MEASURING THIN FILM INTERFACE STRENGTH BY LASER INDUCED STRESS WAVES

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1 MEASURING THIN FILM INTERFACE STRENGTH BY LASER INDUCED STRESS WAVES Rahul R. 1, R. Kitey 2 Department of Aerospace Engineering IIT Kanpur, Kanpur, India ABSTRACT A hybrid experimental/numerical approach is applied to assess the dynamic interface strength of epoxy film, deposited on fused quartz substrate. Interfacial failure is initiated by subjecting substrate/film interface to high amplitude short duration stress wave, developed using pulsed laser ablation technique. The magnitude of loading is controlled by varying the incident energy of laser pulse. Optical micrographs show the development of cracks along with a failure envelop in the epoxy layer at failure initiation. The size of failure domain increases with increasing laser fluence. The tearing failure is observed with partially or completely blown out epoxy film at higher laser fluences. The substrate stress history is obtained by performing interferometric measurements on fused quartz/aluminum calibration sample. A one dimensional transient stress analysis is conducted to study the wave propagation in fused quartz/epoxy specimen where the substrate stress history is imposed as the boundary condition. The fused quartz/epoxy interface stress corresponds to the failure initiation (adhesion strength) is inferred from the simulations. Keywords thin film; laser spallation; interface strength; film interface; wave propagation. NOMENCLATURE : Wave length of laser beam t : time I : Intensity of interference fringes N : Fringe order : The density of layered material C d : Longitudinal wave speed sub : Substrate stress int : Interface stress 1 Sr. Project Associate 2 Assistant Professor and corresponding author, kitey@iitk.ac.in (Ph: , Department of Aerospace Engineering, IIT Kanpur India )

2 I. INTRODUCTION Adhesive bonded joints are extensively used in aerospace applications since several aircraft components are smaller in sizes and/or have complex shapes which cannot be fastened to another structure by riveting, bolting or welding techniques. The reliability of these joints is inevitably governed by the interfacial properties such as the adhesion strength and the interfacial fracture toughness. The failure behavior of such multilayer systems further complicates when the interfaces are subjected to extreme dynamic loading conditions in which case the interface and the bond layer geometries in addition to the properties of the constituent layers play significant roles in fracture mechanisms. Conventional adhesion measurement techniques such as lap shear (ASTM D1002), peel/t-peel (ASTM D1876) or butt joint tests only provide average macroscopic or sometime just the qualitative information about the failure pattern. In no way they give insights into the microscopic details of the fracture mechanisms. There exist other techniques, such as scratch [1-3], peel [4], stud-pull [5], blister [6], four-point bend [7,8] or indentation [9,10] test, where the interface failure parameters of a thin layer can be independently characterized. Although simple to perform, these tests subject the interface to very high stress level that result in large amount of plastic deformation (especially in peel test). The resulting measurements tend to be mostly qualitative because the stress fields are difficult to analyze and separate from the measured adhesion energy. The characterization is very difficult, often impossible, when the films are ductile or strongly adhered. The commonly used nano-indentation and four-point bend techniques also possess several complications when applied to measure interfacial fracture toughness. In many cases the failure at the interface-of-interest in four-point-bend test could not be induced due to a lower cohesive toughness of the film compared to its interfacial adhesion [11]. The inherent plasticity involved in nano-indentation complicates quantitative analysis, especially when multilayer films are under consideration. Also, the fracture response during experiments was seen to be dependent upon the film thickness and the residual stresses. In contrast to the above mentioned conventional test methods where the test samples are loaded quasi-statically, laser-generated high amplitude acoustic stress pulse dynamically loads the film interface in non-contacting mode in the laser spallation technique [12 23]. The stress pulses with a sharp temporal rise and fall times subject the thin film interface to strain rates of the order of 10 7 /s. The laser spallation method was first introduced by Yang [12] and Vossen [13]. Gupta and coworkers [14-17] further developed the technique and coupled it with interferometry to obtain thin

film interfacial strength. Wang and Sottos [18] and Kitey and co-workers [22] have demonstrated that the method could be adapted to load the film interface in both mixed-mode and pure shear.

