Joydeb Mukerji

Influence of grain size, free silicon content and temperature on the strength and toughness of reaction-bonded silicon carbide

Abstract The flexural strength and fracture toughness ( K IC ) of reaction-bonded silicon carbide (RBSC) made from α-SiC grains of 0·2 and 23·65 μm with a free Si content of 16·5 and 26% and 16·5 and 24%, respectively, were measured from room temperature to 1370°C (1350°C in the case of K IC ). The modulus of rupture (MOR) increased to 1300°C, followed by a sharp decrease to 1370°C. The K IC increased with temperature, from room temperature to 1350°C. The RBSC made from 0·2-μm α-SiC grains was superior in MOR K IC to that made from 23·65-μm α-SiC. The RBSCs with varying free Si contents were made by varying the pressure and composition during fabrication, and were tested for room temperature MOR and K IC , which decreased linearly with volume per cent of free Si.

Characterization of silicon nitride single crystals and polycrystalline reaction sintered silicon nitride by microhardness measurements

Identification of α- and β-phases of Si3N4 single crystals grown from Si melt could be made with the help of Vickers microhardness measurements. The effect of chemical additives, e.g. metallic Fe and BaF2, on the microhardness of Si3N4 was also determined. Different constants involved in the empirical Meyer relationship between load and indentation diameters could be correlated with the porosity and microhardness of Si3N4 single crystals and polycrystalline, reaction sintered Si3N4.

Wear of nitrogen ceramics and composites in contact with bearing steel under oscillating sliding condition

The wear of nitrogen based ceramics in contact with bearing steel balls (SAE 52100) was investigated under reciprocating sliding conditions at a velocity of 0.1 m/s, and 20 N, 40 N and 100 N load. Three types of ceramics were studied: viz. hot pressed silicon nitride (:HPSN) sintered with selected liquid in the system yttria-aluminium nitride-silica and composites of HPSN with BN and TIG; SiAlON formulated with different amounts of alumina and silica; and alumina-titanium nitride composite having 60mol% TIN. Among the HPSM composites, HPSN 15 vol% TiC had the lowest average wear factor (K) of 2.0 x 10(-5) mm(3)/m/N. The SiAlONs, in general, had the highest K and this increased with increasing O concentration. The lowest K value for SiAlON ceramic was 6.9 x 10(-5) mm(3)/m/N. The high K value of SiAlON is attributed to O substitution, which promotes adhesive wear resulting in mutual material transfer. The lowest wear of all the ceramics tested was found for the alumina-titanium nitride composite, the wear factor being of the order of 4.4 x 10(-7) mm(3)/m/N, which is one order of magnitude lower than the other nitrogen ceramics. In the case of HPSN and alumina-titanium nitride composites, abrasive wear and wear due to microfracture were the main wear mechanisms. Wear of the steel ball was lowest when in contact with HPSN-TiC composites, being almost equal to that of the ceramic, whereas the alumina-titanium nitride composite wore away the steel almost 20 times faster than the ceramic and will thus be unsuitable as a counterface material for 52100 steel

Effect of crystal orientation, structure and dimension on vickers microhardness anisotropy of β-, α-Si3N4, α-SiO2 and α-SiC single crystals

Abstract Variation of Vickers microhardness on planes parallel to [VMH(∥C)] and perpendicular to the C-axis [VMH(⊥C)] of hexagonal β-Si 3 N 4 , α-Si 3 N 4 , α-SiO 2 (α-quartz) and α-SiC(6H) single crystals has been studied. The VMH (∥ C ) VMH (⊥ C ) ratios have been correlated with a c ratios, a and c being unit cell parameters of the single crystals. VMH(⊥C) was found to be independent of orientation of Vickers indenter on basal plane (0001) of α-SiC, while VMH(∥C) was found to vary with Vickers indenter direction on prismatic plane (10-10) of α-SiC. VMH(∥C) of β-Si 3 N 4 single crystals was found to depend on the aspect ratio of β-Si 3 N 4 , while VMH(⊥C) was found to be independent of the dimension of basal plane. VMH(⊥C) of α-SiC single crystals was also found to be independent of the dimension of basal plane of α-SiC.

