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Yi Gu School of Materials Science and Engineering, Central South University, Changsha 410083, China

Fuqin Zhang Science and Technology on High-Strength Structural Materials Laboratory, Central South University, Changsha 410083, China

Tongxiang Liang State Key Laboratory of New Ceramic and Fine Processing, Tsinghua University, Beijing 100084, China (Received 13 January 2017; accepted 8 May 2017)

Boron carbide (B4C) powder was consolidated at 45 MPa by Spark Plasma Sintering (SPS) for 20 min from 1450 to 2000 °C. The density of the B4C reached 99.6% at 2000 °C. A continuum model was applied to describe the densification mechanism of B4C powder under SPS conditions. The shrinkage rate was sensitive to particle size and temperature. The effect of porosity on thermal diffusion was significant, especially for small particle sizes. It appears that there is Joule heating, discharge, and electromagnetic field involved during the SPS of B4C. The current can enhance the sintering process, and it can obviously reduce the creep activation energy.

I. INTRODUCTION

Boron carbide (B4C) is an attractive covalently bonded ceramic with low density (2.52 g/cm3), high melting point (2450 °C), high Young’s modulus (.435 GPa), and especially high Vickers hardness (.27 GPa). Furthermore, B4C has good chemical inertness, low thermal expansion coefficient (5.7
106/°C), and high neutron absorption cross-section. Due to these outstanding properties, B4C ceramic has great potential for many applications such as neutron absorber, blasting nozzles, wear-resistant components, and especially light armor.1–7 However, it’s difficult to obtain high density pure B4C by pressureless sintering methods due to the high of covalency of bonds between B and C atoms in B4C (.90%).8 Thus high temperatures close to the melting point of B4C are required to achieve complete densification, which results in grain coarsening and degenerated material’ properties. Hot-pressing (HP) can lower the consolidation temperature of B4C ceramics but the relative density only reaches 92–98% when the pressure and temperature are 30–40 MPa and 2000– 2100 °C respectively.7 Sintering aid additions like pure metal (Al, Ti, Fe, Ni, Cu, Cr), metal oxides and metal salts (MgO, Fe2O3, Al2O3, TiO2, Na2SO4), metal carbide and boride (TiC, TiB2, SiC, CrC, CrB) are reported to lower Contributing Editor: Gary L. Messing a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.205

the sintering temperature of B4C ceramics.9–13 However, owing to the complex composition and reaction between carbide and sintering aids, the purity of final produce limits its application in nuclear industries. Spark Plasma Sintering (SPS), also known as electricdischarge sintering or field-assisted sintering, is a fast sintering technology that combines the application of pressure and current directly or indirectly to consolidate the powder. It was suggested that due to the presence of a plasma between powder particles, the powder surface can be activated and thus the powd