Keywords: surface resistivity/conductivity, organic thin films, digital shearography technique, electrochemical ... electrical resistivity of the coated film, in cm.
Measurement of Surface Resistivity/Conductivity of Different Organic Thin Films by a Combination of Optical Shearography and Electrochemical Impedance Spectroscopy Khaled HABIB
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Vol. 20, No. 6 (2013)
OPTICAL REVIEW Vol. 20, No. 6 (2013) 500–503
Measurement of Surface Resistivity/Conductivity of Different Organic Thin Films by a Combination of Optical Shearography and Electrochemical Impedance Spectroscopy Khaled HABIB Materials Science and Photo-Electronics Lab., IRE Program, EBR Center, KISR, P. O. Box 24885 SAFAT, 13109 Kuwait (Received July 26, 2013; Accepted August 21, 2013) Shearography techniques were applied again to measure the surface resistivity/conductivity of different organic thin films on a metallic substrate. The coatings were ACE premium-grey enamel (spray coating), a yellow Acrylic lacquer, and a gold nail polish on a carbon steel substrate. The investigation was focused on determining the in-plane displacement of the coatings by shearography between 20 and 60 C. Then, the alternating current (AC) impedance (resistance) of the same coated samples was determined by electrochemical impedance spectroscopy (EIS) in 3.0% NaCl solution at room temperature. As a result, the proportionality constant (resistivity or conductivity = 1/surface resistivity) between the determined AC impedance and the in-plane displacement was obtained. The obtained resistivity of all investigated coatings, 40:15 106{24:6 109 cm, was found in the insulator range. # 2013 The Japan Society of Applied Physics Keywords: surface resistivity/conductivity, organic thin films, digital shearography technique, electrochemical impedance spectroscopy (EIS), carbon steel
1.
was used again to determine the resistivity of several organic coatings, along with the application of the EIS technique. Here, jZj is the AC impedance (resistance), which can be determined by the technique of EIS, at a very low frequency, in 3.0% NaCl solution at room temperature.4,5) is the electrical resistivity of the coated film, in cm. A is the surface area of the sample, in cm2 , and d is the surface (in-plane) displacement of the coated samples, which can be obtained by shearography.3) Eventually, a correlation can be developed between the determined (AC) impedance (Z) (from EIS) and the surface (in-plane) displacement, d, of the coated samples (by shearography), from Eq. (1). It is worth mentioning that no data on the values of () were found in the literature for the same coatings using other methods, i.e., direct current (DC) methods.6) There are disadvantages of using DC electrochemical methods for measuring the surface resistivity and surface conductivity of coated samples as compared with the combination of shearography and EIS. DC electrochemical methods produce heat that might affect the measurements of the surface resistivity and surface conductivity of coated samples as compared with the combination of shearography and EIS. This due to the fact that the organic coatings, i.e., epoxy base (thermosetting) resins, are well known to have extremely high resistivity values,6) DC ¼ 3:8 1015 cm. Also, the combination of shearography and EIS results in a powerful 2D microscopy technique for monitoring the surface strain of coated samples, on a microscopic scale.
Introduction
It is well established that metrological methods utilizing coherent light such as speckle interferometry are very useful in measuring the deformation of surfaces of different objects under various conditions.1) However, few investigations have determined the failure behavior of different coatings on metallic samples thus far. In fact, the author2) was the first to apply the technique of shearography to develop a failure criterion of different organic coatings on metallic samples, over the temperature range between 20 and 60 C. The failure criterion was developed by determining the critical (steady state) value of the thermal expansion coefficients of coatings. The value of the thermal expansion coefficients of coatings was derived from the slope of the plot of the thermal deformation (strain) versus the applied temperature. The integrity of the coatings with respect to time was assessed by comparison of the measured coefficients of thermal expansion (CTE) with the critical (steady state) or asymptotic value of CTE. By shearography, the measurement of coating properties could be performed independent of parameters, such as UV exposure, humidity, and presence of chemical species, which may normally interfere with conventional methods of assessing the integrity of coatings. Therefore, one may measure the CTE of coatings, regardless of the history of the coating, to assess the integrity of coatings. In this investigation, the same procedures for determining the values of the resistivity/conductivity of several coatings3) were used by the combination of shearography and electrochemical impedance spectroscopy (EIS). 2.
3.
