Key Engineering Materials Vols. 510-511

Abstract: Magnetic nanocomposites are offering a variety of novel features and tune able properties, mainly depending on particle size, cation distribution, morphology and porosity of the prepared materials. The aim of this research work is to understand the effects of Mn doping on the microstructures and hence consequences on the electrical transport properties with shift of cation distribution in CoFe₂O₄. Co_1-xMn_xFe₂O₄ nanocrystallite particles with stoichiometric proportion (x) varying from 0.0 to 1.0 were prepared by co-precipitation method. X-ray diffraction patterns confirmed the FCC spinel structure of synthesized particles. The crystal structure is found to be inverse cubic spinel with a space group Fd3m and the lattice constants ranges from 8.36 Å to 8.46 Å The crystallite sizes were calculated from the most intense peak (311) using the Debye-Scherrer formula for all the samples those were synthesized at reaction temperature of 70°C. Then samples were sintered at 600°C for 3 hours, characterized by X-ray diffraction at room temperature and DC electrical resistivity measurements were done as a function of temperature by two-probe method from 370 K to 690 K. The measurements showed that DC electrical resistivity decreased with increase in temperature ensuring the semiconductor nature of the material in this temperature range. DC electrical resistivity results were discussed in terms of polaron hopping model under the effects of cation distribution. AC electrical properties were also analyzed. All the observed properties were correlated with observed microstructures.

Abstract: Surface layer of D2 tool steel was subjected to laser surface melting using continuous wave 2.5kW CO₂ laser in point source melting mode. The processing parameters were varied to achieve a uniform depth of around 2 mm. Microstructural study revealed epitaxial growth of fine dendritic structure with secondary dendrite arm spacing in the range of 20-25 µm. The phases in the parent annealed sample were BCC ferrite and chromium rich M₇C₃ carbide. The major phase after laser treatment was austenite and M₇C₃. The average hardness of annealed sample was 195 HV which increased to 410 HV after laser melting. Corrosion studies in 2% HCl solution exhibited a drastic improvement in corrosion resistance in laser treated samples. Improvement in properties is attributed to the refinement and uniformity of microstructure in the rapidly solidified surface. The case of a moving heat source was subjected to computer aided simulation to predict the melt depth at different processing conditions in point source melting mode. The calculated depths using the model, in ABAQUS software was found in good agreement with the experimental data.

Abstract: Mechanical and ballistics properties are a measure of the performance of AP/Al/HTPB composites. These properties are influenced by the characteristics of inorganic crystalline oxidizer and aluminum particles. This paper reports the effect of the particle size distribution on mechanical and burning properties of 87% solid loaded composites. Composites were fabricated by mixing the ingredients in the planetary mixer followed by curing at an elevated temperature for several days. Multimodal particle size distribution was used in which coarse to fine ratio was varied proportionally. The burning rate was measured using strand burner at various pressures. Mechanical properties of the composites were investigated by using tensile tester. Results revealed that tensile strength and the burning rate increased while elongation at maximum stress decreased with the increase in the quantity of fines oxidizer. Whereas the variation in the particle size of the aluminum do not have any pronounced effect on the burning as well as mechanical properties of the composite.

Abstract: The present work is focused on a new approach for the development of Fe-Cr-Co based permanent magnets. Fe-Cr-Co alloy was prepared by using tri arc melting technique under inert atmosphere of Argon. Solution treatment was done at a temperature of 1250°C for five hours followed by water quenching and then a single step thermo-magnetic treatment (TMT) was applied at predetermined cooling rates. The influence of TMT and cooling rates on the final magnetic properties of the alloy were investigated. The results reveal that microstructure and magnetic properties were sensitive to both cooling rates & TMT and can be optimized by controlling the processing conditions. The optimum magnetic properties in the alloy with two different cooling rates of 1°C per minute and 2°C per minute were obtained as (i) 1010 Oe (H_c), 9400 G (B_r), 3.4 MGOe (BH_max) (ii) 810 Oe (H_c), 10590 G (B_r), 3.6 MGOe (BH_max) respectively. The above method provides a quick and low cost manufacturing route for the Fe-Cr-Co based permanent magnets with comparable magnetic properties to that of Alnico with added advantage of having high ductility.

