Papers by Author: Cosme Roberto Moreira Silva

Abstract: In this work itria stabilized ZrO2 based ceramics for dental applications has been sintered and characterized in terms of physical and mechanical properties. Nanostructured blocks were sintered at 1400 0C and microstrutured blocks sintered at 1600 0C. Both nanostructured and microstructured materials were characterized in terms of densification, crystalline phases, mechanical properties and microstructure. Fracture toughness and four point bending strength were evaluated and compared. For the nanostructured zirconia, the reduced grain size allowed the increase of its toughening capacity, generated from maximization of volumetric fraction of retained tetragonal zirconia particles. For this material higher bending flexural strength is related to induced nucleation of microcracks, increase of energy absorption during crack propagation and developed compressive surface stress. The fracture toughness obtained at nanostructured samples sintered at 1400 0C is approximately 20% higher when compared to microstrucutured samples. These results represent the toughening ability of nanostructured zirconia, originated from higher amount of retained tetragonal phase and grain boundary microcracks. The higher Weibull moduli in this case are indicative of material reliability improvement and these results are correlated to grain size and its influence at mechanical strength.

Abstract: Industrial applications of partially stabilized zirconia (PSZ) has increased substantially recently, considering its excellent thermal stability, strength and ionic conductibility. Its main application includes oxygen sensors and fuel cells. In this work zirconia ceramic powder is produced by Pechini process, mixing citric acid and ethylene glycol, adding yttrium and zirconium precursors. All the process was carried out with complete agitation. The obtained powder after this procedure is heat treated at 650°C during two hours. Characterization was performed by infra-red spectroscopy, x-ray diffractometry and quantitative analysis by Rietveld Method. The results show tetragonal and monoclinic phases, with nanometric crystallite.

Abstract: Applications of stabilized zirconia include uses related to its low thermal conductivity, strength and electrical properties. Rare earth oxides (REO) were used as additives to stabilize appropriate crystalline phases at zirconia compacts. In this work zirconia based solid electrolytes were sintered with 10, 15 and 20 wt % of REO as sintering aids. Samples were prepared by uniaxial cold press and sintered at 1400 °C, followed by electric properties determination via impedance spectroscopy. Complimentary characterization was made by X-Ray diffractometry and quantitative phase analysis by Rietveld refinement. The ionic conductivity is affected by the amount of REO additive and this effect is correlated to the existing tetragonal and cubic phases for each composition

Abstract: Beta titanium alloys parts are used on advanced aerospace systems because of their high strength to weight ratio and excellent corrosion resistance. Production of powder metallurgy titanium alloys components may lead to a substantial reduction in the cost, compared to those produced by conventional cast and wrought processes, because additional working operations and material waste can be avoided. In this work, beta Ti-45Nb and Ti-50Nb were produced by the blended elemental technique, followed by uniaxial and cold isostatic pressing with subsequent densification by sintering. Sintered samples were characterized for phase composition by XRD, microstructure by SEM, hardness by Vickers indentation, specific mass by the Archimedes method and elastic modulus by resonance ultrasound. The sintered samples presented only the beta phase, higher hardness and lower elastic modulus when compared to Ti6Al4V alloy and experimental specific mass value near theoretical specific mass. These characteristics are adequate for application on several aerospace parts.

Abstract: The formation of PbTe intermetallic compound by mechanical alloying (MA) has been investigated. The elemental starting materials were 99.5% pure lead and tellurium, with a sieve size of 80 and 200 mesh, respectively. A SPEX 8000 shaker milling was used to perform the MA, using WC balls as milling media in a cylindrical hardened tool steel vial. X-ray diffraction analysis was performed with a profile-fitting program, to evaluate time evolution of the alloy formation. An exotermical reaction occurs on PbTe formation, with entalphy H= - 16.3 Kcal/mol. The *T value is confirmed by the heat exchange equation *Q = |*Hf | =* i (mici ) *T, where the summation comprises the mass and specific heat of vial, balls and powder material. For the standard milling conditions employed, the PbTe formation occurs at aproximately 90 seconds of milling, when using charge ratios between 3:1 and 7.5:1. However, for lower charge ratios (8:1 to 10:1), isolated reactions at the mixture occurs, but the amount of material is not enough to raise the temperature of adjacent regions, and the propagation of the reaction is avoided. There is therefore a minimum amount of powder (“critical mass”), and below this value the reaction will not be self-sustained.

