Thermodynamics of the oxidation of ZrB 2 – TiB 2 , ZrB 2 –SiC and ZrB 2 – B 4C ceramics

Abstract

The thermodynamics of the oxidation of three-high temperature ZrB 2-based ceramics (ZrB 2–TiB 2, ZrB 2–SiC and ZrB 2–B 4C) has been studied in order to find the stability domain of zirconium diboride, in terms of temperature, partial pressure of oxygen and composition, in which it is protected against oxidation. In the case of the ZrB 2–TiB 2 binary system, a plot of log pO2vs. 1/T in the temperature range of 500–2000 K and another plot of pO2 (× 10 14) vs.xTiB2 for T=2000K are made taking into account the two-extreme possibilities of no solubility and 100% solid solubility between ZrB 2 and TiB 2, respectively. A plot of log pCOvs. log pO2 is made for 1773 K for the systems ZrB 2–SiC and ZrB 2–B 4C. It was found that the ZrB 2–TiB 2 ceramics does not have sufficient oxidation resistance in the temperature range of 500–2000 K. ZrB 2 of ZrB 2–SiC ceramics can be protected under 1 atmosphere oxygen or in air if the liquid borosilicate (with the chosen composition, 70% B 2O 3–30% SiO 2), which is an intermediate product, provides a kinetic barrier to the continuation of oxidation by forming an impervious layer on the exposed surfaces. In contrast, the ZrB 2–B 4C ceramics does not produce the borosilicate upon oxidation. In view of the volatility of pure liquid B 2O 3, it is recommended that the ZrB 2–B 4C ceramics can be used at a lower temperature, perhaps below 1373 K, when the vapour pressure of B 2O 3 is significantly small.