Development and characterization of sintered ceramic materials from waste zirconia based powder mixtures with transition metal additives

Ceramic materials have an ever – increasing importance among high technology materials due to their superior mechanical, thermal and electrical properties. Although conventional ceramic materials do not lose past significance, the demand for new technologies requires new ceramic materials to be processed. Initially, these ceramics were used for high performance applications but later they were developed with the aim of combining different properties. Advanced ceramics which are produced by synthetic chemicals are new members of ceramic family. In the beginning, this ceramics were manufactured for various high performance industrial applications. In recent years, ceramics have been developed specially for multifunction goals to combine the conflincting properties in materials. Generally, multifuntionality is obtained by either decrasing a feature for accepting other properties in expense of high performance , or combining different materials in composite structure. These are common studies to produce big components on a large scale. However, the design becomes more productive when the particle size of components decreases due to the material having more features than it had before. Hence, conflicting properties can be developed with different ways. For this reason, interesting specifications can be advanced for various structural, functional, and biomedical applications. Zirconia exists in nature as two mineral form: baddeleyite (ZrO2) and zircon (ZrSiO4). Pure zirconia has three crystallographic structures which are monoclinic, tetragonal, and cubic. Zirconia transforms from monoclinic structure to tetragonal structure at 1170 ˚C. Then, zirconia transforms into cubic phase at 2370 ˚C. During this transformations, generated volume changes limit the usage of pure zirconia. To eliminate this limitation, zirconia is stabilized with various oxides such as; yttria, ceria, magnesia, calcia. Stabilized zirconia is used in varied applications and is an unrivalled advanced technology material in most applications. Yttrium oxide (Y2O3) is used for stabilizing of pure zirconia at room temperature and forming multiphase material. This multiphase material is named as yittria stabilized tetragonal zirconia polycrystals (Y – TZP). High starting strength and fracture toughness of yttria stabilized zirconia polycrystals originates from the physical properties of partial stabilized zirconia. The tensile stress acting on crack tips result in transformation of metastable tetragonal zirconia to monoclinic phase. In the end of this transformation, increase of 3 – 5% in volume and 16% shear stress occurs. This increase in volume generates local compressive strength at the crack tip and around it and this compressive strength removes the tensile stress acting on the crack tip. This physical property is named transformation toughening and retards crack propogating. Under high stress crack propogating may proceed. Toughening mechanism does not prevent crack propogation, just makes it difficult. Zirconia has an important place among advanced technology materials due to its superior properties such as: high melting temperature, high resistance to acidic chemicals, high abbrasion and corrosion resistance, low thermal expansion coefficient, high fracture toughness and ionic conductivity at elevated temperatures. Accordingly, it has a broad area of usage because of these properties. Zirconia can be used as a refractory material, in heater elements, as an isolation material, in abrassives and in production of cutting tools, extrusion dies, abrrasion resistant machine components, coatings for thermal barriers, production of ceramic filters, fuels cells, piezoelectrical and electro – optical circuits, capacitors, turbine blades etc. In this thesis study, it is aimed for production of ceramic materials from waste zirconia powders of dental laboratories and transition metal additives, and investigation of the resultant effects on mechanical properties by perfoming mechanical tests and characterization methods. Furthermore, effect of cold isostatic press (CIP) is studied with the purpose of upgrading mechanical properties. Initially, pre – sintered blocks supplied from dental laboratories were mixed with waste powders which are also supplied from the same dental laboratories. Mixed waste powders were characterized using X – ray fluorescence (XRF) analysis and X – ray diffraction (XRD) analysis. Also, particle size measurements of mixed waste powders and transition metal powders were practiced. Mixed waste powders and transition metal (Ni and Co) powders with diffrerent weight percent ratios were mechanically milled for 30 hours in the presence of isopropyl alcohol in a high energy planetary ball mill. The ball to powder ratio was chosen 7:1. As a binder, polyvinyl alcohol was just used for the specimen in which transition metals were not added into (3 – YSZ). Mechanically milled powders were characterized using XRF and XRD analysis and also particle size of the powders was measured. After this characterization processes, bulk specimens were formed in cylindirical press mold with axial press under 130 MPa pressure from mechanically milled powders. CIP were applied to a group of this specimens under 400 MPa pressure. Bulk 3 – YSZ specimens were heated with 2 ˚C/min speed up to 650 ˚C and for 2 hours as hold time in order to debind polyvinyl alcohol completely at this temperature. Specimens formed with axial press and both axial and cold isostatic presswere sintered at 1500 ˚C for 2 hours. Sintered specimens were characterized using XRD and SEM/EDS analysis, microhardness test, compressive strength test, and fracture toughness test. Results of the studies show that, CIP has a possitive effect on mechanical properties of the materials due to the more compact and homogenous structure obtained after CIP application. As a result, mechanical properties of the specimens which were formed by both axial press and CIP are better than the specimens formed with just axial presses. Addition of transition metals to 3 mole Y2O3 stabilized ZrO2 lead up to decrease in microhardness of the specimens. 8 Ni (1293±76.96 HV) which has the highest microhardness value beside 3 – YSZ among all specimens which was formed with just axial press also has the highest micro hardness value (1463±86.25 HV) beside 3 – YSZ among specimens formed with both axial press and CIP. Within the specimens formed with just axial press except 4 Ni 4 Co and 8 Ni, addition of transition metals to 3 mole Y2O3 stabilized ZrO2 enhances compressive strentgh. 6 Ni 2 Co has the highest compressive strentgh value which is 2189.04±265.69 MPa. Among the specimens formed with both axial press and CIP, 3 – YSZ has the highest value (3277.98±325.21 MPa) of the compressive strength. Addition of transition metals to 3 – YSZ CIP decreases the compressive strentgh. Beside 3 – YSZ CIP, 8 Ni with the lowest compressive strength value within the specimens formed with just axial press has the highest compressive strength value which is 2503.47±328.00 MPa within the specimens both axial press and CIP. The fracture toughness of the specimens increases with addition of transition metals to 3 mole Y2O3 stabilized ZrO2. Only, the specimen which has just Ni rather than addition of both transition metals has lower fracture toughness than (4.91±1.02 MPa.m1/2) 3 – YSZ CIP (5.07±2.29 MPa.m1/2). 6 Ni 2 Co CIP has the highest fracture toughness value which is 6.73±0.47 MPa.m1/2. The fracture toughness of 6 Ni 2 Co formed with both axial press and CIP is higher than the fracture toughness of 6 Ni 2 Co formed with just axial press. This shows that CIP has a positive effect on the fracture toughness of the materials. Moreover, the fracture toughness of the 6 Ni 2 Co is higher than the fracture toughness of the other specimens formed with both axial press and CIP. Addition of transition metals to 3 mole Y2O3 stabilized ZrO2 caused decrease in elastic modulus of all specimens, but 8 Ni CIP. The elastic modulus of the 8 Ni CIP (291.368 GPa) is higher than the elastic modulus of the 3 – YSZ CIP (281.868 GPa).