Reaction sequences in the synthesis of silicon nitride from quartz

Carbothermal reduction and nitridation of pure silica have been performed to synthesize pure Si3N4 powder. Iron was used to serve as a catalyst. The reduction reaction was studied with respect to different parameters such as temperature, soaking time and gas flow rate, etc. Almost pure Si3N4 powder with predominating β-phase could be synthesised at 1540°C. Below this temperature SiC and Si2N2O are the associated phases. SiC is unstable to Si3N4 in nitrogen atmosphere and disappears at 1540°C or under a longer holding at 1440°C. There is a critical flow rate of the nitrogen gas of 1·32 × 104 litre m−2 h−1, above which the yield on nitridation almost flattens. It has been suggested through thermodynamic arguments that the Si3N4 formation at 1440°C takes place through the intermediate formation of SiC and Si2N2O as follows: SiO2(s ) + 2SiC(s) + 2N2(g) = Si3N4(s) + 2CO(g) SiO2(s) + Si2N2O(s) + 3C(s) + N2(g) = Si3N4(s) + 3CO(g) At 1540°C the following reaction may predominate: 3SiO2(s) + 6C(s) + 2N2(g) = Si3N4(s) + 6CO(g)

Reaction of CsNO3 and RbNO3 with natural aluminosilicates

Abstract Reaction of CsNO 3 and RbNO 3 with naturally occuring kyanite, kaolin and pyrophillite have been studied using DTA and TGA apparatus. The reaction products are the alumino di- and tetrasilicate either appearing alone or in combination depending on the composition of the starting mixture. A linear rate law was found to be valid for alkali nitrate-metakaolin reaction up to 100 percent conversion.

The comparative behaviour and some parameters on the carbothermal reduction and nitridation of aluminosilicates

Abstract A few natural aluminosilicates with variable Al:Si ratios of 1:2, 1:1 and 2:1 have been subjected to carbothermal reduction and nitridation process. β ′-Sialon with different z -values and in different amounts are formed associated with α -Al 2 O 3 . The degree of substitution in β ′-sialon is always lower than that predicted theoretically. Among all the aluminosilicates under study, kaolin is the best producer of sialon. The reactivity of the reduction and nitridation process increases with increasing Al:Si ratio of the starting raw aluminosilicates. The pressnce of iron is essential, which possibly acts as nitrogen carrier.

Dependence of strength and toughness of reaction bonded silicon nitride and composites on fabrication routes

Abstract Nitridation characteristics of silicon compacts made by uniaxial and isostatic pressing and slip casting, respectively, have been studied. α and β-Si3N4 were the major phases in uniaxially and isostatically pressed specimens, and α phase being over 90%. In slip cast specimens around 20% Si2N2O was detected. The uniaxially pressed and nitrided samples showed a regular rise in MOR and KIC up to 1200°C, falling thereafter. Temperature-independent values were observed for isopressed samples. Slip cast specimens on the other hand showed a rise with temperature, and sharply increased after showing a dip at 1200°C. The MOR and KIC values did not fall at 1400°C. Si3N4SiC composites show improved values of MOR and KIC when SiC is 5 vol%;Si3N4BN (3·6 vol%) and Si3N4TiC (5 vol%) show an inflection in the MOR and KIC curves above 1200°C which do not deteriorate even at 1400°C. Oxidation, pore size distribution and nature of phases present in RBSN and composites have been considered for explanation of the observed behaviour.

Oxidation kinetics of reaction-sintered silicon carbide

The oxidation kinetics of reaction-sintered silicon carbide has been studied over the temperature range 1200° to 1350°C. The material has a bulk density of 3·00 g/cm3 and the unreacted Si content is 22·5% (v/v). The activation energy for oxidation is 28·75 ± 2·61 kcal/mol. It is proposed that the diffusion of oxygen through the growing oxide film is the rate-controlling process.

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.