3 film interfacial strength. Wang and Sottos [18] and Kitey and co-workers [22] have demonstrated that the method could be adapted to load the film interface in both mixed-mode and pure shear. The method was applied in past to characterize metallic, ceramic and polymer film interfaces [12-23] with the film thickness ranging from few hundred nanometers to few millimeters. In current investigation the adhesion strength of epoxy layer deposited on fused quartz substrate is quantified by using hybrid experimental/numerical approach. First a failure is initiated at fused quartz/epoxy (FQ/EP) interface by employing laser induced stress waves. The stress history in the substrate of a fused quartz/aluminum (FQ/Al) calibration sample is separately obtained by performing interferometric measurements for the same loading configuration (critical laser fluence) that cause a failure to induce in FQ/EP specimen. The obtained stress history is then used as a boundary condition in a one dimensional wave propagation analysis to infer the stresses at FQ/EP interface at the onset of failure. II. EXPERIMENTAL SETUP AND MEASUREMENTS A. Optical measurements A detailed schematic of the laser spallation setup along with a Michelson Interferometer is shown in Fig.1. The specimen, illustrated in the figure, consists of a substrate S with a film F on top and the backside of the substrate has a 400 nm thick aluminum film constrained by a 2 m waterglass layer. A 5 ns Gaussian pulse of variable energy content (0-300 mj) from Q-switched Nd:YAG laser ( = 1064 nm) is focused onto the absorbing (aluminum) layer using an infrared achromatic lens. The ablation of aluminum layer develops a compressive acoustic pulse in the Reference mirror IR lens S F Optical lens Ar-ion laser Nd:YAG laser Thin film Specimen Beam-splitter Oscilloscope Photodiode Figure 1. Interferometric set-up for measuring (transient) free surface displacements.

4 substrate with a shape similar to the Gaussian profile of the incident Nd:YAG laser pulse. The substrate pulse, transmitted across the substrate/film interface, reflects back as a tensile wave upon reaching the free surface of the film, subjecting the interface to mode-i loading. The laser energy is gradually increased to amplify the developed interfacial stresses until the interfacial failure is instigated. In situ out-of-plane displacement measurement at the free surface of the film is performed by focusing a continuous laser beam at the point-of-interest (See, Fig. 1). The reflected beam from the specimen surface produces interference fringes when combined with unperturbed laser beam reflected from the reference mirror. The variation of fringe intensity I(t), recorded using a photodiode and a 2.5 GHz ultra high frequency oscilloscope, is related to the fringe order N(t) by, where I t I I I I (1) 2 2 max min max min ( ) sin 2 N( t) Imax and Imin are maximum and minimum intensities of the fringe in consideration, respectively. The out-of-plane displacement history, u(t), is calculated from the Doppler shift [24], u( t) N( t), (2) 2 where is the wave length of the probe beam (514 nm) Since, the time required for the wave to propagate through the film is significantly smaller compared to the duration of the pulse, the following equations based on thin film assumptions is employed to evaluate substrate stress ( sub ) and interface stress histories ( int ) [18], sub 1 2 t C t h int film 2 d 2 u t sub u, (3) t (4) The above mentioned quantitative stress analysis cannot be applied directly to assess the interface stress of the epoxy layer due to its non-reflective surface. Therefore, first the interferometric measurements are performed on FQ/Al calibration sample and then the acquired (substrate) stress profile is used in the wave propagation analysis (described below) for determining the interface stress history.

B. Wave propagation analysis A one dimensional transient finite element model is developed to study the wave propagation in layered specimen.

Instead, the substrate stress history measured directly from calibration experiments is imposed as boundary condition for assessing interface stresses.

The specimen geometry is meshed using elements of size h/100 where h is the thinnest layer dimension (h = 50 m in current simulations).

The simulations are performed using a time step t/5 where t is the minimum of wave travel time in the elements of considered layers.

5 B. Wave propagation analysis A one dimensional transient finite element model is developed to study the wave propagation in layered specimen. The non-linear characteristics of fused quartz are not explicitly included in the analysis. Instead, the substrate stress history measured directly from calibration experiments is imposed as boundary condition for assessing interface stresses. The top and the bottom surface of the layered specimen are assumed in plane strain with traction free boundary conditions. The specimen geometry is meshed using elements of size h/100 where h is the thinnest layer dimension (h = 50 m in current simulations). The element size is chosen based on a convergence study. An explicit direct integration scheme with a forward difference time steps is applied for transient analysis. The simulations are performed using a time step t/5 where t is the minimum of wave travel time in the elements of considered layers. The layer properties used in the interferometric measurements and in the computational analysis is tabulated in Table 1. (a) Failure initiation zone (b) Cracks in the epoxy layer Zone of interfacial fracture Initial cracks in the epoxy layer 100 m 200 m (c) FQ substrate (d) Epoxy film 200 m 500 m Figure 2. Effect of laser fluence on the failure of FQ/EP interface. The illustrated micrographs correspond to the, (a) 64 mj/mm 2, (b) 75 mj/mm 2, (c) 86 mj/mm 2, and (d) 113 mj/mm 2 incident Nd:YAG laser fluence.

