Key Takeaways & Executive Findings
- •• • DFT calculations reveal that water forms O–H···Se hydrogen bonds with WSe2 at bond lengths of 3.02–3.25 Å, which are significantly longer (weaker) than O–H···S bonds with WS2 (2.80–2.98 Å), leading to a 35% lower interlayer sliding energy barrier for WSe2 under humid conditions. • • Selenium doping of WS2 (W–S–Se coating) achieves an 18% reduction in coefficient of friction and a 78% decrease in wear rate at 40% relative humidity compared to pristine WS2, demonstrating a viable strategy to mitigate moisture-induced degradation. • • The improved tribological performance is attributed to weakened interfacial hydrogen bonding, which reduces the energy penalty for interlayer shear, as confirmed by DFT and sliding friction experiments. • • The W–S–Se coating forms a reoriented, crystalline transfer layer with insignificant oxidation, indicating that the lubricious mechanism is preserved even in humid air, offering a robust solution for solid lubrication in ambient environments.
Abstract
The degradation of the tribological performance of WS2 in humid environments represents a persistent scientific and practical challenge, limiting its application scope despite its excellent lubricity in inert atmospheres. While the superior moisture tolerance of WSe2 has been recognized, the fundamental atomic-scale mechanisms governing this difference remain inadequately understood. This work addresses this critical knowledge gap by revealing that this disparity originates from the distinct hydrogen bond strengths formed at the material–water interface. Through integrated density functional theory (DFT) calculations and experimental validation, we quantitatively demonstrate that water molecules form significantly weaker O–H···Se hydrogen bonds with WSe2 (bond lengths: 3.02–3.25 Å) compared to O–H···S bonds with WS2 (2.80–2.98 Å). This fundamental difference manifests functionally as a 35% lower interlayer sliding energy barrier for WSe2 under humid conditions, providing the first atomistic explanation for its sustained lubricity. Leveraging this mechanistic insight, we propose and validate a novel materials design strategy: selectively doping the WS2 lattice with selenium to engineer its interfacial chemistry. The developed W–S–Se coating exhibits remarkable performance, achieving an 18% reduction in the coefficient of friction and a 78% decrease in the wear rate at 40% RH compared to pristine WS2. Extensive characterization confirms the formation of a reoriented, crystalline transfer layer with insignificant oxidation. This study establishes a new paradigm for solid lubricant design, shifting the focus from conventional microstructure optimization toward direct atomic-level engineering of interfacial water interactions, opening avenues for developing advanced lubricants operable across diverse environmental conditions.
1. Introduction
Transition metal dichalcogenides (TMDs) such as WS2 and MoS2 are widely used as solid lubricants in aerospace and nuclear industries due to their low friction in vacuum and dry environments. However, their tribological performance severely degrades in humid air, with the coefficient of friction of WS2 increasing from 0.04 in dry nitrogen to 0.10–0.15 at 60% relative humidity. This moisture sensitivity has long been attributed to oxidation and water adsorption, but the atomic-scale mechanisms remain poorly understood, hindering the rational design of moisture-tolerant coatings.
This study identifies the root cause: the strength of hydrogen bonds formed between water molecules and the chalcogen atoms. Water forms stronger O–H···S bonds with WS2 than O–H···Se bonds with WSe2, resulting in a higher energy barrier for interlayer sliding in humid conditions. By selectively doping WS2 with selenium, the interfacial chemistry is engineered to weaken these hydrogen bonds, thereby preserving lubricity. The developed W–S–Se coating demonstrates a significant reduction in friction and wear at 40% RH, offering a new paradigm for designing solid lubricants that operate reliably in ambient environments.
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Yuqian Huang, Bin Zhang, Zhiwei Wang, Zaixiu Yang, Zhenwei Niu, Kaixiong Gao, Junyan Zhang, Goksel Hizli, Kürşat Kazmanlı, Ahmet T. Alpas (2026). Selenium-doped WS2 for improved humid-air lubricity via weakened interfacial hydrogen bonding. SinoTechIntel Verified Research. https://doi.org/10.26599/FRICT.2026.9441261
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Frequently Asked Questions
What is the fundamental mechanism by which selenium doping improves the lubricity of WS2 in humid air?
DFT calculations show that water forms weaker O–H···Se hydrogen bonds (bond lengths 3.02–3.25 Å) with WSe2 compared to O–H···S bonds (2.80–2.98 Å) with WS2. This weaker interaction reduces the interlayer sliding energy barrier by 35% under humid conditions, facilitating easier shear and lower friction.
How does the W–S–Se coating perform in terms of friction and wear compared to pristine WS2 at 40% RH?
The W–S–Se coating achieves an 18% reduction in coefficient of friction and a 78% decrease in wear rate relative to pristine WS2 at 40% RH, as measured in sliding friction experiments.
What is the role of the transfer layer in the tribological performance of the W–S–Se coating?
Characterization confirms the formation of a reoriented, crystalline transfer layer with insignificant oxidation. This layer maintains the low-shear characteristics of the TMD structure, contributing to the sustained low friction and wear resistance.
Are the DFT predictions experimentally validated, and what is the significance of the bond length differences?
Yes, the DFT predictions are validated by sliding friction experiments. The longer O–H···Se bonds indicate weaker hydrogen bonding, which correlates with a lower energy barrier for interlayer sliding and thus better lubricity in humid conditions.
What are the potential limitations or scalability challenges of selenium doping for industrial applications?
While the study demonstrates promising results, scalability depends on the availability and cost of selenium precursors and the ability to control doping uniformity in large-scale deposition processes. Further research is needed to optimize doping levels and assess long-term durability under varying humidity and load conditions.
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