Papers by Author: Kurt Strecker

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Authors: Claudinei dos Santos, Kurt Strecker, M.J.R. Barboza, Sandro Aparecido Baldacim, Francisco Piorino Neto, Olivério Moreira Macedo Silva, Cosme Roberto Moreira Silva
Abstract: The creep behavior of hot-pressed Si3N4 ceramics was investigated. The proposal of this work is to investigate the use of yttrium-rare earth oxide mixture, CRE2O3, produced at FAENQUIL, as sintering additive, since the cost of production of this material is 80% inferior to Y2O3. These ceramics were obtained by uniaxial hotpressing using different additive contents and mixtures (CRE2O3-Al2O3 or CRE2O3- AlN). Compressive creep tests were carried out at 13000C and 300 MPa, in air. The Si3N4-CRE2O3-Al2O3 ceramics demonstrated that the creep resistance is inversely proportional to the additive content. Mixtures with high intergranular phase content presented low creep resistance due to high oxidation and more pronounced softening of the intergranular phase. Si3N4-CRE2O3-AlN ceramics demonstrated better creep resistance with a steady-state creep rate of 7 x 10-8 s-1. This behavior is related to the a-SiAlON content, a solid solution of Si3N4 that incorporates a great fraction of intergranular phase, decreasing the amount of intergranular phase.
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Authors: Claudinei dos Santos, Kurt Strecker, M.J.R. Barboza, Sandro Aparecido Baldacim, Francisco Piorino Neto, Olivério Moreira Macedo Silva, Cosme Roberto Moreira Silva
Abstract: a−SiAlON (a’) is a solid solution of a−Si3N4, where Si and N are substituted by Al and O, respectively. The principal stabilizers of the a’-phase are Mg, Ca, Y and rare earth cations. In this way, the possible use of the yttrium-rare earth oxide mixture, CRE2O3, produced at FAENQUIL, in obtaining these SiAlONs was investigated. Samples were sintered by hotpressing at 17500C, for 30 minutes, using a sintering pressure of 20 MPa. Creep behavior of the hot-pressed CRE-a-SiAlON/b-Si3N4 ceramic was investigated, using compressive creep tests, in air, at 1280 to 1340 0C, under stresses of 200 to 350 MPa, for 70 hours. This type of ceramic exhibited high creep and oxidation resistance. Its improved high-temperature properties are mainly due to the absence or reduced amount of intergranular phases, because of the incorporation of the metallic cations from the liquid phase formed during sintering into the Si3N4 structure, forming a a’/b composite.
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Authors: Claudinei dos Santos, Kurt Strecker, Francisco Piorino Neto, Cosme Roberto Moreira Silva, Flávia A. Almeida, Rui F. Silva
Abstract: The objective of this work was to evaluate the creep behaviour of Si3N4 based ceramics obtained by uniaxial hot-pressing. As sintering additive, an yttrium-rare earth oxide solid solution, designed RE2O3, that shows similar characteristics to pure Y2O3, was used. Samples were sintered using high-purity α-Si3N4 powder, with additive mixtures based on RE2O3/Al2O3 or RE2O3/AlN, at 5 and 20 vol.%, respectively. The sintered samples were characterized by X-ray diffractometry, scanning electron microscopy and density. Specimens of 3x3x6 mm3 were submitted to creep tests, under compressive stresses between 100 and 350 MPa at temperatures ranging from 1250 to 13750C in air. Samples with RE2O3/Al2O3 showed β-Si3N4 as crystalline phase, with grains of high aspect ratio, and a relative density around 99% of the theoretical density. The Si3N4/RE2O3/AlN samples presented α-Si3N4 solid solution, designed α-SiAlON, with a more equiaxed microstructure and slightly lower relative density (96-98%). The results of creep tests indicated that these ceramics containing α-SiAlON are the more creep resistant, with steady-state creep rates around 10-4 h-1, with stress exponents (n) in the range 0.67-2.53, indicating grain boundary sliding as the main creep mechanism.
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Authors: Claudinei dos Santos, Kurt Strecker, M.J.R. Barboza, Francisco Piorino Neto, Olivério Moreira Macedo Silva, Cosme Roberto Moreira Silva
Abstract: Commercial α−Si3N4, Al2O3 and a mixed yttrium and rare earth oxides, RE2O3, were used as starting-powders. Powder batches were milled using different Al2O3/RE2O3 contents, as additive. Hot-pressing was done at 1750oC-30 min-20MPa in N2 atmosphere. Specimens neat to 6x3x3mm3 were polished and characterized by XRD and SEM. Specimens were submitted to creep tests, under compressive stresses between 100 and 350 MPa at temperatures ranging from 1250 to 1300oC in air. Higher additive amounts resulted in larger grains of higher aspect ratios and in a decreased anisotropy in the hot-pressed ceramics. The compressive creep behavior depends on the intergranular phase content. While higher amounts of additives resulted in higher creep rates, • ε , and higher stress exponents, n, the activation energy Qss, has been inferior for samples with lower additive contents. Grain sliding has been identified to be the predominant mechanism responsible for creep deformation of these ceramics.
