Aniruddha Samanta

Decomposition reactions in the SiC-Al-Y-O system during gas pressure sintering

Abstract The substantial densification, that occurred in the SiC–Al–Y–O system was explained in the present work by analysing possible chemical reactions and their dependence on initial particle associations, i.e. homogeneity of mixing, the physical and chemical state of additives, pressurised sintering environment over the reactants and temperature of sintering. Hydroxyhydrogel powder precursors were found to be better than mechanically mixed SiC–YAG powder and pre-forming of YAG by holding the specimens at the temperature of 1400°C for 2 h were found to be the best. Decomposition reactions within the system could be controlled by using finer SiC and applying gas pressure over the reactants.

Retention of SiC during development of SiC-MxSiyOz composites [M = Al, Zr, Mg] by reaction bonding in air

Abstract Oxidation of SiC is a major constraint during development of metal oxide–silicon carbide composites when processed in oxygen containing environment such as in air. In the present investigation, Mg+2, Al+3 and Zr+4 hydrogels were used as a source of respective oxide and oxidation of SiC in each system was studied. A three-stage mechanism was found to be operative in Al+3 and Zr+4 systems where oxidation at the initial stages was found to be controlled by the nature of the polynuclear complexes formed on the surfaces of SiC particles. At the intermediate stage a transition from polynuclear complex to metal silicate protective layer formation changes the oxidation characteristics. Finally the metal silicates provided the ultimate protection. Mg+2 was found to be ineffective. The extent of retention of SiC in the final composites could be premonitored by controlling the amount and the nature of complexing cations.

Near net shape SiC–mullite composites from a powder precursor prepared through an intermediate Al-hydroxyhydrogel

Abstract SiC–mullite composites with low dimensional changes on sintering were fabricated by partial oxidation of SiC followed by reaction bonding using hydroxyhydrogel derived alumina keeping SiC as dispersed phase in the intermediate gel-like mass as starting material. The sintering-induced shrinkage is compensated by volume expansion caused by the silicon carbide oxidation-induced-volume expansion. This work describes a new processing route to fabricate SiC–mullite composites where in situ formation of mullite takes place that proceeds at much lower temperatures (⩽1600°C) than in normal ceramics processing routes. Specimens containing Al2O3⩽40% are not suitable due to the formation of large amount of low eutectic aluminosilicates at processing temperatures. Specimens containing ⩾40% Al2O3 yield different aluminosilicates, mainly mullite, which acted as a retarder for oxidation of SiC and at the same time it helped to consolidate SiC compacts. Final phases in fired compact were identified by XRD and different phases present were calculated on the basis of X-ray diffraction results.

SiC–YAG sintered composites from hydroxy hydrogel powder precursors

Abstract Yttrium hydroxide and aluminium hydroxide hydrogel were derived from yttrium nitrate and aluminium nitrate through hydroxy hydrogel route in which SiC particles were kept dispersed. The gel-like mass was heat treated at 900°C in ambient atmosphere followed by heat treatment at 1400°C in Ar atmosphere. The specimens were then sintered in the temperature range of 1800–1950°C in Ar atmosphere with 30 min soaking. The phases were identified by XRD analysis. Microstructure of the sintered materials were analysed by scanning electron micrograph. With this new method of preparation of powder precursors, the process of sintering was easier and almost theoretical density was achieved with moderate hardness. The mechanism of densification was postulated to be a solid-state initiated liquid phase sintering and the overall process of which was activated by the reactive species formed from hydroxy hydrogel powder precursors.

Prevention of Oxidation of SiC through Reaction Bonding in the SiC-Mullite Composite System by Using the Technique of Intermediate Gel Formation

Reaction bonding of silicon carbide-mullite composites with varying SiC to mullite ratio is studied. The oxidation of silicon carbide to cristobalite could be controlled by increasing the material that gives mullite after heat treatment. In the present study, the best composition was found to be that which produces SiC and mullite in the ratio of about 1:1.

