Mechanical surface treatments - Principles

Statistical analyses of the causes of failure of mechanical components show that, in the vast majority of cases, failure is due to part fracture with surface initiation. The quality of the surface layer is therefore an essential factor in the mechanical integrity of mechanical structures. Indeed, surface zones are often the most stressed due to the stress concentrations imposed by the geometry of a mechanical part, which features holes, notches and other geometric discontinuities. What's more, with the exception of mechanical contact stresses, mechanical and thermomechanical stresses are often greatest at the surface, such as bending and torsion, and thermal shock. Even in the case of tensile stress, surface roughness generates a concentration of local stresses, which increases the level of mechanical stress. The surface of a mechanical part is also a zone of contact with hostile environments such as air, for oxidation problems, and corrosive media. It is also the part of a component where fretting, wear and friction, seizure and matting occur. Combining all these unfavorable factors, the surface of a mechanical part is a particularly vulnerable zone of interest to mechanics for mechanical design, and to materials specialists for improving mechanical and overall performance.

To improve surface properties, a large number of manufacturing processes are available, including thermal and thermochemical treatments, vapor deposition (PVD and CVD), thermal spraying and mechanical surface treatments. In this dossier, we will focus on the last category of treatments.

Mechanical surface treatments are processes that improve the performance of materials through a combined action of surface hardening, structural modification and the introduction of residual compressive stress through heterogeneous plastic deformation on the surface of mechanical components. The most commonly used treatments are shot peening, roll peening, hammering, laser shock, and the generation of nanostructures by random plastic deformation introduced at the surface of materials. The basic principle is the application of pressure to the surface of a material to cause plastic deformation, either by a forming tool, as in shot peening or roller peening, or by a shock wave, as in laser shock treatment. This plastic deformation is not homogeneous throughout the depth of the part, starting from the treated surface. This type of treatment generates residual compressive stresses that are often favorable for fatigue and corrosion resistance. Following plastic deformation, the material may harden through surface work hardening and/or may reduce grain size or generate phase transformation. These structural changes are also favorable in most cases with regard to mechanical stresses such as fatigue, wear and friction....