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Induced electrodeposition of nanostructures as nanowires and nanotubes consisting of cobalt-based multilayers for MEMS applications

Final Report Summary - NANOALLOY (Induced electrodeposition of nanostructures as nanowires and nanotubes consisting of cobalt-based multilayers for MEMS applications)

The first objective of the NANOALLOY project for the reporting period is linked to electrodeposition of Co-W alloys with different tungsten content on plane substrates from environmentally friendly citrate-borate electrolytes under direct current (DC) and pulse current (PC) modes. In order to determine optimal electrodeposition parameters (pH, solution composition, duration of pulses and pause, potentiostatic / galvanostatic mode), structural, tribological, thermal stability and magnetic characterisation as a function of electrodeposition parameters has been done. Nanocrystalline Co-W alloys were electrodeposited at 60 degrees of Celsius from citrate-borate electrolyte at pH 6.7 and 8.0. The effect of electrodeposition conditions in DC and PC modes on tungsten content in the alloys and their structure were investigated. The tungsten content in the alloys was varied between 13 and 36 at.%, and it defines the grain size of the Co-W coatings irrespectively on the deposition mode or current density allowing to obtain this particular content of alloy. A grain size 27 - 40 nm was obtained for lower content of tungsten, and 3 - 6 nm is characteristic for as-deposited coatings containing more than 23 at.% W. The formation of W solid solution in Co and Co3W phase is discussed. The transition from nanocrystalline to polycrystalline structure of Co-W coatings with as-deposited grain size of 3 - 6 nm was obtained by annealing. In coatings containing 24.5 at.% W, a well-developed polycrystalline structure of Co3W appears at 400 degrees of Celsius with (200), (201) and (220) textures whose remain stable up to 1 000 degrees of Celsius. The grain size of coatings containing 24.5 at.% W increases from 3 - 5 nm up to the 26 - 30 nm after annealing at 400 degrees of Celsius. Whereas, Co-W coatings with tungsten content of 34.4 at.% W maintain their nanocrystallinity at 400 degrees of Celsius, and transforms to the polycrystalline structure at 600 degrees of Celsius with an increase of grain size up to 45 nm.

The results obtained during Co-W coatings preparation were integrated for Cu/Co-W multilayers electrodeposition in collaboration with TEMADEP project. Cu/Co-W multilayered coatings were electrodeposited from a single bath for the first time. The layered coatings with 5 to 200 nm of individual layers were produced. The current density for Co-W rich layers has been changed to detect the influence on tungsten content in the electrodeposits. The tribological and mechanical properties of multilayered Co-W/Cu coatings have been investigated. These coatings were compared with Co-W coatings electrodeposited from a similar bath. The hardness of Cu/Co-W multilayers varied with the bi-layer period and the electrodeposition parameters. The Cu/Co-W multilayers show in most cases an increase in hardness compared to the Co-W monolayers electrodeposited under the same conditions. In dry-sliding fretting tests, Cu/Co-W multilayers withstand well at low normal forces (2N), but at high forces of 10 N the wear resistance of these coatings depends strongly on the presence of copper in the electrodeposited layers.

The second objective concerns the priority to establish a joint cooperation with Interuniversity Microelectronic Centre (IMEC), Leuven in the field of Co-W alloys applications. In that respect, in the reporting period of the project research efforts were concentrate to study the synthesis and the characteristics of thin films and coatings consisting of Co-W alloys that are especially appropriate for use in microelectromechanical systems (MEMS) where microstructures with high mechanical strength and wear resistance coupled to a high thermal resistance and a good resistance against plastic deformation are required. The main interest was to produce microbumps of nanocrystalline Co-W alloys obtained by induced electrodeposition, and to unravel the relationship between structural properties and synthesis conditions. The already obtained Co-W microbumps were optimised in order to achieve the pre-requisites for microbumps applicatios such as void-free and uniform structures. In that respect collaboration with IMEC was successful and generated a new platform for Co-W alloys application.

The third objective was devoted to a progressive research on the synthesis by induced electrodeposition of Co-W nanowires. The filling of nanopores present as arrays in anodised aluminium oxide (AAO) was investigated in-depth. Ordered, uniform Co-W nanowire arrays were fabricated by a simple electroplating method in high aspect-ratio anodic aluminum oxide (AAO) membrane. Electrodeposition (potentiostatic, galvanostatic and pulse electrodeposition) of Co-W nanowires was carried out at different temperatures (20 - 60 degrees of Celsius) from the citrate-borate electrolyte at pH 5, 6.7 and 8. It has been show that the most uniform growth of Co-W nanowires can be achieved at pH 5.

In collaboration with NANOCOAT project and Marie Curie fellow at KU Leuven Dr Ivan Buijnsters the fourth objective was developed as application of AAO for versatile applications where good mechanical strength, wear and corrosion resistance, and hydrophobicity of AAO can be of use. The first step was the synthesis and properties investigations of AAO. Thus, well-ordered nanoporous AAO film was obtained with relative small pore diameter (16 and 27 nm) and investigated to understand the wear behaviour and mechanical properties. It was been shown that AAO subjected to mesoscale and macroscale fretting tests formed very fine debris on the surface that cover the pores with thin tribolayer. Debris which are produced during sliding on the surface fill in the pores followed by formation of a tribolayer which reduces the coefficient of friction. Hardness and Young’s modulus of highly ordered alumina were measured by nanoindentation. The hardness and elastic modulus values depend on the loads applied. Hardness can reach 10 GPa at 2 mN on membrane of 16 nm pore diameter. The second step was preparation and structural analyses of AAO arrays with relatively large pore sizes (100 nm) for surface wetting studies has been done. Contact angles as large as 135 degrees have been achieved based on pore opening step (= large hydrophobicity). Surface modification after pore opening by treatments in lauric acid and silane led to near-superhydrophobic AAO arrays with contact angles close to 150 degrees.

The fifth objective was linked to evolvement of the fellow in creating new opportunities for a further exploration of materials science and versatile collaborations:

(a) through the submission of new Seventh Framework Programme (FP7) projects: MRC-IAP (ERA-WIDE), Oil&Sugar (IRSES), EC-MODEL and IAPSEC (2 REGPOT projects);
(b) participation with oral presentation and key lectures at international conferences, including invited presentations at University Autonoma de Barcelona, Spain, Vilnius University (Nanoschool), Lithuania, Northeastern University, United States of America (USA) (TEMADEP project meeting);
(c) organisation of international Workshop at KU Leuven with participation student and researches from Spain, Lithuania, USA, France, Moldova, Belgium, Russia.