Today, thermal spray coatings are utilized in several different industries. These coatings consist of wire and molten powder which are exposed to plasma or oxy-fuel combustion.
The fire from the spray device will power the mixture that has been heated, and once it has been sprayed onto metal, the mixture will retain a firm coating.
Thermal spray coatings are used in a wide spectrum of many useful applications, which can include protecting airplanes, buildings, and other structures from extreme temperatures, chemicals, or environmental conditions such as humidity and rain.
In this article, we will discuss what thermal spraying is and how to do it, and its application and benefits.
What is Thermal Spray Coating?
Thermal spray coating is a process that sprays coating materials using the pressure of a high-heat gas. The material then bonds to the surface, forming a coating that grants properties such as corrosion protection, thermal resistance, environmental protection, lubricity, wear resistance, and electrical/thermal conductivity.
Coating materials usually come in powder or wire form through a feeder and may include metals, alloys, ceramics, cermets, carbides, plastics, or composites. The spray gun, also known as the spray torch, is the main tool used to operate. It can be operated manually or by a robot arm.
Thermal spray systems are widely used in engineering for aerospace, mechanical, marine, and automotive applications. They are also used for electronics, biomedical as well as several other applications.
Several criteria are used to evaluate the quality of the coating, such as bond strength, porosity, oxidation, hardness, and roughness. The coating’s thickness can be as small as 20 microns, or it can be several millimeters thick.
How to do Thermal spraying?
Thermal spraying is a generic category of coating processes that apply a consumable as a spray of finely divided molten or semi-molten droplets to produce a coating.
It is distinguished by its ability to deposit coatings of metals, cermet’s, ceramics, and polymers in layers of substantial thickness, typically 0.1 to 10mm, for engineering applications.
Almost any material can be deposited so long as it melts or becomes plastic during the spraying operation. At the substrate surface, the particles form ‘splats’ or ‘platelets’ that interlock and build up to give the coating.
The deposit does not fuse with the substrate or have to form a solid solution to achieve a bond. This is a significant feature of thermal spraying compared to many other coating processes, particularly arc welding, brazing, and laser coating processes.
The bond between a thermally sprayed coating and the substrate is primarily mechanical, and not metallurgical or fused. Adhesion to the substrate will depend on the condition of the substrate surface, which must be clean and roughened by grit blasting or machining prior to spraying.
Thermal spraying processes have been widely used for many years throughout all the major engineering industry sectors for component protection and reclamation. Recent equipment and process developments have improved the quality and expanded the potential application range for thermally sprayed coatings.
Types Of Thermal Spray Coating Processes
Several variations of thermal spraying are distinguished:
- Plasma spraying
- Detonation spraying
- Wire arc spraying
- Flame spraying
- High-velocity oxy-fuel coating spraying (HVOF)
- High-velocity air fuel (HVAF)
- Warm spraying
- Cold spraying
- Spray and Fuse
In classical (developed between 1910 and 1920) but still widely used processes such as flame spraying and wire arc spraying, the particle velocities are generally low (< 150 m/s), and raw materials must be molten to be deposited.
Plasma spraying, developed in the 1970s, uses a high-temperature plasma jet generated by arc discharge with typical temperatures >15,000 K, which makes it possible to spray refractory materials such as oxides, molybdenum, etc.
Here are we discuss the 5 most common processes to generate thermal spray coatings in detail:
#1. HVOF (High-Velocity Oxy-Fuel Spraying).
HVOF is a process that makes use of a torch that allows the flame to spread whenever the nozzle is used. This creates rapid acceleration which speeds up the particles in the mixture. The end result is an exceptionally thin coating that is evenly applied.
Despite being thin, this coating is strong and adheres well. Its resistance to corrosion is better than plasma coatings, but it is not well suited for high temperatures.
#2. Combustion Flame Spraying.
Combustion flame spraying is an excellent option for surfaces that aren’t designed to handle extreme stress. The coating which results from this process is not strongly attached to the surface since the spraying mechanism is powered by a lower flame velocity.
The flame will be generated via oxygen which has been combined with fuel, and this will melt the mixture. Combustion flame spraying is popular for low-intensity applications due to its low cost.
#3. Plasma Spraying.
Plasma spraying makes use of the plasma torch as the main tool for heating and spraying the coating. After the powder material has been melted down, it is then placed on the product in a manner that is similar to combustion flame spraying.
The coatings which result from plasma spraying maybe a few micrometers thick to a few millimeters thick. While the powder is the most widely used material, metals and ceramics are also used. The plasma spraying process is highly popular due to its adaptability.
#4. Vacuum Plasma Spraying.
Vacuum plasma spraying is done in a controlled environment but utilizes low temperatures. This maintains the vacuum while also reducing damage to the material. A variety of gas combinations can be used to get the necessary pressure for spraying.
Vacuum plasma spraying is used for items such as car bumpers, the dashboard, or housings for door mirrors. This process can also be used for the pre-treatment of polyethylene moldings, which provides adhesion for epoxy adhesives that are water-based.
#5. Two-Wire Electric Arc Spraying.
This spraying method utilizes an arc point that is created between two wires which are electrically conductive.
Melting will occur at the point where the wires connect. The arc allows for heating which in turn creates deposition and melting, similar to combustion flame spraying which is used with a torch.
Compressed air will be used for spraying the coatings. This procedure is popular due to its cost-effectiveness, and will typically use aluminum or zinc as the base material.
Advantages of Thermal Spray
- Wide range of coating materials – there is a vast array of different materials which can be turned into high quality coatings via the thermal spray process. These can include; metal, alloy, ceramic, plastic, and polymer and can be in the form of powder, rod, or wire. The coating material can be specifically selected for each individual substrate and the job that it has to do and so a near perfect match can often be made between the two.
- Wise variety of substrate materials – as long as the material to be coated can withstand the heat of the thermal spray process then almost any material can be coated using this method. However, there are also a number of specific spray coating methods which in fact use much lower temperatures and so the range of substrate materials is even further increased.
- Extends the lifespan of the substrate – a strong and efficient thermal spray coating will extend the lifespan of that substrate by providing an effective barrier against erosion, decomposition and other forms of surface damage. Thermal spraying allows much thicker coatings to be applied (typically up to 10 mm) in higher deposition rates than other coating methods, which provide a much more effective barrier against wear and tear.
- Reduced cost – in many cases repairing an object by applying a thermal spray coating to it is much cheaper than completely replacing it. Also, thermal spray coatings tend to be much more efficient and waste less of the coating material than with other methods so are a much more viable option when more expensive coating materials are involved.
Disadvantages of Thermal Spray
- Disguises the substrate – as thermal spray is so efficient in many cases it is impossible to tell what material the substrate was made of after the coating process, unless stringent records are kept.
- Cannot precisely evaluate effectiveness – once the thermal spray coating has been applied it is often difficult to tell exactly how well the coating has gone on, other than by a visual assessment.
- Costly set up – some methods of thermal spray coatings require very expensive apparatus, which can result in a high initial set up cost.