L.D. Favro, H.I. Jin, T.Ahmed, X.Wang, P.K. Kuo and R.L. Thomas

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1 INFRARED THERMAL WAVE STUDIES OF COATED SURFACES L.D. Favro, H.I. Jin, T.Ahmed, X.Wang, P.K. Kuo and R.L. Thomas Department of Physics and Institute for Manufacturing Research Wayne State University Detroit, MI INTRODUCTION In this paper we discuss the recent results of our infrared thermal wave studies of plasma-sprayed coatings, coatings on automotive metal panels, and metal to polymer adhesive joints. The purpose of this study is to develop a reliable NDE technique to assess the quality and test the integrity of these coatings and bonding. The experimental method is a pulsed heating and synchronous infrared thermal wave detection technique commonly referred to as box-car thermal wave video imaging. [1-3] The thermal wave propagation times for plasma-sprayed coatings and adhesive bonds are usually long enough for many NDE applications that the method used here can be considered to be a real time technique. In the IR thermal wave imaging method (Fig. 1) the sample surface is pulse-heated by a bank of flash lamps, and the thermal response of the surface is monitored as a function of time and space by means of an infrared video camera. The resulting video signal is sent to a fast signal processing system and averaged synchronously on a pixel-by-pixel basis. The final data are displayed in the form of 2-dimensional or 3-dimensional gray scale or pseudocolor images of subsurface thermal features. THERMAL BARRIER COATED PISTON HEADS The application of infrared thermal imaging to study the integrity of plasma sprayed zirconia thermal barrier coatings on metal piston heads was introduced in the 1989 QNDE conference. [4] As an example ofthe results of this ongoing study, in Fig. 2 we show an optical image of a piston after the engine test. While the image shows a pattern of carbonization correlated with the positions of the fuel injector nozzles, there are no visible coating defects. However, a thermal wave image of the same piston, shown in Fig. 3, displays a clear subsurface delamination at the center of the piston and two weaker subsurface delaminations on the rim. This image was obtained by setting one electronic time-gate whose window corresponds to the time of maximum thermal image contrast (about 1.5 sec). This time is determined by the time of propagation of the thermal wave from the surface to the defect and back again. A second gate is set at a considerably longer time, corresponding to a background image. These two images have been subtracted in real time and then synchronously averaged over several repetitive pulses of the flash lamps; the resulting image shown in Fig. 3. This subtraction and averaging process effectively removes artifacts which originate in the emissivity variations associated with the optical pattern shown in Fig. 2, leaving only the portions of the image resulting from the subsurface defects. A three- dimensional perspective plot of a thermal wave image of a second thermal barrier coated piston head is shown in Fig. 4. This piston showed visible damage in the form of spalled areas in the center. In the thermal wave image we can see additional damage in the form of delarninations around the spalled area at the center and three other regions on the rim. Review oj Progress in Quantitative Nondestructive Evaluation, Vol. lob Edited by D.O. Thompson and D.E. Chimenti, Plenum Pres'" New York

$Q \\\1 :::d 111/ radiation Subsurface d~ SPECIMEN -~ ---$ ee 01 the sample Thermll w.v. reu.ctlon Fig. 1.

2 Q \\\1 :::d 111/ radiation Subsurface d~ SPECIMEN -~ --- Pulsed or modulated heat so\,jrce on the surt.ee 01 the sample Thermll w.v. reu.ctlon Fig. 1. Schematic diagram of the thennal wave imaging process. Fig. 2. Optical image of a thermal barrier coated piston after an engine test. Fig. 3. Thennal wave image of the piston in Fig

Fig. 4. A 3-D thennal wave image of a thennal barrier coated piston showing delarninations around the central spalled region and on the rim.

properties of the coating and substrate and on their thicknesses.

To illustrate this method we studied three paint panels of the same paint, but different paint thicknesses.

3 Fig. 4. A 3-D thennal wave image of a thennal barrier coated piston showing delarninations around the central spalled region and on the rim. PAINT PANELS In our infrared thermal wave imaging method, the time evolution of the thermal image for a coated sample depends on the thermal properties of the coating and substrate and on their thicknesses. Therefore, if the thermal properties are known or measured separately, it is possible to use the measurement to determine coating thickness. To illustrate this method we studied three paint panels of the same paint, but different paint thicknesses. A schematic diagram of the sample is shown in Fig. 5. The four figures which follow illustrate the time dependence of the thermal wave image for the different paint images. C-Clamp 3.58 mils 5.51 mils Fig. 5. A schematic diagram of the three paint panels. 1203

Figure 6 is a thermal wave image with a first gate delay time of 17 ms.

