Researcher:
Balcı, Özge

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Özge

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Balcı

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Balcı, Özge
Balcı, Özge Çağıran

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2016 - 2019222020 - 202315

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    Effect of ambient and cryogenic milling on the microstructure and properties of tungsten matrix composites fabricated by activated sintering
    (Sivas Cumhuriyet Üniversitesi, 2019) Ağaoğulları, Duygu; Öveçoğlu, M. Lütfi; Department of Chemistry; Department of Chemistry; Balcı, Özge; Somer, Mehmet Suat; Researcher; Faculty Member; Department of Chemistry; College of Sciences; College of Sciences; 295531; 178882
    Tungsten matrix composites reinforced with TiB2 and Y2O3 particles were fabricated by milling under ambient/cryogenic conditions and Ni activated sintering. Powder blends constituting the W - 1 wt. % Ni - 2 wt. % TiB2 - 1 wt. % Y2O3 composition were mechanically milled for 12 h under ambient condition or cryomilled for 10 min or sequentially milled under ambient and cryogenic conditions. Milling was carried out in a high-energy ball mill under ambient condition whereas cryogenic milling was conducted in externally circulated liquid nitrogen. Milled powders were compacted using a hydraulic press and the pellets were sintered at 1400°C for 1 h under Ar / H2 gas flowing conditions. The effects of different milling types on the microstructural and mechanical properties of the sintered composites were investigated. After sintering, in addition to dominant W phase, small amounts of WB and NiW phases were detected in all sintered samples. The application of cryomilling after milling at ambient condition provided the disappearance of the clustered TiB2 and Y2O3 particles in the sintered sample: They were located at the grain boundaries of W1Ni matrix and homogeneously distributed through the microstructure. Sequentially milled and sintered composite had the highest relative density (95.77 %) and the highest microhardness (7.23 GPa) values among the samples. Nanoindentation tests showed that there was an improvement in the hardness and elastic modulus of W matrix phase, which yielded the values of 8.9 and 373.7 GPa, respectively. / Özet: Bu çalışmada, Ni ile aktive edilerek sinterlenen tungsten esaslı matrisin TiB2 ve Y2O3 partikülleri ile takviye edilmesiyle, tungsten esaslı kompozit malzeme üretimi gerçekleştirilmiştir. W - % 1 ağ. Ni - % 2 ağ. TiB2 - % 1 ağ. Y2O3 kompozisyonundan oluşan toz harmanları, normal koşullarda 12 sa mekanik olarak öğütülerek ya da 10 dk kriyojenik ortamda öğütülerek ya da normal ve kriyojenik şartlarda ardışık olarak öğütülerek hazırlanmıştır. Normal şartlarda öğütme yüksek enerjili bir değirmende uygulanırken; kriyojenik şartlarda öğütme sıvı azot ile dışarıdan çevrelenen bir sistemde yapılmıştır. Öğütülmüş tozlar hidrolik pres kullanılarak preslenmiş ve pekiştirilen bünyeler Ar/H2 gazaltı şartlarında 1400°C’de 1 sa sinterlenmiştir. Farklı öğütme koşullarının sinterlenen kompozit malzemelerin mikroyapısı ve özellikleri üzerindeki etkisi incelenmiştir. Sinterlenme sonrasında, baskın W fazına ek olarak düşük mikarda WB ve NiW fazları oluşumu gözlemlenmiştir. Normal şartlarda öğütme sonrası uygulanan kriyojenik öğütme, sinter malzemelerin mikroyapısındaki TiB2 ve Y2O3 partiküllerin topaklanmasının yok olmasına neden olmuştur: Partiküllerin W1Ni matrisinin tane sınırlarında ve homojen olarak mikroyapıda dağılması sağlanmıştır. Ardışık olarak öğütülmüş ve sinterlenmiş kompozitler, numuneler arasında en yüksek rölatif yoğunluk (% 95,77) ve mikrosertlik değerlerini (7,23 GPa) sunmuştur. Nano-indentasyon testleri sayesinde, W matris fazının sertlik ve elastisite modül değerlerinin sırasıyla 8,9 ve 373,7 GPa değerlerine kadar iyileştirildiği kanıtlanmıştır.
