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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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    Microstructural and mechanical characterization of ceria-stabilized tetragonal zirconia/alumina composites produced through a segregated-network approach for ceramic bushing applications
    (Springer, 2020) Yurdakul, Arife
    The aim of this study was to develop ceramic bushings used in automotive engineering. To achieve this, 12 mol% ceria (CeO2)-stabilized tetragonal polycrystalline zirconia (12Ce-TZP)/10-20 wt% alumina (Al2O3) composites were designed by using a segregated-network approach. They were subsequently produced by wet-mixing, cold isostatic pressing (CIP), computer numerical control (CNC) machining, binder burnout, and sintering at 1550-1600 degrees C for 1-3 h. Physical, mechanical, and microstructural properties of 12Ce-TZP/Al2O3-sintered composites were characterized using the Archimedes' principle, Vickers hardness (HV), indentation fracture toughness (K-Ic), flexural strength (sigma), X-ray diffraction (XRD), ultra-high-resolution scanning electron microscopy (UHR-SEM), and energy-dispersive X-ray spectroscopy (EDX) analyses. According to the overall results, 12Ce-TZP/Al2O3 composites were sintered up to 99.5% of theoretical density, when sintering temperature and dwell time were increased. 12Ce-TZP/10 wt% Al2O3/1600 degrees C/2 h composite, showing high mechanical properties in HV = 9.52 +/- 0.09 GPa, K-Ic = 15.44 +/- 0.15 MPa m(1/2), and sigma = 955.41 +/- 15 MPa, was considered the most appropriate composition for ceramic bushing production. XRD analyses indicated that 12Ce-TZP/Al2O3 composites consisted of tetragonal zirconia (t-ZrO2) and corundum (alpha-Al2O3) phases, while 12Ce-TZPs were found to contain only t-ZrO2 phase with no trace of monoclinic zirconia (m-ZrO2). UHR-SEM investigations revealed that the microstructural evolution of 12Ce-TZP/Al2O3 composites was observed as an interpenetrated intragranular-type through the formation of a segregated-network structure. In addition, energy-absorbing mechanisms, i.e., crack propagation hindrance, crack blunting, crack bridging, crack deflection, and stress-induced t-ZrO2 -> m-ZrO2 phase transformation were seen to govern the enhancement of mechanical properties. It is thought that results presented herein are also significant for new commercial applications of 12Ce-TZP/Al2O3 composites rather than other biomaterials.
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    Nano ölçekli koyu siyah renkli TZP (Tetragonal Zirkonya Polikristal) tozlarının tek kademede hidrotermal yöntemle üretilmesi
    (2018) Yurdakul, Arife
    Bu çalışmanın temel amacı, ilk kez ileri teknoloji malzeme üretim tekniklerini kullanarak homojen renk dağılımlı ve üstün mekanik özelliklere sahip siyah renkte ZrO 2 seramik malzemesinin üretimini sağlamaktır. Bu amaçla Fe, Co, Ni, Mn içeren geçiş metal oksitleri (Fe, Co, Ni, Mn) kullanılarak Al 2 O 4 spinel yapıya sahip ıtriyum katkılı zirkonya başlangıç tozu hidrotermal yöntemle ikinci bir prosese ihtiyaç duyulmadan tek aşamada sentezlenmiş ve düşük sinterleme sıcaklığında kararlı siyah ZrO 2 seramiklerin üretimi sağlanmıştır. Böylelikle maliyetin azalması ile birlikte siyah renkte ZrO 2 kristallerinde homojen renk dağılımı gözlenmiştir. Ayrıca, tek kademede sentezlenen siyah renkteki ZrO 2 tozlarını granül hale getirilmeden önce katkı malzemelerinin ilavesi ile seramik üretimini gerçekleştirmek amacıyla uygun teknik ile şekillendirilip, yüksek tokluk ve sertlik değerlerinde üretilebilirliği araştırılmıştır. Deneysel sonuçlar ile siyah renkli ZrO 2 numunesine ait ortalama tokluk değerinin 11 MPa.m 1/2 ve sertlik değerinin ise 9.5 GPa olduğu kanıtlanmıştır. Bu değerler, siyah renkli ZrO 2 üretiminde yapılan literatür çalışmaları arasında en yüksek değeri göstermektedir. Bu yüzden, yüksek aşınma direnci, sertlik gerektiren uygulamalar ve ağır yük altında çalışabilecek alternatif seramik malzeme olarak kabul edilebilir.
