Tribological behaviour of Fe₃C-CNT bilayers grown by plasma-induced metal dusting: Insights into the effect of nanotubes catalyst particles on friction and wear

This work investigates the tribological behaviour of bilayer surfaces formed by vertically aligned carbon nanotubes (VACNT) films grown on top of cementite (Fe₃C) layers. Four Fe₃C‑VACNT surfaces with different morphological and structural aspects were developed using plasma carburizing associated with the metal dusting phenomenon. Their tribological performance was assessed by reciprocating dry sliding tests using a ball-on-flat configuration. The combined analysis of the wear tracks through optical and scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Raman spectroscopy indicate that the catalyst particles of the carbon nanotubes are an essential factor controlling the tribosystem. Lubrication and wear depend on the ratio between the nanotubes and their catalyst particle lengths (L_N/L_P). VACNT films with L_N/L_P >20 promoted COF ~0.08 and reduced the wear rate of the ferrous substrate up to 70%. Wear rates showed an exponential reduction by increasing L_N/L_P. However, the VACNT film with L_N/L_P <10 induced severe abrasive wear since the beginning of the tribotests, leading to worse tribological performance than a reference surface composed of a single Fe₃C layer. Furthermore, the initial crystallinity of the VACNT films did not affect the solid lubrication or wear resistance of the surfaces.