Investigating Carbon Coating on Ni-Invar and Ti–6Al–4V Surfaces for Low Friction Performance

This study explored a sustainable method for depositing carbon-based coatings on Ni-Invar and Ti-6Al-4V metallic substrates using a pack carburization process with cyanide-rich <i>Manihot esculenta</i> (cassava) leaves as the carbon source. Comprehensive experimental and computational analyses were performed to investigate the composition, structure, and tribological performance of the coatings. Characterization using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, scanning electron microscopy (SEM), and lateral force microscopy (LFM) revealed that the Ni-Invar substrate developed turbostratic multilayer graphene coatings with minimal carbide formation, achieving ultralow coefficient of friction (COF) values of 0.08 (macroscale) and 0.0033 (nanoscale). In contrast, Ti-6Al-4V substrates formed coatings rich in titanium carbide and metal carbonates with more disordered carbon structures, resulting in higher COF values of 0.1 and 0.0147, respectively. Monte Carlo simulations illustrated an island growth mechanism on Ni-Invar driven by dominant carbon-carbon interactions, while Ti-6Al-4V exhibited uniform layer growth due to stronger carbon-substrate affinity. Density Functional Theory (DFT) calculations provided further insight, showing that Ti had a strong affinity for graphene, with a binding energy of -1.1440 × 10³ eV, whereas Ni and Fe exhibited stronger self-affinity (-1.6250 × 10³ and -1.7190 × 10³ eV, respectively), favoring metal-metal bonding over carbon-metal bonding. These integrated experimental and numerical analyses offer detailed insights into carbon coating structures and their growth mechanisms. The results underscore the critical role of substrate chemistry in determining coating morphology and tribological behavior, highlighting Ni-Invar as a promising candidate for achieving superlubricity through carbon-based surface engineering.