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    One-step hydrothermal synthesis of yttria-stabilized tetragonal zirconia polycrystalline nanopowders for blue-colored zirconia-cobalt aluminate spinel composite ceramics
    (Elsevier Sci Ltd, 2019) Yurdakul, Arife; Gocmez, Hasan
    A novel approach for the preparation of blue-color giving zirconia nanopowders by doping of 3 mol% Y2O3 through simple one-step hydrothermal process is proposed. A blue-color giving yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) powders were prepared by urea-based solution, zirconium acetate, CoCl2 center dot 6H(2)O and AlCl3 precursors in hydrothermal vessel at 24 h, 150 degrees C and 20 bars. Based on the results, the synthesized bluecolor giving Y-TZP nanopowders have entirely tetragonal structure as mono phase with 3.8 +/- 0.2 nm average grain size and (Y, Co, Al)(x)ZrO2; x <= 0.03 at. with chemical composition. Thermal treatment was also applied to synthesized Y-TZP powders at 1200 degrees C and 1450 degrees C to observe the color evolution. Only sharp blue was obtained in Y-TZP powders resulting the development of zirconia-cobalt aluminate spinel (ZrO2-CoAl2O4) composite ceramic structure for both temperatures after heat-treatment. Herein, not only formation of CoAl2O4 but also incorporation of cobalt (Co) and aluminum (Al) into the Y-TZP grains plays a critical role on evolving of blue color. This synthesized Y-TZP nanopowders can be a good candidate for one-step production of blue-color sintered ZrO2-CoAl2O4 spinel composite ceramics in numerous ceramic applications due to their superior structural and functional properties.
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    Production and characterization of ceramic cushings from Alumina-Toughened yttrium-stabilized tetragonal zirconia composites for projection welding applications
    (Springer Heidelberg, 2020) Yurdakul, Arife
    In current study, for the first time, it was aimed to produce the ceramic bushings for use of projection bolt welding process in automotive industry. Here, they were manufactured from ready-to-press binder containing granulated alumina-toughened zirconia (ATZ) powders consisted of 20 wt.% alumina (alpha-Al2O3) and 3 mol.% yttria-stabilized tetragonal zirconia polycrystalline components. The ATZ ceramic bushings were densified by pressureless sintering between 1500 and 1600 degrees C with 25 degrees C increments for 2 h under air atmosphere. Physical, mechanical, tribological and microstructural properties of ATZ composites were characterized by different tests, analyses and techniques, i.e., Archimedes' principle, Vickers hardness (HV), indentation fracture toughness (K-Ic), specific wear rate (W), X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy. Considering overall results, ATZ ceramic bushings were successfully sintered up to 99% of theoretical density by increasing sintering temperature. The maximum HV and K-Ic values of ATZ composites were determined as 14.62 +/- 0.02 GPa at 1500 degrees C and 6.10 +/- 0.04 MPa m(1/2) at 1600 degrees C, respectively. XRD patterns revealed that ATZ composites comprised of primary t-ZrO2 and secondary alpha-Al2O3 phases with no traces of m-ZrO2 and c-ZrO2. SEM observations showed the homogenous microstructures with high densification including no porosity; however, a slight degree of normal grain growth for t-ZrO2 and alpha-Al2O3 grains. ATZ ceramic bushings also exhibited high wear resistant property with W = 4.71 x 10(-9) m(3) N-1 m(-1) value. They welded over the 500.000 +/- 2.000 bolts to metal sheets in the automatic projection welding machines. This makes the ATZ composites a superior material for projection bolt welding process.
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    The usage of newly developed glass fibre in cement structure and their characterization
    (Elsevier Sci Ltd, 2018) Yurdakul, Arife; Dölekçekiç, Emrah; Günkaya, Göktuğ; Kavaş, Taner; Karasu, Bekir
    In this work, recently developed Zrnl glass fibres belonging to the SrO-Mn2O3-Fe2O3-MgO-ZrO2-SiO2 (SMFMZS) system have been used as reinforcing materials in cement. The highest (2.610 N/mm(2)) compressive strength value was obtained with the addition of 3 wt% of Zrn1 fibres into Portland cement. The microstructural behaviour of these fibres along with commercially available ones in the cement structure were investigated using a scanning electron microscope (SEM). Microstructures were fully compatible with mechanical results obtained. Eventually, it was determined that Zrn1 fibre can be a good candidate for cement reinforcement leading to high strength values. (C) 2018 Elsevier Ltd. All rights reserved.