Hybrid coupling layers for bulk metallic glass adhesion
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Solutions to the technical challenge of bonding and joining bulk metallic glasses have long been sought after due to the exceptional property sets displayed by this class of engineering materials. Here, we demonstrate the ability to deposit a compositionally and functionally graded hybrid coupling layer using sol–gel processing methods to promote adhesion at the metallic glass–epoxy interface. In this study, we fine-tune the molecular composition by varying the sol Zr:Si ratio, altering film properties that consequently influence crack path selection at the interface. When optimized, up to 3-fold improvements in the adhesive/cohesive properties of these structural bond lines can be attained, with the highest GC values correlating with cohesive cracking through the hybrid. We also demonstrate the ability of these hybrid structures to significantly reduce the influence of moisture-assisted degradation as evidenced by reductions in crack growth rates of over two orders of magnitude and increased threshold limits.
I. INTRODUCTION
Bulk metallic glasses have demonstrated significant potential for use in advanced engineering applications due to their extraordinary properties including high strength, moderate toughness, good wear resistance, and high strength-to-weight ratio.1–3 However, integration of these materials into engineering structures has been limited by challenges in adhesion and joining. Conventional welding techniques lead to recrystallization of the amorphous glass local to the weld spot, whereas bolted joint techniques introduce stress concentration sites which drive shear localization, fatigue, and related fracture processes. Standard epoxy bonding techniques have been used to limited success; however, recent developments in compositionally graded hybrid materials may be capable of achieving the robust and reliable bonds required for high-performance structural applications. Hybrid organic–inorganic thin films processed via sol– gel chemistry have demonstrated novel properties and functionalities4–6 that lend themselves to use in a wide variety of applications including adhesive coupling layers for structural joints and laminates.7–11 By fine tuning synthesis and processing parameters, hybrid thin films can be developed to exhibit high-performance adhesion capable of withstanding in-service stresses and degradation from environmental stresses. In the present study, we synthesized and deposited a compositionally and functionally graded hybrid coupling film from a metal alkoxide and an epoxy-functionalized silane, which serves as an adhesion promotion layer a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.331 3164
J. Mater. Res., Vol. 28, No. 22, Nov 28, 2013
http://journals.cambridge.org
Downloaded: 01 Dec 2014
between the metallic glass substrate and epoxy resin. Using the well-established double cantilever beam (DCB) testing method, we investigated the effects of Zr:Si precursor ratios on the debond energy of the metallic glass–hybrid–epoxy interface and corr
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