Some factors influencing the formation of reaction-bonded silicon nitride

Several salient factors influencing the formation of reaction-bonded silicon nitride (RBSN) compacts have been studied. These include the effects of mullite and alumina furnace tubes typically employed during high-purity nitridation studies, pre-sintering of green silicon compacts, free powder versus compact nitridation, and the influence of metal/metal oxide additions. The latter studies have provided experimental evidence for enhancement due to dissociated nitrogen, and suggest that (1) -Si₃N₄ formation does not necessarily require a liquid phase, (2) atomic nitrogen stimulates -phase formation, and (3) the liquid phase provides an efficient source for volatile silicon, promoting -Si₃N₄. These conclusions are consistent with accepted mechanisms for the formation of the two phases.     

The permittivity and dielectric loss of reaction-bonded silicon nitride

Using recently developed coaxial line methods values of permittivity and dielectric loss have been determined over the frequency range 0.5 to 7 GHz for a series of reaction-bonded silicon nitride specimens in which the degree of nitridation has been varied. For fully nitrided material (having a weight gain of 62% and a volume porosity of 19%) the measured permittivity was 4.60 and was almost independent of frequency; fitting both the permittivity and loss data to the Universal Law of dielectric response confirmed that the limiting condition of lattice loss applied withn=0.98±0.02. Reduction of the degree of nitridation caused progressive increases in permittivity and loss, both of which closely approached the quoted values for pure silicon at weight gains below about 40%.     

Effect of frequency on ambient temperature fatigue crack growth in a silicon carbide reinforced silicon nitride composite

A detailed study on a silicon nitride reinforced with silicon carbide whiskers has been undertaken on room temperature fatigue during static and dynamic loading at constant ΔK. It is shown that sub-critical crack growth rates are lower when the material experiences sustained far field loading than during cyclic far field loading. The increased crack growth rate during cyclic loading is attributed to a wedging effect within the crack wake causing an increase in the tensile stress and resultant increased micro-cracking ahead of the crack tip. This additional micro-structural damage leads to enhanced sub-critical crack growth rates during cyclic loading. The asperities that are responsible for the wedging effect are attributed to the isolation of small portions of material due to branching of small cracks and by degradation of the bridging SiC whiskers and Si₃N₄ grains within the crack wake.     

Effects of interface coating and nitride enhancing additive on properties of Hi-Nicalon SiC Fiber reinforced reaction-bonded silicon nitride composites

Strong and tough Hi-Nicalon SiC fiber reinforced reaction-bonded silicon nitride matrix composites (SiC/RBSN) have been fabricated by the fiber lay-up approach. Commercially available uncoated and PBN, PBN/Si-rich PBN, and BN/SiC coated SiC Hi-Nicalon fiber tows were used as reinforcement. The composites contained 24 vol% of aligned 14 m diameter SiC fibers in a porous RBSN matrix. Both one- and two-dimensional composites were characterized. The effects of interface coating composition, and the nitridation enhancing additive, NiO, on the room temperature physical, tensile, and interfacial shear strength properties of SiC/RBSN matrix composites were evaluated. Results indicate that for all three coated fibers, the thickness of the coatings decreased from the outer periphery to the interior of the tows, and that from 10 to 30 percent of the fibers were not covered with the interface coating. In the uncoated regions, chemical reaction between the NiO additive and the SiC fiber occurs causing degradation of tensile properties of the composites. Among the three interface coating combinations investigated, the BN/SiC coated Hi-Nicalon SiC fiber reinforced RBSN matrix composite showed the least amount of uncoated regions and reasonably uniform interface coating thickness. The matrix cracking stress in SiC/RBSN composites was predicted using a fracture mechanics based crack bridging model.     

