Papers by Author: N. Vouroutzis

Abstract: A study of the structural and mechanical properties of nanocrystalline TiAlSiN gradient coatings deposited by cathodic arc deposition techniques at 500 °C and post-annealed at 525 °C is presented. Analysis of the coatings, chemical composition and microstructure revealed that the coatings have a structure based on (Ti, Al)N nanocrystals with an average size of 10 nm embedded in an amorphous Si₃N₄ phase. The study of the mechanical properties showed that post-annealing causes improvement and increase of the coatings hardness. A maximum hardness of 48 GPa and elastic modulus of 560 GPa were measured. Also, excellent adhesion to the WC-Co substrate was observed in the post-annealed coatings.

Abstract: High strength of Yttria stabilized tetragonal zirconia polycrystal (Y-TZP) zirconia ceramics are sensitive to low temperature degradation (LTD) that leads to exaggerated tetragonal to monoclinic transformation and a surface to depth propagating degradation that diminishes their mechanical properties. In vitro tests for accelerating ageing have been proposed for the prediction of zirconia ceramics clinical performance. The aim of the present work was to investigate the in vitro ageing of a cold isostatic-pressed zirconia ceramic for all ceramic restorations. Bar-shaped specimens milled from a zirconia block (Ivoclar IPS e.max ZirCAD) were sintered to full density, mirror-polished and cut into two equal pieces. One piece was used as control while the other was subsequently aged (steam 134°C / 2 bars / 10 hours). Atomic Force Microscopy (AFM) was used to evaluate the surface profile and the micro-structural features before and after ageing. Fourier Transform Infrared Spectroscopy (FTIR) and X-Ray Diffraction Analysis (XRD) were used to determine the degree of the m-ZrO₂ transformation. Ageing resulted in an increase of the surface roughness, while the formation of monoclinic spots on the surface of the specimens was verified by surface uplifts in the AFM images. The peaks of the m-phase were clearly observed in the FTIR spectra while an average increase of 16% w.t. of the m-ZrO₂ phase was recorded by XRD. Although, no sound lifetime predictions can be made from accelerated tests, based on the ISO standard that imposes that the m-phase should not exceed 25% wt after 5h at 134 °C and 2 bar pressure , it can be concluded that the tested ceramic resisted an extreme transformation that could negatively affect its clinical performance.