de Souza Lamim T., Andrioni T.D., Pigosso T., Martinez Martinez D., Simões Abreu C., Bendo T., Binder C.
Tribology International, vol. 224, art. no. 112283, 2026
This study investigates the self-lubricating behaviour of bilayers formed by vertically aligned carbon nanotube (VACNT) films grown on top of a cementite (Fe<sub>3</sub>C) layer. The tribological performance of four bilayers with distinct characteristics grown on steel was evaluated using reciprocating dry sliding tests in a ball-on-plane configuration. Incremental-load tests assessed the durability of the self-lubricating regime, while constant-load tests ranging from 15 mN to 7 N explored the load effect on the lubrication mechanisms. Results reveal a combined effect of Fe<sub>3</sub>C interface roughness (Sk) and nanotube structure (L<sub>N</sub>/L<sub>P</sub>) on tribological behaviour, described by Sk<sup>a</sup>× (L<sub>N</sub>/L<sub>P</sub>)<sup>b</sup>. Rougher Fe<sub>3</sub>C layers enhance lubricant retention at the interface and tribofilm regeneration, whereas higher L<sub>N</sub>/L<sub>P</sub> ratios provide a greater supply of lubricant. The scuffing resistance is controlled by both parameters, but with a stronger influence of the cementite roughness ( a = 0.74, b = 0.39), showing the critical role of the interface under severe contact conditions. In contrast, wear rate is primarily controlled by the VACNT structure with a minor contribution from interface ( a = 0.07, b = 0.42). Constant-load tests showed that at low loads (15 and 40 mN), thick, weakly disordered tribofilms are formed and primarily act as load-bearing layers, delaminating without establishing efficient lubrication and showing higher shear strength. Higher loads induce rapid transformation into thin, highly disordered carbon tribofilms, resulting in stable low-friction behaviour. In addition, VACNT characteristics affect the kinetics of tribofilm evolution.