ROS mediated high anti-bacterial efficacy of strain tolerant layered phase pure nano-calcium hydroxide

The present work provides the first ever report on extraordinarily high antibacterial efficacy of phase pure micro-layered calcium hydroxide nanoparticles (LCHNPs) even under dark condition. The LCHNPs synthesized especially in aqueous medium by a simple, inexpensive method show adequate mechanical properties along with the presence of a unique strain tolerant behaviour. The LCHNPs are characterized by FTIR, Raman spectroscopy, XRD, Rietveld analysis, FE-SEM, TEM, TG-DTA, surface area, particle size distribution, zeta potential analysis and nanoindentation techniques. The LCHNPs have 98.1% phase pure hexagonal Ca(OH)2 as the major phase having micro-layered architecture made up of about ~100-200nm thick individual nano-layers. The nanomechanical properties e.g., nanohardness (H) and Youngs modulus (E) of the LCHNPs are found to have a unique load independent behavior. The dielectric responses (e.g., dielectric constant and dielectric loss) and antibacterial properties are evaluated for such LCHNPs. Further, the LCHNPs show much better antibacterial potency against both gram-positive e.g., Staphylococcus aureus (S. aureus) and gram-negative e.g., Pseudomonas putida (P. putida) bacteria even in dark especially, with the lowest ever reported MIC value (e.g., 1 μg ml-1) against the P. putida bacterial strain and exhibit ROS mediated antibacterial proficiency. Finally, such LCHNPs has almost ~8-16% inhibition efficacy towards the development of biofilm of these microorganisms quantified by colorimetric detection process. So, such LCHNPs may find potential applications in the areas of healthcare industry and environmental engineering.

OXIDATION BEHAVIOUR OF SILICON CARBIDE : GEL DERIVED OXIDE COMPOSITE SYSTEMS

Consolidation of composites in the system SiC—Al2O3, SiC—ZrO2 and SiC—MgO in relation to oxidation characteristics of silicon carbide during firing in the ambient atmosphere were studied. A12O3, ZrO2 and MgO additives were derived through sol-gel route. SiC—Al2O3 system was found to be more resistant towards oxidation of SiC where ultimate fired phases were silicon carbide and mullite with traces of cristobalite as detected by XRD. Both Al2O3, and ZrO2 are prospective, but MgO in the present system is not effective.

Nano- and micro-tribological behaviours of plasma nitrided Ti6Al4V alloys

Plasma nitriding of the Ti-6Al-4V alloy (TA) sample was carried out in a plasma reactor with a hot wall vacuum chamber. For ease of comparison these plasma nitrided samples were termed as TAPN. The TA and TAPN samples were characterized by XRD, Optical microscopy, FESEM, TEM, EDX, AFM, nanoindentation, micro scratch, nanotribology, sliding wear resistance evaluation and in vitro cytotoxicity evaluation techniques. The experimental results confirmed that the nanohardness, Youngs modulus, micro scratch wear resistance, nanowear resistance, sliding wear resistance of the TAPN samples were much better than those of the TA samples. Further, when the data are normalized with respect to those of the TA alloy, the TAPN sample showed cell viability about 11% higher than that of the TA alloy used in the present work. This happened due to the formation of a surface hardened embedded nitrided metallic alloy layer zone (ENMALZ) having a finer microstructure characterized by presence of hard ceramic Ti2N, TiN etc. phases in the TAPN samples, which could find enhanced application as a bioimplant material.

Nanomechanical responses of human hair

Here we report the first ever studies on nanomechanical properties e.g., nanohardness and Young׳s modulus for human hair of Indian origin. Three types of hair samples e.g., virgin hair samples (VH), bleached hair samples (BH) and Fe-tannin complex colour treated hair samples (FT) with the treatment by a proprietary hair care product are used in the present work. The proprietary hair care product involves a Fe-salt based formulation. The hair samples are characterized by optical microscopy, atomic force microscopy, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy (EDAX) genesis line map, EDAX spot mapping, nanoindentation, tensile fracture, and X-ray diffraction techniques. The nanoindentation studies are conducted on the cross-sections of the VH, BH and FT hair samples. The results prove that the nanomechanical properties e.g., nanohardness and Young׳s modulus are sensitive to measurement location e.g., cortex or medulla and presence or absence of the chemical treatment. Additional results obtained from the tensile fracture experiments establish that the trends reflected from the evaluations of the nanomechanical properties are general enough to hold good. Based on these observations a schematic model is developed. The model explains the present results in a qualitative yet satisfactory manner.