Oxidation of zirconium diboride with niobium additions

Oxidation of ZrB2 ceramics containing Nb additions at 1500 °C resulted in the formation of a two-layer oxide scale. The outer surface was partially covered by a glassy layer containing B2O3 with smaller amounts of Nb and Zr oxides dissolved into it. With increasing exposure time, evaporation of B2O3 from the outer layer resulted in precipitation of oxide particles in the receding glassy phase. Between the outer layer and the unoxidized (Zr,Nb)B2 was a porous layer that consisted of particles containing Zr, Nb, and O. The formation of Nb2Zr6O17 was observed in the porous oxide layer. Since this compound is solid at the oxidation temperature, liquid phase sintering of the ZrO2 scale was not possible. However, dissolution of Nb into B2O3 increased the stability of the liquid/glassy layer, which acted as a barrier to the transport of oxygen at higher temperatures compared to the scale formed on nominally pure ZrB2.