Microstructural, Thermal, Mechanical, and Tribological Properties of Al-2 wt.% CNT Nanocomposites Prepared by Mechanical Alloying, Conventional Sintering, and Spark Plasma Sintering

This study investigates the effect of short milling times (15, 30, and 60 min), conventional sintering (CS), and spark plasma sintering (SPS) on the microstructure, mechanical, and tribological properties, as well as thermal behavior of Al-2 wt.% CNT nanocomposites. A uniform dispersion of CNTs within the aluminum (Al) matrix was achieved without CNT damage, after 60 min of milling, accompanied by an increase in the lattice parameter to 0.4052 ± 10⁻⁴ nm, and a drop in the crystallite size to ~ 323 nm. Thermal analysis reveals that the exothermic and endothermic peaks arise from the microstructure of the milled powders. After heating to 700 °C and cooling to room temperature, a thin layer of molten Al coats the CNTs, which becomes more pronounced during a second heating and cooling cycle. The in situ XRD reveals that the lattice parameter increases linearly with increasing temperature. Sintering at 600 °C for 30 min (CS-600/30) resulted in the formation of Al₂O₃, increased hardness to 90.4 HV, and enhanced resistance to creep. In the SPS-processed samples, poor CNT dispersion, combined with a low sintering temperature (S-15-500), resulted in favorable tribological performance, yielding a low coefficient of friction (µ = 0.232) and a low wear rate of 7.4 × 10⁻⁶ mm³/Nm. The highest overall mechanical performance was achieved for the CS-600/30-60 sample. The low wear rates, which range from 7 × 10⁻⁶ to 7.5 × 10⁻⁴ mm³/N.m, indicating superior wear resistance and excellent tribological performance of the Al-2 wt.%CNT composites.