LEE- Malzeme Bilimi ve Mühendisliği-Yüksek Lisans
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ÖgeAttempts to re-evaluate waste thermoplastic polyurethane (TPU)(Graduate School, 2025-01-16)Polymers' excellent mechanical and thermal qualities, lightweight nature, and economical manufacturing have made them essential materials in many different sectors. However, there are major environmental issues with waste management and large-scale polymer production, which highlights the need for sustainable recycling solutions. Thermoplastic polyurethanes (TPUs), which are widely used in industries including consumer goods, automotive, and medicine, are known for their flexibility, toughness, and chemical resistance. In order to encourage sustainable material development, this study aims to enhance the revaluation of TPU waste through chemical modification and blending procedures. The environmental problems caused by polymeric waste are highlighted in this study of the literature, especially in the case of TPU, which has been widely used in the consumer goods, automotive, and medical sectors. TPU is a good option for recycling because of its thermoplastic characteristics, mechanical strength, and durability. However, the process is made more difficult by its structural complexity and the presence of additives. Studies has investigated polymer blending and chemical modification as approaches to enhance the properties of TPU and make recycling less challenging. The rheological, mechanical, and thermal properties of TPU are enhanced via chemical modification. Through a variety of processes, additives such diisocyanates (polymerized-methylene diphenyl diisocyanate (PMDI), and hexamethylene diisocyanate (HDI)), Joncryl ADR 4468, pyromellitic dianhydride (PMDA), and other different additives have an impact on the properties of TPU. Waste of TPU could also have effectively revaluate through blending it with polymers including polylactic acid (PLA), polyamide (PA), polymethyl methacrylate (PMMA), and polybutylene terephthalate (PBT). This method improves mechanical properties including toughness, flexibility, and impact resistance while addressing PLA's shortcomings, such as brittleness, low melt strength, and slow degradation. However, compatibility problems with PLA/TPU blends can result in phase separation as well as limitations. Compatibilizers such as Joncryl optimize mechanical and thermal qualities by enhancing phase compatibility and blend morphology. The objective of this thesis is to develop sustainable materials with improved mechanical, thermal, morphological, and rheological properties by improving the reutilization of TPU waste through chemical modification and blending techniques. Due to TPU's great resistance to degradation and growing environmental concerns about its recycling and disposal, improved methods are required to increase its reusability. Through the chemical modification of TPU with different additives and also blending the waste TPU with biodegradable polymers like PLA, this work aims to address these problems. The experimental process has been divided into two parts: the chemical modification and blending. TPU wastes have been melt processed with additives (PMDI, HDI, Joncryl ADR 4468, and PMDA) at 0.5% and 1% by weight for five minutes at 200°C and 100 rpm in an internal melt mixer. Rheological, mechanical, thermal, and properties of the samples were determined using small amplitude oscillatory shear (SAOS) rheometer, tensile and hardness tests, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and gel permeation chromatography (GPC). PLA/TPU blends were made with 20% waste TPU and 80% PLA in a twin screw extruder. Joncryl was utilized as a compatibilizer in certain samples. Mechanical, thermal, and morphological properties of the samples were investigated using tensile, impact, and hardness tests, DSC, TGA, and scanning electron microscopy (SEM). Chemical modification study findings shown significant increases in TPU properties depending on the type and concentration of additions. particularly when diisocyanates are incorporated, resulting in improved mechanical performance (tensile strength and elongation at break) and melt strength (complex viscosity) while preserving thermal stability. However, PMDA-modified TPU resulted in lower mechanical performance because to hydrolysis-induced degradation. This finding highlights the potential of the modifications to enhance material performance without compromising