In situ microscopic surface characterization studies of polymeric thin films during tensile deformation using atomic for
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Atomic force microscopy (AFM) has been used to study the cracks developed on thin-film coatings on a polymer substrate subjected to external tension. To conduct in situ tensile tests in AFM, a special stage has been built. A new technique to image the same control area at different strains was developed and used to study the propagation of a crack with increasing strain in magnetic tapes. Metal particulate tapes developed numerous cracks of shorter length, perpendicular to the loading direction. In contrast, metal-evaporated tapes developed cracks that extend edge to edge. The variation of the crack width and the spacing with strain were measured and explained with the help of models based on elasticity.
I. INTRODUCTION
A thin film of higher elastic modulus on a lower modulus substrate such as polymer is often encountered in many applications. For example, magnetic recording tapes1 are a composite of either polymer or metal on a polymer substrate; transparent gas barrier films2 are formed by a ceramic coating on a polymer and tribological coatings of diamondlike carbon (DLC) coatings3 on metallic substrate. When subjected to stress, the substrate could undergo a significant extension before failure. The coating cannot match the substrate stretching and fails at strains lower than the failure strain of the substrate. The failure could be either adhesive or cohesive. In adhesive failure, the coating delaminates, whereas in cohesive failure, multiple cracking of the film occurs. In multiple cracking, the coating cracks perpendicular to the direction of the applied tensile stress. Some of the parameters that affect crack spacing are the thickness of the coating and the substrate and the relative elastic moduli. Multiple cracking has been studied with the optical microscope by Wojciechowski and Mendolia4 in metallic coating on a polymer substrate and by Yanaka et al.2 in ceramic coating on the polymer substrate. They observed that the spacing between the cracks is roughly uniform and decreases with increasing strain. Wojciechowski and Mendolia4 applied the shear lag model developed by Averston and Kelly5 for multiple fractures of brittle fibers in fiber reinforced composites. Yanaka et al.2 modified this model to take into consideration the intrinsic stresses present in the film as a result 844
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J. Mater. Res., Vol. 16, No. 3, Mar 2001 Downloaded: 18 Mar 2015
of coating process. Volynskii et al.6 used scanning electron microscopy (SEM) to study the multiple cracking of platinum film on a polyethylene terephthalate (PET) substrate. Bordet et al.7 studied the effect of substrate roughness on the cracking behavior of thin films by using in situ tensile tests in SEM. In this study we developed a technique using the atomic force microscopy (AFM) and apply it to study the multiple cracking in various magnetic tapes. Sub-nm resolution of the AFM allows observations of nano-scratches and precludes any sample preparations. Magnetic tapes consist of a polymeric substrate, generally PET with a thi
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