Spherical-Impact Damage and Strength Degradation in Silicon Carbide Whisker/Silicon Nitride Composites

The silicon carbide (SiC) whisker reinforcement of silicon nitride (Si₃N₄) improves fracture strength and toughness, hardness, and Young's modulus, resulting in higher resistance of the composites to sphere penetration and crack initiation at spherical impact. Sintered Si₃N₄ shows an elastic/plastic response and initiates median/radial cracks at 100 m/s impact velocity. SiC-whisker/Si₃N₄ composites, on the other hand, demonstrate an elastic response, with Hertzian cone crack initiation, only when impact velocity exceeds 280 m/s. The SiC-whisker/Si₃N₄ composites thus exhibit improved strength degradation versus critical impact velocity characteristics because of improved mechanical properties provided by the SiC whiskers.     

High-Temperature Strength of Sinter-Forged Silicon Nitride with Lutetia Additive

A sinter-forging technique was successfully applied to fabricate a silicon nitride with a lutetia (Lu₂O₃) additive. The sinter-forged specimen had a strongly anisotropic microstructure where rodlike silicon nitride grains preferentially aligned perpendicular to the forging direction. The specimen exhibited superior strength of ∼700 MPa at 1500°C. This strength was highest when compared with previous silicon nitrides at temperatures >1400°C. Such superior high-temperature strength was attributed to grain alignment as well as to the refractory grain-boundary glassy phase and the existence of glass-free grain boundaries.     

Wetting and adhesion of Si on Si3N4 and BN substrates

Wetting, adhesion and reactivity are the principal factors determining the capability of a solid to be used as mould material. In this work wetting of silicon and boron nitrides by molten silicon is studied in neutral gas atmosphere by the sessile drop technique at temperatures close to the silicon melting point. Adhesion is qualified by the behaviour of solidified droplets under the effect of thermo-mechanical stresses generated during cooling at room temperature. The reactivity at silicon/nitride interfaces is studied by scanning electron microscopy and EDX-microanalysis.     

R-Curve Behavior and Strength for In-Situ Reinforced Silicon Nitrides with Different Microstructures

R -curves for two in-situ reinforced silicon nitrides A and B of nominally the same composition are characterized using the Griffith equation and indentation fracture mechanics. These R -curves are calibrated against fine-grained silicon nitrides which have a known chevron-notch (long-crack) toughness and with a nearly flat R -curve behavior. Silicon nitride A, with its coarser microstructure and higher chevron-notch toughness, shows lower resitance to crack growth than silicon nitride B if the crack size is less than ∼200 μm. These results are consistent with the indentation–Strength measurements which show a crossover of strength between the two materials at an indentation load between 49 and 98 N, and below the crossover A has a lower strength. The toughening behavior is explained using an elastic-bridging model for the short crack, and a pullout model for the long crack. The effects of R -curve properties on design are discussed.     

Formation and reactivity of nitrides. III. Boron,aluminium and silicon nitrides

The reactivities of boron, aluminium and silicon nitrides have been compared. Samples of these nitrides have been converted to oxides by being calcined in air. Changes in phase composition, surface area, crystallite and aggregate sizes have been correlated with oxidation time and temperature. Crystallites of alumina, α-Al₂O₃, split off from the remaining aluminium nitride before they sinter and inhibit further oxidation. The diboron trioxide, B₂O₃, and silica, SiO₂ (α-cristobalite), immediately act as mineralisers for the remaining boron and silicon nitrides, and progressively retard the oxidations.     

氮化硅薄膜制备方法现状综述

氮化硅薄膜是一种多功能材料，在许多领域有着广泛的应用。本文系统综述氮化硅薄膜的性质、结构、应用及各种制备方法，并对今后的研究作了展望。     

High-Temperature Chemical Phenomena Affecting Silicon Nitride Joints

Knudsen cell mass spectrometry was used to study the chemical processes responsible for joint degradation in joined silicon nitride ceramics. Vapor species present above two commercial hot-pressed silicon nitrides and above three joining glasses were identified, and partial pressures were estimated at 1480 K. Oxide vaporization products related to reducing conditions were observed. The implications of these results on proposed silicon nitride joining processes are discussed. It appears that oxygen potential gradients within both glazed and unglazed hot-pressed Si₃N₄ samples are responsible for the enhanced vaporization rates of the sample and the observed instability of glazed joints at high temperatures. Observed vaporization behavior of oxide additives correlates well with that predicted for the chemically reducing environment of Si₃N₄.     