its fundamental characteristics. Experiments carried out in the second stage of the study indicated that, the use of waste TPU contributed to improving the disadvantageous properties of PLA. Based on the results of various mechanical analyses, the structure obtained by blending PLA and TPU exhibited a decrease in tensile strength due to the immiscibility between PLA and TPU. However, the presence of TPU led to a more ductile structure, reflected by an increase in elongation at break values, while hardness values decreased. Additionally, TPU enhanced the impact resistance properties of PLA. The incorporation of Joncryl ADR 4468 as a compatibilizer improved phase compatibility, thereby enhancing the mechanical properties. Blend structure created by pre-blending TPU with Joncryl (PLA/(TPU/J)*) resulted in only a slight increase in tensile strength and modulus values. SEM analysis revealed that phase separation occurred due to the immiscibility of PLA and TPU, with TPU appearing as droplets inside the PLA matrix. The addition of Joncryl increased phase compatibility, resulting in finer and more uniform microstructures by interacting with PLA and TPU to lower interfacial tension and increase viscosity by branching in the PLA matrix. Stronger interphase bonding and a more stable structure were confirmed in PLA/(TPU/J)* blends. The microstructure was further improved by pre-blending TPU with Joncryl, which improved its dispersion inside the blend. The presence of TPU improved PLA's thermal stability and processability by increasing the degree of crystallization while decreasing its total crystallinity, according to thermal analysis. Tg of PLA was lowered by TPU's plasticizing effect, which may reduce brittleness. Joncryl enhances the overall performance of the blend and increases the maximum degradation temperatures, especially when blended with TPU before that, which greatly increases thermal stability. This study emphasizes the possibilities of chemical modification and blending methods for tackling TPU waste recycling concerns. While the incorporation of additives enhances the rheological and mechanical property of TPU, blending it with PLA tackles PLA's limitations, and the two methods promote the revaluation of waste TPU. The findings emphasize the importance of optimizing process parameters, selecting suitable materials, and evaluating alternating additives for enhanced performance. These efforts contribute to the development of sustainable materials, reduction of polymer waste, and promotion of a circular economy.
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ÖgeInvestigation of mechanical and thermal properties of TiB2 coating grown on Ti-6Al-4V VIA CRTD-Bor(Graduate School, 2025-01-10)Surface modification methods are extensively used in modern engineering to promote surface properties. Various surface modification methods and agents are utilized to conduct the process of surface treatment. The agents and methods vary in each other via their different advantages and disadvantages. Hence, among all surface modification methods and agents titanium diboride and CRTD-Bor technique possess the uttermost advantages. TiB2 is considered a desired transition metal boride for most engineering enthusiasts since it performs advanced electrical, thermal, and mechanical properties. The main advantage of the TiB2 coatings are well known as their excellent hardness and wear resistance against sliding surfaces. Moreover, the high-temperature stability of titanium diboride coatings makes it required for high-temperature applications. The boriding process is conveniently conducted by governing the pack and past process techniques. However, in modern days these techniques have several drawbacks, mostly about time and hazardous by-products. When compared with the convenient techniques, the CRTD-Bor method offers; ▪ Shortened process times ▪ Thicker coating layer ▪ Elimination of hazardous by-products ▪ Aligned stoichiometric compliance with the desired coating compound. Hence, when the process advantages and outstanding properties of TiB2 are considered, the application of titanium diboride coatings via the CRTD-Bor technique may be useful for aerospace applications where high-temperature stability and excellent mechanical properties are desired to be combined. In this study, a thin film layer of TiB2 is coated onto Grade-5 titanium (Ti-6Al-4V) substrate material. After several condition and coating trials, 15 minutes of electrolysisxii and 30 