Hard Coating of Aircraft Parts

Hard coating refers to surface treatments applied to aircraft components to enhance wear resistance, corrosion protection, and durability in extreme operating conditions. These coatings are especially important for high-stress components such as landing gear, engine parts, fasteners, and structural elements.

Types of Hard Coatings Used in Aerospace

1. Hard Anodizing (Type III Anodizing)

• Process: An electrochemical treatment that forms a thick, hard aluminum oxide layer on aluminum parts.
• Benefits: ✔ High wear resistance. ✔ Excellent corrosion protection. ✔ Improved lubrication retention (ideal for moving parts).
• Applications: Landing gear, hydraulic components, and aircraft skins.

2. Hard Chrome Plating

• Process: Electroplating that deposits a layer of chromium onto metal surfaces.
• Benefits: ✔ Exceptional hardness and low friction. ✔ Corrosion and oxidation resistance. ✔ Restores worn parts to original specifications.
• Applications: Hydraulic actuators, landing gear struts, and engine components.
• Challenges: Contains hexavalent chromium, a hazardous material being phased out.

3. Tungsten Carbide Coatings (HVOF Thermal Spray)

• Process: High-Velocity Oxygen Fuel (HVOF) spraying applies tungsten carbide particles to metal surfaces.
• Benefits: ✔ Extreme hardness (comparable to diamonds). ✔ High-temperature resistance. ✔ Outperforms hard chrome plating in durability.
• Applications: Jet engine blades, landing gear, and high-wear components.
• Alternative to: Hard chrome plating due to environmental concerns.

4. Titanium Nitride (TiN) & Other PVD Coatings

• Process: Applied via Physical Vapor Deposition (PVD), forming a thin but extremely hard ceramic-like layer.
• Benefits: ✔ Ultra-hard, scratch-resistant surface. ✔ Reduces friction and enhances wear resistance. ✔ Heat-resistant, making it ideal for high-temperature applications.
• Applications: Cutting tools, turbine blades, and fasteners.

5. Electroless Nickel Coating (ENC)

• Process: A chemical plating process that deposits a uniform layer of nickel-phosphorus alloy.
• Benefits: ✔ Even coating thickness, even on complex shapes. ✔ Hardness similar to hard chrome. ✔ Corrosion and chemical resistance.
• Applications: Fuel system components, hydraulic parts, and fasteners.

6. Ceramic Coatings

• Process: Plasma spraying or sol-gel application of ceramics like Zirconia or Alumina.
• Benefits: ✔ High thermal barrier properties. ✔ Exceptional wear resistance. ✔ Used in extreme environments.
• Applications: Jet engine components, heat shields, and exhaust systems.

Aerospace Applications of Hard Coatings

✔ Landing Gear & Hydraulic Cylinders – Requires extreme wear resistance and low friction. ✔ Jet Engine Components – Protects against heat and oxidation. ✔ Aircraft Fasteners & Structural Components – Improves fatigue strength and prevents galling. ✔ Turbine Blades & Compressor Disks – Enhances thermal and wear resistance. ✔ Fuel & Hydraulic Systems – Protects against corrosion and fluid erosion.

Advantages of Hard Coatings for Aircraft Parts

✅ Increased Wear & Abrasion Resistance – Extends the lifespan of components. ✅ Superior Corrosion Protection – Prevents rust and oxidation. ✅ Reduced Friction & Improved Lubrication – Enhances performance of moving parts. ✅ Thermal Resistance – Protects against extreme heat in jet engines. ✅ Restoration of Worn Parts – Some coatings allow for refurbishment instead of replacement.

Challenges & Considerations

• Environmental Regulations – Hard chrome plating is being replaced due to toxicity.
• Coating Thickness Control – Some coatings may alter tolerances.
• Cost Factor – Advanced coatings like PVD and HVOF are expensive but provide long-term benefits.

Industry Standards & Compliance

• AMS 2469 – Hard anodizing for aerospace applications.
• AMS 2438 – HVOF tungsten carbide coatings.
• AMS 2406 – Electroless nickel plating.
• MIL-STD-1501 – Hard coatings for aerospace components.