Planar defects, widely observed in diamond and boron nitride, have been shown to markedly enhance hardness and toughness, yet the mechanical role of inversion domain boundaries (IDBs) in metastable wurtzite boron nitride (w-BN) remains largely unexplored. In this work, we present a comprehensive investigation of the mechanical response of IDBs, combining first-principles cleavage and stacking fault energetics with the Peierls-Nabarro framework of dislocation resistance. To resolve the mechanical response specific to the IDB-affected region, localized energetics are introduced in terms of area-normalized energy changes evaluated for cleavage or sliding events confined to the IDB region. The IDB-affected region acts as a weakening origin: both cleavage and slip resistances, for deformation either across the IDB or along the IDB, are reduced relative to bulk w-BN. Consequently, the IDB vicinity becomes the preferred site for crack initiation and dislocation activation. Electronic-structure analyses further reveal suppressed deformation-induced charge transfer across the IDB, which reduces the energetic cost of bond distortion, and less negative ICOHP values indicate weakened covalent bonding at the IDB, thereby facilitating local failure. Together, these results provide a microscopic picture of how dense IDBs could contribute to the reduced strength and apparent hardness of w-BN, given that IDBs are ubiquitous in synthesized w-BN, even though an ideal, defect-free w-BN crystal may possess a high intrinsic hardness.

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• Defects
• Hardness
• Mechanical & acoustical properties
• Plastic deformation
• Plasticity
• First-principles calculations