Fabrication And Characteristics Of TiB₂-BN-SiC Composite Ceramics

TiB2-BN (TB) composite ceramics are of high-conductivity, heat-shock resistance, machinability, corrosion resistance, etc. and have been applied widely as the high-temperature structural ceramics, conductive evaporation boat and the high-temperature electrode materials. But how to further promote the compositive property of TB composite ceramics is the key point in current research work. In this work, the TB composites were fabricated by hot pressing sintering method using different types of TiB2 and BN powders. The effect of raw powder characteristics to densification, microstructure and properties of TB composites were studied. On this basis, TiB2-BN-SiC (TBS) composite ceramics also were prepared by the same method. The influence of phase composition and sintering processes on the microstructure and properties of TBS composites were studied.Characteristics of TiB2 and BN raw powders have significant effect on the densification, microstructure and properties of TB composites prepared by hot pressing. The experimental results are as followed:(1) The particle size and grain morphology of TiB2 powders have major impact on the structure and properties of TB composite ceramics. The electric conductivity of samples increases with the decrease of grain size. When the size distributions are similar, TiB2 powders with much smoother surface have greater densification rate, so as to obtain higher density, mechanical propertied and thermal conductibity. After hot-pressed at 1850℃for 1.5 h, the open porosity, bending strength, elastic modulous, thermal conductivity (637 K) of TB composites are 11.95%,95.28 MPa,47.16 GPa and 33.8 W/m respectively, which are all much higher than that of the samples prepared with the short hexagonal columnar TiB2 powders. The analysis of microstructure of the materials indicate that TiB2 grains with smaller size is very favorable for forming the conductive pathways and improving the conductivity; However, TiB2 grains with larger size are likely to exist as a large isolated island-like. TiB2 grains with short hexagonal are easily to be partially aggregated and off-contact, forming pores and reducing the density, as well as mechanical properties and thermal conductivity of the samples.(2) The specific surface area (BET) and oxygen-impurity content of BN powders have major inpact on the structure and properties of TB composite ceramics. When the oxygen content is similar, as the densification rate rise up, the density and mechanical properties of the sample get enhanced as the increase of the BET. Analysis of microstructure of the materials indicates that BN powders with the high specific surface area and lamellar-structure have greater sintering activity, easily forming large lamellar-structure in the direction perpendicular to the pressure. After hot-pressed at 1850℃for 1.5 h, bending strength, elastic modulous, thermal conductivity (637 K) and mean thermal expansion coefficient (298-1373 K) of TB composites are 219.71 MPa,105.36 GPa,13.8 W/m.K and 3.84x10-6/K, respectively.BN powders with 4.92 wt.% in oxygen-impurity can significantly raise the density rate, improve thermal conductivity, and decrease the sintering temperature. Under the same process, the sample can obtain the highest density when the open porosity is only 1.16%.The analysis of microstructure of the materials shows that BN powders with 4.92 wt.%in oxygen content transform into shape of large flakes from the floc-structure, uniformly distributing around TiB2 grains, and there are oxygen impurities exist in the grain boundary between TiB2 and BN.The phase composition of TiB2, BN and SiC and sintering processes on the microstructure and properties of TBS composites prepared by hot pressing have a significant effect. The experimental results are as followed:(1) The TBS samples mechanics, electrical conductivity and thermal resistance decreased as the of BN volume content increase. When the volume ratio of TiB2 and SiC was 6:1, meanwhile volume fraction of BN was 72-75%, the resistance mutation and the materials is converted from the conductor to semiconductor, due to percolation threshold between conductive phase and insulative phase in the TB system. The content of SiC has no significant influence on the density and mechanical properties of samples, however, the electric conductivity, thermal conductivity and thermal expansion coefficient decreased with the increase of SiC.(2) The density, thermal conductivity and thermal expansion coefficient of TBS composite ceramics get enforced with the increase of sintering temperature and length of sintering time, while the electrical conductivity and mechanical properties decrease. The microstructure analysis indicate that the high sintering temperature and long sintering time will make the grain growth abnormal, grain growth of BN into sheet structure, which would increase the conductivity of the barrier. Meanwhile, larger grains of TiB2 and SiC make the proportion of grain boundaries decrease, the initial crack size shorter and decreasing critical fracture stress reduce. When the volume ratio between TiB2and SiC was 6:1 and the volume fraction of BN was 72%, after hot-pressed at 1750℃for 1 h, the conductivity and mechanical properties of the TBS sample are the best. The resistivity, bending strength and elastic modulus were 24 mΩ·cm,235.07 MPa and 96.47 GPa, respectively.