Abstract: In general, semiconductor materials for thermoelectric generation prepared by vacuum metallurgy shows a relatively high value of figure-of-merit. However, differences in some properties of alloys elemental constituents can cause processing problems. Recently, Mechanical Alloying (MA) has been used to produce polycrystalline thermoelectric materials, such as (Bi,Sb)2 and (Te,Se)3(1). The industry is using this process since early 70’s to produce oxide dispersion strengthened alloys and those with widely different melting temperatures (2) In the present work, Si0.80Ge0.20 alloys were prepared via Mechanical Alloying (MA), using 99.9 % pure silicon and germanium powders, with a sieve size of 100 mesh. The MA has been performed, for several balls - to - powder ratio, in a SPEX 8000 vibratory high energy milling with tungsten carbide balls. Time for alloy formation was in a range from 3 to 9 hours, corresponding to charge ratio of 12:1 and 4:1, respectively. After two hours of processing time, the grinding temperature reached 80 0 C, and remained at this level until the end of the process. It was possible to follow the SiGe alloy formation by x-ray diffractometry, as the peak lines positions of elemental Si and Ge were continuously shifted, and end up to merge into a single broad peak. There was a convergence of the individual lattice parameters of Si and Ge to a single value of 5.470 A, measured within the limit of  0.005 A. For the Si0.80Ge0.20 system evaluated in this work, the alloying progress occurred continuously, and changed inversely with charge ratio.

Abstract: The proposal of this research has been the study of the plasma spayed coating on creep of the Ti-6Al-4V, focusing on the determination of the experimental parameters related to the first and second creep stages. Yttria (8 wt %) stabilized zirconia (YSZ) (Metco 204B-NS) with CoNiCrAlY ( AMDRY 995C) has been plasma sprayed coated on Ti-6Al-4V substrate. Creep tests with constant load had been done on Ti-6Al-4V coated samples, the stress level was from 250 to 319 MPa at 600 °C. Highest values of tp and the decrease of the second stage rate had shown a better creep resistance on coated sample. Results indicate that the coated sample was greater than uncoated sample, thus the plasma sprayed coating prevent the sample oxidation efficiently.

Abstract: Alfa/beta titanium alloys have been intensely used for aerospace and biomedical applications. Production of powder metallurgy titanium alloys components may lead to a reduction in the cost of parts, compared to those produced by conventional cast and wrought (ingot metallurgy) processes, because additional working operations (machining, turning, milling, etc.) and material waste can be avoided. In this work, samples of Ti- 10, 15Nb (weight%) alloys were obtained by the blended elemental technique using hydride-dehydride (HDH) powders as raw material, followed by uniaxial and cold isostatic pressing with subsequent densification by sintering carried out in the range 900–1500 °C. These alloys were characterized by X-ray diffractometry for phase composition, scanning electron microscopy for microstructure, Vickers indentation for hardness, Archimedes method for specific mass and resonance ultrasound device for elastic modulus. For the samples sintered at 1500°C it was identified  and  phases. It was observed the influence of the sintering temperatures on the final microstructure. With increasing sintering temperature, microstructure homogenization of the alloy takes place and at 1500 °C this process is complete. The same behavior is observed for densification. Comparing to the Ti6Al4V alloy properties, these alloys hardness (sintered at 1500 °C) are near and elastic modulus are 18% less.

Abstract: Ti-35Nb-7Zr-5Ta alloy is considered an attractive material for implants manufacture due to an excellent combination of properties, including high mechanical and corrosion resistance, beyond the lowest elastic modulus among the titanium alloys. The alloy processing by powder metallurgy (P/M) eases the obtainment of parts with near-net shape forming and low production costs. Samples were produced by mixing of initial metallic powders followed by uniaxial and cold isostatic pressing with subsequent densification by sintering between 800-1600 °C, in vacuum. The isochronal sintering demonstrated to be efficient for the study of the microstructural evolution. The samples presented high densification and adequate microstructure. The results show that a beta-homogeneous microstructure is obtained in the whole sample extension when sintered at high temperatures beyond that P/M technology allows an effective porosity control.

Abstract: Powder metallurgy (P/M) of titanium alloys may lead to the obtainment of components having weak-to-absent textures, uniform grain structure and higher homogeneity compared with conventional wrought products. The production of the Ti-13Nb-13Zr alloy by P/M starting from blended elemental (BE) powders is a cost-effective route considering its versatility and also for allowing the manufacture of complex parts. This alloy due its high biocompatibility and lower modulus of elasticity is a promising candidate for implants fabrication. Samples were produced by mixing of initial metallic powders followed by uniaxial and cold isostatic pressing with subsequent densification by sintering in order to identify the microstructural evolution. Sintered samples were characterized for phase composition, microstructure, microhardness and density. The surface topography of the samples was studied by means of atomic force microscopy (AFM). It was shown that the route is adequate to reach high densities with homogeneous microstructure. Representative AFM images allowed distinguishing a lamellar structure caused by the different phases that are present in the surface of the specimens.