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Laser induced decohesion of coatings: probing by laser ultrasonics
Ultrasonics 40 (2002) 765–769 www.elsevier.com/locate/ultras
Laser induced decohesion of coatings: probing by laser ultrasonics G. Rosa a
a,*
, R. Oltra
a,*
, M.-H. Nadal
b
Laboratoire de Recherches sur la R eactivit e des Solides, UMR 5613 CNRS, Universit e de Bourgogne, UFR Sciences et Techniques, 9 Avenue A. Savary, B.P. 47 870, 21078 Dijon Cedex, France b CEA, Centre de Valduc, DRMN, 21120 Is-sur-Tille, France
Abstract The aim of the present study is to investigate a conventional laser-ultrasonics technique for the determination of intrinsic properties of oxide coatings and their adhesion strength on a metallic substrate. The good agreement between experiments and computations in an epicenter configuration allows determining the longitudinal wave velocity as well as the Young’s modulus of the oxide coatings versus the porosity. For a critical value of the laser energy, a breakdown at the coating–substrate interface is generated by the laser irradiation. The critical tensile stress field developed at the coating/substrate interface, which leads to the interfacial fracture, can be easily calculated. The value of the practical adhesion which is defined is found to be in accordance with those obtained by classic contact techniques (tensile adhesion test, indentation, bending test). Finally, this work demonstrates that this quantitative, contactless test fits well to simultaneously characterise the oxide coatings and evaluate the coating–substrate adhesion. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Oxide coatings; Laser ultrasonics; Adhesion
1. Introduction The laser-ultrasonic technique also defined as ‘‘laser ultrasonics’’ (contactless non-destructive testing method) can provide important information on materials characterisation [1,2], e.g. microstructural characteristics [3]. This technique may be also used to measure acoustic velocities, in ceramic–metal composites [4], in metals at elevated temperatures [5], whilst their greatest shortcoming is their poor sensitivity compared to conventional contact systems. If the laser-ultrasonic technique has been widely used for the different applications concerning bulk materials or composites, at ambient and elevated temperatures, in the case of the plasma sprayed coatings on a metallic substrate, only some experiments have been done with this technique [6]. However, the effect of a constraining layer on the generation efficiency of the longitudinal pulse and the shear pulse, within a metal
*
Corresponding authors. Fax: +33-3-8039-6132. E-mail address: [email protected] (R. Oltra).
irradiated by a laser beam has been monitored by detecting longitudinal and shear transducers respectively [7–10]. The aim of the present study was to investigate a laser-based technique for the determination of intrinsic properties of oxide coatings and their adhesion strength on a metallic substrate. With regard to adhesion characterization, conventional laser-ultrasonic technique has been used to detect debonding in multilayer structures and which involves an adhesion parameter between layers [11]. In this paper, the same approach was followed in terms of probing but the laser was also used for the generation of the decohesion of the coating itself. The acoustic response of a transparent and porous ceramic coating on a metallic substrate was studied, under different regimes of pulsed laser irradiation/material interaction (thermo-elastic interactions, fracture of the coating–substrate interface and coating expulsion) but in the absence of ablation and subsequent plasma formation. For the determination of intrinsic properties of coatings and their adhesion strength, a model was developed and validated in the case of alumina coatings deposited on stainless steel substrates by atmospheric plasma spraying (APS).
0041-624X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 1 - 6 2 4 X ( 0 2 ) 0 0 2 0 9 - 3
766
G. Rosa et al. / Ultrasonics 40 (2002) 765–769
2. Experimental Al2 O3 coatings of different thickness and of different pore volume fraction deposited on 5-mm thick stainless steel substrates, by APS were tested. The experimental arrangement based on conventional laser ultrasonics, used for the determination of intrinsic properties of alumina coatings and their adhesion strength, is described in a previous paper [12]. A nanosecond pulsed Nd:YAG laser (k ¼ 1064 nm), was used for the irradiation of the ceramic coatings and the acoustic signal was recorded in real time with the aid of a laser heterodyne interferometer (k ¼ 632 nm) placed at the free rear surface of the substrate. Experiments were carried out applying energies from 5 up to 30 mJ, which provided mean incident energy densities from 0.3 up to 1.7 J cm2 . At the wavelength of the laser irradiation used, all the alumina coatings examined were transparent, whilst the absorption depth was 21 nm below the ceramic/metal interface. Taking into account the value of the reflectivity (0.8), the laser energy densities which are absorbed at the inner ceramic/metal interface (from 0.06 up to 0.35 J cm2 ) are always below the ablation threshold value of the stainless steel (0.65 J cm2 ).
