Advanced hydroxyapatite- DLC bilayer coatings for improved performance of NiTi biomedical alloys

Enhancing the performance and biocompatibility of NiTi alloys is crucial for advancing biomedical implant technology. This study aimed to develop and evaluate a novel dual-layer composite coating comprising diamond-like carbon (DLC) and hydroxyapatite (HA) on NiTi alloy, focusing on corrosion resistance, biocompatibility, and antibacterial properties. We applied the coating using plasma immersion ion implantation and deposition (PIII&D) combined with the sol-gel technique, creating a 1.65 μm thick DLC layer and an 800 nm thick HA layer. Corrosion tests were conducted in simulated body fluid (SBF) at 37 °C and 40 °C for 120 h, with electrochemical measurements assessing corrosion potential (Ecorr) and current density (icorr). Biocompatibility was evaluated through nickel ion leaching tests, and antibacterial efficacy was determined using E. coli. Results showed remarkable improvements in corrosion resistance compared to single-layer DLC coating, with Ecorr increasing by 36 mV and icorr decreasing by 0.15 μA. cm⁻² at 37 °C. At 40 °C, Ecorr improved by 298 mV and icorr reduced by 0.17 μA. cm⁻². The enhanced protection was attributed to calcium-phosphate precipitation, forming a protective layer on the HA surface. Nickel ion leaching decreased by 47 %, and antibacterial activity against E. coli improved by 39 % compared to the uncoated NiTi substrate. These findings suggest that the hydroxyapatite-DLC composite coating offers a promising multifunctional solution for enhancing NiTi biomedical implants, potentially extending implant longevity by up to 30 % and significantly improving patient outcomes. © 2025 Elsevier Ltd and Techna Group S.r.l.