Topotactic transformations in Hf-Al-C MAX phase compounds: Synthesis and characterization of nanolaminated Hf2AlC, 2 AlC, Hf3AlC2 3 AlC 2 and Hf5Al2C3 5 Al 2 C 3

The synthesis of high-purity M n+1 AX n (MAX) phase ceramics in the Hf-Al-C ternary system from nearstoichiometric 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 5 Al 2 C 3 'superstructure', showing that it comprises alternating Hf2AlC 2 AlC and Hf3AlC2 3 AlC 2 atomic stackings. For the first time, the Hf5Al2C3 5 Al 2 C 3 complex structure was associated with the topotactic transformation of Hf2AlC 2 AlC into Hf3AlC2, 3 AlC 2 , which is observed upon heating the powder compact to temperatures higher than 1500 degrees C; moreover, an inverse decomposition reaction of Hf3AlC2 3 AlC 2 into Hf2AlC 2 AlC was observed as result of further heating the powder compact to temperatures exceeding 1600 degrees C. The crystal structure and lattice parameters of the Hf5Al2C3 5 Al 2 C 3 'superstructure' were determined. MAX phase ceramics containing up to 40-45 wt% Hf3AlC2/Hf2AlC 3 AlC 2 /Hf 2 AlC were produced with HfC as the main competing phase. The hardness and damage tolerance of these MAX phase ceramics were also evaluated.