Sol-enhanced Electroless Plating of Ni-P-TiO2 Composite Coatings

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Chapter 3 Experimental Design and Procedures

Introduction

Electroless/electro- plating has been a commercially important and versatile surface coating/finishing technique [168-169]. Metal/alloy-based coatings have been widely deposited on the surface of working parts to improve their corrosion and wear resistance, or to modify the magnetic and other physical properties [3, 169-170]. In order to achieve better properties, the deposited metals/alloys were modified by codepositing second phase particles in the matrix, which are called composite coatings. The traditional composite coating method uses a solid particle mixing process: The second-phase particles are mixed and suspended into the plating solution, and then the solid particles and the metal ions co-deposit onto the specimens/parts to form composite coatings [3]. The metal/alloy-based composite coatings were early fabricated from suspensions of relatively large-sized (typically micrometre level) particles of carbides [33], oxides [34], diamond [35],and Teflon [36-37]. More recently, there has been an increasing emphasis on co-depositing Ni ions and superfine or nano-sized particles to synthesize a new structure – nano-composite coatings. The superfine/nano-particles are dispersed into the Ni matrix, providing significantly improved properties, such as hardness and wear resistance [4-8]. The strengthening mechanisms for nano-composite coatings can be interpreted based on the dislocation model such as the Orowan theory [9]. In this theory, the critical condition for a dislocation to bypass the particles in its glide plane is to bend the dislocation to a semicircular between the particles. The dislocation with its dipoles annihilated can move forward while dislocation loops are left behind surrounding each particle [9-11]. The Orowan’s criterion indicates that the mechanical properties of composite coatings increase with decreasing mean planar inter-particle spacing and particle size.Based on the above theory, incorporation of second-phase superfine/nano-particles can be much more effective than micro-sized particles in reinforcing the composite coatings. Theoretically, if the second-phase nano-particles are highly dispersed in composite materials, the strong interaction between dislocations and the nano-particles can block the movements of dislocations, leading to a significant improvement of mechanical strength [171].In order to achieve good dispersion of the nano-particles, powder suspension has to be physically maintained in a solution by either vigorous agitation, air injection, ultrasonic vibration or adding surfactants. However, it is always difficult for nano-particles to achieve good suspension because they have very large surface areas. The high surface energy tends to cause agglomeration of nano-particles in composite coatings. Therefore, it has been a challenge to prepare highly dispersive nano-particles reinforced composites or composite coatings.Sol-gel process has been widely applied to prepare uniform nano-sized particles [134-139].Typically the hydrolysis and condensation reactions take place in the sol-gel process to form metal oxides or their composite nano-particles [142-143]. The sol-gel process can be regarded as an in-situ liquid phase synthesis of uniform nano-particles at room temperature, providing a precursor to introduce nano-particles into the composite electrodeposition. In the electroplating process, the metal ions discharge on the surface of the cathode through migration and diffusion in the electrolyte [3], which can provide an opportunity for the in-situ formed nano-particles to integrate immediately into the alloy deposit. In the present study we presented a novel sol-enhanced composite plating technique by adding a transparent TiO2 sol into the traditional electroless/electro- plating solution [18]. This method led to highly dispersive distribution of TiO2 nano-particles in the coating matrix, avoiding the agglomeration of TiO2 nano-particles. The mechanical properties were therefore significantly improved.

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Table of Contents
Abstract
Acknowledgements
Table of Contents
Chapter 1 Introduction
1.1 Background
1.2 Objectives
1.3 Thesis outline
Chapter 2 Literature Review
2.1 Introduction
2.1.1 Electroless plating
2.1.2 Electroplating
2.2 Deposition of composite coatings
2.2.1 Electroless composite plating
2.2.2 Composite electroplating
2.3 Strengthening mechanisms in composite coatings
2.3.1 Grain refinement strengthening
2.3.2 Dispersion strengthening
2.4 Synthesis of nano-particles by sol-gel processes
2.4.1 Preparation and reactions of sol solutions
2.4.2 Microstructures of nano-particles
2.5 Problems and challenges in composite plating
2.6 Summary
Chapter 3 Experimental Design and Procedures
3.1 Introduction
3.2 Experimental design
3.2.1 The principle of the design
3.2.2 The system design
3.3 Experimental procedures
3.3.1 Preparation of composite coatings
3.3.2 Characterization of microstructures
3.3.3 Mechanical properties test
3.3.4 Tensile strength measurement
3.3.5 Characterization of the electrochemical behaviours
3.4 Summary
Chapter 4 Sol-enhanced Electroless Plating of Ni-P-TiO2 Composite Coatings
4.1 Introduction
4.2 Experimental
4.3 Results
4.3.1 Surface and cross-sectional morphologies of coatings
4.3.2 Morphologies and distributions of TiO2 nano-particles in the composite coatings
4.3.3 Phase structures of coatings
4.3.4 Mechanical properties of coatings
4.4 Discussion
4.4.1 Formation mechanisms for the sol-enhanced Ni-P-TiO2 composite coatings
4.4.2 Reinforcement mechanisms for the sol-enhanced Ni-P-TiO2 composite coatings.
4.5 Summary
Chapter 5 Sol-enhanced Electroplating of Ni-TiO2 Composite Coatings: Effects of the Sol Concentration
5.1 Introduction.
5.2 Experimental
5.2.1 Preparation of coatings
5.2.2 Characterization of coatings
5.2.3 Electrochemical analysis
5.3 Results
5.3.1 Surface and cross-sectional morphologies
5.3.2 Microstructure characterization by TEM
5.3.3 The content of TiO2 particles in composite coatings
5.3.4 Phase structures
5.3.5 Mechanical properties
5.3.6 Electrochemical characterization
5.4 Discussion
5.4.1 Studies on the mechanism of the sol-enhanced composite plating
5.4.2 Effect of the sol concentration
5.5 Summary.
Chapter 6 Sol-enhanced Electroplating of Ni-TiO2 Composite Coatings: Studies on the Electrochemical Process
6.1 Introduction
6.2 Experimental
6.2.1 Electrodeposition of coatings
6.2.2 Characterization of coatings
6.3 Results
6.3.1 Microstructure characterization of coatings
6.3.2 Phase structures
6.3.3 Mechanical properties
6.4 Discussion
6.4.1 Grain growth during the sol-enhanced electroplating process
6.4.2 The electrochemical process in the sol-enhanced composite plating
6.5 Summary
Chapter 7 Sol-enhanced Electroplating of Ni-TiO2 Composite Coatings: Thermal Stability and Tensile Properties
7.1 Introduction
7.2 Experimental
7.3 Results
7.3.1 Surface and cross-sectional morphologies
7.3.2 Microstructures of the composites
7.3.3 Phase structure
7.3.4 Grain size
7.3.5 Microhardness
7.3.6 Tensile test
7.4 Discussion
7.4.1 Grain growth
7.4.2 Fracture mechanisms
7.5 Summary
Chapter 8 Conclusions and Future Work
8.1 Conclusions.
8.2 Future work
References