Kajal Kumar Dhargupta

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.

Gas pressure sintering of β SiC–γ-AlON composite in nitrogen/argon environment

A SiC and γ AlON system was studied using β-SiC powder and a preformed spinel between Al2O3 and AlN corresponding to γ-AlON. A wide range of composition from 10 to 90 wt.% γ-AlON with β-SiC was studied. The composite was sintered to near pore-free dense sintered product for a wide range of composition ranging from 10 to 55 wt.% of γ-AlON content in both a N2 and an Ar atmosphere under a gas pressure of 6 bar. The effect of sintering atmosphere on the sintered properties such as weight loss, percentage linear shrinkage, percentage apparent porosity, and bulk density of the fired samples were determined. Weight loss of the specimen was 4.3–5.5 wt.% in a nitrogen and 6.2–10.75 wt.% in an Ar atmosphere indicating substantial supression of dissociation reaction during sintering in nitrogen atmosphere. Microhardness of the sintered specimen measured by indentation under a load of 1 N was 16.5–29.81 GPa. Phase analysis by XRD studies and the other properties indicated solid solution formation between SiC and γ-AlON.

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.

Rheological Behaviour of Silicon Carbide Suspension in Relation to Particle Size Distribution

Rheological behaviour of an aqueous suspension of silicon carbide in presence of a dispersing agent has been studied to analyze the important factors related to casting. Shear stress τ and rate of shear γ were found to follow power law with two constants n and k both of which were dependent on particle size distribution.

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.