From Ductile to Brittle: Defect-Engineered Mechanical Properties of Metal-Organic Frameworks

The introduction of defects in metal-organic frameworks (MOFs) is an effective method to improve the performance of MOFs in many applications, but it also compromises the mechanical properties of MOFs. Thus, a comprehensive understanding of the mechanical properties of defective MOFs becomes important for the defect engineering in MOFs. Herein, using the in situ compression tests, we directly observe very different mechanical responses in HKUST-1 MOFs with various defect concentrations. The elastic-plastic deformation followed by a ductile flattened failure is found in the defective HKUST-1 with a small defect concentration, while a hyperelastic-plastic behavior accompanied by the brittle fracture failure could occur in the HKUST-1 with a large defect concentration. The strong dependence of deformation and failure behaviors of defective HKUST-1 crystals on the defect concentration is ascribed to the change in their local deformation mechanism and stress distribution with varying defect concentration, according to the analysis by finite element and molecular dynamics (MD) simulations. Our compression experiments and MD simulations also indicate a significant reduction in both Young's modulus and yield strength of HKUST-1 with growing defect concentration, which agrees well with the theoretical predictions of micromechanics theory. This study is expected to provide a more precise understanding of the mechanical properties of defective MOFs.