Abstract: An engine bearing was failed during service. During inspection, ball and races of rear bearing were found with wear marks. The bearing was subjected to metallurgical failure analysis. Inclusion level in the balls of bearing was observed higher; these inclusions were rich in Mo and V. The inclusions being the potential sites of the stress concentration caused the subsurface fatigue cracks in the bearing balls. The bearing was failed due to the severe spalling produced under subsurface fatigue leading to the subsequent wear of the raceways.

Abstract: 0-3 nanocomposites of PVDF with varying volume percents of strontium doped PZT have been synthesized using a combination of a modified solution casting process and the hot press technique. The nanocrystalline PZT used prepared from the sol gel route and was in the calcined state. An increase in the volume percent of PZT caused an increase in the dielectric constant, dielectric loss and piezoelectric charge coefficients. The experimental dielectric constants gave a perfect fit to the theoretical Furukawa model. However, no significant trend is perceived for the dielectric loss. SEM and XRD aided in determining the morphology and the crystalline phases in PVDF, PZT and PVDF/PZT composite. TG/DSC was done to confirm the percentage crystallinity of PVDF. A comparison with samples prepared from the conventional solution casting process has also been given. Composites prepared from the modified solution casting process using nanocrystalline strontium doped PZT give better piezoelectric properties as compared to earlier works.

Abstract: Multiferroic bismuth manganites (BiMnO₃) possess both ferromagnetic and ferroelectric properties. Their electrical properties can be controlled by doping for useful applications. In this work single-phase cobalt doped bismuth manganite nanoparticles having general formula BiMn_1-xCo_xO₃ (x=0, 0.2, 0.4, 0.6) were synthesized by co-precipitation method. Structural properties like lattice parameters and crystallite size of samples were determined by the data obtained by X-rays diffraction. The dielectric constant (ε) and dielectric loss tangent (tanδ) of samples were investigated as a function of frequency from 20Hz-3MHz using ac measurement data. For all the compositions dielectric constant was decreased with increasing frequency, however it increased with the increase in cobalt content. However cobalt addition causes a decrease in loss tangent as compared to pure BiMnO₃ composition. The origin of this behavior is discussed in terms of Maxwell-Wagner and Koops model. Substitution of Mn with Co, in BiMnO₃-based compounds is supposed to cause better properties in terms of tangent loss.

Abstract: Natural rubber (polymer designation cis-1-4 polyisoprene, is obtained from the sap ("latex") of several rubber-yielding plants (e.g., HeveaBrasiliensis and Partheniaargentatum) by coagulation with chemicals, drying, electrical coagulation, and other processes. Foamy or sponge structure of natural rubber (NR) is very useful in aerospace and as auxetic materials (exhibiting negative poisons ratio) for use in application of homeland security.The main aim of this research to estimate the influence of carbon black on mechanical properties, curing and viscosity variation behaviors of the natural rubber based composites. Different amounts of carbon black were used along with other fillers. The influence of carbon black on curing behavior and mechanicalproperties of natural rubber foams was investigated at different feedingratios of the carbon black. The physical properties of the foamed NRs were then measured as a function of carbon blacks content. The mechanical properties of the foamed NRs such as tensile strength,strength at break and modulus,were gradually increased with increasingcarbon black content whereas elongation decreasedat break.

Abstract: WC-Co cermet coatings were produced on AISI 321 stainless steel samples by air plasma spraying system. In this regard, the coatings were deposited by varying the spraying distance i.e. 80 mm and 100 mm. It was observed that spraying distance play an important role on the final properties of the WC-Co cermet coating. The coatings were characterized by optical and electron microscopy, microhardness testing and X-Ray Diffractometry. A remarkable micro-structural difference was observed between the two coatings. It was observed that the coatings produced at 80 mm having more porosity and un-melted particles as compared to that produced at higher distance. Similarly, the change in concentration of metallurgical phases was also observed.

Abstract: Alumina was coated on Ti6Al4V alloy by atmospheric plasma spraying. Surface and cross sectional analysis of the coating by SEM showed that diameter and thickness of splat was greatly influenced by arc current and stand-off distance. In turn, this variation in splat morphology had effect on evolution of α-Al₂O₃ and γ-Al₂O₃ phases as revealed by XRD. Higher proportion of required γ-Al₂O₃ phase was achieved at stand-off distance of 130 mm and arc current of 500A.