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CNAA / Theses / 2012 / June /

Induced codeposition of nanocrystalline Co-W coatings and their mechanical properties


Author: Belevschi Stanislav
Degree:doctor of chemistry
Speciality: 02.00.05 - Electrochemistry
Year:2012
Scientific adviser: Alexandru Dicusar
doctor habilitat, professor, Institute of Applied Physics, Academy of Sciences of Moldova
Institution: Institute of Applied Physics, Academy of Sciences of Moldova
Scientific council: DH 02-02.00.05-27.03.08
Institute of Applied Physics, Academy of Sciences of Moldova

Status

The thesis was presented on the 14 June, 2012
Approved by NCAA on the 9 October, 2012

Abstract

Adobe PDF document0.98 Mb / in romanian
Adobe PDF document0.93 Mb / in russian

Keywords

induced codeposition, anomalous deposition of alloys, Co-W alloys, citric electrolyte, gel-chromatography, impedance, hydrodynamics, ionic mass transfer, microhardness, wear resistance, long term operation of electrolyte.

Summary

The thesis is written in Russian, consists of introduction, six chapters, general conclusions, recommendations and a list of references. The work contains 142 pages of text, 74 figure, 9 tables, reference list, which includes 123 sources.

Publications: the obtained results were published in 20 scientific papers (10 articles and 10 abstracts).

Field of research: Electrochemistry.

The aim of the research: was the complex investigation of induced codeposition mechanism of Co-W coatings obtaining from citrate electrolyte and determining the conditions of electrodeposition that provide the coatings the properties that could compete with the hard chromium electroplating coatings.

The scientific novelty and originality of the work: for the first time it is demonstrated that citrate electrolyte used for electrodeposition of Co-W alloy is a mixture of complex compounds, whose composition is determined by the pH. At high pH values, its main component is heteropolynuclear complex with a molecular weight over 1200 g / mol. The totality of the results obtained by different methods (gel-chromatography, voltammetry, the methods of physicochemical hydrodynamics, determination of the composition of coatings, the current efficiency, etc.), can conclude that the chemical composition of electrodeposited Co-W coatings is determined by the heteropolynuclear complex composition on the one hand and the pH near-electrode layer on the other. However, the pH near-electrode layer depends on the rate of the parallel hydrogen evolution reaction (defined by the potential of electrodeposition and the hydrodynamic conditions). The increasing of the pH near-electrode layer shifts the chemical equilibrium toward to the formation of complex products with high molecular weight. It was confirmed the existence of heteropolynuclear Co-W-citrate complex compound, where the atomic ratio of Co:W is equal to 1:1.

Solved scientific problem: The experimental proof of the fact that the formation of cobalt-tungsten coatings from citric electrolyte is the result of electrochemical reduction of polynuclear heterometallic complex.

The research object is the chemical composition of citrate electrolyte (identification of the contained complexes) and induced codeposition of Co-W coatings from citrate electrolyte. The determination of the influence of the degree of the electrodeposition conditions on the composition, structure and properties of the Co-W alloys.

The theoretic value of the research: The obtained results can be considered as fundamentally new. They can explain the nature of the induced anomalous codeposition of W with Co, and can be a basis for developing a mechanism of induced codeposition of iron group metals with refractory metals (W, Mo, Re) from citrate and gluconate baths.

The implementation and practical value of the research: the results of this research can be considered fundamental for the development of electrochemical obtaining technology of hardening coatings on the basis of Co-W alloys, that reduces the environmental pollution in comparison with the technology of the hard chromium plating coatings obtaining.