Al2O3   ZrO2   CeO2 kompozitlerinin spark plazma sinterleme (SPS)yöntemi ile üretimi ve karakterizasyonu
Production and characterization of Al2O3   ZrO2   CeO2 composites by spark plasma sintering (SPS)
Yazar: BETÜL ZEYNEP BİLGİÇ
Danışman: PROF. DR. GÜLTEKİN GÖLLER
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Seramik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2012
Konu: Mühendislik Bilimleri = Engineering Sciences ; Seramik Mühendisliği = Ceramic Engineering
Dizin: Alümina = Alumina ; Seramik kompozitler = Ceramic composites ; Seryum dioksit = Cerium dioxide ; Sinterleme = Sintering ; Zirkonyum oksit = Zirconium oxide

Özet:
Yüksek mukavemet ve sertlik, kimyasal stabilite, ısıl dirence ve sürünme direnci gibi mükemmel mekanik özelliklerden dolayı yapısal seramik malzemeler son yıllarda daha çok incelenmeye başlanmıştır. Mükemmel özelliklerinin yanında, düşük kırılma tokluğu ve pratik uygulamalarda kullanılmasını sınırlayan zayıf şekillenebilme özelliği seramik malzemelerin sahip olduğu dezavantajlardandır. Kompozit malzeme yapısının iyileştirilmesiyle birlikte kullanılan yöntemlerde de gelişmeler gözlenmiştir.Alümina (Al2O3) yüksek sertlik, ısıl kararlılık, yüksek korozyon dayanımı ve biyolojik uyumluluk özelliklerine sahiptir. Bu özelliklerinin yanında, alüminanın sahip olduğu düşük eğme mukavemeti ve düşük kırılma tokluk değeri kullanılabilirliğini sınırlandırmıştır. Kırılma tokluğunu arttırmak amacıyla birçok teknik geliştirilmiştir. Yapısına alüminadan yüksek eğme mukavemeti ve kırılma tokluğu değerine sahip zirkonyum oksit (ZrO2) katılması bu tekniklerden birisidir.ZrO2 esaslı seramik malzemelerin kullanılması sırasında karşılaşılan en büyük dezavantaj tetragonal fazdan monoklinik faza dönüşüm sırasında birim hacmin artmasıyla birlikte mukavette görülen azalmadır. Stabilizatörlerin de katkısıyla dönüşüm sonucunda oluşan mukavemet azalmasını en aza indirmek için yarı kararlı t-ZrO2 (tetragonal)'den kararlı m-ZrO2 (monoklinik) faza dönüşüm engellenerek, tetragonal faz mikroyapıda tutulur.İtriyum oksit (Y2O3), sahip olduğu tokluk ve tetragonal fazın yapıda kalmasına olanak veren etkisi sebebiyle zirkonya seramiklerinde en yaygın şekilde kullanılan stabilizatördür. Yorulma ömrü ve kırılma tokluğunu arttırmak amacıyla alümina seramiklerine itriyum oksit ile stabilize edilmiş zirkonya (YSZ) katılmaktadır.Nadir toprak ailesinin en çok bulunan elementi seryumdur. Yakıt hücreleri, optik filmler, parlatma malzemeleri, gaz sensörleri ve biyomedikal alanlarda kullanılabildikleri için seryum bazlı malzemeler geniş uygulama alanlarına sahiptir. Seryum dioksitin (CeO2) sahip olduğu yüksek korozyon direnci, termal kararlılık ve aynı zamanda hücre canlılığını arttırma özelliği CeO2 kullanımına yönelik çalışmaları arttırmaktadır.Gerçekleştirilen deneysel çalışmalarda, yüksek sertlik ve yüksek kırılma tokluk değerlerine sahip Al2O3-YSZ ikili ve Al2O3-YSZ-CeO2 üçlü kompozitlerinin spark plazma sinterleme (SPS) yöntemi ile üretilmesi amaçlanmıştır.Deneysel çalışmalar kapsamında Al2O3-YSZ-CeO2 kompozitleri SPS tekniği ile üretilmiştir. İlk aşamada hacimce farklı oranlarda (%5, 15 ve 25) YSZ içeriği ile hazırlanan Al2O3 ? YSZ kompozitlerine ikinci aşamada ağırlıkça %3 ve %5 CeO2 katılmıştır. CeO2 ilavesinin Al2O3-YSZ-CeO2 kompozitlerinin yapısında oluşturduğu mikroyapısal ve mekanik etkiler incelenerek karakterizasyonu yapılmıştır.Tek eksenli uygulanan basınç altında yapılan sinterleme işleminde ark yaratmak amacıyla darbeli doğru akım kullanılarak sinterleme işlemi yapılmıştır. Spark plazma sinterleme yöntemiyle yapılan üretimde, 50 mm çapında kalıplara yerleştirilen tozlar 1350oC ve 1400oC sıcaklıklarında 5 dakika süresince 40 MPa basınç altında ve vakum ortamında sinterlenmiştir. Sinterlenen numunelerin densifikasyon davranışları incelenmiş, mikroyapı ve mekanik analizleri yapılmıştır.Deneysel çalışmalar sonucunda Al2O3-YSZ kompozitlerine ait en yüksek Vickers sertlik değeri 19,95 GPa değerinde 85A15Z kompozitinden elde edilmiştir. Al2O3-YSZ kompozitleri içerisinde en yüksek kırılma tokluk değeri 4,1 MPa·m1/2 ile 75A25Z kompozitine aittir. Saf Al2O3'e YSZ ilavesi, oluşan kompozitlerin Vickers mikrosertlik ve kırılma tokluğu değerini arttırmıştır. Artan sertlik ve kırılma tokluk değerinin YSZ'nin göstermiş olduğu dönüşüm toklaşması özelliğinden kaynaklandığı düşünülmektedir.Al2O3-YSZ kompozitlerinin mekanik özelliklerine CeO2'nin etkisi incelendiğinde, CeO2 oranı %3'ten %5'e çıkarıldığında sertlikte azalma gözlenmiştir. En yüksek Vickers mikrosertlik değeri 85A15Z3C numunesinden 18,9 GPa olarak ölçülmüştür. CeO2'nin kırılma tokluğuna etkisi incelendiğinde CeO2 miktarı arttıkça kırılma tokluğunda azalma gözlenmiştir. Mikroyapı incelemeleri sonucunda kırılma tokluğunda meydana gelen düşüşün seryum alüminat tanelerinde sapmaya uğramadan geçen çatlak ilerleyişinden kaynaklandığı düşünülmektedir.Hücre kültürü testlerine göre osteoblast hücreleri Al2O3-YSZ ve Al2O3-YSZ-CeO2 numuneleri bulunan kültür ortamında büyümeye devam etmiş, olumlu veya olumsuz herhangi bir etkileşime girmediği görülmüştür.

