Thermal Spraying Processes

Thermal Spray

A common feature of all thermal spray coatings is their lenticular or lamellar grain structure resulting from the rapid solidification of small globules, flattened from striking a cold surface at high velocities. In the simplest terms possible, thermal spray coating involves heating a material, in powder or wire form, to a molten or semi-molten state.

The material is propelled using a stream of gas or compressed air to deposit it, creating a surface structure on a given substrate. The coating material may consist of a single element, but is often an alloy or composite with unique physical properties that are only achievable through the thermal spray process.

Thermal coatings are a highly cost-effective way to add superior performance qualities to a given substrate. Coatings can be metallic, ceramic, plastic or any combination desired to meet a broad range of physical criteria. The coating materials can be applied using different processes.

Thermal coating methods utilise fuel combustion, plasma spray and electric arc delivery systems. Coatings can be applied under standard atmospheric conditions or in specialised, highly controlled atmospheric environments. Coatings can be applied manually or with the automated precision of software-driven robotics. Many industries use our thermal spray coatings to extend product life, increase service performance and reduce production and maintenance costs.

Thermal spray coatings can be the most cost-effective means of protecting substrate surfaces from wear or corrosion. Other primary uses of thermally sprayed coatings include dimensional restoration, maintaining precise clearances, and modifying thermal and electrical properties.

1.  Fusion
2.  Automisation and speed up
3.  Temperature and speed control
4.  Coating building

1.  Transport 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 Coating Processes

  • Wire Flame Spray
  • Wire Arc Spray
  • Powder Flame Spray
  • HVOF Spray (High Velocity Oxy Fuel)
  • Detonation Spray
  • Spray and Fuse

Coatings Sprayed using Thermal Spray Process

  • Metals : Aluminium, Copper, Nickel, Molybdenum, etc.
  • Alloys : Steel, Babbit, Monel, Inconel, Stellite, Colmonoy etc.
  • Ceramics : Aluminium Oxide, Chromium Oxide, Alumina Titania, Zirconium Oxide etc.
  • Cermets and Carbides : Tungsten Carbide, Chromium Carbide, MCrAlY, CoCrAlY etc.
  • Abradables

Properties Provided by Thermal Spray Coatings

#Tribological (wear resistance)

  • Resist abrasive wear occurring when a harder surface slides over a softer surface and when abrasive grains are present between the surfaces, also when fibres or threads run over surfaces at high speeds.
  • Resist adhesive wear occurring when two surfaces slide against each other and fragments from one surface adhere to the other.
  • Resist fretting wear when repeated loading causes cyclic stresses which induces the surface to break up and loose large fragments.
  • Resist cavitation wear when liquid flow causes mechanical shocks to the surface.
  • Resist erosive wear when a gas or liquid carrying entrained particles impinges on a surface with velocity.

#Corrosion resistance

  • Resist atmospheric environments such as salt, industrial or rural atmospheres.
  • Resist immersion environments such as salt water, hot fresh water, potable and nonpotable fresh water.
  • Resist actions of chemicals such as oils, fuels and solvents.
  • Resist the action of foodstuffs without altering either their taste or chemical composition.

#Heat & Oxidation resistance

  • Inhibit heat transfer between the part and a high temperature environment. Protect a substrate against high temperature oxidation.
  • Protect a substrate exposed to hot corrosive gases.

#Electrical conductivity or resistivity

  • Transmit electric current. Insulate against the passage of electric current.
  • Shield against EMI / RFI. Shield against radiation by resisting passage of thermal neutrons or gamma rays.

#Abradable or abrasive

  • Clearance control for sealing parts, where controlled preferential abrasion takes place on contact with a mating part.

#Forming textured surfaces

  • To achieve the required friction or traction during operation
  • To enhance the grip between textured surface and body in contact

#Restoration of dimension

  • Rebuilding of areas which are excessively worn out
  • Under-layer to provide better adhesion between substrate and top layer

Wire Flame Spray

The 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) & atomised using compressed air to form a fine spray. When the spray contacts the prepared surface material, the fine molten droplet rapidly solidify forming a coating.

