탄화규소의 고온염소반응으로부터 생성된 탄소막의 제조 및 특성평가 = Fabrication and Characteristics of Carbon Films Derived from High Temperature Cl2 Gas Reaction with Silicon Carbide

다국어 초록 (Multilingual Abstract)

Carbon is one of the most promising materials for nanotechnology applications. Various arrangements of carbon atoms result in amazingly different physical and chemical properties, offering a great flexibility in the design of new carbon-based materials. Silicon carbide-derived carbon is a new form of carbon which can be produced by etching silicon carbide in mixed gas environment containing chlorine and hydrogen at elevated temperatures much lower than those used for sintering or graphitization. Thus, a number of articles concerning carbide-derived carbon (CDC) have been published so far. This dissertation is divided in three parts: sliding wear of silicon carbide modified by etching with chlorine at various temperatures, effect of hydrogen on the mechanical and structural properties of SiC-derived carbon films, and tribology of carbon layers fabricated from SiC exposed to different H2/Cl2 gas mixtures.
To begin with, the effect of reaction temperature on the formation of a carbon layer on the surface of SiC has been investigated. Subsequently, the tribological properties of the formed carbon layers were studied. The experimental procedure involved exposing reaction-bonded SiC balls to a flowing gas mixture of 5% Cl2, 2.5% H2, and Ar at a high temperature of 800 oC, 1000 oC, or 1200 oC. A ball-on disk tribometer was used to investigate the friction and wear behavior of the treated specimens. While partially unreacted SiC phases were observed in the layer modified at 800 oC, rhombohedral graphite crystals were formed in the layer modified at 1200 oC. Compared to untreated SiC, the treated SiC materials were found to have relatively low friction coefficients and better wear resistance. Increasing the treatment temperature was found to improve the tribological performance of the resulting surface-modified SiC balls. A possible reason for this tribological improvement has been discussed based on the observed carbon phases.
In the second part of this dissertation, sintered SiC materials were treated with various compositions of chlorine-hydrogen gas mixtures at a temperature of 1000 oC for 20 hrs. Then, it focused on how the presence of hydrogen has key functions for structural and mechanical properties of modified carbon films on silicon carbide. Based on the structural analysis of carbon films produced, conversion from silicon carbide proceeded faster with increasing chlorine content at a high temperature, and besides, the chlorination reaction rate decreased with increasing hydrogen content of mixtures, as determined by XRD and SEM. The correlation between the degree of crystallinity and mechanical properties of carbon films produced depended obviously on the presence of hydrogen in terms of the kinetics of chlorination reaction. The action of chlorine on carbides led to a pore formation due to the physical transformations in the structure, not chemical transformation. Based on nanoindentation measurements, the hardness and elastic modulus of carbon films transformed by high-temperature chlorination of silicon carbide with varying hydrogen content of mixtures were determined. The hardness and elastic modulus of the carbon films treated with only Cl2 and Cl2-H2 of gas mixtures were related to the surface and structural properties as well as the film thickness of modified carbon films. Besides, an addition of hydrogen to a gas mixture led to an increase in plasticity (56.71 % in indentation deformation) of modified carbon films.
Silicon carbide-based ceramics are some of the best materials for tribological applications. However, their tribological properties still need to be improved for certain uses under severe conditions. In the last part of this dissertation, carbon layers were produced by the exposure of ball and disc type SiCs to various compositions of chlorine-hydrogen gas mixtures at a temperature of 1000 oC for 20 hrs. After the chlorination of the silicon carbide materials, the modified layers were characterized by XRD, Raman, and FE-SEM. The effect of the hydrogen gas content on the carbide-derived carbon (CDC) layers and their resulting tribological properties have been investigated. The tribological behaviors of the CDC layers were studied using a ball-on-disk tribometer. Silicon nitride and chlorinated silicon carbide balls were selected as the counterpart material. The results showed that the wear resistance and frictional coefficients of the surface-modified ball and disk-type SiCs were significantly improved compared to those of untreated silicon carbide specimens. Increasing the hydrogen content of the gas mixture improved the tribological performance of the resulting carbon layers. The use of a higher applied load also improved the tribological performance. The possible mechanisms responsible for the tribological properties of the carbon layers have been discussed.

국문 초록 (Abstract)

탄화규소의 고온염소반응으로부터 생성된
탄소막의 제조 및 특성평가

CVD, PVD 등과 같은 증착방법을 이용하여 제조된 세라믹이나 금속모재의 탄소막 코팅은 마찰특성을 향상시켜 재료의 내구성을 높여줌으로써 tribology 응용분야에 효과적으로 이용되고 있다. 그러나 이와 같은 코팅 방법은 코팅과 모재간의 접착력, 잔류응력의 문제를 갖고 있기 때문에 그 응용이 제한되어 왔다. 본 연구에 적용된 CDC (Carbide-Derived Carbon) 코팅방법은 실질적으로 탄화규소의 표면층에서부터 모든 깊이까지 고온 염소 처리함으로써 탄소로 상전이 되어 탄소층을 만드는 방법을 말한다. 그래서 요구되는 탄소막을 위해 실리콘 등 금속원자를 선택적으로 에칭시키는 본 방법을 이용하면, 생성 박막의 두께 및 표면조도를 자유롭게 조절할 수가 있게 된다. 그러므로 본 연구는 탄화규소볼을 이용하여 고온 염소처리시켜 탄소상으로 개질된 표면층을 얻었다. 이때 800℃-1200℃ 등의 온도 변화에 따른 탄소의 결정상태변화를 관찰하였고 이러한 변화가 tribology 특성에 미치는 영향을 평가하였다. 또한 반응가스로 사용된 염소와 수소 가스비를 달리하여 탄소층을 얻었으며 기계적 물성 및 마찰마모 특성을 평가하여 비교분석하였다. 이때, XRD, HRTEM, Raman 분석을 통해 생성된 탄소막이 나노결정질 및 비정질 흑연상임을 확인하였다. 나노인덴테이션법으로 생성 탄소막의 기계적 물성을 측정하였으며, 개질된 박막의 tribology 특성을 관찰하기 위하여 ball-on-disk 방식으로 마모실험을 실시하였다. 이렇게 고온에서 염소처리법으로 얻어진 박막의 tribology 특성평가 결과로는 표면개질 전과 비교해 볼 때 탄소로 표면개질된 탄화규소의 마모-마찰 특성이 현저히 향상되는 경향을 보여, 온도 및 가스조성비의 변화에 따른 탄소층의 결정상태와 그에 따른 tribology 특성과의 상관관계에 대해 비교 분석하였다.