minutes of electrolytic holding were implemented to the substrate specimens at 1000° C. Afterwards, solution treatment and aging and heat treatment were implemented to the specimens to eliminate the drawbacks of high-temperature electrolysis application. The attained coating on the substrate material was near 3 microns thick TiB2 and well aligned with the stoichiometric patterns. To obtain the effects of titanium diboride coating on Grade-5 titanium alloy a series of test campaigns was held. In particular, roughness measurements, tensile tests, high cycle fatigue tests, high-temperature oxidation tests, thermal radiation emittance/reflectance tests, and wettability tests were executed with the borided and non-borided (bare, lean) specimens. Roughness measurements of borided specimens showed 0.461µm for Ra (average roughness) compared with the bare specimen roughness of 0.479 µm for Ra. In addition to the average roughness values, RZ values of both types of specimens have been obtained as 2.5076 µm for borided and 2.883 µm for bare specimens. Thus, 15 minutes of electrolytic coating and 30 minutes of holding in the molted salt bath process generated no adverse effect on roughness values. Tensile tests have been carried out at 3 different temperature ranges by the specific aerospace regulations. No adverse effect of boriding on the tensile properties is obtained at the end of the campaign with ultimate tensile strength results of 1112 MPa- 1155 Mpa for -55°C, 1014 MPa, and 1014 MPa for 23° C, and 849 MPa-873 MPa for 180°C respectively for bare and borided specimens. High cycle fatigue test has been carried out by implementing various stress amplitudes with a stress ratio of R 0.1. Borided specimens showed a significant amount of decrease when compared with the bare specimens. This event was linked with columnar-like microstructure and dendritic morphology of the coating. High-temperature oxidation tests were executed at 700 C°, 800 C°, and 1000 C° respectively for 150, 28, and 24 hours. For the 700 C° test, a 5.07 rate of weight change for bare specimen/coated specimen has been examined. For 800 C° and 1000 C° tests, 1.4 and 1.6 rates of weight change for bare specimen/coated specimen have been found respectively. In addition to that, the overall reaction energy between the implemented temperature ranges was evaluated as -255.12 kJ for bare and -456.46 kJ for coatedxiii specimens. Hence 1.8 times better performance of titanium diboride coating is discussed for the overall reaction energy. Thermal emissivity test has been executed to characterize the radiation dissipation form of the borided specimens. Three types of specimens have been used; bare, Type 1 borided (unwashed) with a darker surface color, and Type 2 borided (washed) with a lighter surface color in comparison with the Type 1 borided specimen. Resulted revealed that, for the average of 20° and 60° incidence angle, Type 1 specimen showed an emissivity value of 0.91, Type 2 specimen showed an emissivity value of 0.42 and bare specimen showed an emissivity value of 0.28 between the wavelength of 3-5 µm which is named as MWIR (mid-wave infrared wavelength). This wavelength was discussed as important for jet-engine interior radiance dissipation and remarked as vital for low observability requirements of aerospace applications. The optimum value which combines emissivity with low observability is found for the Type-2 specimen. Wettability tests have been conducted to see the reaction of the coating for possible secondary surface treatments such as dying or secondary coatings. Results showed a contact angel of nearly 50 degrees for bare specimen and nearly 20 degrees for the coated specimen. Cross-checks have been done from different locations of specimens to validate the results. The hydrophilicity of TiB2 was way higher than that of bare specimen which can be correlated to better capacity for secondary operations. The driving force for the above test group has been sourced from the huge literature gap for TiB2 coatings. The whole test group generated unique findings and hopefully will be beneficial for later studies for coating candidates on aerospace applications.
