Thermomechanical properties and fatigue of nanocrystalline Ni/Cu electrodeposits
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Thermomechanical properties and fatigue of nanocrystalline Ni/Cu electrodeposits Olivier Arnould1, Olivier Hubert and François Hild LMT-Cachan, ENS Cachan/CNRS UMR 8535/Université Paris VI 61, avenue du Président Wilson 94235 Cachan Cedex, France ABSTRACT This study deals with the long-term reliability of a high precision pressure sensor using bellows mainly made of electroplated Ni. Thermomechanical properties of this deposit are obtained by several experiments and compared to theoretical models, computations and other authors' results. Bellows are expected to stay in service for many decades, thus their high cycle fatigue behavior has to be known. Stress-life fatigue curve for crack initiation and fatigue crack growth in the electroplated Ni are measured and identified using numerical computations. Results are compared with other results obtained on similar Ni electrodeposits. Normalized stress-life fatigue curve shows no specific nanosize effects. INTRODUCTION This papers deals with the study of aging modes of a high precision pressure sensor. It uses bellows to convert pressure variations into a displacement (figure 1a). The stiffness and airtightness of bellows require to use a Ni/Cu/Ni layered material (overall thickness ~50µm) obtained by electroforming (figure 1b). The main aging mode of this layered material [1] that affects the bellows airtightness is due to high cycle fatigue crack initiation at the surface of the nickel layers and, to a lesser extent, on its propagation through them. The fatigue behavior of the Ni deposits is linked to their microstructure and the associated (thermo)mechanical properties that depend on the electrodeposition process. NI DEPOSIT CHARACTERIZATIONS Microstructure analysis Nickel is electrodeposited in a sulfamate bath ([Ni(SO3NH2)2·4H20] = 600g/l, pH = 4, θ = 50°C) with a low DC-current density (4 ≤ i ≤ 10mA/cm2). Deposits are electroplated on a rotating aluminum mandrel first coated with a copper layer. Both are removed at the end of the electrodeposition process to obtain the bellows. Under these conditions, nickel deposits have a fine columnar structure parallel to the electroplating direction (figure 1b) with clusters (figure 2a, [2]) made of micrometer domains of low-misoriented grains (figure 2b). XRD measurements and TEM observations (figure 2c) lead to a mean grain size of about 60nm, small second order residual stresses and a very low dislocation density. Moreover, the plating conditions induce a specific inhibited growth of the nickel layers associated with hydrogen absorption [3]. This yields a (relatively low) 110 fiber texture along the electroplating direction (figure 1c) and a high (nano)porosity, f > 5% (density measurements), of the deposits. 1
now at LMGC, Université de Montpellier II, cc 048, Place Eugène Bataillon, 34095 Montpellier Cedex 05, France E-mail : [email protected]
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Figure 1. a) General view of electroplated bellows. b) Micrograph of a cross-section obtained in an SEM after chemical etching: fine columnar microstruct
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