Theoretical Analysis In this investigation, the established relation; ¼
jZjA d
Experimental Methods
In a similar fashion to the previous study,3) uncoated and coated metallic samples of a carbon steel, UNS No. 1020, were used in this investigation. The chemical composition of the carbon steel was 0.18–0.23% C, 0.3–0.6% Mn, and balanced Fe. The carbon steel samples were fabricated in
3)
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0-2.66 x100µm/mm
a
b
c
d
e
Fig. 1. (Color online) 2D shearographs of the carbon steel sample covered by with gray coating at temperatures of approximately (a) 20, (b) 30, (c) 40, (d) 50, and (e) 60 C. The magnitude of the scale is ð0{2:66Þ 100 m/mm (0.0–2.66% strain). 3 Strain (x100um/mm)
a rectangular form with dimensions of 5 10 0:15 cm3 . All samples were covered on one side with different coatings. The coatings were ACE premium-grey enamel (spray coating), a yellow acrylic lacquer, and a gold nail polish. The reason behind leaving one side (10 5:0 cm2 ) of the samples uncoated is to allow direct heating of the uncovered side in a furnace while testing the painted side of the samples at high temperatures. The temperature of the furnace was monitored by a digital thermocouple, attached to the uncovered side of the samples facing the inside of the furnace, and a temperature controller. The sample surface was heated efficiently using forced air flow from a blower connected to the furnace. The blower was programmed to reach a desired temperature and then automatically shut down. At the desired temperature, i.e., 20, 30, 40, 50, or 60 C, a shearograph of the sample was recorded using a shearographic camera. In this study, a shearographic camera with a T.V monitoring system was used to facilitate recording of the real-time shearographs of the samples during the high temperature test. Details of the optical setup are given elsewhere in the literature.3) The size of the recorded shearograph was 10 3:3 cm2 from the coated side of the sample. The sample was mounted in a slot in the furnace wall. The uncoated side of the sample faced the air flow from the blower in the furnace. The shearographic camera was made by Steinbichler Optotechnik GmbH, in Germany. The obtained shearographs of the samples were analyzed to provide thermal deformations (strain). The surface displacement, d, of all coatings was determined as a function of thermal deformation (strain) versus the applied temperature range. EIS measurements were performed against a saturated Calomel electrode (SCE) in accordance with procedures described elsewhere.4,5) Standard electrochemical cells were used; each cell consisted of a 1000 cm3 flask, a reference electrode (a SCE), a counter electrode (made of platinum wire), and a working electrode (formed by each of the coated samples and the uncoated carbon steel sample). The exposed surface area of all samples was 3.14 cm2 . In this study, EIS measurements were conducted using a potentiastat/galvanostat made by Gamry instruments to obtain impedance spectra. The EIS spectra of all investigated coated samples were determined in 3% NaCl solution. The AC impedance was obtained at low frequency on the basis of the extrapolation of the intersection line at a frequency equal to 0.16 Hz from the x-coordinate to the y-coordinate in Bode plots. Bode plots are basically the logarithm of the impedance (Z) (Y-coordinate) and phase () (Y-coordinate) plotted versus the logarithm of the frequency (X-coordinate). All the AC impedances of the investigated coated samples were determined by using a software (developed by Gamry instrument), using the data fitting method of the paint model of a semicircle. Also, to plot the complex plane (Nyquist) and Bode plots, the frequency range was chosen to be between 10000 and 0.01 Hz. The obtained data of the AC impedance of all investigated coated samples, were used to determine the AC impedance, jZj, represented in Eq. (1).
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y = 0.0467x - 0.3
2 1.5 1 0.5 0 0
20
40
60
80
Temperature (°C)
Fig. 2. (Color online) Plot of temperature versus strain of carbon steel sample covered with the grey coating.
4.
Results and Discussion
Figures 1(a)–1(e) illustrate examples of shearographs1) of the sample of the carbon steel coated by the ACE premiumgrey enamel (coating) at 20, 30, 40, 50, 60 C, respectively. From Figs. 1(b) and 1(c), colorful fringes, black-light bluedark-blue fringes, were observed to gradually appear in the shearographs, indicating a localized thermal deformation from ð0{1Þ 100 m/mm (0.0–0.1% strain) at 30 and 40 C, respectively. This appearance of colorful fringes in the carbon steel covered with the gray coating was due to the higher thermal expansion coefficient of the carbon steel covered with the gray coating than the thermal expansion coefficients of the uncovered carbon steel.3) In contrast, Figs. 1(d) and 1(e) exhibit uniform fringes of different colors (from light blue to white spectra) in the shearographs, indicating that a steady thermal deformation from ð0:33{ 2:66Þ 100 m/mm, has developed in the shearographs at 50 and 60 C, respectively. From the shearographs in Figs. 1(a)–1(e) one can conclude that the coated sample of the carbon steel has gone through a thermal deformation ranging from ð0{2:66Þ 100 m/mm, with an average thermal expansion coefficient equivalent to Ave:gc ¼ 0:0467 100 m/mm C, with a standard deviation equivalent to 20%, over a temperature range from 20–60 C. The plot of the temperature versus the strain of the carbon steel sample covered with the grey coating is shown in Fig. 2.