3. Results and discussion 3.1. Laser-material interactions When a laser source is used to irradiate a transparent ceramic coating deposited onto a metallic substrate, the pulsed laser irradiation creates an instantaneous thermal and acoustic source at the absorption depth of the substrate near the ceramic/metal interface. The propagation of the acoustic waves through the volume of the examined material induces the normal displacement of the free surface of the substrate. In some experiments, the specific time profile of the epicentral displacement generated by laser irradiation of a constraining layer consisting of a transparent material placed over the irradiated face of the target [13], or a polymer coating [10], has been clearly exhibited, but the relation between the normal displacement at the epicenter and the intrinsic properties of coatings and their adhesion has not been determined. The normal displacement at the epicenter (Fig. 1a) was recorded for a transparent coating on a metallic substrate using the contactless experimental arrangement. This typical waveform at the epicenter versus time (the origin of time is the time when the laser beam irradiates the system), exhibits multiple oscillations appearing at regular intervals. The first of the acoustic response recorded for an alumina coating on stainless steel substrate (Fig. 1b) was found to be indicative of the
Fig. 1. (a) Typical waveform recorded at the epicenter for a transparent coating on a metallic substrate. (b) Acoustic response corresponding to the arrival of the longitudinal wave for an alumina coating on a stainless steel substrate (coating thickness 150 lm, coating pore volume fraction 6%, Dt: 100 ns).
interaction between the laser radiation and the system examined. 3.2. Energy thresholds and spatial localization of the damage As detailed elsewhere [12], investigating the evolution of the amplitude of the first two peaks of the displacement (Fig. 2a) recorded at the epicenter, denoted L1 and L2 respectively, with the laser beam energy applied, it was found that, for coatings of the same thickness and porosity: (a) In the range of laser beam energies promoting thermo-elastic interactions, e.g. absorbed energies up to 3.5 mJ, the amplitude ratio (L2 =L1 ) was found to be constant. (b) In the range of laser beam energies leading to interfacial fracture, e.g. absorbed energies from 3.5 up to 4.8 mJ, the amplitude ratio (L2 =L1 ) slightly decreased with increasing laser beam energy. (c) In the range of laser beam energies generating expulsion of the coating, e.g. absorbed energies higher than 4.8 mJ, the second peak of displacement almost
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allowed to determine the period of time during which the event took place. As the interface damage can be detected from the evolution of the amplitude of the second longitudinal displacement (L2 ), this period corresponds to the time interval Ds between the arrival of the first two peaks (L1 , L2 ), as recorded in the acoustic signal. Ds depends on both the thickness and the longitudinal wave velocity of the coating. By example, the case of an 80-lm-thick alumina coating, with a pore volume fraction of 6%, the interfacial fracture takes place within an interval of 50 ns from the beginning of the laser pulse. 3.4. Intrinsic properties and practical adhesion estimation
Fig. 2. Evolution of (a) the amplitude ratio (L2 =L1 ) and (b) the interfacial area of debonding with the laser beam energy applied (coating thickness: 80 lm, coating pore volume fraction: 6%). Each point corresponds to a different irradiated area.
