• P K Das

      Articles written in Bulletin of Materials Science

    • Optimization of time-temperature schedule for nitridation of silicon compact on the basis of silicon and nitrogen reaction kinetics

      J Rakshit P K Das

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      A time-temperature schedule for formation of silicon-nitride by direct nitridation of silicon compact was optimized by kinetic study of the reaction, 3Si + 2N2 = Si3N4 at four different temperatures (1250°C, 1300°C, 1350°C and 1400°C). From kinetic study, three different temperature schedules were selected each of duration 20 h in the temperature range 1250°-1450°C, for complete nitridation. Theoretically full nitridation (100% i.e. 66.7% weight gain) was not achieved in the product having no unreacted silicon in the matrix, because impurities in Si powder and loss of material during nitridation would result in 5–10% reduction of weight gain.

      Green compact of density < 66% was fully nitrided by any one of the three schedules. For compact of density > 66%, the nitridation schedule was maneuvered for complete nitridation. Iron promotes nitridation reaction. Higher weight loss during nitridation of iron doped compact is the main cause of lower nitridation gain compared to undoped compact in the same firing schedule. Iron also enhances the amount of Β-Si3N4 phase by formation of low melting FeSix phase.

    • Study of indentation induced cracks in MoSi2-reaction bonded SiC ceramics

      O P Chakrabarti P K Das S Mondal

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      MoSi2–RBSC composite samples were prepared by infiltration of Si–2 at.% Mo melt into a preform of commercial SiC and petroleum coke powder. The infiltrated sample had a density > 92% of the theoretical density (TD) and microstructurally contained SiC, MoSi2, residual Si and unreacted C. The material was tested for indentation fracture toughness at room temperature with a Vicker’s indenter and KIC was found to be 4.42 MPa√m which is around 39% higher than the conventional RBSC material. Enhancement in indentation fracture toughness is explained in terms of bowing of propagating cracks through MoSi2/SiC interface which is under high thermal stress arising from the thermal expansion mismatch between MoSi2 and SiC.

    • Effect of microstructure on the high temperature strength of nitride bonded silicon carbide composite

      J Rakshit P K Das

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      Four compositions of nitride bonded SiC were fabricated with varying particle size of SiC of ∼ 9.67, ∼ 13.79, ∼ 60 𝜇 and their mixture with Si of ∼ 4.83 𝜇 particle size. The green density and hence the open porosity of the shapes were varied between 1.83 to 2.09 g/cc and 33.3 to 26.8 vol.%, respectively. The effect of these parameters on room temperature and high temperature strength of the composite up to 1300°C in ambient condition were studied. The high temperature flexural strength of the composite of all compositions increased at 1200 and 1300°C because of oxidation of Si3N4 phase and blunting crack front. Formation of Si3N4 whisker was also observed. The strength of the mixture composition was maximum.

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