목차 (Table of Contents)

• ABSTRACT
• LIST OF TABLES
• LIST OF FIGURES
• LIST OF ABBREVIATIONS
• 1. INTRODUCTION 1
• 2. REVIEW OF LITERATURE 5
• 2.1. Silicon Carbide 5
• 2.1.1. Historic Overview of Silicon Carbide 5
• 2.1.2. Crystal Structure and Properties 8
• 2.2. The Carbon Polymorphs 18
• 2.2.1. The Element Carbon 18
• 2.2.2. Diamond 24
• 2.2.3. Graphite and Graphene 30
• 2.2.4. Diamond-Like Carbon and Amorphous Carbon 36
• 2.2.5. Closed-Shell Carbon Structures 41
• 2.3. Carbon Coating Methods 46
• 2.3.1. Existing Techniques for Carbon Coating 46
• 2.3.2. Disadvantages of Current Processes 48
• 2.4. Selective Etching of Carbides by Halogens 50
• 2.4.1. Historic Overview of CDC Studies 50
• 2.4.2. Thermodynamic Simulations of Carbides 60
• 2.4.3. Nanoporous Structure and Absorption Properties 68
• 2.4.4. Physical Properties of CDC Coatings 71
• 2.5. Tribology 76
• 2.5.1. Introduction 76
• 2.5.2. Theory of Friction 76
• 2.5.3. Friction and Wear of Carbon Materials 78
• 2.5.4. Friction and Wear of Silicon Carbon 81
• 2.5.5. Friction and Wear of CDC Coatings 82
• 2.6. Objectives of the Study 85
• 3. MATERIALS AND METHODS 88
• 3.1. Experimental Set-Up for Chlorination 88
• 3.1.1. 50 ID Furnace Using Digital Mass-Flow-Controllers 88
• 3.1.2. 76 ID Furnace with a Gas Distribution System 91
• 3.2. Characterizations of SiC-Derived Carbon Films 94
• 3.2.1. Structural and Surface Morphology Analysis 94
• 3.2.1.1. X-ray Diffraction (XRD) 94
• 3.2.1.2. Micro-Raman Spectroscopy 94
• 3.2.1.3. Surface Roughness: AFM and Alpha Step 103
• 3.2.1.4. BET Surface Area Analysis 103
• 3.2.2. Microstructural Analysis of SiC-Derived Carbon Films 103
• 3.2.2.1. Field Emission Scanning Electron Microscope 103
• 3.2.2.2. High-Resolution Transmission Electron Microscope 104
• 3.2.3. Physical Properties of SiC-Derived Carbon Films 110
• 3.2.3.1. Load-Displacement Curves in Nanoindentation Tests 110
• 3.2.4. Tribological Properties of SiC-Derived Carbon Films 112
• 3.2.4.1. Frictional Coefficient and Wear Resistance 112
• 4. RESULTS AND DISCUSSIONS 114
• 4.1. Carbon Films Synthesized at Various Temperatures 114
• 4.1.1. Identification of Modified Carbon Films on SiC Balls 114
• 4.1.1.1. Reactants Effect: Pure Cl2 and Cl2/H2 Blend 114
• 4.1.2. Microstructure of Modified Carbon Films Transformed 122
• 4.1.3. Tribological Properties of Modified Carbon Films 125
• 4.1.3.1. Wear Properties 125
• 4.1.3.2. Frictional Properties 125
• 4.1.3.3. Worn Surfaces after Testing 133
• 4.1.4. Summary 134
• 4.2. Carbon Films with Different Hydrogen Contents 135
• 4.2.1. Analysis of SiC-Derived Carbon Films from SiC Powders 135
• 4.2.1.1. Raman Scattering Studies 135
• 4.2.1.2. HR-TEM Studies of Growth Mechanism 137
• 4.2.2. Analysis of Carbon Films on SiC Disks 140
• 4.2.2.1. Effect of Hydrogen on Structure of Carbon Films 140
• 4.2.2.2. Effect of Hydrogen on Surface Roughness 144
• 4.2.2.3. Effect of Hydrogen on Pore Formation 147
• 4.2.3. Mechanical Properties of Carbon Films 149
• 4.2.3.1. Load-Displacement Curves 149
• 4.2.3.2. Plasticity 151
• 4.2.3.3. Hardness and Elastic Modulus 153
• 4.2.4. Tribological Properties of SiC-Derived Carbon Films 160
• 4.2.4.1. Tribology: wear and friction 160
• 4.2.4.2. Worn Surfaces after Testing 163
• 4.2.5. Summary 165
• 5. POTENTIAL APPLICATIONS 167
• 6. CONCLUSIONS 168
• 7. REFERENCES 169