Suman K Mishra

Sintering studies on ultrafine ZrB 2 powder produced by a self-propagating high-temperature synthesis process

A detailed study on the sintering behavior of zirconium diboride powder produced by the self-propagating high-temperature synthesis (SHS) process was carried out in the temperature range of 1500-1800 degreesC. The fine powder prepared by the SHS process exhibited excellent sinterability and could be sintered at 1800 degreesC for 1 h to approximately 94% of the theoretical density. The apparent activation energy of densification in the range of 1500-1800 degreesC was estimated to be 248 +/- 4 kJ mol(-1). A zirconium dioxide layer formed on the surface of the sintered body and was attributed to boron oxide formation during sintering and concurrent surface oxidation by the oxygen generated from the reduction of boron oxide in the carbonaceous atmosphere. Sintering aids like Fe and Cr appeared to help in densification of ZrB2 powder.

Cavitation Erosion in Hydraulic Turbine Components and Mitigation by Coatings: Current Status and Future Needs

Cavitation erosion is a frequently observed phenomenon in underwater engineering materials and is the primary reason for component failure. The damage due to cavitation erosion is not yet fully understood, as it is influenced by several parameters, such as hydrodynamics, component design, environment, and material chemistry. This article gives an overview of the current state of understanding of cavitation erosion of materials used in hydroturbines, coatings and coating methodologies for combating cavitation erosion, and methods to characterize cavitation erosion. No single material property fully characterizes the resistance to cavitation erosion. The combination of ultimate resilience, hardness, and toughness rather may be useful to estimate the cavitation erosion resistance of material. Improved hydrodynamic design and appropriate surface engineering practices reduce damage due to cavitation erosion. The coatings suggested for combating the cavitation erosion encompasses carbides (WC Cr2C3, Cr3C2, 20CrC-80WC), cermets of different compositions (e.g., 56W2C/Ni/Cr, 41WC/Ni/Cr/Co), intermetallic composites, intermetallic matrix composites with TiC reinforcement, composite nitrides such as TiAlN and elastomers. A few of them have also been used commercially. Thermal spraying, arc plasma spraying, and high velocity oxy-fuel (HVOF) processes have been used commercially to apply the coatings. Boronizing, laser surface hardening and cladding, chemical vapor deposition, physical vapor deposition, and plasma nitriding have been tried for surface treatments at laboratory levels and have shown promise to be used on actual components.

Microstructure evolution during sintering of self-propagating high-temperature synthesis produced ZrB2 powder

A transmission electron microscopy investigation on the microstructure evolution of the sintered compact of zirconium diboride was carried out. Grain boundary phases, structure, and morphology of the grains and defect structures were studied to understand the sintering behavior of these borides. The impurities, such as Fe, were found to stay at the triple junction point as (Fe, Zr)(2)B. Different types of defect structures, such as dislocations, twins, and faceted growth, were observed. No diffusion of Fe into ZrB2 could be observed, but the Zr diffusion into Fe was established.

EFFECT OF NICKEL ON SINTERING OF SELF-PROPAGATING HIGH-TEMPERATURE SYNTHESIS PRODUCED TITANIUM CARBIDE

A detailed study of the sintering behavior of titanium carbide (TiC) powder synthesized by the self-propagating high-temperature synthesis (SHS) route was carried out with varying percentages of nickel. With an increase in the nickel content, porosity was found to increase along with Ni2.67Ti1.33 secondary phase at grain boundary. A 90-92% dense sintered body of TiC was produced from SHS-produced powder at 1800 degrees C with 10 wt% nickel.

Self-propagating high-temperature synthesis of a zirconium diboride-alumina composite: a dynamic X-ray diffraction study

A time-resolved X-ray diffraction study during the formation of a zirconium diboride (ZrB2)–alumina (Al2O3) composite from zirconium dioxide, Al and boron oxide by self-propagating high-temperature synthesis (SHS) has been carried out. During the formation of the composite, solid solutions of Al in ZrB2 form, with a slight change in the lattice parameters of ZrB2, during the SHS reaction and finally form a ZrB2–Al2O3 composite powder. Dendritic growth of ZrB2 was also observed during the synthesis.

