Effect of standoff distance on the mechanical and biological behaviors of seashell-derived hydroxyapatite coatings deposited on Ti-alloy plates by flame spraying

Abstract

This work investigates the effect of standoff distance (SOD) on the mechanical and biological properties of hydroxyapatite (HA) coating deposited on Ti-6Al-4 V substrates by spraying seashell-derived HA powder using flame spraying. The results are compared with the coating obtained by spraying phase-pure commercial HA powder. The SOD varies from 80 to 120 mm to obtain successful coatings, which are characterized by microstructure, phase composition, surface roughness (Ra), porosity, adhesion strength, fracture toughness, wear resistance, and in-vitro biocompatibility in simulated body fluid (SBF). The stable and metastable HAs, with TCP phase, are formed in the synthesized coatings, whereas only the stable HA phase is formed in commercial HA. The lower SOD of 80 mm has enabled the deposition of well-spread splats, which have reduced the surface roughness and porosity of the coating, thereby improving coating adhesion. It also shows the highest fracture toughness, hardness, and wear resistance among all synthesized HA. However, commercial HA with high crystallinity shows better tribo-mechanical properties than the synthesized HA. In contrast, the biocompatibility of seashell-derived HA is much higher than that of commercial HA. Coating porosity and metastable/TCP phases play crucial roles in apatite growth. Therefore, seashell-derived HA from SOD of 120 mm shows remarkably high biocompatibility with fast apatite growth. For improved mechanical properties, lower SOD may be preferred for HA deposition, whereas for enhanced biocompatibility, higher SOD should be chosen. Commercial HA may be a great choice for mechanical applications, but seashell-derived HA can be a better choice for biological applications. © The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2026.