disappeared but no ablation phenomenon occurred. The amplitude ratio (L2 =L1 ) reached a null value and finally total expulsion of the coating took place. The above observations indicated that the amplitude ratio (L2 =L1 ) can provide a reliable criterion for the laser effects on the integrity of the irradiated systems and, consequently, it can be used for determination of the energy thresholds, which lead to interfacial fracture and to coating expulsion. Moreover, the debonded area (noted A) at the interface can be calculated [12] with the relation (1): A ¼ Sð1 bÞ
The stress field, which led to debonding, was estimated as the combined effect of a thermal and an acoustic source, both localised at the near interface region. Due to the Gaussian distribution of the laser beam, the most heavily loaded point of the interface was the one corresponding to the center of the irradiated area of the coating (or to the epicenter of the coated system). Thus, the following analysis was focused on the evolution of the stresses, which were developed at this point during the previously determined interval Ds. The stresses were determined by a semi-analytical model [14], adapted to a coated substrate, suitable to calculate the in- and out-of-plane components of the mechanical displacement versus time. The modelling combines the classical elasticity equations and is valid both for the thickness of the coating and the substrate. The two layers are assumed to be perfectly joined at the interface. The components of the displacement for an epicenter configuration were measured and are compared with computations for the set of samples (Fig. 3). The good agreement between experiments and computations in an epicenter configuration allows determining the mechanical, thermal and optical characteristics of the ceramic coating.
ð1Þ
where S is the irradiated area and b is defined by fitting of the experimental waveform with the waveform calculated by an analytical model [12]. As an example, in Fig. 2b the debonded area are plotted against the laser beam energy applied and the energy absorbed by the substrate, for the case of a 80lm-thick alumina coating with a porosity of 6%. 3.3. Time delay for the damage generation For laser beam energies resulting in interfacial fracture, e.g. absorbed energies from 3.5 up to 4.8 mJ, the real-time monitoring of the interfacial debonding [12]
Fig. 3. Comparison of the normal component of the displacement at the epicenter between the experiment ( ) and the model (—) for alumina coating with a thickness of 80 lm.
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G. Rosa et al. / Ultrasonics 40 (2002) 765–769
3.4.2. Calculation of the fracture toughness of the interface From the rzz determination, the fracture toughness of the interface, as expressed by the critical stress intensity factor (KIC ), was calculated for a laser beam energy of 20 mJ, taking into account the size of the debonding area (Fig. 2b) and using the relation (2): KIC
Fig. 4. Dependence of the velocity of the longitudinal waves and of the Young’s modulus of the ceramic coatings on the pore volume fraction.
For example, in Fig. 4, the so-obtained velocity of the longitudinal wave, as well as the Young’s modulus of the coatings were plotted versus the coating porosity. These values were found to be in the same order of magnitude as obtained in other works on the characterisation of thermal-sprayed ceramic coatings [15,16]. Those parameters are used for the modelling of the components of stress field at the interface ceramic coating/substrate. 3.4.1. Calculation of stresses due to thermal and acoustic effects By computing rzz , the stress value involving the interfacial fracture is given by the tensile stress (positive value). For an 80-lm-thick alumina coating, the interfacial fracture takes place within an interval Ds of 50 ns from the beginning of the laser pulse; the stress component is 13 MPa at 12 ns (Fig. 5). We have to note here that the interfacial fracture is not taken into account in the model.
Fig. 5. Temporal evolution of the normal stress (rzz ), at the point of the interface corresponding to the center of the area (alumina coating with a thickness of 80 lm).
pffiffiffi 3Pl a ¼Y 2BW 2
ð2Þ
where P is the normal load resulted in debonding; a, the crack length and Y, l, B, W, factors correlated to the geometry of the specimen and the loaded area [17]. The so-obtained value of KIC (0.3 MPa m1=2 ) of plasma-sprayed alumina coatings onto metallic substrates is in excellent agreement with other values, concerning three points bending and double cantilever beam tests [18]. However, comparing with the results obtained by scratch test [19] and indentation technique [20], this value is low. This fact could be attributed to the ceramic-like behavior of the interface: it presents low mechanical strength in tensile loading (proposed technique, three points bending and double cantilever beam tests) and high mechanical strength in compressive loading (scratch test and indentation technique). The critical strain energy release rate (GIC ) was calculated using the typical relation (3) and found to be 2.86 J m2 , value in the same order of the values obtained by three points bending test [18]. 2 GIC ¼ KIC
1 v2 E
ð3Þ
Compared with other tests used for the estimation of the adhesion of plasma-sprayed coatings, the proposed technique presents the advantages of a rapid and noncontact application, providing information not only about the strength of the interface, but also about the Young’s modulus of the coating and/or its porosity. The proposed technique could be applied on specimens of various dimensions, whilst the commonly used tensile and three points bending tests require specimens of a standard geometry. It could be applied for coatings having thickness in a large range, from 30 up to 400 lm, whilst the indentation test is suitable only for thick coatings (>300 lm). The real-time recording of the interfacial damage and the precise determination of the debonding area eliminate the step of the posterior observation, required in the cases of indentation or laser spallation techniques. Finally, its direct application on the system to be examined, without the need of a bonding agent, as in the case of tensile adhesion test (ASTM C633-79), allows testing coatings of elevated porosity.
