What Is Stellite Coating? Cobalt-Based Wear and Galling Resistance

What Is Stellite Coating?

Stellite coating is a cobalt-chromium-tungsten (Co-Cr-W) alloy coating applied to a component surface to resist galling, wear and corrosion, particularly in metal-to-metal sliding or seating contact and at temperatures that would soften a hardened steel. "Stellite" is the trade name for a family of cobalt-based alloys — originally developed by Elwood Haynes in the early 1900s — built around a cobalt-chromium solid-solution matrix strengthened by hard chromium-carbide (and, in tungsten-bearing grades, tungsten-carbide) particles. Unlike a hardened steel, Stellite's hardness comes from that carbide-in-matrix structure rather than from a martensitic transformation, so it doesn't need heat treatment to reach working hardness and doesn't soften the way tempered steel does as temperature climbs.

The common grades used in industrial coating work are Stellite 6 (roughly Co-28Cr-4.5W-1.2C), the general-purpose workhorse balancing wear, corrosion and galling resistance; Stellite 12 (roughly Co-30Cr-8.5W-1.4C), with a higher carbide fraction for greater hardness and wear resistance at some cost to toughness; and Stellite 21 (roughly Co-27Cr-5.5Mo-0.25C), a lower-carbon, molybdenum-bearing grade chosen for better machinability and toughness where extreme hardness isn't the priority. For where Stellite sits alongside HVOF, D-Gun, plasma spray and metalizing in the broader thermal spray family, see our guide on what thermal spray coating is.

How Stellite Coating Is Applied

As a thermal spray coating, Stellite is most commonly applied by plasma spray, where an electric arc ionises gas into a plasma jet that melts the Co-Cr-W powder and propels it onto the substrate, and by flame spray or HVOF, which use combustion rather than plasma to melt and accelerate the powder. Plasma spray is the standard route for Stellite because the higher flame temperature fully melts the cobalt matrix and carbide particles, producing a denser, better-bonded coating than flame spray, while HVOF's higher particle velocity gives a finer, lower-porosity structure again at somewhat higher process cost. As-sprayed Stellite coatings typically test in the range of roughly 35-47 HRC (approximately 350-470 HV) depending on grade and process — lower than a tungsten carbide coating's hardness, which is expected, since Stellite is chosen for galling and toughness characteristics that pure hardness numbers don't capture.

It's worth distinguishing thermal spray Stellite coatings from plasma-transferred-arc (PTA) or laser-cladded Stellite overlays, which are welding processes rather than thermal spray. PTA and laser cladding produce a thicker, fully metallurgically fused deposit and are common for heavy-duty valve seat overlay, while thermal-sprayed Stellite (plasma, flame, or HVOF) suits thinner wear coatings on shafts, sleeves, rings and other components where a mechanically bonded coating is the right fit. As with any thermal spray job, the surface is grit-blasted to the correct profile before spraying and precision-ground to final dimension afterward.

Why Stellite Resists Galling and High-Temperature Wear

Galling is the adhesive wear failure that happens when two metal surfaces in sliding or oscillating contact momentarily weld together at high-friction points and tear apart, leaving torn, roughened surfaces that rapidly worsen. Cobalt-based alloys resist this failure mode unusually well because cobalt's crystal structure shifts between face-centred-cubic and hexagonal-close-packed forms under mechanical stress, generating a stacking-fault structure at the surface that behaves in a self-lubricating way and resists the adhesive pickup that causes galling in most steels and even in many hard-chrome and carbide surfaces run against themselves. This is the specific property that sets Stellite apart from tungsten carbide or chromium carbide coatings, which resist abrasive and erosive wear extremely well but were never designed to solve a galling problem.

Because Stellite's hardness comes from carbides dispersed in a cobalt-chromium matrix rather than a heat-treated martensitic structure, it also retains useful hardness, wear resistance and the high chromium content's passive-film corrosion resistance at temperatures up to roughly 650-700°C — well above what a hardened steel can sustain, and useful in service conditions like engine exhaust valve seats that combine heat, corrosion and repeated metal-to-metal seating impact.