Investigations on precipitation characteristics in a high strength low alloy (HSLA) steel

The nature of precipitation and the solutionizing temperature of the precipitates are crucial factors in controlling the strength of high strength low alloy steels. Niobium, vanadium and titanium are the elements most commonly added to HSLA steels to produce fine precipitates and grain refinement. In addition to these, other alloying additions such as chromium, molybdenum and manganese are also made. Copper is added to provide age hardening. Addition of copper is accompanied by a proportional level of nickel in these steels. Such a complex scenario of alloying additions can influence the mass transfer kinetics as well as equilibrium precipitation. Therefore, steels of similar chemistry can exhibit different precipitation behaviour as has been already observed in some cases. Investigations made on the precipitation behaviour, in the austenitic range, of two steels are reported in this communication. The precipitates were studied for their morphology, size and chemistry. SEM along with EDX has been employed in characterizing these precipitates. A few TEM results have been included. The solutionizing temperatures of the precipitates were computed from thermodynamic models and compared with experimental observations.

Electrical behaviour of PMN–PT–PVDF nanocomposite

Nanocomposites of polyvinyldene fluoride (PVDF) and a solid solution of lead magnesium niobate and lead titanate (0.65PMN–0.35PT) with varying composition ratios were prepared by the hot-press technique. The phase structure and morphology were studied by thermal analysis (DTA), x-ray diffraction and scanning electron microscopy. The PVDF sample showed an exothermic peak due to the crystallization of the PVDF phase, but with the addition of PMN–PT powders an extra peak appeared due to the crystallization of the pyrochlore phase present in the PMN–PT powder. The crystallite size of the prepared samples was found to be between 40 and 60 nm. The value of the relative permittivity increased with the increase in the ceramic concentration. The presence of a single semicircle confirmed the presence of the bulk effect only. The bulk conductivity indicated an Arrhenius type thermally activated process. The ac conductivity spectrum obeyed the Jonscher power law.

Hard and soft multilayered SiCN nanocoatings with high hardness and toughness

Alternate hard and soft layers increase deformation accommodation as thin hard layers slide relative to each other due to shear deformation of low modulus layers. However, the processing of suchmultilayers is challenging. In the present paper the alternating soft and hard multilayered SiCN coating deposited by magnetron sputtering has been studied and presented. A hardness and modulus of 37GPa and 317GPa with elastic recovery of 62% are achieved by alternate hard and soft layer of Si-C-N by magnetron sputtering. The trilayer films sustained even 2000 gf under indentation without failure though substrate plastically deformed. The fracture toughness value KIC was measured to be 9.5-10MPa m1/2, significantly higher than many reported hard coatings.

Sintering characteristics of Y-Ba-Cu oxide-Ag-x superconductors under argon atmosphere

Sintering studies on Y-Ba-Cu oxide (YBCO)-Ag-x (x = 0, 0.6, and 1.0 mol) powder were carried out in argon atmosphere to understand the role of silver addition on the densification behavior of these materials. The increase of sintered densities of the compacts with silver addition in argon atmosphere contradicted our earlier observation on sintering of YBCO-Ag-x powder compacts in air, where the densities decreased for x > 0.6. Thermogravimetric (TG) studies under argon atmosphere indicate a continuous decrease of mass on heating suggesting an enhanced rate of oxygen removal from the YBCO matrix that facilitated the sintering in argon atmosphere. Sintering studies of YBCO-Ag-x powder compacts in argon in conjunction with earlier observations in air has substantiated our claim that high-temperature oxygen desorption by the silver from the YBCO matrix to the sintering atmosphere controls the rate of densification for these superconducting composites. However, the apparent activation energies for sintering, suggest that the sintering process is controlled by yttrium ion diffusion along bulk and grain boundaries.

Cladding of Tungsten Carbide and Stellite Using High Power Diode Laser to Improve the Surface Properties of Stainless Steel

Surface engineering is one of the most viable methods, in addition to development of new alloys and equipment design, to minimize degradation due to cavitation erosion, and corrosion. Laser surface cladding is relatively a newer engineering technique to produce metallurgically bonded coating for industrial applications due to its inherent benefits. Present paper reports the results obtained on the laser cladding of stainless steel with tungsten carbide (WC) and stellite alloy powder using high power diode laser (HPDL) at various laser parameters. Cladded specimens were characterized for erosion, and corrosion resistance. Both WC and stellite cladding have increased the erosion resistance of stainless steels. WC cladding was found to reduce the corrosion resistance of steel while stellite showed it to increase significantly.