How is the surface treatment of self-locking tuning screw done after production and processing?
The surface treatment of self-locking tuning screws (for RF/microwave/precision adjustment) is based on five requirements: conductivity, corrosion resistance, wear resistance, self-locking and accurate size, and the process is highly customized. The following is clear according to the general production process of the industry.
A, general process (all surface treatment must go)
Pretreatment (deciding success or failure)
Degreasing → hot water washing → cold water washing → pickling activation → neutralization → pure water rinsing.
Thread root, chamfer and blind hole must be thoroughly cleaned.
Brass/copper alloy: slightly corroded by weak acid; Stainless steel: electrolytic polishing/pickling passivation
Base coat (optional)
Brass: pre-copper plating 0.5~1μm → enhancing the adhesion of the upper coating.
Carbon steel/stainless steel: flash nickel plating 0.3~0.8μm → prevent copper infiltration and improve bonding.
Functional coating (core)
Silver/gold/nickel/hard chromium/dacromet/nylon coating, etc.
aftertreatment
Dehydrogenation (electroplating must be done)
Anti-discoloration (silver, copper)
Drying → inspection (thickness, adhesion, salt spray, conduction)
Second, the mainstream surface treatment scheme (by function)
1. RF/microwave conductive type (most commonly used: base station, cavity filter)
① Silver plating (industry standard)
Base material: H62/H65 brass (preferred) and bronze.
Thickness: 3~8μm (common); 8~15μm (high frequency/high wear resistance)
Technology:
Degreasing → Pre-copper plating → Silver plating → Anti-discoloration (silver immersion protective agent/passivation)
Advantages: excellent conductivity, low high frequency loss and stable contact resistance.
note:
It must be treated with anti-discoloration, otherwise it is easy to blacken in wet environment.
The coating thickness should be reserved for thread matching (especially for M1~M3 micro screws).
② Gold plating (high-end/aerospace/long life)
Thickness: 0.5~2μm (gold layer)+1~3μm of nickel bottom.
Advantages: no oxidation, strong corrosion resistance and extremely low contact resistance.
Application: satellite communication, military industry, high temperature and high humidity.
③ Nickel plating (semi-conductive/general precision)
Thickness: 5~12μm (semi-bright/bright nickel)
Advantages: moderate hardness, wear resistance, moderate corrosion resistance and stable size.
Application: general tuning, non-high frequency precision adjustment
2. Wear-resistant+self-locking type (frequent screwing and self-locking)
① hard chromium plating (HV800~1000)
Thickness: 3~8μm (not too thick, otherwise the thread will be stuck)
Technology:
Polishing (Ra≤0.4)→ degreasing → chromium plating → dehydrogenation (200~220℃×2~4h)
Advantages: extremely wear-resistant, long service life and high dimensional accuracy.
Application: industrial tuning screw, repeated adjustment mechanism
② nylon coating (self-locking and locking coating)
Location: thread profile surface only (not full surface)
Thickness: 5~15μm (precise thickness control)
Technology:
Cleaning → electrostatic spraying/dipping nylon → curing (150~180℃)
Function:
Elastic deformation of nylon during tightening → stable friction → self-locking and anti-loosening.
Noise reduction, anti-seizure, temperature resistance ≥120℃
Application: self-locking tuning screws for communication cavities and instruments.
3. High corrosion resistance/hydrogen-free brittleness type (outdoor/high strength/salt spray)
① dacromet (zinc-aluminum coating)
Thickness: 6~10μm
Advantages: no hydrogen embrittlement, salt spray up to 500~1000h, good temperature resistance.
Application: high-strength steel tuning screws, outdoor equipment
② Stainless steel passivation (304/316)
Process: pickling → nitric acid+citric acid passivation → washing and drying.
A compact Cr₂O₃ film is formed, and the salt spray is 48~200h (depending on the material).
4. Low cost/indoor general use
Blacking+oil immersion: carbon steel, indoor, low requirements.
Galvanizing+blue-white/colored zinc: general anticorrosion and low cost.
Note: High-strength (≥10.9) common zinc plating (hydrogen embrittlement risk) is strictly prohibited.
III. Special Precautions for Self-locking Tuning Screw (Key)
Dimensional accuracy control (most stringent)
Coating/coating must be included in thread tolerance.
M1~M4 micro specifications: coating thickness **≤3~5μm**, otherwise it will not be screwed in.
Thick coating (hard chromium, hot dip zinc) must be plated first and then rubbed/brushed.
Hydrogen embrittlement control (high strength must be done)
After carbon steel/alloy steel electroplating: dehydrogenate and bake for 200~230℃×2~4h.
Grade 10.9 and above: priority is given to dacromet/phosphating, and ordinary electrogalvanizing is prohibited.
Full coverage of thread root
Complex threads, blind holes and deep holes: chemical plating/barrel plating is preferred to avoid missing plating.
Self-locking coating (nylon) is only used for tooth shape.
The head/stem is not painted to ensure the appearance and assembly.
The friction coefficient is stable (0.12~0.18), and the self-locking torque is consistent.
Common combinations of conductive+self-locking composite treatment:
Silver plating+local nylon coating (tooth type)
Nickel plating+nylon coating
→ Give consideration to conductivity, corrosion resistance, self-locking and wear resistance.
Environmental protection and certification
RoHS compliance: hexavalent chromium, lead and cadmium are prohibited.
Trivalent chromium is used for passivation, and hexavalent chromium is prohibited.
Fourth, the selection of quick look-up table (directly applied)
form
Application scenarios recommend core advantages of surface treatment.
The RF cavity/filter is plated with silver for 3~8μm+,which has excellent anti-tarnish and conductivity and high frequency.
Military/aerospace/long-life gold plating with 0.5~2μm+nickel bottom is non-oxidizing and highly reliable.
Frequent adjustment of wear-resistant hard chromium plating 3~8μm+hydrogen removal, super wear-resistant and long service life.
Self-locking and anti-loosening thread nylon coated with 5 ~ 15μ m is stable for self-locking and anti-loosening.
High strength/salt spray dacromet has no hydrogen embrittlement and high corrosion resistance.
General precision indoor nickel plating 5 ~ 10μ m is balanced and stable in size.
Five, a sentence summary
Surface treatment of self-locking tuning screw = clean pretreatment+accurate functional coating+hydrogen removal+anti-discoloration+self-locking coating (as required), and the core is to meet the five requirements of conductivity, wear resistance, corrosion resistance, self-locking and size at the same time.
