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Theoretical hardness calculated from crystallo‐chemical data for MoS2 and WS2 crystals and nanostructures

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Abstract:

The calculation of the hardness of Mo and W disulfides using a crystallo‐chemical model provides a unique opportunity to obtain separate quantitative information on the maximum hardness H max governed by strong intra‐layer covalent bonds acting within the (0001) plane versus the minimum hardness H min governed by weak inter‐layer van der Waals bonds acting along the c‐axis of the hexagonal lattice. The penetration hardness derived from fundamental crystallo‐chemical data (confirmed by experimental determinations) proved to be far lower in MS2 (M = Mo, W) than in graphite and hexagonal BN, both for H max (H graph/H MoS2 = 3.85; H graph/H WS2 = 3.60; H hBN/H MoS2 = 2.54; H hBN/HWS2 = 2.37) as well as for H min (H graph/H MoS2 = 6.22; H graph/H WS2 = 5.87; H hBN/H MoS2 = 4.72; H hBN/H WS2 = 4.46). However, the gap between H max and H min is considerably larger in MS2 (M = Mo,W), as indicated by H max/H min being 279 in 2H‐MoS2, 282 in 2H‐WS2, 173 in graphite and 150 in hBN. The gap was found to be even larger in MS2 (M = Mo, W) nanostructures. These findings help to explain the excellent properties of MS2 (M = Mo, W) as solid lubricants in high tech fields, either as bulk 2H crystals (inter‐layer shear and peeling off lubricating mechanisms), or especially as onion‐like fullerene nanoparticles (rolling/sliding mechanisms).

Document Type: Research Article

DOI: https://doi.org/10.1107/S0108768112033149

Affiliations: Faculty Materials Science and Engineering, University Politehnica of Bucharest, Bucharest 060042, Romania

Publication date: 2012-10-01

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