Papers by Author: Cosme Roberto Moreira Silva

Abstract: Wear resistance of tool steels can be increased with deep cryogenic treatment (DCT) application. Mechanisms related to DCT are still not completely understood. Microabrasive wear resistance of cryogenically treated samples of AISI D2 steel was evaluated in terms of austenitization temperature at heat treatment cycle and quenching steps related to DCT. X-ray difractometry, scanning and optical microscopy and quantitative evaluation of carbides with image analysis were carried out aiming material characterization. For samples subjected to higher austenitization temperatures, the DCT treatment does not increase abrasive wear resistance. For samples treated at lower austenitization temperature, the DCT treatment results on 44% increase at abrasive resistance. This effect is correlated to the increase of the amount of fine carbides distributed at samples matrices cryogenically treated.

Abstract: Solid electrolytes based on stabilized zirconia have been studied a long time ago in its cubic phase because of its electrical properties, which make them excellent candidates to be used in applications such as oxygen sensors and solid oxide fuel cells [1], [2]. Lambda sensor or oxygen sensor, as it is also known, is a device that measures the oxygen concentration of the gases that flow through the exhaust pipe. Physically, the lambda sensor has two electrodes. The outer which is exposed to the exhaust gases and the inner to the air (reference) [3]; these electrodes are made, generally, of porous platinum. The ceramic material, i.e., zirconium oxide, is placed in between the electrodes, so the oxygen ions can move from one electrode to another. As one of the electrodes is exposed to the reference gas, the voltage generated is a measure of the concentration of oxygen in the exhaust gases [4].

Abstract: Oxygen ion conductors of zirconia based ceramics are a class of materials with technological applications in several application areas: sensors of chemical species, oxygen pumps, solid oxide fuel cells among others [1]. For these applications, the zirconia must possess the fluorite type crystal structure, or close to it. Such oxides with this structure are the classic oxygen ion conductors [2]. The fluorite structure consists of a cubic lattice of oxygen ions surrounded by cations. The cations are arranged in a face centered cubic structure with anions occupying tetrahedral positions. This leads to an open structure with large empty octahedral interstices.

Abstract: Mean stress on fatigue strength of Ti-35Nb-7Zr-5Ta, used in the manufacture of orthopedic prosthesis, was evaluated. Samples of Ti-35Nb-7Zr-5Ta were pressureless sintered and tested using microhardness (Vickers) and four point bending fatigue apparatus. Characterization was carried out using optical microscopy, scanning electron microscopy, EDS analysis, oxygen analysis and density. The density analysis shows almost 98% of density and microscopy reveals some precipitates of α phase and presumably ω phase at the grain and grain boundaries. The bending fatigue resistance limit reached (Se (10⁶) 88 MPa), due to, presumably, the influence of coarse α phase precipitates at grain boundaries as well as the high oxygen content in the material after sintering (0.96 %). In this study, Kwofie model best explains the influence of mean stress on fatigue strength of this alloy.

Abstract: Production and processing of titanium alloys are expensive and its alloys show low wear resistance. Substrates of Ti-40Zr, used for orthopedic implants, were obtained from the elemental mixture of hydrogenated powders, followed by a sequence of cold uniaxial and isostatic pressing and vacuum sintering. Aiming wear resistance increase, samples were coated by physical vapor deposition using electron beam technique (EB-PVD). Multilayer coatings with three different configurations were undertaken: Ti/TiN, Ti/TiN/ZrN and Ti/TiN/ZrN/TiN. Micro-abrasive wear tests were conducted by ball-cratering, with abrasive slurry of silicon carbide (SiC). The wear craters were measured at intervals corresponding to increments in the sliding distance of approximately 14 m. Range of the normal force were between 0.35 and 0,45 N. Scanning electron microscopy was used to evaluate the wear crater and the wear mode. All experiments showed three-body abrasive wear and a reduction of one order of magnitude in wear coefficient for two coated samples: 2,58∙10^-12 m³(Nm)^-1 to substrate, 5,4∙10^-13 m³(Nm)^-1 to Ti+TiN film and 7,22∙10^-13 m³(Nm)^-1 to Ti+TiN+ZrN film. In the Ti+TiN+ZrN+TiN experiment the wear coefficient of film is nearer to the substrate, 7,58∙10^-13 m³(Nm)^-1. The multi-layers containing ZrN presented wear coefficient higher than Ti/TiN films, possibly due to existing residual stresses.