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ÖgeImproving electrolyte performance of PEO by addition of LLZTO nanofillers in solid state battery applications(Graduate School, 2024-07-14)With the increasing demand for energy storage technologies, traditional lithium-ion batteries becoming inadequate and require enhancement. The rising trend of electric cars constitutes a significant portion of battery usage of today. Considering the needs of electric vehicles, higher energy density, higher power density and improved safety have become key areas for improvement in lithium-ion batteries. Extensive research has been conducted on various approaches to enhancing lithium-ion batteries, and studies on electrolyte have led to the discovery of solid state batteries. Solid-state batteries differ from traditional batteries by using a solid electrolyte instead of a liquid one. This solid material also acts as a separator to prevent electrode contact. Inorganic crystalline ceramics, glassy materials, and organic polymers can be considered as solid electrolyte materials, with high ionic conductivity being the most crucial requirement. While traditional liquid electrolytes have an ionic conductivity of 10-2 S cm-1, solid electrolytes are expected to have conductivities above 10-4 S cm-1 at room temperature to be suitable for commerc fillial battery applications. Ceramics like LLTO, LLZTO, and Li7P3S11 meet this requirement at room temperature however their application as solid electrolyte is limited due to their brittle nature. On the other hand polymers can be a good candidate considering their flexible structure. However, they typically have low ionic conductivities around 10-10 to 10-7 S cm-1 at room temperature, which considered as the drawback of polymer materials for to be utilized as solid electrolytes. Composite electrolytes emerge as a solution to this problem by combining a polymer matrix with ceramic fillers to create a conductive pathway. This structure retains the mechanical flexibility of polymers while benefiting from the high ionic conductivity of ceramics. This method also can solve, if not reduce the effects of, dendrite formation, a significant issue in lithium-ion batteries, by ensuring uniform current distribution and preventing lithium ion accumulation. In this study a composite solid electrolyte with a polymer matrix and ceramic nanoparticles is formulated and fabricated. Polyethylene oxide (PEO) served as the polymer, and Lithium Lanthanum Tantalum Zirconate (LLZTO) nanoparticles were used as the ceramic additive, with Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as the lithium salt. Various LLZTO concentrations, 40%, 45%, and 50%, were tested for their effects on ionic conductivity and transfer numbers. For the production of the composite electrolyte samples, solution casting method has been employed Characterization of the produced electrolytes involved techniques like Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric Analysis (TGA), X-ray Diffraction (XRD), Linear Sweep Voltammetry (LSV), Chronoamperometry (CA), and Electrochemical Impedance Spectroscopy (EIS). These tests are used for obtaining xxii the data that is necessary for calculation of parameters such as ionic conductivity, transfer numbers and examining the electrochemical and thermal stability of the samples. Results showed that LLZTO addition improved the ionic conductivity of the PEO with 10% (wt) up to 1.76×10-5 S cm-1 and transfer number up to 92%. Although it is observed that these values are retreat with increasing LLZTO contents. This effect is believed to be related with several factors. Surface roughness of composite electrolyte increases with LLZTO content. This is expected to be related with the declined surface contact of electrolyte with the stainless steel plates that used in the measurements. Also, with the increasing nanofiller content, fillers are tend to agglomerate and this resulted with lower surface area of polymer/ceramic interface. The electrochemical stability window for all samples exceeded 5V and nanofiller addition results with increasing ESW. FTIR and XRD analyzes indicated that LLZTO reduced crystallinity of PEO, enhancing amorphous characteristics, which likely contributed to improved ionic conductivity. Additionally, TGA results demonstrated that LLZTO increased the thermal stability of PEO from 357 °C to above 380 °C.
