In thermal spraying processes melted or heated materials, usually powder or wire, are sprayed onto a surface to form a protective coating. A jet propels droplets of melted materials towards a substrate, where the droplets flatten and solidify rapidly.
The thermal spraying research lab at the Production Technology Centre in Trollhättan is unique in Sweden. There is equipment performing the four most common thermal spraying processes, plasma spraying, flame spraying, high velocity oxy-fuel spraying (HVOF) and wire arc spraying. During the spraying process the spray gun is usually manipulated by a robot and the work piece to be coated is mounted on a robotically controlled turntable.
In addition the lab has advanced equipment for different kinds of measurements.
In plasma spraying the material to be deposited is introduced into a plasma jet, emanating from a plasma torch. The heat is extreme and because of this the method is suitable for spraying ceramic materials. Plasma spraying forms porous coatings which for example can protect against heat.
Flame spraying uses the heat from the combustion of a fuel gas, usually acetylene or propane, with oxygen to melt the coating material. It is suitable for spraying lower melting point materials since flame temperatures are comparatively low. It forms porous coatings.
High-velocity oxy-fuel spraying, HVOF
In the HVOF-process a mixture of gaseous or liquid fuel and oxygen is fed into a combustion chamber, where it is ignited and burned continuously. The resultant hot gas emanates through a nozzle and travels through a straight section. A powder feed stock is injected into the gas stream. The gas and the sprayed particles reach very high speeds, sometimes up to 1000 meter per second. Because of this the kinetic energy of the particles is very high and the resultant coating on the substrate becomes extremely dense with very high adhesion. HVOF coatings are excellent protection against corrosion.
Wire arc spraying
In the wire arc spraying process two consumable metal wires are fed independently into a spray gun. These wires are then charged and an arc is generated between them. The heat from this arc melts the incoming wire, which is then entrained in an air jet from the gun. The molten material is deposited onto the substrate. Wire arc spraying is a relatively simple and inexpensive method and it has wide uses.
There is a whole range of measuring equipment in the thermal spraying lab. The two most important are:
Feedstock particle scanner
This equipment scans particles before spraying. It measures their size and the proportions of particles of different sizes. It is also possible to determine their shape. They could be round, oval or maybe angular. All these factors are of great importance for the quality of the resultant coating.
In-flight particle measurement system
This system measures temperature, velocity and size of the in-flight particles during spraying. When various parameters of the process are changed, it is possible to see how this affects the particles during spraying.
Measurements of sprayed components are done in the nearby Material lab.
Room number: O3-203
Room number: J3-203