Thermal effects on the mechanical properties of SiC fibre reinforced reaction-bonded silicon nitride matrix composites

The elevated temperature four-point flexural strength and the room-temperature tensile and flexural strength properties after thermal shock were measured for ceramic composites consisting of 30 vol% uniaxially aligned 142 m diameter SiC fibres in a reaction-bonded Si₃N₄ matrix. The elevated temperature strengths were measured after 15 min exposure in air at temperatures upto 1400 ° C. The thermal shock treatment was accomplished by heating the composite in air for 15 min at temperatures up to 1200 ° C and then quenching in water at 25 ° C. The results indicate no significant loss in strength properties either at temperature or after thermal shock when compared with the strength data for composites in the as-fabricated condition.     

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

Identification of - and -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.     

The temperature and frequency dependencies of permittivity and dielectric loss in reaction bonded silicon nitride

The temperature dependencies of the permittivity and dielectric loss of reaction bonded silicon nitride (RBSN) have been measured between 20 and 900° C at frequencies covering the range from 3 Hz to 300 kHz. Above about 300° C both parameters have a large effect. Analysis of the permittivity data in terms of the theory discussed by Jonscher and by Dissado and Hill predicts loss peaks at 48 Hz and 8 Hz at temperatures of 900 and 800° C, respectively. These values are in close agreement with those found independently from direct observation of the temperature and frequency variations of dielectric loss. On the assumption that a thermal activation process is responsible for the temperature dependence of the loss peak frequency, the associate activation energy is found to be about 1.94 eV.     

Bifurcation analysis of the triaxial test on rock specimens. A theoretical model for shape and size effect

Summary In this paper a bifurcation analysis of the triaxial compression test on rock specimens is presented. Rock behavior is modelled by a deformation theory of elasto-plasticity for a pressure sensitive, cohesive-frictional, dilatant material with microstructure. In particular a Cosserat continuum model for a granular rock is developed. Shape and size effects are discussed on the basis of an analysis of diffuse bifurcation of triaxial tests on a particular sandstone [25].     

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 FeSi_x phase.     

温度对碳热还原/氮化法合成氮化硅的影响

碳热还原/氮化合成Si3N4在1 300~1 600℃下N2或N2-H2混合气中进行。反应物由非晶Si O2与C粉以1∶4.5摩尔比混合、压片。产生的CO由红外传感器监测,样品中氧、氮、碳含量由LECO元素分析仪测得,混合物各相由X射线衍射(XRD)检测。Si O2还原反应在1 300℃以下开始,速率随温度升高增大;温度高于1 570℃时,速率因反应物表面被生成物覆盖降低。由于还原产物CO平衡分压差别小,选择生成Si3N4或Si C的临界温度不明显。碳热还原/氮化法合成氮化硅的原理需进一步探讨。     

Residual stress and damage effect on integrity of ground silicon nitride

Ceramic workpiece integrity and residual surface stresses generated by single pass diamond grinding were evaluated for three flaring cup wheels and four machine-loop stiffnesses. Stresses in silicon nitride bars ground on one face were characterized by X-ray diffraction, strength by four-point bending, and grinding damage depth by scanning electron microscopy. A custom-built workpiece holder was used to tune the grinding machine-loop stiffness. Electrolytic in-process dressing was applied to one of the wheels to provide stable cutting conditions. The experimental results indicate machine stiffness does not have significant influence on flexural strength, but rather affects the depth of cut. All ground surfaces have some degree of damage and residual stress, and differences are revealed between wheel bonds and grit sizes. The competing phenomena of strength enhancement due to residual stress and strength degradation due to damage are discussed.     

Effects of temperature and strain rate on fracture properties of amorphous silicon nitride

The high temperature mechanical behaviours of silicon nitride are difficult to be investigated because it is hard to measure these structural changes in the mechanical test. Atomistic simulation is a proper way to investigate the structural-mechanical mechanism at high temperatures. In this paper, the structural and fracture properties of amorphous silicon nitride (a-Si₃N₄) were studied at different temperatures. The simulation results consist with the experiments on radial distribution function, temperature-dependent yield stress and Young??s modulus. Moreover, the effects of strain rate on mechanical properties were also studied, the results show a higher strain rate lead to larger yield stress and Young??s modulus.