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ÖgeEffects of pressure and bias voltage on the morphology and properties of refractory WNbMoV high entropy thin films coated via magnetron sputtering(Graduate School, 2024-07-09)The synthesis and characterization of thin film materials have garnered significant attention in advanced technological field nowadays. Development of advanced thin film materials with superior properties is crucial for progress in various technological fields. Among these, refractory high entropy thin films have emerged as innovative materials due to their exceptional properties. The deposition of HEAs as thin films has garnered significant attention as it enables the fabrication of advanced functional coatings with tailored properties. Recent research has focused on understanding the influence of deposition parameters on microstructure and properties, exploring various HEA compositions, and developing innovative applications in fields like electronics, energy, and biomedical engineering. High-entropy thin films offer exceptional properties, making them suitable for various applications. These include protective coatings, electronics, biomedical implants, energy technologies, and components for the aerospace and automotive industries. In this study, by using magnetron sputtering technique, equimolar WNbMoV refractory high entropy thin film coatings were deposited on the silicon wafer substrate which had been coated with chromium metal as an intermediate layer between the substrate and the thin film. The effects of bias voltage and pressure of the chamber on the physical and mechanical properties of coated films during the magnetron sputtering process were investigated. X-Ray diffraction experiments were carried out for phase analysis, determining the experimental components and grain sizes were calculated by using the Williamson-Hall method, ORIGINPro and CALPHAD softwares. Scanning electron microscopy (SEM) and EDS mapping analyses were used to evaluate the microstructural properties and XRF analyses were used to investigate elemental distribution, respectively. In addition, we studied the Surface morphology by AFM (Atomic Force Microscopy). Hardness and Electrical Resistance of the samples was measured using nanoindentation and relative equipment. X-ray diffraction (XRD) analysis confirmed a dominant body-centered cubic (BCC) solid solution phase, aligning with expectations for WNbMoV HEAs, but also revealed minor oxide phases due to oxygen contamination. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) showed a dense columnar microstructure with varying surface roughness depending on deposition conditions. Notably, applying a -80V bias voltage resulted in smoother and denser coatings due to increased energy and directionality of sputtered particles. Nanoindentation tests revealed an inverse relationship between film hardness and working pressure, with -80V bias voltage enhancing hardness due to grain refinement. Energy-dispersive X-ray spectroscopy (EDS) confirmed uniform elemental distribution within the film. Electrical conductivity was influenced by phase composition and microstructure, with oxide phases decreasing conductivity and denser microstructures improving it. UV-Vis spectroscopy showed tunable optical properties, with increased working pressure decreasing transmittance. The sample produced at 1.5 Pa pressure and 0V bias exhibited complete transparency, likely due to excessive oxidation or stoichiometry deviations. The results show that by applying bias and pressure, there are alterations in the produced thin films thickness. This suggests a direct correlation between the applied parameters and hardness values. By applying the -80 V bias, the grain size of samples decreased from 10 to 5.2 nm and the hardness of the films increased from 350 to 301 HVs, respectively. Furthermore, when the pressure increased from 0.5 Pa to 1.5 Pa, the thickness of films decreased approximately from 1.26 to 1.11 m. This project offers unique insights into experimental studies. We've gained groundbreaking knowledge about WNbMoV high-entropy alloy thin films, understanding how processing, structure, and properties are interconnected. This allows us to tailor these films for applications like wear-resistant coatings. Future research should focus on optimizing deposition to improve film quality.