6 Substrate Stress ( GPa ) Interface Stress (MPa) Photodiode Output ( mv ) Displacement ( nm ) III. RESULTS AND DISCUSSION The epoxy, prepared by mixing diglycidyl ether of bisphenol-a (DGEBA) resin with methyl tetra hydrophthalic anhydride (TETA) curing agent, is spin coated on top of 1 mm thick fused quartz disk. The laser fluence is gradually increased until the failure is detected. The micrographs of laser induced interfacial failure in 50 m thick epoxy layer are shown in Fig. 2. The crack development in the epoxy layer is first observed at 64 mj/mm 2 laser fluence (See, Fig. 2(a)). The close looped dark spot around the crack indicates failure at the FQ/EP interface. The failure zone is better visible in the next image (See, 2(b)) that corresponds to the comparatively higher laser fluence, 75 mj/mm 2. A partial and completely blown out epoxy film is observed when the laser fluence is further increased to, 86 mj/mm 2 (Fig. 2(c)) and 113 mj/mm 2 (Fig. 2(d)), respectively. 8 7 (a) Fringe Intensity (b) time (ns) (c) time (ns) (d) time (ns) time (ns) Figure 3. Interferometric measurements performed on a fused quartz/aluminum calibration sample. (a) Displacement fringes recorded using photo-diode, (b) corresponding out-of-plane displacement history, (c) the associated substrate stress history ( sub ), and (d) the interface stress history ( int ) at fused quartz/epoxy specimen computed by performing one dimensional transient wave propagation analysis.

7 The interferometric measurements, shown in Fig. 3, are from the calibration experiment performed on FQ/Al specimen at 64 mj/mm 2 laser fluence. A sharp turn-around point at ~ 318 ns in the recorded interferogram (Fig. 3(a)) indicates the arrival of decompression shock [18-23]. The out-of-plane displacement history (Fig. 3(b)) at the free surface of the Al layer is calculated by analyzing interferometric fringes illustrated in Fig. 3(a), using Eqs. (1) and (2). The displacement monotonically increases until the turning point is reached. The substrate stress, sub, is evaluated from the displacement history by applying Eq. (3). The substrate stress history in Fig. 3(c) shows a characteristic linear ramp attaining a peak value of 0.36 GPa which is followed by a sharp rising stress curve signifying the arrival of decompression shock. Layer Density (kg/m 3 ) Longitudinal wave speed (m/s) Shear wave speed (m/s) Fused Quartz Epoxy Aluminum Table 1. Layer properties used in interferometric measurements and in the wave propagation analysis The interface stress history is then assessed by simulating wave propagation in the FQ/EP layered geometry. The computations are again associated to 64 mj/mm 2 laser fluence where the substrate stress history from Fig. 3(c) is applied as the loading condition in the simulation. Figure 3(d) shows the stress history at the FQ/EP interface. A negative interface stress builds when the substrate stress pulse reaches the interface. The shape of the interfacial stress between 144 and 155 ns is quite similar to the incident substrate stress pulse (Fig. 3(c)) except with a smaller magnitude. The compressive stress pulse upon reflection from the free epoxy surface reverses the sign, subjecting the interface to a tensile stresses between 173 and 185 ns. Negligible interface stress between 155 and 173 ns indicates that there is no wave interaction around at/in the proximity of the interface due to thick epoxy layer. The peak of tensile interfacial stress (~ 250 MPa) attained at ~ 180 ns, referred as the adhesion strength of the epoxy film. It should be noted that the damping effect and the scattering of energy in epoxy layer are neglected in the simulations. IV. CONCLUSION The dynamic interface strength of epoxy film, deposited on fused quartz substrate, is determined by employing laser spallation technique. A high amplitude short duration acoustic pulse is

8 developed in the substrate by pulsed laser ablation (Laser Spallation) technique for loading the film interface at ~10 7 /s strain rate. The laser fluence is gradually increased until the optical imaging indicates failure initiation. The size of the interfacial failure domain increases with increasing laser fluence. The tearing failure is observed with partially or completely blown out epoxy film at higher laser fluences. The stress history in fused quartz is obtained by performing interferometric measurements in a fused quartz/aluminum calibration sample. The strength of the failed interface is inferred from the one dimensional wave propagation analysis where the incident stress history corresponds to the critical laser fluence is imposed as loading condition. From the analysis the fused quartz/epoxy interfacial adhesion strength is determined to be ~ 250 MPa. ACKNOWLEDGEMENT Authors would like to thank Aeronautics Research & Development Board (ARDB) for supporting this research through grant DARO/08/ /M/I. REFERENCES [1] Laugier, M., The development of scratch technique for the determination of the adhesion of coatings, Thin Solid Films, vol. 76(3), pp , [2] Kinbara, A. and Baba, S., Adhesion measurement of non-metallic thin films using a scratch method, Thin Solid Films, vol. 163, pp , [3] Bull, S.J. and Rickerby, D.S., New developments in the modeling of the hardness and scratch adhesion of thin films, Surface and Coating Technology, vol. 42(2), pp , [4] Kim, K.S., and Kim, J., Elasto-plastic analysis of the peel test for thin film adhesion, J. Eng. Mater. Tech. Trans. ASME, vol. 110(3), pp , [5] Hull, T.R., Colligon, J.N. and Hill, A.E., Measurement of thin film adhesion, Vacuum, vol. 37, pp , [6] Hinkley, J., A blister test for adhesion of polymer films to SiO2, J. Adhesion, vol. 16, pp , [7] Vlassak, J.J., Lin, Y. and Tsui, T.Y., Fracture of ongoing glass thin films: environmental effects, Mater. Sci. Eng. A, vol. 391, pp , 2005.

