Sintering behavior of B4C–CrB2 ceramics

Abstract

Boron carbide (B4C) exhibits outstanding properties such as high hardness, high elastic modulus, wear resistance, high melting point and low density which make it a suitable materials for various industrial applications [1]. However, disadvantages of B4C are its poor sinterability caused by the low self-diffusion coefficient as well as relatively low strength and fracture toughness. Usually, dense B4C ceramics without additives are prepared by means of hot-pressing above 2100 ◦C [2], which is relatively expensive and limits sample shape. Several additives have been examined in order to obtain dense B4C ceramics by a pressureless sintering process [2–4]. Kanno et al. stated that Al, TiB2 and AlF3 were effective additives and that high relative density of 95% was achieved for B4C ceramics with Al addition sintered at 2200 ◦C [3]. The effect of carbon addition on the densification of B4C has also been studied [2, 4–7]. A relative density of 96.4% was obtained by pressureless sintering of B4C at 2150 ◦C doped with carbon [7]. Skorokhod et al. prepared B4C–TiB2 composites with densities higher than 99% by reaction pressureless sintering of B4C with the addition of TiO2 and C at temperatures from 1900 ◦C to 2050 ◦C [8]. It has been demonstrated that, as in the densification of non-oxide ceramics, such as silicon carbide and silicon nitride [9, 10], liquid phase sintering can also be an effective method for the fabrication of dense B4C ceramics. Relative densities higher than 95% were achieved for a B4C–TiB2–1 mass% Fe system sintered in a temperature range of 2150 ◦C to 2175 ◦C [11]. It was suggested that the iron-rich phase formed a liquid phase and contributed to the densification. However, investigation on the liquid phase sintering of B4C based ceramics is very limited. Results of surveying phase diagrams suggest that CrB2 may be a candidate as an effective additive to achieve the liquid phase sintering of B4C based ceramics due to the presence of the CrB2–B4C eutectic liquid phase formed at the relatively low eutectic temperature of 2150 ◦C as shown in Fig. 1 [12]. In the present study, the sintering behavior of B4C–CrB2 ceramics was examined by thermomechanical analyzer (TMA) measurement in order to discuss the possibility of CrB2 additives for pressureless liquid phase sintering. The starting B4C powder (Grade 3000F, Elektroschmelzwerk Kempten GmbH, Germany) had a ∗Present Address: Research Center, Denki Kagaku, Kogyo K. K., Machida-city, Tokyo 194-8560, Japan. E-mail: suzuya-yamada@denka.co.jp. mean particle size of 0.43 μm and a specific surface area of 15.3 m2/g (BET). The impurities of the B4C powder were oxygen (2.0 mass%), Fe (140 ppm), and Al (50 ppm). The B4C powder was blended with a CrB2 powder with a mean particle size of 3.5 μm (Japan New Metals Co., Ltd., Japan) using a planetary ball mill. The amount of CrB2 was 20 mol%. The powder mixture was formed into disk specimens of 11 mm in diameter and 6 mm in thickness by uniaxial pressing at a pressure of 20 MPa, followed by isostatic pressing at a pressure of 200 MPa. Relative densities of the green compacts were about 56%. The dimensional change during heating was determined using TMA (SETSYS TMA 24, Setaram, France). The temperature was monitored by a thermocouple. The specimen was heated up to 2350 ◦C at a heating rate of 10 ◦C/min, then immediately cooled down to room temperature at a rate of 30 ◦C/min in a helium gas atmosphere. The densities of specimens were determined by the Archimedes’ method. Phase identification was performed by X-ray diffractometry (XRD: RINT 2500, Rigaku, Japan) with Cu Kα radiation. Microstructural analysis was carried out using scanning electron microscopy (SEM: JSM 6320/JSM 5600, Jeol, Japan).