The present work describes a synthesis route for bulk Ta4AlC3 MAX phase ceramics with high phase purity. Pressure-assisted densification was achieved by both hot pressing and spark plasma sintering of Ta2H, Al and C powder mixtures in the 1200-1650°C range. The phases present and microstructures were characterized as a function of the sintering temperature by X-ray diffraction and scanning electron microscopy. High-purity -Ta4AlC3 was obtained by hot pressing at 1500°C for 30 min at 30 MPa. The β-Ta4AlC3 allotrope was observed in the samples produced by SPS. The Young's modulus, Vickers hardness, flexural strength and single-edge V-notch beam fracture toughness of the high-purity bulk sample were determined. The thermal decomposition of Ta4AlC3 into TaCx and Al vapour in high (10 -5 mbar) vacuum at 1200°C and 1250°C was also investigated, as a possible processing route to produce porous TaCx components.

New bulk MAX phase-based ceramics were synthesized in the Ta-Hf-Al-C and Ta-Nb-Al-C systems. Specifically, (Ta1-x,Hfx)4AlC3 and (Ta1-x,Nbx)4AlC3 stoichiometries with x = 0.05, 0.1, 0.15, 0.2, 0.25 were targeted by reactive hot pressing of Ta2H, HfH2, NbH0.89, Al and C powder mixtures at 1550°C in vacuum. The produced ceramics were characterized in terms of phase composition and microstructure by X-ray diffraction, scanning electron microscopy, electron probe microanalysis and scanning transmission electron microscopy. The investigation confirmed the existence of such M-site solid solutions with low solute concentrations, as predicted by first-principles calculations. These calculations also predicted a linear trend in lattice parameter evolution with increasing Hf concentration, in agreement with the experimental results. In order to increase the low phase purity of the produced ceramics, Sn was added to form (Ta1-2 x,Hfx)4(Al0.5,Sn0.5)C3 and (Ta1-x,Nbx)4(Al0.5,Sn0.5)C3 double solid solutions, thus resulting in a higher content of the 413 MAX phase compounds in the produced ceramics.

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