Enhanced reactive plasma species ingesting helium for surface nitriding of 3-D printed Ti-6Al-4V alloy

This study reveals the role of helium seeding in nitrogen plasma to enhance reactive plasma species, particularly nitrogen ions and radicals, which contribute to the nitriding process of additively manufactured Ti-6Al-4V alloy. At first, operating parameters including gas composition (combination of helium and nitrogen), gas fill pressure, and discharge power were analyzed using optical emission spectroscopy (OES). After establishing optimal discharge conditions, the sample of Ti-6Al-4V alloy was exposed to a plasma environment for different treatment times. The phase changes, surface morphology, and elemental composition of the nitride surfaces were then evaluated by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX). The distribution of crystalline phases in the Ti-6Al-4V alloy had an impact on the alloy's chemical and physical characteristics, making it suitable for medical implant applications. The XRD spectrum of the surface nitrided titanium alloy after 15 h of the ion nitriding confirmed the formation of the tetragonal ε-Ti2N phase owing to the reaction of α-Ti and plasma N, apart from the underneath nitrogen-rich α-Ti (hcp) diffusion zone enhancing surface hardness. The EDX results show that longer nitriding time allowed nitrogen ions to diffuse deeper into the lattice, increasing nitrogen content. Finally, the scanning electron microscope (SEM) photographs illustrated the surface morphology and showed a gradual buildup of grainy filling in the empty region of the material, demonstrating that the smoothness and texture of the surface have improved over longer treatment times. The AFM results show the 3-D imaging to mirror the surface morphology of the plasma nitrided samples with significant reduction in the surface roughness. The characteristic bands of the Raman spectrum confirm the formation of the titanium nitride surface layer, whereas Vickers microhardness testing shows a significant increase in the surface hardness for the sample nitrided for 15 h. [Display omitted] •Enhanced reactive plasma species with helium gas seeding in nitrogen plasma•Spectral measurements of electron temperature and density for varying conditions•Plasma surface nitriding of 3D-printed Ti-6Al-4V alloy and surface analysis•Surface investigations of microstructures, texture, roughness, and morphology