G. Rosa et al. / Ultrasonics 40 (2002) 765–769
4. Conclusions In the present study, the potential of a contactless laser technique for the characterisation of transparent ceramic coating on a metallic substrate has been shown. The proposed technique is based on the simultaneous laser induced and laser detection of damages in such coated systems. The irradiation of transparent ceramic coatings using nanosecond pulsed laser radiation, combined with the in real-time recording at the epicenter of the laser-generated waves, allows the determination of the intrinsic properties of the coating, as well as the in real-time detection of laser-generated interfacial damages. The critical tensile stress field developed at the coating/substrate interface, which leads to interfacial fracture, can be easily determined as the combined effect of thermal and acoustic stresses. Compared to the conventional adhesion tests commonly used until now, the proposed technique presents several advantages, concerning the rapid and contactless application on coated substrates of various dimensions and it is suitable even for porous and relatively thin plasma-sprayed oxide coatings.
Acknowledgements The authors would like to express their deep thanks to C. Coddet and S. Costil of Universite de Technologie de Belfort—Montbeliard for the elaboration of the samples.
References [1] C.B. Scruby, L.E. Drain, Laser Ultrasonics: Techniques and Applications, Adam Hilger, New York, 1990, p. 356. [2] J.-P. Monchalin, Progress towards the application of laserultrasonics in industry, in: D.O. Thompson, D.E. Chimenti, Thompson (Eds.), Review of Progress in Quantitative Nondestructive Evaluation, vol. 12, Plenum Press, New York, 1993, p. 495. [3] A.K. Mukhopadhyay, K.K. Phani, An analysis of microstructural parameters in the minimum contact area model for ultrasonic velocity–porosity relations, J. Eur. Ceram. Soc. 20 (2000) 29. [4] J.D. Aussel, J.P. Monchalin, Precision laser-ultrasonic velocity measurement and elastic constant determination, Ultrasonics 27 (1989) 165.
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[5] C.B. Scruby, B.C. Moss, Non-contact ultrasonic measurements on steel at elevated temperatures, NDT Inter. 26 (1993) 177. [6] G. Rosa, P. Psyllaki, R. Oltra, A pulsed laser technique to evaluate the adhesion strength of oxide films on metallic substrates, in: K.L. Mittal (Ed.), Adhesion Measurement of Films and Coatings, vol. 2, 2001, pp. 277–290. [7] R.J. Von Gutfeld, Thermoelastic generation of elastic waves for non-destructive testing and medical diagnostics, Ultrasonics (1980) 175. [8] R.J. Dewhurst, D.A. Hutchins, S.B. Palmer, C.B. Scruby, Quantitative measurements of laser-generated acoustic waveforms, J. Appl. Phys. 53 (1982) 4064. [9] D.A. Hutchins, R.J. Dewhurst, S.B. Palmer, Laser generated ultrasound at modified metal surfaces, Ultrasonics 19 (1981) 103. [10] D.A. Hutchins, Mechanisms of pulsed photoacoustic generation, Can. J. Phys. 64 (1986) 1247. [11] M. Choquet, D. Lvesque, M. Massabki, C. Nron, N.C. Bellinger, D. Forsyth, C.E. Chapman, R. Gould, J.P. Komorowski, J.-P. Monchalin, Laser-ultrasonic detection of hidden corrosion in aircraft lap joints: results from corroded samples, in: D.O. Thompson, D.E. Chimenti (Eds.), Review of Progress in Quantitative Nondestructive Evaluation, vol. 20, AIP, 2001, p. 300. [12] G. Rosa, P. Psyllaki, R. Oltra, Simultaneous laser generation and laser ultrasonic detection of the mechanical breakdown of a coating-substrate interface, Ultrasonics 39 (2001) 355. [13] S.J. Davies, C. Edwards, G.S. Taylor, S.B. Palmer, Lasergenerated ultrasound: its properties, mechanisms and multivarious applications, J. Phys. D: Appl. Phys. 26 (1993) 329. [14] G. Rosa, M.