What Is Thermal Spray Coating? The Engineer's Complete Guide

What Is Thermal Spray Coating?

Thermal spray coating is an industrial surface-engineering process in which a feedstock material — metal powder, wire, or rod — is heated to a molten or semi-molten state and then propelled at high velocity onto a prepared substrate. On impact, each particle flattens into a thin splat, bonds to the surface, and rapidly solidifies. Successive splat layers build up a dense, adherent coating that protects the underlying component against wear, corrosion, erosion, or heat without melting or distorting the base metal.

The process encompasses a family of five distinct technologies — flame spray, High-Velocity Oxygen Fuel (HVOF), Detonation Spray (D-Gun), plasma spray, and arc spray (metalizing) — each delivering a different combination of coating density, bond strength, and material capability. Choosing the right process is an engineering decision, not a commodity purchase: a D-Gun tungsten carbide coating on a pump shaft sleeve has virtually nothing in common with a zinc arc-spray on a bridge girder, even though both are 'thermal spray'. Understand the differences and you understand the technology. Our thermal spray technologies page details every process we operate.

The Five Thermal Spray Processes Compared

All five processes share the same physics — heat plus kinetic energy converts feedstock into a bonded layer on the substrate — but they differ sharply in gas temperature, particle velocity, achievable bond strength, and coating porosity. The table below summarises industry-standard performance benchmarks for each process.

| Process | Heat Source | Particle Velocity | Bond Strength | Porosity | Primary Use | |---|---|---|---|---|---| | Flame Spray | Fuel/O₂ combustion | 80–150 m/s | 10–30 MPa | 10–15% | Zinc/bronze on large structures | | HVOF | Supersonic fuel/O₂ | 600–900 m/s | 60–80 MPa | < 2% | WC-CoCr hard coatings, hard-chrome replacement | | D-Gun (Detonation Spray) | Detonation shock wave | 700–1000 m/s | > 80 MPa | < 1% | Highest-density wear coatings, precision components | | Plasma Spray | DC plasma arc (>8000 °C) | 200–500 m/s | 35–70 MPa | 2–10% | Ceramics, thermal barrier coatings | | Arc Spray (Metalizing) | Electric arc between wires | 100–200 m/s | 10–20 MPa | 5–15% | Large-area corrosion protection — zinc, aluminium |

D-Gun and HVOF sit at the high-performance end. Their supersonic particle velocities produce mechanically denser, harder, and better-bonded coatings than flame or arc spray can achieve. Plasma spray occupies a unique niche: its plasma arc reaches above 8 000 °C, hot enough to melt refractory ceramics such as yttria-stabilised zirconia that no other process can handle. For an in-depth look at how HVOF works, see our guide on what is HVOF coating.

How the Process Works: From Powder to Bonded Layer

Regardless of process, thermal spray follows the same four-step sequence. First, the substrate is grit-blasted to a surface roughness of Ra 6–10 µm, removing all contaminants and creating the mechanical keys that anchor the coating. This is the most-often-skimped step, and inadequate surface preparation is the leading cause of premature coating failure. Second, the feedstock — a precisely sized powder or wire — is fed into the heat source, where it melts or softens within milliseconds. Third, carrier gas or combustion products accelerate the particles toward the substrate at velocities ranging from 80 m/s for flame spray to more than 1 000 m/s for D-Gun. Fourth, each particle impacts the surface, flattens to roughly five times its original diameter, and solidifies in microseconds, bonding mechanically and — in high-velocity processes — partially metallurgically to the layers beneath.

Post-spray finishing is almost always required. As-sprayed surfaces are rough (typical Ra 4–12 µm) and may contain interconnected porosity that allows corrosive fluids to wick through to the substrate. Precision cylindrical or surface grinding brings running faces to the required dimensional tolerance; vacuum impregnation or polymer sealing closes surface-connected porosity where sealing is necessary for corrosive service. Skipping post-spray finishing is the second-most-common thermal spray specification error.

What Materials Can Be Thermally Sprayed?

Thermal spray is material-agnostic to a degree no other coating technology matches. Any material that melts before it decomposes — and has a vapour pressure low enough to remain a stable liquid at the nozzle — can in principle be sprayed. In industrial practice, coatings draw from four main families.

Carbide cermets — tungsten carbide (WC-CoCr, WC-Co) and chromium carbide (Cr₃C₂-NiCr) — are the workhorses of wear protection. WC-CoCr applied by HVOF or D-Gun achieves hardness up to approximately 1 400 HV and is the internationally accepted hard-chrome replacement on hydraulic rods, pump shafts, and roll surfaces. Chromium carbide is chosen where service temperatures exceed 450–500 °C, because WC begins to oxidise above that threshold. Cobalt alloys such as Stellite cover galling resistance and moderate-temperature erosion on valve trim and turbine components. Ceramics — alumina (Al₂O₃), chromium oxide (Cr₂O₃), and yttria-stabilised zirconia — address electrical insulation, extreme temperatures, and sliding abrasion. Pure metals and alloys (zinc, aluminium, 316 stainless, Inconel, NiCrAlY) handle corrosion protection and bond-coat applications for ceramic top coats.

