Viscoelastic mapping of spruce-polyurethane bond line area using AM-FM atomic force microscopy

Amplitude modulation-frequency modulation atomic force microscopy (AM-FM AFM) with the capability of mapping simultaneously the topography and the nanomechanical properties of materials provides elastic information including Young's modulus, and loss tangent as viscoelastic information. The AM-FM mode operating principle involves the excitation and detection of two cantilever resonances simultaneously. Amplitude modulation (AM), the first resonance, is used for tapping mode imaging, while a higher resonance mode is operated in frequency modulation (FM). At resonance, the cantilever frequency and phase respond sensitively to changes in sample properties to provide a direct measurement of substrate mechanical properties and compositional information that only requires tuning of the cantilever resonance to provide quantitative information. In this study for the first time, by using an AM-FM AFM, the viscoelastic properties of the spruce wood and polyurethane adhesive (PUR) bond line area as a part of cross laminated timber (CLT) system were investigated. It was found that PUR adhesive rarely penetrates into the wood, and in contrast to viscous properties (tan δ), the elastic properties increase from the wood part towards the adhesive part in the bond line. AM-FM AFM technique proved to be able to effectively map viscoelastic properties of bond line region.