Thermodynamics of oxidation and nitridation reactions of boron carbide

Raman Centre for Applied and Interdisciplinary Sciences, 16A, Jheel Road, Kolkata-700 075, India
E-mail: cv@rcais.res.in, ritabratabhowmick@gmail.com, kghoshcu@gmail.com
Present address: Department of Chemistry, Techno India-Batanagar, Kolkata-700 141, India
Manuscript received 16 May 2018, revised 25 May 2018, accepted 26 May 2018

Boron carbide, an extremely hard material that is used in lightweight body armour and nuclear reactors, requires to be sintered
at high temperature to form appropriate shapes. The quality and hardness of the product is affected by oxidative processes
and reaction with traces of nitrogen in air. Information on the thermodynamics of such processes is quite limited. In
this work, computational study of the thermodynamics of reactions of B₄C with oxygen and nitrogen has been undertaken.
This includes derivation of equilibrium constant (log k)-temperature relations and equilibrium O₂/N₂ activities-temperature relations
in the region 273–2300 K, for all possible reactions with O₂ and N₂. Reaction phase diagrams were then derived to
depict the reaction that is thermodynamically most favourable under the T-P_O2/N2 conditions. All computations were done with
a program in Mathematica, developed for this purpose. Results reveal that log k values for all oxidative reactions are positive
and those producing the trioxide (B₂O₃) are energetically most favoured. Reaction phase diagrams show that B₄C is stable
only at extremely low O₂ pressures and under high concentrations of gaseous components; the B moiety is more susceptible
to oxidisation compared to C moiety. When heated, initially B₂O₃ is formed regardless of concentration of gases. Similar
behaviour is observed in N₂, where BN is produced at the lowest temperatures but is stable at higher temperatures. It is concluded
that a surface layer of B₂O₃ produced initially serves as a protective shield against catastrophic oxidation of B₄C