-H. Nadal, R. Oltra, Towards a new non-destructiveevaluation of coating-substrate adhesion by laser-ultrasounds, Proc. 2000 IEEE International Ultrasonics Symposium, vol. 1, 2000, pp. 717–720. [15] A. Pajares, W. Lanhua, B.R. Lawn, N.P. Padture, C.C. Berndt, Mechanical characterization of plasma sprayed ceramic coatings on metal substrates by contact testing, Mat. Sci. Eng. A 208 (1996) 158. [16] S.H. Leigh, C.C. Berndt, Modelling of elastic constants of plasma spray deposits with ellipsoid-shaped voids, Acta Mater. 47 (1999) 1575. [17] C.K. Lin, C.C. Berndt, Measurement and analysis of adhesion strength for thermally sprayed coatings, J. Thermal Spray Technol. 3 (1994) 75. [18] A.G. Evans, Fracture mechanics determinations, in: R.C. Bradt, D.P.H. Haselman, F.F. Lange (Eds.), Fracture Mechanics of Ceramics, vol. 1, Plenum Press, 1974, pp. 17–48. [19] F. Beltzung, G. Zambelli, E. Lopez, A.R. Nicoll, Fracture toughness measurement of plasma sprayed ceramic coatings, Thin Solid Films 181 (1989) 407. [20] H.M. Hawthorne, L.C. Erickson, D. Ross, H. Tai, T. Troczynski, The microstructural dependence of wear and indentation behaviour of some plasma-sprayed alumia coatings, Wear 203/204 (1997) 709.
Laser induced decohesion of coatings: probing by laser ultrasonics G. Rosa a
a,*
, R. Oltra
a,*
, M.-H. Nadal
b
Laboratoire de Recherches sur la R eactivit e des Solides, UMR 5613 CNRS, Universit e de Bourgogne, UFR Sciences et Techniques, 9 Avenue A. Savary, B.P. 47 870, 21078 Dijon Cedex, France b CEA, Centre de Valduc, DRMN, 21120 Is-sur-Tille, France
Abstract The aim of the present study is to investigate a conventional laser-ultrasonics technique for the determination of intrinsic properties of oxide coatings and their adhesion strength on a metallic substrate. The good agreement between experiments and computations in an epicenter configuration allows determining the longitudinal wave velocity as well as the Young’s modulus of the oxide coatings versus the porosity. For a critical value of the laser energy, a breakdown at the coating–substrate interface is generated by the laser irradiation. The critical tensile stress field developed at the coating/substrate interface, which leads to the interfacial fracture, can be easily calculated. The value of the practical adhesion which is defined is found to be in accordance with those obtained by classic contact techniques (tensile adhesion test, indentation, bending test). Finally, this work demonstrates that this quantitative, contactless test fits well to simultaneously characterise the oxide coatings and evaluate the coating–substrate adhesion. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Oxide coatings; Laser ultrasonics; Adhesion
1. Introduction The laser-ultrasonic technique also defined as ‘‘laser ultrasonics’’ (contactless non-destructive testing method) can provide important information on materials characterisation [1,2], e.g. microstructural characteristics [3]. This technique may be also used to measure acoustic velocities, in ceramic–metal composites [4], in metals at elevated temperatures [5], whilst their greatest shortcoming is their poor sensitivity compared to conventional contact systems. If the laser-ultrasonic technique has been widely used for the different applications concerning bulk materials or composites, at ambient and elevated temperatures, in the case of the plasma sprayed coatings on a metallic substrate, only some experiments have been done with this technique [6]. However, the effect of a constraining layer on the generation efficiency of the longitudinal pulse and the shear pulse, within a metal
*
Corresponding authors. Fax: +33-3-8039-6132. E-mail address: [email protected] (R. Oltra).
