Topotactic transformations in Hf-Al-C MAX phase compounds: Synthesis and characterization of nanolaminated Hf2AlC, Hf3AlC2 and Hf5Al2C3

The synthesis of high-purity Mn+1AXn (MAX) phase ceramics in the Hf-Al-C ternary system from near-stoichiometric feedstock powder mixtures has been exceptionally challenging due to the rapid concurrent formation of persistent, ultrafine HfC impurities. This work synthesized ceramics containing the nanolaminated Hf5Al2C3 'superstructure', showing that it comprises alternating Hf2AlC and Hf3AlC2 atomic stackings. For the first time, the Hf5Al2C3 complex structure was associated with the topotactic transformation of Hf2AlC into Hf3AlC2, which is observed upon heating the powder compact to temperatures higher than 1500 °C; moreover, an inverse decomposition reaction of Hf3AlC2 into Hf2AlC was observed as result of further heating the powder compact to temperatures exceeding 1600 °C. The crystal structure and lattice parameters of the Hf5Al2C3 'superstructure' were determined. MAX phase ceramics containing up to 40–45 wt% Hf3AlC2/Hf2AlC were produced with HfC as the main competing phase. The hardness and damage tolerance of these MAX phase ceramics were also evaluated.