A GUide to Thermal Spray
What is Thermal Spray?
Thermal spraying techniques consist of a heat source such as a flame and a coating material, typically in a powder or wire form, that is melted into small droplets and sprayed onto surfaces at high velocity. This industrial coating process is also sometimes referred to as “spray welding.”
Thermal sprayed coatings are usually applied to metal substrates and are sometimes applied to plastic or ceramic base materials. They enhance the performance of the component by providing improved wear resistance, corrosion resistance or thermal barriers, and are a versatile and effective alternative to other coating methods. For engineering applications, thermal coatings are distinguished by their abilities to apply metals, cermets, ceramics, and polymers with minimal porosity and very high bond strengths without melting or substantially heating the substrate.
Coating quality is usually evaluated by calculating porosity, oxide content, macro and micro-hardness, surface roughness, and bond strength. As the particle velocities increase, the coating quality usually increases as well.
Different processes of thermal spraying include:
- Plasma spraying
- High-velocity Oxy-fuel (HVOF)
- Detonation spraying (D-gun)
- Wire arc spraying
- Cold spray
- Flame spraying
- High-velocity air fuel (HVAF)
- Spray and fuse
Benefits of Thermal Spray
Thermal spraying provides the option to apply an array of coating materials such as ceramics, metals, alloys, cermets, carbides, polymers, and plastics. Thermal spray coatings allow design and construct a component using a substrate and a coating material that cannot be accomplished by either alone. They also allow the use of inexpensive base materials in demanding wear/corrosion applications by putting the required mechanical and corrosion-resistant properties “at the surface”. Thousands of applications in virtually every industry employ the cost-effective benefits of thermal spray technology.
Tight control of spray parameters allows application of a range of coating thicknesses, from very thin, dense coatings to thick, high cohesions coating applied at high deposition rates and thicknesses. While the majority of thermal spray coatings are mechanically bonded to the substrate, some coatings can be heat-treated to achieve an even stronger bond to the substrate. Thermal spray also affords the option to spray coating materials with a higher melting points than the substrate. The majority of parts can be sprayed with minimal-to-no preheat or post-treatment, and the ability to control heat input into the base metal is very well understood and, therefore, very controllable. Parts can be coated “new” or rebuilt/refurbished at a price that is typically a fraction of the wrought material or replacement price. The exceptional versatility and cost-effectiveness of thermal spray coatings makes them an attractive tool for engineers designing new wear/corrosion-resistant components as well as M&R professionals managing equipment uptime while managing a budget.
Since the launch of thermal spray coatings in the middle of the 20th century, plant superintendents and maintenance personnel have depended on thermal spray technology as a valuable way to improve and enhance critical components for light and heavy industrial equipment. Thermal spray technologists constantly advanced thermal spray technology by creating new delivery systems and materials (mostly powders), and perhaps most importantly, perfecting the application know-how that provides potential new and exciting applications.
Thermal Spray Processes
Our primary thermal spray processes are detonation, flame, arc, plasma, and high-velocity oxygen fuel (HVOF).
This is a proprietary process that was invented at Praxair Surface Technologies. Gas and powder are combined in controlled detonations and blasted at supersonic speeds onto the part. Common materials are tungsten carbide or chrome carbide, metallic alloys, ceramics, and cermets.
This process is low cost and can easily be performed in the shop or onsite. It is also known as oxy/acetylene combustion spray and is the earliest thermal spray technique that was developed 100 years ago. Oxygen and fuel gas such as acetylene, propane, or propylene is fed into a torch and lit into a flame. Wire or powder can be injected into the flame where it melts and is then thermally sprayed onto the workpiece. Stainless steels, nickel, aluminides, Hastelloy alloys, tin, and babbitt metal (a tin-based alloy) are a few of the materials that flame spraying can be applied to.
During this process, two wires are brought into contact with each other at the nozzle at the same time. When the electrical load is placed on the wires, it causes the tips of the wires to melt once they touch. Atomizing gases like air or nitrogen are used to clear out the molten material off the wires to move it to the workpiece. Arc spraying is relatively cost-effective and readily functional in the field. Velocities that have low particles allow for high-maximum coating thickness for any specified material. Materials that are typically applied by arc spraying include stainless steels, Hastelloys, nickel aluminides, zinc, aluminum, and bronze.
Plasma spraying is usually considered to be the most multifaceted of the thermal spray processes. Gases like argon and hydrogen are passed through a torch throughout the plasma spraying operation. The gases are dissociated and ionized through an electric arc. The atomic components recombine past the nozzle and they give off an immense amount of heat. The plasma core temperatures are usually more than 10,000 ℃, which is more than the average melting temperature of any material. The powder is injected into the plasma plume where it is melted and projected toward the workpiece.
High-Velocity Oxygen Fuel
This process is relatively new and has propelled the thermal spray application range into places that used to be unreachable. In HVOF spraying, a gaseous fuel like hydrogen or liquid fuel like kerosene, is mixed with oxygen and combusted inside the combustion chamber of the torch at high pressure. The powder is injected into the flame and heated and accelerated due to supersonic speed of the gas’s velocity. This produces very dense coatings by thermal spray available. The HVOF process is the preferred technique for thermal spraying wear and corrosion-resistant carbides as well as Hastelloy, Tribaloy, and Inconel alloys.
Thermal spraying application improvements include:
- Wear resistance & control:
- Abrasion resistance
- Adhesion (friction) resistance
- Oxidation resistance
- Cavitation resistance
- Erosion resistance
- Metal fatigue replacement
- Corrosion resistance & protection
- Mechanical property enhancement