What Dry-Film Lubrication Is: The Transfer-Film Principle

Dry film lubrication (DFL) fixes a solid lubricant—most typically molybdenum disulfide and graphite—onto the part surface with a binder resin, curing into a thin, dry lubricating film. Unlike oil lubrication, which separates contact faces with an "oil film", it reduces friction through the solid lubricant's own layered crystal structure: both MoS2 and graphite are hexagonal layered crystals with extremely weak bonding between layers, so the layers slide easily under load, lowering the friction coefficient.

More important is the transfer film effect: during rubbing, the solid lubricant in the dry film transfers from the coating onto the mating-part surface, forming a continuous lubricating layer on both sides. After that, even if the original coating wears away somewhat, this transfer film continues to reduce friction, achieving long-term self-lubrication. This is the fundamental reason a dry film is "applied once and effective for a long time", and its core advantage over grease that dries out and drains away.

The practical consequence is that dry-film lubrication is chosen not for its absolute lowest friction number but for tribological consistency over years of service. A coated thread behaves the same on the first tightening and on the fifth, even after dust, oxidation or partial wear—something an oil film cannot promise. That is why dry films are preferred wherever a joint must keep a known preload and be reusable without re-lubrication, such as in tightened structural joints and serviceable assemblies.

Molybdenum Disulfide (MoS2): The First Choice for Heavy Load and Vacuum

Molybdenum disulfide is a black, silver-grey powder with easy interlayer sliding; it performs especially well under high contact pressure—which is why it is called a "heavy-load lubricant". When a fastener is tightened, the thread-flank contact pressure is very high, and ordinary oil is easily squeezed out of the boundary, whereas an MoS2 dry film maintains a very low friction coefficient under boundary lubrication and heavy load, with typical static/dynamic friction coefficients as low as about 0.05–0.10.

MoS2 also has a unique advantage: it still works in vacuum. This is exactly the opposite of graphite (see next section), which is why MoS2 dry films are widely used in "no-air" service such as aerospace, vacuum equipment and satellite mechanisms. On fasteners, its most direct value is anti-galling: high-strength steel and stainless-steel bolts easily cold-weld and seize under high-temperature repeated disassembly; the MoS2 dry film separates the mating faces so the bolt can still be removed.

Graphite: The "Atmosphere-Type" Lubricant That Depends on Moisture

Graphite is also a layered solid lubricant; it is cheap, conductive, and has a low friction coefficient in ordinary atmospheric environments. But graphite has a frequently overlooked precondition: its lubrication depends on moisture (or oxygen) from the air adsorbing between the layers. Once it enters a high vacuum, dry or inert-atmosphere environment, the graphite layers lose their "lubrication" moisture, the friction coefficient instead rises sharply, and abrasive wear can even occur.

There is therefore a rule of thumb in engineering: for atmosphere, room temperature, conductive requirements or cost sensitivity choose graphite; for vacuum, dry, heavy load or high temperature choose molybdenum disulfide. YF Zhichengjia makes heavy use of solid-lubrication concepts in cold-heading wire preparation and surface treatment, while the finished fastener's dry-film coating is chosen among MoS2, graphite and PTFE according to the final service environment.

How to Choose MoS2 vs Graphite: One Comparison Table

Typical Applications and Application-Control Key Points

• High-strength-steel / stainless-steel bolt anti-seizure: for 10.9/12.9-grade and stainless bolts under repeated disassembly or high temperature, an MoS2 dry film prevents cold-weld seizure and eases later removal.
• Heavy load, boundary lubrication: for high-clamping-pressure joints in chassis, wind power and construction machinery, MoS2 maintains a stable friction coefficient.
• Vacuum / aerospace mechanisms: choose MoS2, avoid graphite.
• Conductive or low-cost needs: in atmosphere room-temperature service, a graphite-based dry film is an option.
• Application control: before coating, thoroughly degrease, phosphate or blast-activate to ensure film adhesion; film thickness must be uniform—too thick jams thread go/no-go gaging, too thin gives insufficient life; curing temperature must not damage the already heat-treated high-strength-steel substrate.