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

The synthesis of high-purity M_n+1AX_n (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.