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    Effect of milling type on the microstructural and mechanical properties of W-Ni-ZrC-Y2O3 composites
    (Elsevier, 2017) Ağaoğullari, Duygu; Öveçoğlu, M. Lütfi; Department of Chemistry; Balcı, Özge; Researcher; Department of Chemistry; College of Sciences; 295531
    This study reports the effect of milling type on the microstructural, physical and mechanical properties of the W-Ni-ZrC-Y2O3 composites. Powder blends having the composition of W-1 wt% Ni-2 wt% ZrC-1 wt% Y2O3 were milled at room temperature for 12 h using a Spex(TM) 8000D Mixer/Mill or cryomilled in the presence of externally circulated liquid nitrogen for 10 min using a Spex(TM) 6870 Freezer/Mill or sequentially milled at room temperature and cryogenic condition. Then, powders were compacted in a hydraulic press under a uniaxial pressure of 400 MPa and green bodies were sintered at 1400 degrees C for 1 h under Ar/H-2 atmosphere. Phase and microstructural characterization of the milled powders and sintered samples were performed using X-ray diffractometer (XRD), TOPAS software, scanning electron microscope/energy dispersive spectrometer (SEM/EDS), X-ray fluorescence (XRF) spectrometer and particle size analyzer (PSA). Archimedes density and Vickers microhardness measurements, and sliding wear tests were also conducted on the sintered samples. The results showed that sequential milling enables the lowest average particle size (214.90 nm) and it is effective in inhibiting W grain coarsening during sintering. The cryomilled and sintered composite yielded a lower hardness value (5.80 +/- 0.23 GPa) and higher wear volume loss value (149.42 mu m(3)) than that of the sintered sample after room temperature milling (6.66 +/- 0.39 GPa; 102.50 mu m(3)). However, the sequentially milled and sintered sample had the highest relative density and microhardness values of 95.09% and 7.16 +/- 0.59 GPa and the lowest wear volume loss value of 66.0 mu m(3).
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    Crystalline Co-Fe-b nanoparticles: Synthesis, microstructure and magnetic properties
    (Elsevier, 2019) Schmidt, Marcus; Bobnar, Matej; N/A; N/A; Department of Chemistry; Department of Chemistry; Khoshsima, Sina; Altıntaş, Zerrin; Somer, Mehmet Suat; Balcı, Özge; Researcher; Researcher; Faculty Member; Researcher; Department of Chemistry; Koç University AKKİM Boron-Based Materials & High-technology Chemicals Research & Application Center (KABAM) / Koç Üniversitesi AKKİM Bor Tabanlı Malzemeler ve İleri Teknoloji Kimyasallar Uygulama ve Araştırma Merkezi (KABAM); Koç University AKKİM Boron-Based Materials & High-technology Chemicals Research & Application Center (KABAM) / Koç Üniversitesi AKKİM Bor Tabanlı Malzemeler ve İleri Teknoloji Kimyasallar Uygulama ve Araştırma Merkezi (KABAM); N/A; N/A; N/A; N/A; College of Sciences; College of Sciences; N/A; N/A; 178882; 295531
    A new approach for in-situ synthesis of crystalline Co-Fe-B nanoparticles was presented in which low temperature methods were developed by using metal chlorides and NaBH4 in an inorganic molten salt environment. Effects of different reaction systems/conditions on the phase formation, thermal behavior and microstructure were investigated. The melting point of reactants and impurities in final powders were reduced by the use of molten salt technique. After a reaction of CoCl2, FeCl3 and NaBH4 at 850 degrees C in sealed tubes, CoB and Fe3B phases formed separately. After a reaction under Ar flow; however, CoFeB2 solid solution nano powders were obtained in one step at 850 degrees C with an average size of 60 nm. After annealing at 1100 degrees C, stable and highly crystalline (CoFe)B-2 solid solution phase with a Co:Fe molar ratio of 1:1 was achieved. As-synthesized particles exhibited ferromagnetic property, and possessed a narrow hysteresis curve characteristic of soft magnetic materials. Extended reaction temperature from 650 to 850 degrees C is seen to produce coercivity enhancement up to 500 Oe without significant reduction in saturation magnetization. On the other hand, after an annealing process and subsequent phase and chemical change, crystalline (CoFe)B-2 particles exhibited superparamagnetic property.
