The interlayer shear strength of molybdenum disulfide (MoS2) is the core physical quantity that determines its solid lubrication performance. As a hexagonal layered crystal, MoS2 features S-Mo-S tri-layer structural units where intra-layer bonding is strong covalent, while interlayer bonding relies solely on weak van der Waals forces. This asymmetric bonding enables interlayer sliding under minimal shear stress. The interlayer shear strength of MoS2 is typically only 0.5 to 3.0 MPa, far below the fracture strength of intra-layer covalent bonds — this enormous disparity is the physical origin of its low-friction behavior.

I. Physical Mechanism of Interlayer Shear Strength

MoS2 crystals belong to the P63/mmc space group, with lattice parameters a=3.15 Å and c=12.30 Å, where the spacing between adjacent S-Mo-S layers is approximately 3.49 Å. The intra-layer Mo-S bond length is 2.41 Å with bond energy of several hundred kJ/mol, whereas the interlayer van der Waals interaction energy is only about 0.08 to 0.12 J/m² — a difference of roughly three orders of magnitude. When an external force is applied along the basal plane, interlayer van der Waals bonds break first, causing relative sliding between crystal planes, while intra-layer covalent bonds remain essentially unaffected. This deformation mode of "interlayer sliding without intra-layer rupture" causes the material to exhibit extremely low shear resistance at the macroscopic scale.

II. Relationship Between Shear Strength and Friction Coefficient

According to tribological principles, friction force F equals shear strength τ_s multiplied by real contact area A_r. The interlayer shear strength of MoS2 is only 0.5 to 3.0 MPa, approximately 1/50 to 1/100 of the shear strength of metal soap thickeners in conventional lubricating greases. Under identical loads, the real contact area remains relatively small due to the material's high elastic modulus (approximately 40 GPa). Together, these two factors determine the low friction coefficient of MoS2 — as low as 0.002 in vacuum environments, and approximately 0.02 to 0.06 in ambient air. Humidity is a critical external factor affecting shear strength: water molecules from the environment intercalate between layers, increasing interlayer bonding forces, which can raise shear strength by 2 to 3 times and correspondingly increase the friction coefficient.

III. Measurement Methods for Interlayer Shear Strength

Experimental determination of interlayer shear strength primarily employs nanoindentation scratch testing and tape peel methods. In nano-scratch tests, a diamond probe is drawn across the MoS2 surface under controlled loading, and the lateral force is measured to calculate shear strength. Published literature reports that the interlayer shear strength of 2H-MoS2 single crystals is approximately 2.8±0.3 MPa (Nature Communications, 2017). In polycrystalline powder compacts, measured values can drop to 1.0 to 2.0 MPa due to non-uniform grain orientation distributions. Density functional theory (DFT) calculations yield theoretical values of approximately 0.5 to 1.0 MPa, consistent with experimental values in order of magnitude, with differences primarily attributable to defects and grain boundaries in real crystals.

IV. Implications for Engineering Applications

In engineering practice, interlayer shear strength directly affects the wear life of MoS2 coatings. Sputtered MoS2 coatings exhibiting strong (002) basal plane preferred orientation — meaning interlayer slip planes parallel to the coating surface — can achieve friction coefficients 40% to 60% lower than randomly oriented coatings. Therefore, controlling deposition temperature, sputtering power, and substrate bias parameters to obtain preferred orientation is a critical pathway for enhancing lubrication performance in coating processes. In powder metallurgy self-lubricating bearings, the orientation consistency of MoS2 particles similarly affects the overall anti-friction performance of the material, and mixing and compaction processes should promote alignment of flake particle basal planes parallel to the friction surface.

Conclusion

The interlayer shear strength of MoS2 is determined by its S-Mo-S layered crystal structure and weak interlayer van der Waals bonding, with both theoretical and experimental values falling in the 0.5 to 3.0 MPa range. This value is approximately three orders of magnitude lower than the intra-layer covalent bond strength, constituting the microscopic physical foundation of MoS2's low friction coefficient. Understanding the relationships between interlayer shear strength and crystal orientation, environmental humidity, and material defects enables more targeted optimization of MoS2 lubrication performance in coating preparation and composite material design.

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