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X-ray analysis of residual stress gradients and textures in thin coatings
The increasing interest in coatings is understandable for their importance in microelectronic technology and in surface treatments of various materials to get good wear and corrosion resistance. The deposition of thin layers on a substrate material often causes a specific stress and texture state, depending strongly on the process parameters and finally on the thickness of the coatings. X-ray diffraction technique is a suitable and well-known non destructive method to analyse residual stresses and texture in thin coatings. The depth of penetration of X-rays is the focus of attention of the analysis of surface layers, because gradients of stress and of texture are often observed in the coating and in the substrate material. The measured values represent absorption weighted effective parameters, different from the the true values in a special depth. For this reason in X-ray studies of polycrystalline surface layers, it is necessary to know the thickness of the layer from which thediffra ction pattern has been obtained and to vary the depth of penetration in a wide range. To overcome these problems the coatings can be investigated using: - Bragg-Brentano diffraction in symmetric case and different X-ray radiations, having a remarkable different penetration depth and - Bragg-Brentano diffraction in asymmetric case (based on the grazing incidence technique) in order to produce a very small penetration of X-rays and to get a value of stress and texture very close to the surface. The pros and cons of the two mentioned techniques to determine stress gradients and textures in thin coatings are shown and discussed. For example, in this paper results are presented using X-ray diffraction to determine residual stress and texture state in PVD (physical vapour deposition) hard coatings of thickness renging from 0,5 micro meters to 5 micro meters on steels S6-5-2 and 100Cr6. During the last years, a lot of work in the field of stress and texture analysis has beenperformed on har d coatings made be physical vapour deposition. We would like to mention some authors: Vavolda; Perry and Chollet; Mayr and Hirsch; Eigenmann, Scholtes and Macherauch; and Birkhölzer and Hauk. An interesting side effect concerning the interpretation of the residual stresses was the comparison of experimantal results with the results of numerical finite element calculations within the meaning of hybrid methods.