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    Synthesis of cobalt-nickel-boron based composite powders using metal chloride powder blends
    (TANGER, 2018) N/A; N/A; N/A; Department of Chemistry; Department of Chemistry; Khoshsima, Sina; Altıntaş, Zerrin; Somer, Mehmet Suat; Balcı, Özge; Researcher; Researcher; Faculty Member; Researcher; Department of Chemistry; Koç University AKKİM Boron-Based Materials & High-technology Chemicals Research & Application Center (KABAM) / Koç Üniversitesi AKKİM Bor Tabanlı Malzemeler ve İleri Teknoloji Kimyasallar Uygulama ve Araştırma Merkezi (KABAM); N/A; N/A; College of Sciences; College of Sciences; N/A; N/A; 178882; 295531
    This study reports an alternative route for obtaining crystalline metal boride composite powders at low temperatures using various chemical reactions. The ternary system of Co-Ni-B was studied by using anhydrous metal chlorides and sodium borohydride powder mixtures. The reactions were carried out in a sealed reactor under autogenic pressure, placed in a chamber furnace. The unwanted chloride phases were removed by hot water leaching after reaction. Some of the purified powders were annealed at 1100 °C to improve the crystallinity. Effects of different reaction conditions on the formation and microstructure of the final powders were investigated. Phase, chemical and microstructural characterizations and particle size measurements of the synthesized and annealed powders were conducted using X-ray diffractometer (XRD), X-ray fluorescence spectrometer (XRF), scanning electron microscope (SEM/EDX) and dynamic light scattering technique. The results revealed the positive effect of inorganic molten salt mixture (LiCl/KCl eutectic mixture) on the formed phases during the reaction between CoCl 2 , NiCl 2 and NaBH 4 powder blends. After their reaction at 750°C in a sealed reactor under autogenic pressure, crystalline cobalt-nickel-boron based composite powders were achieved with an average particle size of 60 nm.
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    Mechanical activation-assisted autoclave processing and sintering of HfB2-HfO2 ceramic powders
    (Elsevier Sci Ltd, 2016) Akçamlı, Nazlı; Ağaoğulları, Duygu; Öveçoğlu, M. Lütfi; Duman, İsmail; Department of Chemistry; Balcı, Özge; Researcher; Department of Chemistry; College of Sciences; 295531
    This study reports on the synthesis and consolidation of HfB2-HfO2 ceramic powders via mechanical activation-assisted autoclave processing followed by pressureless sintering (PS) or spark plasma sintering (SPS). HfCl4, B2O3 and Mg starting powders were mechanically activated for 5 min to obtain homogeneously blended precursors with active particle surfaces. Autoclave synthesis was carried out at a relatively low temperature at 500 degrees C for 6 or 12 h. As-synthesized powders were purified from reaction by-products such as MgO and MgCl2 by washing and acid leaching treatments. The characterization investigations of the as-synthesized and purified powders were performed by using an X-ray diffractometer (XRD), stereomicroscope (SM), scanning electron microscope (SEM) and particle size analyzer (PSA). The purified powders with an average particle size of about 190 nm comprised the HfB2 phase with an amount of 79.6 wt% in addition to the HfO2 phase and a very small amount of Mg2Hf5O12 phase after mechanical activation for 5 min and autoclave processing for 12 h. They were consolidated at 1700 degrees C both by PS for 6 h and SPS for 15 min. The Mg2Hf5O12 phase decomposed during sintering and