Yucel Gencer

Microarc oxidation discharge types and bio properties of the coating synthesized on zirconium

This study is an attempt for gaining a better understanding on relationship between microarc oxidation (MAO) coating discharge types and bioactivity of an oxide-based coating synthesized on a Zr substrate. The discharge types and the coating growth mechanism were identified by the examination of the real cross-section image of the coating microstructure. The coating was conducted by using MAO in an electrolyte containing Na2SiO3, Ca(CH3COO)2 and C3H7Na2O6P, for different durations of 2.5, 5, 15, and 30mins. The effect of the process duration on the different discharge model types (Type-A, B, and C) and bioactivity of the coatings were investigated by using X-ray Diffractometry (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy-Energy-Dispersive X-ray spectroscopy measurements (SEM-EDS) and Optical Surface Profilometry (OSP). It was found that the increasing MAO duration resulted in thicker and rougher coatings. The XRD data revealed that all the samples prepared at different process durations contained the t-ZrO2 (tetragonal zirconia) phase. During the MAO process, non-crystalline hydroxyapatite (HA) formed, which was confirmed from the FTIR data. The surface morphology, the amount and distribution of the features of the coating surface were modified by increasing voltage. The simulated body fluid (SBF) tests showed that the more bioactive surface with more HA crystals formed owing to chemical composition and high surface roughness of the coating. The pore, crack and discharge structures played a key role in apatite nucleation and growth, and provided ingrowth of apatite into discharge channels on the coating surface.

Characterisation of boride layer formed on Fe–Mo binary alloys

The influence of molybdenum on the boronising behaviour of iron (with addition of 1, 2, 4, 8 and 16 wt-% Mo) was investigated by performing pack boronising at 1100°C for 3 h. The borided samples were characterised using SEM–EDS, EPMA, XRD, microhardness tester and profilometer. FeB and Fe2B polyphase exists in all boronised pure Fe and Fe–Mo alloys. Mo2FeB2 precipitates were detected in the boride layer and transition zone. The increasing content of Mo in Fe–Mo alloys resulted in a decrease in the boride layer thickness. The average microhardness value of boride layers formed on pure Fe and Fe–16Mo alloys were measured as 1750 and 2050 HV, respectively, while approximate microhardness value of Mo2FeB2 precipitates was measured as 2600 HV. It was also observed that the increasing Mo content in Fe–Mo alloys resulted in an increment in the surface roughness from approximately 0.8 to 2.7 μm.

The Effect of Sodium Silicate Concentration on the Properties of the Coating Formed on Pure Zirconium by Microarc Oxidation Coating Technique

In this study, zirconium oxide coatings were formed on pure zirconium by microarc oxidation technique with the electrolytes containing KOH and different amounts of sodium silicate (0-40 gr/lt) for the same coating duration of 2 hours. The microstructure, surface roughness, phase content and chemical composition of the coatings were characterized by using scanning electron microscopy, profilometery and X-ray diffractometry. It was found that the coatings on surface of zirconium consist of monoclinic ZrO2 (m-ZrO2) and tetragonal ZrO2 (t-ZrO2) phases and the addition and increasing sodium silicate concentration in the electrolyte increases amount of t-ZrO2 phase. The coatings were well adhered to Zr substrate with some cracks and porosities in the coating for all concentrations of sodium silicate. The coating thickness and surface roughness increased with sodium silicate concentration in the electrolyte. A glaze like Si rich structure and its increase with Si rate was evident in the outermost region of the coating.