Fracture behavior of MOF monoliths revealed by nanoindentation and nanoscratch

TL;DR

This study investigates the fracture behavior of MOF monoliths using nanoindentation and nanoscratch techniques, revealing shear faults as primary failure modes and highlighting the high crack resistance of MIL-68 and MOF-808 due to their nanostructure.

Contribution

It provides the first detailed analysis of crack initiation and propagation in MOF monoliths, emphasizing the role of nanostructure in mechanical resilience.

Findings

Shear faults are the main failure mechanism in MOF monoliths.

MIL-68 and MOF-808 exhibit high resistance to cracking.

Nanostructure contributes to enhanced mechanical stability.

Abstract

Monolithic metal-organic frameworks (MOFs) represent a promising solution for the industrial implementation of this emerging class of multifunctional materials, due to their structural stability. When compared to MOF powders, monoliths exhibit other intriguing properties like hierarchical porosity, that significantly improves volumetric adsorption capacity. The mechanical characterization of MOF monoliths plays a pivotal role in their industrial expansion, but so far, several key aspects remain unclear. In particular, the fracture behavior of MOF monoliths has not been explored. In this work, we studied the initiation and propagation of cracks in four prototypical MOF monoliths, namely ZIF-8, HKUST-1, MIL-68 and MOF-808. We observed that shear faults inside the contact area represent the main failure mechanism of MOF monoliths and are the source of radial cracks. MIL-68 and MOF-808 showed a remarkably high resistance to cracking, which can be ascribed to their consolidated nanostructure.