S., (1999). Investigtion on mechnism of plstic deformtion by digitl speckle pttern interferometry. Exper. Mech. 39, Hung, Y. M. Y., Dhun, S., (1998). Technique for rpid inspection of hermetic sels of microelectronic pckges using sherogrphy. Opt. Eng. 37(05), Lee S.M., (1991), Interntionl encyclopedi of composites, 4, VCH Publishers. Leendertz J. nd Butters J., (1973). An imge-shering speckle-pttern interferometer for mesuring bending moments, J. Phys. E., 6, Løkberg O. nd Slettemoen G., (1987). Bsic electronic speckle pttern interferometry, In: Applied optics nd opticl engineering. Acdemic Press. Ms, A. M.; Somers, P. A.A. M.; (1997). Two-dimensionl deconvolution pplied to phse-stepped sherogrphy. Opt. Lsers Eng. 26, Owner-Peterson M., (1991). Decorreltion nd fringe visibility: on the limiting behviour of vrious electronic speckle pttern correltion interferometers. J. Opt. Soc. Am. A, 8, Scle, F. L., Hong, S. S., Cloud, G. L., (1998). Whole-field strin mesurement in pin-loded plte by electronic speckle pttern interferometry nd the finite element method. Exper. Mech. 55, Sirohi, R. S., Ty, C. J., Shng, H. M., Boo, W. P., (1999). Non-destructive ssessment of thinning of pltes using digitl sherogrphy. Opt. Eng. 38(09),

7 CAPTIONS TO FIGURES Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Schemtic representtions of ESPI nd Sherogrphy mesurements of defect. A schemtic of the propriety therml brrier coting system investigted Imges resulting from ESPI: () Fringe pttern, (b) Phse mp, (c) Filtered phse mp, nd (d) Unwrpped phse. Imges resulting from Sherogrphy: () Fringe pttern, (b) Phse mp, (c) Filtered phse mp, nd (d) Phse unwrpping. A smple of post-mortem exmintion of the therml brrier coting system investigted. -7-

8 LIST OF TABLES Coting System Detils Composition Thickness Therml brrier ZrO 2 24% MgO 0.1 mm Binding lyer Ni Al Mo 0.3 mm Interphse lyer High purity Copper 2 mm Substrte Brss 18 mm Tble 1 Detils of coting system investigted -8-

9 Absolute mesurement ESPI or HI Initil position After deformtion position Grdient mesurement Sherogrphy x Mesured vlue Shering of imge x Fig. 1 Fig. 2-9-

10 () (b) (c) (d) Fig

11 () (b) (c) (d) Fig

12 Therml brrier coting: 0.1 mm ZrO 2-24% MgO 0.3 mm Ni-Al -MO 2 mm Cu Brss csting 18 mm thick Disbond re Front view Side view Fig

Numerical Study Of Coated Electrical Contacts

Numerical Study Of Coated Electrical Contacts Excerpt from the Proceedings of the COMSOL Conference 21 Pris Numericl Study Of Coted Electricl Contcts Per Lindholm Mchine Design KTH Brinellvägen 83 SE-144 Stockholm per@md.kth.se Abstrct: Electricl