Plasma-Enhanced Graphene Coatings on Ti-6Al-4V: Insights from Non-Destructive Characterization

In this work the deposition of graphene coatings on substrates of an ELI grade Ti-6Al-4V alloy was carried out using the Plasma Enhanced Chemical Vapor Deposition (PECVD) technique. The purpose of this study was to improve the surface properties of the material. The characterization of the material was carried out by non-destructive techniques, such as Raman Spectroscopy and Thermoelectric Potential. A preliminary characterization of Ti substrates was carried out by Raman spectroscopy. Conversely, thermoelectric potential tests were conducted using three distinct tip systems and four different temperature gradients. Lastly, some surface roughness measurements were conducted on all samples, both coated and uncoated. Graphene micro-structured coatings were obtained using a plasma activated mixture of hydrogen and methane gases with equimolar feed ratio (1:1 H₂:CH₄) at a temperature of 850°C and a plasma exposure of 150 Watts and duration of 15 minutes. Raman spectra verified the presence of uniform micrometric graphene on the surface of Ti substrates. Graphene-coated Ti-6Al-4V ELI substrates exhibited absolute thermoelectric potentials between 1.9 and 3.4 μV/K, indicating metallic-like behavior and suitability for thermoelectric sensing. In the eddy current analyses, it was found that the 50 kHz frequency provided the highest sensitivity for differentiating between samples. An inverse relationship was identified between substrate thickness and phase angle, and a direct relationship with calculated electrical conductivity. This direct relation is attributed to penetration depth and interactions due to the chemical nature of the substrate and coating. Despite a slight increase in surface roughness after graphene deposition, values remained comparable to the base alloy, preserving compatibility for biomedical integration. The Au-Cu testing system demonstrated enhanced sensitivity to Seebeck coefficient variations, suggesting its promise for high-resolution thermal sensing. These results support potential applications in implantable or wearable temperature sensors, energy harvesting devices, and smart biomedical interfaces. The thickness of the graphene coating was also characterized by SEM, which showed that the films deposited by PECVD are about 1 micron thick.