Abstract: The endurance of components made of aluminum and aluminum alloys is often limited by their low yield strength and by their low wear resistance. The aim of this paper is to investigate the effect of different methods that can improve wear resistance of aluminum alloys. As a first approach, a highly wear resistant chromium nitrite layer was deposited by plasma vapor deposition on the surface of the aluminum alloy AA 6101-T4. In the second method, an ultra-deep cryogenic treatment was selected. Both methods have been previously used to improve the wear resistance of other harder substrate materials, like tool steel. To investigate the impact of the two methods on the wear resistance of such alloy, micro abrasive wear tests were carried out and an analysis based on the Archard’s law was considered. The results showed a decrease of the wear rate by 29% and 26% for the coated and for the cryogenically treated specimens, respectively, when compared to the as received material. The work also investigated the performance of three different methods (Allsopp, Double Intercept and Polynomial AT) usually considered to calculate the wear rate of coated samples. The three methods presented similar measures of wear rate for the substrate and for the coating

Abstract: The incremental sheet forming allows the production in high quality of extremely complex pieces of steel, aluminum, titanium and many other materials, in order to reduce time and costs of production and cost of tooling used in low production. Working with tools of various diameters and a suitable machine programming is possible to produce complex parts of small lots in few hours, which in conventional stamping process would take days. This paper aims to introduce process parameters and analyze results from practice tests and computational analysis by changing the rotation, advancement in XY and Z advancement, tooling options, fixture plate and forming tool for producing pieces of varying size and complexity in the shortest time and at the highest quality possible. Thus, presents the incremental sheet forming as an alternative for rapid prototyping, as well as its devices and tools.

Abstract: The zirconia in its cubic phase (C-ZrO2) has gained scientific and technological interest because it has high ionic conductivity and is useful in applications where the transport of oxygen ions prevails, for example, in the oxygen sensors and solid oxide fuel cells [1,. In the pure zirconia, the Zr4+ ion is too small to sustain the fluorite structure at low temperatures, so it has to be partially replaced by a higher atomic radius cation and lower valence number, for example, the Y3 +, Mg2 +, Ca2 + and the rare earth cations TR3 +, [. Currently there are several synthesis methods used to obtain cubic zirconia, the most popular being the mixture of oxides and coprecipitation used industrially in the research labs, but these methods provide powders with different characteristics which will be decisive for a specific application. In this context, the objective of this study was the preparation of homogeneous mixtures of zirconia-rare earth in different concentrations in order to stabilize the C-ZrO2, using the technique of heterogeneous coprecipitation for potential applications in oxygen sensors.

Abstract: In this paper we propose the stabilization of zirconium oxide with controlled additions of a rare earth elements concentrate, in the system ZrO2: ƞ wt% Re2O3 (with ƞ=5.36, 10.47, 13.74, 16.91 e 20) where Re₂O₃ is a rare earth elements concentrate composed mainly of 76.88% of yttrium oxide, 12.1% of Dysprosium oxide, 4.04% of Erbium oxide and 1.94% of Holmium oxide. The synthesis method used was the Pechini method. The results show that additions of 5.36 and 20 wt% of the concentrate are enough to stabilize the tetragonal and cubic zirconia phases respectively, and that zirconium oxide polymorphs can coexist with additions within these limit. In the characterization of the obtained powders are presented and discussed the following results: differential scanning calorimetry, transmission electron microscopy and X-ray diffraction. Also, it was necessary to make analysis by Rietveld refinement because they had severe overlap in the diffraction peaks. One of the most relevant results is obtaining a raw material, cheap to be used in many technological applications.

Abstract: Mean stress on fatigue strength of Ti-35Nb-7Zr-5Ta, used in the manufacture of orthopedic prostheses, was evaluates. Samples of Ti-35Nb-7Zr-5Ta were pressureless, sintered and tested using microhardness (Vickers) and four point bending fatigue conditions. Characterization was carried out using optical microscopy, scanning electron microscopy, EDS analysis, oxygen analysis and density. The microstructural analysis shows low densification after sintering (almost 86% of theoretical) and some precipitates of α phase and presumably ω phase at the grain and grain boundaries. The bending fatigue resistance limit reached (Se (10⁶) = 90 MPa), due to, presumably, the influence of the medium-high porosity, α phase precipitates at grain boundaries as well as the high oxygen content in the material after sintering (0.96 %). In this case, Kwofie model best explains the influence of mean stress on fatigue strength of this alloy.