Materials Science Forum Vols. 498-499

Abstract: Organophilic Bentonities clays are widely used for industrial purposes, as components of petroleum drilling fluids as well as in cosmetics, lubricants adhesives and paints. Most recent they have been used as fillers in polymer nanocomposites. In this work, three organophilic bentonitic clays from the distric of Boa Vista and one from Campina Grande, PB, Brazil, were synthesized and characterized. The four clays were treated with a quaternary chloride ammonium salt- dimethyl benzyl ammonium alkyl chloride (Dodigen 1611) provided by Chemco. The cation exchange and swelling capacities of the modified clays were determined and the clays were characterized by FTIR, Thermal Analysis (DTA, TGA) and X-ray diffraction. The results showed the modification of all four clays by the quaternary ammonium salt to be successful.

Abstract: It is well known that colloidal powder particles (between 1 mm and 0.001 mm) tend to agglomerate due to electrostatic forces. Then assuring an optimal dispersion condition is essential for good particle-size analysis results, since aggregates or weak agglomerates can be measured as single particles. In this paper the particle size distribution of an alumina powder A1000SG (ALCOA) was measured using distinct dispersion procedures. Distilled water was used as dispersant liquid in the pure state and with additives (citric acid and Duramax D-3005). Dispersion by supersonic vibration was also investigated, but only the application time was varied. Particle size analysis was accomplished by laser scattering technique and the dispersion condition was evaluated through zeta potential. The results showed that the Duramax’s electrosteric impediment is more efficient than citric acid’s electrostatic force, thereby providing better dispersion. Although useful, the supersonic vibration was not good enough to assure an optimal dispersion, at least for the material tested here.

Abstract: This article shows some fabrication aspects related to the obtention of sintered valve seat insert. This insert was made of a mixture of high-speed steel powders and iron powders plus NbC. This is a new development aiming the substitution of Co alloys currently used for valve seat inserts. The physical properties, mechanical properties and machining behaviour are discussed. The machining characteristics in terms of tool wear, cutting forces and chips morphology of the insert was compared to available commercial insert. The machining results indicate that the material under development has potential for commercial application and shows good machining evidences, in terms of equivalent cutting forces for ceramic tool. In addition, the machining using hard metal tool was susceptible to hardness variation observed for the material under development. Therefore, the machining tests point out the necessity for a microstructure homogenisation of the obtained material.

Abstract: This study investigates the in vitro corrosion and cytotoxicity response of AISI 316L stainless steel produced by powder injection molding (PIM) technology in a solution that simulates physiological fluids (MEM) by electrochemical techniques and neutral red uptake cytotoxicity assay. The results were compared with those of AISI 316L produced by conventional metallurgy. Both steels showed high corrosion resistance and no toxic effect in the cytotoxicity test. The corrosion products were analyzed by instrumental neutron activation analysis (INAA). The surfaces of the alloys were evaluated before and after corrosion test by scanning electron microscopy and a passive behaviour was indicated supporting the results from other techniques.

Abstract: In this study a corrosion protection method was used with AISI 304 SS filters and its effect on their corrosion resistance was evaluated in a sodium chloride solution (0.5 mol/L). The corrosion resistance of the surface treated filters was investigated at increasing temperatures, from 22 oC to 100 oC. It was found that the treatment improved the corrosion resistance of the filters at all temperatures tested. The film formed was maintained on the filters surface during the whole period of test (3 weeks). Surface analysis of untreated and treated filters, after corrosion tests, supported the indication of the beneficial effect of the corrosion protection treatment evaluated in this study.

Abstract: In this study, the corrosion behaviour of a sintered commercial Nd-Fe-B magnet has been investigated in naturally aerated and deaerated 3.5 (m/v)% sodium chloride solutions. Magnetized specimens were used in this investigation. The corrosion behaviour of the specimens was monitored at increasing test times by means of electrochemical impedance spectroscopy (EIS). Potentiodynamic polarization curves and surface observation by scanning electron microscopy (SEM), were also carried out after 30 days of immersion in the test solutions. Experimental results indicated that the corrosion resistance was affected by aeration of the solution. The corrosive attack penetrates deeply into the specimens and, consequently, the magnetic properties decreased with immersion time. The magnetic properties most influenced by corrosion were the remanence (Br) and the maximum energy product (BHmax), both presenting a reduction corresponding to 6%, after 30 days of immersion.

Abstract: The creep behavior of hot-pressed Si3N4 ceramics was investigated. The proposal of this work is to investigate the use of yttrium-rare earth oxide mixture, CRE2O3, produced at FAENQUIL, as sintering additive, since the cost of production of this material is 80% inferior to Y2O3. These ceramics were obtained by uniaxial hotpressing using different additive contents and mixtures (CRE2O3-Al2O3 or CRE2O3- AlN). Compressive creep tests were carried out at 13000C and 300 MPa, in air. The Si3N4-CRE2O3-Al2O3 ceramics demonstrated that the creep resistance is inversely proportional to the additive content. Mixtures with high intergranular phase content presented low creep resistance due to high oxidation and more pronounced softening of the intergranular phase. Si3N4-CRE2O3-AlN ceramics demonstrated better creep resistance with a steady-state creep rate of 7 x 10-8 s-1. This behavior is related to the a-SiAlON content, a solid solution of Si3N4 that incorporates a great fraction of intergranular phase, decreasing the amount of intergranular phase.

Abstract: Deposits of the Fe-6.5wt%Si alloy produced by spray forming were annealed at temperatures between 900 and 1300oC, during 1h in vacuum and quenched in oil at temperatures between 300 and 700oC, separately. Magnetic properties, singular microstructure and random crystallographic texture were measured. After annealing at 1250°C for 1h under vacuum, the average grain size is of 500 μm, the grain orientation is random and the magnetic properties were: power loss of 1.30 W/kg, maximum permeability of 15400 and coercive force of 40 A/m, at B=1 T, f=60 Hz by using 0.60 mm thick rings for all studied samples. Higher annealing temperatures cause no decreasing of these properties. After quenched at 700°C, an improvement the magnetic properties where detected due to antiphase domain B2 growth. The magnetic properties were: power loss of 1.59 W/kg, maximum permeability of 12300 and coercive force of 76 A/m, at B=1 T, f=60 Hz.

Abstract: MnZn ferrites are conventionally produced by the ceramic method that involves the solid state reaction of metallic oxides or carbonates at high temperatures. The particles obtained by this method are rather large and non-uniform in size. In order to overcome the difficulties arising out of the ceramic process, the coprecipitation method has been used as an alternative route to produce chemically homogeneous powders with fine particle size. In this work MnZn ferrites powders were produced by the coprecipitation method. The calcination conditions, such as temperature (900oC to 1100oC) and atmosphere (air and nitrogen), were investigated. X ray diffractometry, scanning electron microscopy, thermomagnetic analysis and vibration sample magnetometry were used to characterize the obtained samples. The results indicated that when the samples were calcined in nitrogen atmosphere, the ferrite formation occurred at low calcination temperatures and presented better magnetic properties than those calcined in air.

Abstract: Permanent magnets were produced from annealed alloys using the hydrogenation, disproportionation, desorption and recombination (HDDR) process. The influence of Pr concentration on the magnetic properties of these magnets was studied. Under the present processing conditions Pr has a significant influence on the magnetic behaviour of these magnetic materials. Microstructural examinations revealed that free iron was completed eliminated from all studied alloys with annealing and the grain size changed somewhat with praseodymium content. It has also been shown that to obtain bonded magnets with optimum magnetic properties the praseodymium content in these alloys should be around 13.5 at%.