Materials Science Forum Vol. 802

Abstract: Powder Metallurgy (PM) Techniques consists in a suitable technique to process composites materials. A specific PM technique of mechanical alloying developed to produce new materials in the solid state is a well known route to obtain aluminum alloys metal matrix composites. Aluminum alloys metal matrix composites allies the good properties of aluminum and its alloys but with poor mechanical properties and the reinforcement of ceramics phases which add better mechanical properties to these alloys. The research of this materials processing by PM techniques presented new materials with improved properties. In this work an AA2124 aluminum alloy was reforced by particulated silicon nitride a kind of ceramic phase. The powders were mixed and then processed by mechanical alloying in a SPEX vibratory type mill. Then the powders obtained were compacted and vacuum sintered. The sintered composites were characterized by means of Scanning Electron Microscopy (SEM) plus Energy Dispersive Spectroscopy (EDS) and Vickers hardness tests to evaluate the mechanical behavior.

Abstract: Extensive work has been performed on WC-Co hard metals for mining tools, tool inserts and other components. Cobalt is widely used as the binder metal because it’s good wetting behavior and solubility. However, the cost is high. Fe-Cr-Ni alloys show similar characteristics to Cobalt regard to melting temperature and crystal structure. Additionally, Fe-Cr-Ni alloys are a less expensive and nontoxic alternative. The present work analyze sintering characteristics of the WC / Fe-Cr-Ni alloys composite and development of new sintered hard metal composite for mining tools. The composite WC-316L was processed via Powder Metallurgy and sintering was performed at different temperatures: 1200^oC, 1300^oC and 1400^oC rate and 20^oC and isotherm 1hour in vacuum atmosphere. The composite sintered WC-316L was characterized by XRD, SEM and Vickers micro hardness test. The samples processed at temperatures of 1200^oC and 1300^oC showed considerable porosity, heterogeneity microstructure, low density relative and low Micro hardness, 300 HV and 700 HV, respectively. The samples sintered at temperature 1400^oC showed higher homogeneity microstructure compared to the samples sintered at temperatures of 1200⁰C and 1300⁰C, higher density relative, 86%, and micro hardness value compatible with the composite WC-Co, 1890 HV.These analyzes infer that stainless steel can be used instead of cobalt.

Abstract: Functionally graded materials (FGM) based on stainless steel and ceramic materials have inspired researchers to combine properties and features which are not present in conventional composites, and are considered to be an alternative in the production of motors parts, cutting tools and coatings for reactors. The addition of metal and ceramic in a graded structure allows the integration of distinct properties that combine advantages of metallic and ceramic materials. Ceramic components withstand high temperatures and present high corrosion resistance, while metallic ones provide higher mechanical resistance, in particular ultimate tensile strength and fracture toughness. In this work, composites with variable levels of 316 Stainless Steel and Ytria-stabilized Zirconia, were prepared and characterized, in order to determine the thermal behavior of each composition, aiming the optimization of sintering of pieces with chemical composition gradation.

Abstract: High performance nanostructured light metals and alloys are very interesting for replacing conventional heavier materials in many industrial components. High Energy Ball Milling and Cryomilling are useful techniques to obtain nanocrystalline powders. In this work the effect of several milling conditions such as rotation speed, time, ball to powder ratio and temperature on the crystallite and particle size and morphology in pure aluminum are presented. X-Ray Diffraction, Laser Diffraction and Scanning Electron Microscopy are used. High energy ball milling at ambient and cryogenic temperature of Al powders rapidly leads to a nanometer size down to about 35 nm. High ball to powder ratio promotes both low crystallite and particle size. Small crystallite size like 18 nm and particle size as 4 μm were achieved in the most energetic conditions at ambient temperature. Isopropyl alcohol used as liquid media and protective atmosphere has a strong influence on the results depending on the milling temperature of Al.

Abstract: Mixing and sintering aluminium and 20% mechanically alloyed Fe/B nanoparticles provokes the formation of intermetallics in the aluminium matrix when following a powder metallurgy route. Materials were sintered in a wide range of temperatures (from 600 to 1100 oC). Previous studies have shown that these materials present neither important dimensional changes during sintering nor significant differences in mechanical properties. However, sintering temperature strongly affects corrosion resistance and hot rolling capability. Low sintering temperature provides nanocomposites with lower corrosion properties and hot rolling capability. In this work the nanocomposites obtained at different temperatures were characterized by X-ray diffraction. This technique allows following the formation of different intermetallics at each temperature, since being non-equilibrium processes, the use of ternary phase diagram of these elements is not possible.