irradiated by a laser beam has been monitored by detecting longitudinal and shear transducers respectively [7–10]. The aim of the present study was to investigate a laser-based technique for the determination of intrinsic properties of oxide coatings and their adhesion strength on a metallic substrate. With regard to adhesion characterization, conventional laser-ultrasonic technique has been used to detect debonding in multilayer structures and which involves an adhesion parameter between layers [11]. In this paper, the same approach was followed in terms of probing but the laser was also used for the generation of the decohesion of the coating itself. The acoustic response of a transparent and porous ceramic coating on a metallic substrate was studied, under different regimes of pulsed laser irradiation/material interaction (thermo-elastic interactions, fracture of the coating–substrate interface and coating expulsion) but in the absence of ablation and subsequent plasma formation. For the determination of intrinsic properties of coatings and their adhesion strength, a model was developed and validated in the case of alumina coatings deposited on stainless steel substrates by atmospheric plasma spraying (APS).
0041-624X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 1 - 6 2 4 X ( 0 2 ) 0 0 2 0 9 - 3
766
G. Rosa et al. / Ultrasonics 40 (2002) 765–769
2. Experimental Al2 O3 coatings of different thickness and of different pore volume fraction deposited on 5-mm thick stainless steel substrates, by APS were tested. The experimental arrangement based on conventional laser ultrasonics, used for the determination of intrinsic properties of alumina coatings and their adhesion strength, is described in a previous paper [12]. A nanosecond pulsed Nd:YAG laser (k ¼ 1064 nm), was used for the irradiation of the ceramic coatings and the acoustic signal was recorded in real time with the aid of a laser heterodyne interferometer (k ¼ 632 nm) placed at the free rear surface of the substrate. Experiments were carried out applying energies from 5 up to 30 mJ, which provided mean incident energy densities from 0.3 up to 1.7 J cm2 . At the wavelength of the laser irradiation used, all the alumina coatings examined were transparent, whilst the absorption depth was 21 nm below the ceramic/metal interface. Taking into account the value of the reflectivity (0.8), the laser energy densities which are absorbed at the inner ceramic/metal interface (from 0.06 up to 0.35 J cm2 ) are always below the ablation threshold value of the stainless steel (0.65 J cm2 ).
3. Results and discussion 3.1. Laser-material interactions When a laser source is used to irradiate a transparent ceramic coating deposited onto a metallic substrate, the pulsed laser irradiation creates an instantaneous thermal and acoustic source at the absorption depth of the substrate near the ceramic/metal interface. The propagation of the acoustic waves through the volume of the examined material induces the normal displacement of the free surface of the substrate. In some experiments, the specific time profile of the epicentral displacement generated by laser irradiation of a constraining layer consisting of a transparent material placed over the irradiated face of the target [13], or a polymer coating [10], has been clearly exhibited, but the relation between the normal displacement at the epicenter and the intrinsic properties of coatings and their adhesion has not been determined. The normal displacement at the epicenter (Fig. 1a) was recorded for a transparent coating on a metallic substrate using the contactless experimental arrangement. This typical waveform at the epicenter versus time (the origin of time is the time when the laser beam irradiates the system), exhibits multiple oscillations appearing at regular intervals. The first of the acoustic response recorded for an alumina coating on stainless steel substrate (Fig. 1b) was found to be indicative of the
Fig. 1. (a) Typical waveform recorded at the epicenter for a transparent coating on a metallic substrate. (b) Acoustic response corresponding to the arrival of the longitudinal wave for an alumina coating on a stainless steel substrate (coating thickness 150 lm, coating pore volume fraction 6%, Dt: 100 ns).
