What Are HVOF Parts and Why Do They Perform Differently?
Not all coated parts are built the same way. HVOF parts are components that have been surface-treated using the High Velocity Oxygen Fuel process, where a combustion mixture of oxygen and fuel (typically hydrogen, propane, or kerosene) propels molten or semi-molten powder particles at supersonic speeds toward the substrate surface.
The result is a coating with exceptionally dense microstructure. Porosity levels in HVOF coatings typically fall below 1%, compared to 5 to 15% in conventional flame or arc wire spray. That density is not cosmetic. It directly translates to how well the part resists abrasion, erosion, and chemical attack over its service life.
HVOF is a type of thermal spray process, which means all HVOF parts are technically thermal spray parts, but not all thermal spray parts are HVOF-coated. That distinction matters when you’re specifying parts for high-load environments.
The Broader Family: Understanding Thermal Spray Parts
Thermal spray parts cover a wide range of coating technologies applied through thermal energy. The major processes include:
Plasma Spray uses an electrically generated plasma arc to melt powders at temperatures exceeding 15,000°C. It handles high-melting-point ceramics like alumina and zirconia well, which makes it the standard choice for thermal barrier coatings in gas turbines.
Arc Wire Spray feeds two conductive wires into an arc, melting them and propelling the droplets with compressed air. It is fast, cost-effective, and works well for zinc or aluminum anti-corrosion coatings on structural steel.
Cold Spray does not melt the particles at all. Instead, it accelerates solid particles at very high velocity so they bond through kinetic energy on impact. Researchers at institutions like the U.S. Army Research Laboratory have explored cold spray for repairing aluminum airframe components without heat distortion.
Each process produces a different type of thermal spray part with different performance characteristics. The right choice depends entirely on the substrate material, the service environment, and the property being protected.
Where HVOF Parts Outperform Standard Thermal Spray Parts
Here’s where it gets interesting. When comparing HVOF parts to plasma or flame spray alternatives for wear-intensive applications, HVOF wins on three measurable fronts.
Bond Strength: HVOF coatings routinely achieve bond strengths above 70 MPa. Plasma spray parts typically land in the 35 to 55 MPa range. For rotating shafts, pump impellers, or valve seats that experience cyclic mechanical stress, that gap is significant.
Coating Hardness: WC-Co (Tungsten Carbide Cobalt) HVOF coatings can reach 1,100 to 1,400 HV on the Vickers hardness scale. This is why HVOF-coated pump sleeves and hydraulic rod components outlast hard chrome plating in abrasion tests, while also avoiding the environmental concerns associated with hexavalent chromium.
Oxidation Control: The supersonic particle velocity in HVOF means particles spend less time in the hot combustion zone. Less dwell time means less in-flight oxidation, which preserves the powder’s original chemistry in the final coating. Plasma spray parts coating tungsten carbide, by contrast, often show higher decarburization rates.
Manufacturers like Oerlikon Metco and Praxair Surface Technologies have built entire product lines around HVOF feedstock powders precisely because the process consistently delivers coatings that hold up in oil and gas, aerospace, and power generation environments.
When Standard Thermal Spray Parts Make More Sense
HVOF is not always the right call. Plasma spray parts remain the better option when you need thick ceramic coatings, particularly yttria-stabilized zirconia (YSZ) thermal barrier coatings on turbine blades. The higher flame temperatures in plasma spray are needed to fully melt ceramics that HVOF cannot process reliably.
Arc wire spray parts are the practical choice when you’re coating large surface areas on a budget, such as bridge decks or structural steel, where porosity is acceptable and throughput matters more than microstructural density.
The substrate also matters. Thin-walled components or heat-sensitive alloys may not tolerate the thermal input of any spray process without careful parameter control.
Frequently Asked Questions
Q: What are HVOF parts used for? A: HVOF parts are commonly used in aerospace (landing gear components, compressor blades), oil and gas (pump sleeves, valve stems), and industrial equipment (paper mill rolls, printing cylinders) where wear resistance and corrosion protection are critical.
Q: How do HVOF parts compare to hard chrome plated parts? A: HVOF coatings, especially WC-Co formulations, match or exceed hard chrome in wear resistance and bond strength while avoiding the hexavalent chromium waste streams associated with electroplating. Many aerospace OEMs now specify HVOF as the preferred chrome replacement.
Q: What is the typical coating thickness for HVOF parts? A: HVOF coatings are generally applied between 0.1 mm and 0.5 mm thick. Thicker deposits are possible but may introduce residual stress. Most functional coatings for wear applications sit in the 0.2 to 0.3 mm range.
Q: Are thermal spray parts suitable for food or medical applications? A: Some thermal spray coatings, particularly alumina and certain carbide blends, are used in food processing equipment. Suitability depends on the specific coating material, porosity level, and any regulatory standards that apply.
Q: How is substrate preparation handled before thermal spray coating? A: Grit blasting is the standard first step, creating a roughened surface profile that promotes mechanical adhesion. Cleanliness of the substrate, correct blast profile, and minimal delay between blasting and spraying all directly affect the final bond strength of the thermal spray part.