RESUMO
The effect of a boronizing and siliciding process on CoCrFeNiHf0.1-0.42 high entropy alloys was examined in this study. When increasing the amount of added Hf in CoCrFeNiHfx, the structure of the alloys gradually transformed from single-phase FCC to firstly Ni7Hf2 + FCC, and finally to C15 Laves and FCC phases. The boronizing/siliciding process resulted in the formation of a silicon-rich layer and a boride layer (BL). Increasing the amount of Hf in the alloys resulted in a decrease in the combined layer thickness, which was measured for CoCrFeNi, CoCrFeNiHf0.1, CoCrFeNiHf0.2, and CoCrFeNiHf0.42 to be 70 µm, 63 µm, 20 µm, and 15 µm, respectively. In contrast, the thickness of the transition zone/diffusion zone increased with more Hf in the alloys. While silicon atoms were gathered close to the BL, they were not transferred into the CoCrFeNi substrate. In contrast to the observation for CoCrFeNi, Si atoms penetrated through the Ni-rich phase (Ni7Hf2) in the CoCrFeNiHfx alloys. Furthermore, the Cr-B rich area (Cr5B3) in the coating limited the transport of Si into the CoCrFeNiHfx substrates. XRD analysis showed that the BL contained Ni2Si, FeB, Fe2B, Co2B, and Cr5B3 phases.
RESUMO
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.
