TYPES OF THERMAL SPRAY COATIGS
The Combustion Wire Thermal Spray Process formerly known as Metallizing, Flame Spraying and Metal Spraying Processes was first invented in 1910 by Schoop in Switzerland.
The flame spraying process is basically the spraying of molten metal* onto a surface to provide a coating. Material in wire form is melted in a flame (oxy-acetylene flame most common) and atomized using compressed air to form a fine spray. When the spray contacts the prepared surface of a substrate material, the fine molten droplets rapidly solidify forming a coating.
This process carried out correctly is called a “cold process” (relative to the substrate material being coated) as the substrate temperature can be kept low during processing avoiding damage, metallurgical changes and distortion to the substrate material. This flame spraying process has been extensively used in the past and today for machine element work and anti-corrosion coatings.
*Ceramics and cermets can be used in rod or composite wire form.
Common materials for flame spraying:
- Zinc and aluminium for anti-corrosion cathodic coatings on steel
- Nickel/aluminium composite wire for bond coats and self-bonding coatings
- Molybdenum for bond coats
- Molybdenum for hard bearing applications, excellent resistance to adhesive wear, used on piston rings, synchromesh cones and journals
- High Chromium steel for many applications requiring hard and wear resistant coating
- Bronzes, babbitt for bearing applications
- Stainless steels, nickel and monel for anti-corrosion and wear
- Aluminium, nickel/aluminium for heat and oxidation resistance
- Low capital investment
- Simple to operate
- Wire form cheaper than powder
- Deposit efficiency very high
- Possibly still best for applying pure molybdenum coatings for wear
- Portable system
- Preheating facility built in, unlike arc spraying
- Possible to use system in areas without electricity supply
- Limited to spraying materials supplied in wire or rod form
- Not capable of the low oxide, high density and high strength coatings of plasma and HVOF
The Plasma Spraying Process is basically the spraying of molten or heat softened material onto a surface to provide a coating. Material in the form of powder is injected into a very high temperature plasma flame, where it is rapidly heated and accelerated to a high velocity. The hot material impacts on the substrate surface and rapidly cools forming a coating. This process carried out correctly is called a “cold process” (relative to the substrate material being coated) as the substrate temperature can be kept low during processing avoiding damage, metallurgical changes and distortion to the substrate material. The plasma gun comprises a copper anode and tungsten cathode, both of which are water cooled. Plasma gas (argon, nitrogen, hydrogen, helium) flows around the cathode and through the anode which is shaped as a constricting nozzle. The plasma is initiated by a high voltage discharge which causes localized ionization and a conductive path for a DC arc to form between cathode and anode. The resistance heating from the arc causes the gas to reach extreme temperatures, dissociate and ionize to form plasma. The plasma exits the anode nozzle as a free or neutral plasma flame (plasma which does not carry electric current) which is quite different to the Plasma Transferred Arc coating process where the arc extends to the surface to be coated. When the plasma is stabilized ready for spraying the electric arc extends down the nozzle, instead of shorting out to the nearest edge of the anode nozzle. This stretching of the arc is due to a thermal pinch effect. Cold gas around the surface of the water cooled anode nozzle being electrically non-conductive constricts the plasma arc, raising its temperature and velocity. Powder is fed into the plasma flame most commonly via an external powder port mounted near the anode nozzle exit. The powder is so rapidly heated and accelerated that spray distances can be in the order of 25 to 150 mm. The plasma spraying process is most commonly used in normal atmospheric conditions and referred as APS. Some plasma spraying is conducted in protective environments using vacuum chambers normally back filled with a protective gas at low pressure; this is referred as VPS or LPPS.
Plasma spraying has the advantage that it can spray very high melting point materials such as refractory metals like tungsten and ceramics like zirconia unlike combustion processes. Plasma sprayed coatings are generally much denser, stronger and cleaner than the other thermal spraying processes with the exception of HVOF and detonation processes. Plasma spray coatings probably account for the widest range of thermal spray coatings and applications and make this process the most versatile.
Disadvantages of the plasma spraying process are relative high cost and complexity of process.
The HVOF (High Velocity Oxygen Fuel) Thermal Spray Process is basically the same as the combustion powder spray process (LVOF) except that this process has been developed to produce extremely high spray velocity. There are a number of HVOF guns which use different methods to achieve high velocity spraying. One method is basically a high pressure water cooled combustion chamber and long nozzle. Fuel (kerosene, acetylene, propylene and hydrogen) and oxygen are fed into the chamber; combustion produces a hot high pressure flame which is forced down a nozzle increasing its velocity. Powder may be fed axially into the combustion chamber under high pressure or fed through the side of laval type nozzle where the pressure is lower. Another method uses a simpler system of a high pressure combustion nozzle and air cap. Fuel gas (propane, propylene or hydrogen) and oxygen are supplied at high pressure, combustion occurs outside the nozzle but within an air cap supplied with compressed air. The compressed air pinches and accelerates the flame and acts as a coolant for the gun. Powder is fed at high pressure axially from the centre of the nozzle. The coatings produced by HVOF are similar to those produce by the detonation process. Coatings are very dense, strong and show low residual tensile stress or in some cases compressive stress, which enable very much thicker coatings to be applied than previously possible with the other processes.
The very high kinetic energy of particles striking the substrate surface does not require the particles to be fully molten to form high quality coatings. This is certainly an advantage for the carbide cermet type coatings and is where this process really excels.
HVOF coatings are used in applications requiring the highest density and strength not found in most other thermal spray processes. New applications, previously not suitable for thermal spray coatings are becoming viable.
In the Arc Spraying Process a pair of electrically conductive wires is melted by means of an electric arc. The molten material is atomized by compressed air and propelled towards the substrate surface. The impacting molten particles on the substrate rapidly solidify to form a coating. This arc spraying process carried out correctly is called a “cold process” (relative to the substrate material being coated) as the substrate temperature can be kept low during processing avoiding damage, metallurgical changes and distortion to the substrate material.
Arc spray coatings are normally denser and stronger than their equivalent combustion spray coatings. Low running costs, high spray rates and efficiency make it a good tool for spraying large areas and high production rates.
Disadvantages of the arc spraying process are that only electrically conductive wires can be sprayed and if substrate preheating is required, a separate heating source is needed.
The main applications of arc spraying are anti-corrosion coatings of zinc and aluminium and machine element work on large components.
Also known as Powder Flame Spraying and LVOF (Low Velocity Oxygen Fuel Process). This flame spraying process is basically the spraying of molten material onto a surface to provide a coating. Material in powder form is melted in a flame (oxy-acetylene or hydrogen most common) to form a fine spray. When the spray contacts the prepared surface of a substrate material, the fine molten droplets rapidly solidify forming a coating. This flame spraying process carried out correctly is called a “cold process” (relative to the substrate material being coated) as the substrate temperature can be kept low during processing avoiding damage, metallurgical changes and distortion to the substrate material.
The main advantage of this flame spraying process over the similar combustion wire spray process is that a much wider range of materials can be easily processed into powder form giving a larger choice of coatings.
Disadvantages – The flame spraying process is only limited by materials with higher melting temperatures than the flame can provide or if the material decomposes on heating.