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Thermal Spray Principles and Techniques

All thermal spraying processes are based on the same principle of heating and melting a feed stock material, typically in the form of powder or wire before accelerating it to a high velocity and then allowing the particles to strike the substrate surface. The particles will then re-solidified on the substrate.

The main feature of all thermal spray coatings is their lamellar grain structure resulting from the rapid solidification of small globule particles, flattened from striking a cold surface at high velocities.

The coating is formed when millions of particles are deposited on top of each other. These particles are mechanical or metallurgical bonded to the substrate.

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For better understanding, the following sketches she the different steps of thermal spraying.

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1 – Fusion

2 – Atomisation and speed up

3 – Temperature and speed control

4 – Coating building

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1 – Transfer of the spraying particles

2 – Impact on the surface

3 – Thermal transfer from particles to substrate

4 – Solidification and contraction of particles

5 – Mechanical bond

6 – Local fusion

 

Thermal Spray Techniques

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Flame Spray

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Among the thermal spray methods, flame spray is the simplest and the cheapest way to deposit coatings. The feed stock material is fed into the flame in the form of a powder, wire or rod-flame.

Common fuel gases include hydrogen, acetylene, propane and natural gas. The process relies on the chemical reaction between oxygen and the fuel of combustion to produce a heat source which in turn creates a gas steam. The compressed gas steam is then used to atomise the molten metal and accelerate particles onto the substrate.

The main advantage of the combustion powder thermal spray process is that a wide range of materials can be easily processed into powder from giving a larger choice of coatings.

The flame spray process is limited by materials with higher melting temperatures than the flame. The lower temperatures and velocities (40m/s) associated with conventional flame spraying typically result in higher oxides, porosity and inclusions in coatings. The bond strengths and adhesive strengths are also low between the coating and substrate.

An excellent use for flame spray is for producing coatings where good wear resistance and excellent impact resistance are required such as with agricultural harvesting components and oil drilling parts.

 

Plasma Spray

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In the plasma spray process, the plasma is initialised by a high voltage discharge which causes localised ionisation and a conductive path for a DC electric arc to form in between two electrodes (comprises of a copper anode and tungsten cathode) to generate a stream of high temperature ionised plasma gas, which usually consists of either argon/hydrogen or argon/helium. As the plasma gas is heated by the arc, it expands and is accelerated through a constricting nozzle, creating velocities up to MACH 2. The coating material, in powder form, is carried in an inert gas stream into the plasma jet where it is heated and propelled towards the substrate.

Plasma spraying produces a high quality coating by a combination of a high temperature (15,000°C), high thermal energy of the plasma jet, a relatively inert spraying medium and quite high particle velocities, typically 200-300m/s.

Plasma spraying has the advantage that it can spray very high melting point materials such as refractory materials including tungsten, tantalum, ceramic oxides, and ceramics like zirconium. Because plasma-arc spraying is the most versatile of all the thermal spray processes it can be found in the widest range of industries. Plasma spray coatings are used commonly for applications in aerospace, automotive, medical devices, agriculture, etc.

 

High Velocity Oxygen Fuel (HVOF) Spray

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High-velocity, oxy-fuel, (HVOF) utilizes confined combustion and an extended nozzle to heat and accelerate the powdered coating material. HVOF devices operate at hypersonic gas velocities greater than MACH 5 before impact into the substrate. The high combustion pressure together with extreme velocities produce high particle velocity and resulting in high coating quality that are very dense with low oxide content and very well adhered in the as-sprayed condition. The low oxides (porosity levels typically less than 0.5%) are due partly to the speed of the particles spending less time within the heat source and partly due to the lower flame temperature (around 3,000°C) of the heat source compared with alternative processes.

The technique uses the combustion of gases, such as propane, propylene, hydrogen, or a liquid fuel such as kerosene. Fuel and oxygen mix and atomise within the combustion arc under conditions that monitor the correct combustion made and pressure. The very high kinetic energy of particles striking the substrate does not require the particles to be fully molten to form high quality HVOF coatings. This is certainly another advantage for the carbide cermet type coatings and is where this process excels.

Some HVOF coatings can be sprayed very thick due to the exceptionally high velocities producing coatings in compression instead of tension. HVOF coatings can be found in several diverse industries needing wear resistance such as agricultural and construction equipment, food processing, aerospace, etc.