TiB₂-based Ceramic Composites Fabricated Using Glassy Powder As Sintering Aid By SPS

Ti-Al intermetallics, especially the Ti₃Al（α2） and TiAl（γ）, possess good compatibility with ceramics because of their semi-ceramic characteristics. They are playing significant roles in the fields of aerospace industry and armored materials due to their high-temperature performance, oxidation resistance, good rigidity, and low density. Advanced ceramic materials were widely used because of their excellent properties like high strength, high hardness,resistance to high temperature, low linear expansion coefficient and low density, etc. It may become a feasible method to improve the sintering characteristics and mechanical properties of TiB₂ ceramics through combining the new sintering process, spark plasma sintering（SPS）,with adding metallic sintering aids.Firstly, Ti₃Al matrix alloy Ti_60.8Al₂₅Nb₁₀V₃Mo₁B_0.2（Ti₃Al-m, Ti₃Al-m-A after annealing）and TiAl matrix alloy Ti_48.1Al_46.5Nb₃Cr₂W_0.2B_0.2（TiAl-m, Ti₃Al-m-A after annealing） were chosen as the research objects. The impacts of the sintering temperature on microstructure,phase composition and mechanical properties of the Ti-Al intermetallics after mechanical alloying and SPS were studied, in order to provide a basis for the subsequent preparation of bulk TiB₂-based ceramics composites. After adding the ball-milled sintering aids（3 wt.% and10 wt. %） and the annealed counterparts powders（10 wt.%） into the TiB₂ powders, SPS was used to prepare the TiB₂-based ceramics composites at various sintering temperatures. The effects of Ti-Al intermetallics sintering aids on the densification, microstructure, phase composition and mechanical properties of the TiB₂ composites powders were explored.The 70h-milled Ti₃Al-m alloy powders consists of glassy matrix and nanocrystalline Ti₃ Al, and transformed into Ti₂ AlNb and Ti₃ Al phases after SPS. The SPS-induced phases were equiaxed below 1200℃, while transforming into lamellar structure sintered at 1200℃.The TiAl-m bulk alloys are composed of Ti₃ Al and TiAl phases with a lamellar structure, and the spacing increases with the increasing sintering temperature. The compressive fracture strength and fracture strain increases with the sintering temperature, and reached the highest values at 1200℃, 2414 MPa and 24.8% for the SPSed Ti₃Al-m bulk alloys, 2155 MPa and22.1% for The SPSed TiAl-m bulk alloys, respectively. However, the yield strength and microhardness decreases with the sintering temperature. The existence of micro-cracks and holes on the surfaces of the fractured bulk alloys prepared below 1200℃ is observed and the dimples’ characteristics is most obvious for the prepared bulk alloy at 1200℃.For the two groups of samples with 3 wt.% sintering aids, only TiB₂ phase can bedetected in XRD patterns, while the diffraction peaks of Ti（C, N, O） are obvious in the bulk alloys with 10 wt.% addition. Moreover, Ti₃ Al is detected in the XRD patterns of the TiB₂-10 wt.%（TiAl-m） added bulk alloys sintered under 1500℃, but decomposed with the increasing sintering temperature. An upward trend of the relative density and hardness of the TiB₂-based ceramics composites（3 wt.%Ti₃Al-m, 10 wt.%Ti₃Al-m, 3 wt.%TiAl-m and 10 wt.%TiAl-m）were observed with the increase sintering temperature. The highest relative density is 97.58%,99.96%, 98.26%, and 97.37% for the four kinds of bulk samples, respectively. In addition to TiB₂-10 wt.%（TiAl-m）, the fracture strength and fracture strain of other TiB₂-based ceramics composites increases to the maximum at 1600 ℃, and began to drop with the further increasing sintering temperature. Under the same sintering process, the relative density of TiB₂-based ceramics composites with glassy sintering aid is higher than that with crystalline sintering aid. Compared with the annealed crystalline counterparts powders, glassy powder are confirmed to be more advantageous to promote the sintering densification behavior of TiB₂ ceramics powders. Besides, compared with the TiB₂-10 wt.%（TiAl-m） added sample, the mechanical properties（fracture strength 454.42 MPa and fracture toughness 5.24 MPa·m1/2）of the TiB₂-10 wt.%（Ti₃Al-m） added sample are better, and its microstructure is more uniform and dense. The reason is that the TiAl-m completely transforms into liquid phase（mainly Ti and Al）, which lose the effect as a sintering aid to improve the sintering properties of the TiB₂ powders. When the sintering temperature increases, the grain growth increases slightly, and the fracture mode gradually transforms from intergranular to transgranular.