Whether worn or spalled, a sprayed coating that has failed in service is usually far from being scrapped as wasted. It is common for the coating to be repaired to extend the life of the part. There are several ways that this can be done and an understanding of the options will allow a sensible decision to be made as to the best course of action.
Check Whether Overspraying Is Feasible
Inspecting the existing coating: If the coating has deteriorated but is still strongly attached to the substrate and has not become contaminated then it may be possible to overspray a new coating on top of the old, provided that the maximum total build thickness for that coating has not been exceeded.
Strip and Grit-Blast Before Recoating
If the coating needs to be removed from the part, sand blasting the surface to Sa 2.5 using an aluminium oxide grit of around 60 to 80 mesh is sufficient for most applications. A bond coat of Ni-Al or Ni-Cr is then applied to the blasted surface and allowed to dry before applying the top coat. This will provide a hard wearing surface that will resist wear and heat.
Flame Spray Versus HVOF
Flame spray is generally used for repairs where heat input is less critical and there are relatively large clearances. However, for higher wear and/or higher pressure applications, the deposits produced by HVOF (High-Velocity OxyFuel) spraying are denser, harder and have lower porosity than those produced by flame spraying. For Thermal Spray Coating, visit poeton.co.uk/surface-treatments/thermal-metal-sprays/.
Cold Spray for Heat-Sensitive Parts
This process is used for heat sensitive parts such as aluminium, copper and other thin walled components where the potential for warping is high.
Dimensional Build-Up on Worn Shafts
The ability of thermal spray coating to build up to precise dimensions makes it an ideal repair for worn shafts and bore surfaces. Depositions of 0.5 mm to 1.5 mm are typical. A well applied spray coating can be finished ground to exacting dimensions to return the worn part to original tolerance. Equal buildup on both sides of the shaft is recommended to avoid unbalanced conditions.
Workshop operations carry fumes, noise and fire risks. Any repair facility carrying out thermal spray processes must ensure that they are operating in a safe environment and comply with health and safety requirements for the processes used.
With a little planning and the correct process, worn components can be returned to full service at a fraction of the cost of replacement.