Molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) are the two most widely used layered-structure solid lubricants in industrial applications. Both belong to the transition metal dichalcogenide family and share a similar S-Me-S layered crystal structure, but they differ significantly in friction coefficient, thermal stability, load-bearing capacity, and cost. In applications such as vacuum lubrication, high-temperature conditions, and heavy-load equipment, the correct choice between MoS₂ and WS₂ directly affects equipment reliability and maintenance costs.

Crystal Structure and Lubrication Mechanism Comparison

Both MoS₂ and WS₂ belong to the hexagonal crystal system 2H phase, composed of S-Me-S trilayers stacked with weak van der Waals forces between layers. The interlayer spacing of MoS₂ is approximately 0.615 nm, and WS₂ is approximately 0.618 nm. The lubrication mechanism is identical for both: under shear force, interlayer sliding occurs, converting macroscopic friction into microscopic interlayer sliding to achieve low friction.

Key physical parameters of both materials:

- MoS₂: density 4.80-5.06 g/cm³, Mohs hardness 1.0-1.5, friction coefficient 0.02-0.06

- WS₂: density 7.50 g/cm³, Mohs hardness 1.0-2.0, friction coefficient 0.04-0.08

WS₂ has approximately 50% higher density than MoS₂. At the same volume addition in grease, WS₂ requires greater mass. When adding 3% by mass in grease, the actual volume concentration of WS₂ is lower than MoS₂, requiring volume compensation.

Thermal Stability Differences

Thermal stability is a core selection criterion for solid lubricants. MoS₂ begins oxidizing to MoO₃ at 350°C in air and can be used stably up to 1100°C in vacuum. WS₂ offers superior heat resistance, with oxidation onset at approximately 450-500°C in air and stability up to 1200-1300°C in vacuum.

In aerospace vacuum applications, WS₂'s high-temperature stability advantage is pronounced. The European Space Agency (ESA) Lubricoat project demonstrated that WS₂ coatings maintained friction coefficients below 0.04 after 10,000 sliding cycles in vacuum at 10⁻⁶ Pa. However, in humid air environments, WS₂ friction coefficients rise to 0.10-0.15, while MoS₂ maintains 0.06-0.10 under the same conditions, and MoS₂'s moisture sensitivity can be effectively suppressed by adding antioxidants such as Sb₂O₃.

Load-Bearing Capacity and Wear Rate

Four-ball test (ASTM D2596) data shows that lithium grease with 3% MoS₂ achieves a sintering load (PD) of 3088N, while the same addition of WS₂ achieves 2695N — MoS₂'s extreme pressure performance exceeds WS₂ by approximately 15%. This difference relates to MoS₂'s lower interlayer shear strength — approximately 25 MPa for MoS₂ versus 35 MPa for WS₂.

In reciprocating wear tests, MoS₂ wear rate decreases logarithmically with increasing load, maintaining stable lubrication at contact stresses above 1500 MPa. WS₂ exhibits faster initial run-in characteristics, forming transfer films approximately 30% faster than MoS₂, but MoS₂ transfer films demonstrate longer service life in continuous operation.

Cost and Market Supply

Global annual production of MoS₂ is approximately 4,000-5,000 tons, with major producers in China, the United States, and Chile. WS₂ global annual production is only about 200-300 tons, with highly concentrated supply. In terms of pricing, industrial-grade MoS₂ (98-99% purity) costs approximately 30-60 RMB/kg, while WS₂ (99% purity) costs approximately 300-500 RMB/kg — nearly 10 times higher.

This price differential dictates the division of application domains between the two materials. MoS₂ is widely used in automotive grease, powder metallurgy, plastic modification, and coatings for large-scale industrial applications. WS₂ is primarily used in aerospace fastener coatings, vacuum equipment bearings, and high-end racing components where cost sensitivity is low.

Selection Recommendations

Choose MoS₂ for: grease additives at ambient to 350°C, powder metallurgy self-lubricating bearings, plastic wear-resistant modification, and dry film lubrication coatings. Choose WS₂ for: high-temperature vacuum environments (>350°C), aerospace fastener anti-seize agents, and conditions requiring higher oxidation resistance temperatures. For applications requiring both low cost and high-temperature stability, consider mixing MoS₂ and WS₂ to leverage the synergistic effects of both materials, balancing performance and cost.

Tags: molybdenum disulfide tungsten disulfide MoS2 WS2 solid lubricant performance comparison 摩擦系数 thermal stability extreme pressure anti-wear