Computational Study of Low Interlayer Friction in Ti_n+1C_n (n = 1, 2, and 3) MXene

The friction of adjacent Ti_n+1C_n (n = 1, 2, and 3) MXene layers is investigated using density functional theory (DFT) calculations and classical molecular dynamics simulations with ReaxFF potentials. The calculations reveal the sliding pathways in all three MXene systems with low energy barriers. The friction coefficients for interlayer sliding are evaluated using static calculations. Both DFT and ReaxFF methods predict friction coefficients between 0.24 and 0.27 for normal loads less than 1.2 GPa. The effect of titanium (Ti) vacancies in sublayers and terminal oxygen (O) vacancies at surfaces on the interlayer friction is further investigated using the ReaxFF potential. These defects are found to increase the friction coefficients by increasing surface roughness and creating additional attractive forces between adjacent layers. However, these defective MXenes still maintain friction coefficients below 0.31. We also consider functionalized Ti₃C₂ MXene terminated with -OH and -OCH₃ and find that compared to the -O-terminated surface both groups further reduce the interlayer friction coefficient to 0.10-0.14.