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ÖgeCoCrFeNiAlx (x=0.2; 0.6; 1.0; 1.5) yüksek entropili alaşım sistemlerin mekanik alaşımlamasının optimizasyonu ve B4C takviyesinin spark plazma sinterleme prosesine etkisinin araştırılması(Lisansüstü Eğitim Enstitüsü, 2025-01-29)Yüksek entropili alaşımlar (YEA), en az beş farklı elementin yapıya dahil olmasıyla elde edilen malzeme grubudur. Alaşım sisteminde bulunan, veya eklenecek olan her bir elementin, malzemenin belirli özelliklerini geliştirmesi beklenir. Mevcut alaşıma, çeşitli takviye malzemeleri (bor karbür, silisyum karbür vb.) veya oranı %5'ten daha az olacak şekilde minör olarak adlandırılan alaşım elementleri de eklenerek, yüksek mukavemet, yüksek sertlik, yüksek aşınma direnci, iyi seviyede korozyon ve oksidasyon direncine sahip bir malzeme elde edilebilmektedir. Bu sayede, yüksek entropili alaşım sistemlerinin, üstün performans gerektiren çeşitli alanlara yönelik (havacılık, otomotiv vb.) geliştirilmesine devam edilmektedir. YEA sistemlerinin kendine has bazı özellikleri bulunmaktadır. Bu özellikler, dört temel etki olarak adlandırılmakta olup, yüksek entropi etkisi, yüksek latis distorsiyonu, yavaş difüzyon etkisi ve kokteyl etkileridir. İlgili parametreler sayesinde, malzemelerin temel özellikleri ve davranışları hakkında bilgi edinilebilmektedir. YEA sistemlerinde en çok kullanılan üretim yöntemleri, genellikle üç başlık altında değerlendirilmektedir. Bu yöntemler; katı hal, sıvı hal ve gaz hal olmak üzere sınıflandırılmaktadır. Sıvı hal yöntemlerinde en çok kullanılan vakum ark ergitme prosesidir. Bu yöntemin temel sınırlaması, homojenliğin elde edilmesinin uzun vakitler gerektirmesidir. Homojenliğin sağlanması için, yöntemin birden fazla kez tekrarlanması gerekebilmektedir. Gaz hal üretim yöntemleri, genellikle YEA film kaplamaları üretiminde tercih edilmektedir. Katı hal üretim yöntemlerinde ise mekanik alaşımlama (MA) prosesi yer almaktadır. Mekanik alaşımlama prosesinde, toz, bilye ve proses kontrol maddesinin içerisinde olduğu, genellikle paslanmaz çelikten oluşan bir kap sistemi kullanılır. Yüksek enerjili değirmenlerde gerçekleştirilen proseste, çarpışmanın kuvvetiyle, toz parçacıkları plastik olarak deforme olur ve deformasyon sertleşmesine uğrayıp ile parçacıkların kırılması sağlanır. MA sayesinde, vakum ark ergitme yönteminin sınırlamalarından olan homojen mikroyapı iyi bir şekilde elde edilebilmektedir. Malzemede istenen yoğunluğun elde edilebilmesi için, MA işlemini takiben spark plazma sinterleme ile malzemeler şekillendirilerek, yüksek yoğunluk değerleri elde edilebilmektedir. SPS işlemi, düşük voltajlı, doğru akımlı, darbeli akımla aktive edilen bir basınçlı sinterleme tekniği olarak da bilinir ve yüksek sıcaklıklarda bile malzemelerin çok kısa sürelerde sinterlenip, yoğunlaştırılması sağlanır. YEA sistemlerine takviye malzemesi olarak eklenebilen, bor karbür (B4C), elmas ve kübik bor nitrürün ardından bilinen en sert üçüncü malzemedir. Bor karbür, düşük yoğunluğu (2,52 g/cm3), yüksek sertliği (29.1 GPa), yüksek ergime sıcaklığı (2450°C), yüksek elastik modülü (448 GPa), yüksek nötron emilim kesiti (600 barns) ve mükemmel termoelektrik gibi birçok çekici kombinasyonu sebebiyle yüksek performans uygulamaları için uygun bir malzemedir. Bu kapsamda, bor karbür, nükleer endüstride, personel ve araç güvenliği için zırh, roket yakıtı vb. uygulamalarda kullanılmaktadır. Çalışma kapsamında CoCrFeNiAlx (x=0,2 0,6 1,0 1,5) yüksek entropili alaşım sistemleri, mekanik alaşımlama yöntemi ile farklı sürelerde (2, 4, 6 ve 8 saat), sabit rpm dönüş hızında (800) üretilmiştir. Üretilen toz alaşımının partikül boyut dağılımları belirlenmiş ve X-ışını difraktormetre analizi (XRD) ile faz analizleri gerçekleştirilmiştir. Sinterleme işleminden önce, ThermoCalc yazılım programında yapıda hangi fazların oluşabileceğine ilişkin faz tahmin analizi gerçekleştirilmiştir. MA yöntemiyle üretilen toz alaşımlarının şekillendirilmesi için spark plazma sinterleme (SPS) prosesi gerçekleştirilmiştir. Sinterleme işleminde başlangıç tozu olarak 20 saat 300 rpm hızında öğütülmüş CoCrFeNiAl0,2 ve CoCrFeNiAl1 alaşım sistemleri kullanılmıştır. Bor karbür takviyeli alaşım sistemlerinde, sinterleme işleminden önce, öğütülmüş YEA tozları ve bor karbürün daha iyi homojen dağılımı için turbulada 6 saat süreyle karıştırma işlemi gerçekleştirilmiştir. Bu kapsamda, hem takviyesiz hem de değişen oranlarda (hacimce %2 ve %4) bor karbür takviyeli CoCrFeNiAl0,2 ve CoCrFeNiAl1 alaşım sistemleri değişen sıcaklıklarda (845, 900, 1000°C), sabit basınçta (40 MPa) ve sabit sinterleme sıcaklığında (3 dakika) SPS yöntemiyle şekillendirilmiştir. SPS işlemlerinden sonra, malzemelerin yoğunluk ölçümleri, XRD yöntemiyle faz analizleri, taramalı elektron mikroskobu ve enerji dağılım spektrometresi ile mikroyapı karakterizasyonları gerçekleştirilmiştir. Mekanik testler kapsamında, malzemelerin Vickers mikrosertlik ölçümleri, aşınma ve basma testleri yapılmıştır. Malzemelerin termal davranışları hakkında bilgi sahibi olabilmek için termogravimetrik analiz (TGA) ve diferansiyel termal analiz (DTA) işlemleri gerçekleştirilmiştir. Partikül boyut analizlerinde, genellikle 2-4 saat aralığında partikül boyutlarında bir azalma görülmüştür. Burada mekanik alaşımlama mekanizmalarından kırılma, soğuk kaynağa göre daha baskındır. 