Reaction-bonded Silicon Nitride Research Articles

The Effect of Weak Interface on Transverse Properties of a Ceramic Matrix Composite

AbstractExperimental studies conducted at the NASA Lewis Research Center on silicon carbide reaction-bonded silicon nitride composite system (SiC/RBSN) led to a significant observation regarding their unidirectional tensile properties. It was found that transverse stiffness and strength were much lower than those predicted from existing analytical models based on good interfacial bonding. Since the composite system was designed to have weakened interfaces to improve toughness, it was believed that these weakened interfaces were responsible for the decrease in transverse properties.To support this claim, a two dimensional finite element analysis was performed for a transverse representative volume element. Specifically, the effect of fiber/matrix displacement compatibility at the interface was studied under both tensile and compressive transverse loadings. Interface debonding was represented active gap elements connecting the fiber and matrix.The analyses show that the transverse tensile strength and stiffness are best predicted when a debonded interface is assumed for the composite. In fact, the measured properties can be predicted by simply replacing the fibers by voids. Thus, the following two conclusions are drawn from the present study: (1) little or no interfacial bonding exists in the SiC/RBSN composite; (2) an elastic analysis can predict the transverse stiffness and strength.

Assessment of Strength by Young's Modulus and Porosity: A Critical Evaluation

Literature data on strength, porosity, and Young's modulus at room temperature of reaction‐bonded silicon nitride, sintered silicon nitride, and hot‐pressed silicon nitride have been fitted into available and proposed strength‐porosity relationships. In general, the Lewis method of iterative least‐squares fitting in these relationships has been found to be better than conventional linearized least‐squares fitting. Further a semiempirically proposed strength–Young's modulus relationship has been found to predict strength more precisely than the conventional strength‐porosity relationship.

High-Strength Reaction-Bonded Silicon Nitride

High-purity, small-diameter silicon powders made from laser-heated SiH{sub 4} have been used to fabricate 76% dense reaction-bonded silicon nitride (BSN) samples with a fine, uniform microstructure. Room-temperature strength values were 75% greater than conventionally processed RBSN (83% dense), with toughness and hardness values being {approx}10% greater. These high strengths result from uniformly distributed, small pores (<15 {mu}m in diameter), made possible from the combination of ideal powders and careful post-synthesis processing.

Influence of some Factors on Strength and Fracture Toughness of Reaction Bonded Silicon Nitride

Influence of density, Young's modulus, temperature and other experimental variables on strength and fracture toughness of reaction bonded silicon nitride (RBSN) was studied. Fractographic study was made to identify the nature of flaws. The density dependence of strength could be expressed through proposed empirical equations. Strength of RBSN was proportional to Young's modulus. In the range RT-1400°C some RBSN had constant strength, while others had a peak in strength at 1200°C followed by degradation at 1400°C.The density dependence of fracture toughness could be expressed through some equations. Fracture toughness of RBSN had direct dependence on strength and Young's modulus. High temperature fracture toughness and strength were directly dependent on each other. Fracture toughness of RBSN at RT increased as notch tip radius was increased in the range 79–225 μm, but it did not show any systematic variation as the normalized notch length was varied.

High temperature Young's modulus of reaction-bonded Si 3N 4, liquid phase sintered Si 3N 4 and sialon

Temperature dependence (RT-1400 C) of Young's modulus and strength of reaction-bonded silicon nitride (RBSN), liquid phase sintered silicon nitride (LPSSN) and Sialon has been investigated. Although strength and Young's modulus are related through the equation σ = 7.39 × 10 −6 E 1.41 the influence of temperature on strength and Young's modulus is different. For RBSN strength increases with temperature, reaches a maximum at 1200 C and then decreases. But Young's modulus steadily decreases with temperature. For LPSSN and Sialon both Young's modulus and strength degrade with temperature. Details of Young's modulus data have been presented and their implication in assessing temperature dependence of strength discussed.

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

Nitridation characteristics of silicon compacts made by uniaxial and isostatic pressing and slip casting, respectively, have been studied. α and β-Si 3N 4 were the major phases in uniaxially and isostatically pressed specimens, and α phase being over 90%. In slip cast specimens around 20% Si 2N 2O was detected. The uniaxially pressed and nitrided samples showed a regular rise in MOR and K IC 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 K IC values did not fall at 1400°C. Si 3N 4SiC composites show improved values of MOR and K IC when SiC is 5 vol%;Si 3N 4BN (3·6 vol%) and Si 3N 4TiC (5 vol%) show an inflection in the MOR and K IC 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.