bulk samples only had the HfB2 and HfO2 phases. The bulk properties of the sintered samples were characterized in terms of microstructure, density, microhardness and wear characteristics. The HfB2-HfO2 ceramics consolidated by PS exhibited poor densification rates. A considerable improvement was obtained in the relative density (similar to 91%), microhardness (similar to 16 GPa) and relative wear resistance (2.5) values of the HfB2-HfO2 ceramics consolidated by SPS. (C) 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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    Autoclave processing and sintering of ZrB2 -ZrO2 powders and investigation of microstructural and some mechanical properties of bulk products
    (TENMAK Bor Araştırma Enstitüsü, 2017) Akçamlı, Nazlı; Ağaoğulları, Duygu; Öveçoğlu, M. Lütfi; Duman, İsmail; Department of Chemistry; Balcı, Özge; Researcher; Department of Chemistry; College of Sciences; 295531
    In this study, the production of ZrB2 -ZrO2 powders and their sintered bodies was aimed by utilizing mechanical activation assisted autoclave process and pressureless sintering (PS) or spark plasma sintering (SPS) techniques. ZrCl4 , B2 O3 and Mg powders were used as starting materials and the powder mixtures were mechanically activated for 5 min by using high energy ball milling system. Mechanically activated powders were heated in an autoclave at 450°C for 12 h and subsequently purified by HCl leaching. Pure powders were consolidated by PS (1700°C, 6 h) and SPS (1700°C, 60 MPa, 15 min) to obtain the bulk samples. Characterization studies of the powders and sintered products involve the XRD, SEM, SM, OM techniques, particle size and Archimedes density measurements, microhardness and wear tests. Synthesized powders comprise of ZrB2 and ZrO2 phases with 78.5 % wt. ZrB2 content and have an average particle size of 203 nm. The bulk samples produced by PS and SPS techniques have relative densities of 92.8 % and 99.3 %, respectively and the microhardness reaches to the value of 12.6 GPa. The bulk sample fabricated by SPS technique having a wear volume loss of 8,52x10-3 mm3 reveals a smooth wear track instead of micro-grooves along the sliding direction. / Bu çalışmada, ZrB2 -ZrO2 tozlarının ve sinter ürünlerinin, mekanik aktivasyon destekli otoklavda sentezleme ve basınçsız sinterleme (PS) veya spark plazma sinterleme (SPS) teknikleri kullanılarak üretimi amaçlanmıştır. Hammadde olarak ZrCl4 , B2 O3 ve Mg kullanılmış olup, toz karışımları yüksek enerjili öğütme sisteminde 5 dk süre ile mekanik olarak aktive edilmiştir. Mekanik olarak aktive edilmiş toz karışımları, 450°C’de 12 sa süre ile otoklavda reaksiyona tabi tutulmuş ve ardından HCl çözeltisi ile liç işlemi uygulanarak saflaştırılmıştır. Saf tozlar, basınçsız sinterleme (1700°C, 6 sa) ve spark plazma sinterleme (1700°C, 60 MPa, 15 dk) yöntemleriyle yığın yapıya getirilmiştir. Elde edilen toz ve sinter ürünlerin karakterizasyonu, XRD, SEM, SM, OM, partikül boyutu, Arşimet yoğunluk ölçümü, mikrosertlik ve aşınma testi çalışmalarını kapsamaktadır. Elde edilen tozlar, ağırlıkça % 78,5 ZrB2 fazı ihtiva eden ZrB2 -ZrO2 tozları olup hiçbir empürite içermemektir ve 203 nm ortalama partikül boyutuna sahiptir. PS ve SPS metotları ile elde edilen yığın yapıdaki ürünler, sırasıyla % 92,8 ve % 99,3 rölatif yoğunluk değerlerine sahiptir ve mikrosertlik ortalama 12,6 GPa değerine ulaşmaktadır. SPS ile elde edilen numunedeki aşınma hacimsel kayıp 8,52x10-3 mm3 olup; aşınma izi, derin oyuklar yerine malzemenin yüzeysel olarak aşındığını göstermektedir.
