Internal stress left after spraying. Quenching of hot splats and thermal mismatch on cooling usually leave tension, while high-velocity impacts in HVOF and cold spray peen the coating into compression. It is checked with Almen strips sprayed on one side, whose arc height shows stress level and sign, or by curvature, X-ray diffraction or hole drilling. Too much tension cracks or delaminates thick coatings and can cut fatigue life of the part.
Almen strip arched the wrong way, we've gone tensile on this carbide, thinner passes, more cooling air, then check residual stress again.
Almen strips are cheap insurance on fatigue-critical parts: spray one with every lot and a parameter drift shows up before the part cracks.
Also heard as
- coating stress
- internal stress
Related terms
Mud cracking
A network of surface cracks that looks like dried mud, from tensile stress in a coating.
Delamination
Separation of a coating from its substrate or of one coating layer from another.
Cooling air
Jets of compressed air or CO2 aimed at the part during spraying to keep its temperature in check.
Cold spray
Process that fires solid metal powder at supersonic speed so it bonds by impact rather than melting.