Relationships between processing, microstructure and properties of dense and reaction-bonded silicon nitride

Some aspects of processing, microstructure and properties of the various types of silicon nitride are discussed. Special emphasis is placed on the relationships between powder properties, process conditions, densification and microstructure, as well as the interdependence between microstructure and properties. After summarizing the areas of crystal structure and thermodynamic properties, and processing of the different types of Si3N4, the state-of-the-art of dense and reaction-bonded silicon nitride is given. For both types the formation mechanisms and microstructure, relationships between powder properties, additives (in the case of dense Si3N4), process conditions, and densification and microstructure, as well as data and microstructural effects of various mechanical, thermal and thermo-mechanical properties, are outlined. Advanced processing techniques, such as sintering, gas-pressure sintering, post-sintering, and the different routes of hot-isostatic pressing (starting with powder compacts, reaction-bonded Si3N4 or pre-sintered Si3N4 and the resulting properties, are discussed.

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.

The nature of sialon joints between silicon nitride based bodies

Quite strong joints between silicon nitride based bodies have been made by incorporating a layer of aluminium and oxides between the bodies and heating in a nitriding atmosphere. The joints are resistant to thermal shock and maintain their strength at 1200° C. Microscopic, DTA and X-ray diffraction studies indicated that sialon phases are present in the joints, and that the bonding reaction involves the reduction of Si3N4 by aluminium and the subsequent renitriding of the resultant silicon, as well as the simultaneous nitriding of a portion of the aluminium. Transmission electron microscopy of a joint between hot pressed and reaction bonded silicon nitrides showed that 15R aluminium nitride polytype sialon was present on the reaction bonded side of the joint and s′-sialon on the hot pressed side.

Analysis of grain boundaries for reaction-bonded silicon nitride with yttria addition

Silicon nitride is often the developmental ceramic of choice for high-temperature engine components. Sintering this ceramic, however, is very difficult due to the covalent nature of the bonding. Complicating the matter, Si3N 4 tends to dissociate at temperatures above 1700 ° C. Dissociation can be impeded by application of a nitrogen overpressure, while the general sintering problem is overcome by the addition of small amounts of various oxides. These sintering aids react with the silicon nitride and each other to form liquid phases, allowing sintering at temperatures between 1600 and 1900°C. Common additives include magnesia, yttria, alumina, and their various combinations. Normal additive content is in the range of 1 to 10% by weight. Although these sintering aids produce highly dense structures, they also lead to the formation of secondary phases along grain boundaries. The resulting materials are very strong at room temperature, but they rapidly lose their strength at high temperature (> 1000°C) because high-temperature mechanical properties such as creep are controlled by the grain boundary phases. The grain-boundary phases, in turn, are determined by the quantity and combination of sintering aids, by the firing cycle, and by the densification process (e.g. hot-pressed or reaction-bonded). Thus, by correlating the microstructure with the physical properties and processing/composition with microstructure, the effect of processing and compositional factors upon the physical properties can be determined and controlled. Although MgO was initially used as a sintering aid in the past, Y203 and YaO3-A1203 yield greater hightemperature strength [1, 2]. Yttria is an attractive additive because at low temperatures it reacts with the SiO2 (and some Si3N4) on the surface of the Si3N4 to form a liquid, greatly enhancing the sintering rate [3]. At higher temperatures when densification is complete, the liquid reacts with more Si 3 N 4 to give a highly refractory bonding phase along the grain boundaries. Tsuge et al. [4] showed that crystallization of the grain-boundary phase could improve high-temperature strength. Investigations of Si3N4-Y203 and Si3Na-Y203SiO2 phase diagrams [3, 5, 6] have been used to predict and explain the various grain-boundary constitutents and resulting properties. Using hot-pressed materials, Lange and co-workers [5, 7] found that the phases within the Si3N4 Si2N20 Y2Si207 compatibility triangle (in the Si3N4 Y203 SiO2 phase diagram) were extremely oxidation resistant, while Si3Y203N4, YSiOzN (K-phase), YxoSiyO23N4 (H-phase) and Y48i207N2 (J-phase) readily oxidize at 1000 ° C. Richer-

Fracture of flash oxidized, yttria-doped sintered reaction-bonded silicon nitride

The oxidation behaviour of a slip cast, yttria-doped, sintered reaction-bonded silicon nitride after “flash oxidation’ was investigated. It was found that both the static oxidation resistance and flexural stress rupture life (creep deformation) were improved at 1000° C in air compared to those of the same material without flash oxidation. Stress rupture data at high temperatures (1000 to 1200° C) are presented to indicate applied stress levels for oxidation-dependent and independent failures.

Electron paramagnetic resonance in reaction-bonded silicon nitride

Electron paramagnetic resonance in reaction-bonded silicon nitride

Dynamic fracture toughness

Dynamic fracture toughness versus crack velocity relations of Homalite-100, polycarbonate, hardened 4340 steel and reaction bonded silicon nitride are reviewed and discrepancies with published data and their probable causes are discussed. Data scatter in published data are attributed in part to the observed fluctuations in crack velocities. The results reaffirmed our previous conclusion that the dynamic fracture toughness versus crack velocity relation is specimen dependent and that the dynamic crack arrest stress intensity factor is not a unique material property.