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    EPR investigation of point defects in HFB2 and their roles in supercapacitor device performances
    (Aip Publishing, 2022) Buldu-Akturk, Merve; Erdem, Emre; Department of Chemistry; Balcı, Özge; Researcher; Department of Chemistry; College of Sciences; 295531
    Boron-based materials have various attractive properties and gained increased attention in recent years as promising materials for energy storage applications. Despite vast literature on structural and mechanical properties of transition metal diborides, hafnium diboride (HfB2) in particular, research that addresses the use of HfB2 as an electrode for supercapacitor devices is lacking. Herein, we report both the synthesis and characterization of HfB2 and its electrochemical performance as the electrode for all-in-one symmetric and asymmetric supercapacitor devices. HfB2 powders were synthesized by mechanical activation assisted carbothermal reduction of hafnium oxide and boron oxide precursors. To improve the electrochemical energy storage performance of the electrodes, point defects (either Hf or B vacancies/interstitials) were formed in HfB2 through annealing at different temperatures (1450 and 1650 & nbsp;C) under a flowing Ar atmosphere. The origin of point defects and their localization on the surface in HfB2 were identified using electron paramagnetic resonance (EPR) spectroscopy and discussed both from chemical and materials point-of-view. The defective HfB2 electrode exhibited higher performance than that of the non-defective one with specific energy and power densities of 0.144 W h kg(-1) and 33.3 W kg(-1); specific charge-discharge capacities of 0.32 and 0.31 mA h g(-1); and 115.5%, 106.2%, and 84.1% retention of the initial capacitances, respectively. The relation between the defect content and the improved supercapacitor performances was explained by employing several structural (x-ray diffractometer and x-ray fluorescence), electronic (EPR), and electrochemical (potentiostatic electrochemical impedance spectroscopy, cyclic voltammetry, galvanostatic cycling with potential limitation) characterization tools.& nbsp;& nbsp;Published under an exclusive license by AIP Publishing.
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    The synthesis of binary and ternary cobalt based metal borides by inorganic molten salt technique
    (TENMAK Bor Araştırma Enstitüsü, 2020) N/A; N/A; Department of Chemistry; Department of Chemistry; Altıntaş, Zerrin; Khoshsima, Sina; Somer, Mehmet Suat; Balcı, Özge; Researcher; Researcher; Faculty Member; Researcher; Department of Chemistry; Koç University AKKİM Boron-Based Materials & High-technology Chemicals Research & Application Center (KABAM) / Koç Üniversitesi AKKİM Bor Tabanlı Malzemeler ve İleri Teknoloji Kimyasallar Uygulama ve Araştırma Merkezi (KABAM); N/A; N/A; College of Sciences; College of Sciences; N/A; N/A; 178882; 295531
    Crystalline metal boride powders were synthesized via low temperature method in inorganic molten salt medium, and binary and ternary metal boride composite powders were investigated using anhydrous metal chlorides and sodium borohydride powder mixtures. The reactions were carried out in an aluminum crucible placed in a silica tube under argon which was put in a vertical tube furnace. At the end of the reaction, the resulting powder mixture was leached with hot water to remove any undesirable chloride phases. In order to improve crystalline properties, some of pure powders were selected and annealed at 1100°C. Characterization of synthesized and annealed powders was carried out using X-ray diffractometer (XRD), X-ray fluorescence spectrometry (XRF), scanning electron microscopy (SEM / EDX) and dynamic light scattering technique (DLS). The results showed the positive effect of inorganic molten salt technique (LiCl/ KCl eutectic mixture) on the formation of phases during the reaction between CoCl2 , NiCl2 and NaBH4 powder mixtures. Following the reactions at between 750-950 °C, the binary and ternary metal boride powders consisting of CoB-Ni2 BCoBx , CoB-Ni4 B3 ve CoB-NiB-Ni2 Co0.67B0.33 phases were obtained. The measured particle size of the final particles had an average of 60 nm. / Öz: Kristalin metal borür tozları inorganik ergimiş tuz ortamında düşük sıcaklık yöntemi ile sentezlenmiş, ikili ve üçlü metal borür kompozit tozlarının eldesi susuz metal klorürler ve sodyum borhidrür toz karışımları kullanılarak incelenmiştir. Reaksiyonlar, argon altında dikey bir tüp fırında silika tüp içine yerleştirilmiş alüminyum pota içinde gerçekleştirilmiştir. Reaksiyon sonunda elde edilen toz karışımına sıcak su ile liç işlemi yapılarak istenmeyen klorür fazları giderilmiştir. Seçilen bazı saf tozlar, kristalin özelliği geliştirmek amacıyla 1100°C’de tavlama işlemine tabi tutulmuştur. Sentezlenen ve tavlanmış tozların karakterizasyonu, X-ışını difraktometresi (XRD), X-ışını floresans spektrometresi (XRF), taramalı elektron mikroskobu (SEM/EDX) ve dinamik ışık saçma tekniği (DLS) kullanılarak analiz edilmiştir. Sonuçlar, inorganik ergimiş tuz tekniğinin (LiCl/KCl ötektik karışımı) CoCl2 , NiCl2 ve NaBH4 toz karışımları arasındaki reaksiyon sırasında oluşan fazlar üzerindeki olumlu etkisini ortaya koymuştur. 750-950°C sıcaklıkları arasında gerçekleşen reaksiyonlarda CoB-Ni2 B-CoBx , CoB-Ni4 B3 ve CoB-NiB-Ni2 Co0.67B0.33 fazlarını içeren ikili ve üçlü metal borür tozları nano boyutta elde edilmiştir. Sentezlenen tozların partikül boyutu ortalama 60 nm civarında hesaplanmıştır.
