Yunus Azakli

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.

Microstructural characterisation of borided binary Fe–W alloys

ABSTRACT Binary Fe–W alloys with 1, 2, 4, 8, 12 and 16 at.-% W additions were prepared to investigate the effect of tungsten on the boronising behaviour by performing pack boronising at 1100°C for 3 h. The borided samples were characterised using SEM, SEM-EPMA, X-ray diffraction, microhardness tester and profilometer. The formation of the FeW2B2 and FeWB precipitates in the main boride layer, the transition zone was verified, and their number and size increased in the main boride layer and transition zone with increasing amount of W in Fe–W binary alloys. The cross-sectional etching with strong acid solution and SEM examination of borided samples revealed the 3D morphology of the FeWB boride crystals with various shapes in the transition zone. The thickness of the boride layer decreased while the average microhardness values of the main boride layer and transition region increased with increasing W contents in the Fe–W binary alloys.