Creep of hot-pressed silicon nitride

Creep tests were undertaken on hot-pressed silicon nitride in the temperature range 1200 to 1400° C. The activation energy for creep was determined to be 140 kcal mol^?1 and the stress exponent of creep rate was 1.7. The creep behaviour is ascribed to grain-boundary sliding accommodated by void deformation at triple points and by limited local plastic deformation. Electron microscopic evidence supporting this mechanism is presented.     

Proof-testing of hot-pressed silicon nitride

Proof-testing was investigated as a method for insuring the reliability of hot-pressed silicon nitride in high temperature structural applications. The objective of the study was to determine if the strength distribution of a population of test specimens could be truncated by proof-testing. To achieve this objective the strength of silicon nitride was measured at 25° C and 1200° C, both with and without proof-testing. At 25° C, however, the strength distribution was effectively truncated by proof-test ing. At 1200° C, however, the effectiveness of proof-testing as a means of truncating the strength distribution was determined by the resistance of the silicon nitride to oxidation. Although oxidation removes machining flaws that limit the strength of silicon nitride, long-term exposure to high temperature oxidizing conditions resulted in the formation of surface pits that severely degraded the strength. Provided the effects of high temperature exposure are taken into account, proof-testing is shown to be useful for truncating the strength distribution of hot-pressed silicon nitride at elevated temperatures.     

Phenomenology of fracture in hot-pressed silicon nitride

Fracture phenomenology in hot-pressed silicon nitride has been studied fractographically as a function of flaw size, temperature and loading rate. Surface cracks of controlled size were introduced using the microhardness indentation technique. At room temperature, the fracture stress was found to depend on initial crack size according to the Griffith relationship and extrapolation of the data indicated that inherent processing flaws of the order of 12 to 24 m are strength-controlling in virgin material. Using a simplified Griffith approach, the fracture surface energy, , at 20° C for hot-pressed Si₃ N₄ is about 22 000 erg cm^–2. Two mechanistic regimes were manifest in the temperature dependence of the fracture stress. A mixed mode of fracture consisting of transcrystalline and intergranular crack propagation occurred up to 1100° C; at 1200° C and above, subcritical crack growth (SCG) occurred intergranularly and the extent of SCG increased with increasing temperature. Similarly, the extent of SCG decreased with increasing loading rate.     

Creep and strain recovery in hot-pressed silicon nitride

It is observed that creep response in hot-pressed silicon is characterized by two parallel phenomena; one accounts for a persistent non-recoverable plastic deformation and the other for a transient viscoelastic recoverable deformation. The persistent creep component is time-dependent, and apparently follows parabolic time kinetics. It is further observed that creep is characterized by a power law stress exponent of about 4 and an activation energy of 848 kJ mol^–1. The viscoelastic recoverable component of strain is found to be independent of the total plastic strain in the material. The recovery rate at any given time is directly proportional to the preceding creep stress and therefore can be considered linear viscoelastic. The creep compliance of the viscoelastic transient is temperature-dependent with an activation energy of about 722 kJ mol^–1. It is further observed that the viscoelastic recovery is characterized by a spectrum of retardation times and can be modelled by a series of Kelvin analogue models. Finally, the viscoelastic recovery and the viscoelastic component of subsequent creep appear to be inversely related and apparently obey Boltzman superposition. A model is developed for the creep and recovery behaviour of hot-pressed silicon nitride consistent with all experimental observations and based in relative grain motion accommodated by the fluid grain-boundary glass liquid flow, cavitation and wedge opening.     

Oxidation kinetics of hot-pressed silicon carbide

The oxidation of silicon carbide, hot-pressed with 4 wt % Al₂O₃, in 1 atm dry oxygen follows classical parabolic behaviour with an activation energy of 481 kJ mol^–1 in the temperature range 1200 to 1400° C. The oxide film consists predominantly of cristobalite and a glassy phase in which additive (Al) and various impurity elements (Fe, Na, K, etc) concentrate. The desorption of CO(g) from the SiC/SiO₂ interface appears to be oxidation rate controlling.     

Origin of strength anisotropy in hot-pressed silicon nitride

Three- and four-point room-temperature bend tests were carried out on specimens of a hot-pressed silicon nitride oriented parallel and perpendicular to the hot-pressing direction. Scanning electron microscopy was used to identify and measure the fracture-initating flaws, thus enabling the fracture toughness to be calculated. The strength anisotropy exhibited by this material was attributed to variation in the severity of the inherent flaws in the material with orientation, with the fracture toughness showing no significant anisotropy in this case.     

Determination of v-K-curves of hot-pressed silicon nitride at elevated temperatures

The problem of the peeling of an elastic layer from the rigid wall of a cylindrical cavity is considered. A closed form analytical solution is obtained and numerical results are presented revealing characteristic features of the phenomenon.

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Résumé On considère le problème du pelage d'une couche élastique à partir de la paroi réputée d'une cavité cylindrique. On obtient une solution analytique de forme fermée; les résultats numériques qui sont présentés révèlent les composantes caractéristiques du phénomène.

