What LC Coating Actually Is: Two Parallel Process Routes

As basic connecting elements, fasteners' corrosion resistance, wear resistance, lubrication, and anti-loosening / anti-galling performance are often determined not by the base material but by the surface coating. LC coating is YF Zhichengjia's umbrella term for a family of composite functional coatings developed for high-precision, high-load scenarios such as 3C consumer electronics, new energy, energy storage, automotive, humanoid robots and AI servers, targeting common problems such as precision thread jamming, coating peeling, insufficient wear resistance, salt-spray failure, vibrational loosening, and stainless-steel seizure. In engineering it splits into two independent, mature process families; before selecting, you must first make clear which one you are using:

• Vacuum LC-DLC hard coating: In a vacuum chamber, high-purity acetylene and methane are used as the core carbon source, paired with high-purity argon and nitrogen as shielding gas. According to the base material (stainless steel, boron steel, alloy steel, heat-treated high-strength steel), the doping ratio of titanium, silicon and tungsten nano-alloys is adjusted on demand; raw-material purity is ≥99.99%, and a diamond-like carbon film is grown by arc / magnet-filtered ion deposition. The film is fine and particle-free, and suits M0.8 and larger micro-grade fine threads.
• Polymer dry-film LC lubrication coating: Using an eco-friendly lubricating resin base, compounded with nano solid-lubricant powder and a low-temperature curing agent, the film is formed by spray impregnation and low-temperature curing, with no oil residue left behind. It focuses on electrical insulation, torque stability and medium corrosion protection, and suits high-volume production of all categories of standard fasteners.

The two routes share the same pre-cleaning and quality-control system, but their performance positioning differs: the former leans toward "hard, wear-resistant, low friction", while the latter leans toward "insulating, torque-stable, hydrogen-embrittlement-free". For the performance parameters below, unless otherwise stated, the vacuum-route values come from YF Zhichengjia's mass-production LC-DLC process, while the dry-film route is distinguished by insulation and assembly consistency.

Film Formation: From Plasma Activation to Magnet-Filtered Deposition

The thread root, flank and crest of a fastener are geometrically complex; even slight coating unevenness causes "thread jamming" or local wear-through. The LC process therefore uses a custom six-step flow, all at low temperature, that does not change base-material hardness or torque:

1. Multi-stage precision pre-cleaning: A double pretreatment of multi-stage ultrasonic degreasing plus vacuum plasma activation thoroughly removes oil, wax, oxide scale and dust from the thread surface. Plasma activation builds a micro-rough interface on the thread surface, greatly improving coating adhesion and reducing peeling from the source.
2. Customized fixturing by part type: Dedicated openwork jigs and individual hangers are designed for micro fine-thread screws, high-strength bolts, and irregular thin-wall parts; individual parts are isolated to eliminate stacking impact and protect the dimensional precision of tiny threads.
3. Low-temperature gradient preheating at 160–220°C: Low-temperature constant heating adapts to aluminum-alloy thin-wall parts, quench-hardened steel, and high-strength alloys, without softening the material or producing stress deformation.
4. Magnet-filtered nano-uniform deposition: Relying on magnet-filtered multi-arc ion deposition, nano carbon ions uniformly cover the thread root, flank and crest; film thickness is uniform and controllable, avoiding locally too-thick jamming or too-thin poor wear resistance.
5. Staged low-temperature stress relief: Vacuum LC is naturally cured at constant temperature, while dry-film LC is cured by low-temperature baking; internal stress is released in steps, reducing the risk of cracking and spalling under thermal cycling and long-term vibration.
6. Full-dimensional outgoing quality control: 100% visual inspection plus sampling of film thickness, adhesion, friction coefficient, neutral salt spray, hardness and tensile strength; test data is issued for every batch.

Film thickness is precisely adjustable between 0.3 and 3µm according to service conditions: an ultra-thin 0.3–1µm layer is used for M0.8 and larger micro fine threads to eliminate jamming, while a thickened 2–3µm layer is used for heavy-duty high-strength bolts, trading wear resistance for life. All finished parts stably pass thread go/no-go gage inspection, with no secondary thread repair needed.

Key Performance Parameters: Hardness, Friction Coefficient and Salt Spray

For engineers, whether a coating is worth its cost depends on three groups of hard indicators. The table below compares the vacuum LC-DLC mass-production process with the polymer dry-film route:

Stable friction coefficient is the core reason LC coating is chosen by automated lines: the tightening-torque spread of the same bolt batch is narrowed, and combined with torque-clamp-force testing it improves preload consistency. At the same time, low μ fundamentally suppresses fretting wear and seizure in stainless-steel and high-strength-steel threads. Once the film is dense, neutral salt spray reaches 500–1500 h (tested per GB/T 10125 neutral salt-spray conditions), far better than conventional electroplating and blackening, suiting in-vehicle, outdoor energy storage and other high-corrosion environments.

LC vs Zinc Plating / DLC / Dacromet: The Boundaries in One Table

Typical Applications and Selection Recommendations

• M0.5–M5 micro precision screws, AI-server small bolts: choose a 0.3–1µm ultra-thin vacuum LC-DLC; smooth, no jamming, no dimensional deformation.
• 8.8/10.9/12.9/14.9-grade automotive chassis and energy-storage high-strength bolts: choose a 2–3µm thickened heavy-duty film for anti-fretting and long-term anti-loosening.
• 6061/7075 aluminum and magnesium-alloy thin-wall screws: the low-temperature process does not soften the base material and eases light-alloy galvanic corrosion.
• 304/316 stainless-steel screws: vacuum LC-DLC thoroughly solves assembly locking and seizure, while also improving oxidation and corrosion resistance.
• NEV battery / electronic-control joints needing insulation: choose polymer dry-film LC for electrical isolation + stable torque + medium corrosion protection.