In simple terms, thermal spraying may be defined as the process where a particular heated and melted material is sprayed onto a substrate surface for a variety of purposes. Various types of thermal spray techniques are used in different industries. Thermal spraying is mostly used for producing coatings on various structural materials.
These coatings are made of different materials and are used for providing protection against high temperatures, erosion, corrosion, wear and these are also used for changing the appearance as well as electrical properties of the substrate surface. There are a number of advantages of using thermal spraying.
One of the biggest advantages is that only lightweight equipment is needed and the process is extremely flexible which also reduces the overall processing time. It is easy to control the coating thickness. Almost all types of substrate materials can be coated with the help of these techniques.
Various Types of Thermal Spraying Techniques
One of the more simple techniques used for thermal spraying is the flames technique. It also is the cheapest technique to deposit material onto the substrate. Various forms of feedstock such as rod-flame, wire or powder can be used in this method. As far as the fuel gases are concerned, the commonly used gases include natural gas, propane, and hydrogen.
The basic technique behind this method is that when the fuel and oxygen combines, they produce a heat source that creates gas steam. This gas steam is used for atomizing the molten metal and the molten metal is then accelerated in particle form onto the substrate. One of the biggest advantages of this technique is it allows the use of a wide range of feedstock materials by processing them into powder form. In other words, this particular technique offers a large choice of coating materials.
There are also some limitations of this process. The biggest limitation is that there are some materials that have a higher melting temperature as compared to the temperature of the flame. Due to the low velocity which is restricted to 40 m/s and lower temperatures, this process usually results in higher porosity, oxides, and inclusions in the coating on the substrate. The adhesiveness strength, as well as the bond strength, is also low as compared to some of the other techniques used these days.
As far as the application of this technique is concerned, it is extremely useful for coatings where excellent impact resistance and wear resistance is required such as in oil drilling parts and agricultural harvesting components.
Another popular type of process is the plasma spray process. In this process, a high-voltage discharge is used to initialize the plasma that results in localized ionization. An electric arc is formed between two electrodes made of tungsten and copper. It results in the generation of steam of ionized plasma gas at a very high temperature. This gas is usually made of argon/hydrogen. The superheated gas expands and is accelerated when it passes through a constricting nozzle, where it can reach a velocity of Mach 2.
In this technique, the coating material is carried in powder form and is carried through an inert gas stream into this plasma jet. Here it is heated by the plasma jet and propelled over the substrate. The temperature can reach up to 15,000 degrees C and this is the reason that this process results in a high-quality coating. The high velocity of the coating particles which is usually 200 to 300 meters per second also plays a part in the quality of the coating.
One of the biggest advantages of this coating technique is that it can be used for spraying coating materials with an extremely high melting point is such as ceramic oxides, tungsten as well as zirconium among others. This is a highly versatile spraying technique and this is the reason that it is used in a wide range of industries. This technique is commonly used in automotive, agricultural, medical device and aerospace industry.
The high-velocity oxygen fuel spray is another technique that is widely used in a variety of industries. As the name suggests, this technique uses confined combustion and an extended nozzle for heating and accelerating the coating material. The velocity of gas particles in this device may reach greater than Mach 5 before the particles hit the substrate. Due to the high velocity and high combustion pressure, this technique produces a very high-quality coating which is extremely dense and has very low oxide content.
In this technique, various combustion gases are used including hydrogen and propane among others. Since the coating particles hit the substrate with a very high kinetic energy, these do not need to be in fully molten form for high-quality coating. Due to the exceptionally high velocities, some of the coatings can be sprayed extremely thick. This technique is used in a variety of industries such as food processing, construction equipment, agricultural equipment and aerospace industry among others.
Overall, these are some of the major types of thermal spraying techniques. The technique for a particular application is chosen on the basis of requirements and budget.