Highlights

Every year, ELETTRA publishes 'ELETTRA Highlights' reporting a selection of the work done at the Laboratory during the last twelve months.
In this page you will find a brief presentation of the experiments done at the MCX beamline and present in ELETTRA Highlights, along with other relevant results obtained at the beamline.

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.

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.

Retrieve article

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)
doi 10.4028/www.scientific.net/MSF.681.115

Non destructive XRD investigations on grisaille paint layerson stained glass windows

Non destructive and innovative XRD experiments on the MCX beamline allowed us to recognize the alteration products on the grisailles surface and to propose a mechanism for the formation of alteration patinae.

L. Nodari et al., Elettra Highlights

Since the end ofXIX century the study of glass corrosion and its weathering products was considered of great interest for Cultural Heritage Science. In the last decades the interest of the scientific community was mainly focused on the characterization of glass corrosion products, their formation processes, and on the development of new systems for the protection of the glass surface. An interesting and breakingresearch field, still nowadays almost unexplored, regards the investigations on stained glass windows alteration, especially on those painted by the grisaille method. Grisaille can be defined as a painting mixture formed by a finely powdered pigment (mainly transition metal oxides) and highly fusible glass (lead rich glass) forming a coloured layer applied on the glass surface, the adherence to the glass being guaranteed by a low temperature firing of the grisaille without melting the glass support.

Under a chemico-physical point of view, the obtained material is a system in which the crystalline phases (pigments) are embedded in an amorphous phase (low melting glass) which acts both as a dispersing medium and as an agent fixing the pigments onto the substrate (the stained glass window). The interaction of this surface with the environment (i.e. the effect of pollutant agents) promotes the formation of alteration crusts involving both grisaille paint layer and glass window surface. The aim of the present manuscript deals with the characterization of the alteration products of ancient and modern grisaille paint layers. The understanding of the complex chemical and structural nature of alteration products leads to an exhaustive comprehension of the deterioration processes and will be helpful to propose a correct project for the conservation and restoration of grisaille stained glass windows.

Ultima modifica il Martedì, 24 Gennaio 2012 12:40