Mechanical surface treatments — Principles

Statistical analyses of the causes of mechanical component failure show that, in the vast majority of cases, failure is due to the breakage of the part with surface initiation. The quality of the surface layer is therefore an essential factor in the mechanical integrity of mechanical structures. In fact, surface areas are often the most stressed due to stress concentrations imposed by the geometry of a mechanical part that has holes, notches, and other geometric discontinuities. In addition, with the exception of mechanical stresses caused by contact, mechanical and thermomechanical stresses are very often at their highest on the surface, for example bending, torsion, and thermal shock. Even for tensile stress, surface roughness generates a concentration of local stresses that increases the level of mechanical stress. The surface of a mechanical part is also an area of contact with the hostile environment, such as air, which can cause oxidation problems, and corrosive environments. It is also the part of a component where fretting, wear and friction, seizing, and galling occur. Combining all these unfavorable factors, the surface of a mechanical part is a particularly vulnerable area that is of interest to mechanical engineers for mechanical design and materials specialists for improving mechanical and overall performance.

To improve surface properties, there are a number of manufacturing processes available, such as thermal and thermochemical treatments, vapor deposition (PVD and CVD), thermal spraying, and mechanical surface treatments. In this article, we will focus primarily on the latter category of treatments.

Mechanical surface treatments are processes that improve the performance of materials through a combination of surface hardening, structural modification, and the introduction of residual compressive stresses via heterogeneous plastic deformation on the surface of mechanical components. The most commonly used treatments are shot peening, roller burnishing, hammering, laser shock, and the generation of nanostructures through random plastic deformation introduced to the surface of materials. The basic principle is to apply pressure to the surface of a material to cause plastic deformation, either by means of a shaped tool, as in shot peening or roller burnishing, or by means of a shock wave, as in laser shock treatment. This plastic deformation is not homogeneous across the depth of the part from the treated surface. This type of treatment generates residual compressive stresses that are often beneficial for fatigue and corrosion resistance. Following plastic deformation, the material may harden due to surface work hardening and/or may reduce grain size or generate a phase transformation. These structural changes are also beneficial in most cases with...