Menu

Thermal Spraying Processes

THERMAL SPRAYING PROCESSES

What is 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.

TECHNOLOGY_1

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

TECHNOLOGY_2

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, Monel, Babbit, etc..
– CERAMICS: Aluminium Titania, Aluminium/Chrome/Zirconium Oxide
– CERMETS and CARBIDES: Tungsten Carbide, Chrome Carbide, 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

Restoration of dimension

Wire Flame Spray

 

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 dropletsw rapidly solidify forming a coating.

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_flame_spray

Characteristics of Wire Flame Spray:

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

Wire Arc Spray

 

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.

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

 

wire_arc_spray

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

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.

powder_flame_spray

 

Characteristics of Powder Flame Spray

 Material Form  Powder
 Heat Source  Oxy-Fuel Combustion
 Flame Temperature (°C)  3000
 Gas 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

Plasma Spray

 

AIR 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.plasma_spray

Plasma Spray Process

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.

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.

Characteristics of Air Plasma Spray: 

 Material Form  Powder
 Heat Source  Plasma Flame
 Flame Temperature (°C)  12,000 to 16,000
 Gas Velocity (m/sec)  ~ 500
 Porosity (%)  2 to 10
 Coating Adhesion (MPa)  40 to 70

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

HVOF Spray

 

The HVOF (High Velocity Oxy-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.

HVOF Spray

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 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 solidy 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 have a perfect alternative for Hard Chrome platings.

 

Widely used Carbide Coatings Sprayed by HVOF Process:
– Tungsten Carbide: Excellent Wear and Abrasion Resistance
– Chrome Carbide: Excellent Corrosion Resistance

<p align=”justify”>Carbide powder are available in many variants for meeting requirements of specific applications. Good quality Metal & Alloy coatings can also be achieved using HVOF.</p>

Characteristics of HVOF Coatings:

 Material Form  Powder
 Heat Source  Accelerated Oxy-fuel flame
 Flame Temperature (°C)  > 3000
 Gas Velocity (m/sec)  700 to 1200
 Porosity (%)  < 1
 Coating Adhesion (MPa)  > 70

Advantages:

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

Disadvantages:

Process should preferably be automated
Very high noise emissions

Detonation Spray

 

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.

de_gun

 

Characteristics of Detonation Spray Coating:

 

 Material Form  Powder
 Heat Source  Charge Detonation
 Flame Temperature (°C)  3000 to 4000
 Gas Velocity (m/sec)  > 1500
 Porosity (%)  < 1
 Coating Adhesion (MPa)  > 70

Advantages:

– Excellent bond strength
Very dense coatings
Porosity less than 1%
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
Process can be easily automated