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2018-09-13 - Colloque/Présentation - poster - Anglais - page(s)

Montagne Alex, Bonin Luiza, Mughal M.Z., Sebastiani Marco, Bemporad Edoardo, Vitry Véronique , Iost Alain, Staia Mariana, "Mechanical behavior of electroless nickel coatings" in Indentation 2018, Liège, Belgique, 2018

  • Codes CREF : Chimie des surfaces et des interfaces (DI1327), Mécanique appliquée générale (DI2100), Mécanique des milieux continus (DI1243)
  • Unités de recherche UMONS : Métallurgie (F601)
  • Instituts UMONS : Institut de Recherche en Science et Ingénierie des Matériaux (Matériaux)
  • Centres UMONS : Ingénierie des matériaux (CRIM)

Abstract(s) :

(Anglais) Electroless nickel coatings are of interest for tribological applications. They are the subject of many researches since they are seen as potential substitutes for hard chrome plating (hexavalent chromium, toxic and carcinogenic). The objective of this study is to explore the mechanical properties of nickel-boron (NiB) coatings in order to validate their use as a substitute for traditional chrome coatings. A specific emphasis is given to the study of both hardness and residual stresses, which have a strong influence on the coating adhesion to the substrate and, hence, on the mechanical performance of the coated system. The determination of residual stresses in the coatings is not an easy task. The conventional technique based on X-ray diffraction offers a submicron spatial resolution, but requires a crystalline material. On the other hand, the Stoney's method links the curvature of a sample to the residual stresses, but it requires a particular shape of the substrate or a controlled thinning of the coating. The nanoindentation technique can also be used to determine the residual stresses by studying the shift of the load-displacement curves obtained on stressed and completely relaxed materials. The main difficulty of the latter method is to obtain an identical material in terms of microstructure, but free of stresses. STRAIN RELAXATION An original technique based on the principle of incremental hole, reported at the micrometer scale, has been proposed to study the residual stresses [1]. Its principle is to drill a ring around a micropillar by means of focused ion beam techniques. Images of the surface are obtained at different milling depths using a scanning electron microscope. Images are then processed by means of digital image correlation software developed at the University of Roma Tre, Italy. Reference dots on the surface are tracked from one milling step to another. The strain relaxation is deduced from the dots shifting and the residual stresses are calculated using a modified Hook’s relation. The electroless nickel coatings are almost free of residual stresses with values not exceeding 200 MPa. However, it is shown that the stress distribution in the coating is heterogeneous in terms of intensity and sign. Furthermore, the microstructure of the coating seems to partially relax the stresses at large length scale. Colloque Indentation 2018 11 – 14 Septembre 2018 Liège Groupe d’Indentation Multi Echelle INDENTATION TESTS Figure 1: a) optical image of a cross-section NiB deposited onto a steel sample, with indentation array, b) corresponding hardness. Nanoindentation tests were carried out on a cross section of the coating. Figure 1 shows the array of indents (1.a) as well as the corresponding hardness profile (1.b) for the as-plated NiB coating. The evolution of hardness is related to the different materials indented: the substrate, the coating and the resin. If a closer look is given to the results corresponding to the coating, it is observed that the hardness value remains constant throughout the whole thickness. A mean value of 10 ± 1 GPa is calculated. Tests have been performed on the as-plated and heat treated NiB coatings. The hardness clearly increased with the heat treatment, from 10 to 15 GPa. However, there is a small evolution of the residual stresses with the heat treatment. The hardness evolution is then linked to the microstructural changes that take place during the heat treatment and not linked to the presence of residual stresses. CONCLUSION Residual stresses in these electroless nickel coatings seem to be very low and have almost no influence on the hardness measured by nanoindentation. The high resolution of the ring-core technique highlights heterogeneities of stress distribution in the coating observed at short length scale. References [1] A. M. Korsunsky, M. Sebastiani, and E. Bemporad, Focused ion beam ring drilling for residual stress evaluation, Mater. Lett., 63 (22), pages 1961–1963, 2009 substrate coating resin a