PROCESSING AND PROPERTIES OF BORON CARBIDE WITH HAFNIUM DIBORIDE ADDITION

 Boron carbide is a promising candidate for many high performance applications in nuclear and defence sectors because of its unique characteristics. A combination of low density (2.51 g∙cm-3), high elastic modulus (460 GPa) and high hardness (38 GPa) enables B4C to find application in defence sector as an armour material. 10B isotope of B4C offers significant neutron absorption cross-section for both thermal and fast neutrons and thus plays a major role in nuclear industries as neutron detectors, control rods and shielding materials. Refractory nature of B4C necessitates a high sintering temperature close to 2273 K which makes densification difficult and often deteriorates mechanical properties of the material due to the grain coarsening effects. Also, the extreme brittleness/poor fracture toughness of B4C limits the wide spread application of the material. Different sintering methods that assists the densification of high temperature ceramics are the use of submicron particles, binder additions and adoption of advanced sintering process like spark plasma sintering (SPS). Most of the research work has been devoted towards binder based additions as it is reported to enhance both densification and mechanical properties of B4C.  In this study, hafnium diboride (HfB2) is chosen as a ceramic binder, as it possesses attractive properties like high melting point, high hardness, high elastic modulus and neutron absorption cross-section  Limited reports were available on the densification of B4C with pre-synthesized diboride powders (TiB2, ZrB2 and CrB2) as ceramic additives.The present paper gives the results of investigations carried out on the effect of HfB2 addition on the densification and properties of B4C by hot-pressing method.

Certain advantages can be observed in sintering of B4C with the addition of HfB2 as compared to HfO2. Though the in-situ processing route has proven to fabricate high dense B4C composites, it often causes deviations from its stoichiometric composition. This can be realized based on the reported changes in lattice parameters of B4C phase in the sintered product with respect to oxide additions. Since both boron and carbon atoms taking part in the reduction of oxides, B/C ratio of B4C will no longer remain the same as that of initial or starting material composition. In such cases, precise control over the material chemistry is often difficult. Further, the incomplete or partial completion of chemical reactions between B4C and oxides would also cause compositional uncertainties in the material. B4C finds many uses in nuclear industry as a neutron control rods, detectors etc. . The discrepancies that arise from compositional uncertainties, particularly impurities and non-stoichiometry are of primary concern for nuclear applications. Hence applications of this type, demand materials whose compositions and impurity levels are precisely known . Table 2 shows the measured lattice parameters of B4C phase that remain unaffected with respect to HfB2 addition. Hence the direct additions HfB2 could be preferred over the HfO2 addition while fabricating boron carbide-hafnium diboride composites for nuclear applications.

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