What is HVOF?

• HVOF is a thermal spray process where fuel (hydrogen, kerosene, propane, or propylene) is mixed with oxygen and combusted in a chamber.
• The combustion gases are expelled at supersonic speeds through a nozzle, carrying powdered coating materials (typically metals, alloys, or carbides) at high velocity (500–1000 m/s).
• The particles impact the substrate, plastically deform, and build a very dense, strongly adhered coating.
• Deposition temperatures are lower than plasma spraying, so oxidation and phase changes in the coating material are reduced.

Applications in Aerospace

HVOF coatings are crucial in aircraft and space components where high performance and durability are required:

1. Landing Gear Replaces hard chrome plating (due to environmental restrictions on hexavalent chromium). Common coatings: Tungsten Carbide-Cobalt (WC-Co), WC-Co-Cr, or Cr₃C₂-NiCr. Provides resistance to wear, erosion, corrosion, and fretting.
2. Engine Components Turbine shafts, bearings, compressor seals, and blades use HVOF coatings for wear, oxidation, and thermal barrier properties. Coatings can include superalloys, MCrAlY (where M = Ni, Co), and carbides.
3. Actuation Systems Hydraulic cylinders and rods protected with HVOF WC-based coatings against sliding wear and fluid corrosion.
4. Structural & Other Uses Protection of aluminum structures from corrosion. Bond coats for thermal barrier systems. Restoration/repair of worn components to reduce replacement costs.

Advantages of HVOF vs Alternatives

• Superior hardness & wear resistance vs plasma spray or flame spray.
• Dense, low-porosity coatings (<1%), which enhances corrosion protection.
• High bond strength (60+ MPa, often exceeding substrate yield strength).
• Environmentally friendly alternative to electroplated hard chrome.
• Lower residual stresses, enabling thicker coatings without cracking.

Regulatory & Industry Adoption

• Driven largely by REACH and OSHA restrictions on hexavalent chromium, HVOF is now the industry standard replacement for hard chrome in aerospace.
• Major aerospace OEMs (Boeing, Airbus, Rolls-Royce, GE Aviation) have fully qualified HVOF coatings for both OEM production and MRO (maintenance, repair, overhaul).

Limitations

• High equipment and operating cost compared to electroplating.
• Line-of-sight process, so coating complex geometries is difficult.
• Requires surface preparation (e.g., grit blasting).
• Thickness typically limited to <1 mm.

HVOF thermal spray has become an essential aerospace coating technology, especially for landing gear, engines, and hydraulic systems, offering durability, environmental compliance, and performance improvements over older methods like chrome plating. It is one of the key enablers of next-generation, eco-compliant aerospace materials engineering.