Properties of Electrodeposited Metals and Alloys

W. H. Safranek, The Properties of Electrodeposited Metals and Alloys, 2nd ed., American Electroplaters and Surface Finishers Society, Orlando, Fla.,... 

The key factors that control the rate of electrodeposition and the structure, physical properties, uniformity, and composition of electrodeposited metals and alloys are (1) thermodynamics (where the electric potential is based on the standard electromotive series) (2) electrode kinetics (which may vary with the structure of the electrodeposit) and (3) mass transport (which is important at high current densities, where the delivery of reactant to the cathode surface affects the local deposition rate and the structure of the deposit). 

Many books have been dedicated over the years to the topic of electrodeposition (see, for example, Refs. 1-6). These books deal with a variety of sub-topics such as surface preparation of the substrate prior to deposition, thermodynamics and kinetics of electrodeposition, the reactions that take place on an atomistic level, the mechanisms of growth, the effect of bath chemistry and operating conditions, the deposition of specific metals and alloys, the structure and properties of deposits, etc. 

The properties of electroplated gold, as often is the case for electrodeposited metals and alloys, are scattered in a number of technical and scientific publications, the only comprehensive review being the chapter devoted to it in the book of Safranek [7]. 

Electroplating—the process of electrodeposition onto a metallic substrate of a thin adherent layer of a metal or alloy having desirable chemical, physical and/or mechanical properties. 

Let us add here that the fabrication of polycrystalline semiconductive films with enhanced photoresponse and increased resistance to electrochemical corrosion has been attempted by introducing semiconductor particles of colloidal dimensions to bulk deposited films, following the well-developed practice of producing composite metal and alloy deposits with improved thermal, mechanical, or anti-corrosion properties. Eor instance, it has been reported that colloidal cadmium sulfide [105] or mercuric sulfide [106] inclusions significanfly improve photoactivity and corrosion resistance of electrodeposited cadmium selenide. 

Electroless deposition as we know it today has had many applications, e.g., in corrosion prevention [5-8], and electronics [9]. Although it yields a limited number of metals and alloys compared to electrodeposition, materials with unique properties, such as Ni-P (corrosion resistance) and Co-P (magnetic properties), are readily obtained by electroless deposition. It is in principle easier to obtain coatings of uniform thickness and composition using the electroless process, since one does not have the current density uniformity problem of electrodeposition. However, as we shall see, the practitioner of electroless deposition needs to be aware of the actions of solution additives and dissolved O2 gas on deposition kinetics, which affect deposit thickness and composition uniformity. Nevertheless, electroless deposition is experiencing increased interest in microelectronics, in part due to the need to replace expensive vacuum metallization methods with less expensive and selective deposition methods. The need to find creative deposition methods in the emerging field of nanofabrication is generating much interest in electroless deposition, at the present time more so as a useful process however, than as a subject of serious research. 

Relatively little attention has been devoted to the direct electrodeposition of transition metal-aluminum alloys in spite of the fact that isothermal electrodeposition leads to coatings with very uniform composition and structure and that the deposition current gives a direct measure of the deposition rate. Unfortunately, neither aluminum nor its alloys can be electrodeposited from aqueous solutions because hydrogen is evolved before aluminum is plated. Thus, it is necessary to employ nonaqueous solvents (both molecular and ionic) for this purpose. Among the solvents that have been used successfully to electrodeposit aluminum and its transition metal alloys are the chloroaluminate molten salts, which consist of inorganic or organic chloride salts combined with anhydrous aluminum chloride. An introduction to the chemical, electrochemical, and physical properties of the most commonly used chloroaluminate melts is given below. 

A comparison was made between Ni and Co diffusion barriers produced by electroless, electro-, and evaporation deposition (64). This comparison shows that only electrolessly deposited metals and alloys, at a thickness of 1000 im, have barrier properties for Cu diffusion. For Co(P) 1000-pm-thick barriers, annealed for 14h, the amount of Cu interdiffused into Co(P) is less than 1 at %. Thicker barriers of Ni(P), Ni(B), and Co(B) are required for the same degree of Cu interdiffusion. The same metals, if electrodeposited, both do and do not have inferior barrier properties. This... 

There are four types of fundamental subjects involved in the process represented by Eq. (1.1) (1) metal-solution interface as the locus of the deposition process, (2) kinetics and mechanism of the deposition process, (3) nucleation and growth processes of the metal lattice (M a[tice), and (4) structure and properties of the deposits. The material in this book is arranged according to these four fundamental issues. We start by considering the basic components of an electrochemical cell for deposition in the first three chapters. Chapter 2 treats water and ionic solutions Chapter 3, metal and metal surfaces and Chapter 4, the metal-solution interface. In Chapter 5 we discuss the potential difference across an interface. Chapter 6 contains presentation of the kinetics and mechanisms of electrodeposition. Nucleation and growth of thin films and formation of the bulk phase are treated in Chapter 7. Electroless deposition and deposition by displacement are the subject of Chapters 8 and 9, respectively. Chapter 10 contains discussion on the effects of additives in the deposition and nucleation and growth processes. Simultaneous deposition of two or more metals, alloy deposition, is discussed in Chapter 11. The manner in which... 

Mechanical properties of multilayered structures were only investigated for electrodeposits containing Cu/Cu-Ni layers. Following mechanical properties were mainly investigated Young s modulus [58], hardness [75], and tensile strength [67,75-77]. It was shown that all investigated properties depend on the thickness of the individual layers and that in all cases multilayered structures showed better properties than that of pure metals and/or their alloys. 

The influence of the deposition parameters on the surface morphology of chemically deposited metals and alloys is significantly more complex than that described for its electrodeposition counterpart. The surface morphology of chemically deposited metals and alloys depends on many parameters such as concentrations of the metal ions and reducing agent, pH, temperature, and mixing. Further studies are definitely required for a more systematic and precise description in order to achieve the desired physicochemical properties and a required surface morphology. 

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