Papers by Author: Vijaya Agarwala

Abstract: In this study, an attempt has been made to develop electroless Ni-P-ZrO₂ nanocomposite coatings on mild steel substrate where the reinforced nanosized ZrO₂ particles were prepared by mechanical milling using high energy planetary ball mill. An alkaline bath was used with a suspension of ZrO₂ particles (4 g/L) for the synthesis of Ni-P-ZrO₂ composite coating. The surface morphology, size range and phase analysis of as-prepared ZrO₂ particles were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and the properties of the coatings such as hardness and wear (ball-on-disc) were investigated and compared with Ni-P deposits. The results showed that as-prepared ZrO₂ particles exhibit irregular shaped and size ranges from 14 to 17nm. After heat treatment (400°C,1h), the microhardness and wear resistance of the coatings are significantly improved. The Ni-P-ZrO₂ nanocomposite coatings exhibit enhanced wear resistance over NiP coating.

Abstract: Since the introduction of Electroless (EL) coating in 1946 by Brenner and Riddle, the process has been the subject of steady growth. It is one of the most elegant methods available for the production of alloy coatings on surface. The technique involves the autocatalytic reduction, at the substrate/solution interface, of cations by EL bath released from suitable chemical reducing agents. EL coating technique is simple one, as can be manifested just by controlling pH and temperature of the coating bath. Such coatings are reported to provide excellent physical and mechanical properties. The electroless coatings are being studied at Indian Institute of Technology Roorkee since 1985. The structural and morphological behavior of Ni-P coatings for different phosphorous contents has been extensively studied. Sub-micron size coating islands and their transformations have been deduced. The metallography studies using hot stage within TEM to follow the phase transformations occurring at various temperatures have been studied for Ni-B EL coatings. The realization of mechanical bonding along with chemical bonding between EL coating and the substrate has been explained by coated copper on ceramic powder. As a forward step towards composite coatings, Ni-P-C, Ni-P-Al₂O₃, Ni-P-ZrO₂ has been developed by EL co-deposition technique. Ag-graphite coatings produced by EL technique exhibits nearly five times higher wear resistance and nearly two times better corrosion resistance apart from a good electrical conductor. The tribological behavior of electroless Ni-P-X and Ni-P coatings on steel and aluminium substrates in different conditions i.e., as coated, heat treated at various temperatures at different extents of time with different normal loads, have been studied in terms of dry sliding friction and wear against counter face of case hardened steel. In Ni-P-X nanocoating (X= ZrO₂-Al₂O₃-Al₃Zr), X has been produced in-situ and are of nanosize particles. Such coating could be done on carbon fibre of 7µ diameter uniformly. Ni-P-ferrite nanocoatings with thickness less than nearly 1mm thick, is exhibiting good absorption of microwave in the range of 12-18 GHz which can be exploited for radar applications. Micro-thickness coatings are paying ways to nanocoatings. Nanocoatings are the coatings in which either the thickness of the coating is in nanolevel or second phase that dispersed in the coat matrix is of nanosize. To further explore the field of EL nanocomposite coatings, now days, a work on EL Ni-P-ZnO, TiO₂, Al₂O₃, ZrO₂ and Ni-B-ZrO₂ for its mechanical properties has been carried out.

Abstract: Zirconia-reinforced nickel–phosphorus matrix (Ni–P–ZrO2) nanocomposite coatings were prepared by electroless (EL) coating technique and compared with plain Ni–P deposits. Nano-sized second phase ZrO2 particles (5–10nm) were synthesized by coprecipitation method. An alkaline hypophosphite EL bath is used with a suspension of zirconia nano-particles (4 g/L) during the process for the synthesis of Ni-P-ZrO2 composite coating at temperature 90±2°C and pH 9.0±2 respectively. The coating’s surface morphologies were examined under Scanning electron microscope (SEM), the average globular size of about 50μm was observed having the lateral growth followed by vertical growth. Phase analysis by XRD confirmed the presence of Ni and ZrO2.The grain size within the globules had been calculated by Scherrer formula= 0.9λ/βcosθ.The heat treatment (400 0C, 1 h) enhances the mechanical properties like hardness of the composite deposits.

Abstract: Hot working of titanium aluminide made through reaction synthesis (RS) has been studied. Hot isostatic pressing under argon, isothermal pressing under vacuum, open die forging and pack forging was carried out. Hot worked billet samples were characterized for density and microstructure evolution. Presence of refined grains and dynamically recrystallized grains were observed. In case of open die forging, presence of twins was also found. From the present study, near isothermal working through near conventional processes has been confirmed. However, role of isothermal working in obtaining crack free sound deformed product has also been noted.

Abstract: Titanium aluminide intermetallic was made through reaction synthesis (RS) process using elemental powders. Pressure assisted synthesis was carried out at high temperature under vacuum. Ti powder with two different particle sizes (200μm and 30μm average) were used in RS. Synthesized blocks were homogenized and characterized for chemical homogeneity, density, phase formation and microstructure evolution. Products near to theoretical density have been obtained with uniform chemistry after homogenization. Al₃Ti as a major phase along with TiAl as minor phase was confirmed after RS and TiAl along with Ti₃Al was observed after homogenization. Homogenization cycle was found to be different for the alloys made through different Ti particle sizes. Significant role of Ti particle size has been observed in this pressure assisted RS process.