Abstract:
Investigating structural ceramic materials have been more popular recently because of their superior mechanical properties such as high strength and hardness, chemical stability, heat resistivity and creep-resistance. However, its low fracture toughness and poor machinability feature limits the use of practical applications is one of the disadvantages of ceramic materials. The methods that are developed in order to achieve the desired properties should contain developing the composite material structure first.Alumina (Al2O3) has a high hardness, high thermal stability, high corrosion resistance and good biocompatibility properties. However, its low flexural strength and low fracture toughness value restrict the availability of biomedical applications. In order to increase fracture toughness of alumina, various techniques have been investigated. One of these techniques is adding zirconia (ZrO2) into alumina structure because of the being inert in-vivo, having higher flexural strength and higher fracture toughness value when compared to alumina. Thus, it has high strength enough to handle and transfer the loading forces coming from joint and muscle.The major disadvantage during the use of ceramic materials based on ZrO2 is increasing the volume during transformation of the tetragonal phase to monoclinic phase and the decrease in strength with density. With the help of stabilizers, phase transformation from semi-stable (tetragonal) t-ZrO2 to stable (monoclinic) m-ZrO2 is prevented and kept tetragonal phase in microstructure.Yttria (Y2O3), remains the most widely used stabilizer in zirconia ceramics, due to the effect of its toughness and structure of tetragonal phase. Therefore, zirconia stabilised with yttria, is mixed with alumina seramics. Increased fatigue life and fracture toughness of alumina and zirconia composites (Al2O3-YSZ) stabilized with 3mol% Y2O3 is used the areas that exposed to constant load, such as hip replacement and knee combinations as hard tissues or surgical or dental implant used.Cerium is the most abundant element in rare-earth family. Ceria based materials are used in many areas such as fuel cells, optical films, polishing and biomedical applications due to their widespread applications. Cerium dioxide (CeO2), with its high corrosion resistance, thermal stability and enhanced cell viability and function, have encouraged the development of biomaterials containing CeO2. According to osteoblast cell culture tests, addition of 1 wt% CeO2 to glass ceramics showed better cell viability. In this study, CeO2 is used as a secondary phase with ytrria stabilized zirconia instead of stabilizing agent.Al2O3-YSZ-CeO2 composites owe their high strength to the phase of toughening alumina with YSZ. When the structure stimulated within the help of the notch opening for the purpose of finding the critical fracture toughness of the structure, crack passes between the layers or damage the layers. Changing the direction of the crack depends on the position of the layers. This mechanism leads to a decrease in crack propogation energy and increase in fracture toughness.In the studies, with high hardness and high fracture toughness values, as well as high biocompatibility with the value of the mechanical properties of Al2O3 and Al2O3-YSZ-CeO2 triple stressed composites, can be used as load-bearing parts of the implant material is planned to develop an alternative material.Producing Al2O3-YSZ binary and Al2O3-YSZ-CeO2 ternary composites with high hardness and high fracture toughness values by spark plasma sintering (SPS) method is aimed.By the scope of the studies, Al2O3-YSZ-CeO2 composites were produced using spark plasma sintering (SPS) method. After the first stage which is preparing Al2O3-YSZ composites with different volume ratio (5, 15, and 25 vol% YSZ) and joining 3 and 5 wt% CeO2. Finally, characterization of Al2O3-YSZ-CeO2 composite systems characterization is done by examining the generated microstructural and mechanical effects. Moreover, cell culture tests were carried out in order to determine the biocompatibility of composites.Powders that are mixed by a ball mill with ethanol environment for 24 hours in order to achieve homogeneous mixture were subjected to the SPS by placing in graphite molds with 50 mm in diameter. Graphitic sheet was placed between powder and die in order to have