放电等离子烧结B4C基陶瓷的致密化、微观结构和机械性能

摘要

Boron carbide (B4C) ceramic has exhibited a unique set of properties such as high melting point (2450℃), low density (2.52 g/cm3, high hardness (26-32 GPa, after diamond and cubic nitride), high elastic modulus (450 GPa), and high temperature semiconductor. B4C is one of the hardest materials and has been used as light weight armors, grinding and cutting tools, abrasive materials, blasting nozzles, neutron radiation absorbent in nuclear reactor and as a wear resistance components. Due to his high hardness (26-32 GPa) and low density of 2.52 g/cm3 enable it very outstanding for armor applications. All of these applications of boron carbide are restricted because the densification of pure B4C to high density has proved very complicated. It is difficult to densify B4C to high density due to the presence of strong covalent bonding, high resistance to grain boundary sliding and low plasticity and low superficial tension in the solid state. Furthermore, the presence of oxide layer (B2O3) coating on the B4C powder surface slows down the consolidation process via evaporation and condensation reactions. Therefore, the sintering of B4C by conventional sintering (hot-pressing, pressureless sintering, etc.) methods is, very high sintering temperature around 2000-2300℃ is required with and without any sintering additives which leads to rapid grain coarsening of B4C grains and is still an expensive approach for its consolidation. The motivation of the work demonstrated in this dissertation was to employ the different sintering additives to B4C with an objective to obtain dense compact at relatively lower sintering temperature by applying the novel sintering technique of spark plasma sintering. In past, much attention has been given to spark plasma sintering (SPS) for consolidation of poorly sinterable substances such as carbides, nitrides and borides at lower sintering temperatures. SPS technique is highly capable of generating highly dense compacts with cleaner grain boundaries and smaller grain size of ceramics. In the first part of this dissertation, B4C based ceramics were fabricated with different Fe3Al additions as sintering aids through spark plasma sintering (SPS) technique at low temperature of 1700 ℃ under vacuum with applied pressure of 50 MPa and held at 1700 ℃ for 5 min. The effect of iron aluminide (Fe3Al) additions on the densification, sinterability, microstructure and mechanical properties of B4C based ceramics has been investigated. The mixtures of B4C and Fe3Al underwent a chemical reaction which resulted in metal borides (FeB, AlB10) and B4C were considered as chief crystallographic phases. The specimen with 7 wt％ of Fe3Al addition to boron carbide had 32.46 GPa Vickers hardness, 483.40 MPa flexural strength, and 4.1 MPa.m1/2 fracture toughness. In the second part of this dissertation, B4C powders were sintered without any sintering additives using the technique of spark plasma sintering at 1700℃ within a short consolidation time of 3, 5, 7, and 9 min with applied pressure of 80 MPa .This work was aimed to obtain dense pure B4C compact at lower temperature for nuclear application, where high purity boron carbide is needed. The fast heating rate (200℃/min) and high pressure (80 MPa) can hold back the grain coarsening process of boron carbide grains and thus enhancement in densification and mechanical properties were observed. No grain boundary films were observed by TEM and HRTEM investigation, suggesting that boron carbide powders can self-bond without the assistance of additives. In the third part of this dissertation, Dense B4C compacts were fabricated by spark plasma sintering (SPS) technique in the presence of Si as a sintering additive. The sinterability improved by adding small amount of Si due to the formation of liquid Si during sintering and then molten Si reacted with free carbon supplied by B4C. The addition of Si as a sintering aid to the B4C was found to alter the fracture mode from purely transgranular to a mixture of transgranular and intergranular fracture. In the last part of this dissertation, B4C/TiB2 composites were fabricated from raw mixtures of B4C and TiH2 by spark plasma sintering. X-ray diffraction analysis demonstrated that a chemical reaction took place between B4C and TiH2 which resulted in B4C/TiB2 composites. TEM investigation revealed the presence of amorphous carbon at the grain boundaries of the B4C/TiB2 composites. The in situ reaction between B4C and TiH2 produced elemental carbon and TiB2, both of them aided the sintering process. The effect of TiH2 addition on the microstructure and mechanical properties of B4C matrix was studied. Microstructrual coarsening was restrained by the incorporation ofTiB2 particles and its mechanical properties were consequently enhanced.

目录

封面

声明

英文摘要

目录

Chapter 1 Introduction

1.1 Introduction

1.2 Crystal Structure of B4C

1.3 Preparation \(synthesis\) of B4C Powders

1.4 Main Industrial Applications of B4C

1. 5 Facing problem for Densification of B4C

1.6 Scope and study of the present research work

Chapter 2 Sintering methods for Boron Carbide

2. 1 Sintering

2. 2 Sintering Techniques for Consolidation of B4C

Chapter 3 Microstructure and Mechanical Properties of B4C based ceramics with Fe3Al as Sintering Aid by Spark Plasma Sintering

3.1 Introduction

3.2 Experimental Procedures

3.3 Results and discussion

3.4 Conclusions

Chapter 4 Densification behavior, Microstructure and Mechanical properties of Spark Plasma sintered B4C without any sintering additives

4.1 Introduction

4.2 Experimental Procedures

4.3 Results and discussion

4.4 Conclusions

Chapter 5 B4C/SiC composite, formed from B4C and Si by spark plasma sintering.

5.1 Introduction

5.2 Experimental procedures

5.3 Results and discussion

5.4 Conclusions

Chapter 6 B4C/TiB2 composite prepared from the raw mixtures of B4C and TiH2by spark plasma sintering

6.1 Introduction

6.2 Experimental procedures

6.3 Results and discussion

6.4. Conclusions

Chapter 7 Summary and Recommendations for Future Research

7.1 Summary

7.2 Recommendations for Future research work

参考文献

致谢

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