     

Crystallization of grain boundary phases in hot-pressed silicon nitride materials

Multicomponent oxide additions were used to replace magnesia as the densification aid for hot-pressing silicon nitride and silicon nitride-alumina materials. Crystallization of the silicate glass phases present after hot-pressing was encouraged by the use of an in situ heat treatment process. The phase constitutions of the materials produced by this method were determined using X-ray powder analysis. The kinetics of the densification of the silicon nitride materials were investigated, and these materials were examined using an X-ray diffractometer to study the textures produced by hot-pressing.     

High temperature oxidation of hot-pressed aluminium nitride by water vapour

Hot pressed AIN without additives was oxidized et 1100 to 1400°C in dry air, wet air and wet nitrogen gas atmospheres with 1.5 to 20 kPa of water vapour pressure. AIN was oxidized by both air and water vapour, and formed -Al₂O₃ film on the surface above 1150°C. The oxidation kinetics in air were parabolic end were promoted by water vapour. On the other hand, the oxidation kinetics in wet nitrogen were linear below 1250°C and parabolic above 1350°C. The oxidation rate in wet nitrogen was much greater than that in wet air. The rate of oxidation increased with increasing temperature until 1350°C, and then decreased. The parabolic rate constant decreased with increasing temperature and increased linearly with increasing water vapour pressure. The linear rate constant at 1150 to 1250° C increased with increasing the temperature with the apparent activation energy of 250 kJ mol^–1. The relation between the linear rate constant and water vapour pressure was of the Langmuir type.     

Modeling the strain-rate dependence of fatigue life of hot-pressed silicon nitride

This paper presents the results of a micromechanical model used to explain the strain-rate dependence of the compression fatigue lives of amorphous and crystalline grain boundary phase; denoted by ABP and CBP silicon nitrides, respectively. When the strain-rate is changed from 400 to 0.01/s, the fatigue lives of both materials, evaluated at a peak stress of 3.2 GPa, increased by more than two orders of magnitude (Sharma et al. (1996a,b)). The model is based on the dynamic and quasi-static microstructural damage mechanisms observed in both materials. The microstructure of ABP and CBP silicon nitrides is modeled as a simple composite in which silicon nitride grains are embedded in a continuous network of the grain boundary phase. Since the subsurface fatigue cracks in both materials nucleate mainly from the contact region between silicon nitride grains, contact stresses between adjacent silicon nitride grains are obtained, and the frequency dependence of the fatigue lives of ABP and CBP silicon nitrides is explained on the basis of the strain-rate sensitivity of the grain boundary phase.     

Effect of composition on the fracture strength of hot-pressed silicon nitride

The hot-pressing behaviour and fracture of silicon nitride as a function of composition was investigated. Two compositions were studied: 95 wt% Si₃ N4 + 5 wt% MgO, Material A, and 88 wt% Si₃N₄ + 2wt% MgO + 10wt% Y₂O₃, Material B. The effect of composition, hot-pressing time and temperature on the properties of the product were examined. The room-temperature fracture strengths of the two materials were evaluated using the diametral-compression disc test.     

Measurement of viscosity of the grain-boundary phase in hot-pressed silicon nitride

An internal friction technique has been used to measure the viscosity of the grain-boundary amorphous phase in commercial hot-pressed silicon nitride. The viscosity in the region of the glass transition (850 to 900° C) was approximately 5×10¹⁵ P per unit thickness (cm) of the grain boundary, with an apparant activation energy of 163 kcal mol⁻¹.     

Sodium-assisted oxidation of reaction-bonded silicon nitride

The oxidation of reaction-bonded silicon nitride in air, and with small amounts of sodium carbonate applied to the sample surface, has been studied. The action of the alkali is to cause short-term enhanced oxidation,which is terminated when specific compositions of the product sodium silicate glass are attained. These correspond closely to liquidus compositions in the Na₂O-SiO₂ system, and it is postulated that the retardation in the oxidation rate at this stage is due to the formation of a stable tridymite film at the silicon nitride-glass interface. The implications for the high temperature stability of reactionbonded silicon nitride components in alkali contaminated atmospheres are discussed.     

Dependence of high-temperature tensile strength on displacement rate for hot-pressed silicon nitride

Tensile tests of hot-pressed silicon nitride were conducted in a wide range of displacement rates from 0.0005 to 50.0 mm min^–1 at 1260 and 1380 ° C, and the dependence of strength on displacement rate was discussed in conjunction with the change of fracture surface features. As displacement rate was lowered, strength was degraded and stress-displacement diagrams altered from linearity to non-linearity, accompanying appearances of yielding phenomena. Strength was largely dependent on displacement rate in the high-rate region and rather independent in the low-rate region. The fracture surface features were drastically changed from brittle fracture to subcritical crack-growth fracture with decreasing displacement rate. A further increase in stress following the lower yielding point in the low-rate region was attributed to increments of fracture resistance due to the pull-out contribution of elongated grains.