Abstract: Nanoparticles have large surface area, which gives them more pronounced effects. Silver nanoparticles, for example, have pronounced biocidal effects, since they can inactivate certain enzymes and alter the DNA synthesis of some microorganisms. In this context, the study of the synthesis and characterization of nanoparticles becomes potentially important. The aim of this work was to synthesize nanoparticles and to characterize them in order to contribute to the development of synthesis and characterization of nanomaterials. Several methods are used for the synthesis of silver nanoparticles; however, in this study we used the Turchevich method which makes use of a reduction reaction using sodium citrate as the reducing agent, and silver nitrate as starting material. After being synthesized, nanoparticles were analyzed with the technique of transmission electron microscopy (TEM). Also, a curve of the size distribution of the particles formed was obtained. This distribution was obtained through a nanoparticle filtration equipment connected to a particle counter-SMPS (Scanning Mobility Particle Sizer).

Abstract: Boron-doped diamond (BDD) films were grown with different grain sizes. The films were deposited on silicon substrate after a suitable pre-treatment in a hot filament assisted by chemical vapor deposition (CVD) reactor in Ar/H₂/CH₄ gas mixtures. The addition of argon to the growth gas mixture clearly revealed the transition from nanocrystalline (BDND) to ultrananocrystalline (BDUND) diamond films. Raman spectroscopy results of BDD, BDND and BDUND exhibited a good quality diamond films considering the diamond defined peak. Fourier Transform Infrared Spectroscopy (FTIR) spectra indicated carbonyl groups and B-C in BDND and BDUND films, while the microcrystalline BDD films showed only C-H bonds and boron-carbon (B-C), without the presence of oxygen and unsaturated species. Therefore, the carbonyl presence in the nanocrystalline films is mainly due to oxidation of transpolyacetylene present at the film grain boundaries. The transition became pronounced in the gas mixture with 60% of Ar, and the microcrystalline films were totally transformed in nanocrystalline diamond at 70% of Ar.

Abstract: The growth of diamond films in metastable condition occurs in two steps: nucleation and growth of crystals. Studies have shown that the nucleation process is the most critical step and essential to optimize the properties of the diamond, but its understanding is still very limited. Furthermore, the nucleation process is directly related to the pretreatment applied to the surface of the substrate: cleaning the surface and seeding. When the substrate is silicon, it is cleaned with acetone and scratching with diamond particles dispersed in a suitable solvent followed by ultrasonic agitation (nucleation rate = 10⁹ part/cm²). However, research has demonstrated that the use of diamond nanoparticles (ND) prepared with the use of a powerful ultrasound (750W) provides nucleation density much higher (10¹² part/cm²) compared to that processed with larger size particles. This work demonstrates that diamond films prepared with ND different solutions exhibit differences in relation to diamond films prepared using diamond particle dispersed in an organic solvent. Morphological analysis and the quality of the films were evaluated by Scanning Electron Microscopy, Optical Perfilometry and Raman Scattering Spectroscopy.

Abstract: In this work, the achievement and characterization of boron-doped nanocrystalline diamond films is presented. A series of experiments varying boron doping levels from 2,000 to 30,000 ppm and film growth times during 6, 10 and 16 h were performed. These films were analyzed by Scanning Electron Microscoy (SEM), Atomic Force Microscopy (AFM), Raman spectroscopy and Cyclic Voltammetry (CV) measurements. The results showed that the films presented two morphologies: ultra and nanocrystalline diamond. From Raman spectroscopy, the doping level increase for all the films, independent of growth time, increased the boron acceptor number and it was confirmed by Mott-Schottky plot (MSP). Electrochemical response showed the influence of boron content in the work potential window, mainly for films grown during 6 h. However, the reversibility was almost independent on the boron content for samples grown during 16 h.

Abstract: The production and characterization of porous silicon (PS) samples were studied as well as their use as substrates to grow boron doped nanocrystalline diamond (NCD) films. PS represents a suitable material for diamond growth due to its large number of nucleation sites and surface area, becoming an excellent material for porous electrodes. NCD films were grown by chemical vapor deposition (CVD) technique by balancing H₂/CH₄/Ar gas mixture, at two different boron levels. Doping was conducted by an additional hydrogen line passing through a bubbler containing B₂O₃ dissolved in methanol. Two ratios of boron/carbon were used of 2000 and 20000 ppm in the bubbler solution. Scanning electron microscopy, Raman spectroscopy and X-ray diffraction were used to characterize the films as well as the PS substrate. Results showed that it is possible to obtain NCD films on PS substrate with good quality at different doping levels.