Papers by Author: Uilame Umbelino Gomes

Abstract: The carbides of refractory metals like tungsten carbide (WC), tantalum carbide (TaC) and niobium carbide (NbC), has been extensively studied due to their applications in several areas of industry, because of their specific properties; such as high melting point, high hardness, wear resistance, oxidation resistance and good electrical conductivity. The tungsten carbide, particularly, is generally used at hardmetal industries due to its high hardness and wear resistance. New synthesis techniques have been developed to reduce the synthesis temperature of refractory metal carbides using more reactive precursors and gas-solid reactions for carbon reduction. The result is producing pure carbides suitable properties for production of high quality cemented carbides and more selective catalysts. In this work, pure and nanostructured WC was obtained from the ammonium paratungstate hydrate (APT), at low temperature and short reaction time. Hydrogen (H₂) and methane (CH₄) were used as a reducing gas and carbon source, respectively. The precursor and obtained product were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results obtained by diffraction of X-rays showed that complete reduction and carburization of APT have been took place resulted in pure WC formation. The average crystallite size was in nanometer order reaching values of approximately 20.8 nm and a surface area (BET) of 26.9 m2/g.

Abstract: The technical porcelain is usually obtained by mixing different raw materials, which are generally clay, kaolin, quartz, and a fluxing agent, in appropriate amounts. These porcelains are used in the production of electric and electronic devices for several applications, ranging from high and low power capacitors to low, medium, high and extra high voltage insulators. In this work, we studied technological properties of dielectric porcelain, obtained from raw materials found in large quantities and excellent quality in the Rio Grande do Norte state in Brazil. Test samples were produced by powder technology for two different compositions: composition A (20% clay, 28% kaolin, 32% feldspar and 20% quartz) and composition B (15% clay, 31% kaolin, 33% feldspar and 21% quartz), and sintered at temperatures, T1=1150°C, T2=1200°C, T3=1250°C, T4=1300°C and T5=1350°C with isotherms P1=0.5 hours, P2=1 hour, P3=1.5 hours and P4=2 hours. Then after sintered, the sample microstructure was characterized by scanning electron microscopy (SEM) and analysis of dielectric strength with direct and alternating voltage. The best results for electrical properties were obtained in temperature of 1250oC for composition A, and confirms the electrotechnical porcelain production feasibility.

Abstract: The ceramic sector is important to the economy in Rio Grande do Norte, Brazil. Overall, however, the manufacturing process is imperfect; leading to inappropriate use of raw materials, waste of energy, environmental degradation, nonconformity to the Brazilian and international standards, etc. So, its crucial improves the produced bricks. This work aims to contribute to quality control of the ceramic industry, raising the values of physical properties of their products. The main parameters that influence these values were determined by Weibull statistics. Ceramics were studied by measuring porosity, water absorption, linear shrinkage, bulk density and mechanical strength. Mechanical strength test data resulted in Weibull distributions. The Weibull statistic parameter was calculated to best evaluations of ceramic quality. The results support our initial considerations about current stands of low quality of ceramic products from Rio Grande do Norte and underline the importance of testing and improving these products.

Abstract: Particle reinforced metal matrix composites have received considerable interest over many years and continue still under constant development to gain wider industrial applications. New technique of production of carbetos of refractory metals (WC, NbC, TaC, Ta_xNb_y) has been developed, synthesizing nanostructured carbides that show improvement of diverse properties of the materials to the gotten ones for the conventional processes. The properties of sintered composites are determined not only by the nature and quality of the raw materials employed but also by microstructure and porosity resulting from the processing techniques e sintering method. In this study, additions of 20 wt% NbC nanoparticles or micro-particles in the ferrite matrix were performed with the aim of improving the mechanical and use properties. Ancorsteel Fe 1000B powder from Hoengans Corp. was used together with the graphite, and a small amount of Fe₃P, to induce liquid phase sintering. NbC nanoparticles or micro-particles were inserted into the Fe 1000B matrix by wet grinding (acetone) in a mill of planetary type of high energy. The angular velocity of the mill was kept constant at 300 rpm with milling time of 10 hours. The composites powders milled were annealed at 900 ° C for 1 hour under flowing hydrogen e argon, and a priori were pressed into cylindrical pellets under 600 MPa and sintered the plasma. Finally, the sintered pellets were evaluated through the testing: SEM, microhardness and density. It was noticeable the behavior of the composites Fe 1000B - NbC was affected by the content of nanoparticles of NbC added as well as by processing parameters, particularly plasma sintering.

