Techniques

Thermal spraying is a technique whereby a metal, a metal alloy or metal compound in wire or powder form, is melted and sprayed onto a surface using a thermal spray gun. This is done so that the component's usability is enhanced by the addition of coatings with improved properties.

Principle of thermal spraying2

With only one exception, thermal spraying is a cold process. That is to say that the workpiece undergoes a slight rise in temperature due to the heat transfer from the molten droplets when they land the surface. The conventional workpiece temperature of between 50° to 200° C cannot cause any deformation, change in structure or dilution. As a result, the base material always retains its original mechanical properties.

The structure of a cold-sprayed layer is lamellar, which is somewhat similar to a cast iron structure. Micro pores and oxide inclusions are partly the reason why some sprayed metal layers have a low frictional coefficient and high hardness. Microporous sprayed layers are also very good at retaining certain lubricants, thus preventing wear caused by metal/metal contact.

Cross Section of Coating2

The thermal spraying technique uses a group of materials that are known as self-fluxing alloys or hard metals, and, after spraying, are given a heat treatment at 1050° - 1150° C. The structure of these hot-sprayed layers is homogeneous and, as a result, the mechanical strength is significantly higher. 

The base material for hot-sprayed layers is nickel, chromium, or cobalt, possibly with the addition of tungsten carbides, and these can be supplied in a hardness of Rc 30 to 75. These types of fused layers have a very high resistance to chemical corrosion, abrasion, heat oxidation, impact and/or heavy mechanical stress.

There are 5 main thermal spray techniques for applying the coatings. Griekspoor has the in-house expertise to apply every one of these techniques.

 These are:

  1. Atmospheric Plasma Spraying (plasma spraying)
  2. HVOF
  3. Autogenous wire spraying (including metal spraying)
  4. Autogenous powder spraying (5P)
  5. Electric wire spraying

 

Plasma Spraying

Plasma spraying

In a plasma spray gun a plasma gas is ionized between an anode and a cathode. The plasma reaches temperatures between 15,000 and 20,000°C. A coating powder is injected into the plasma jet by a carrier gas, where it melts before depositing on the…
HVOF spraying

HVOF spraying

When HVOF (High Velocity Oxygen Fuel) spraying fuel and oxygen are ignited. The flow of gas expands through a special nozzle and reaches a speed of more than 1,500 m/s. The coating powder is injected axially in the gas flow reaching extremely high…
Flame wire spraying

Flame wire spraying

Autogenous wire spraying or wire flame spraying is the oldest process in thermal spray technology. Using an electric or air motor a material is transported as a wire through a spray gun and melted down centrally in a burning gas-oxygen mixture…
Flame powder spraying

Flame powder spraying

In this process the spray material is fed in powder form through the gun using a transport gas injecting it centrally into a burning gas/oxygen mixture, where it is melted and delivered onto the substrate.
Arc wire spraying

Arc wire spraying

Electric wire spraying is a technique in which two electrically conductive wires are transported through a spray gun. An electric arc is produced between the ends of the two wires causing them to melt. Compressed air is used to transport the molten…
Lasser cladding

Laser cladding

Lasercladding (laser welding) strongly resembles traditional welding of metals. The outcome, however, is metallurgically more related to flame sprayed and then fused coatings. The result of which is a metallurgic connection of the coating to the…