Relaxation of residual stresses on the near surface of carbon steel substrates due to plasma cleaning

Plasma cleaning has been used as a treatment prior to PVD coating processes in order to improve the film adhesion. The variation of the cleaning parameters, such as, the bias power, the chamber pressure and the process time, modifies the energy of the argon ion bombardment. In this work it is presented a study in order to determine the influence of these cleaning parameters on the surface of AFNOR 20M5 steel substrates. Particularly, it is presented how the argon bombardment affects the relaxation of residual stresses profiles on the near surface of the substrates. The residual stress profiles were determined by X-Ray Diffraction with Cu Kα and Cr Kα radiation using the conventional sin²ψ method and the pseudo-grazing incidence method. Concerning the experimental results, it is very clear from all data that the compressive residual stress value found in the substrate before plasma cleaning, due to the mechanical polishing, was relaxed after cleaning. The application of the pseudo-grazing incidence method enabled the characterisation of the extreme surface (≈0.1-0.5 μm), and it seems that the residual stresses state in this region is dependent on the pressure chamber, if a convenient bias power is applied. However the study had shown that thermal effects due to the prior heating of the substrates, carried out just before the plasma cleaning, seams to be important on the residual stress relaxation over a higher depth range.