Flame spraying processes
The term flame spraying covers all thermal spraying processes in which a coating material is heated – and usually also melted – in a flame and then sprayed onto a pre-blasted workpiece surface.
The coating material can be supplied in powder, rod, cord or wire form. A further distinction is made between processes in which the material is accelerated and projected onto the surface solely by the flame, and those in which an additional atomising gas – typically compressed air – is used.
The properties of flame-sprayed coatings vary significantly depending on the specific process and spray parameters.
Alongside plasma and laser spraying, Häuser & Co GmbH also employs the following flame spraying processes for selected applications:
Wire flame spraying
In wire flame spraying, the coating material in wire form is fed into a flame and melted. The molten particles are then accelerated by an atomising gas (usually compressed air) and projected onto the component surface. Variants of this process include rod and cord flame spraying. Typical materials are aluminium and zinc–aluminium alloys, which are used for atmospheric corrosion protection.
Powder flame spraying
The principle of powder flame spraying is very similar to wire flame spraying; the key difference is that the coating material is supplied in powder form. This allows processing of alloys that cannot be manufactured as wire or rod.
High-velocity oxy-fuel (HVOF) spraying
A further development of powder flame spraying is high-velocity oxy-fuel spraying (HVOF). In this process, the molten powder particles are accelerated in the gas stream to several times the speed of sound. The resulting coatings are highly dense with very low porosity, which makes them harder and less ductile. These properties make HVOF coatings particularly suitable for wear-protection applications, for example using chromium-oxide-based alloys. Depending on the gun configuration, not only combustible gas mixtures are used, but also liquid fuels such as petroleum, kerosene or ethanol.
Detonation spraying
Detonation spraying is another variant of powder flame spraying. Here, the powder coating material is fed into a combustion chamber, where a gas mixture is detonated at high frequency by an electrical ignition. The resulting shock wave, generated by the rapid expansion of the gas mixture, accelerates the molten powder particles to velocities of up to around 600 m/s. Owing to its specific coating characteristics, detonation spraying is used today mainly for turbine components. In many other applications, it has been largely superseded by advanced HVOF processes.
Because of the exothermic reactions during combustion in flame spraying and the associated undesirable heat input into the workpiece, oxygen-reactive materials and alloys, as well as temperature-sensitive components, can only be coated with difficulty – or not at all – using these processes.
Cold-gas spraying
To address this, cold-gas spraying has been developed as an additional process variant. Here, the gas temperatures generated are typically slightly below the melting point of the coating material. However, the injected powder particles are accelerated to such high velocities that, unlike in other thermal spraying processes, they form a dense, strongly adherent coating on impact with the substrate – even without prior softening or melting. Typical applications are found in the automotive industry and in turbine engineering.