interaction between the laser radiation and the system examined. 3.2. Energy thresholds and spatial localization of the damage As detailed elsewhere [12], investigating the evolution of the amplitude of the first two peaks of the displacement (Fig. 2a) recorded at the epicenter, denoted L1 and L2 respectively, with the laser beam energy applied, it was found that, for coatings of the same thickness and porosity: (a) In the range of laser beam energies promoting thermo-elastic interactions, e.g. absorbed energies up to 3.5 mJ, the amplitude ratio (L2 =L1 ) was found to be constant. (b) In the range of laser beam energies leading to interfacial fracture, e.g. absorbed energies from 3.5 up to 4.8 mJ, the amplitude ratio (L2 =L1 ) slightly decreased with increasing laser beam energy. (c) In the range of laser beam energies generating expulsion of the coating, e.g. absorbed energies higher than 4.8 mJ, the second peak of displacement almost
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allowed to determine the period of time during which the event took place. As the interface damage can be detected from the evolution of the amplitude of the second longitudinal displacement (L2 ), this period corresponds to the time interval Ds between the arrival of the first two peaks (L1 , L2 ), as recorded in the acoustic signal. Ds depends on both the thickness and the longitudinal wave velocity of the coating. By example, the case of an 80-lm-thick alumina coating, with a pore volume fraction of 6%, the interfacial fracture takes place within an interval of 50 ns from the beginning of the laser pulse. 3.4. Intrinsic properties and practical adhesion estimation
Fig. 2. Evolution of (a) the amplitude ratio (L2 =L1 ) and (b) the interfacial area of debonding with the laser beam energy applied (coating thickness: 80 lm, coating pore volume fraction: 6%). Each point corresponds to a different irradiated area.
disappeared but no ablation phenomenon occurred. The amplitude ratio (L2 =L1 ) reached a null value and finally total expulsion of the coating took place. The above observations indicated that the amplitude ratio (L2 =L1 ) can provide a reliable criterion for the laser effects on the integrity of the irradiated systems and, consequently, it can be used for determination of the energy thresholds, which lead to interfacial fracture and to coating expulsion. Moreover, the debonded area (noted A) at the interface can be calculated [12] with the relation (1): A ¼ Sð1 bÞ
The stress field, which led to debonding, was estimated as the combined effect of a thermal and an acoustic source, both localised at the near interface region. Due to the Gaussian distribution of the laser beam, the most heavily loaded point of the interface was the one corresponding to the center of the irradiated area of the coating (or to the epicenter of the coated system). Thus, the following analysis was focused on the evolution of the stresses, which were developed at this point during the previously determined interval Ds. The stresses were determined by a semi-analytical model [14], adapted to a coated substrate, suitable to calculate the in- and out-of-plane components of the mechanical displacement versus time. The modelling combines the classical elasticity equations and is valid both for the thickness of the coating and the substrate. The two layers are assumed to be perfectly joined at the interface. The components of the displacement for an epicenter configuration were measured and are compared with computations for the set of samples (Fig. 3). The good agreement between experiments and computations in an epicenter configuration allows determining the mechanical, thermal and optical characteristics of the ceramic coating.
ð1Þ
where S is the irradiated area and b is defined by fitting of the experimental waveform with the waveform calculated by an analytical model [12]. As an example, in Fig. 2b the debonded area are plotted against the laser beam energy applied and the energy absorbed by the substrate, for the case of a 80lm-thick alumina coating with a porosity of 6%. 3.3. Time delay for the damage generation For laser beam energies resulting in interfacial fracture, e.g. absorbed energies from 3.5 up to 4.8 mJ, the real-time monitoring of the interfacial debonding [12]
Fig. 3. Comparison of the normal component of the displacement at the epicenter between the experiment ( ) and the model (—) for alumina coating with a thickness of 80 lm.
