  The synthesis of metal-organic frameworks (MOFs) in a monolithic morphology
is a promising way to achieve the transition of this class of materials from
academia to industrial applications. The sol-gel process has been widely
employed to produce MOF monoliths. It is relatively cheap and simple compared
to other techniques (e.g., mechanical densification) and moreover it allows to
produce "pure" monoliths, i.e., without the need of using binders or templates
that could affect the functional properties of the MOF. Understanding the
mechanical properties of these monoliths is crucial for their transit to
practical applications. We studied the mechanical behavior of two zeolitic
imidazolate frameworks (ZIF-8 and ZIF-71) by means of instrumented
nanoindentation and atomic force microscopy (AFM). Tip Force Microscopy (TFM),
an extension of AFM, was used to reveal the surface nanostructure of the
monoliths. We employed finite-element (FE) simulations alongside the
experiments, to establish a suitable constitutive model and determine an
improved estimate of the yield stress of ZIF monoliths. NanoFTIR was
subsequently used to pinpoint local structural alteration of the framework in
the contact area. The combination of TFM, FE simulations, and nanoFTIR enabled
us to identify the mechanical deformation mechanisms in monolithic ZIF
materials: grain boundaries sliding is dominating at low stresses, then
breakage of chemical bonds and a partial failure of the framework occurs,
eventually leading to a densification of porous framework at the contact zone.
Finally, we measured the fracture toughness using a cube corner indenter to
study the resistance of monoliths against cracking failure.