Stellite vs Tungsten Carbide vs Chromium Carbide

Stellite, tungsten carbide and chromium carbide are all thermal spray materials used for wear protection, but they solve different failure modes and shouldn't be treated as interchangeable "hard coating" options.

| Property | Stellite (Co-Cr-W) | Tungsten Carbide (WC-CoCr) | Chromium Carbide (Cr3C2-NiCr) | |---|---|---|---| | Typical hardness (thermal spray) | ~350-470 HV (35-47 HRC) | 1,200-1,350 HV | 750-900 HV | | Galling resistance | Excellent — the defining property | Poor to moderate against itself | Moderate | | Abrasive wear resistance | Good, not the hardest option | Highest in the family | Very good | | Max useful temperature | ~650-700°C | ~450-500°C | ~800-850°C | | Corrosion resistance | Very good (high Cr content) | Good (with Cr in binder) | Good | | Typical duty | Valve seats/trim, pump wear rings, exhaust valves | Pump shafts, hydraulic rods, valve trim | Boiler tubes, ID/FD fans, hot rolls |

Choose Stellite when the dominant failure mode is galling or seizing between mating metal surfaces, especially with corrosion or elevated temperature in the mix. Choose tungsten carbide when the job is pure abrasive or sliding wear resistance below ~450-500°C — see our tungsten carbide coating guide. Choose chromium carbide when the job runs hotter than tungsten carbide can tolerate — see our tungsten carbide vs chromium carbide guide for that decision in full.

Where Stellite Coating Is Used

Stellite coating is specified wherever galling, corrosion and moderate-to-high-temperature wear occur together — conditions where a plain hardened steel or even a harder carbide coating would seize or corrode before it wore out:

• Valve seats, discs and trim in process, power and oil & gas valves, where metal-to-metal seating under repeated closure would gall a lesser material
• Pump wear rings, sleeves and other close-clearance components running metal-to-metal against a mating part rather than against an abrasive slurry
• Steam and gas turbine components exposed to erosion and moderate heat where corrosion resistance also matters
• Internal combustion engine exhaust valve seats and faces, combining high temperature, corrosive combustion byproducts and repeated seating impact
• Hot-forming and extrusion tooling surfaces that see sliding contact under heat

Selecting the Right Stellite Grade

Work through these points before specifying a Stellite coating and grade:

• Confirm the actual failure mode first — if the component wears from abrasion or slurry erosion rather than galling or seizing, tungsten carbide or chromium carbide will usually outperform Stellite for the same coating cost
• Default to Stellite 6 for general-purpose galling, corrosion and moderate wear duty unless a specific reason points elsewhere — it's the most widely used and best-balanced grade
• Step up to Stellite 12 only where wear resistance needs to be pushed harder and the reduced toughness is acceptable for the application
• Consider Stellite 21 where machinability and toughness matter more than maximum hardness, or where a lower-carbon, tungsten-free composition is specifically called for
• Specify plasma spray for the denser, better-bonded coating on critical components; flame spray is acceptable for lower-duty, cost-sensitive parts
• Confirm the sustained and peak operating temperature — Stellite's ~650-700°C ceiling is well above tungsten carbide's but still finite, and components running hotter need a different material entirely
• Allow adequate grinding stock in the drawing, since thermal spray coatings are applied thicker than finished size and precision-ground to dimension

Common Mistakes When Specifying Stellite Coating

These errors show up repeatedly in enquiries and are worth catching before a part goes out for coating:

• Specifying Stellite for a pure abrasive-wear application where tungsten carbide would give longer wear life for similar or lower cost — Stellite's advantage is galling and corrosion resistance, not maximum hardness
• Specifying tungsten carbide or chromium carbide on a component whose real failure mode is galling between mating metal surfaces — a harder coating doesn't fix an adhesive-wear problem the way a cobalt alloy does
• Assuming all Stellite grades are interchangeable — Stellite 6, 12 and 21 differ meaningfully in hardness, toughness and machinability, and the wrong grade underperforms even though the coating is technically "Stellite"
• Confusing thermal-sprayed Stellite with PTA or laser-cladded Stellite weld overlay — the two processes suit different thickness ranges and duty levels and are not simply interchangeable specifications
• Ignoring peak transient temperature and checking only the steady-state duty point against Stellite's ~650-700°C practical ceiling

Get a Quote for Stellite Coating

Lotus Surface Tech is an ISO 9001:2015-certified thermal spray facility in Chennai, Tamil Nadu, established in 2015, applying Stellite and other wear- and corrosion-resistant coatings alongside our tungsten carbide, chromium carbide and metalizing capability, with capacity for components up to Ø800 mm x 7 m and in-house precision grinding. Full process capability is on our technologies page, and industry-specific applications including power, process and oil & gas are covered on our industries page.

If you're deciding between Stellite and a carbide coating for a specific component, our tungsten carbide vs chromium carbide guide covers the carbide side of that decision in depth. When you're ready to move forward, contact us to get a quote — share your part drawing, the failure mode you're seeing in service, and the operating environment, and we'll recommend the right material and process.