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

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 Al${}_2$O${}_3$, LaPO${}_4$, and TiO${}_2$. Consequently, the newly synthesized MAX phases Nb₂NiC and Nb₂CoC are promising candidates for TBC applications.