Development of sintered ceramic based materials from seydi̇şehi̇r alumina powders with nickel and cobalt additives and pertinent characterization investigations

Ceramics has an ever-increasing importance among high technology materials due to their superior mechanical, thermal and electrical properties. Although it is hard for conventional ceramic materials to maintain their past significance further, developing technologies demand new ceramic materials to be processed. Initialy, these ceramics were used for high performance applications but later they were developed with the aim of combining different properties. The manufacturing process plays an important role on final microstructure and properties when ceramic materials are in question. For a specific production method a minor change in the processing condition may result in strong influences on these features. Ceramic materials which are formed through powder compaction methods, often contain large pores in their sintered bodies that are responsible for reduction of the fracture toughness. Typically, the structure of the powder granules is the reason of these large pores as the incompelete adhesion between particles causes void spaces to form at center region and grain boundaries in green compacts. Unfortunately, these defects persist, in fact grow during sintering process generating fracture origins that trigger loss of fracture strength in sintered material. Thus, it becomes quite an important task to maintain both the structure and compaction characteristics of these granules in order to ensure reduction in size of the pores to provide improved strength for ceramics. Alumina, which is a member of advanced ceramic materials has a prominent place due to its cost-effectiveness, high abrassion resistance, acidic and alkaline resistance at high temperatures, good thermal conductivity, high strength and stiffness, and purtiy ranging from 94% to 99.8% on a basis of high temperature performance demand. Today alumina is primarily used in high temperature furnaces, cutting tools, abrrasion resistant tubes, thermometer sensors, electronic industries, laboratory equipment, high voltage isolators, implants and prothesis, armor applications and grinding media. Despite its superior properties, due to the ionic and covalent bonds in its structure alumina has a rather limited dislocation movement and low fracture toughness as a consequence. In order to upgrade these mechanical properties, studies often concentrate on metalic or intermetallic compounds addition to alumina matrix composites. It is highlighted that for further improvement in mechanical properties use of components in nanometer scales is necessary. This thesis study is build on preliminary experiments carried out following the 2013 study 'Examination of Binary Systems of Alumina – Transition Metal As Armor Material' by Ahmet Burçin Batıbay. Utilizing Bayer alumina powders from Seydişehir Plant of Eti Aluminum Co.Inc., which is one of the local raw materials, sintered ceramic materials with transition metal and rare earth oxide additives were developed via powder metallurgy practices. The preliminary experiments were focused on improvement of sintering conditions necessary for liquid phase sintering of these ceramics with ductile transition metal additives based on studies in literature and the effect of eliminating the use of binders in mechanical milling processes on final porosity, microstructure, and thereby mechanical properties of sintered specimens taking advantage of these binding phases. In literature studies, insufficent wetting of the structure via transition metal addition to alumina during liquid phase sintering below relevant temperatures, likewise negative effects of sintering additive rare earth oxides on sintering mechanism of alumina below specific temperatures, direct influence of manufacturing and compaction methods of ceramic materials on microstructure and properties, utilizing the calcined alumina with low soda content obtained after Bayer process in development of advanced ceramic materials have become the main motivations of this thesis study. In the scope of this study, the influence of compaction method and addition of La2O3 as a rare earth oxide additive with Ni and Co as transition metals on final properties were surveyed by characterization studies, mechanical tests and microstructure examinations. Sintering temperature was chosen as 1700°C regarding the effect of additives on sintering of alumina with the help of literature studies. Aside from specimens formed with uniaxial pressing machine, specimens preformed with unixal press and later on compacted via cold isostatic press (CIP) were also prepared, inspecting the impact of compaction method on final properties of the material produced. Prior to powder production via mechanical milling, the starting material Seydişehir alumina powder was calcined at 1200°C for an hour in order to remove volatile materials and obtain the complete transformation of the structure to stable corundum phase for sintering process. Following calcination, XRF, XRD and particle size measurement analyses of the powder material were carried out. Ni and Co transition metals were used up to ~wt. 8% as various combinations with ~wt. 1% La2O3 and without binder additives in powders produced by mechanical milling for preparation of bulk specimens. However, polyvinyl alcohol was used as a binder in undoped Seydişehir Al2O3 powders with no ductile phase content. Powder mixtures were mechanically milled in the presence of isopropyl alcohol for 30 hours with a 7:1 ball to powder ratio in a high energy ball mill. After mechanical milling XRF, XRD and particle size measurement analyses of dried powders were performed. Pellet shaped specimens were formed under 130 MPa pressure using a uniaxial pressing machine. For determining the impact of compaction method, specimens with the same chemical compositions preformed at 130 MPa pressure by uniaxial press and later on compacted under 400 MPa pressure via CIP were also produced. Debinding procedure was executed for undoped Seydişehir Al2O3 specimens with a heating up speed of 2 ˚C/min up to 650°C and a hold time of 2 hours. All prepared specimens were sintered under Ar and H2 atmosphere at 1700°C for 4 hours. Characterization studies include size and density measurements of sintered specimens, XRD analyses, optical microscopy examinations, SEM/EDS analyses. Mechanical tests performed in order to determine mechanical properties consist of microhardness measurements, compressive strength and fracture toughness tests. Addition of transition metals and rare earth oxide to calcined Bayer alumina had a possitve effect on material properties, especially fracture toughness. Mechanical properties of the specimens which are formed by CIP showed better improvement compared with the specimens formed by axial presses. However, the applied pressure during forming of the specimens, especially the during CIP application was high enough for ceramic based materials which have been resulted in new defects in some sintered bodies and resulted in poor performance in mechanical tests and variation between results. For better results and further studies, application of appropriate forming processes and pressure with the characteristics of the starting powder granules, also control of grain growth regarding the high sintering temperature are recommended.