Synthesis and Tribological Assessment of Carbon Nanostructure Coatings Directly Grown on AISI 1018 Low Carbon Steel Substrates

This study explores the direct synthesis of carbon nanostructures on AISI 1018 low-carbon steel via spray pyrolysis, using α-pinene—a turpentine-derived terpene—as a sustainable carbon source. Two synthesis routes were evaluated: a catalyst-aided approach using ferrocene and a catalyst-free approach. Both methods yielded homogeneous coatings; however, the addition of ferrocene produced carbon nanotube (CNT) films (~70 μm), while the catalyst-free method resulted in carbon nanofiber (CNF) films (~50 μm). Tribological testing revealed that CNF coatings maintained a consistently low friction coefficient of ~0.12. In contrast, CNT coatings exhibited higher friction, increasing from 0.15 to 0.35 under loads of 2–5 N. SEM and Raman spectroscopy of the wear tracks suggest that CNFs retain their crystalline structure during friction, whereas CNTs become increasingly defective, leading to higher friction levels.