Atmospheric pressure plasma jet deposition of nanostructured hydroxyapatite coatings with thickness dependent behavior

Hydroxyapatite (HAp) is used as a bioactive coating because of its similarity to bone mineral, but conventional deposition methods struggle to control thickness, crystallinity, and adhesion. In this work, nanostructured HAp coatings are deposited from an aqueous suspension using an atmospheric-pressure plasma jet (APPJ), in which an ultrasonic nebulizer injects the mist into the plasma, activating the stainless-steel substrate and promoting particle self-assembly. Continuous coatings with bone-like Ca–P–O composition and elongated HAp nanocrystals (~120 × 20–25 nm) are obtained, and the thickness increases linearly from ~3 to ~9 μm with deposition time. XRD, Raman, and XPS confirm preservation of the hydroxyapatite phase without detectable secondary calcium phosphates, while contact angles (~27°) indicate a hydrophilic surface. Tribological tests show that thin (~3 μm) coatings act as adherent anti-wear layers under 1 N load, whereas thicker (~6–9 μm) coatings undergo controlled delamination and form a third-body HAp film that lowers the coefficient of friction at 2 N. APPJ enabled low-temperature fabrication of HAp coatings with thickness-dependent tribological behavior, relevant for protective and biomedical surfaces requiring controlled wear and bone-like chemistry. • APPJ deposits hydroxyapatite coatings from aqueous nanoparticle suspensions. • Coating thickness is tuned linearly from ~3 to ~9 μm by plasma deposition time. • APPJ HAp coatings preserve bone-like Ca–P–O and nanocrystalline morphology. • Thin (~3 μm) coatings act as adherent anti-wear layers under 1 N load on steel. • Thicker (~6–9 μm) coatings delaminate controllably and have lower friction.