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Chapter 1
Introduction
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1.1 Metal Matrix Nanocomposites
Metal matrix composites (MMCs) refer to a kind of material in which rigid ceramic reinforcements are embedded in a ductile metal or alloy matrix. MMCs combine metallic properties (ductility and toughness) with ceramic characteristics (high strength and modulus), leading to greater strength in shear and compression, near-isotropic as well as excellent high-temperature properties [Tjon 2000]. An optimum set of mechanical properties can be achieved when fine and thermally stable ceramic particulates are dispersed uniformly in the metal matrix. Efforts have been made to meet such requirements.
A variety of processing techniques have evolved over the last decade in an effort to optimize the structure and properties of ceramic phase reinforced MMCs. In most cases, the nanostructured materials are fabricated by vacuum deposition, sputtering, sol–gel, spray pyrolysis, etc [Glei 1989]. All these methods require a high degree of process control, which could be accomplished only with the use of expensive equipment and involve a huge wastage of materials. Compared to many other methods, electrodeposition is an inexpensive process, offers a simple and viable alternative to these complicated and expensive high-temperature or high vacuum deposition processes [Tao 2006]. The electrodeposition technique is extensively reported in the literature for the processing of metal matrix nanocomposites (MMNCs). With the emergence of nanostructured materials in recent years, electrodeposition techniques have provided a route to a variety of new nanomaterials. With the trend towards miniaturization, electrodeposition has established itself as the manufacturing technology of choice [Land 2002].
Pulse current electrodeposition, in which current is imposed in a periodic manner with a rectangular waveform, is a powerful means for controlling the electrocrystallization process and producing deposits with unique structure and properties. Many nanocrystalline metals, alloys and composites have been produced by pulse electrodeposition successfully [Chan 2008].
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1.2 Why copper based composites?
The extensive usage of Cu and its composites has increased in recent years because of the attractiveness of this material for applications that require high electrical conductive layers and interconnect lines. In order to enhance the reliability and the response speed of microelectronics, copper has been employed to replace aluminum and tungsten as the interconnect material for printed circuit boards (PCBs) and integrated circuit chips [Schu 2001].
Copper is a highly promising alternative material to aluminum-based alloys for the new generation of ultra large scale integration (ULSI) circuits owing to its low electrical resistivity and high electromigration resistance [Kang 2008]. Copper thin films are also used in the multilayer sandwiches of giant magneto resistive (GMR) hard disk read heads. The electrodeposited Cu can be deposited with small grain size, thus providing smooth films that can be used in void-free filling of high aspect ratio trenches.
Electrodeposited copper films have been extensively used due to their morphological characteristics, microstructure, and enhanced mechanical properties.
The composites, prepared by copper deposition on steel substrates, are materials of interest with high strength, conductivity, and excellent corrosion resistance. These composites are commonly used in the communication and electrical power industries as copper has good physical properties, in particular, the low bulk electrical resistivity and superior resistance to electromigration. Copper deposits have also found applications in nickel and chromium plating as an undercoat. Copper matrix composites have the potential for use as wear resistant and heat resistant materials; brush and torch nozzle materials and for applications in electrical sliding contacts such as those in homopolar machines and railway overhead current collector systems where high electrical/thermal conductivity and good wear resistant properties are needed. High thermal conductivity coupled with improved strength and wear resistance was also reported for Cu-based composites with refined microstructures developed as candidate materials for use in advanced rocket nozzle combustion liners.
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1.3 Motivation for the present study
The electrolytic deposition of a metallic matrix containing a dispersion of fine inert particles has been under continuous development since 1970 [Low 2006]. The increasing interest in electrolytic deposition is largely due to the flexibility of the process and the favoring economics of the electroplating of composites. The production of composite coatings by electrolytic codeposition of fine inert, semi conductive and conductive particles with metal from plating baths has occupied the attention of numerous investigators from all over the world [Stan 1996, Thie 2007]. The interest shown in these inclusion deposits was due to their various intrinsic engineering applications. Despite the potential interest in such deposits and many years of work in this field, only a few large- scale commercial baths have been developed so far for industrial production of such composites. The smaller inert particles are embedded into the deposition layer with greater difficulty because of the inert particle dispersion difficulties in the electrolyte.
The volume content of the nano-particles in the composite coating was very low in many researches under common operating conditions.
Dispersoid particles such as oxides, carbides, and borides, which are insoluble in the copper matrix and thermally stable at high temperature, are being increasingly used as the reinforcement phase. Cu-matrix composites are promising candidates for sliding contact applications, where high electrical/thermal conductivity and good wear resistance are needed. Many manufacturing processes have been used for producing such composites.