9 [8] Tsui, T.Y., McKerrow, A.J. and Vlassak, J.J., The effect of water diffusion on the adhesion of organosilicate glass film stacks J. Mech. Phys. Solids, vol. 54, pp , [9] Kriese, M.D., Gerberich W.W. and Moody, N.R., Quantitative adhesion measures of multilayer films: Part I. Indentation mechanics, J. Mater. Res., vol. 14(7), pp , 1999a. [10] Kriese, M.D., Gerberich W.W. and Moody, N.R., Quantitative adhesion measures of multilayer films: Part II. Indentation of W/Cu, W/W, Cr/W, J. Mater. Res., vol. 14(7), pp , 1999b. [11] Dauskardt, R.H., Lane, M., Ma, Q. and Krishna, N., Adhesion and debonding of multi-layer thin film structures, Eng. Fract. Mech., vol. 61, pp , [12] Yang, L.C., Stress waves generated in thin metallic films by a Q-switched ruby laser, J. App. Phys., vol. 45(6), pp , [13] Vossen, J.L., Measurements of film-substrate bond strength by laser spallation. Adhesion measurement of thin films, thick films and bulk coatings, ASTM STP 640, American Society for Testing and Materials, Philadelphia, 1978, pp [14] Gupta, V., Argon, A.S., Cornie J.A. and Parks, D.M., Measurement of interface strength by laser pulse-induced spallation, Mater. Sci. Eng. A, vol. 125, pp , [15] Gupta, V. and Yuan, J., Measurement of interface strength by the modified laser-spallation technique: II. Application to metal/ceramic interfaces, J. App. Phys., vol. 74, pp , [16] Gupta, V., Yuan J. and Pronin, A., Recent developments in the laser spallation technique to measure the interface strength and its relationship to interface toughness with applications to metal/ceramic, ceramic/ceramic and ceramic/polymer interfaces, J. Adhesion Sci. Tech., vol. 8, pp , [17] Jain, A., Gupta, V. and Basu, N., A quantitative study of moisture adsorption in poly-imide and its effects on the strength of the polyimide/silicon nitride interface, Acta Mater., vol. 53, pp , [18] Wang, J., Sottos, N.R. and Weaver, R.L., Tensile and mixed-mode strength of a thin filmsubstrate interface under laser induced pulse loading, J. Mech. Phys. Solids, vol. 52, pp , [19] Kandula, S.S.V., Hartfield, C.D., Geubelle, P.H. and Sottos, N.R., Adhesion strength measurement of polymer dielectric interfaces using laser spallation technique, Thin Solid Films, vol. 516, pp , 2008.

10 [20] Wang, J., Weaver, R.L. and Sottos, N.R., A parametric study of laser induced thin film spallation, Exp. Mech., vol. 42(1), pp , [21] Wang, J, Weaver, R.L. and Sottos, N.R., Laser induced decompression shock development in fused silica, J. App. Phys., vol. 93(12), pp , [22] Kitey, R., Geubelle P.H. and Sottos, N.R., Mixed-mode interfacial adhesive strength of a thin film on an anisotropic substrate, J. Mech. Phys. Solids, vol. 57, pp , [23] Kitey, R., Sottos, N.R. and Geubelle, P.H., A hybrid experimental/numerical approach to characterize interfacial adhesion in multilayer low-k thin film specimens, Thin Solid Films, vol. 519, pp , [24] Barker, L.M., Laser interferometry in shock-wave research, Exp. Mech., vol. 12, pp , [25] Pierce, S.G., Culshaw, B., Philp, W.R., Lecuyer F. and Farlow, R., Broadband lamb wave measurements in aluminum and carbon/glass fiber reinforced composite materials using noncontacting laser generation and detection, Ultrasonics, vol. 35, pp , [26] Viktorov, I.A., Rayleigh and lamb waves: Physical theory and applications, New York Press, 1967.

Influence of Ductility Ingredients of Structural Adhesives on Fracture Energy under Static Mixed-Mode Loading 1. Introduction

contents Introduction xi CHAPTER 1 Influence of Ductility Ingredients of Structural Adhesives on Fracture Energy under Static Mixed-Mode Loading 1 Introduction 2 Fracture Energy Values in Adhesives 2 Methods