Structure–Barrier–Release Relationships of Low-Temperature ALD ZrO2 Thin Films on Co–Cr Alloys: Reducing Long-Term Co and Cr Ion Release in Artificial Saliva

Co–Cr alloys are widely used in biomedical devices owing to their excellent mechanical properties and corrosion resistance; however, sustained metal-ion release in physiological environments remains an important materials concern. Low-temperature atomic layer deposition (ALD) enables the formation of conformal ceramic barriers that can restrict long-term metal-ion transport. In this study, nominally ~50 nm ALD-derived ZrO2 coatings were deposited on Co–Cr substrates manufactured by investment casting (CAST), CAD/CAM milling (MILLED), and laser powder bed fusion (PRINTED). The coatings were characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy, transmission electron microscopy with selected-area electron diffraction (TEM/SAED), and X-ray photoelectron spectroscopy (XPS), while the electrochemical response was evaluated by open-circuit potential (OCP) and electrochemical impedance spectroscopy (EIS). Long-term barrier performance was quantified by inductively coupled plasma atomic emission spectroscopy (ICP-AES) as cumulative Co and Cr release for all manufacturing routes and Mo release for the Mo-containing CAST and PRINTED alloys after 28 days in artificial saliva at 37 °C. ZrO2 deposition reduced cumulative Co and Cr release for all manufacturing routes. Mo release was also reduced in the Mo-containing alloys, with Mo remaining below the analytical detection limit (<0.005 µg cm−2) for the PRINTED + ZrO2 system. Electrochemical measurements further demonstrated manufacturing-route-dependent differences in barrier response between the initial and 28-day exposure states. Overall, the findings establish a structure–barrier–release framework linking coating formation and electrochemical response with cumulative ion release, and identify 28-day cumulative ion release as a practical functional descriptor for comparing the long-term performance of ALD-derived ceramic barriers on Co–Cr alloys.