Production of titanium-based coatings by arc spray method and its characterization

Implants are biomaterials designed to restore, support, or reconstruct the function of damaged or lost tissues within the human body. These materials are continuously exposed to physiological fluids, and their biocompatibility depends strongly on their surface characteristics and corrosion behavior under such conditions. Titanium (Ti) and its alloys have been widely used as implant materials due to their high corrosion resistance, mechanical strength, and excellent biocompatibility. However, under certain physiological conditions, Ti-based materials may release metal ions, which can trigger allergic or inflammatory responses. Therefore, improving the surface biocompatibility of metallic implants through coating technologies has become a major area of research. In this context, the present study focuses on the production and characterization of titanium-based coatings on AISI 316L austenitic stainless steel substrates using the Electric Arc Spray (EAS) process. The motivation was to produce coatings with minimal oxidation and metallic character, thereby enhancing the surface properties of stainless steel for potential biomedical applications. The process parameters — including arc current (150–200 A), voltage (25–35 V), nozzle type (HV and Focus), atomization gas type (Air, Ar, N₂, Ar+5%H₂), and gas pressure — were systematically varied to investigate their effects on coating structure and performance. Prior to spraying, all substrates were mechanically polished, degreased with ethanol, and grit-blasted to ensure strong mechanical anchoring. The spraying operations were carried out using a Sulzer Metco SmartArc system equipped with a programmable KUKA robotic arm to maintain a constant spray distance of 120 mm and uniform deposition paths. The feedstock material was commercially pure titanium (Grade 2) wire. The produced coatings were characterized comprehensively to determine the relationships between process parameters, microstructure, and resulting properties. X-ray diffraction (XRD) analysis was used to identify the phase composition, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were used to evaluate the microstructure, lamellar stacking, and chemical distribution. Vickers microhardness and adhesion strength (ASTM C633) were measured to assess mechanical performance. Ball-on-disk wear tests (ASTM G99) were performed to study the tribological behavior under a 5 N load and a 1000 m sliding distance. The findings revealed that microstructure, phase formation, and mechanical properties were strongly influenced by the arc power and gas atmosphere. • In argon atmospheres, metallic Ti phases were preserved, resulting in ductile and dense coatings with moderate hardness (~780 HV0.3). • In air, oxidation reactions dominated, forming TiOₓ phases that increased hardness (~1430 HV0.3) but also induced brittleness. • In nitrogen atmospheres, TiN-rich coatings were obtained, exhibiting the highest hardness (~1740 HV0.3) and superior wear resistance, though with limited ductility due to the brittle nature of nitride phases. It was also observed that increasing atomization gas pressure enhanced coating density and reduced porosity, improving mechanical integrity. However, excessive reactivity (in air or nitrogen) promoted oxide/nitride formation, which limited adhesion strength. Nozzle geometry played a decisive role in controlling particle velocity and deposition efficiency, which in turn affected coating density, hardness, and adhesion strength. • The HV nozzle provided higher particle velocities and denser lamella structures, resulting in higher hardness and better adhesion. • The Focus nozzle, on the other hand, yielded smoother surfaces and more homogeneous lamella stacking but slightly lower hardness. Arc current and voltage significantly affected the melting behavior and deposition efficiency. Lower settings (≤175 A, ≤30 V) caused incomplete melting and higher porosity, whereas higher settings (200 A, 35 V) promoted full melting, yielding denser and better-bonded coatings. However, excessive heat input in reactive atmospheres led to increased oxidation, requiring a balance between thermal energy and gas selection. The adhesion strength of the coatings was strongly influenced by both the atomization gas type and the resulting microstructural characteristics. • The highest apparent adhesion strength (47 MPa) was measured for coatings produced in air atmosphere using a Focus nozzle (35 V / 200 A). However, this elevated value was attributed not to improved metallurgical or mechanical bonding, but rather to excessive adhesive penetration into the porous coating structure during the ASTM C633 test, leading to an artificially increased adhesion strength measurement. • Coatings produced in argon showed moderate adhesion (~34 MPa), while those deposited under Ar+5%H₂ or N₂ atmospheres showed reduced bonding (15–23 MPa) due to increased brittleness and interface porosity. The wear behavior of coatings strongly correlated with hardness and phase content. Argon-sprayed coatings exhibited wider wear tracks (~931 µm) and ductile deformation, consistent with metallic Ti. Air-sprayed coatings, containing TiOₓ phases, showed narrower tracks (~715 µm) and improved wear resistance due to the protective nature of oxides. Nitrogen-sprayed coatings, dominated by TiN, exhibited the best wear resistance, with a narrow wear scar (~540 µm) and minimal debris formation. The wear mechanisms were primarily abrasive and three-body, consistent with Archard's wear model, where increased hardness led to improved wear resistance. Overall, the study demonstrated that the electric arc spray process parameters have a critical influence on coating quality, microstructure, and mechanical response. The gas atmosphere and nozzle type were identified as the most dominant factors controlling phase evolution, porosity, and adhesion behavior. In conclusion, the Electric Arc Spray (EAS) process has been identified as a low-cost, industrially scalable, and efficient coating method for producing titanium-based surfaces on stainless steel substrates. Although coatings produced in air and nitrogen atmospheres exhibited high hardness and wear resistance, their brittle nature limits direct biomedical use. Nevertheless, through proper parameter optimization and gas control, EAS offers a promising route for producing dense, adherent, and biocompatible Ti coatings, potentially extending its applicability to biomedical implant manufacturing.