Fracture toughness of a metal-organic framework glass

Abstract

Metal-organic framework glasses feature unique thermal, structural, and chemical properties compared to traditional metallic, organic, and oxide glasses. So far, there is a lack of knowledge of their mechanical properties, especially toughness and strength, owing to the challenge in preparing large bulk glass samples for mechanical testing. However, a recently developed melting method enables fabrication of large bulk glass samples (>25 mm³) from zeolitic imidazolate frameworks. Here, fracture toughness (K_Ic) of a representative glass, namely ZIF-62 glass (Zn(C₃H₃N₂)_1.75(C₇H₅N₂)_0.25), is measured using single-edge precracked beam method and simulated using reactive molecular dynamics. K_Ic is determined to be ~0.1 MPa m^0.5, which is even lower than that of brittle oxide glasses due to the preferential breakage of the weak coordinative bonds (Zn-N). The glass is found to exhibit an anomalous brittle-to-ductile transition behavior, considering its low fracture surface energy despite similar Poisson's ratio to that of many ductile metallic and organic glasses.

Fig. 1. Molecular dynamics simulations of ZIF-62 glass structure and mechanical properties.

a Comparison between experimental (blue) and simulated (orange) differential correlation function (D(r)) of ZIF-62 glass averaged over eight performed quenches. Data have been shifted vertically by +2 for easier comparison. b Example of a stress (σ) vs. strain (ε) curve used in the estimation of the simulated ultimate strength (main figure) and modulus (inset). c Structural representation of an induced precrack in the glass network. Colored spheres represent carbon (red), hydrogen (gray), nitrogen (green), and zinc (blue). Cutoffs for bonds are 2.0 Å for C–C and C–N bonds, 3.0 Å for Zn–N bonds, and 1.5 Å for C–H bonds. d Simulated stress–strain curve of the precracked ZIF-62 glass (blue line, enlarged in inset) used in the estimation of fracture toughness. The green and orange lines represent stress–strain curves for disordered a-SiO₂ and calcium aluminosilicate, respectively. Note that these simulations featured a slightly different crack-to-box ratio (~0.33) compared to that in this study (~0.41). Source data are provided as a Source Data file.

Fig. 2. Fracture toughness and strength measurement of ZIF-62 glass.

a Example of an indented single-edge precracked beam (SEPB) specimen of ZIF-62 glass with dimensions of 1.5 × 1.9 × 10 mm³. The indentation line is enlarged and shown in the inset. b Post-fractured SEPB specimen. c Example of load-deflection curve of three-point bending on SEPB specimen. d Example of load-deflection curve of three-point bending on non-cracked specimen. Source data are provided as a Source Data file.

Fig. 3. Fracture mechanism of ZIF-62 glass and comparison with other material families.

a Structural representation of the crack propagation in the precracked ZIF-62 glass upon increasing strain (ε). Colored spheres represent carbon (red), hydrogen (gray), nitrogen (green), and zinc (blue). b Enlarged view of the Zn–N bond before (left) and after breaking (right). c Comparison of theoretically predicted and experimental fracture toughness (K_Ic) for a range of glass and glass-ceramic materials. The theoretical prediction is explained in the text. Figure is adopted with data from ref. , in addition to data for silicate and borate glasses, oxycarbide glass-ceramics, and the present ZIF-62 glass. All the experimental K_Ic values are from self-consistent methods such as SEPB, chevron notched beam, and surface cracked in flexure, with an error smaller than ±0.05 MPa m^0.5. d Relationship between fracture surface energy (γ) and Poisson’s ratio (ν) for a range of materials. Figure is adopted with the data from refs. ^,, and extended with additional data for metallic glasses^–, silicate glasses^,, borate glasses^–, chalcogenide glasses^,,, phosphate glasses^,, fluoride glasses^,, oxycarbide glasses and glass ceramics, tellurite glass, and the present ZIF-62 glass. e Ashby plot of the relation between K_Ic and Young’s modulus (E) for a range of materials. The figure is adopted with data from ref. and extended with that of the present ZIF-62 glass. Source data are provided as a Source Data file.