Fracture-induced amorphization of polycrystalline SiO2 stishovite: a potential platform for toughening in ceramics

Silicon dioxide has eight stable crystalline phases at conditions of the Earth's rocky parts. Many metastable phases including amorphous phases have been known, which indicates the presence of large kinetic barriers. As a consequence, some crystalline silica phases transform to amorphous phases by bypassing the liquid via two different pathways. Here we show a new pathway, a fracture-induced amorphization of stishovite that is a high-pressure polymorph. The amorphization accompanies a huge volume expansion of ~100% and occurs in a thin layer whose thickness from the fracture surface is several tens of nanometers. Amorphous silica materials that look like strings or worms were observed on the fracture surfaces. The amount of amorphous silica near the fracture surfaces is positively correlated with indentation fracture toughness. This result indicates that the fracture-induced amorphization causes toughening of stishovite polycrystals. The fracture-induced solid-state amorphization may provide a potential platform for toughening in ceramics.

| Results of X-ray diffraction measurements obtained at P02.1 at PETRA III, Germany. a, Representative X-ray diffraction patterns of stishovite polycrystals synthesized at different temperatures at 15 GPa. All the peaks shown in this figure are explained by stishovite. Note that peak width decreases with synthesis temperature. b, Data to show presence of a small amount of coesite (0.2 vol%) in a sample synthesized at 1200°C. Samples synthesized above 1300°C are pure stishovite polycrystals. c, Refined crystallite size as a function of synthesis temperature.  Stishovite polished surface, a polished surface of a stishovite poly-crystal synthesized at 1700°C. Stishovite fracture surface, a fracture surface of a stishovite poly-crystal synthesized at 1300°C. Amorphous backtransformed from St, a stishovite poly-crystal synthesized at 1600°C was heated up to 1000°C in air and the stishovite was transformed to an amorphous phase. Thin amorphous layers that are observed by TEY-XANES measurements on the polished and fracture surfaces cannot be detected by conventional Raman spectroscopy. The excitation laser is a Nd:YAG laser (wave length = 532 nm). All the spectra were obtained at Raman Spectroscopy Lab, GFZ Potsdam, Germany.   (c, d). a, a WC material, TF05 (Fujilloy, Vickers hardness (H V ) = 22.4 GPa, IF-K 1c = 6.2 MPa m 1/2 ). b, a transparent polycrystalline alumina synthesized using a spark plasma sintering equipment [S1, S2]: H V = 20.6 ± 0.3 GPa and 2.3 ± 0.1 MPa m 1/2 . c, a stishovite polycrystal synthesized at 1300°C. d, a stishovite polycrystal synthesized at 1900°C (c and d are the same as Fig. 5c and Fig. 5d in the main text. These are shown for comparison). All the indentation traces were obtained at an applied load of 98 N and the scale bars represent 100 m.

Details of XANES simulation
The XANES simulations were carried out by first-principles method with a core-hole effect included. [S3] The calculated models were based on 72-atom 2×2×3 and 2 ×2×2 supercell for stishovite and α-quartz, respectively. The atomistic structure of models were calculated with the projector augmented wave (PAW) method [S4] implemented in Vienna ab-initio simulation package (VASP) code. [S5, S6] The cut-off energy of the plane waves was set to 600 eV. The k-point sampling was carried out by Γ-centered 2×2×2 Monkhorst-Pack mesh. [S7] The exchange-correlational functional was given by the generalized gradient approximation proposed by Perdew, Burke and Ernzerhof (GGA-PBE).
[S8] The structural parameters were fully relaxed until the residual forces and stress became less than 0.03 eV/Å and 0.3 GPa. In this condition, the lattice constants were overestimated by about 2%, but this was usual GGA tendency. The convergence tests were carried out by comparing more severe conditions, 800 eV of cut-off energy and 4×4×4 k-mesh and the total energy was converged within 5 meV atoms.
Theoretical Si-K XANES spectra were calculated by augmented plane-wave + local orbital (APW+lo) method implemented in WIEN2k code [S9] with a core-hole effect included. The cut-off parameter, R MT ·K MAX , were set to 6.0 bohr·Ry 1/2 . The R MT s were set to 1.30 and 1.60 bohr for Si and O. The models, k-mesh and exchange-correlational functional were the same conditions as those in the structural calculations. The calculated XANES spectra were broadened by Lorentz function with the natural width of Si-K shell.
[S10] The transition energy was corrected via the alignment of the strongest peak of each substance. We also performed convergence tests by comparing more severe conditions, 7.0 bohr·Ry 1/2 and 4 ×4×4 k-mesh. It was confirmed that spectra were quantitatively well converged in the present conditions.