Reaction sintering and mechanical properties of B 4 C with addition of ZrO 2
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The effect of ZrO2 addition on sintering behavior and mechanical properties of both hot-pressed and pressureless-sintered B4C was investigated. The addition of ZrO2 improved the densification behavior of B4C remarkably via a reaction with the B4C to form ZrB2 at elevated temperatures. When B4C was densified at 2000 °C by hot pressing, only a small amount (approximately 2.5 vol%) of ZrO2 was necessary to achieve a full densification. Excellent mechanical properties (hardness, elastic modulus, flexural strength, and fracture toughness) were observed in those specimens. As the amount of ZrO2 was increased further, the mechanical properties were reduced, except for the fracture toughness, apparently due to the formation of too much ZrB2 in the specimen. Without the applied pressure, larger amounts of ZrO2 should be added to obtain a body with high relative density. When the B4C was sintered at 2175 °C with addition of 10 vol% ZrO2, the specimen has a density higher than 95% of the theoretical, and hardness and flexural strength of 25 GPa and 400 MPa, respectively.
I. INTRODUCTION
Due to their outstanding properties such as extremely high hardness, wear resistance, low specific weight, and resistance to chemical attacks, B4C ceramics are regarded as having great potential for applications in wearresistant parts and for armor materials.1–4 Moreover, their nuclear properties (high cross section for neutron absorption and resistance to irradiation) have made them an attractive choice for neutron-absorbing materials.5,6 However, one of the major obstacles for wider applications of B4C in industry is its relatively low strength and fracture toughness as well as difficulties related to its fabrication.7 Because pure B4C is very difficult to sinter with density higher than approximately 80% of the theoretical, a variety of second phases were added as sintering aids. Even with sintering additives, in most cases, hot pressing should be done at extremely high temperatures (approximately 2200 °C) to obtain a fully dense B4C body.8 Significant improvement in sinterability was observed when a small amount of carbon was added before hot pressing.9 Other additives (such as Al, Fe, Ti, B, TiB2, and SiC) have also been used, with various degrees of improvement in sintering behavior and mechanical proper-
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 15, No. 11, Nov 2000
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ties.10–21 However, addition of metallic phases with low melting temperatures generally reduces the physical and mechanical properties of hard ceramics. Also the improvement in sintering behavior of B4C by addition of nonoxide ceramics was rather restricted. Oxides were scarcely used as sintering aids for B4C, mainly because of the chemical instability of B4C with respect to many oxides.22,23 In other words, B4C reacts with other oxides during sintering to form new compounds that might be detrimental to the unique properties of B4C. However, recently, we observed tha
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