This is to certify that the thesis entitled "The study of hydroxyapatite and hydroxyapatite-chitosan composite coatings on stainless steel by electrophoretic deposition method". Hydroxyapatite is one of the most important mineral components of bone, due to which it exhibits excellent biocompatibility. Electrophoretic deposition is a gentle procedure that does not involve extreme temperatures and has the advantage of producing a stoichiometric, uniform coating of the desired thickness.
A coating of HA and composite coating of stoichiometric hydroxyapatite and chitosan were fabricated on medical grade stainless steel (SS 316L). The SEM analysis showed that at a constant voltage, the layer deposition increases with an increase in the layer time, while when the layer time is kept constant, the fineness of the layer increases with an increase in the voltage. The coating processes have the advantage of changing the properties of the surface of the material without affecting the bulk properties.
The alternative to the coating process is to use a highly corrosion resistant alloy, but the use of highly corrosion resistant alloys will increase the cost of the implants making them economically unfeasible. It has the advantage of being able to produce uniform deposits with high microstructural homogeneity; it also provides ample control over the thickness of the deposit. Hydroxyapatite is the main mineral component of bone and therefore hydroxyapatite as a biomaterial shows good biocompatibility and bone adhesion.
In the present study, a coating of HA and a composite coating of hydroxyapatite chitosan were applied to stainless steel substrate by means of electrophoretic deposition method.
LITERATURE SURVEY
Coating metal implants with bioactive HA leads to a rapid bond between the hydroxyapatite and the surrounding bone tissue. These are non-metallic coatings obtained on the metal surface in the form of compounds of the base metals. Physical vapor deposition (PVD) - the process that involves evaporation of the coating material in a vacuum, transport of the vapor to the substrate and condensation of the vapor on the surface of the substrate.
Chemical Vapor Deposition (CVD) - A process in which a coating is formed on a hot substrate surface placed in an atmosphere of a mixture of gases as a result of a chemical reaction or decomposition of the gases on the substrate material. Thermal Sputtering – The deposition of a high temperature sputtered metal delivered to the surface of a substrate in a high velocity gas stream. Electrophoretic deposition is a process in which charged ions in a solution are attracted and deposited on an oppositely charged electrode using an electric field.
There can be two types of electrophoretic deposition processes based on the electrode on which the coating is produced. The process leads to the creation of stoichiometric films whose composition depends on the stoichiometry of the. The main driving force for electrophoretic deposition is the charge on the particle and its electrophoretic mobility in the solvent under the presence of the applied electric field.
The process-related parameters are deposition time, applied voltage, concentration of the solid in suspension and conductivity of the substrate. When stronger electric fields are used, the deposition rate increases, but the quality of the deposition is affected. The rate at which the particles are deposited on the electrodes depends on their electrophoretic mobilities; it becomes an important factor when the volume fraction of the solids is low.
When the volume fraction of the solids is high, they settle equally. However, when the volume fraction of the solids is low, the particles settle at a rate proportional to their individual electrophoretic mobility (Vandeperre, Biest and Clegg). 300 ml of the final solution was prepared by mixing the solutions obtained in the above two steps.
The deposition was carried out at room temperature from 150 ml of the solution (according to the desired coating) in a 250 ml beaker. The SEM analysis of the sample was performed on JEOL JSM 6480 lv to study the microstructure of the coating.
RESULTS AND DISCUSSION
Introduction
- Characteristics of the HA layer on 316L Stainless steel
- Microstructural analysis
- Analysis of pore structure
- Phase Analysis
Figures 4.2 (a), (b) and (c) show scanning electron micrographs of a HA-coated 316L stainless steel sample for three different time periods (10, 15 and 20 minutes, respectively) at 30 volts. The images show that the HA is deposited in a feathery form with good coverage over the entire surface. HA crystal growth is more pronounced in the 20-min sample compared to the 10- and 15-min samples, leading to a better coating on the surface as seen in the images.
This better coverage of the coating in the 20 min treated sample is due to a proper growth of the HA crystals with longer time, which also results in giving a good thick layer on the surface. The average pore size (calculated on the basis of simple mathematical averages) decreases from 316 nm to 253 nm. This is due to the fact that growth of the HA crystals occurs with longer deposition times and the crystals cover the maximum area of the implant surface leaving very small holes.
From the analysis of the XRD data obtained from the scraping of the coating on the sample surface, it was found that the peaks obtained were consistent with the standard peaks of HA in the JCPDS data book. The coating thus formed consists mainly of HA as verified from the X-ray diffraction pattern. No other lower calcium compounds are detected in the data, confirming that the purity of the layer in terms of HA is almost 99.
- Characteristics of the HA-chitosan layer on 316L Stainless steel
- Microstructural analysis
- Analysis of pore structure
- Phase Analysis
From figures 4.6 (a), (b) and (c) it is observed that the roughness of the layer is reduced with increase in stress. This is because at a constant deposition time, with increase in the applied potential difference, there is more nucleation of HA-chitosan composite crystals on the stainless steel surface leading to the formation of finer coatings. With increase in stress, there is a slight decrease in the size of the pores formed.
The coating was found to consist of small hydroxyapatite crystals incorporated into the continuous film of chitosan. From the analysis of the XRD data obtained from the surface of the sample, it was found that the obtained peaks match the standard peaks of HA and chitosan in the JCPDS data book. Therefore we can conclude that neither HA nor chitosan lose their phase purity after deposition and the presence of both in the coating is verified by the XRD pattern.
CONCLUSION