better conductivity and remove easily sample from die after sintering. A uniaxial pressure of 40 MPa was applied with a pulsed direct current (12 ms/on, 2 ms/off) during the SPS process. The pulsed direct current flows through the graphite die, the punches and the powder. The temperature was measured by an optical pyrometer focused onto the sintered sample through a small hole in the graphite die surface. Shrinkage of the sample during the SPS process was monitored by the displacement of the punch rods.Relative density values of the samples studied, the microstructure and phase analysis was performed after sintering in a vacuum under 40 MPa pressure and with temperature range of 1350°C and 1400°C for 5 min. Afterwards, microhardness and fracture toughness were measured and cell culture tests were carried out by the immersed samples in the simulated body fluid solution, nearly equal to human blood plasma.Samples were identified by X-ray diffraction (XRD) analysis. An XRD machine was used with CuK? radiation, at a scanning speed of 2?min-1 and in the 2? range from 20? to 80?. Bulk densities of the sintered samples were measured by using the Archimedes method and converted to relative density using theorical densities of Al2O3 (3.97 Mg/m3), YSZ (6.05 Mg/m3) and CeO2 (7.65 Mg/m3).The sintered samples were polished to 1?m. The hardness was determined by Vickers indentation on polished surfaces. Vickers hardness was determined from a minimum of 20 indents using a load of 9.8 N and the fracture toughness KIC was calculated under load of 19.6 N.Microstructure of monolithic Al2O3 consisted of both large and small equiaxed grains 0.5?3 mm in size and straight grain boundaries. The fracture surfaces of Al2O3-YSZ composites containing 5 vol% YSZ indicated that grain growth of Al2O3 was inhibited because of the fact that small zirconia grains were found at the grain boundaries or triple junctions of Al2O3 grains.Elongated grains of CeAl11O18 was observed in Al2O3 matrix. For Al2O3-YSZ composites containing 3 and 5 mass% CeO2, the size of elongated CeAl11O18 grains increased with increasing CeO2 content.Al2O3-YSZ composites without CeO2 had higher hardness than monolithic Al2O3 and Al2O3-YSZ-CeO2 composites. Hardness of Al2O3-YSZ composites decreased from 20.0 GPa to 19.0 GPa when the content of ZrO2 increased from 15 to 25 vol%. Moreover,The presence of YSZ particles results in an improved toughness than pure Al2O3. At the same time, the fracture toughness of the composites increased from 3 to 4.1 MPa·m1/2 with increasing volume fraction of YSZ from 5 to 25 vol% and the highest value of fracture toughness, 4.1 MPa·m1/2, was achieved with the addition of 25 vol% YSZ. To examine the effect of the addition of 3-5 wt% CeO2 into the Al2O3?YSZ composites containing 5, 15 and 25 vol% YSZ are studied. The highest hardness value, 18.9GPa, were obtained with the addition of 3 wt% CeO2 for the 85A15Z3C composite. However, the hardness of Al2O3-YSZ composites significantly decreased with the addition of CeO2 and further addition resulted in lower hardness values. This could be attributed to the formation of elongated CeAl11O18 grains due to reduction of CeO2 to Ce2O3 and segregation of Al2O3. Fracture toughness of the composites containing 3 and 5 mass% CeO2 increased with increasing YSZ content. Because of the fact that the expansion of the inclusions when the zirconia activate the tetragonal to monoclinic phase transformation, having highest rate of YSZ into Al2O3?YSZ composites with or without CeO2 provides highest fracture toughness values. However, Al2O3-YSZ-CeO2 composites had lower fracture toughness value when compared to Al2O3-YSZ composites.Cell viability and alkaline phosphates in the presence of 85A15Z, 85A15Z3C, and 85A15Z5C samples were studied against controls. Cell viability/proliferation was not prevented by the presence of samples. Cell viability in the presence and absence of samples increased within the first 3 days and the similar increase ratio was observed when samples compared to control.When alkaline phosphatase activities were measured at the end of the incubation period, materials did not obstruct alkaline phosphate production. Moreover, samples containing 3 and 5 mass% CeO2 showed greater activity than 85A15Z composite without CeO2.The cell culture test (cell viability and alkaline phosphatese tests) showed that Al2O3-YSZ ceramics containing %15 YSZ with 3 and 5 mass% CeO2 have good biocompability and there was no unaccepted observation as a function of ceria content. Moreover, Al2O3-YSZ-CeO2 composites containing 3-5 mass% CeO2 showed almost same cell viability and alkaline phosphatese activity compared control and also better than without CeO2.