What is Plasma Spraying? The Process Uncovered

You know the drill, don't you? Manufacturers just swap out parts whenever they get a bit worn out. But what if you could get those parts to last several times longer by giving them a coating that's actually tougher than the original material? That's exactly what plasma spraying does, and strangely enough, it's one of the most commonly used thermal spray methods out there.

The way it works is by firing a stream of coating material through a hot plasma jet reaching temperatures over 10,000°C, and what you get is a protective layer that can shrug off extreme wear, corrosion, and heat without breaking a sweat. In this article, we'll take a closer look behind the scenes :

• What plasma spraying is, and how it really works
• The many different techniques you can use for plasma spraying
• A step-by-step guide on how each of those techniques is actually done

Let's dive in.

How Plasma Spraying Actually Works

Plasma spraying uses an electrically generated plasma arc to melt coating materials and then shoot them onto a surface at high speed. The coating material, fed into the system as a fine powder, passes through a plasma jet where it turns into a liquid almost instantly before bonding to the surface.

To break it down:

• Gas gets ionized inside the spray gun's nozzle to create the plasma jet - this converts a gas like argon or nitrogen into an ultra-hot stream
• Powder gets injected right into this plasma stream, where it gets melted down in a split second
• Particles get accelerated & pushed towards the workpiece at high speed, where they then flatten out on impact
• Layer builds up as millions of flattened particles stack and interlock across the surface

Key Components of the System

Every plasma spray setup relies on a few critical parts working together.

The coating doesn't just stick by itself - when the molten particles hit the prepared surface at high speed, they fill in every tiny groove and pore. This mechanical sticking is what gives plasma-sprayed coatings their staying power, even in really harsh operating conditions.

The surface prep before spraying, usually just a bit of grit blasting, makes a huge difference in how well the final coating will perform over time

Plasma Spray Techniques You Need to Know

Not all plasma spraying methods work the same way. The technique you use depends on the coating material, the operating environment of the finished part, & how tight the quality tolerances need to be. Each method tweaks the standard plasma spray setup in a specific way to control the coating's density, purity, & microstructure.

Here are the primary techniques used across industrial applications today:

• Atmospheric Plasma Spraying (APS) is the most widely used technique & is performed under normal open-air conditions for general-purpose wear & corrosion-resistant coatings
• Vacuum Plasma Spraying (VPS) runs inside a low-pressure vacuum chamber, producing high-purity coatings on reactive or oxidation-sensitive materials
• Suspension Plasma Spraying (SPS) uses a liquid suspension feedstock instead of powder & creates ultra-fine, nanostructured coatings with columnar microstructures
• Solution Precursor Plasma Spraying (SPPS) uses a chemical solution as feedstock to produce thin, highly uniform ceramic coatings at the nanoscale level

APS is the workhorse of the group & handles the widest range of coating materials. VPS strips oxygen from the equation entirely, which makes it the go-to option for coatings that would degrade through oxidation during spraying. SPS & SPPS swap traditional powder feedstock for liquid-based alternatives, giving engineers finer control over coating thickness & grain structure at the nanoscale.

Each Technique From Start to Finish

Now that we know what each plasma spray method brings to the table, let's walk through the actual process behind all four. The steps might look similar at first - but it's the details that separate one technique from another that really matter.

1. Atmospheric Plasma Spraying (APS)

APS is the most straightforward of the four techniques. It runs under normal open-air conditions, which keeps setup costs low & makes it flexible for a wide range of materials.

• Step 1 – Surface Prep: The substrate gets cleaned & abraded with grit blasting to roughen the surface at a microscopic level. This gives the molten particles something to grip onto.
• Step 2 – Plasma Generation: A DC electric arc fires between the cathode & anode inside the spray gun, ionizing a gas like argon. To boost energy enough to melt high-temperature materials, a secondary gas is added & changes the thermal properties of the stream. This creates a plasma plume.
• Step 3 – Powder Injection: The fine powder is actually pumped into that plasma jet through a gas stream. That powder is a pretty fine texture (20–90 µm particles), and the extreme heat from the plasma jet causes it to melt almost instantly.
• Step 4 – Deposition: Those molten droplets start flying towards the substrate at high speed. When they hit the surface, they flatten into little "splats" and then solidify in a split second. We get millions of these tiny splats that pile on top of each other, layer by layer, to form the coating.

Pro tip: To be honest, the shape those splats take on impact is a lot more important than most people give it credit for. Disc-shaped splats make for a coating that's a whole lot better at sticking to the substrate and produces fewer bubbles than the splat-shaped ones. The real trick to controlling this is keeping an eye on the temperature of the substrate while you're spraying.

2. Vacuum Plasma Spraying (VPS)

VPS pretty much follows the same basic idea as APS, but the whole process happens inside a sealed vacuum chamber. And that makes an absolute huge difference in coating purity and density.

• Step 1 – Chamber Evacuation: We pull a vacuum in that chamber down to a pretty low pressure – less than 0.1 mbar. That gets rid of any oxygen or hydrogen that might be lurking in there and trying to mess up our coating.
• Step 2 – Filling with an Inert Gas: Once the vacuum is reached, we pump in some inert gas (usually argon) to bring it up to the right operating pressure - that's usually somewhere between 30–900 mbar, depending on what we're trying to do.
• Step 3 – Cleaning the Substrate with a Reverse Arc: Before we even start spraying, we send a reverse arc between the plasma torch and the workpiece. This gives the substrate a good clean at an atomic level, gets rid of those oxide layers, and leaves the coating with a much better bond than you'd get from blasting alone.
• Step 4 – Plasma generation and Powder Injection: The plasma arc gets lit up, and we pump powder into the jet just like in APS. But the environment being inert here means the molten particles can make their way to the substrate without oxidizing during their journey.
• Step 5 – Coating Build-Up: Since it's a vacuum, these particles hit the substrate at a higher velocity, and we end up with coatings that are a lot denser, with fewer oxides and better bonding.

