Fabrication And Properties Researches Of B4_C/LAS And LAS/B₄C Composites

In this paper, boron carbide reinforced lithium aluminosilicate(LAS) glass-ceramics(B4C/LAS) were prepared by sol-gel technique and vacuum hot-pressing method. The effects of B4 C content and pressing temperature on microstructure, mechanical properties and thermal properties were studied. The reaction between the B4 C and LAS glass was discussed in detail. In addition, using LAS as sintering additive during the sintering of boron carbide was also discussed. The main results of this study are as follows:LAS precursor gels were prepared by sol-gel method using B4 C powder, aluminium nitrate, lithium nitrate, silicat sol, and ammonia as start materials. Then, boron carbide reinforced lithium aluminosilicate(LAS) glass-ceramics(B4C/LAS) were fabricated by vacuum hot-pressing method. With increasing sintering temperature and the content of B4 C, the relative density of glass ceramics increases due to decreasing of glass viscosity. β-spodumene and Li2 Si O3 phase were detected by XRD in LAS glass-ceramics without B4 C addition. Doping B4 C hindered the transformation of Li2 Si O3, and benefited the formation of β-spodumene crystals. When the content of B4 C was increased to 10 wt.%, the maximum content of β-spodumene was reached with an average size of 2~2.5μm.Based on the standpoint of thermodynamics and kinetics, the solid-state reactions between boron carbide and LAS glass matrix were analyzed. It was found that B4 C reacted with LAS glass to form B2O3 and Li C6, and the content of B4 C would affect the degree of these reactions. When the content of B4 C was increased to 5wt.%, almost B4 C transformed into B2O3 glass phase. As the content of B4 C was increased to 10 wt.%, there were enough B4 C to react with glass matrix and to remain appropriate content of B4 C. And the microstructure that the Li C6 crystal layer surrounded a single crystalline boron carbide was observed. With increasing B4 C to 15 wt.%, so much glass matrix were consumed that the content of β-spodumene will decreased slightly. Meanwhile, a microstructure that B4 C crystal were surrounded by glass phase was observed.The effects of content of boron carbide and sintering temperature on the microstructure, interface structure, mechanical and thermal properties were discussed. B2O3 produced by reaction was favorable to the densification of B4C/LAS glass-ceramics composites due to its very low viscosity at high temperature. Furthermore, the liquid B2O3 glass phase can serve as diffusion paths, which was beneficial to the formation of β-spodumene. It was observed that the interface between glass matrix and B4 C played an important role in mechanical properties. The interface structure that boron carbide was surrounded by Li C6 crystal can improve the mechanical properties by deflecting cracks due to the lamellar structure of Li C6. Its maximum flexural strength and fracture toughness was 160.58±18.86 MPa and 2.7±0.27MPa?m1/2 respectively. Thermal conductivity increased with the increasing of pressing temperature. The LAS glass-ceramics hot pressed at 1100°C with 5wt.% B4 C demonstrated very low thermal conductivity, where the value was in the range of 0.52~0.57 W/(m?K). The content of B4 C has significant effects on the CTE of LAS glass-ceramics, and the value of coefficient of thermal expansion of LAS glass-ceramic increased with the increasing of content of B4 C.LAS/B4 C ceramics were prepared by sol-gel and hot-pressing method. During hot-pressing, the reaction between LAS and B4 C to form Si C occurred. When the content of LAS was increased to 30 wt.%, boron carbide sintered at 1500°C has a maximum flexural strength of 77.3±11.5MPa and fracture toughness of 2.5±0.25 MPa.m1/2.