Shear strength measures a material’s resistance to forces causing layers or planes to slide past each other. For tungsten disulfide (WS₂), its shear strength is notably low, typically ranging from 0.1 to 0.2 GPa, a property that underpins its exceptional lubricity. This low value arises from its layered hexagonal structure: strong covalent W-S bonds within each layer contrast with weak van der Waals forces between layers, allowing easy interlayer sliding under shear stress.
The shear strength is quantified via tribological tests or nanoindentation, where a probe slides across WS₂ surfaces or penetrates layers. In-plane shear modulus (related to strength) is higher (~10-15 GPa), but interlayer shear dominates in bulk WS₂, aligning with its friction coefficient (0.03-0.06 in vacuum). Compared to MoS₂ (~0.1-0.15 GPa), WS₂’s shear strength is similar, though slight differences stem from tungsten’s higher atomic mass and bond energy. Nanoscale forms like WS₂ nanotubes exhibit higher effective shear strength (~1-2 GPa) due to curvature, but bulk values remain low.
Environmental factors influence shear strength minimally below decomposition (1250°C in inert conditions, 510°C in air). In vacuum, it’s stable, optimizing lubricity; in air, humidity slightly increases shear resistance (to 0.2-0.3 GPa) by adsorbing water, stiffening interlayer interactions. Oxidation above 510°C in air transforms WS₂ to WO₃, raising shear strength dramatically (~5-10 GPa) but eliminating its lubricating function. Load and sliding speed have modest effects—higher loads align layers, reducing shear strength slightly, enhancing performance.
This low shear strength is critical for WS₂’s use in bearings, gears, and coatings, where it forms a low-friction transfer film. Unlike structural materials (e.g., steel, shear strength ~200 MPa), WS₂ sacrifices strength for lubricity, making it unsuitable as a standalone mechanical component but ideal for reducing wear in dynamic systems.
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