HYDROXYAPATITE POWDER AND FILMS ON METALLIC ALLOYS WITH CERAMIC BUFFER LAYERS

Abstract

Hydroxyapatite (HA) is similar to the mineral part of human calcified tissues and therefore has a wide range of applications like treatment of bone defects in the form of powder; and as coatings on metallic implants for enhanced osseointegration. The synthesis of HA crystals with dimensions close to those found in natural tissues has been researched in recent years to produce highly biocompatible synthetic products. Similarly, bioactive coatings with adequate roughness, enhanced wear and corrosion resistance, good adhesion and resistant to the diffusion of harmful metallic ions are desirable. The present work comprises synthesis and characterization of nanostructured HA powder by wet precipitation route with and without surfactant and then employing this powder for preparing thin films on metallic substrates with buffer layers based on alumina and calcium zirconate. The buffer layers were synthesized by plasma spraying while HA top layer was produced by three techniques: namely, dipping method, pulsed laser deposition and biomimetic growth. The intention was to develop desirable (mentioned above) features in the powder and coatings. The buffer layers of bioinert ceramics (alumina and calcium zirconate) may be useful to stop the diffusion of harmful metallic ions from the implants and a thin HA top layer is considered helpful to induce early bone apposition. The HA crystals prepared with surfactant were finer, more pure and possessed higher surface area. The employment of 60 mM surfactant concentration produced 10–25 nm diameter and 75–100 nm long HA crystals which are close to the ones found in natural calcified tissues. The thermal treatment of HA nano crystals at 900 °C reduced the surface area considerably and increased the degree of crystallinity. Alumina coatings were deposited on Ti6Al4V alloy at different process variables for the purpose of obtaining maximum γ-Al2O3 phase in the as sprayed coating and to understand their effects on coating microstructure, average roughness, adhesion and tribology. In this regard, the coating sprayed at a higher stand-off distance had more γAl2O3 phase and high bonding strength due to reduced coating thickness but its wear resistance was found poor. The bonding strength between alumina and Ti6Al4V alloy was greater than 16 MPa. The rough alumina surface was used to deposit thin HA films by dip coating and pulsed laser deposition techniques. It was observed that HA film enveloped completely the troughs and crests of the underneath rough alumina surface in both methods but with varied morphologies. The presence of HA was corroborated by XRD and Raman spectroscopy in both films. The XRD analysis also indicated low crystallinity and limited thickness of the films. Calcium zirconate (CaZrO3) was deposited on stainless steel (316L) at arc currents of 400, 500 and 600 A. It was found that the arc current had a profound effect on thickness, microstructure, phase evolution, crystallinity and wear behavior of the coatings. The detailed XRD analysis of (002) and (200) reflections regarding the ferroelectric CaZrO3 phase indicated that the coatings were in the textured state. The bone like HA was obtained on CaZrO3-coated stainless steel samples by a novel evaporation based biomimetic method as evidenced by SEM, XRD, EDS and Raman analysis.