Whether a worn part can be repaired depends largely on its design and application. If a part such as a shaft, bearing housing, pump casing or cylinder bore is worn but the parent metal is sound then the part can be cost effectively restored to original dimensions using thermal spraying.
Deciding Whether a Part Is a Candidate
There are some parts that are just not worth being re-machined and to be re-sprayed. Shafts, bearing housings, pump casings and also cylinder bores are typical parts for repair by thermal spraying.
Preparing the Surface
The most critical part of this restoration is the preparation of the worn area. The surface of the worn area is grit blasted to form a surface with a roughness to enable the sprayed material to “grip” to the surface. If any contamination such as oil, oxide or scale is left on the surface then this will weaken the adhesion of the sprayed material. Therefore, the surface is prepared immediately prior to spraying.
Building Up the Material
A variety of different thermal spray materials are available. Typically they are supplied as either a powder or wire, which are then melted and propelled to the surface using a variety of different methods. As the material hits the surface it flattens and forms part of the coating. The coating is typically built up in a large number of very thin layers. This allows very close control of the amount of coating material that is required to build up a specific thickness, typically between 0.1 mm and 1 mm. There are a number of different processes used to perform thermal spraying, and the main ones are described on TWI’s technical knowledge pages: Thermal spraying processes.
Post-Spray Machining
After spraying, the part is then ground or machined back to the original dimensions. In most cases a good quality thermal spray coating can be machined to a tolerance of ±0.025mm after finishing grinding. If you want Thermal Sprays, www.poeton.co.uk/surface-treatments/thermal-metal-sprays/ is a good place to start.
Restoring worn components by thermal spraying offers such huge cost savings that when done properly, the component can be returned to full service life.
