Role of friction and loading parameters in four-point bend adhesion measurements
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Kyunghoon Kim Department of Mechanical Engineering, Stanford University, Stanford, California 94305-2205
Christopher S. Litteken and Reinhold H. Dauskardta) Department of Materials Science and Engineering, Stanford University, Stanford, California 94305-2205 (Received 29 June 2007; accepted 12 September 2007)
The effects of salient testing parameters on four-point adhesion measurements of thin-film structures on silicon substrates were systematically studied. These included specimen geometry, applied displacement rate, and load point separation. Measured fracture energy values, Gc, were observed to increase as the ratio of applied moment arm to specimen thickness was decreased beyond a value of ∼4, particularly for specimens with Gc > 5 J/m2. Testing parameters that affect the steady-state crack velocity were also found to affect reported Gc values. The resulting trends in Gc values are shown to be related to loading-point friction and environmentally assisted cracking effects. Good practice testing guidelines are suggested to improve the accuracy and precision of four-point bend measurements.
I. INTRODUCTION
The four-point bend adhesion technique initially developed for bimaterial interface fracture energy measurements1,2 is a widely used experimental method for characterizing adhesion and cohesion in thin-film structures. 3 – 6 It provides accurate and reproducible measurements over a wider range of adhesion values than other thin-film adhesion techniques and has been applied to thin-film structures containing metals, 7 glasses,8,9 polymers,10 and even biomaterials.11 The technique allows determination of the interfacial fracture energy, Gc, and its dependence on the phase angle of loading, , which is a measure of the relative shear to tensile opening stresses at the crack tip. Within the framework of conventional fracture mechanics,12 the debond driving energy is represented by the strain energy release rate, G(), and the interface fracture energy is equated to the critical strain energy release rate Gc() at incipient crack growth. By varying the relative thicknesses and elastic properties of the substrate materials in the four-point specimen, a range of applied loading mode mixities can be obtained.1,2
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0001 J. Mater. Res., Vol. 23, No. 1, Jan 2008
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One major advantage of the specimen geometry in which the thin films are sandwiched between elastic substrates is that the constraint imposed by the substrates prevents residual stresses in the thin films from relaxing and contributing to the measured Gc values.3 Moreover, plastic deformation of ductile films is confined to the sandwiched thin-film stack. This allows for straightforward analytical determination of G in terms of global parameters, such as the applied load, loading geometry, and the elastic properties of the substrates. Thus the technique is well suited to the study of many thin-film technol
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