Effect of boron on the thermodynamic stability of amorphous polymer-derived Si-(B-)-CN ceramics

The reason for the higher thermal persistence of amorphous polymer-derived SiBCN ceramics (T ∼ 1700-2000 °C) compared to SiCN ones (T ∼ 1500 °C) has been a matter of debate for more than a decade. Despite recent experimental results which indicate a major kinetic effect of boron on the thermal persistence of the ceramics, no experimental investigation of the thermodynamic stability of the materials has been reported. In this work, we present measured energetics of a series of the amorphous ceramics with various boron contents (0-8.3 at.%) using high-temperature oxidative drop-solution calorimetry. Through measurement of the drop-solution enthalpies in molten sodium molybdate at 811 °C, the formation enthalpies of the amorphous ceramics from crystalline components (SiC, BN, Si ₃N ₄, C) at 25 °C were obtained and found to be between -1.4 and -26.6 kJ g-atom ^-1. The determined enthalpy data plus the estimated positive entropy of formation values point to the thermodynamic stability of the amorphous ceramics relative to the crystalline phases, but such stabilization diminishes with increasing boron content. In contrast, the higher boron content increases the temperature of Si ₃N ₄ crystallization despite less favorable energetics for the amorphous phase, implying more favorable energetics for crystallization. Thus the so-called "stability" of SiBCN ceramics in terms of persistence against Si ₃N ₄ crystallization appears to be controlled by kinetics rather than by thermodynamic stability.