Thermal Spray Coating Explained: Types, Functions and Applications

Thermal spray coating is a process that applies a new surface to a part, much like a high-tech painting process. It is a surface modification technique that allows manufacturers to give metal castings new and improved properties without changing the original casting material. This process is a form of thermal spraying, which applies a thin layer of material onto a substrate. When applied to castings, this coating can add properties like extreme durability, heat protection, or resistance to chemicals. This article will explain the fundamental process of thermal spray, and then detail the main types of thermal spray coating, their specific functions, and their many applications in different industries.

How Thermal Spray Coating Works

The process of creating a thermal coating on a metal casting involves a few steps. It is based on a simple concept: heating a material and then spraying it onto a surface. The way the material is heated and propelled determines the final properties of the thermal spray coating. Understanding this basic process helps to understand why the different types of coating methods produce different results.

The Fundamental Process

The core of thermal spray is a process of heating and propelling. The process begins with the preparation of the metal casting’s surface. The casting is first thoroughly cleaned to remove any dirt, oil, or grease. Then, the surface is roughened, often by a process like grit blasting. This roughened surface helps to create a good mechanical grip for the thermal coating.

Next, a raw material, called a feedstock, is prepared as a powder, wire, or rod. This feedstock is fed into a special thermal spray gun. Inside the gun, a heat source—such as a flame, a high-temperature plasma arc, or an electric arc—raises the temperature of the material to a molten or semi-molten state. The choice of heat source impacts how the material melts. Simultaneously, a high-velocity gas stream, which can be compressed air or an inert gas, atomizes the molten material into a fine spray of tiny particles. The stream propels the heated particles at high speeds toward the prepared surface of the metal casting.

The Resulting Thermal Coating

The moment the tiny, heated particles hit the surface of the casting, they flatten into thin, disc-like shapes called “splats” due to their high velocity. Upon impact, the particles cool and solidify almost instantly. The roughened surface of the metal casting is what allows the splats to interlock, forming a strong mechanical bond. This is a physical bond, not a fusion or chemical bond, which is why the underlying material of the casting is not changed.

As the thermal spray gun continues to move, more and more particles impact the surface, with new splats layering on top of the previously deposited ones. This continuous layering process builds a dense, cohesive structure that makes up the final coating. This structure is often described as being lamellar, or layered. The final properties of this thermal spray layer, such as its density, hardness, and porosity, depend directly on the type of feedstock material used and the specific thermal spraying method applied. This strong bond gives the casting its new surface properties without changing the underlying material.

Types of Thermal Spray Coatings

After learning about the general process, it’s helpful to understand the different methods used. The various types of thermal spray coating are categorized by their heat source and the way the particles are propelled. Each method creates a unique coating with distinct properties, making certain processes better for specific materials or applications, especially when working with different metal castings. The choice of which type to use depends on the desired final property and the type of material being applied.

Plasma Spray

The plasma spray process uses a high-temperature plasma jet, which is created by passing a gas like argon, helium, or nitrogen through an electric arc. The temperatures within this plasma can be extremely high, allowing it to melt materials with very high melting points, such as ceramics, refractories, and certain metals. Powder feedstock is injected into the plasma stream, where it melts and is carried by the gas jet toward the metal casting. The high velocity of the particles results in a very dense and well-bonded thermal coating with low porosity. This type of coating is often used for demanding applications on castings that require heat resistance, corrosion resistance, or a hard surface.

High-Velocity Oxy-Fuel (HVOF)

The HVOF thermal spraying method is known for producing dense, high-quality coatings. It works by mixing a fuel gas, such as propane, hydrogen, or natural gas, with oxygen in a combustion chamber. This mixture is ignited and burned at a high pressure, creating a high-speed flame that travels down a nozzle. The powder feedstock is fed into this gas stream, where it heats and accelerates. The particles reach very high velocities, often supersonic speeds, before they hit the metal casting. The high kinetic energy of the particles creates a dense thermal coating with a very strong bond to the substrate. HVOF is widely used for applying materials like carbides and certain alloys to castings for wear and corrosion protection.

Arc Spray

Arc spray is a type of thermal spray coating that uses an electric arc as its heat source. This process uses two electrically conductive wires as the feedstock. An electric arc is created between the two wires, causing them to melt at their tips. A high-pressure gas jet, often compressed air, then atomizes the molten metal and propels it toward the metal casting. This thermal spraying method is fast and cost-effective for applying metallic coatings. It is commonly used for applying thick layers of zinc or aluminum to large metal castings for corrosion protection, or for repairing worn components by building up the surface with a new layer of metal.

