Papers by Author: 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 kgf.mm-2 and fracture toughness of 8.3 MPam1/2.

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.

Abstract: In-situ measurements of acoustic emission (AE) in self-mated tribological pairs of CVD diamond coated silicon nitride (Si3N4) were made with the purpose of investigating the relationship between AE signal and friction events. A good correlation is found between the energy dissipation/emission processes, therefore enabling the possibility of monitoring the different friction regimes occurring during the sliding contact of microcrystalline diamond (MCD) coatings. Deposition of MCD on flat and ball-shaped Si3N4 samples was accomplished using microwave plasma assisted chemical vapour deposition (MPCVD) with H2/CH4 gas mixtures. The friction behaviour of self-mated MCD coatings was assessed using a reciprocating ball-on-flat geometry. The tests were run in ambient atmosphere without lubrication, the frequency (1Hz) and stroke (6mm) were kept constant while the applied normal load varied in the range 10-80N. The microstructure, surface topography and roughness of the MCD coatings were characterised by SEM and AFM techniques. The diamond quality was assessed from micro-Raman spectroscopy. The friction evolution was characterised by a short running-in period where the main feature is a sharp peak reaching values as high as approximately 0.6 followed by a steady-state regime with very low values in the range 0.03-0.04.

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 7.5
cm. 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.

Abstract: Superconducting fibres of Bi-Sr-Ca-Cu-O system were grown by the conventional Laser Floating Zone (LFZ) process and by the new technique of Electrical Assisted Laser Floating Zone (EALFZ). The presence of an electric field during solidification process significantly changes the fibre microstructure with Bi2Sr2Ca2Cu3O10 nominal composition. The higher cristallinity of the asgrown EALFZ fibres, resulting from the current application during solidification, leads to a transformation rate enhancement during subsequent annealing. A strong correlation between the transport properties and the final microstructure of the annealed fibres is observed. The highest critical current density of the EALFZ fibres is obtained after heat treatment at 845°C while for the LFZ ones similar values are obtained only after annealing at 860°C.