Micromechanical behavior of polycrystalline metal - Organic framework thin films synthesized by electrochemical reaction

We have studied the mechanical properties of an archetypical metal-organic framework (MOF) polycrystalline thin-film material, termed HKUST-1 or Cu₃(BTC)₂, which was synthesized by means of electrochemistry. We demonstrate that the average crystal size and surface coverage of electrochemically grown thin films, with associated coating thickness and surface roughness, can be controlled by adjusting not only the reaction time but also the anodic substrate surface characteristics. The polycrystalline films were characterized via scanning electron microscopy, optical three-dimensional profilometry, atomic force microscopy, and X-ray diffraction. Using an instrumented nanoindenter, we performed fine-scale nanoscratch experiments under two distinct test modes: (i) ramp-load and (ii) pass-and-return (cyclic wear), to establish the underpinning failure mechanisms of MOF coatings with varied average thicknesses (∼ 2-10 μm). Our results reveal that the ramp-load approach is ideal to pinpoint the critical force required to debond films from the substrate, and the pass-and-return method has the propensity to crush polycrystals into a compacted layer on top of the substrate, but cause no film debonding even at a high number of cycles. Notably the film-to-substrate adhesion strength of electrochemical coatings could be enhanced with increasing HKUST-1 film thickness (∼μm), while the attachment of polycrystals is weakened when grown on smoother substrates.