Types of Thermal Spray Coating

Thermal spraying is a series of processes in which a finely divided metallic or non-metallic surface material, in a molten or semi-molten state, is dropped onto a prepared substrate to form a sputter deposit. The surface materials can be in the form of powders, sticks, strings or wires, and the various spraying processes are generally classified into two groups according to how they are exothermic: combustion or electric. According to the method of energy production, thermal spray processes can be divided into three main groups.


Commonly known as Cold Spray or Cold Gas Dynamic Spray (CGDS), these products use converging-diverging nozzles to create a high-velocity gas stream used to accelerate powder particles during the atomization process. In this case, only cold-formed metal or alloy particles are sprayed without oxidation during the process.


Three types of combustion spray guns are:


Flame spray torch operate at atmospheric pressure, using mainly oxyacetylene mixtures, and reach combustion temperatures of up to about 3000 K. The spray material is introduced axially. Flame velocities of less than 100 m/s are characteristic of this process and use powder or wire, rods or strings.

2.1.1 Powder Flame Spraying

This process is "a thermal spray process in which the material to be sprayed is in powder form." This is one of the simplest and oldest injection molding processes. The powder is fed by a simple powder hopper or a more complex powder feeder through the central orifice of the nozzle, where it is heated by an oxy-fuel flame and transported to the workpiece by a carrier gas and hot gas. 

Flame temperature and enthalpy are determined by fuel gas composition (CxHy) and oxidizer flow rate (oxygen or air). This process takes place at atmospheric pressure. It should be remembered that 1 mole of air contains only 0.2 moles of O2 as an oxidizing agent. it includes used for combustion, 0.8 mol N2 do not burn or heat. So for the same amount of O2 and hydrocarbon molecules, flames using air as an oxidizing agent have a lower bulk temperature.

2.1.2 Wire, Rod, or Cord Flame Spraying

This process is "injection process in which the raw material is in the form of wire or rod" or wire. This was the first injection process. 1912 The installation consists of a nozzle that burns a mixture of acetylene and oxygen. At the nozzle surface, the flame extends to the air cap. The wires, rods or cords are fed axially continuously and the tip melts in the flame. A stream of compressed air surrounding a flame atomizes and creates matter flows in a continuous stream. 

The advantage of this system is that only the molten droplets travel to the substrate. If the material is fed too fast, the wire (rod or wire) will come out of the gun cap. Another advantage of this process is that the nozzle flame is concentric with the wire or rod or wire to maximize uniform heating. For example, Saint Gobaincan's Master Jet injects metal wires, core wires, flexicord and ceramic cores powered by electric motors. The gun is lightweight and easy to use for manual spraying of complex shapes. Of course, you can also equip the robot with a weapon. An example of a copper coating can be seen in Figure 1.

Figure 1 NiCrBSi (self-flowing alloy) coated paper machine roll with two powder flame guns


Combustion of hydrocarbon molecules (CXHY) is performed using chamber oxygen or air at pressures between 0.24 and 0.82 MPa, or slightly more oxidizer for high-performance kerosene-fueled guns. Laval convergent-divergent nozzles follow the chamber and achieve very high gas velocities (up to 2000 m/s). 

Typically, either axially or radially injected powder is used, or both, depending on the gun design. More recently, liquid (slurry or solution) injection has been developed primarily for axial injection. Some guns are designed to use real wires or threads. The first HVOF guns were introduced by Browning and Whitfield in the early 1980s. Particles are introduced axially upstream of the nozzle (at pressure powder supply is required) as shown in Figure 2 or radially downward from the nozzle (for normal powder feed)

Figure 2 Scheme of the HVOF process based on theJet Kote gun



Detonation is mainly carried out using acetylene or hydrogen-oxygen mixtures (including nitrogen to change the detonation parameters) in sealed tubes. The shock wave from the combustion of a highly compressed explosive medium leads to a high pressure wave (about 2 MPa) that repels particles heated by the combustion gases. Gas velocities of over 2000 m/s have been achieved. Unlike the two previous units, where the fuel gas and powder are continuously supplied to the gun, the fuel gas and powder of the D gun are supplied in repeated cycles at a frequency of 3-100 Hz. Schematic of the D-gun is shown in Figure 3.

Figure 3 Scheme of the D-gun



This is done by arc or plasma with 4 different types of torches.


Direct Current (d.c.) plasma torches creates a plasma beam from a continuously flowing gas heated by an electric arc in a nozzle (see Chapter 7 for details). They work with Ar, Ar-H2, Ar–He, Ar–He–H2, N2, and N2-H2 mixtures that cause temperatures above 8,000 K (up to 14,000 K) and subsonic velocities between 500 and 2,800 m/s. When used at atmospheric pressure, the process is commonly referred to as Atmospheric Plasma Spraying (APS) and under mild vacuum conditions is commonly referred to as Vacuum Plasma Spraying (VPS). In some cases, a modification of the anode nozzle design allows these torches to produce a supersonic plasma beam. Most applications use powders, but more recently solution and suspension precursors have been used.

Figure 4 RF induction of Tekna plasma-spraying torch



Radio Frequency (R.F.) An inductively coupled plasma torches in which a plasma is formed as a result of electromagnetic coupling of energy in a discharge cavity. Electroless plasma guns that can be used to create plasmas from a variety of gases, including inert, reducing, or oxidizing gas mixtures such as Ar and Ar/H2, Ar/H2/He, Ar/O2, air etc. The total volume of the generated plasma is large, the energy density is low, and the velocity is low (10-100 m/s). The bulk temperature of the plasma is usually in the range of 6000-9000K. Induction plasma torches have the ability to axially inject powders or slurries into the discharge cavity. Induced plasma atomization is usually performed under reduced pressure and is commonly referred to as vacuum induction plasma spraying (VIPS). A typical induction plasma torch design developed by Tekna Plasma Systems Inc. is shown in Figure 4.


Instead of using dedicated electrodes, an arc is created between two continuously moving wires (one negative and one positive). The melted end of the wire falls in small drops (a few tens of μm) using atomizing gas sprayed between two wires. The wire must be made of an extensible material or a flexible sheath filled with a non-extensible material such as ceramic powder. These are commonly known as core wire.


The principle is the same as the d.c blown arc. Plasma torches are, in most cases, necessarily metal, except that an arc is passed between the substrate and the floating electrode, which is the anode. The transported arc causes local melting of the substrate, and the heated particles in the plasma column are attached to the molten pool through the transported arc. This process is basically similar to the welding process. Figure 5 shows a typical PTA gun.

Figure 5 Scheme of plasma-transferred arc (PTA) deposition


That's the basic description of the types of Thermal Spray Coating, where we are a major player in the industry, especially in the KIIC Karawang area, Indonesia.

For those of you who represent factories that need this service, please contact us PT Tocalo Indonesia.

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