Some materials show up in a grinding shop and immediately change the rules. Thermal spray coatings — especially HVOF tungsten carbide — are exactly that kind of material. You touch a conventional wheel to them, and within seconds you can see it is not going to work. The wheel rubs instead of cuts. The surface heats up. The coating, which someone just spent serious money to apply, is at risk of being ruined before it ever goes into service.

We have spent years helping customers find the right diamond wheel for these coatings. Here is what we have learned along the way.

Why thermal spray coatings are different

A thermal spray coating is not like a piece of solid steel. It is a built-up layer, particle by particle, applied by heating feedstock material and spraying it at high velocity onto a prepared surface. The most common process for rolls and wear parts is HVOF — High-Velocity Oxygen Fuel — which produces coatings that are dense, well-bonded and extremely hard.

Typical HVOF coating materials include:

• Tungsten carbide (WC-Co, WC-CoCr): Hardness typically HV 1100–1400. Used on process rolls, pump sleeves, hydraulic rods and anywhere extreme wear resistance is needed.

• Chromium carbide (Cr₃C₂-NiCr): HV 800–1100. Common in high-temperature wear applications, such as boiler tubes and furnace rolls.

• Ceramic coatings (Al₂O₃, Cr₂O₃, Al₂O₃-TiO₂): HV 800–1200. Used for electrical insulation, corrosion barriers and wear-resistant surfaces in textile and paper machinery.

Conventional abrasives — aluminium oxide, silicon carbide — simply cannot cut these coatings efficiently. The abrasive grains dull almost instantly. The wheel glazes. Heat builds up at the contact point. This heat can cause cracks in the coating, pull out individual spray particles, or create burn marks that compromise the surface integrity the coating was meant to provide.

For thermal spray coatings, the abrasive choice is straightforward: diamond. Nothing else has the hardness to cut tungsten carbide, chromium carbide or sprayed ceramics cleanly and efficiently.

The bond must match the job

Choosing diamond is only half the decision. The bond holding those diamond grains has a huge effect on how the wheel cuts, how long it lasts, and what kind of surface finish it produces. Two bond types cover the vast majority of thermal spray coating grinding: resin bond and vitrified bond. They are not competitors — they are partners, each better suited to a specific stage of the job.

Resin bond diamond wheels — the finishing specialist

Resin bond wheels cut with a softer action, which makes them an excellent choice for finishing and polishing passes on thermal spray coatings. The resin bond has a small amount of give, meaning the wheel absorbs minor vibration and produces consistent surface finishes — typically Ra 0.2 µm or better when paired with the right grit in the final polishing stage.

For shops that are new to grinding thermal spray coatings, resin bond is also a practical starting point. It is more forgiving of machine condition and parameter setup, and it dresses easily when needed.

Vitrified bond diamond wheels — the productivity driver

In production environments where cycle time and tight dimensional control matter, vitrified bond diamond wheels are the proven workhorse. Their porous structure carries coolant into the grinding zone and evacuates swarf effectively — important when grinding coatings that produce fine, abrasive dust. A vitrified wheel typically cuts faster and holds profile longer than a resin wheel, which is why many roll shops have moved to vitrified diamond for HVOF-coated rolls that need strict cylindricality and roundness.

The trade-off is that vitrified wheels are stiffer and less forgiving of vibration. The machine needs to be in good condition and parameters must be properly set. But once dialled in, the productivity gain is real.

Metal bond diamond wheels — for roughing and extreme life

Metal bond wheels are extremely durable and keep their shape for a very long time. They are sometimes used for rough grinding passes on thick thermal spray coatings where the priority is stock removal and wheel life. However, they cut with more force and generate more heat, which means coolant delivery must be excellent and dressing must be kept up. Metal bond is rarely used for finishing thermal spray coatings because it tends to produce a rougher surface and requires more careful parameter control to avoid thermal damage.

Grit selection for thermal spray coatings

The grit size determines how much material you remove per pass and what the final surface looks like. For thermal spray coatings, we typically recommend a staged approach, starting from a moderate grit for roughing and working down.

Practical grinding parameters that work

You do not need a research paper to get good results on thermal spray coatings. Start here and adjust based on your own setup.

• Wheel speed: 18–25 m/s for resin bond diamond wheels. Vitrified bond can run slightly higher, 20–30 m/s.

• Depth of cut (roughing): 0.01–0.03 mm per pass. Start light and increase only if the wheel and coating allow it.

• Depth of cut (finishing): 0.003–0.01 mm per pass. Light cuts are essential for surface finish and to avoid pulling particles out of the coating.

• Spark-out passes: 3–8 passes at zero infeed to clean up deflection and improve finish.

• Coolant: High flow, water-based emulsion (3–5%) directed squarely at the grinding contact zone. Coatings like tungsten carbide generate fine dust that must be flushed away continuously.

Things that go wrong and how to fix them

"The coating is chipping or pulling out."
This is often caused by excessive depth of cut, too coarse a grit for the coating thickness, or a wheel bond that is too hard and rubbing instead of cutting. Reduce the infeed per pass, dress the wheel to expose fresh diamond, and check that the coating itself was applied correctly — poor bond between coating and substrate will show up here.

"The wheel wears out too fast."
Thermal spray coatings are abrasive, and some wheel wear is normal. But if wear is excessive, check your wheel speed — too low a speed makes individual diamond grains take heavier cuts. Also verify that you are using the correct bond grade for your coating hardness.

"I am getting burn marks."
A burn on a thermal spray coating is bad news. It usually means the wheel is loaded and rubbing. Dress the wheel. Increase coolant flow. Reduce the depth of cut. If the problem persists, consider switching to a vitrified bond wheel, which runs cooler thanks to its natural porosity.

"The surface finish is not good enough."
Move to a finer grit for the final pass. Ensure spark-out passes are included. Check for vibration or spindle runout — thermal spray coatings are sensitive to any movement that transfers to the surface.

Why the grinding wheel is the decision that makes or breaks a thermal spray job

Applying a thermal spray coating is an investment. The coating material costs money. The spraying equipment costs money. The downtime while the part is out of service costs money. None of that investment pays off if the finish grinding step goes wrong.

A diamond grinding wheel — matched correctly to the coating material, the bond type, the grit size and the grinding stage — is what turns a rough as-sprayed surface into a precision-engineered working layer.

If you are grinding HVOF tungsten carbide, chromium carbide, ceramic coatings, or any other thermal-sprayed surface, and you are not sure your current wheel is giving you the best results, reach out. Tell us your coating material, your machine and the surface finish you need. We will recommend a diamond wheel specification that fits the job — because we have seen these coatings before, and we know what works.

Let us help you get the finish right. Contact our technical team with your coating details, and we will put the right diamond wheel on your grinder.