Powder metallurgy (PM) self-lubricating bearings are functional components that utilize sintering to form interconnected pores in a metal matrix, enabling long-term self-lubrication after oil impregnation. The role of molybdenum disulfide (MoS₂) as a solid lubricant additive in these bearings goes far beyond simple friction reduction. In production practice, bronze-based oil-impregnated bearings with 1%-2% MoS₂ addition achieve a limiting pv value of 2.94 MPa·m/s (experimental data from Huang Zhaoxuan et al., Nanjing University of Science and Technology), with friction coefficients as low as 0.038—a performance level approaching that of some rolling element bearings.

The Triple Role of MoS₂ During Sintering

The functions of MoS₂ in PM bearings must be understood across three stages: powder mixing, sintering, and service.

**Mixing Stage: Improving Powder Flowability and Filling Uniformity.** The layered crystal structure of MoS₂ (S-Mo-S, with van der Waals bonding between layers) gives it an inherently low shear characteristic. During powder mixing, it reduces inter-particle friction among metal powders. When iron-based or copper-based powders are mixed in a blender at 200-400 rpm, adding 1% MoS₂ significantly reduces powder segregation and improves hopper discharge uniformity. This is critical for subsequent compact density consistency—density variations exceeding 0.1 g/cm³ lead to localized porosity or blistering after sintering.

**Sintering Stage: In-Situ Formation of Anti-Friction Phases.** The behavior of MoS₂ at sintering temperatures (780-820°C for copper-based, 1120-1150°C for iron-based) is complex. Since copper-based sintering temperatures are below the decomposition temperature of MoS₂ (approximately 1185°C), MoS₂ largely retains its structural integrity during sintering, forming a dispersed solid lubricant phase in the interstices between metal particles. During high-temperature sintering of iron-based bearings, some MoS₂ decomposes into Mo and S, where S reacts with Fe to form FeS (Mohs hardness 2.0), which also serves as a soft anti-friction phase in the matrix. This "in-situ reaction forming lubricant phase" mechanism is why non-acid-leached MoS₂ (free of HCl/HF residues) is preferred in PM applications—HCl molecules intercalated in the layers of acid-leached MoS₂ release H₂S and Cl₂ during sintering, which not only corrodes furnace linings but also causes compact blistering and cracking, increasing the reject rate by over 25%.

**Service Stage: Continuous Transfer Film Supply.** During initial operation, lubricating oil in the pores seeps out under capillary action to form an oil film. When the pv value increases and the oil film ruptures, the dispersed MoS₂ particles in the matrix are dragged onto the friction surface under frictional force, forming a MoS₂ transfer film approximately 0.1-1.0 μm thick. This film transforms metal-to-metal contact into MoS₂-MoS₂ interlayer slip, reducing the friction coefficient from 0.15-0.30 (metal dry friction) to 0.02-0.06.

Addition Amount Selection: The 1%-2% Process Window

More MoS₂ is not always better. Systematic experiments at Nanjing University of Science and Technology demonstrate that in bronze-based oil-impregnated bearings, increasing MoS₂ mass fraction from 1.0% to 2.0% actually decreases the limiting pv value. The reason is that excessive MoS₂ reduces matrix continuity, decreases pore connectivity and oil content, and increases sintering shrinkage, causing dimensional deviations.

In practice, a composite addition of 1.0% MoS₂ + 1.0% graphite delivers the best overall performance—MoS₂ provides a solid lubrication baseline under extreme pressure conditions, while graphite improves friction performance at low speeds. Together they reduce the friction coefficient to 0.038. However, graphite content exceeding 1.5% significantly weakens the matrix mechanical properties (crushing strength decreases by 15%-20%).

For iron-based oil-impregnated bearings, MoS₂ addition is typically controlled at 0.5%-1.5%. Beyond this range, excessive S released during sintering reacts with Fe to form a continuous FeS network, embrittling the matrix and reducing the bearing crushing strength K value from ≥200 MPa to below 150 MPa.

Particle Size Selection and Mixing Process

MoS₂ particle size directly affects its dispersion in the matrix. Coarse powder with D50 of 12-30 μm (corresponding to 500-750 mesh) is suitable for conventional mixing of iron-based and bronze-based bearings, offering good mixing uniformity and low agglomeration tendency. Ultra-fine powder with D50 of 3-5 μm has a larger specific surface area but requires 30%-50% longer mixing times; otherwise, local agglomeration occurs, creating soft spots after sintering.

The industry standard GB/T 23271-2023 for molybdenum disulfide specifies MoS₂ content ≥98.5%, Fe ≤0.25%, and H₂O ≤0.15% for PM applications. Iron content control is particularly critical—exceeding Fe limits indicates insufficient molybdenite concentrate grade or inadequate purification, which can result in hard FeS segregation points in sintered bearings, accelerating wear of mating components.

Summary

The role of MoS₂ additive in PM self-lubricating bearings spans three critical stages: powder mixing uniformity, sintering reaction control, and service-phase transfer film formation. The 1%-2% addition window, preference for coarser powder grades, and low-iron-content quality requirements are three core parameters that process engineers must manage. Selecting MoS₂ raw materials purified by physical flotation processes, avoiding acid residue interference during sintering, is the fundamental condition for ensuring oil-impregnated bearing yield rates and friction performance.

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