EFFECTS OF PLASMA CLEANING OF A CARBON STEEL SUBSTRATE STUDIED BY PSEUDO-GRAZING INCIDENCE X-RAY DIFFRACTION

摘要

Plasma cleaning has been used as a pre-treatment prior to PVD coating processes in order to improve the film adhesion. The biased surface of 20M5 steel substrate exposed to the plasma is subjected to sputtering. 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. The aim of this work is to show how the residual stress profile in the near surface of substrate is affected by cleaning parameters. The residual stress profiles in the cleaning 20M5 steel substrates were determined by XRD with Cu kα and Cr kα radiation using the conventional sin~2ψ method and the pseudo-grazing incidence method. The pseudo-grazing incidence method enables the evaluation of the residual stresses for different penetration of the X-rays, thus a profile of residual stress can be obtained. The conversion of the τ-profiles (stress vs. penetration depth, τ) to z-profiles (stress vs. depth curves z) has been extensively treated by several authors. This conversion is delicate and often unstable. It is out of the present work. Concerning the experimental results, residual stress value of the order of -190 MPa was observed at 5.8 μm mean depth in the polished 20M5 steel substrate. After cleaning, the data had shown a decrease to values comprised between -60 MPa and -100 MPa depending on cleaning parameters. The results obtained with Cu kα wavelength shown a residual stress value of the order of -500 MPa at 1.54 μm that relax to -200 MPa after cleaning. In the extreme surface (≈0.1-0.5μm), it seems that the residual stresses state is dependent on the pressure chamber, if a convenient bias power is applied. It is very clear from the profiles that the compressive residual stress value found in the substrate before cleaning due to the mechanical polishing was relaxed after cleaning. However the study had shown that thermal effects seems to be important on the residual stress relaxation over a higher depth range.