Elastic grain interaction in electrodeposited nanocomposite Nickel matrix coatings

https://doi.org/10.1016/j.surfcoat.2011.10.056Get rights and content

Abstract

The effect of SiC and Al2O3 nanoparticles in electrodeposited Nickel coatings was studied by X-ray Diffraction methods, in terms of size and texture of the crystalline domains as well as residual stress and elastic constants. SiC nanoparticles finely disperse and affect grain growth significantly, determining the rise of a strong 100 crystallographic texture with equiaxed domains; whereas Al2O3 nanoparticles tend to agglomerate, only resulting in a precipitation hardening-driven stiffening effect of the metallic matrix. Concerning residual stress analysis, the known elastic grain interaction models seem not suitable to this kind of deposits: a poor modelling of the experimental data is attributed to an inaccurate assessment of the elastic constants and too restrictive hypotheses. More reliable and conclusive results are obtained by an in-situ XRD stress analysis during four point bending, which provides a direct measurement of the effective X-ray elastic constants. The measurement technique is briefly introduced and applied to the electrodeposited Nickel matrix nanocomposite coatings.

Highlights

► Nanocomposite, electrodeposited Nickel coatings on steel were studied by XRD. ► In situ XRD during four point bending gives direct information on the elastic grain interaction. ► SiC and Al2O3 nanoparticles have quite a different effect on microstructure and elastic properties. ► SiC nanoparticles promote a [001] texture with small equiaxed domains in the metal matrix. ► Al2O3 nanoparticles tend to agglomerate and only cause precipitation hardening.

Introduction

Nickel matrix composite coatings have long been studied and employed to enhance wear and corrosion resistance of galvanic products, their history dating back to the early 70s [1]. Recently [2], [3], [4] interest has been shown for composites featuring dispersed nanoparticles, whose mean size can be as small as 20 nm, as they display enhanced properties in respect to their micrometre-scale equivalents. The large matrix–particle interface area per volume fraction of the precipitate phase has a marked effect on the final product microstructure, in terms of grain size distribution and preferred orientation, and hence on the final elastic properties and residual stress state.

Residual stresses are a key feature of thin films and coatings, as in most cases they develop naturally during the deposition process [5]. In particular, electrodeposited Nickel-based coatings develop different stress states depending on deposition parameters such as current density and pulsation, coating thickness, etc. Additionally preferred orientation, of either crystallographic or morphological nature, can be found in different amounts, from having a nearly randomly oriented polycrystal to displaying sharper fibre texture poles [6], [7], [8]. A precipitate phase is known to affect preferred orientation, as well as residual stress state, as a consequence of the surface interactions which occur during electrocrystallization [9]; moreover, the particle content and distribution can be tailored by varying the particle nature and deposition parameters.

Nanocomposite Nickel matrix nanocomposite coatings thus make an ideal case of study for stress–texture relationships [10]. In the present work we investigate three different Nickel matrix coatings [3] including a pure Nickel one and two coatings respectively with SiC and Al2O3 nanoparticles. A new approach to study the elastic grain interaction model and the residual stress state is presented.

Section snippets

Methods

X-ray Diffraction (XRD) is conveniently used for stress and texture characterisation, thanks to the non-contact and non-destructive nature [11]. In addition, XRD is sensitive to the anisotropy of the elastic response of a material. As a peculiarity and possible drawback, XRD only measures an average strain (in terms of lattice d-spacing relative shift) over the crystallites whose atomic planes are in diffraction condition, i.e., orthogonal to the scattering vector in the diffractometer setup. A

Experimental

Three different 20 μm thick Nickel matrix coatings were grown on a low-carbon steel sheet based on a known deposition procedure [3]: one of pure Nickel coating with a columnar structure (1); one with well dispersed SiC nanoparticles (2); and one with agglomerated Al2O3 nanoparticles (3). Coatings have been deposited using a standard Watt's bath, having the following composition: 240 g/l NiSO4, 45 g/l NiCl2, 30 g/l H3BO4, at 45 C under galvanostatic control applying 1 Hz pulse current. 0.1 g/l sodium

Results and discussion

The cross section of samples 2 and 3, and a TEM micrograph of sample 2 are presented in Fig. 1. The different powder agglomeration is noticeable: Ni + SiC appears homogeneous and almost featureless, whereas alumina agglomerates are clearly visible as dark dots in optical micrographs. Silicon carbide nanoparticles, instead, can only be seen by TEM as small, white crystals at grain boundary. TEM micrographs also display arrays of stacking faults (e.g., in the top-left grain), likely introduced by a

Concluding remarks

The present work shows the important effect of SiC nanoparticles on the coating microstructure (and hence the properties) of a polycrystalline Ni coating, the change being significantly more pronounced than with the addition of Al2O3. SiC shows good nanoparticle dispersion, as well as remarkable effects on Ni grain growth, effectively promoting the development of a 100 texture. SiC nanoparticles have an opposite effect on morphological texture, which displays larger, equiaxed domains with

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