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    Fracture toughness enhancement of yttria-stabilized tetragonal zirconia polycrystalline ceramics through magnesia-partially stabilized zirconia addition
    (Vietnam Natl Univ, 2020) Söylemez, Bilal; Şener, Ercan; Yurdakul, Arife; Yurdakul, Hilmi
    In this work, for the first time, we report a novel method on the fracture toughness enhancement of 3 mol % yttria-stabilized tetragonal zirconia polycrystalline (3Y-TZP) ceramics through the incorporation of 8 mol % magnesia-partially stabilized zirconia (8 Mg-PSZ) powders having high fracture toughness. Highly densified composites (x3Y-TZP/y8Mg-PSZ; where x and y vary between 0.25 and 0.75 wt. %) were obtained with a relative density over 99% by pressureless sintering. Relative density, Vickers hardness (HV) and indentation fracture toughness (K-Ic) were significantly improved by sintering temperature and dwell-time increment. Specifically, HV and K-Ic values of 0.5(3Y-TZP)/0.5(8 Mg-PSZ) composite sintered at 1500 degrees C-2h were increased by 7% and 30%, respectively, compared to that of 3Y-TZP. Sintered bodies consisted of c-ZrO2, t-ZrO2 and m-ZrO2 phases without any new phase formation. m-ZrO2/c-ZrO2+t-ZrO2 volumetric phase ratios changed with the increase of sintering temperature and time. Stress-induced t-ZrO2/m-ZrO2 -> m-ZrO2 phase transformation within c-ZrO2 grains in 8 Mg-PSZ was the main mechanism for toughness enhancement. Energy absorbing mechanisms, e.g., crack-bridging, crack-deflection and crack branching were also found to contribute the blunting of cracks. It is thought that our approach presented herein can be considered not only fracture toughness enhancement but also other properties in various materials for functional and structural purposes. (C) 2020 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi.
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    Preface to the special issue on "The Third International Hydrogen Technologies Congress (IHTEC 2018), March 15-18 2018, Alanya-Antalya, Turkey"
    (Pergamon-Elsevier Science Ltd, 2019) Solmaz, Ramazan; Yurdakul, Hilmi
    [No abstract available]
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    Öğe
    Unveiling the reaction products in heat treated Si3N4-Ti joined ceramics by transmission electron microscopy
    (Springeropen, 2019) Tunçkan, Orkun; Yurdakul, Hilmi; Turan, Servet
    Joining is a crucial process for the production of complex-shaped advanced engineering materials. Deep understanding of ceramic-metal interfaces during joining or following heat-treatment steps is therefore of important concern in designing the new systems. Capacitor discharge joining (CDJ) method was firstly carried out to compose the ceramic-metal joint material by silicon nitride (Si3N4)-titanium (Ti) constituents. Afterwards, heat treatment was performed on the Si3N4-Ti joints in air atmosphere at 1000 degrees C temperature to reveal the interface reactions and phases. Reaction layer that occurred between the Si3N4 and Ti interfaces and new phase formations were examined by transmission electron microscopy (TEM)-based various imaging and chemical analysis techniques. Electron transparent samples for TEM characterization were prepared by focused ion beam (FIB) milling and lifting method. Based on the detailed TEM results, Si and N diffusion arising from the Si3N4 ceramic was observed towards Ti metal foil side and further interacted with Ti atoms. The upshot of current diffusion was that Ti3N2 reaction layer with 50 nm thickness was formed at the interface while titanium silicon nitride (Ti6Si3N) matrix phase including dendritic-shaped Ti2N grains occurred in the Ti interlayer. It is believed that our TEM-based microscopy results not only provide the knowledge on ceramic-metal joint materials by CDJ method, but also contribute new insights on the development of various new joint systems.