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G. Rosa et al. / Ultrasonics 40 (2002) 765–769
3.4.2. Calculation of the fracture toughness of the interface From the rzz determination, the fracture toughness of the interface, as expressed by the critical stress intensity factor (KIC ), was calculated for a laser beam energy of 20 mJ, taking into account the size of the debonding area (Fig. 2b) and using the relation (2): KIC
Fig. 4. Dependence of the velocity of the longitudinal waves and of the Young’s modulus of the ceramic coatings on the pore volume fraction.
For example, in Fig. 4, the so-obtained velocity of the longitudinal wave, as well as the Young’s modulus of the coatings were plotted versus the coating porosity. These values were found to be in the same order of magnitude as obtained in other works on the characterisation of thermal-sprayed ceramic coatings [15,16]. Those parameters are used for the modelling of the components of stress field at the interface ceramic coating/substrate. 3.4.1. Calculation of stresses due to thermal and acoustic effects By computing rzz , the stress value involving the interfacial fracture is given by the tensile stress (positive value). For an 80-lm-thick alumina coating, the interfacial fracture takes place within an interval Ds of 50 ns from the beginning of the laser pulse; the stress component is 13 MPa at 12 ns (Fig. 5). We have to note here that the interfacial fracture is not taken into account in the model.
Fig. 5. Temporal evolution of the normal stress (rzz ), at the point of the interface corresponding to the center of the area (alumina coating with a thickness of 80 lm).
pffiffiffi 3Pl a ¼Y 2BW 2
ð2Þ
where P is the normal load resulted in debonding; a, the crack length and Y, l, B, W, factors correlated to the geometry of the specimen and the loaded area [17]. The so-obtained value of KIC (0.3 MPa m1=2 ) of plasma-sprayed alumina coatings onto metallic substrates is in excellent agreement with other values, concerning three points bending and double cantilever beam tests [18]. However, comparing with the results obtained by scratch test [19] and indentation technique [20], this value is low. This fact could be attributed to the ceramic-like behavior of the interface: it presents low mechanical strength in tensile loading (proposed technique, three points bending and double cantilever beam tests) and high mechanical strength in compressive loading (scratch test and indentation technique). The critical strain energy release rate (GIC ) was calculated using the typical relation (3) and found to be 2.86 J m2 , value in the same order of the values obtained by three points bending test [18]. 2 GIC ¼ KIC
1 v2 E
ð3Þ
Compared with other tests used for the estimation of the adhesion of plasma-sprayed coatings, the proposed technique presents the advantages of a rapid and noncontact application, providing information not only about the strength of the interface, but also about the Young’s modulus of the coating and/or its porosity. The proposed technique could be applied on specimens of various dimensions, whilst the commonly used tensile and three points bending tests require specimens of a standard geometry. It could be applied for coatings having thickness in a large range, from 30 up to 400 lm, whilst the indentation test is suitable only for thick coatings (>300 lm). The real-time recording of the interfacial damage and the precise determination of the debonding area eliminate the step of the posterior observation, required in the cases of indentation or laser spallation techniques. Finally, its direct application on the system to be examined, without the need of a bonding agent, as in the case of tensile adhesion test (ASTM C633-79), allows testing coatings of elevated porosity.
G. Rosa et al. / Ultrasonics 40 (2002) 765–769
4. Conclusions In the present study, the potential of a contactless laser technique for the characterisation of transparent ceramic coating on a metallic substrate has been shown. The proposed technique is based on the simultaneous laser induced and laser detection of damages in such coated systems. The irradiation of transparent ceramic coatings using nanosecond pulsed laser radiation, combined with the in real-time recording at the epicenter of the laser-generated waves, allows the determination of the intrinsic properties of the coating, as well as the in real-time detection of laser-generated interfacial damages. The critical tensile stress field developed at the coating/substrate interface, which leads to interfacial fracture, can be easily determined as the combined effect of thermal and acoustic stresses. Compared to the conventional adhesion tests commonly used until now, the proposed technique presents several advantages, concerning the rapid and contactless application on coated substrates of various dimensions and it is suitable even for porous and relatively thin plasma-sprayed oxide coatings.
Acknowledgements The authors would like to express their deep thanks to C. Coddet and S. Costil of Universite de Technologie de Belfort—Montbeliard for the elaboration of the samples.
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