Abstract: The present investigation is on the microstructure evolution and hardness of powder metallurgically processed Al- 0.5 wt.%Mg base 10 wt.% short steel fiber reinforced composites. The 0.38 wt.% C short steel fibers of average diameter 50μm and 500-800μm length were nitrided and chromized in a fluid bed furnace. Nitriding was carried out at 525°C for 90, 30 and 5 min durations. Chromizing was performed at 950°C for 53 and 7 min durations, using thermal reactive deposition (TRD) and diffusion technique. The treated fibers and resulting reaction interfaces were characterized using metallographic, microhardness and XRD techniques.

Abstract: M-type strontium hexaferrite nanocrystals of radar absorbing material (NRAM) i.e. SrFe12O19 were synthesized by modified flux method that combine the controlled chemical co-precipitation process for nucleation and complete uniform growth during annealing with NaCl flux. Uniform structural morphological transformation of nanocrystals from needle to hexagonal prism faces were noticed after annealing with increasing of heat treatment (HT) temperature from 800 to 1200°C for 4h. X-ray diffraction (XRD) results show the formation of various phases with increase in annealing temperature. The crystallinity and crystallite size are found to increase with increase in heat treatment temperature (15-40nm). The superparamagnetic behavior of strontium hexaferrite is confirmed by vibrating sample magnetometer (VSM) wherein it was noticed that magnetic field (10000 gauss max) did not have any effect on these materials. The transformation of magnetic properties i.e. from superparamagnetic to ferromagnetic behaviour after heating at various HT temperatures have been revealed by hysteresis loops under VSM study. The increase in saturation magnetization from 2.44 to 75.03 emu/gm is observed. Formation of ultrafine particles has been confirmed through field emission scanning electron microscope (FESEM). About 5 to10% of the needle like crystals in the ‘as synthesized’ condition were transformed to hexagonal pyramidal structure and most of the crystals are found to have plate like hexagonal structures with increase in heat treatment temperatures. The effect of such systematic morphological transformation of nanocrystals was observed in reflection loss properties in X band (8-12 GHz). The maximum reflection loss of -20.05, -24.31, -24.16, -25.22, -25.12, -24.01 dB at 8.1, 8.6, 9.2, 9.6, 10.7, and 12 GHz respectively are observed for the material heat treated at 1200°C. A significant increment from - 6.5 to -25.22 dB at 9.6 GHz in reflection loss (RL) is noticed due to symmetric morphological growth of RAM nanocrystal during annealing.

Abstract: ZnO nanostructures of different morphologies were grown in a controlled manner by a novel sonication assisted sol gel process. Variation in the alcoholic medium resulted in nanorods and flower like ZnO nanostructures. Flower like ZnO nanostructures with a root size of 1m and a tip of 150nm were synthesized using ethanol as solvent, while ZnO nanorods with diameter less than 60 nm and length greater than 1m were synthesized using methanol as solvent. The samples were characterized by X-ray diffraction and Scanning Electron Microscopy. The XRD results show the formation of highly crystalline ZnO with no detectable level of impurity. In this paper, a possible mechanism for the variation of morphology with the change in solvent has been discussed. The effect of solvent vapour pressure on the sonication process has been explained. The solvent vapour pressure has an effect on the intensity of sonication which affects various chemical reactions taking place during the formation of ZnO. The effect of ethylenediamine on the chemical reactions leading to formation of ZnO nanostructures has also been analyzed.

Abstract: Nanocrystalline strontium hexaferrite (SrFe12O19) and barium hexaferrite (BaFe12O19) powders were synthesized by co-precipitation method. The ‘as synthesized’ powders were heat treated (HT) at different temperatures ranging from 800 to 1200°C at a heating rate of 30°C /min in nitrogen atmosphere. Decomposition behaviour and the phases associated therein are investigated by thermal analysis (DTA/DTG/TG) and X-ray diffraction (XRD). Formations of ultrafine particles have been confirmed through field emission scanning electron microscop (FESEM). The superparamagnetic behavior of both, barium and strontium hexaferrite is confirmed by vibrating sample magnetometer (VSM). The increase in saturation magnetization from 1.94 to 31.05 emu/gm in case of barium hexaferrite and from 2.44 to 43.38 emu/gm for strontium hexaferrite is observed with HT temperatures. The changes in coercivity and remanence with HT temperatures for both the ferrites are analysed.

Abstract: Nano radar absorbing material (NRAM) i.e. BaMe2Fe16O27 (Me2+=Fe2+) powder (10 nm) is coated with amorphous Ni-P nano layer (5-10 nm) by using electroless (EL) technology to develop EL Ni-P/NRAM nanocomposite powder. The experimental processes parameters and EL Ni-P bath composition were optimized to obtain the deposition. As-deposited nanocomposite powder was microwave annealed (MWA) with increasing radiation power from 160 to 760 watts for 5 minutes. The surface morphology, elemental contents, phase transformation and magnetic properties of NRAM powders were examined under field emission scanning electron microscope (FESEM), transmission electron microscope (TEM), X-ray diffraction (XRD) and vibrating sample magnetometer (VSM) respectively. Maximum reflection loss (RL) 33.75 dB at 15.80 GHz for nanocomposite powder MWA at 760 watt was obtained the absorption range under −15 dB is from 13.76 to 16.77 GHz with 2 mm thickness layer in Ku Band. Excellent microwave absorption properties due to accurate electromagnetic (EM) match in the nanocomposite microstructure, a strong natural resonance and multipolarization. Such (Ni+ Ni3P)/NRAM nanocomposite powders may be attractive candidates for EM absorption.