Residual stress and texture gradients in protective nickel coatings

The beamline MCX offers a perfect design and geometrical set-up for residual stress and texture analysis by X-ray diffraction. This work presents an example of application of X-ray diffraction to the study of residual stress and texture gradients in electrodeposited Nickel coatings.

M.Ortolani et al., Mater. Science Forum, 681, 115 (2011).

Thin films and coatings often show preferred crystalline orientation and a residual stress state as a consequence of the competitive grain growth process taking place during the deposition on the substrate. Moreover, as the film thickness increases and substrate ceases to directly interact with newly formed layers, gradients of preferred orientation and stress can be generated.
Residual stress and texture measurements are conveniently performed by means of X-ray diffraction, as the technique is contact-free and non destructive, hence it does not alter the sample during the observation. However, diffraction does not measure stresses directly, but rather strains; to retrieve the former, an appropriate constitutive equation needs to be applied, implying knowledge of the material’s elastic properties and mechanism of grain interaction. The task is made complicated by possible preferred orientation and properties gradients, as well as by the lack of a-priori information on the specific properties of the thin film, which are frequently different from those of the corresponding bulk materials. So far, fairly complex models have been proposed to account for the elastic grain interaction in thin film residual stress analysis, but the agreement with the experimental data, in many cases, is steel poor. An elegant solution consists of performing an in-situ mechanical testing on specimens during X-ray diffraction strain measurement, so to allow a simultaneous determination of residual stress and elastic constants, thus providing a complete outlook of the component’s mechanical properties.

As a case study, stress and texture gradients were measured on an electrodeposited Nickel coating on steel. These coatings are very common in corrosion protection applications, and constitute an ideal case study as their residual stress state, as well as preferred orientation of crystalline domains sensibly vary as a function of deposition conditions. In this case, the 20μm thick film was deposited in a standard Watt’s bath, under pulsed current and continuous solution stirring to promote homogenization and minimize porosity. X-ray stress and texture measurements have been repeated using an incident beam photon energy of 8.3 and 8.4 keV, respectively (the Kα absorption edge of Nickel being 8.33 keV).
Measurements showed that, while a slight texture gradient can be appreciated after observing the pole figure peak width decreasing with beam penetration, stress gradient is non-existent. This information, which would have been impossible to achieve with conventional laboratory instruments, provides useful feedback to the coating technologists, to optimize the deposition process and resulting properties of the protective coatings.

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Thin Film Stress and Texture Analysis at the MCX Synchrotron Radiation Beamline at ELETTRA;
M. Ortolani, C.L. Azanza Ricardo, A. Lausi and P. Scardi,
Materials Science Forum 681, 115-120 (2011)
Ultima modifica il Giovedì, 13 Giugno 2019 11:22