• Tungsten carbide (WC-CoCr, WC-Co) — maximum wear resistance up to ~450 °C, hard-chrome replacement
• Chromium carbide (Cr₃C₂-NiCr) — high-temperature wear and oxidation resistance up to ~900 °C
• Stellite (Co-Cr-W alloys) — galling resistance, valve trim, turbine blades
• Ceramics (Al₂O₃, Cr₂O₃, YSZ) — electrical insulation, thermal barriers, abrasion
• Metallic alloys (316SS, Inconel, NiCrAlY) — corrosion resistance and bond coats
• Pure metals (Zn, Al) — cathodic protection of structural steel by arc spray

Industrial Applications: Where Thermal Spray Earns Its Keep

Thermal spray coatings are applied wherever a component faces aggressive wear, corrosion, erosion, or heat that the base material alone cannot withstand. In the Indian industrial context — where monsoon humidity accelerates corrosion, power plant duty cycles are demanding, and replacement lead times for imported components can stretch to months — the economics of thermal spray repair are particularly compelling. Restoring a worn pump shaft sleeve by HVOF rather than replacing it typically costs 70–90% less than a new part and returns the component to original dimensional tolerance within one to two working days.

Applications span every major industry. Oil and gas operators coat valve gates, choke stems, and downhole tool surfaces with WC-CoCr to resist sand-laden erosive flows. Power plant engineers apply ceramic plasma spray as thermal barrier coatings on combustion hardware, and chromium carbide on induced-draft fan blades and boiler tubes operating at elevated temperature. Pump manufacturers and repair shops specify HVOF WC on shaft sleeves, plungers, and impeller wear rings. Steel and paper mills coat roll surfaces for web-handling and calendering, where dimensional precision and wear life directly govern production yield. Full process and capacity details are on the Lotus Surface Tech technologies page.

• Pump shaft sleeves and plungers — HVOF WC-CoCr for mechanical seal-face wear
• Hydraulic cylinder rods — HVOF as a REACH-compliant replacement for hard chrome plating
• Boiler tubes and ID/FD fan blades — Cr₃C₂-NiCr for high-temperature erosion
• Roll surfaces in steel and paper mills — WC or ceramic for precision web handling
• Valve trim and gate valves — Stellite for galling and erosion resistance
• Structural steel (offshore platforms, bridges, storage tanks) — arc-sprayed zinc or aluminium for long-life cathodic protection

How to Choose the Right Thermal Spray Process: A Selection Checklist

Process selection starts with three questions: What failure mode are you solving for? What temperature will the coating see in service? And what dimensional tolerance must be held after post-spray finishing? The checklist below covers the most common decision branches encountered in plant and maintenance engineering. When service conditions are complex — multiple wear modes, combined corrosion and abrasion, variable temperatures — share the part drawing and failure analysis with a coatings engineer before committing to a specification.

• Maximum wear resistance at ambient to ~450 °C → WC-CoCr by HVOF or D-Gun
• Service temperature 450–900 °C (fan blades, boiler tubes) → Cr₃C₂-NiCr by HVOF or plasma spray
• Thermal barrier or ceramic insulation needed → yttria-stabilised zirconia by plasma spray, with NiCrAlY bond coat
• Lowest possible porosity and highest bond strength → D-Gun (porosity < 1%, bond strength > 80 MPa)
• Replacing hexavalent hard chrome (REACH/RoHS compliance) → HVOF WC-CoCr is the industry-standard replacement
• Large structural areas requiring corrosion protection → arc spray with zinc or aluminium (metalizing)
• Galling resistance on valves or turbine components → Stellite by plasma spray or HVOF

Common Mistakes When Specifying Thermal Spray Coatings

Thermal spray is a mature, well-understood technology, but specification errors are costly because they surface at the coating stage — or worse, in service. The following failure modes appear repeatedly in coatings engineering reviews.

• Specifying 'thermal spray' without naming the process — flame spray and D-Gun produce entirely different bond strengths and porosity; a process-agnostic spec is not a valid engineering document
• Inadequate surface preparation — grit blasting to Ra 6–10 µm is mandatory; a contaminated or insufficiently roughened surface produces weak adhesion regardless of which process follows
• Selecting coating thickness by cost alone — too thin and the coating wears through at the seal face before the next planned shutdown; too thick and residual tensile stresses risk spalling at the substrate interface
• Treating hard-chrome replacement as a direct dimensional swap — HVOF WC coatings are harder and denser than electroplated chrome, but require different grinding wheels (cubic boron nitride or diamond) and sealing procedures; copy the dimensions, not the finishing method
• Omitting a sealer on porous coatings used in corrosive service — flame and arc spray coatings are inherently porous; without a polymer or inorganic sealer, corrosive fluids permeate to the substrate and undercut the coating from within
• Applying ceramic plasma coatings without a metallic bond coat — a ceramic top coat bonded directly to steel without an intermediate NiCrAlY or NiAl bond coat will delaminate under thermal cycling due to coefficient-of-thermal-expansion mismatch

Get a Quote for Thermal Spray Coating

Lotus Surface Tech is an ISO 9001:2015-certified thermal spray coating facility in Chennai, Tamil Nadu, established in 2015. We operate five coating lines — Super-D-Gun, HVOF, plasma spray, flame spray, and arc spray (metalizing) — with capacity to handle components up to Ø800 mm × 7 m. Our engineers work with plant maintenance teams and procurement departments across power generation, oil and gas, pumps, paper, and heavy industry to specify the correct process, apply the coating to drawing, and grind to final tolerance.

If you are evaluating a hard-chrome replacement programme, restoring a worn or corroded component, or specifying a coating for a new design, visit our technologies page for process-by-process capability detail. When you are ready to move forward, contact us to get a quote — bring the part drawing, the failure mode, and the service environment, and we will recommend the right process and give you a firm price.