The first practical decision when considering thermal spray is the choice of the spray material. This material will be used to protect a surface against wear or corrosion and there are a number of different materials, each suited to a number of different problems. A brief guide to the main groups of materials will help to clarify the choice of material for specific applications.
Zinc and Zinc-Aluminium Alloys
Zinc and Zinc-Aluminium Alloys – These are the most common corrosion protection coating on structural steelwork, such as steel bridges, steel buildings and other steel fabricated items. They are sacrificial and will corrode before the underlying steel, provided there is some exposed steel present even if the coating has been damaged. The addition of aluminium to zinc increases the coating’s adhesion and life.
Aluminium
Aluminium coatings are used for protection against high-temperature oxidation. Typical applications for this type of coating are exhaust systems, furnace parts and other articles which are used in the petrochemical industry and are carried by pipelines. The coating, when heated, forms a stable oxide layer that acts as a protective barrier and prevents the underlying metal from oxidising. This type of coating is generally suitable for use up to temperatures of around 550°C.
Tungsten Carbide
Tungsten carbide is the hardest wearing of all thermal spray coating materials and is most frequently used on surfaces subject to extreme abrasion. Such items include pump impellers, cutting tools, wear plates and any sliding wear surfaces where other coatings would quickly wear away. It is essential that someone with a full understanding of the thermal spray process fundamentals is specifying the hardest wearing coating and that it is applied correctly in order to achieve the maximum hardness of the coating.
Ceramic Oxides
A variety of ceramic oxide materials are available. Alumina coatings are typically used for electrical insulation applications, while zirconia is used as a thermal barrier coating on parts such as those found on gas turbine engines and in the aerospace industry. In these applications, the coating helps to prevent heat from transferring to the substrate.
Stainless Steel and Nickel Alloys
Spraying stainless steel or nickel alloys is ideal for repair of wear on surfaces of shafts, bores and bearing housings, allowing them to be machined back to original dimensions.
Thermal Spray Coating is covered in more detail at https://www.poeton.co.uk/surface-treatments/thermal-metal-sprays.
Molybdenum
Because Molybdenum has a naturally low friction coefficient it is an ideal material for a number of wearing applications. It is commonly used for the facing surfaces of piston rings, for wearing surfaces of bearings, and for synchroniser rings in gearboxes. In all of these instances the main aim is to try to reduce the surface-to-surface friction, and by using Molybdenum the engineer is able to achieve this.
Match the material to the job in hand and then it will be relatively easy to detail the rest of the specification.