A comparative study of structural, mechanical, and thermal shock properties in ZrB$_2$-B$_4$C-SiC-LaB$_6$ functionally graded and non-graded composites

The rapid progress in space exploration and hypersonic aviation has intensified the demand for materials capable of withstanding extreme temperatures (>2000 °C) to protect underlying metallic structures. Ceramic matrix composites, especially carbon–carbon (C/C) composites, with protective SiC coatings, possess good oxidation resistance, but their effectiveness is limited to ∼1600 °C due to the active oxidation of SiC above 1500 °C. This limitation drives the need for new materials for next-generation hypersonic thermal protection systems. ZrB$_2$-B$_4$C-SiC-LaB$_6$ composites are attractive for ultra-high-temperature applications, and they are known to provide oxidation resistance at > 2000 °C. In this study, ZrB$_2$-B$_4$C-SiC-LaB$_6$ composites of two different configurations, 3-layered functionally graded material (FGM) and equivalent nongraded composite (NGC), were fabricated using hot pressing. A detailed comparative analysis of the structural, mechanical, and thermal shock behaviour of the FGM and NGC was performed. The interlayer regions of the FGM remained intact even after the 3-point bending test and thermal shock from ΔT = 800 °C. ... mehrThe highest average bending strength (∼ 376 MPa) was achieved in the FGM when the ZrB$_2$-rich layer experienced tensile stress, which was ∼ 20 % higher than the NGC samples. The thermal shock behavior of the composites was studied using a combined 3-point bending test and non-destructive ultrasonic phase spectroscopy. While the critical thermal shock temperature was similar for both material types (∼ 350 °C), the overall thermal shock resistance of the NGC material was marginally better owing to the favorable thermal residual stress distribution in the as-fabricated materials.