In general most metal matrix composites are produced by squeeze or stir casting, spray forming or by powder metallurgy techniques. In these methods the reinforcements are incorporated into the matrix by ex situ methods. The reinforcement particulates are usually coarse and rarely below 5 µm. They tend to agglomerate together leading to non- homogenous distribution and poor wettability of reinforcement oxides, which badly influences the mechanical and electrical properties of obtained composites. A few trials such as mechanical alloying or rapid solidification have been tried to overcome the agglomeration of reinforcements and obtain dispersed nanoparticles, but have often shown a contamination and poor economical efficiency. Secondary processes such as
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extrusion and other forming techniques were also used to overcome these drawbacks but they have often showed limited success.
Although pulsed current (PC) electrodeposition is one of the effective methods to produce nanocrystalline materials, the properties of such fabricated nanocrystalline electrodeposits are less investigated in the literature compared to those made by CVP or the condensation method. Some studies of the common composites including copper graphite and copper-MoS2 were reported [Nick 2009]. In the case of carbide and oxide reinforced copper matrix composites, several studies have been reported on SiC and Al2O3 reinforced copper matrix composites [Buel 1983, Gan 2005, Lekk 2009]. The literature on the use of other carbides and oxides for reinforced Cu-matrix composites is very limited. However, there are no reports available on the Cu/CeO2 composites on mechanical, microstructural and tribological behavior. There are no published data available on the influence of incorporated ceria particles and their amount in coatings on the quality of copper composites, i.e. on their mechanical, electrical, and corrosion performances.
In this dissertation, the endeavor is devoted to the fabrication of the dense nanocrystalline copper based nanocomposites using the pulsed co-electrodeposition technique. The different microstructures are realized by varying the concentration of CeO2 nanoparticles, in the electrolyte. The CeO2 nanoparticles are produced by combustion synthesis technique. The experimental characterizations using electron microscopy and other techniques are performed to characterize the microstructures at the nano-level. The present investigation is aimed at a study of the effect of nanocrystalline ceria on the microstructure, mechanical, wear and corrosion behavior of electrocodeposited copper/ceria composite.
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1.4 Objectives of the Thesis
The main objectives of the present investigation are
• To synthesize and characterize the nanosized CeO2 reinforcements using combustion synthesis process.
• Pulsed co-electrodeposition of Cu-CeO2 nanocomposite from the electrolytes consisting of different concentrations of CeO2.
• To evaluate the microstructure, mechanical and electrical resistivity properties of Cu/CeO2 nanocomposites.
• To evaluate the friction and wear behavior of Cu/CeO2 nanocomposites.
• To evaluate the corrosion behavior of Cu/CeO2 nanocomposites.
1.5 Contributions of the Thesis
The present work has made the following contributions.
(1) This study has confirmed that the nanosized ceria powder can be produced effectively by combustion synthesis technique.
(2) It has been shown that the copper-ceria nanocomposites can be synthesized using pulse electrodeposition method.
(3) The results of this study indicate that the incorporation of ceria in the copper matrix leads to higher microhardness of the composite.
(4) It has been demonstrated that the reinforcement of ceria leads to remarkable improvement in wear and corrosion resistance of the composite due to its grain refinement, particle distribution and increase in hardness.
(5) This study proposes the optimum content of ceria in the copper-ceria nanocomposite for the enhancement of various properties.
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1.6 Organization of the Thesis
The thesis has been organized in five chapters. The contents of each chapter are presented in brief.
Chapter 1 gives the brief description of the metal matrix composites, importance of copper based composites and motivation to pursue the present work. The objectives of the present investigation are also presented in this chapter.
Chapter 2 provides a thorough and critical review of the existing literature in the field of electrodeposition of MMNCs. The present study has been inspired by the achievements of previous investigations, and has addressed the codeposition of inert nanoparticles in the copper matrix.
Chapter 3 deals with the detailed description of experimental procedure to achieve the goals of this study. The steps involved in the synthesis of ceria powder, pulsed electrocodeposition of copper/ceria nanocomposites, microstructural characterization, wear and corrosion testing methods have been discussed in detail.
Chapter 4 presents the synthesis of nanosized ceria powder through aqueous combustion synthesis technique and also the synthesis of Cu-CeO2 nanocomposites through pulsed electrocodeposition method. It deals with the results of the microstructural characterization of the composites, evaluation of hardness and electrical resistivity of the deposits. The effect of the addition of ceria nanoparticles in the copper matrix on the wear and corrosion behavior of the Cu/CeO2 nanocomposites has been discussed.
Chapter 5 provides the conclusions drawn from the present work and also includes the future scope of the work.
The references cited within each chapter are indexed at the end.
