MXene-derived TiB2 formation in B4C at high temperatures

Two-dimensional (2D) MXenes are nanometer-thick sheets of transition metal carbides, nitrides, or carbonitrides with high-temperature capabilities. MXenes can be used as nanofillers and functional additives in ceramic hybrids, enhancing sintering and mechanical and electrical properties. In this study, we systematically investigate the incorporation of titanium carbide (Ti₃C₂T_x) MXene into micron-sized (~ 6 μm) boron carbide (B₄C) using a one-step electrostatic self-assembly method. We tuned the zeta potential of B₄C and Ti₃C₂T_x MXene solutions, gradually added B₄C into the MXene solution, and prepared green bodies with 1 to 10 vol.% Ti₃C₂T_x. We examined MXene phase stability, reactions, and phase transformation in B₄C via direct current spark plasma sintering at 1925 °C with a pressure of 70 MPa in a vacuum. To understand the reaction pathways, we conducted stepwise sintering from 800 °C to 1800 °C in 200 °C increments. X-ray diffraction and scanning electron microscopy results revealed that Ti₃C₂T_x reacts with B₄C at ~ 1200 °C to form TiB₂, with complete conversion at 1800 °C, resulting in a TiB₂-B₄C structure. For samples with 4 vol.% MXene or higher, B₂O₃ formed above 1200 °C due to MXene oxygen terminations and disappeared above 1800 °C. The overall oxygen content in MXene-containing samples was lower than in B₄C sintered without MXene, indicating its reducing nature as a 2D carbide. MXene also acts as a sintering additive and the relative density was increased by increasing the MXene content, achieving 99% with 10 vol.% MXene. The measured hardness values were 20 ± 1.6 GPa and 41 ± 0.8 GPa for B₄C and B₄C with 10 vol.% MXene Ti₃C₂ samples, respectively. This trend indicates that increasing Ti₃C₂T_x MXene content (1–10 vol.%) in the B₄C matrix enhances hardness due to the formation of TiB₂ phases, which improves densification and reinforces the B₄C matrix. Compared to traditional bulk TiC, 2D MXene sheets provide a larger surface area and uniform coverage of B₄C particles, lowering diffusion energy and enhancing sintering rates. This study demonstrates a new approach to using MXene as a nanometer-thick additive to form uniformly distributed TiB₂ in B₄C to enhance its structural properties.