Year 2017. Volume 37

Abstract

Ceramics for dental applications have become increasingly important in the last decades. Particularly, the introduction of yttria-stabilized zirconia tetragonal polycrystalline (Y-TZP) materials as an alternative to the manufacturing of dental implants and prosthesis has provided a powerful tool to meet the demands required for these replacements in terms of biocompatibility, toughness, hardness and optical properties. Several commercial Y-TZP materials are currently available on the market and strong efforts in research and development facilities are being carried out to improve processing of Y-TZP to fully consolidate odontological pieces. Novel processing methods for ceramic powder sintering, including Y-TZP, aim to reduce processing times and production costs significantly, while maintaining or even improving the resulting microstructure and mechanical properties of the material. One of these methods includes microwave sintering. The purpose of this study is to characterize and compare the resulting properties of Y-TZP materials after conventional sintering and the non-conventional method of microwave heating. In this work one commercial material and one laboratory-synthesized Y-TZP powder are considered. The results suggest that microwave sintering results, generally, in better mechanical properties of the material in terms of hardness and fracture toughness than conventional sintering.

Abstract

In this work, the preparation of Al₂O₃-TiO₂ ceramics by high-energy ball milling varying the molar fraction in 1:1 and 3:1 was investigated. The powder mixtures were processed in a planetary mill at 250rpm and a ball-to-powder weight ratio of 5:1, for 120min. Compacts were obtained by cold pressing at 100MPa. These specimens were heated at 1000°C for 30min to promote the water evaporation, and subsequently sintered at 1500°C for 240min. Samples were characterized by relative density and X-ray diffraction. Hardness and fracture toughness were determined by Vickers Indentation Method. The crystallite sizes were lower than 420 and 560Å in Al₂O₃-TiO₂ and 3Al₂O₃-TiO₂ powders, respectively. After sintering, XRD analysis indicates Al₂TiO₅ and Al₂O₃/Al₂TiO₅ as major crystalline phases for Al₂O₃-TiO₂ and 3Al₂O₃-TiO₂ compositions, respectively. The relative density of the Al₂O₃-TiO₂ ceramics was higher than 90% in both compositions. However, hardness and fracture toughness results of 10.7GPa or 10.5GPa and 3.2MPa.m^1/2 or 2.6MPa.m^1/2 for Al₂O₃-TiO₂ and 3Al₂O₃-TiO₂ mixtures respectively, indicates that microstructure duplex composed by Al₂O₃ and Al₂TiO₅ grains lead to improvement of toughness of these ceramics.

Abstract

ZrO₂(Y₂O₃)-based ceramics with coloring gradient can facilitate the development of dental prosthesis by the improvement of esthetic properties. In this work, ZrO₂ powders with different particle sizes were investigated. White and yellow zirconia powders (TOSOH Corporation-Japan) were characterized by particles size distribution using Nano Sight-LM20 analyzer. Furthermore, samples were characterized by X-Ray diffraction, Scanning Electron Microscopy and relative density. Compacts with two layers, one white and one yellow were uniaxially pressed at 80MPa and sintered at 15300C-120min. The yellow-powder presented average particles size of 180±66nm, while the white-powder presented particles size of 198±73nm. After sintering, full dense ceramics with tetragonal phase were obtained. The linear shrinkage of the yellow and white-layer was 22.75% and 22.05% respectively. This difference in shrinkage is important in the machining of prostheses in ceramic CAD/CAM systems, because they lead to difficulties in adapting this customized prosthesis in patients.

Abstract

Results about the vitrification of several hazardous iron-rich industrial wastes (from productions of Zn and Cu metals, as well as EAFD and MSWA) and the usage of obtained glasses for synthesise of sintered glass-ceramics are summarises. It is shown that the appropriated method of sinter-crystallization mainly depends on the crystallization trend of used glasses. When the parent composition is characterised by formation of a moderate amount of crystal phase low temperature sintering with short crystallization step in the interval 800-900 oC can be used. In addition, attractive granite-like building glass-ceramic can be obtained by mixing fine and coarse glass frits and “free” sintering in refractory moulds at 1000-1050 oC. Contrary, when intensive phase formation carries out the crystallization inhibits the sintering resulting in specimens with scarce degree of densification. However, in some cases a secondary densification process occurs at temperatures near to the eutectic ones, leading to well sintered samples with higher crystallinity and improved mechanical properties. It is also highlighted that the preventing of Fe₂+oxidation by using inert atmosphere during the heat-treatments leads to a decreasing of sintering temperature, finer crystalline structure and additional enhancement of the mechanical properties.

Abstract

Zirconium silicate is generally used as a whitening agent in super white and ultra white porcelain tile  compositions. Depend on the market speculation, this material's price fluctuated. To reduce this negative effect of the material on the our profitability, an alternative whitening compositions is developed. The aim of the study is prepare the alternative whitening composition with an innovative approach such as controlling the amount of the amorphous and crystalline phases in the porcelain tile body and produce the new composition with the same property compared standard body such as water absorption, size, strength, color. And also ensure the sustainable production with the same quality.

Abstract

Spodumene, which is a lithium alumina silicate, has been used as a raw material in the production of thermal shock resistant whitewares and sanitarywares. The presence of spodumene results in enhancement of mullitization and imparts better physical and mechanical properties to ceramics. In this study, the influence of Lithium alumina silicate phases on the mechanical properties of standard porcelain stoneware body was investigated. Especially solid-solid reactions were observed between spodumene and quartz or spodumene and clay. These solid-solid reactions bring about a newly formed lithium alumina silicate (LAS) phases. Spodumene allows the development of a low viscosity liquid phase and results in a decrease in closed porosity, also with increase in bulk density, bending strength and elastic modulus.

