Papers by Author: Rui F. Silva

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Authors: V.M. Silva, A.J.S. Fernandes, Florinda M. Costa, J. Sacramento, Rui F. Silva
Authors: Filipe J. Oliveira, Rui F. Silva, José Maria F. Ferreira, Joaquim M. Vieira
Authors: J.M. Carrapichano, A. Taillaire, Filipe J. Oliveira, Rui F. Silva
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.
Authors: M. Boutinguiza, F. Quintero, Manuel Belmonte, A.J.S. Fernandes, Florinda M. Costa, Rui F. Silva, Juan Pou, Mariano Pérez Amor
Authors: Filipe J. Oliveira, Rui F. Silva, Joaquim M. Vieira
Authors: Filipe J. Oliveira, Rui F. Silva, Joaquim M. Vieira
Authors: Claudinei dos Santos, R.C. Souza, Maria Helena F.V. Fernandes, Nuno A.F. Almeida, Flávia A. Almeida, Rui F. Silva
Abstract: In this study, the influence of La-rich glass addition and sintering conditions on the densification and mechanical properties of 3 mol.%Y2O3-stabilized tetragonal zirconia polycrystals (3Y-TZP) ceramics were evaluated. High-purity tetragonal ZrO2 powder stabilized with 3 mol.% Y2O3 and La2O3-Rich glass were used as starting powders. Two compositions, ZrO2 containing 5 and 10 wt.% of a La2O3-rich glass were studied in this work. The starting powders were mixed/milled by planetary milling, dried at 90°C for 24 hours, sieved through a 60 mesh screen and uniaxially cold pressed under 80 MPa. The samples were sintered in air at 1200, 1300 and 1400°C for 60min, and at 1450°C for 120min, with heating and cooling rates of 10°C/min. Sintered samples were characterized by relative density, X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). The mechanical properties, hardness and fracture toughness, were obtained by Vickers indentation method. Dense sintered samples were obtained for all conditions. Samples sintered at 1300°C for 60 min presented the optimal mechanical properties with hardness of 1170 and fracture toughness of 8.3 MPam1/2.
Authors: Flávia A. Almeida, Helena Bóia, Catarina Santos, Jorge Monteiro, Filipe J. Oliveira, Rui F. Silva
Abstract: The addition of titanium nitride (TiN) particles to a Si3N4 matrix reduces the intrinsic electric resistivity of this ceramic allowing it to be machined by EDM in cutting tools manufacturing. Gains can be expected given the cost reduction by the increase of productivity when shaping these hard to machine ceramic materials. Si3N4 ceramic matrix composites (CMC’s) with 0- 30vol.% of TiN sub-micrometric particles were produced by uniaxial hot pressing (HP) and pressureless sintering (PS). For the PS samples, EDM tests showed that machining of the composites is possible when they contain at least 23vol.% TiN particles what corresponds to a resistivity of 7.5cm. For HP samples at least 30vol.% of TiN is required to get an electroconductive material for EDM machining. This difference is due to the lower temperatures used in the HP process that delay the formation of a conductive network between the TiN particles.
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