Coating a wide variety of materials
Thermal Spray is a category of processes which all use a combination of materials (often in wire or powder form), heat and force in order to coat one material with another. This is often done in order to extend part life and give the attributes of a more expensive metal to a cheaper one. This family of processes includes: HVOF, Flame Spray, Plasma Spray and Wire Arc. All operate on the same principle but use a different technique and are better suited to different materials.
HVOF or High Velocity Oxygen Fuel uses a combination of heat and pressure to coat. Oxygen is mixed with either a liquid or gas and combusted in a chamber where it heats and expands until its forced out at supersonic speeds. HVOF was designed using jet engine technology and can produce velocities of MACH 3. Coating is ground up into powder and shot out at such high speeds it bonds with the metal, forming a coating that flake free and has an incredibly strong bond.
Pros: Produces coatings with a very high hardness rating, low porosity and excellent bond strength.
Cons: Requires specialised staff, facilities and equipment due to its often complex micro-structure.
Key Choice: For coating using any carbide materials. The high velocities involve mean that the carbide phases are less likely to degrade in the coating process.
This follows the same principle of HVOF only at lower velocities. Heat is generated by combusting a fuel gas and oxygen mixture which heats up the coating. Coatings for flame spraying can be either wire or powders. The compressed air is used to propel the coating towards the component. Generally these coatings aren’t as good as HVOF in terms of bond strength and porosity levels but you can create a solid coating at a lower cost point.
Pros: Components with complex geometries can easily be coated, and the cheapest option.
Cons: Inner surfaces are almost impossible to coat, lower bond strength, higher oxide levels and higher porosity.
Key Choice: Oil & Gas customers tend to use flame spray to produce Spray and Fuse Coatings.
Plasma Spraying is considered one of the more flexible thermal spraying techniques. It uses a DC electric arc in order to form a high temperature plasma gas. This in turn heats up the coating powder as its fed into the plasma jet. Inert gas is then fed into a torch which creates the velocity needed to bond the coating to the component. Plasma Spraying is especially good at producing coatings for materials with high melting points.
Pros: A very flexible process which can produce a wide range of coatings, coatings produced are of better quality than flame or arc spraying.
Cons: The Plasma Spray gun itself deteriorates very quickly, making this a more expensive option than others.
Key Choice: Plasma Spray is probably the best option when coating thin film ceramics.
Arc Spraying is the thermal technique that often has the highest productivity rate. A DC electric arc is placed between two wires of coating material. These wires melt which form the spray material and compressed gas is used to compress atomise the coating material and propel it towards the component. This will often give you a better bond and porosity rate compared with flame spraying but can sometimes produce arc light, ozone and fumes.
Pros: Requires lower power and a high deposition rate.
Cons: Not suitable for ceramics or cermets, can only spray electrically conductive materials in wire form. Produces a large amount of dust and fumes.
Key Choice: Good option for metal coatings at lower densities.