A DFT exploration of the stabilities, physical properties, and tensile strength of new synthesized Nb2AC (A=Ni and Co) MAX phases

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Based on first-principles calculations, this study explored the structural stability, elastic anisotropy, tensile strength, and the mechanical, electronic, and thermodynamic properties of the newly synthesized MAX phases Nb2NiC and Nb2CoC. It has been found that these compounds are thermodynamically and mechanically stable, exhibit metallic conductivity, and possess ductile nature. The ultimate tensile strengths of Nb2NiC and Nb2CoC depend on their crystallographic directions, specifically [0001] and [112̄0]. In the [0001] direction, Nb2CoC has a tensile strength of about 36.63 GPa at a strain of 26%, compared to Nb2NiC, which has a tensile strength of 31.70 GPa at a strain of 24%. In the [112̄0] direction, Nb2CoC exhibits a tensile strength of around 23.29 GPa at a strain of 12%, while Nb2NiC has a tensile strength of approximately 18.51 GPa at an strain of 8%. Both Nb2CoC and Nb2NiC demonstrate significant elastic deformation before reaching their ultimate tensile strengths, indicating good ductility. It is noteworthy that Nb2NiC is less elastic than Nb2CoC in both the [0001] and [112̄0] directions, as the elastic constants of Nb2CoC are comparatively higher than those of Nb2NiC. Furthermore, the estimated thermal parameters show that these compounds exhibit a relatively low Debye temperature, a high melting point, low minimum thermal conductivity, and thermal expansion coefficient values that are similar to those of well-established thermal barrier coating (TBC) materials such as Al2O3, LaPO4, and TiO2. Consequently, the newly synthesized MAX phases Nb2NiC and Nb2CoC are promising candidates for TBC applications.