Fabrication And Characteristics Of TiB₂-Al₂O₃ Multiphase Ceramics

TiB₂ exhibits high electrical conductivity, melting temperature and wear resistance, low specific weight and relatively good chemical stability, besides its extremely high hardness and elastic modulus. TiB₂ ceramics are very promising in modern industry. The main problems that limit the application of TiB₂ ceramics include: high fabrication cost, low sintering activity and the inherent brittleness. Adding second ceramic phase, using sintering aids and optimizing fabrication technologies are believed important ways to improve its mechanical properties, sinterability of ceramics and cut down on costing.Using Al₂O₃ as the second phase, the different mixing method was applied by the sintering aids of Y₂O₃ and Ni. The TiB₂-Al₂O₃ multiphase ceramics were fabricated by CIP and the gas pressure sintering (GPS). The influence of the second phase and the sintering aids on the performance of materials and the relationship of material structure and performance were studied, for fabricating the low-cost and mass-titanium diboride materials provided a theoretical basis.Using Y₂O₃ as a sintering aid, the composite powders were prepared by spray drying and the green bodies were fabricated by the CIP and the hot degrease. TiB₂-Al₂O₃ multiphase ceramics were fabricated by the gas pressure sintering at the temperature of 1700℃—1900℃. The results show that the sinterability was advanced by the second phase. While the sintering temperature at 1900℃and the gas pressure of 8Mpa, the relative density of 96% was obtained by adding 50wt%Al₂O₃. The analysis of the microstructure of materials show that the solid state sintering of flux mass transport and diffuse mass transport make an important role in the sintering process. The phenomena that TiB₂ and Al₂O₃ phase concentrated on the some parts occurred, because Al₂O₃ transfer and grow more easily than TiB₂ at the sintering temperature. The remained porosity mainly dispersed in the TiB₂ parts. The YAG (Y₃Al₅O₁₂) phase was formed by Y₂O3 and Al₂O₃ at 1800℃, and the densification was accelerated by the YAG phase between the interface of TiB₂ and Al₂O₃ grains and the porosity of the TiB₂ parts. The problem of the amount of YAG phase still requires the advance study.TiB₂-Al₂O₃ composite powders were prepared by dry-milling and were pressed to form the green bodies by CIP. Then, TiB₂-Al₂O₃ multiphase ceramics were fabricated by the gas pressure sintering, using Ni as an assistant. The microstructure of samples was observed by scanning electron microscope, and the effects of Ni and Al₂O₃ contents and the parameters of sintering on the microstructure and mechanical properties were investigated. The best sintering parameters were confirmed. The results show that using Ni as an assistant, the liquid phase of lower viscidity was formed above the melting point of Ni (>1550℃) , the volatilization of the liquid phase was restrained as the existence of the gas pressure, and the densification was effectively accelerated by liquid phase sintering of dissolve-sedimentation mechanism. But the gas was charged early, the phenomenon that some gases were enwrapped in the specimen occurred and the densification was blocked. The connection between the microstructure and mechanics properties of different Al₂O₃ content was studied. The results show that the better wetting capability of Ni between the TiB₂ and Al₂O₃ grains contributed the equality disperse. At the same time, the growth of TiB₂ and Al₂O₃ grains was restrained obviously. With the increase of Al₂O₃ content, the density of the ceramic, the bending strength and fracture toughness are all enhanced; the Electrical resistance increases; Young's modulus appears a peak value; the Vickers hardness decreases and the Rockwell hardness has little change. The TiB₂-Al₂O₃ ceramic with 30% Al₂O₃ that were fabricated by the gas pressure sintering (1550℃1h, 0.5Mpa+1750℃1h, 8Mpa)has a better property, i.e., with a bending strength of 520 MPa, a Young's modulus of 339 GPa, and a Rockwell hardness HRA of 92.6.With the increase of Al₂O₃ content, the TiB₂ grains are smaller and the deflexion of crack expanding increases. When Al₂O₃ content is 20%, the crack expanding behavior presents transgranular cleavage cracking that due to the bigger TiB₂ grain size, the samples containing 30% and 50%Al₂O₃ show the zigzag intergranular cleavage cracking, because this approach only needs lower energy when the grains are smaller. The deflexion of crack expanding increases the area of crack, so the fracture toughness is effectively enhanced.