Dingyong He

In vitro evaluation of hydroxyapatite coatings with (002) crystallographic texture deposited by micro-plasma spraying

Hydroxyapatite (HA) coatings are usually deposited on the metallic implant to increase the biocompatibility and protect the bloodstream from harmful metal ions. Atmospheric plasma spray (APS) is known as a cost effective deposition method. However, the low crystallinity of APS deposited coating accelerates its dissolution in body fluid. We used micro-plasma spray (MPS) to develop chemically stable HA coatings, and performed APS as reference. Results showed that MPS deposited coatings exhibited (002) crystallographic texture while the reference sample did not. The texture intensity and crystallinity were improved by shortening stand-off distance. These suggested that different formation procedures of HA coatings were involved and three mechanisms were proposed by sketching typical splats. To evaluate the chemical stability of the coatings in a physiological environment, in vitro experiments were performed in Hanks solution. Well crystallized (~100%) HA coating with the strongest crystallographic texture exhibited superior stability up to 14days. Crystals with (002) orientation was more stable than that with (211) orientation. Hence columnar structure with nanopores emerged on the coating surface after incubation, and this may facilitate the future osteoblast growth. Furthermore, HA coating with weak and no crystallographic texture induced apatite layer. However, vertical cracks and cleavage at coating-apatite interface may cause coating separation.

Microstructure and High Temperature Corrosion Behavior of Wire-Arc Sprayed FeCrSiB Coating

AbstractNowadays, wire-arc spraying is an attractive technique to deposit coatings for oxidation and corrosion protection. In this study, a new FeCrSiB-cored wire was designed to produce protective coatings for the components used in high temperature environment by wire-arc spraying. The microstructure, phase composition, microhardness, and high temperature corrosion behavior of the new coating were investigated in comparison with a FeCr coating and a commercial NiCrTi coating. The results showed that the FeCrSiB coating was composed of dense lamella with much fewer oxide inclusions than the FeCr and NiCrTi coatings. The microhardness of the FeCrSiB coating was the highest of the three coatings. Thermogravimetric analysis was used to evaluate the high temperature corrosion behavior of the coatings in mixed salt Na2SO4-25% K2SO4 environment at 650xa0°C under cyclic condition. It showed that the high temperature corrosion resistance of the FeCrSiB coating is significantly better than that of the FeCr coating and approaching to the commercial NiCrTi coating.n

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 1.0xa0kW and spraying stand-off distance from 60 to 110xa0mm: (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.

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 600xa0°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.