Layer-Thickness-Controlled Mechanical Anisotropy, Bonding Hierarchy, Metallic Electronic Structure, and Thermal Response of Ti2AlC, Ti3AlC2, and Ti4AlC3 MAX Phases

Ti–Al–C MAX phases are promising nanolaminated ceramics that combine ceramic-like mechanical strength with metallic electronic behavior, thermal stability, and excellent machinability, making them attractive for demanding structural and functional applications. In this work, density functional theory calculations were performed to systematically investigate the influence of Ti–C layer thickness on the structural, mechanical, electronic, bonding, dynamical, thermal, and optical properties of Ti 2 AlC, Ti 3 AlC 2 , and Ti 4 AlC 3 MAX phases. The calculated formation energies, cohesive energies, elastic constants, and phonon spectra confirm that all three compounds are energetically, mechanically, and dynamically stable. A systematic enhancement in elastic stiffness, cohesive strength, and thermal stability is observed with increasing Ti–C layer thickness, indicating that Ti 4 AlC 3 possesses the greatest mechanical robustness and high-temperature suitability. Electronic band structures and density of states reveal metallic behavior in all phases, dominated by Ti-3d states crossing the Fermi level. Bonding analyses demonstrate that strong covalent/ionic Ti–C interactions coexist with weaker metallic Ti–Al bonding, providing the fundamental origin of the characteristic combination of stiffness, conductivity, and machinability. Optical calculations further confirm metallic optical behavior with strong low-energy conductivity and reflectivity. Overall, this comparative study establishes the critical role of Ti–C layer thickness in governing the multifunctional properties of Ti–Al–C MAX phases and provides useful guidelines for the design and selection of MAX-phase ceramics for high-temperature structural, protective coating, and conductive applications.