Characteristics of Wire Flame Spray :

Material Form Wire
Heat Source Oxy-Fuel Combustion
Flame Temperature (°C) 3000
Particle Velocity (m/sec) < 300
Porosity (%) 10 to 15
Coating Adhesion (MPa) 14 to 21

Advantages :

  • Simple to operate
  • Wire form cheaper than powder
  • Very high deposit efficiency
  • It has a portable system and can be used in areas without electricity
  • Possibly still best for applying pure Molybdenum coatings

Disadvantages :

  • Limited to spraying material supplied in wire or rod form
  • Coatings have higher porosity and lower bond strength
  • Not capable of spraying low oxide, high density & high strength coatings

Wire Arc Spray

This form of thermal spraying uses wire material as a feed stock. An electric arc is used as the heat source. As the wires are fed towards each other, an electric arc is struck between the wires creating a temperature of around 4,000°C.

This temperature causes the tips of the wire to melt and once molten state, a stream of compressed air or inert gas is used to atomise and accelerate the feed metal towards the substrate.

Characteristics of Wire Arc Spray :

Material Form Wire
Heat Source Electric Arc
Flame Temperature (°C) 3600 to 4000
Particle Velocity (m/sec) < 300
Porosity (%) 10 to 15
Coating Adhesion (MPa) 28 to 42

Advantages :

  • Coatings with good characteristics can be achieved
  • Two different wires can be used simultaneously to produce a pseudo alloy
  • Cored wires are also available producing coatings with unique properties
  • Applying coating to large areas is easier by this process
  • Used for dimensional restoration due to higher deposit efficiency

Disadvantages :

  • Coatings achieved are not dense and have porosity of up to 15%
  • Good suface preparation is very essential for good bond strength
  • Coatings limited to materials available in wire form with low melting tempertaure

Powder Flame Spray

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.

The main advantage of this process over the similar wire flame spray process is that a much wider range of materials can be easily processed into powder form giving a larger choice of coatings.

Characteristics of Powder Flame Spray :

Material Form Powder
Heat Source Oxy-Fuel Combustion
Flame Temperature (°C) 3000
Particle Velocity (m/sec) < 300
Porosity (%) 10 to 15
Coating Adhesion (MPa) 14 to 21

Advantages :

  • High production spray rates
  • Easy to operate & portable
  • Large choice of coating materials
  • Can be easily automated

Disadvantages :

  • Porosity in the range of 10 - 15%
  • Lower bond strength in comparison to other process
  • High oxide level for metal deposits

Atmospheric Plasma Spray

In this process the coating material in powder form is melted in a hot plasma flame and propelled on to a prepared substrate surface to form a coating. When the spray contacts the prepared surface of a substrate material, the fine molten droplets rapidly solidify forming a coating.

Characteristics of Air Plasma Spray :

Material Form Powder
Heat Source Plasma Flame
Flame Temperature (°C) 12,000 to 20,000
Particle Velocity (m/sec) 200 to 500
Porosity (%) 2 to 10
Coating Adhesion (MPa) 40 to 70

Plasma spray system consists of a Plasma Spray Gun, Power Source, Powder Feeder, Control unit and Heat Exchanger. Plasma is formed inside the specially designed plasma spray gun, by introducing a high intensity electric arc between a cathode (tungsten electrode) and anode (nozzle) assembly. Plasma forming gas is introduced in this arc, where it gets ionised to form a plasma. When this plasma exits the special design convergent – divergent nozzle, it returns to its original state liberating extreme energy. The coating material in powder form is introduced in this hot flame, where it melts and is propelled by the flame on to the substrate surface.

The energy produced in a plasma is high resulting in very high temperatures at the plasma core. Thus ceramics with very high melting points are ideally sprayed with Air Plasma spray for achieving good quality coatings. The coatings produced by plasma spray are uniform, dense and have comparatively lesser porosity.

