Density functional theory calculation of ideal strength of SiC and GaN: Effect of multi-axial stress

[Display omitted] •Ideal (theoretical) strengths of SiC and GaN under multi-axial loading are evaluated.•Tensile and shear ideal strengths for various crystal orientations are obtained.•Strong effect of multi-axial loading condition on critical stresses is revealed.•The results give valuable insights to fracture and deformation of SiC and GaN. First-principles density functional theory calculations are performed to quantitatively evaluate the ideal strength of SiC and GaN crystals. The effect of transverse normal stress on the tensile strength and that of hydrostatic stress on the shear strength are clarified for various crystal structures: 3C, 2H and 4H for SiC, zincblende (Z) and wurtzite (W) for GaN. The effect of polytype SiC structures on the shear strength of some shear systems appears to be significant, while that on the tensile strength is marginal. While the ideal tensile strength generally decreases by transverse (perpendicular) tension, large transverse compression reduces the strength in some structures and orientations ([110] tension in 3C–SiC and Z–GaN; [21¯1¯0] tension in 2H–SiC, 4H–SiC and W–GaN) due to structural phase transition. Although SiC and GaN are basically brittle materials, our investigation suggests that not only cleavage by tension but also sliding due to shear stress take place depending on the multi-axial stress condition.