放電等離子燒結B4C基陶瓷的致密化、微觀結構和機械性能.pdf

1、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)

2、, 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

3、 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 restricte

4、d 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 slidin

5、g 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

6、 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

7、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 a

8、dditives 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 pla

9、sma 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 clean

10、er 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) tec

11、hnique 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 mechan

12、ical 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 phas

13、es. 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

14、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

15、 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 c

16、arbide 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 witho

17、ut 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 improv

18、ed 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 frac

19、ture 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 s

20、intering. 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 b

21、oundaries 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 mechanic