Pro tip: If you're working with a reactive metal like titanium, VPS is the way to go - if you tried to spray titanium in open air, all the oxide contamination would make your coating a whole lot weaker.

3. Suspension Plasma Spraying (SPS)

SPS swaps out the dry powder feedstock for a liquid containing really fine particles (below 1 µm in diameter). This gives us the ability to produce all sorts of super-fine coating microstructures that regular powder-fed systems just can't match.

• Step 1 – Preparing the Suspension: We take some ultra-fine particles - often below 1 µm - and mix them into a liquid carrier like water or ethanol. It's all about the right balance of solid and liquid - how much of each influences the final coating properties. It just so happens that ethanol-based suspensions make for denser coatings at the same power levels.
• Step 2 – Plasma Generation: We're running this plasma torch at a bit higher energy level in SPS because we need to take care of the liquid carrier first. We need to vaporize that carrier before the plasma can actually interact with our particles. We commonly use gas mixtures to get a stable, high-energy plasma stream.
• Step 3 – Suspension Injection: We pump that liquid suspension into the plasma jet, either radially or axially - it all depends on the system. The plasma does all the work of fragmenting the liquid into tiny droplets, and then, as the liquid phase evaporates, our particles are exposed to the plasma.
• Step 4 – Processing in Flight: Now this is where SPS really gets interesting. These particles that are freed from the liquid melt rapidly and head off to the substrate. Since they're so small, even tiny variations in the plasma flow have a big impact, so it's a bit of a delicate dance to control. This is actually one of the things that makes SPS so special – it gives us unique microstructural features like columnar structures.
• Step 5 – Coating Formation: Our molten nanoparticles hit the substrate and form these tiny splats. Now the coating microstructure - whether it's super porous or extremely dense - that's determined by a number of things, including the spray distance and how concentrated our suspension is. A bit shorter distance and a bit more solid in the suspension, and you're looking at a much denser result.

Pro tip: SPS coatings with columnar microstructures have actually been a real game-changer in aerospace as thermal barrier coatings - they give you that same level of strain tolerance as EB-PVD coatings but at a fraction of the cost.

4. Solution Precursor Plasma Spraying (SPPS)

SPPS takes the liquid feedstock idea to the next level, moving beyond suspending pre-made particles in a liquid. Instead, it starts with a liquid solution where the coating material is still in a chemical form, not yet a solid. The ceramic actually forms during the spraying process itself.

• Step 1 – Mixing the Precursor Solution: The Starting Point: To get the ball rolling, you need to dissolve salts or metal-organic compounds in a solvent like water or ethanol. This creates a uniform solution that is the foundation of your coating. When it comes to thermal barrier coatings, zirconium and yttrium salts are the usual go-to ingredients. Importantly, the chemistry, concentration, and solvent you use all play a big role in determining what your final coating will look like, and what it's made of.
• Step 2 – Plasma Generation: Like with APS, the plasma torch is pretty standard stuff - same equipment, same operating parameters. The only thing that changes is that you need to adjust for injecting liquid into the system.
• Step 3 – Getting the Solution into the Plasma: The precursor solution gets injected into the plasma plume via a high-pressure delivery system. Once those droplets hit the plasma jet, a rapid series of physical and chemical reactions happens that you just don't see with any other plasma spray technique.
• Step 4 – The Secret Sauce that Sets SPPS Apart: This is where things get really interesting. The solution droplets go through a whole series of chemical and physical changes as they fly through the plasma jet - all in the space of a few seconds.
  • The solvent in each droplet dries out\
  • The salts inside the droplet start to come out of solution and form solid particles\
  • These particles then decompose and start to crystallize\
  • The resulting solid particles get all molten again as they travel through the plasma stream

The whole process happens from the moment the solution is injected right up to the moment the material hits the substrate. Depending on the parameters you're using, the particles can arrive at the surface in different states - fully molten, or partially pyrolized.

• Step 5 – The Finished Coating: The processed material ends up on your substrate, creating super-thin coatings with a unique, layered structure that is both high-porosity and low-porosity. This has been shown to be really effective at reducing thermal conductivity in barrier coatings.

A Quick Tip: One of the main advantages of SPPS is just how fast you can test out new coating compositions. By mixing up a solution instead of having to make a custom powder, you can tweak the recipe and spray a test sample in a single session. This cuts down the R&D development time to almost zero.

CY Thermal Spray - Experts in Getting Plasma Spraying Right

You now have a pretty good grasp of how plasma spraying works, from the science behind the process to the details that set APS, VPS, SPS, and SPPS apart. That puts you in a much better position to choose the right technique for your needs.

Here are the key points to remember:

• Plasma spraying uses a plasma jet to melt and deposit materials at high speed
• APS is great for all-purpose industrial coatings because it's versatile and cost-effective
• VPS is the go-to option when you need to work with reactive metals or need high-purity results
• SPS uses liquid suspension feedstock to make really fine, nanostructured coatings
• SPPS forms the ceramic material right during the spraying process from a chemical solution, which lets you test different compositions super fast

We here at CY Thermal Spray have spent the last 40 years perfecting surface coating solutions for industrial machinery components - and do it in over 56 countries worldwide. Got a plasma coating that needs to be done right? Send us your plans and requirements for the part, and we'll match you up with the perfect material and technique to get it done to your exact spec.