Fabrication And Properties Of TiB₂-TiC Ceramics By Mechanical Alloying

TiB2-TiC materials have been widely used for wear parts, arm protecting materials, coating and cutting tools due to their high melting points, high hardness and excellent wear resistance. Furthermore, the excellent electrical conductivity, chemical stability and corrosion resistance also suggest the potential application in the aluminium reduction cell. However, dense TiB2-TiC ceramics are very difficult to be fabricated by conventional sintering techniques due to their high melting points (3225 and 3067℃, respectively) and low self-diffusion coefficients. The high pore content (10-20%) and poor mechanical properties are inevitable using the commercial TiB2 and TiC as the starting materials by conventional sintering technique. The prepration of TiB2-TiC ceramics by hot pressing is effective for improving their densities. The shapes and size scales of TiB2-TiC composites, however, are limited by the hot pressing moulds, widely limiting their applications. Therefore, it is urgent to develop the possible TiB2-TiC composites used in the industry scale. Based on the background mentioned above, dense TiB2-TiC ceramics had been prepared via mechanical alloying and subsequent low temperature pressureless sintetering, which broke through the bondage of traditional hot pressing sintering. It was also of great significance for preparing the different types and bulk components of TiB2-TiC composites. Recently, the stuctrure of rare earth cell involves the parallet and vertical cathode-anode arrangement, which has exposed many drawbacks such as high energy consumption, high pollution and high cost. The great economic and social benefits could be achieved if the cell with submerged cathode was popularized in rare earth electrolysis industry. TiB2-TiC ceramics are promising candidates as cathodes in the cell with submerged cathode due to their excellent electrical conductivity, chemical stability and excellent wettability with the liquid metals. Based on this, the properties such as wettability, corrosion resistance and shock resistance of TiB2-TiC multiphase ceramics related to the cathodes were investigated in the present paper. The application possibility of cell with submerged cathode using TiB2-TiC as a cathode material was also investigated, which was significant for developing the submerging cathode electrolytic cells.TiB2-TiC precusors were synthesized using Ti and B4C powders as the starting materials by mechanical alloying method. The diffraction peak intensity of Ti decreased due to the refinement of crystalline size and the generation of strain with the increasing milling time. The diffraction peak intensity of B4C became relatively weak and even partially disappeared as the milling time increased. The diffraction peaks of TiC were detected after milling for 18h, and the intensity of which became strong with the increasing milling time. The T1B2 phase was formed until the milling time was up to 24h. The grain size of powder mixture after milling for 48h was estimated to range from 30 to 50 nm.The sintering temperature had an important effect on the mechanical properties of TiB2-TiC multiphase ceramics. The relative densities and hardness of TiB2-TiC ceramics improved with the increasing temperature, whereas the bending strength and fracture toughness increased first, and then followed by marginal decrease. The TiB2-TiC ceramics sintered using the 48 h milled powder at 1800 ℃ for 2 h owned the optimum values of mechanical properties, such as densities greater than 98% of the theoretical value, higher bending strength (487MPa), higher hardness (94.7, HRA), and fracture toughness (5.83MPa·m1/2). The mechanical alloying was in favor of not only the microstructures of the TiB2-TiC composites, but also the formation of coherent interface between TiB2 and TiC.Wettability of TiB2-TiC ceramics by NdF3-LiF-Nd2O3 melt showed a dynamic wetting process, during which the wetting was fast at the early stage and then became slowly when the wetting time was up to 8 min. The contact angle was 14.8° at which the three phase equilibrium point was reached. The excellent wettability of the TiB2-TiC ceramics suggested the promising candidates for the cathode materials in the neodymium reduction cell. The average linear thermal expansion coefficient of TiB2-TiC ceramics (6.071×10-6℃) obtained here had an important influence on the thermal shock properties, which provided the proof for sustaining thermal shock of electrolysis cell. The heat conductivity of TiB2-TiC multiphase ceramics measured by static conduction method was λ,=2.1788+0.00767T.The electrochemical potentiodynamic polarization curves and high temperature static immersion methods were employed to investigate the corrosion behaviors of dense TiB2-TiC ceramics aiming at the different mediums at room and high temperatures. The polarization curves of TiB2-TiC composites produced in the acid, alcali and netrual solutions exhibited the passivation responses, indicating the formation of passivation films. The passivation films, however, were sparse, which allowed the ion to diffuse slowly through the electrode surfaces. The corrosion rates of TiB2-TiC multiphase ceramics achieved according to the marginal Ti content were 0.0003544 g·cm-2·h-1 in liquid Al and 0.0041 g·cm-2·h-1 in molten NdF3-Nd2O3-LiF. No reaction was observed between the TiB2-TiC ceramics and melts by EDS and XRD analysis, suggesting that the corrosion of TiB2-TiC composites by liquid Al and molten NdF3-Nd2O3-LiF was physical penetration.The electrical conductivity of NdF3-LiF-Nd2O3 systems increased with the elevated temperature. Furthermore, the electrical conductivity of NdF3-LiF-Nd2O3 system increased with the increasing LiF content, and decreased with the increasing Nd2O3 content at the same temperature. Small-scale submerged cathode cell was designed for neodymium elctrolysis using TiB2-TiC composite as the cathode material. The stable cell voltage could be obtained when the electrolysis temperature was 1060℃ and the electrode distance was 40 mm. The neodymium was the main electrolysis products, and the Ti content of which was 0.023wt%.