Thermal sprayed hydroxyapatite splats: nanostructures, pore formation mechanisms and TEM characterization

Area of Science:

• Materials Science
• Biomaterials Engineering
• Surface Science

Background:

• Hydroxyapatite (HA) coatings are crucial for biomedical implants due to their biocompatibility.
• Understanding the microstructure and porosity of HA coatings is essential for optimizing their performance.
• Thermal spraying techniques are widely used for depositing HA coatings, but controlling pore formation remains a challenge.

Purpose of the Study:

• To characterize the microstructure of thermal sprayed HA splats.
• To investigate the formation mechanisms of micropores within HA splats.
• To explore the influence of subsequent splats on pore evolution and overall coating porosity.

Main Methods:

• Utilized high-velocity oxy-fuel and DC plasma spray techniques for HA splat deposition.
• Employed transmission electron microscopy (TEM) for detailed microstructure analysis.
• Conducted in vitro incubation tests in simulated body fluids (SBF) to study pore evolution.

Main Results:

• Identified amorphous calcium phosphate and tricalcium phosphate phases at splat fringes, indicating HA decomposition.
• Classified HA splat pores into open, sealed, and through-thickness categories.
• Determined that particle velocity, melt state, and starting particle porosity are key factors in micropore formation.
• Observed pore-sealing effects from subsequent splats through liquid filling.

Conclusions:

• The microstructure of HA splats is nanostructured at the fringes and contains larger calcium phosphate grains centrally.
• Micropore formation is governed by splatting dynamics (velocity, melt state) and initial particle porosity.
• Subsequent splats can effectively seal open pores, influencing the final coating porosity.
• Overall coating porosity can be controlled by optimizing particle melt state and velocity during thermal spraying.