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Fabrication and Characteristics of TiB2-Al2O3 Multiphase Ceramics

Author: LiJiaJing
Tutor: FuZhengYi；ZhangJinYong
School: Wuhan University of Technology
Course: Materials Science
Keywords: titanium diboride-alumina multiphase ceramics gas pressure sintering microstructure mechanical properties
CLC: TQ174.75
Type: Master's thesis
Year: 2007
Downloads: 81
Quote: 1
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Abstract


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.

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