Flame Spray

Flame spray is one of the most traditional and simple methods of thermal spraying. It uses a combustion flame, created by burning a fuel gas like acetylene or propane with oxygen. The feedstock, which can be in powder or wire form, is fed into this flame. The flame melts the material, and a stream of compressed gas sprays the molten or semi-molten particles onto the surface of the metal casting. Compared to other thermal spray methods, flame spray typically produces a less dense coating and a lower bond strength. However, it is a versatile process that is used for general purpose applications, such as applying protective coatings for corrosion or for rebuilding dimensions on worn parts.

Functions of Thermal Spray Coatings

The primary purpose of applying a thermal spray coating is to give the surface of a metal casting a new property. This property can be a form of protection or a new functional characteristic. By selecting the right feedstock and spraying method, engineers can create a surface that is better suited for a specific environment or task.

Wear Resistance

Many metal castings are used in machines or environments where they are subject to wear from friction, abrasion, or erosion. Over time, this wear can degrade the casting’s surface and shorten its working life. A thermal spray coating can add a very hard and durable surface layer that protects the softer underlying casting. For example, a hard coating made of tungsten carbide can be applied to a steel pump impeller to protect it from abrasive particles in a fluid. The thermal coating takes the wear instead of the original part, making the entire assembly last much longer.

Corrosion Resistance

Corrosion is a natural process that degrades metals, especially when they are exposed to moisture, chemicals, or saltwater. A thermal spray coating can act as a protective barrier to separate the metal casting from its environment. This is a common application of thermal spraying in marine or chemical processing industries. A thick coating of zinc or aluminum, for example, can be applied to a large metal casting that will be exposed to saltwater. These materials corrode at a slower rate than the steel or iron casting, providing a sacrificial layer of protection that extends the life of the component.

Thermal Barrier

In some applications, a metal casting needs to be protected from extreme heat. While the bulk material might not melt, repeated exposure to high temperatures can cause it to lose strength or deform. A thermal spray coating with insulating properties can be used to protect the surface of the casting from heat. This type of coating is often a ceramic material, like yttria-stabilized zirconia. When applied to components in gas turbines or combustion engines, this layer acts as an insulator. It lowers the operating temperature of the underlying metal casting, which allows the component to function in a hotter environment for a longer period of time without damage.

Electrical Properties

A thermal spray coating can also be used to change the electrical behavior of a metal casting’s surface. Depending on the feedstock material, a thermal coating can be either electrically insulating or conductive. For example, a metal casting that needs to act as a conductor can be given a copper coating to improve its conductivity. Conversely, a casting that needs to be electrically isolated can be given a ceramic layer. This can be useful in applications where a component needs to be protected from stray electrical currents or needs to be a part of an electrical circuit.

Dimensional Restoration

Sometimes, a metal casting may have a dimensional problem, either from wear over time or from a mistake in the manufacturing process. Rather than replacing the entire part, a thermal spray can be used to build up a new surface to the correct dimensions. This process is a common way to repair worn bearing journals, shafts, and other machine components. A new layer of material is applied through thermal spraying, and then machined or ground back to the correct size. This is a cost-effective way to salvage and reuse metal castings that might otherwise be scrapped.

Key Applications of Thermal Spray Coating

Thermal spray coating is used across many industries to give metal castings new and useful surface properties. The choice of thermal spraying method and material depends on the specific needs of the application. Here are some examples of how this technology is used in various fields.

Aerospace

In the aerospace industry, metal castings are used in aircraft engines and other systems where components face extreme conditions. A coating is often applied to these castings to protect them from high temperatures and corrosion. For example, a layer made of a ceramic material can act as a thermal barrier on the surface of a turbine blade. This type of thermal coating allows the engine to operate at very high temperatures, which helps to improve its efficiency.

Automotive

Automotive castings, such as those used in engines and transmissions, are subject to high levels of friction and wear. A thermal spray coating can be applied to these parts to give them a hard, durable surface. For example, a hard layer on the surface of a piston ring can reduce friction and wear on the cylinder walls. This coating helps the engine last longer and can also improve fuel efficiency.

