In any industry that requires the following, coating technologies are essential:
- Enhanced durability.
- Wear resistance.
- Corrosion protection.
Thermal spray and cold spray processes are popular techniques to achieve this. Both are effective in depositing coatings on different substrates but they have different mechanisms, which makes them for certain applications and materials.
What is the Thermal Spray Process?
Thermal spraying involves a group of coating processes where the aim is to use heat to melt or soften a coating material. This is then propelled onto a substrate. This is a broad category that includes techniques such as:
- Plasma spraying.
- Vacuum plasma spraying
- Atmospheric plasma spraying.
These methods are vital for producing thermal spray coatings, which are known for improving surface and coatings technology across a range of industries.
A thermal spray process usually involves high-temperature heat sources – such as an electric arc or a plasma torch – which is used to elevate the temperature of a coating material (this could be metal or ceramic).
The heated coating material then gets projected at high velocities toward the substrate surface, where it quickly solidifies and forms a dense, adhered layer. This often gets referred to as a sprayed coating, and it results in the substrate becoming more resistant to:
- Corrosion.
- Wear.
- Heat.
Types of Thermal Spray Techniques
Different thermal spray techniques vary in the following, which makes each suitable for specific applications:
- Temperature.
- Environment.
- Coating properties.
These are the most prominent thermal spraying techniques:
Plasma Spraying
We use this technique to apply ceramic or metal coatings as this helps them achieve thermal barrier coatings. It involves using a plasma torch that can generate temperatures of up to 15,000°C, which is why it’s ideal for high-melting-point coating materials in particular.
Vacuum Plasma Spraying (VPS)
In VPS, the spraying is done within a vacuum chamber so oxidation can be minimized throughout the coating process. You often see this technique used in aerospace and other high-performance applications where you can’t take chances on coating purity or adhesion.
Atmospheric Plasma Spraying (APS)
APS can be done in the open air, so it’s far more cost-effective and suitable for large-scale applications. Although APS coatings may lack some purity compared to VPS, this method is still effective for applications needing thick, durable layers, such as zinc coatings, for corrosion resistance.
You decide which of these three you use depending on what properties you want the final product to have – though all of them perform well in applications that require high-density, durable coatings. This is why they’re often used in producing industrial coating solutions for sectors such as:
- Automotive.
- Aerospace.
- Power generation.
How Cold Spray Works
Cold spray works differently as it uses high-pressure gasses instead of heat. This propels powder particles at supersonic speeds onto a substrate surface (It’s called “cold” spray as it produces less heat than thermal spray processes).
For this method, particles are accelerated in a nozzle and reach extremely high velocities before colliding with the substrate. The impact causes the particles to deform and bond to the surface without melting. This solid-state process helps retain the original properties of the coating material, ensuring high strength, hardness, and resistance to oxidation.
Key Differences Between Thermal Spray and Cold Spray
These processes are similar but involve some major differences:
Temperature and Energy Sources
Thermal Spray: Needs high temperatures to melt or partially melt the coating materials – heat sources like electric arcs or plasma torches are often used for this.
Cold Spray: The high-speed means you don’t have to melt the particles. This means the properties of the coating materials can be preserved, so it’s a great method for metals like aluminum, titanium, and copper – these would normally oxidize or degrade at high temperatures.
Coating Material Properties
Thermal Spray Coating Materials: Primarily the following:
- Metals.
- Ceramics.
- Carbides.
The high-temperature aspect lets you use a wide range of materials – including those requiring thermal barrier properties, such as ceramic.
Cold Spray Coating Materials: Typically metals and alloys. Due to the lower process temperature, cold spray materials must deform under pressure without melting. Metals like aluminum, titanium, and copper are especially compatible, though composite powders are also becoming more prevalent.
Coating Bond and Structure
Thermal Spray: The melting and rapid cooling of particles in thermal spraying often lead to microstructures with some porosity. These coatings, however, are still highly protective and can be tailored by adjusting process parameters.
Cold Spray: Cold spray coatings are denser with minimal porosity because the material isn’t heated. This results in stronger bonds, and these coatings often have superior corrosion resistance compared to traditional sprayed coatings.
Applications and Limitations
Thermal Spray: Mostly in high-temperature applications – anything where you need the materials to withstand significant heat and wear. For example, this could be turbine components and engine parts or in marine and power generation industries.
Cold Spray: Ideal for applications where the coating material’s properties need to be preserved – this includes:
- Aerospace.
- Defense.
- Electronics.
Benefits and Drawbacks
Each method offers unique benefits and has limitations based on what the material will ultimately be used for:
Thermal Spray Benefits:
- Can produce thick and temperature-resistant coatings.
- Great for various metals and ceramics.
- Suitable for harsh environments.
Thermal Spray Drawbacks:
- High temperatures can lead to oxidation or thermal stress in the coating.
- Not all materials are compatible with the high-heat environment.
Cold Spray Benefits:
- Low-temperature process preserves the properties of coating materials.
- Minimal oxidation and porosity, leading to high-density coatings.
- Enhanced corrosion resistance due to the lack of thermal effects.
Cold Spray Drawbacks:
- Limited to materials that can deform at high impact velocities.
- May require specialized equipment to achieve optimal coating properties.
Choosing the Right Industrial Coating Process
Choosing between these two techniques hinges on whatever application the materials will be used in – this varies across different industries.
Opt for a thermal spray if you need a high-strength, heat-resistant coating. Cold spray is preferable if you need corrosion resistance and to preserve the original materials’ properties.
The Evolution of Thermal and Cold Spray Techniques
Industries are constantly seeking ever-greater performance standards, so thermal spray and cold spray processes are always evolving. Research is always ongoing into developing new thermal spraying techniques and more efficient ways to achieve uniform coatings.
Meanwhile, innovations in cold spray technology are enabling companies to repair and rejuvenate components that would otherwise require replacement.
Both techniques play important roles in modern coating processes – from enhancing the longevity and resilience of parts to opening up more design and functionality opportunities.
