The effect of Ti and TiO 2 additions on the pressureless sintering of B 4 C
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I. INTRODUCTION
THE well-documented, outstanding properties of B4C, such as stiffness, hardness, wear resistance, and chemical inertness, make it a valuable potential material for a variety of applications.[1,2] The realization of this potential is hindered, however, by two major drawbacks, namely, the low fracture toughness of B4C and the very high temperature required for its sintering. Nearly full density cannot be achieved by pressureless sintering and can be attained in pure B4C only by hot pressing above 2300 8C. Covalent bonding prevails in boron carbide and, consequently, pore eliminating mass transport mechanisms such as grain boundary and volume diffusion become effective only at elevated temperature, close to the melting point. However, above 2000 8C, rapid coarsening occurs, resulting in unremovable, entrapped residual porosity[3,4] and in large particle size in the sintered compacts. Sintering schedules at such elevated temperatures require expensive equipment and a very close and continuous control of the sintering process parameters. Thus, there exists a very strong motivation for decreasing the temperature required to attain desired levels of density and strength. Numerous proposals have been put forward in order to lower the sintering temperature of boron carbide. Microwave sintering using 2.45 GHz radiation was used to attain 95 pct of the theoretical density (TD) after a 12-minute treatment at 2000 8C.[5] Initial powders with smaller particle size (,1 mm) can be used to reduce the sintering temperature and reach a high final density. Several studies have shown that many additives reduce the sintering temperature of B4C. Carbon, in particular, has been shown to be an effective sintering additive to boron carbide.[6,7,8] The activated sintering was attributed to the increased surface energy resulting from the reducing effect of carbon on the oxide layers. The flexural strength of pressureless sintered boron carbide with L. LEVIN, Graduate Student, and N. FRAGE and M.P. DARIEL, Professors, are with the Department of Materials Engineering, Ben-Gurion University of the Negev, Beer-Sheva, 84105, Israel. Manuscript submitted January 5, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
carbon additions rarely exceeds 350 MPa. Zachariev and Radev[9,10] reported that TiB2, CrB2, and W2B5 additives improve the sinterability of B4C and inhibit grain growth. Sintering submicron powder with the addition of 1 wt pct Be2C between 2200 8C to 2280 8C resulted in 94 pct density.[11] According to References 12 through 14, 95 to 99 pct density is obtained at 2100 8C to 2200 8C with Al or Alproviding (Al4C3, Al2O3, and AlF3) additives that promote liquid-phase phase sintering. Various additive combinations such as B 1 C, SiC 1 C, SiC 1 Al, and TiB2 1 C were tried.[15,16,17] The results of these studies showed an onset of shrinkage at temperatures that were 200 8C to 300 8C lower than those required for pure B4C. As early as in 1958, Adlassnig[18] studied the effect of the carbon content in the carbide phase and repor
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