Medical

The medical industry uses this coating to modify the surfaces of metal castings for implants. These castings need to be biocompatible and have a surface that promotes integration with the body. A thermal spray coating of a porous material like titanium or a ceramic like hydroxyapatite can be applied. The coated layer provides a surface that allows for bone to grow into the implant, which helps to create a strong and stable connection.

Oil & Gas

Metal castings used in the oil and gas industry often operate in harsh, corrosive environments and are subject to abrasion. A thermal spray coating is applied to components such as pump impellers and valves to protect them from both erosion and chemical degradation. Applying a hard thermal coating of a tungsten carbide or a corrosion-resistant alloy can extend the life of these parts and reduce the need for frequent replacement.

Railway

The railway industry uses thermal spraying to protect metal castings on its equipment. For example, point machine parts, which are used to switch trains from one track to another, are exposed to constant friction, wear from grit, and the elements. This coated layer can be applied to these components to give them a very hard surface that resists abrasion. It can also provide protection from corrosion, which is a major factor in outdoor railway parts and components. Overall, it helps to reduce the maintenance on these parts and improves their reliability.

Which Thermal Spray Coating Should You Choose?

Choosing the right thermal spray method is an important decision that affects the performance and cost of the final product. The choice depends on a number of factors related to the metal casting, the desired coating, and the environment the part will be in. Understanding these factors helps to select the best type of thermal spray for a given project.

Desired Properties of the Thermal Coating

The most important factor is what you want the thermal coating to do. If the goal is to resist extreme wear from friction or abrasion, a dense and hard thermal coating is a good choice. For this, HVOF is a good thermal spraying process as it produces a very dense coating with high bond strength. If the casting needs to resist high temperatures, a coating of a ceramic material applied by plasma spray would be a better option because that process can handle the high melting points of ceramics. For corrosion resistance on large parts, a thick coating of zinc or aluminum applied by arc spray is a simple and cost-effective solution.

Substrate Material of the Casting

The base material of the metal casting plays a big part in which thermal spray method is suitable. Some spraying methods use very high temperatures that could affect the properties of the underlying casting material. Plasma spray, for instance, operates at extremely high temperatures. While this is necessary for melting some materials, it might not be suitable for castings made from alloys that are sensitive to high heat. In these cases, a cooler process like flame spray or arc spray is a better option because they do not heat the casting to the same degree. This type of spraying approach helps to avoid changing the original properties of the casting.

Cost and Production Volume

Economic factors are always a part of the decision. Some types of thermal spraying are more expensive to set up and operate than others. Arc spray is generally a lower-cost thermal spray coating process for applying metallic materials over large areas. This makes it a good choice for high-volume production of castings that only need basic protection. Plasma spray and HVOF, while producing higher-quality coatings, often have a higher cost per part. These coating processes are typically reserved for high-value castings where the superior performance of the coating is needed.

Geometry of the Part

The shape and size of the metal casting also influence the choice of a thermal spray. Some parts, like a flat plate, are easy to coat with any thermal spraying method. However, castings with complex shapes, deep internal cavities, or small features can be difficult to access with a thermal spray gun. Some thermal spray processes have smaller guns or nozzles that can reach into tighter spaces. For very detailed or complex shapes, a thermal spray coating may need to be applied from different angles or in multiple steps to get an even layer.

Conclusion

Thermal spray coating is a versatile surface modification process that gives metal castings new properties. This process involves thermal spraying a feedstock material onto a prepared surface, where it solidifies to create a protective or functional coating.

The different types of thermal spraying, such as plasma spray and HVOF, each produce a unique coating that is useful for specific applications. The functions of different coatings can be diverse, ranging from wear and corrosion resistance to thermal barriers and dimensional repair. These properties have a wide range of applications across many industries, including aerospace, automotive, medical, oil and gas, and railway.

Overall, it is a powerful technology that extends the life and improves the performance of metal castings by giving them a new, high-performance surface. The ability to give a casting the specific properties it needs for a given application makes thermal spraying a valuable process for many industries.

Taiyuan SIMIS Investment Casting Co., Ltd is a capable metal casting foundry in China. We provide not only various casting services, but also additional value-added services such as tooling design, machining, heat treatment, and surface treatment, including coating services like thermal spray coating. We can supply a one-stop solution for all your needs. Contact us today and speak with our experts to start your casting projects.