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    Influence of severe straining and strain rate on the evolution of dislocation structures during micro-/nanoindentation in high entropy lamellar eutectics
    (Elsevier, 2018) Maity, T.; Prashanth, K. G.; Kim, J. T.; Schoberl, T.; Wang, Z.; Eckert, J.; Department of Chemistry; Balcı, Özge; Researcher; Department of Chemistry; College of Sciences; 295531
    Eutectic high entropy composites (EHECs) can exhibit an excellent combination of high strength and high ductility; however, the mechanisms responsible for the strength-ductility trade-off remain unpredicted. The influence of strain rate (epsilon) over dot on the severe deformation imposed by high-pressure torsion (HPT) was used to evaluate the deformation mechanisms for a series of CoCrFeNiNbx (x molar ratio, 0 <= x <= 0.80) EHECs. Systematic and detailed micro-/nanoindentation investigations were performed and the results suggest that strain hardening (Taylor hardening) and grain-boundary strengthening (H-P strengthening) are the predominant strengthening mechanisms. Nanoindentation at different loading conditions (varying (epsilon) over dot) revealed that the measured hardness in the eutectic regime increases gradually because of dislocation-lamellae-interface interactions. Based on the deformation mechanisms operating at different strain rates (epsilon) over dot, the density of geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs), determined by the Nix-Gao approach, are used to explain the strain hardening phenomena. The results reveal that a large volume fraction of lamellae-interfaces accommodate more dislocations upon straining these EHECs. Lamellae-interface GNDs (rho(GG)) are activated at higher strain rates and can be effectively stored, thereby improving the global strain and strain hardening.
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    Room-temperature mechanochemical synthesis and consolidation of nanocrystalline HfB2-HfO2 composite powders
    (Goller Verlag Gmbh, 2018) Akçamlı, Nazlı; Ağaoğulları, Duygu; Öveçoğlu, M. Lutfi; Duman, İsmail; Department of Chemistry; Balcı, Özge; Researcher; Department of Chemistry; College of Sciences; 295531
    This study reports on the in-situ preparation of HfB2-HfO2 composite powders at room temperature by means of mechanochemical synthesis (MCS) from HfCl4-B2O3-Mg powder blends. The effects of milling duration and excess amounts of B(2)O3 and Mg reactants (20 and 30 wt%) on the HfB2 formation mechanism were investigated. After MCS and purification, HfB2, HfO2 and Mg2Hf5O12 phases were obtained. The Mg2Hf5O12 phase decomposed during the annealing treatment conducted at 1000 degrees C under Ar flow. The as-synthesized, purified, annealed and subsequently leached powders were characterized with an X-ray diffractometer (XRD), stereomicroscope (SM), scanning electron microscope (SEM), transmission electron microscope (TEM) and particle size analyzer (PSA). The HfB2-Hf-O2 composite powders with an average particle size of 140 nm and predominantly rounded morphology were consolidated with cold pressing/pressureless sintering (PS) and spark plasma sintering (SPS) techniques. The relative density values of the HfB2-Hf-O2 composites obtained by means of PS (with 2 wt% Co) and SPS techniques were 91.82 % and 93.79 %, respectively. A relatively high densification rate for the HfB2-Hf(O2 )ceramic was achieved by means of Co addition, which was considered a promising sintering aid for HfB2-based ceramics. The HfB2-HfO2 composite sample consolidated with SPS exhibited hardness, wear volume loss amount and friction coefficient values of 18.45 GPa, 4.30 mm(3) and 0.60, respectively.