M. Lekka

Atomic layer deposition: state-of-the-art and research/industrial perspectives

Abstract Interest on nanometric conformal coatings is currently growing across a wide range of applications, from electronic components to corrosion protection, chemical barriers or even wear decrease. Currently, atomic layer deposition (ALD) is one of the most promising nanometric deposition technologies: it offers the possibility to obtain conformal coatings even on very complex tridimensional substrates, with a strict thickness tolerance. During an ALD cycle, only one molecular layer is deposited on the substrate surface, enabling the theoretical possibility to tailor the composition of the deposit up to molecular resolution. In the first part of this review, a brief history of ALD is presented to better understand the evolution of the technique during the past five decades; in the second part, a short description of the main ALD techniques is given, considering their main advantages and drawbacks. In the third part, a short review about ALD applications is presented, and in the fourth part the most useful and used instruments for the analysis and characterization of ALD are listed; some results are discussed and particular emphasis on the corrosion protection of different metallic alloys of common industrial interest is given.

Resistance to localized corrosion of pure Ni, micro- and nano-SiC composite electrodeposits

The aim of this work was the production and characterization of composite Ni matrix electrodeposits. Pure Ni, micro- and nano-SiC Ni matrix composite deposits have been produced from a Watts’s-type electroplating bath under both direct (DC) and pulse current (PC) conditions. The obtained deposits have been characterized regarding their microstructure by scanning electron microscopy (SEM) observations on both top surface and cross-section and their SiC content by energy-dispersive X-ray spectrometry (EDXS) and glow discharge optical emission spectrometry (GDOES) analyses. The resistance to localized corrosion has been evaluated by exposing the samples in a salt spray cabinet and performing visual observation as well as electrochemical impedance spectroscopy (EIS) measurements every five days. Both the use of PC and the codeposition of the nanoparticles lead to a grain refinement of the Ni matrix. The use of the PC did not influence in a significant way the resistance of the pure Ni deposits to the localized corrosion. The incorporation of micro-SiC led to a decrease of the corrosion resistance for the deposits produced under DC, while the microcomposites produced under PC presented a corrosion resistance comparable to the pure Ni deposits. The nanocomposites presented the highest corrosion resistance due to the more compact and fine-grained microstructure. EIS revealed the presence of a localized corrosion attack earlier than the visual observation, giving useful information about the failure mechanism.

Study of the influence of sonication during the electrod­eposition of nickel matrix nanocomposite coatings on the protective properties

Abstract The interest in the electrocodeposition has been revived due the development of new nanotechnology methods for the production of nano-particles. The codeposition of nanoparticles might enhance the mechanical properties of composite deposits without penalizing the corrosion resistance and avoid the formation of an abrasive third body during the wear process. On the other hand, the volume fraction of codeposited nano-particles is generally low and usually inversely proportional to their size. Many efforts have been done in order to improve the codeposition rate of nanoparticles. Most of the research on nickel composites is focused on the electrodeposition process or on the mechanical and wear properties rather than on the corrosion resistance. In this work, the protective properties of nickel matrix nanocomposite coatings have been studied by means of potentiodynamic curves and electrochemical impedance spectroscopy. Two different nanopowders, i.e., silicon carbide and alumina, were added to a Watts type galvanic bath in order to deposit the nanocomposites coatings on steel substrate. Ultrasonic vibrations have been considered as new process parameter to improve the dispersion of the powder into both the plating bath and metal matrix coating and to substitute pitting control agents for the production of defect-free coatings. Unique, functional properties of composite coatings are derived not only from the presence of the particles dispersed in the bulk of the metallic matrix but also on the matrix microstructural changes induced by the interaction between particles and electrocrystallization process. It has been demonstrated that the codeposition of the silicon carbide particles induces an important microstructural refinement. On the contrary, the aluminum oxide powder is strongly agglomerated and only under ultrasonic vibrations can be dispersed. Ultrasounds have a positive effect not only in hindering the porosity but also in dispersing the ceramic powder and increasing the codeposition rate leading thus to the production of protective and very refined coatings. Moreover, the better dispersed powder induces an improvement also in the corrosion protection leading to the formation of a more stable and resistant passive nickel oxide.

Heat treatments hardening effect on Ni–Al composite electrodeposits

ABSTRACT Pure Ni galvanic coatings show good mechanical properties at room temperature but exhibit a progressive decrease by increasing the temperature due to the recrystallisation. A possible solution to hinder the recrystallisation is the co-electrodeposition of metal particles which could lead to the formation at high temperature of Ni alloy coatings with higher mechanical properties. Ni matrix composite deposits containing either micro (4 μm) or nano (130 nm) particles of Al were produced in a parallel plate geometry. The deposits were characterised regarding their microstructure both prior and after heat treatments for 3 h at 400, 600 and 800°C in order to evaluate the formation of Ni/Al phases. This work focused on the evaluation of the hardness on coatings cross-section by means of Vickers microhardness and Berkovich nano-indentation mapping. The results obtained with the two methods have been compared and correlated to the microstructural modifications in order to understand the hardening mechanisms.