Structure Of Hydroxyapatite Coatings Research Articles

Структура и фазовый состав плазменных покрытий гидроксиапатита

Changes in the structure, phase composition of the hydroxyapatite (HA) plasma coating were established during deposition at an initial 20 °C and 550 °C temperature of the titanium substrate and a spraying distance of 95 and 150 mm. The structure of HA coatings was analyzed by SEM and optical microscopy. DSC analysis established the transition temperatures of HA coatings to the equilibrium state. When deposited on a substrate with an initial temperature of 20 °C at a deposition distance of 95 mm, a nanostructure with a crystallite size of 21 nm is fixed in the HA coating. With an increase in the deposition distance to 150 mm, the non-equilibrium phase composition increases, the crystallite size decreases to 12 nm, the HA content decreases from 72 to 61 %, TTCP from 10 to 5 %, and the α-TCP content increases from 17 to 30 %. A non-equilibrium nano-structural state passes into a more equilibrium state with the release of heat at temperatures of 615 – 727 °C in DSC studies. The high-temperature α-TCP phase is not fixed when the coating is deposited onto a substrate with an initial temperature of 550 °C at a spraying distance of 95 mm, the content of TTCP increases by 2 times, the size of the HA phase crystallites reaches 36 nm and their size in the sprayed powder is 75 nm. The HA coating has a dendritic microstructure and does not have a thermal effect during DSC heating at an initial substrate temperature of 550 °C.

Effects of ultrasonic treatment and current density on the properties of hydroxyapatite coating via electrodeposition and its in vitro biomineralization behavior.

In this study, ultrasonic-assisted electrodeposition is used to prepare uniform and smooth hydroxyapatite (HA) coating on the surface of the braids, which can be potentially applied as biodegradable bone scaffolds. During the electro-deposition process, the ultrasonic power (0 W and 20 W) and current density (2.5, 5.0, and 7.5 mA/cm2) are altered in order to evaluate their influences on the properties of HA coating over the braid surface. The field emission scanning electron microscope (FE-SEM) images show that the crystals growth stages of HA include spherical particles, plate-like and needle-like crystals when electro-deposited for 15, 30, 45, and 60 min. In particular, when electro-deposited for 60 min, the needle-like structure of HA coatings shows a significant increase in density as a result of increasing the current density or the employment of ultrasonic treatment. Transmission electron microscope (TEM) results show that when the current density increases from 2.5 mA/cm2 to 7.5 mA/cm2, the grain size decreases from 413.65 ± 63.12 nm to 264.56 ± 65.33 nm. With the aid of ultrasonic treatment, the HA coating demonstrates a calcium‑phosphorus (Ca/P) ratio that resembles that of the human skeleton being 1.67. The X-ray Diffraction (XRD) results show the highest crystallinity reaches 73.52% at 7.5 mA/cm2-20 W. The cavitation effect of ultrasonic can improve the roughness of HA coating distinctively. Finally, the 7-day immersion into simulated body fluid (SBF) demonstrated a denser and finer apatite precipitation on the HA coating fabricated by ultrasonic-assisted electrodeposition. Therefore, this study provides a feasible ultrasonic-assisted electrodeposition method that yields a good morphology of bioactive substances coating over braid or other rugged deposition substrate for potential degradable bone scaffolds.

Atmospheric Plasma-Sprayed Hydroxyapatite Coatings with (002) Texture

Hydroxyapatite (HA) coatings are being widely used in biomedical applications owing to their excellent biocompatibility and osteoconductivity. Recent studies have demonstrated that the crystallographic texture plays an important role in improving the chemical stability and mechanical properties of HA coatings. In this study, optimized APS parameter was selected to deposit HA coatings with strong (002) crystallographic texture, high phase purity and enhanced melting state. Cross-sectional SEM images show uniformly distributed columnar grains perpendicular to the coating surface. To study the formation conditions of columnar grains, coatings with distinct microstructure were deposited with different spray parameters. Moreover, HA coatings were deposited on substrates with varying temperatures such as 25, 300 and 600 °C at a long stand-off distance to evaluate the role of the substrate temperature in the formation of columnar grains. The results indicate that completely molten in-flight particles and slow cooling rate are necessary conditions to form a strong crystallographic texture. The present study suggests that the crystalline structure of HA coatings deposited and formed by APS could be well controlled by modifying spray parameters and substrate temperature.

The Influence of Spray Parameters on the Characteristics of Hydroxyapatite In-Flight Particles, Splats and Coatings by Micro-plasma Spraying

Hydroxyapatite (HA) is one of the most important bioceramic materials used in medical implants. The structure of HA coatings is closely related to their manufacturing process. In the present study, HA coatings were deposited on Ti-6Al-4V substrate by micro-plasma spraying. Results show that three distinct HA coatings could be obtained by changing the spraying power from 0.5 to 1.0 kW and spraying stand-off distance from 60 to 110 mm: (1) high crystallinity (93.3%) coatings with porous structure, (2) high crystallinity coatings (86%) with columnar structure, (3) higher amorphous calcium phosphate (ACP, 50%) coatings with dense structure. The in-flight particles melting state and splat topography was analyzed to better understand the formation mechanism of three distinct HA coatings. Results show that HA coatings sprayed at low spraying power and short stand-off distance exhibit high crystallinity and porosity is attributed to the presence of partially melted particles. High crystallinity HA coatings with (002) crystallographic texture could be deposited due to the complete melting of the in-flight particles and low cooling rate of the disk shape splats under higher spraying power and shorter SOD. However, splashed shape splats with relative high cooling can be provided by increasing SOD, which leads to the formation of ACP.

Effect of gun current on the microstructure and crystallinity of plasma sprayed hydroxyapatite coatings

Hydroxyapatite (HA) is a bioactive material because its chemical structure is close to the natural bone. Its bioactive properties make it attractive material in biomedical applications. Gas tunnel type plasma spraying (GTPS) technique was employed in the present study to deposit HA coatings on SUS 304 stainless steel substrate. GTPS is composed of two plasma sources: gun which produces internal low power plasma (1.3–8 kW) and vortex which produces the main plasma with high power level (10–40 kW). Controlling the spraying parameters is the key role for spraying high crystalline HA coatings on the metallic implants. In this study, the arc gun current was changed while the vortex arc current was kept constant at 450 A during the spraying process of HA coatings. The objective of this study is to investigate the influence of gun current on the microstructure, phase crystallinity and hardness properties of HA coatings. The surface morphology and microstructure of as-sprayed coatings were examined by scanning electron microscope. The phase structure of HA coatings was investigated by X-ray diffraction analysis. HA coatings sprayed at high gun current (100 A) are dense, and have high hardness. The crystallinity of HA coatings was decreased with the increasing in the gun current. On the other hand, the hardness was slightly decreased and the coatings suffer from some porosity at gun currents 0, 30 and 50 A.