Abstract: This work reports an investigation about the influence of the environment of milling on the characteristics of the powders and on the structure and density of sintered samples made of these powders. Mixtures of composition W-30wt%Cu were milled for 51 hours in a high energy planetary mill in dry and wet (cyclohexane) conditions. The milled powders have composite particles. The powders were pressed and sintered at 1050º, 1150º and 1200°C under flowing hydrogen. The isothermal times were 0 minutes for the first two temperatures and 60 minutes for the latter. The samples reached around 95% of relative density. The powders were characterized by means of XRD and SEM. The sintered samples were characterized by means of SEM, optical microscopy and density measurement.

Abstract: Sintered stainless steel has a wide range of applications mainly in the automotive industry. Properties such as wear resistance, density and hardness can be improved by addition of nanosized particles of refractory carbides. The present study compares the behavior of the sintering and hardness of stainless steel samples reinforced with NbC or TaC (particles size less than 20 nm) synthesized at UFRN. The main aim of this work was to identify the effect of the particle size and dispersion of different refractory carbides in the hardness and sintered microstructure. The samples were sintered in a vacuum furnace. The heating rate, sintering temperature and times were 20°C/min, 1290°C and 30, 60 min respectively. We have been able to produce compacts with a relative density among 95.0%. The hardness values obtained were 140 HV for the reinforced sample and 76 HV for the sample without reinforcement.

Abstract: W-Cu composite powders were prepared by high energy milling under two milling environments: cyclohexane and air. Composite particles are formed in both cases. The W particles are fragmented and embedded into the Cu particles. Both, W and Cu, are heavily strained, mainly in the first hours of milling. The composite powder has high homogeneity and is much finer than the original Cu powder. The mean particle size of the powders milled in both conditions is very close, but the wet milling was near 25% longer than dry milling and the size distribution is wider. This is consequence of the higher milling intensity of dry milling.

Abstract: High-energy milling has been used for production of nano-structured WC-Co powders. During the High-Energy Milling, the powders suffer severe high-energy impacts in the process of ball-to-ball and ball-to-vial wall collisions of the grinding media. Hard metal produced from nanostructured powders have better mechanical properties after appropriate sintering process. During the milling the particles size of WC and Co can be reduced and plastic deformed. In the present work, a mixture of WC-10%Co was produced by high energy milling. The starting powders of the WC (0.87 μm - Wolfran Bergau) and Co (0.93 μm - H.C.Starck) were used to produce the hard metal. The influence of the milling time on the particle size distributions and in the lattice strain was investigated. Milling time of the 2, 10, 20, 50, 70, 100 and 150 hours were used. The powders after milling were characterized by X-ray diffraction (XRD) and Scanning Electronic Microscopy (SEM). The results show that 10 h milling were enough to reduce the crystallite size of WC and the increase of the milling time reduces the crystallite size.

Abstract: This work presents the results of a study concerning the influence of the addition of rareearth elements (La2O3 and CeO2) on the sintering of the WC/10Co cemented carbide. Several WC/10Co mixtures containing up to 3 wt.% rare-earth of the cobalt phase were prepared. Specimens were uniaxially pressed at 200 MPa, and sintered in a vacuum furnace at 1400 °C during 60 minutes. The sintering behaviour was accompanied by the linear shrinkage, density, and mechanical strength. The development of the microstructure was followed by XRD and SEM. The results showed that the sintering behaviour of the WC/10Co cemented carbide was influenced by adding of rare-earth element. In addition, the lanthanium oxide addition was more effective on the improvement of the physical-mechanical properties of the studied carbide.

Abstract: Cemented tungsten carbides were produced by liquid-phase sintering. In these work high energy milling (HEM) was used to produce homogeneous and finely grained powder mixtures. The milling effect on the magnetic properties of sintered samples is studied. Different mixtures in same composition (WC-10wt.%Co) were prepared by conventional mixture technique and wet HEM up to 300 hs in Planetary Mill. Magnetic hysteresis measurements on the sintered samples detected a significant increase in the coercitive field and a decrease on the saturation magnetization with milling time increasing. X-ray diffractogram show phase transformations with milling time. The Magnetic properties are correlated with phase relations and microstructural properties of the sintered samples.