High-temperature properties and thermal stability of monophasic Al ₃ BC ₃

This paper describes the detailed high-temperature properties of monophasic Al3BC3, including the thermal expansion characteristics of its crystal structure and thermal shock resistance. We examined the differences in physical properties between Al3BC3 and Al8B4C7, which was previously thought to be Al3BC3. Compared with the thermal conductivity and CTE of Al3BC3 measured in the present study, those reported for Al8B4C7 in a previous study were higher, which is attributable to residues of Al and B. The densification of Al8B4C7 is supported by gas-phase diffusion via Al- and B-based gases or liquid, whereas the sintering of Al3BC3 is dominated by solid-phase diffusion. The bending strength of Al3BC3 was 226 MPa at RT and was maintained even at high temperatures. Al3BC3 was found to exhibit a good thermal shock fracture resistance parameter R’ of 3.3, equivalent to that of Al2O3 and substantially better than that of commonly used engineering ceramics Al4SiC4 and Al6Si2O13, mainly because of the higher thermal conductivity and lower elastic modulus of Al3BC3. The dense Al3BC3 sample exhibited excellent thermal stability under N2 and inert gas atmospheres but was easily oxidized to monophasic Al2O3 at 1300°C under ambient air.