Study on the Microstructure and Properties of Plasma-Sprayed HA/ZrO2/Nb Composite Coatings with Varying Nb Contents

Magnesium alloys are promising biodegradable implants, but rapid degradation and insufficient early-stage bioactivity limit their clinical use. In this study, hydroxyapatite (HA)/zirconia (ZrO2)/niobium (Nb) composite coatings with varying Nb contents (10, 20, and 30%) were deposited on ZK60 magnesium alloys via atmospheric plasma spraying. The coatings retained feedstock phases while forming tetracalcium phosphate, tricalcium phosphate, and Nb2O5 during spraying. Microstructurally, increasing Nb to 30% induced an interconnected porous network, resulting in increased surface microhardness (~426 HV) and hydrophilicity (contact angle ~50°). Electrochemical tests revealed that, among the investigated compositions, the 20% Nb coating exhibited the lowest corrosion current density (Icorr = 9.688 × 10−5 A/cm2), indicating the most favorable electrochemical behavior within the tested Nb-content range. Although further increasing Nb to 30% resulted in a higher Icorr than that of the 20% Nb coating, H1Z3N exhibited the highest day-5 OD450 value and favorable cell spreading. This cellular response may be associated with its higher surface roughness, enhanced wettability, and porous architecture. Overall, the investigated compositions exhibited an observed trade-off between electrochemical behavior and cytocompatibility.