Probal Kumar Das

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

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 andKIC 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.

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

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.

A novel carbon rich crystalline (C) SiC-Si(n) interface using liquid polycarbosilane as a precursor - a unique Schottky junction

This paper presents a novel rectifying interface material using carbon rich crystalline (C)-SiC and n-type Si by a modified CVD technique, using liquid polycarbosilane as a precursor at 900 °C. The equilibrium band diagram and Fermi level alignment was explained using Poissons model and the depletion approximation. The junction capacitance, depletion width and saturation current were evaluated and further discussed from the perspective of temperature dependency. The junction was found to be Schottky in nature, with a large breakdown voltage of 69 V and low space charge. This type of junction material, having good mechanical strength, is promising for high temperature and high power applications.

Indentation size effect and wear characteristics of spark plasma sintered, hard MWCNT/Al2O3 nanocomposites

Magnesia doped multiwalled carbon nanotube (CNT)/α-alumina nanocomposites have been fabricated by spark plasma sintering at 1500°C under 50 MPa in argon. Owing to combined grain refining effect of nanotube and magnesia, nanocomposites possessed smaller matrix grains and extensively lower matrix crystallites than pure alumina. Thermal expansion mismatch between matrix and filler rendered up to four times higher compressive lattice microstrain to the nanocomposites over pure alumina. Despite very low CNT loading (e.g. 0·13 wt-%), nanocomposites offered considerably higher hardness (as high as 24·42 GPa), negligible indentation size effect (Meyer exponent = 1·906 − 1·941) and enhanced elastic response over pure alumina. Up to 0·27 wt-% nanotube loading, much higher wear resistance was observed for the nanocomposites over pure alumina. The presence of uniformly dispersed and structurally intact nanotubes coupled with lower matrix grains and crystallites having compressive lattice strain were the key factors behind achieving such improved mechanical properties of the present nanocomposites.

Sintering and properties of silicon nitride densified with liquids in the system MgOAlNSiO2

Abstract Silicon nitride was pressureless sintered at 1700–1800°C with liquids in the ternary system MgOAlNSiO 2 ; liquids rich in nitrogen and high MgO content sintered readily; Si 3 N 4 of 98% theoretical density was obtained. Heat treatments of the samples were carried out to crystallise the grain boundary phase. Hightemperature flexural strength, fracture toughness and creep of the samples were measured. Increase in nitrogen content with constant MgO:SiO 2 ratio produced products with better thermomechanical properties. Superior products were obtained with high nitrogen and low MgO content in the sintering liquid due to a slower sintering rate and acicular grain growth. The best product had a flexural strength (4-point bending test, 40-20 mm) of 490 MPa and fracture toughness of 6·1 MPam 1 2 . The creep strain rate and stress of a sample sintered with a liquid with a high MgO content were 1·0 × 10 −5 h −1 (at 1200°C) and 100 MPa, respectively.

Sintering of Si3N4 with liquid in the system Ce2O3AINSiO2

Abstract Liquid phase sintering of Si3N4 with melts from the system Ce2O3AINSiO2 has been studied. The glass forming region in this system and the reaction products formed during sintering at 1750–1800°C were analysed. Sintering of Si3N4 with two melt compositions selected from outside the glass forming region yields fully dense Si3N4. Post sintering treatment at 1300°C resulted in devitrification with consequent improvement of high temperature mechanical properties. The mechanical properties of Si3N4 sintered with liquids in the system Ce2O3AINSiO2 were found to be inferior to those of liquids selected from Y2O3AINSiO2, but superior to those selected from the system MgOAINSiO2.

Enhanced mechanical properties of single walled carbon nanotube-borosilicate glass composite due to cushioning effect and localized plastic flow

A borosilicate glass composite has been fabricated incorporating Single Wall Carbon Nanotubes (SWCNT) in the glass matrix by melt-quench technique. Hardness and the fracture toughness of the composite, were found to increase moderately with respect to the base glass. Interestingly one can observe accumulation of SWCNT bundles around the crack zone though no such accumulation was observed in the crack free indentation zone. The enhanced hardness of the composite was discussed by correlating the cushioning as well as toughening behavior of the agglomerated SWCNT bundles. On the other hand enhanced plastic flow was proposed to be the prime reason for the accumulation of SWCNT bundles around the crack, which increases the toughness of the composite by reducing the crack length. Moreover to ascertain the enhanced plasticity of the composite than that of the glass we calculated the recovery resistance of glass and the composite where recovery resistance of composite was found to be higher than that of the glass.

Oxidation of Silicon Nitride Sintered with Yttria and Magnesia Containing Nitrogen Rich Liquid

Oxidation of pressureless sintered Silicon Nitride densified with a Nitrogen rich liquid phase in Y2O3-AlN-SiO2 and MgO-AlN-SiO2 systems have been studied. Thermogravimetric studies indicate a parabollic kinetics. Activation energies are 188 and 430 kJ/mole for the oxidation of materials densified with 11.89 and 23.15%. additives containing Y2O3 and 390 kJ/mole for those densified with 21% additives containing MgO.XRD studies show that the oxidation products are SiO2(Cristobalite), Si2ON2,YAlO3 (above 1250C) and Y2Si2O7 in case of Yttria containing additives and SiO2(Cristoballite),Si2ON2, MgSiO3(Protoenstatite),Mg2SiO4(Forsterite) and MgAl2O4(Spinel) in case of Magnesia containing aditives. XPS study shows the presence of a silicate phase in the outermost laver of the oxidised surface. Oxidation of MgO containing material is higher than that of Y2O3 containing material.

Multi-objective optimisation of abrasive water jet machining responses by simulated annealing and particle swarm

Optimum control parameter settings in complex and stochastic type processes are one of the most challenging problems to the process engineers. As such, effective model development and determination of optimal operating conditions of abrasive water jet machining process (AWJM) are reasonably difficult. In this article new modifications are proposed on two single-objective optimisation techniques, simulated annealing and particle swarm optimisation and applied to optimise two response parameters in AWJM – material removal rate (MRR) and depth of cut (DOC) simultaneously. For the optimisation purpose statistical models are developed from experimental data obtained from AWJ machining of amorphous material – borosilicate glass. Optimum results are validated and it is suggested to employ particle swarm-based proposed multi-objective optimisation technique in present case for newly modelled system due to its fast convergence and low memory space requirement.

DC resistivity of alumina and zirconia sintered with TiC

Pure alumina and zirconia powders were sintered separately with increasing amount of TiC up to ∼ 65 vol.%, as a conducting second phase with an aim to prepare conducting structural ceramics which can be precisely machined by EDM technique. TiC did not help in sintering the parent phase but it decreased the d.c. resistivity of the composite to 1 ohm.cm at ∼ 30 vol.% loading. The conductivity is explained by the effective media and percolation theories.