Abstract

Non-destructive testing techniques are widely used for testing ceramic materials. In our studies, two different types of ultrasonic test methods (A-scan and C-Scan) were investigated as non-destructive testing methods for characterization of porcelain tiles. Tiles were sintered in different temperatures to change their porosity and density properties. By changing of ultrasonic time and velocity related with samples’ some physical properties (such as bulk density, apparent density, apparent porosity (%), water absorption (%)) inspected via contact A-scan ultrasonic test method. The results show that without necessity of traditional test methods, some physical properties of ceramics can be determined by using obtained ultrasonic velocity-bulk density, apparent density, apparent porosity (%) and water absorption (%) calibration plots. Additionally, various defects were inspected in samples by using water immersion ultrasonic C-scan method. These results supported this study to obtain the information about defects’ size and place in the ceramic tiles. To support this non-destructive method results scanning electron microscope (SEM) characterization was done and images give the information about the place of the defect.

Abstract

Durability of building ceramics is determined by phase composition, the presence of various admixtures and porosity structure. Insertion of salts into the ceramic materials of bricks causes a gradual loss of the original properties of the material. The studies presented in this paper relate to the impact of magnesium sulfate salts to the microstructure change of the ordinary ceramic brick treated. Two types of ceramic bricks were selected for the studies with significantly different properties of shard, which through capillary action were brought to full saturation by solutions. Determination of changes in microstructure of the test materials by the process of crystallization of salts derived from solutions and the crystallization of the corrosion products, was carried out by observation of the microstructure in SEM. In selected places, chemical analysis was performed to determine the changes in the phase composition of the test materials. As a very important factor that affects the stability of ceramic brick is its porosity, pore structure determination was performed by mercury intrusion porosimetry. Based on the received results, the categorization of pores was presented with the indication of priority place of accumulation of crystallizing salts.

Abstract

Investigations were undertaken to assess the chemical and technological characteristics in clay products from the Cretaceous deposit (Moroccan Meseta), in order to use them in refractory products. We have focused mainly on this clay because of its high content of alumina, silica and magnesium oxide, which are very favorable for the synthesis of refractory and ceramic materials. That is why the study has used several techniques of analysis (FX, XRD, SEM and technological tests. The samples were dry pressed on clay bodies and sintered at temperatures ranging from 900 to 1200°C. The relationship between water absorption, shrinkage and flexural strength as a function of the firing temperature was examined in order to enhance the quality of the final products and to optimize the production process. The results revealed that these clays have qualities necessary for the manufacture of ceramic and refractory products.

Abstract

Lavrinhas, a city located 234 km northeast of São Paulo, Brazil, presents syenitic rocks with high content of nepheline, whose preliminary field assessment indicated its potential to be used in the production of ceramic products. The paper aimed to study this raw material with fluxing properties and to determine the best processing methods based on mineralogy and texture, in order to achieve better removal efficiency of the nepheline. Petrographic characterization was carried out in two rock samples, as well as the particle size characterization of the untreated material, the split into fractions for magnetic separation tests and optical microscope observation of the products. In addition, chemical and particle size analysis and melting tests were performed. The latter concerned the material melting color evaluation before and after the magnetic separation, as well as its potential use in glazes and engobes, compared to a standard sample of Nepheline, and inferences for the use in porcelain masses. Preliminary tests showed that after the removal of residual iron oxide, the nepheline raw material bearer has potential to be used in the formulation of porcelain masses, replacing other fluxing materials with higher benefit.

Abstract

During the operation of industrial vessels at high temperatures both elastic and inelastic behaviour of refractories may occur under intense thermomechanical loads. The latter one brings about an irreversible strain, which may cause mechanical failure of refractories and be responsible for the opening of joints. Besides material failure under tensile or shear loads, creep of refractories also may contribute to the occurrence of the irreversible strains. For quantitative assessment of the thermomechanical behaviour of an industrial vessel lining by the simulations, the accurate determination and description of creep behaviour of refractories at service related load levels is of importance. In this paper, a newly developed high temperature compressive creep testing equipment is introduced. Compared with conventional creep testing machines it enables the application of higher load levels as they occur under service conditions. A general testing procedure and displacement/time curves for various temperatures and loads are shown. For simulation purposes a Norton Bailey type creep law representing strain hardening is identified from the test results. An inverse evaluation procedure is performed for this purpose which exhibits high efficiency and robustness.

Abstract

A deep understanding of the mould flux effect on the wear mechanisms of Al₂O₃-C nozzles (AG) is relevant to avoid premature damage and to decrease the cost of black refractories in the industry. In this paper, a post mortem study on a nozzle was carried out to identify the causes of the wear mechanisms during the continuous casting of billets. Physical properties such as viscosity and surface tension of the mould fluxes were determined at operation temperature (1550ºC), in order to correlate with microstructural results obtained by light and scanning electron microscopy (SEM). Also dihedral angle f measurements were carried out at high magnification by SEM. Applying EDS analysis the infiltrated mould flux chemical composition was determined. The study was completed by EBSD. The EBSD technique contributed to increase the knowledge on wear mechanisms because of the possibility of identifying and localizing phases together with crystalline condition. The phases, the grain orientations and the properties of grain boundaries, have a large influence on the corrosion behaviour. Therefore, it is essential to have a characterization technique that can provide information such as: grain size, orientation, misorientation angle and the present phases. In this context, EBSD can provide relevant information on crystallographic and structural analysis of AG nozzle including the insert of ZrO₂-C.