At a gate time delay of 50 ms (Fig. 7), the thinnest panel has cooled nearly to the background still warm and not significantly different from one another.

Finally (Fig. 9), at 134 ms, the intermediate panel has almost reached background.

4 Figure 6 is a thermal wave image with a first gate delay time of 17 ms. In this image the thinnest of the panels has cooled noticeably more than the other two, except for a thick spot in the upper right center. At a gate time delay of 50 ms (Fig. 7), the thinnest panel has cooled nearly to the background still warm and not significantly different from one another. In Fig. 8 (84 ms), the thin coating has reached background ~emperature, and the intermediate coating has cooled noticeably relative to the thickest. Finally (Fig. 9), at 134 ms, the intermediate panel has almost reached background. This variation in the time dependence of the cooling curves can be used to measure the paint thickness or to detect defects, e.g. the thick spot in the thinnest of the three panels imaged here. Fig. 6. Thermal wave image of the paint panels at a gate time delay of 17 ms. Fig. 7. Thermal wave image of the paint panels at a gate time delay of 50 ms. 1204

5 Fig. 8. Thermal wave image of the paint panels at gate time delay of 84 ms. Fig. 9. Thermal wave image of the paint panels at Ii gate time delay of 134 ms. POLYMER/METAL BOND In the next example we demonstrate the effectiveness of our IR thennal wave imaging to detect the presence of adhesion defects in two polymer to metal bonds in automotive parts. The first sample is a 0.03" rubber layer bonded to a 0.03" metal part with an area of poor adhesion in the center of the region imaged (see Fig 10). The second sample (Fig. 11) shows a 0.04" teflon layer bonded to a 0.03" metal part, showing a disbond in the upper right center of the hnage. ACKNOWLEDGEMENTS This work was sponsored by the Center for Advanced Nondestructive Evaluation, operated by the Ames Laboratory, USDOE, for the Air Force Wright Aeronautical Laboratories under Contract No. W-7405-ENG-82 with Iowa State University, and by the Institute for Manufacturing Research, Wayne State University. 1205

Fig. 10. The~al wave image of a defect in a 0.03" rubber bonded to 0.03" metal. Fig. 11.

Jin, L.D. Favro, and R.L. Thomas, Journal of NDE, 8(2), 97 (1989). 2. P.K. Kuo, T. Ahmed, L.D. Favro, H.J. Jin, and R.

Feng, P.K. Kuo, J. Hartikainen, and J. Jaarinen, Journal of NDE, 6(4):190 (1987). 4. T. Ahmed, P.K. Kuo, L.D. Favro, H.

6 Fig. 10. The~al wave image of a defect in a 0.03" rubber bonded to 0.03" metal. Fig. 11. Thermal wave image of a defect in a 0.04" teflon bonded to 0.03" metal. REFERENCES 1. P.K. Kuo, T. Ahmed, H.I. Jin, L.D. Favro, and R.L. Thomas, Journal of NDE, 8(2), 97 (1989). 2. P.K. Kuo, T. Ahmed, L.D. Favro, H.J. Jin, and R.L. Thomas, Review of Progress in Quantitative NDE, 8B, 1305, Plenum Press, New York (1989). 3. T. Ahmed, Z.J. Feng, P.K. Kuo, J. Hartikainen, and J. Jaarinen, Journal of NDE, 6(4):190 (1987). 4. T. Ahmed, P.K. Kuo, L.D. Favro, H.J. Jin, P. Chen, and R.L. Thomas, Review of Progress in Quantitative NDE, 9B, 2001, Plenum Press, New York (1990). 1206

PARALLEL BOX-CAR IMAGING OF ADHESION DEFECfS IN PLASMA-SPRA YED

PARALLEL BOX-CAR IMAGING OF ADHESION DEFECfS IN PLASMA-SPRA YED COATINGS P.K. Kuo, T.Ahmed, L.D. Favro, H.J. Jin, and R.L. Thomas, Department of Physics, Wayne State University Detroit, MI 48202 J. Jaarinen