General principle
Thermal spraying encompasses a group of coating processes that aim improving material’s characteristics. Wear and friction resistance, thermal properties, corrosion protection, biocompatibility are sought by these treatments.
All thermal spraying processes use a carrier gas to accelerate and transport finely divided particles of a molten, semi-molten or solid material to form a coating. The particles are deposited on the substrate and the coating forms itself through the accumulation of these particles. The bonds between the substrate and the coating are therefore mechanical bonds.
Materials for thermal spray can be in the form of powder, wire or rod.
The thermal spraying processes are grouped into 3 major categories depending of the energy source: combustion, electrical discharge and kinetic energy.
Surface preparation
Substrates have to be prepared before the treatment to ensure the adhesion of the coating. They are cleaned in solvents, detergents, ultrasounds bath or by temperature rise to remove oil or dirt. The mechanical coating adhesion is due to the roughness of the surface which is made by mechanical preparation as sandblasting.
The parts sides that don’t require a coating are covered by masking tape or mechanical masking. Coating’s structure depends on many factors including the substrate nature and the material to deposit on it.
The choice of the process depends on the substrate characteristics and the coating expected performances.
This process is mainly used to make carbide, metallic and alloys coatings.
The high particles velocity and the flow rate of material allow to build quickly coatings with high performances. It’s often used in aeronautical industry.
Applications are numerous as corrosion and wear protections with the ability to make thick coatings.
An arc is struck between two wires of a coating material and a compressed gas is used to atomize and propel the material towards the substrate. The thickness of coatings are between 0,2 and 3 mm with 5 to 10% of porosity.
Its development is focused on the two characteristics to improve :
- The size of the particles
- Coating oxidation
To create an arc, the materials must be electrical conductor. The most frequently used materials are zinc, aluminium, copper …
Development of flux-cored wires with NiCrAl or NiMoAl alloys or wires with composites allow to elaborate new coatings.
A gas plasma is created by a confined electric arc. It’s strangled in a nozzle from which it escapes with high speed. The powder is injected into this plasma jet using a carrier gas. The melted particles are sprayed on the substrate by this jet.
Many materials can be projected (metals, alloys, carbide, ceramics). The plasma spraying needs several equipments : a torch, control and regulation cabine, a direct current generator and a powder feeder. These devices are entirely automated or robotic.
Automobile or aeronautical industries use this process to fight wear, corrosion, oxidation and friction.
This process can be made under vacuum or controlled atmosphere which allows thick coatings with a lower temperature. It’s an expensive method so the uses are limited in very specific application as boron carbide coatings for nuclear field.
Coldspray is a process used to make coatings but also to repair components in industrial sectors or to create 3D shapes of various geometries.
It belongs to the family of thermal spray methods but differs from other techniques because it’s a solid state process. The particles are accelerated by a supersonic gas jet.
It has many advantages : coatings with high thickness, very low porosity, very low oxidation (compared to others thermal spraying processes) and excellent adhesion.