Influence of HVOF parameters on HAp coating generation: An integrated approach using process maps

TL;DR

This study investigates how HVOF process parameters affect hydroxyapatite coatings, optimizing their properties for biocompatibility through experimental design, process mapping, and biological testing.

Contribution

It introduces an integrated approach combining process maps and experimental design to optimize HAp coating parameters for improved biocompatibility and stability.

Findings

F/O ratio significantly influences coating temperature and velocity.

Optimized parameters yield HAp coatings with >45% crystallinity and ~1.67 Ca/P ratio.

Coatings promote apatite formation and are non-toxic to human osteoblasts.

Abstract

An experimental approach based on design of experiments, process maps and the analysis of deposition first stages to improve the biocompatibility of High-Velocity Oxygen Fuel (HVOF) hydroxyapatite (HAp) coatings is here presented. A two-level design of three factors (23) was performed using the stand-off distance (SOD), the fuel-oxygen ratio (F/O) and the powder feed rate (PFR). The effect of these experimental factors on the first stages of the coating formation was investigated to study the physical state of the particles before and after impacting the substrate. This study allowed the selection of the most suitable deposition parameter combinations to obtain HAp coatings with optimal crystallinity (> 45%), Ca/P ratio (approx. 1.67), phase content (> 95% of HAp), which guarantee the coatings mechanical stability and biocompatibility. The behavior of the coating within simulated body fluid (SBF) and cell culture (hFOB) was studied to analyze the apatite layer formation and the extracts cytotoxicity on human osteoblasts, respectively. The results show that the F/O ratio is the most influential factor on temperature and velocity on the in-flight particles and therefore on the coating properties. The SBF results confirmed the formation of an apatite layer after 14 days of immersion. Finally, the mitochondrial activity, measured by the MTS assay, and cell membrane integrity measures by LDH liberation assays, show that the coating released material does not induce toxicity on the exposed cells.