LAUSR

AbstractThe thermodynamics of the oxidation of three-high temperature $$\hbox {ZrB}_{2}$$ZrB2-based ceramics ($$\hbox {ZrB}_{2}$$ZrB2–$$\hbox {TiB}_{2}$$TiB2, $$\hbox {ZrB}_{2}$$ZrB2–SiC and $$\hbox {ZrB}_{2}$$ZrB2–$$\hbox {B}_{4}\hbox {C}$$B4C) 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 $$\hbox {ZrB}_{2}$$ZrB2–$$\hbox {TiB}_{2}$$TiB2 binary system, a plot of log $$p_{\mathrm{O}_{2}}$$pO2vs. 1/T in the temperature range of 500–2000 K and another plot of $$p_{\mathrm{O}_{2}}$$pO2 ($$\times 10^{14}$$×1014) vs.$$x_{\mathrm{TiB}_{2}}$$xTiB2 for $$T = 2000\hbox { K}$$T=2000K are made taking into account the two-extreme possibilities of no solubility and 100% solid solubility between $$\hbox {ZrB}_{2}$$ZrB2 and $$\hbox {TiB}_{2}$$TiB2, respectively. A plot of log $$p_{\mathrm{CO}}$$pCOvs. log $$p_{\mathrm{O}_{2}}$$pO2 is made for 1773 K for the systems $$\hbox {ZrB}_{2}$$ZrB2–SiC and $$\hbox {ZrB}_{2}$$ZrB2–$$\hbox {B}_{4}\hbox {C}$$B4C. It was found that the $$\hbox {ZrB}_{2}$$ZrB2–$$\hbox {TiB}_{2}$$TiB2 ceramics does not have sufficient oxidation resistance in the temperature range of 500–2000 K. $$\hbox {ZrB}_{2}$$ZrB2 of $$\hbox {ZrB}_{2}$$ZrB2–SiC ceramics can be protected under 1 atmosphere oxygen or in air if the liquid borosilicate (with the chosen composition, 70% $$\hbox {B}_{2}\hbox {O}_{3}$$B2O3–30% $$\hbox {SiO}_{2}$$SiO2), 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 $$\hbox {ZrB}_{2}$$ZrB2–$$\hbox {B}_{4}\hbox {C}$$B4C ceramics does not produce the borosilicate upon oxidation. In view of the volatility of pure liquid $$\hbox {B}_{2}\hbox {O}_{3}$$B2O3, it is recommended that the $$\hbox {ZrB}_{2}$$ZrB2–$$\hbox {B}_{4}\hbox {C}$$B4C ceramics can be used at a lower temperature, perhaps below 1373 K, when the vapour pressure of $$\hbox {B}_{2}\hbox {O}_{3}$$B2O3 is significantly small.