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Strain (um/mm)
180 160 140 120 100 80 60 40 20 0 -20 0
10
20
30
40
50
60
70
Temperature (°C)
Fig. 3. (Color online) Plot of temperature versus strain of carbon steel sample covered with the yellow acrylic lacquer.
Fig. 4. (Color online) Plot of temperature versus strain of the carbon steel sample covered with gold nail polish coating.
Fig. 5. (Color online) Bode plot of carbon steel sample covered with the yellow acrylic lacquer in 3% NaCl solution. The blue plot is the logarithm of impedance (Z) versus the frequency and the green plot is the logarithm of impedance (Z) versus phase angle ().
Table 1. Calculated parameters of coated samples in 3% NaCl solution based on an average of at least two readings maintaining a standard deviation of less than or equivalent to 20%. Sample Carbon Steel Covered with
AC impedance Z ()
Surface displacement d (m)
Resistivity by OI ( cm)
Conductivity by OI (S/cm)
() by other source6) ( cm)
Nothing3) Yellow acrylic lacquer Grey enamel3) Gold nail polish
2:7 101 6:9 108 7:5 105 3:5 108
3300 9240 6164.4 6085.2
1:0 105 24:6 109 40:15 106 19:0 109
1:0 105 4:1 1011 2:5 108 5:3 1011
1 105 None None None
Consequently, the average surface displacement of the carbon steel sample covered with the grey coating is equivalent to Ave:d ¼ 6164:4 m, assuming that the width of the recorded shearograph is d ¼ 3:3 cm. In a similar fashion to the above, the average thermal expansion coefficients of carbon steel samples covered with the yellow acrylic lacquer and gold nail polish coatings were determined. Figures 3 and 4 show the plots of the temperature versus the strain of the carbon steel covered with the yellow and gold coatings, respectively. Consequently, the average surface displacements of the carbon steel samples covered by the yellow and gold coatings are equivalent to Ave:d ¼ 9240 m and Ave:d ¼ 6085:2 m, respectively, assuming that the width of the recorded shearograph is d ¼ 3:3 cm, assuming that d ¼ 3:3 cm.
Figure 5 shows an example of a Bode plot of the carbon steel sample covered with the yellow acrylic lacquer in 3.0% NaCl solution. The determined values of the resistivity = and conductivity = 1= ¼ of the coated samples in different solutions are given in Table 1. It is worth mentioning that during the EIS of the carbon steel (uncovered) sample in 3% NaCl solution, a dissolved oxide layer of the sample was observed in the solution. 5.
Conclusions
The following conclusions were drawn from the above investigation: 1. Shearography produced only a nearly linear relationship between strain and temperature for the carbon steel samples covered with the yellow acrylic lacquer, and gold
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nail polish coatings as shown in Figs. 3 and 4, respectively. The technique produced an average linear relationship between strain and temperature that falls within a standard deviation equivalent to 20%, with an error bar of 40%, for the carbon steel sample covered with the gray coating as shown in Fig. 2. 2. The resistivity values of the carbon steel samples covered with ACE premium-grey enamel,3) a yellow acrylic lacquer, and gold gail polish were determined 40:15 106 , 24:6 109 , and 19:0 109 , respectively. In this investigation, the increase in the obtained resistivity corresponded very well to the increase in the AC impedance (Z) of the coated samples as shown in Table 1. 3. No data on the values of () were found in the literature for the same coatings using other methods, i.e., DC methods. However, the values of () of all investigated coatings, 40:15 106 {24:6 109 cm were found to be in the
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insulator range based on the available data of conductivities in the literature.7) References 1) K. Robert: ERF: Speckle Metrology (Academic Press, New York, 1978) pp. 41–98. 2) K. Habib: Opt. Rev. 18 (2011) 324. 3) K. Habib: Opt. Rev. 20 (2013) 59. 4) EG&G: Application Note AC-1 EG&G (Princeton University Press, Princeton, NJ, 1982). 5) H. Uhlig: Corrosion and Corrosion Control (Wiley, New York, 1971) pp. 1–419. 6) R. Bolz and G. Tuve: CRC Handbook of Tables for Applied Engineering and Science (CRC Press, New York, 1973) 2nd ed., pp. 118, 144, and 154. 7) Web [http://www.britannica.com/EBchecked/media/139/ Typicalrange-of-conductivities-for-insulators-semiconductorsand-conductors].