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Authors: Claudinei dos Santos, L.H.P. Teixeira, Kurt Strecker, Carlos Nelson Elias
Abstract: In this work, the effects of alumina additions on the properties of the ZrO2-Al2O3 ceramic composites were investigated. Samples of ZrO2 with Al2O3 additions varying between 0 and 30wt-% were prepared. The powder mixtures were milled, compacted by uniaxial cold pressing and sintered at 16000C, in air, for 2 hours. The sintered samples were characterized by their relative density, phase composition and microstructure. As mechanical properties at room temperature, their Vickers hardness and fracture toughness were determined: In all sintering conditions and Al2O3 amounts, the samples presented relative density higher that 99%. The Al2O3 addition produces a linear increase of the hardness, reaching values between 1350 and 1610 HV for the addition of 0 and 30% of alumina, respectively. The fracture toughness was near to 8 MPam1/2 in all conditions. The phase composition, microstructure and relative density were correlated in order to interpret the mechanical properties obtained.
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Authors: Claudinei dos Santos, L.H.P. Teixeira, J.K.M.F. Daguano, Kurt Strecker, Carlos Nelson Elias
Abstract: In this work the influence of isothermal sintering time on the microstructural development of ZrO2-Al2O3 composite was studied. Powder mixture of ZrO2 containing 20 wt% Al2O3 was prepared by milling, compaction and sintering at 16000C, in air. The isothermal sintering time at 16000C was varied between 0 and 1440 min. The sintered samples were characterized in terms of phase composition and relative density. Their microstructures were characterized by grain size distributions and average grain size. These results were evaluated using the classic grain growth equation as a function of time, determining the grain growth exponent of these materials. Furthermore, the microstructural aspects were related to the mechanical properties (Vicker’s hardness and fracture toughness) of these composites.
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Authors: Kurt Strecker, Helder B. Costa
Abstract: Basically, a traditional ceramic glaze is a thin layer of glass created on the ceramic surface to enhance the aesthetic appearance and also to waterproof the artifact. Almost all glazes are based on quartz, SiO2, and a flux such as Na2O or K2O, similar to the composition of common glass, which contains approximately 72% SiO2 and 15% Na2O, and is widely used as windows or in glass packaging. The current work investigated the formulation of glazes using recycled waste glass powder for firing temperatures of 1100 and 1250°C. Three transparent base glaze formulations were developed and by adding coloring agents such as iron, cobalt or copper oxide several colored glazes could be successfully produced. In this way, up to 30% of recycled waste glass powder could be incorporated into the glazes.
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Authors: Aloízio Geraldo de Araújo Jr., Leandro José da Silva, Túlio Hallak Panzera, Adriano Galvão da Souza Azevedo, Kurt Strecker
Abstract: It is known that the hydration of cement paste is influenced by a variety of factors, it is also known that some hydration products are gradually dehydrated at elevated temperatures. In doing so, different author studied the dehydration of hydrated cement pastes under different condition. In this work, samples of Hydrated Cement Paste (HCP) were prepared from Portland cement of high initial strength (CP V-ARI) with a water/cement ratio of 0.5. The morphological changes during hydration and dehydration by subsequent heat-treatments were analyzed by X-ray diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). Thermal Gravimetric Analysis (TGA) was used to study the thermal stability of the HCP. Dehydrated cement powder samples (DCP) were obtained heat treating samples of HCP at 300, 500, 700 and 900°C. After 7 days of curing HCP samples exhibited no significant changes in its structure. HCP dehydrated at 500°C showed the absence of Ca (OH)2 and calcium silicate hydrate. At 700°C the formation of β-2CaO.SiO2, 3CaO.SiO2 and CaO is observed. During heat treatment at 900°C the HCP revealed a significant mass loss of 36%.
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Authors: Marcelo José Bondioli, Juliana K.M.Faria Daguano, A.A. Palmeira, Claudinei dos Santos, Kurt Strecker
Abstract: Zirconia is a bioceramic material widely used for dental implants. In this work, the sinterability of nano-crystalline powders has been investigated by dilatometry in the temperature range of 1250 to 1400 0C with isothermal holding times of up to 8h. A slight increase in grain growth and an increasing linear shrinkage have been observed with increasing sintering temperatures. The sintered samples were submitted to Vickers' hardness and KIC tests and the results compared regarding the sintering conditions. It has been verified that satisfactory hardness and fractures toughness have been achieved after sintering above 1300 0C during 8h.
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