Metal parts that have experienced wear can, in many cases, be restored to their original dimension with thermal spray coatings. In fact, many times, the cost of re-grinding or machining worn parts to return them to original specification is less than that of purchasing new parts.
Types of Wear Damage That Suit the Process
Thermal spray can be used to repair parts that have suffered from dimensional loss, surface pitting and scoring, either due to sliding or abrasive wear. Deep cracks and fatigue fractures that extend through the whole thickness of a part are generally not suitable for repair by thermal spray, as the main cause of the failure will still need to be addressed. Surface-level material loss however is ideal for a wear repair using thermal spray.
Which Parts and Base Metals Work Well
Cylinders or shafts are ideal for wear repair, as these can be easily coated and then machined back to precise dimensions. Typical base metals that are suitable for thermal spray coating include steel, cast iron, or even copper alloys. However, it is worth noting that parts with thin walls, or with a highly complex geometry, will require careful consideration prior to coating, including the use of suitable masking and fixturing. It is therefore advisable to speak with the coating supplier regarding the specific part geometry.
Matching the Coating to the Wear Mechanism
The most important factor is the material used for the thermal spray coating. Tungsten carbide coatings are ideal for wear that is caused by abrasive or sliding action and should be applied at a thickness of 0.1mm to 0.5mm. A metallic alloy such as nickel-chrome is best for surfaces that are subjected to fatigue and also require some degree of ductility. A number of different ceramic materials, including alumina and chromia, are ideal for high temperature applications and can provide a heat resistant coating. For more information on thermal spray process families and their properties, refer to TWI’s technical guidance on process selection for thermal spray.
Thickness and Bond Strength Compared with Alternatives
Thermal spray coatings offer bond strengths of 20-70 MPa for wear repair depending on the process used and the coatings that are applied. In contrast, welding can often achieve higher bond strength but can suffer from heat induced distortion. Electroplating is often used for precise, thin layers of coating and in most cases restricted to a few materials. More on Thermal Sprays can be found at https://www.poeton.co.uk/surface-treatments/thermal-metal-sprays/.
What to Expect from Post-Spray Machining
The vast majority of thermal spray deposits are applied in a slightly proud state to enable subsequent surface finishing, typically grinding or turning to precise tolerance. Typical final tolerances for finished ground surfaces are in the order of ± 0.025mm. The machining required to remove sprayed coatings must be included in repair lead time and cost.
Getting the wear mechanism and coating material right for a worn part at the first attempt will save a lot of time compared to a second repair.