ファインセラミックスの研削過程に関する研究

In order to get a foundation for scientific understanding of high precision and high productivity grinding of fine ceramics with metal bond diamond wheel, cylindrical plunge grinding processes of normally sintered silicon nitride, alumina, partially stabilized zirconia and reaction bonded silicon nitride are experimentally analyzed with bronze bond diamond wheels. Main results obtained in this paper are as follows : (1) There are three distinct grinding states in a plunge grinding cycle of normally sintered fine ceramics just as in grinding steels. (2) The ratio of tangential force to normal one is remarkably smaller in grinding fine ceramics than that in grinding steels. (3) The wheel wear in grinding reaction bonded silicon nitride is larger than that in other normally sintered fine ceramics. (4) Grinding ratio becomes lower in the order of partially stabilized zirconia, alumina, silicon nitride, and that of reaction bonded silicon nitride is the smallest of all work materials used in this study.

EFFECTS OF STATIC AND CYCLIC FATIGUE AT HIGH TEMPERATURE UPON REACTION BONDED SILICON NITRIDE

The effects of static fatigue and cyclic fatigue upon Reaction Bonded Silicon Nitride (RBSN) were compared using the statistical distributions of residual strengths measured in tension. The strength-controlling flaws are highly dependent upon frequency. It was found that low frequency cyclic and short-term static loadings blunted surface flaws, whereas high frequency cyclic fatigue affects internal flaw populations. This situation is attributed to oxidation related mechanisms.

ChemInform Abstract: FRACTURE OF YTTRIA‐DOPED, SINTERED REACTION‐BONDED SILICON NITRIDE

AbstractDie Biegefestigkeit eines mit Y2O3 dotierten SRBSN wird als Funktion der Temp. zwischen 20 und 1400°C an der Luft, der angewendeten Spannung sowie der Zeit untersucht.

Post-sintering of reaction-bonded silicon nitride

Abstract Reaction-bonded silicon nitride, RBSN, may be used as the starting material for a post-densification process carried out at about 1800°C in nitrogen at atmospheric pressure. The sintering aids used are similar to those employed in hot pressing and pressureless sintering (e.g. MgO, Y 2 O 3 ). They may be added to the silicon powder before nitridation or, in the case of MgO, by diffusing the vapour into preformed RBSN. High densities (98.5% theoretical) have been achieved accompanied by a shrinkage of only 6.2%. The strength at R.T. was about 1000 MN/m 2 ; the strength at H.T. depends on devitrification. The process thus offers complex components to be formed by injection moulding, with strength values after sintering similar to those of hot-pressed silicon nitride.

Interaction between creep, oxidation and microporosity in reaction-bonded silicon nitride

Abstract Kinetic studies have been performed on primary and secondary (minimal) creep of reaction-bonded silicon nitride in 4-pt-bending tests up to 1500°C. The creep deformation depends strongly on the extent of internal oxidation. In spite of the very marked dependence of the creep rate on material and pretreatment parameters, the stress exponents (n = 1.7–1.8) and activation energies (360–390 kJ/mole) are hardly influenced, suggesting similar creep mechanism. Creep deformation is provided by relative motion and separation of grain boundaries. Oxidation and deformation lead to remarkable changes of the pore size distribution; the creep processes are accompanied by deformation of the pores. The creep rupture strain is very limited in highly creep resistant materials and vice versa. Methods for the determination of oxidation products and oxide profiles along sample cross section have been developed and the chemical changes which the material can undergo during creep are outlined.

Microstructure of Reaction‐Bonded Silicon Nitride Consolidated by Isostatic Hot‐Pressing

Reaction‐bonded Si3N4 specimens which had been isostatically hot‐pressed at 1850°, 1950°, and 2050°C were examined microstructurally and by chemical analyses. Previously reported data indicated an increase in the 1200°C bend strength of reaction‐bonded Si3N4 only after it was pressed at 2050°C. Observations from the present study indicate that this behavior results from the relative oxygen levels of the isostatically hot‐pressed specimens. Calculations show that an Si02 content of 52.2 vol% is required to realize an increase in the 1200°C strength.

Fracture of Yttria‐Doped, Sintered Reaction‐Bonded Silicon Nitride

Flexural strength of an yttria‐doped, slip‐cast, sintered reaction‐bonded silicon nitride was evaluated as a function of temperature (20° to 1400°C in air), applied stress, and time. Static oxidation at 700o to 1400°C was investigated in detail; in tests at 1000°C in air, the material showed anomalous weight gain. Flexural stress‐rupture testing a 800° to 1200°C in air indicated that the material is susceptible to stress‐enhanced oxidation and early failure. Fractographic evidence for time‐dependent and ‐independent failures is presented.