4-6 saat aralığında ise partikül boyutlarında bir artış görülmüştür. Burada ise soğuk kaynağın kırılmaya göre baskın olduğu gözlemlenmiştir. En yüksek yoğunluk değeri (7,87 ± 0,012), 1000°C sıcaklık, 40 MPA basınç ve 3 dakika sinterleme süresinde sinterlenen CoCrFeNiAl0,2 sisteminde elde edilmiştir. Yoğunluk artışının temel sebebinin, sinterleme sıcaklığındaki artışa bağlı olarak por miktarının azalması ve alüminyum oranının düşük olmasından kaynaklandığı görülmüştür. Mekanik alaşımlanmış CoCrFeNiAlx sistemlerinin faz analizlerine bakıldığında tüm alüminyum içeriğinde, YMK ve HMK katı çözeltilerinin bir arada bulunduğu görülmüştür. Elde edilen bu sonuçların termodinamik hesaplamalarla da uyumlu olduğu görülmüştür. Sinterlenmiş numunelerin faz analizlerine bakıldığında ise, eşmolar alaşım sistemlerinin hepsinde YMK ve HMK fazları bir arada bulunurken, ek olarak alüminyum esaslı intermetalik bileşiğin ve işlemler sırasında karbon difüzyonuna bağlı olarak karbürlü yapılarının da bulunduğu görülmüştür. Eşmolar olmayan CoCrFeNiAl0,2 alaşım sistemlerinde ise ağırlıklı olarak YMK katı çözelti fazı ve karbürlü yapılar mevcuttur. Alüminyum oranının arttıkça, tek fazlı bir HMK yapısının oluşacağı bilinmektedir. Dolayısıyla eşmolar olmayan CoCrFeNiAl0,2 sisteminde alüminyum oranının düşük olmasına bağlı olarak ağırlıklı olarak YMK fazının bulunması beklenen bir durumdur. Taramalı elektron mikroskobu ve enerji dağılım spektrometresi ile yapılan mikroyapı karakterizasyonlarıyla, X-ışını difraktormetre analizi sonuçlarının birbiriyle uyumlu olduğu görülmüştür. En yüksek sertlik değeri (4,76 ± 0,17 GPa), 900°C sıcaklıkta – 40 MPa basınçta – 3 dakika sinterleme süresinde sinterlenen takviyesiz CoCrFeNiAl alaşımında elde edilmiştir. Genel olarak, yoğunlaşmanın daha iyi olduğu sıcaklıklarda bor karbür takviyesiyle birlikte sertlik değerleri, takviyesiz sisteme göre eşdeğerdir. En yüksek sinterleme sıcaklığında (1000°C) ise, B4C takviyesiyle birlikte, sertlik değerinde belirgin bir artış görülmüştür. Bunun sebebinin, yüksek yoğunlaşmaya ek olarak, takviyesiz sisteme göre farklı pik açılarında oluşan Fe,Cr esaslı karbür yapılarından kaynakladığı düşünülmektedir. Aşınma testi sonuçlarına göre, 845 ve 900°C'de B4C ilavesiyle, daha düşük aşınma derinliği ve genişliğiyle birlikte ortalama sürtünme katsayısı, hacimsel aşınma kaybı ve spesifik aşınma hızı değerleri azalarak aşınma direncinde iyileşme elde edilmiştir. 1000°C sıcaklıkta ise, B4C ilavesiyle birlikte daha yüksek sürtünme katsayısı, hacimsel aşınma kaybı ve aşınma hızı elde edilmiştir. Bunun sebebinin malzeme yüzeyinde oksit tabakasının meydana geldiği düşünülmektedir. Elde edilen verilerin, profilometre sonuçlarıyla ve optik mikroskop görüntüleriyle uyumlu olduğu görülmüştür. Basma testleri, en yüksek sertlik değerine sahip 900°C sıcaklıkta sinterlenen takviyesiz CoCrFeNiAl alaşımı ve en yüksek yoğunluk değerine sahip 1000°C sıcaklıkta sinterlenen eşmolar olmayan takviyesiz CoCrFeNiAl0,2 alaşım sisteminde gerçekleştirilmiştir. Eşmolar olmayan alaşım sisteminin basma mukavemeti 1388,19 MPa bulunurken, eşmolar alaşım sisteminin ise 395,62 MPa bulunmuştur. Sinterleme sıcaklığının artışıyla beraber basma mukavemeti değerinde de artış görülmüştür. Basma mukavemetindeki artışın, azalan gözenek miktarı ve gözenek boyutuna bağlı olarak gerçekleştiği düşünülmektedir. Termal testler kapsamında, en yüksek sertlik değerine sahip 900°C sıcaklıkta sinterlenen takviyesiz CoCrFeNiAl alaşımı ve %2 bor karbür takviyeli sistemleri için termogravimetri (TG) ve diferansiyel termal analizleri (DTA) gerçekleştirilmiştir. Bu kapsamda her iki alaşım için parabolik hız sabitleri hesaplanmıştır. Takviyesiz alaşımın parabolik hız sabiti, kp=1.21×10−8 (mg²/cm⁴/s), bor karbür takviyeli alaşımın parabolik hız sabiti değeri ise kp≈1.71×10−9 (mg²/cm⁴/s) olarak bulunmuştur. Her iki sistemde de ağırlık artışının az olduğu (takviyesiz sistemde %0,7 takviyeli sistemde ise %0,3) ve parabolik hız sabiti değerlerinin düşük olması sebebiyle oksidasyona karşı iyi bir dirence sahip oldukları söylenebilir. B4C ilavesinin, daha az ağırlık artışına ve hız sabiti değerinin daha düşük olmasına sebep olarak, oksidasyon direncini iyileştirmiştir.