Effect of carbon-nanotube length on friction and wear of polyamide 6,6 nanocomposites

• Storage modulus and thermal conductivity increased when CNTs were added to PA66.
• Friction coefficients decreased with the length of CNTs at elevated temperatures.
• Wear rate was increased beyond 110 °C and influenced by the length of CNTs.
• Improved wear resistance is due to nematic alignment and thermal conductivity of CNTs.

The friction and wear of polyamide 6,6 (PA66) nanocomposites containing 1 wt% of carbon nanotubes (CNTs) with different lengths were studied using a block-on-ring tribometer at different sliding velocities. CNT addition was found to increase storage modulus, loss modulus, and thermal conductivity of the composites, suggesting possible influence to their tribological properties. Tribotest results revealed a substantial effect of the length of CNTs and it was more at elevated temperatures. In the sliding tests performed at temperature below 110 °C the coefficient of friction (COF) tended to be decreased by CNTs addition, while it was slightly increased as the CNT length increased. However, at temperatures beyond 110 °C, the COF increased with the addition of CNTs and increased, even more, with longer CNTs. Wear resistance was also improved by long CNTs, and this improvement was particularly evident at elevated temperatures. TEM investigation of the sliding surface indicated that the improvement of wear resistance and the increased COFs observed at high temperatures could be attributed to the nematic alignment of CNTs along the sliding direction, which was more pronounced with longer CNTs. The marked reinforcing effect of long CNTs was also supported by the morphology of the transfer films and wear debris.