Normal Stress-Dependent Shear Behavior of Typical Cubic Covalent Crystals

The mechanical responses of some typical cubic covalent crystals under combined compression and shear loading are critical to their applications in extreme environments. Here, we investigated using first-principles calculations the pressure-dependent shear deformation behaviors of five representative cubic covalent crystals: diamond, cubic boron nitride (c-BN), silicon carbide (SiC), silicon (Si), and germanium (Ge). The results reveal strikingly different volumetric responses under shear: diamond and c-BN expand, whereas SiC, Si, and Ge contract. Increasing normal stress (σzz) strengthens the shear resistance in diamond and c-BN but reduces it in SiC, Si, and Ge. The ideal shear strength of diamond increases by 8.3% at σzz = 100 GPa, and that of c-BN rises by 4.4% at the same σzz. In contrast, the ideal shear strength of SiC decreases by 39.4% as σzz increases from 0 to 50 GPa, while Si and Ge exhibit reductions of 20.4% (0–10 GPa) and 7.3% (0–5 GPa). These contrasting behaviors stem from the distinct local bond rearrangements and shear transformation pathways. Ultrastrong-bonded covalent crystals (diamond and c-BN) deform mainly through a coordinated transition between shuffle and glide slip modes, while the crystals with moderately strong bonds (SiC, Si, and Ge) display prominent atomic rearrangements. This work clarifies the atomistic mechanisms by which pressure enhances or weakens the ideal shear strength of cubic covalent crystals, providing theoretical insights for tuning their mechanical performance and guiding structural design under complex stress conditions.