Dissolution Behavior Of Coatings Research Articles

Preparation and analysis of chemically gradient functional bioceramic coating formed by pulsed laser deposition

Bioactive ceramic coatings based on calcium phosphates yield better functionality in the human body for a variety of metallic implant devices including orthopaedic and dental prostheses. In the present study chemically and hence functionally gradient bioceramic coating was obtained by pulsed laser deposition method. Calcium phosphate bioactive ceramic coatings based on hydroxyapatite (HA) and tricalcium phosphate (TCP) were deposited over titanium substrate to produce gradation in physico-chemical characteristics and in vitro dissolution behaviour. Sintered targets of HA and α-TCP were deposited in a multi target laser deposition system. The obtained deposits were characterized by X-ray diffraction, fourier transform infrared spectroscopy, scanning electron microscopy and energy dispersive X-ray analysis. Inductively coupled plasma spectroscopy was used to estimate the in vitro dissolution behaviour of coatings. The variation in mechanical property of the gradient layer was evaluated through scratch test and micro-indentation hardness. The bioactivity was examined in vitro with respect to the ability of HA layer to form on the surface as a result of contact with simulated body fluid. It could be inferred that chemically gradient functional bioceramic coating can be produced by laser deposition of multiple sintered targets with variable chemical composition.

Dissolution and mineralization of plasma-sprayed wollastonite coatings with different crystallinity

Wollastonite coatings were prepared by plasma spraying and their behaviour during heat treatment was investigated. The phase compositions and the surface morphologies of these coatings were examined by XRD and SEM. While the as-sprayed coating contained plentiful amorphous phase, the heat-treated coating showed high crystallinity. The dissolution behavior of coatings was evaluated by immersion in TRIS-buffer solution. The surface and cross-section morphologies of the coatings after immersion were observed. The results indicated that the preferential dissolution of the amorphous phase led to the disintegration of the coating. In contrast, the heat-treated coating exhibited lower solubility and remained integer. After immersion, pH changes and the concentrations of Ca and Si in the solutions were also measured. It was found that the wollastonite dissolved incongruently, whereby Ca was released at a higher rate than Si, resulting in the presence of a reactive layer enriched in silica at the coating surface. The difference in bioconductivity of wollastonite coatings with different crystallinity was investigated by immersion in simulated body fluid (SBF) solution. While apatite granules appeared on the as-sprayed coating surface after 1 day of immersion, they could not be found at the surface of the heat-treated coating.

Dissolution behavior of plasma-sprayed hydroxyapatite coatings.

The long-term stability of plasma-sprayed hydroxyapatite coatings is influenced by the dissolution behavior of the coating in in vivo conditions. Plasma-spraying generates a mixture of phases and this study has focused on how the balance of phases affects the in vitro dissolution behavior of the coatings in double distilled-deionized water and in tris-buffer solutions. The pH changes in double distilled-deionized water were monitored, whilst the pH value was maintained at 7.25 for the tris-buffer solution at 37 degrees C with 5% CO2 atmosphere. The phosphate and calcium ions released were measured using UV-Visible Spectrophotometer and Atomic Absorption Spectroscopy respectively. Changes in crystal and surface topology were also studied. The results indicate that the dissolution behavior of the coatings depends on several factors. The rate of release of phosphate ions was found to increase significantly for the tris-buffer solution compared to the deionized water, indicating that the presence of electrolyte constituents affects the dissolution behavior of the coatings. The Ca/P ratio in the tris-buffer solution is approximately three. Increases in the level of crystallinity of the coatings significantly decreased the dissolution rate and hence, the amount of phosphate ions released. The higher the percentage of crystallinity, the higher the stability of the coating under in vitro conditions.