Microstructure and phase transformation of Ti3AC2 (A = Al, Si) in hydrofluoric acid solution

In this paper, Ti(3)AC(2) (A = Al, Si) were prepared by pressureless argon shielding synthesis technique. The microstructure and phase transformation of as-prepared Ti(3)AC(2) (A = Al, Si) in hydrothermal hydrofluoric acid (HF) solution were investigated systematically. Results showed that the obtained Ti3AlC2 and Ti3SiC2 were closely aligned layered structure. In hydrothermal HF solution, Al or Si element was preferentially etched from the layered structure, inducing obvious transformation of microstructure and phase composition. For Ti3AlC2, Al atoms diffused out of the structure and reacted with HF to form AlF3 center dot H2O, which induced the rearrangement of the Ti and C atoms, and finally resulted in the formation of TiCx cubic phase. With the hydrothermal temperature and reaction time increasing, the TiCx phase gradually disappeared and the grain size of AlF3 center dot H2O gradually increased. When Ti3SiC2 was immersed in hydrothermal HF solution, the main products were TiC and SiC. Interestingly, with the hydrothermal treatment temperature and reaction time increasing, TiC gradually disappeared, while SiC nearly kept unchanged. This can be explained that SiC was covalently bonded carbide, while TiC was metallically bonded, having relatively weak bond energy and consequently being unstable in hydrothermal HF solution.