Many variants of ceramics are available that have different properties :

  • Aluminum Oxide : Wear Resitance & Abrasion Resitance
  • Aluminum Oxide + Titanium Oxide (Alumina Titania) : Wear Reisitance & Abrasion Resitance
  • Chrome Oxide : Corrosion, Wear and Abrasion Resistance
  • Zirconium Oxide (Zirconia) : High Temperature Barrier
  • Very good quality Metal, Alloys and Abradable coatings can also be achieved using Air Plasma spray. Air Plasma Spray also has the provision to spray coatings inside holes or pockets.

Advantages :

  • Largest choice of coating materials
  • High production spray rates
  • Process can be automated
  • Good bond strength and low porosity
  • High degree of flexibility
  • Very versatile

Disadvantages :

  • Porosity in the range of 2 - 10%
  • Lower bond strength in comparison to HVOF
  • Not feasible for carbide coatings

HVOF Spray

The HVOF (High Velocity Oxy-Fuel) Thermal Spray Process is basically the same as the conventional Powder Flame Spray process (LVOF) except that this process has been developed to produce extremely high spray velocity.

Characteristics of HVOF Coatings :

Material Form Powder
Heat Source Accelerated Oxy-fuel flame
Flame Temperature (°C) 1,200 to 2,000
Particle Velocity (m/sec) 500 to 900
Porosity (%) < 1
Coating Adhesion (MPa) > 70

Fuel and Oxygen are fed into the combustion chamber where the mixture is ignited. Combustion produces a hot high pressure flame which is forced down a nozzle increasing its velocity. Powder is preferably fed axially into the combustion chamber under high pressure or fed through the side of nozzle where the pressure is lower.

Due to higher velocity the bond-strength of the coatings are higher. The powder to be sprayed are often not melted but accelerated in a high temperature and high velocity gas stream causing the phase of the sprayed material to change from solid to plastic (semi-molten) form. When these particles strike the prepared substrate, they solidify to form a very dense and low porosity coating.

HVOF is best recommended for Carbide matrix coatings. Carbide coatings sprayed by HVOF renders good hardness, wear resistance and abrasion resistance characteristics. HVOF Sprayed Carbide coatings are a perfect alternative for Hard Chrome plating's.

Widely used Carbide coatings sprayed by HP-HVOF Process :

  • Tungsten Carbide : Excellent Wear and Abrasion Resistance
  • Chrome Carbide : Excellent Corrosion Resistance
    Carbide powder available in varying chemical composition and proportion can be used for meeting requirements of specific applications. Good quality Metal & Alloy coatings can also be achieved using HP-HVOF.

Advantages :

  • Coatings are dense with very low porosity
  • Excellent, tenaciously bonded coatings
  • Low oxide metallic coatings
  • Optimised microhardness

Disadvantages :

  • Process should preferably be automated
  • Not feasible to spray small components
  • Not feasible for ceramic coatings
  • Very high noise emission

Detonation Spray

Oxygen and fuel (acetylene most common) is fed into the barrel along with a charge of powder. A spark is used to ignite the gas mixture and the resulting detonation heats and accelerates the powder to supersonic velocity down the barrel. A pulse of nitrogen is used to purge the barrel after each detonation. This process is repeated many times a second. The high kinetic energy of the hot powder particles on impact with the substrate results in build up of a very dense and strong coating.

Characteristics of Detonation Spray Coating :

Material Form Powder
Heat Source Charge Detonation
Flame Temperature (°C) 1,500 to 4,000
Particle Velocity (m/sec) 600 to 1,200
Porosity (%) < 1
Coating Adhesion (MPa) > 70

Advantages :

  • Excellent bond strength
  • Very dense coatings
  • Better coating characteristics of hardness, wear & corrosion resistance
  • Versatile process, ensuring wide range of coatings
  • Low process temperature enables spraying of precision components
  • Higher thickness coatings easily possible

Disadvantages :

  • Very slow process as the coating is intermittent
  • Comparatively low coating efficiency
  • Very high noise emission

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