Nanostructured WC?Co Powders and Bulks via Integrated Mechanical/Thermal Activation and Liquid Phase Sintering

다국어 초록 (Multilingual Abstract)

WC‒Co composites have been widely used as a cutting tools, saw blade, dies and valves in industrial fields where high hardness, toughness and wear resistant properties are required. Due to the strategic importance of the resources for WC‒Co composites, substantial research efforts have been directed towards the synthesis of WC nanopowder and nanostructured WC‒Co composites in order to enhance further their mechanical properties and lifetimes. Nonetheless, the specification of mass production type of WC‒Co composites is now limited to the average particle size of > 200 nm. In this thesis, we suggest a scalable production pathway for an extremely fine WC nanopowder (10.54 nm) and a genuine nanostructured WC‒Co composites (20~30 nm).
First, we systematically studied the mechanism for the reactions and growth of reactants during carbothermal reduction with the experimental and theoretical approaches. The results reveal that the finer the intermediates is, the faster the reactions, the lower the reaction temperature becomes. All the processes during carbothermal reduction are governed by the diffusion of constituent elements. Based on these findings, we successfully synthesized an extremely fine WC nanopowder with an average size of 10.54 nm with a standard deviation of 2.1 nm.
Second, we investigated the mechanism for the grain growth inhibition during liquid phase sintering, and suggest the possibilities for the production of nanostructured WC‒Co composites with an average grain size of 30 nm. By the addition of VC, an extremely unstable nanostructure of WC‒Co is found to have a significantly high thermal stability and an unusual microstructure. The nanostructure with carbide size less than 30 nm was retained for a long time (1h) up to 1300 ºC. The growth of WC in the system is limited by two-dimensional nucleation of WC on (V,W)C layer and consequent diffusion of W through the layer. The high thermodynamic stability of the nanostructure assures the realizable possibility of the mass production of a genuine nanostructured WC‒Co composites.

목차 (Table of Contents)

• PartⅠ 1
• I. Introduction 2
• 1.1 Synthesis of WC nanopowder 2
• 1.2 Purpose of the work 5
• II. Experimental procedure 6
• 2.1 Materials and methods 6
• 2.2 Characterization 7
• III. Result and discussion 8
• 3.1 Reactions of WO3−C mixture during carbothermal reduction 8
• 3.2 Grain growth of intermediates during carbothermal reduction 25
• 3.3 Synthesis of Nanopowders via grain growth controlled method 35
• IV. Conclusion 38
• PartⅡ 39
• I. Introduction 40
• 1.1 Mechanism of grain growth inhibition of WC during Liquid Phase Sintering 40
• 1.2 Synthesis of nano WC-Co composites via liquid phase sintering 43
• 1.3 Purpose of the work 46
• II. Experimental procedure 47
• 2.1 Materials and methods 47
• 2.2 Characterization 49
• III. Result and discussion 50
• 3.1 WC growth in undoped WC–Co 50
• 3.2 Particle growth of VC-doped WC–Co 55
• 3.3 Interface characterization: (V,W)C cubic layer formation 57
• 3.4 Formation of Twins and Ledges 61
• 3.5 Effect of Step Free Energy 65
• 3.6 Effect of Solubility and Diffusivity of W 67
• 3.7 Effect of Interfacial Energy 68
• 3.8 Modeling of particle growth of VC-doped WC‒Co system 69
• 3.9 Synthesis of nano WC-Co via liquid phase sintering 74
• IV. Conclusion 78
• Overall Conclusion 79
• Reference 81
• Abstract (Korean) 89
• Further Works 91