Dynamic Fracture Toughnesses of Reaction-Bonded Silicon Nitride

Dynamic fracture toughness specimens consisting of 5.1-mm thick, modified wedge-loaded, tapered double-cantilever-beam (WL-MTDCB) specimens, which are side-grooved on one side, were used to establish the room-temperature dynamic fracture toughness, K _ID vs crack velocity, a , relations of two reaction-bonded silicon nitrides. The measured dynamic crack extension histories were then used to drive a dynamic finite-element code in its generation mode which computes the dynamic stress intensity factors for a given crack extension. Results indicate that the K _ID vs a relations of reaction-bonded silicon nitrides do not follow the general trend in those relations of brittle polymer and steel. The slow initial crack velocity which was reported for glass was observed again in silicon nitride and resulted in a nonunique K _ID vs a relation, in contrast to the unique K _ID vs a material properties reported for brittle polymers and metals.     

Desintering Process in the Gas-Pressure Sintering of Silicon Nitride

A unique desintering phenomenon has been observed in gas-pressure sintering of silicon nitrides with additives of yttria and alumina. The desintering phenomenon occurred, simultaneous with weight increase, only when a boron nitride crucible was used in combination with the application of high nitrogen pressure (5 MPa). When the nitrogen pressure was low (0.5 MPa), or when the boron nitride crucible was replaced by a graphite crucible, this desintering phenomenon was not observed. These results could be rationalized by the chemical dissolution of nitrogen into the oxynitride melts and the resultant evolution of carbon monoxide. This indicates that the high nitrogen overpressure employed in gas-pressure sintering of silicon nitride ceramics is not always beneficial.     

Strain Tolerant Porous Silicon Nitride

An approach to material strain tolerance, which basically makes it possible to lower the elastic modulus while retaining strength, was experimentally confirmed using as an example a porous silicon nitride composed of oriented anisotropic grains and pores. The porous structure consisting of tightly tangled rodlike grains and anisotropic pores was obtained by using β-Si₃N₄ whiskers. This material exhibited a low Youngs modulus while retaining a relatively high fracture stress, even though it contained 14.4% porosity. Consequently, the strain to failure of silicon nitride was appreciably increased.     

Solution Phase Preparative Routes to Nitride Morphologies of Interest in Catalysis

This review assesses the track record and prospects of non-oxide sol–gel and solvothermal routes to nitride materials for use in catalysis. There is a strongly developing body of synthesis methods that yield highly porous materials, some with engineered pore structures, nanocrystalline materials with high surface areas and anisotropic nanocrystals that have their surface area tuned to a particular crystal face. Most of the existing catalytic work on such solution-derived nitrides has focussed on utilising base properties due to surface bound amide and imide groups in silicon (imido)nitride compositions, but there are opportunities to extend these methods to other interesting nitride compositions.     

Grain boundary glasses in silicon nitride: A review of chemistry,properties and crystallisation

Silicon nitride for engineering applications is densified by liquid phase sintering using oxide additives such as yttria and alumina. The oxynitride liquid remains as an intergranular glass. This paper provides a review of microstructural development in silicon nitride, grain boundary oxynitride glasses and effects of chemistry on properties. Nitrogen increases T_g, viscosities, elastic moduli and microhardness. These property changes are compared with known effects of grain boundary glass chemistry in silicon nitride ceramics where significant improvements in fracture resistance of silicon nitride can be achieved by tailoring the intergranular glass chemistry.Crystallisation of the grain boundary Y–Si–Al–O–N glass phase can improve properties. Nucleation and crystallisation of a Y–Si–Al–O–N glass, similar to that found in grain boundaries of silicon nitride densified with yttria and alumina, can be optimised to form different Y-disilicate polymorphs at different temperatures. One solution to provide a single disilicate phase over a range of temperatures is discussed.     

Interfacial Characterization of Silicon Nitride Powders

The composition of the surface and the behavior in aqueous suspensions of three silicon nitride powders were investigated using electron spectroscopy for chemical analysis (ESCA), potentiometric titrations, leaching experiments, and electrophoretic mobility. ESCA shows that the as-received powders have a surface-layer composition similar to that identified as an intermediate state between silica and silicon oxynitride. The original differences in pH_iep between the three powders disappears by aging the powders. The common pH_iep of 6.8 ± 0.3 for the three powders is interpreted as the equilibrium pH_iep for silicon nitride in aqueous suspensions.     

Grinding performance improvement of laser micro-structured silicon nitride ceramics by laser macro-structured diamond wheels

Silicon nitride ceramics are widely used in various industrial fields because of their excellent characteristics: high hardness, high elastic modulus, abrasion resistance, and high heat resistance. Diamond wheel grinding is the predominant and most productive method to machine silicon nitride ceramics. However, a lot of heat is generated due to high friction between a diamond grinding wheel and extremely rigid silicon nitride during grinding. This causes surface/subsurface damage, wheel wear, etc., which impairs the surface quality of silicon nitride. This impairment can restrict the use of silicon nitride ceramic components. To improve the surface quality and service life of grinding wheels, a laser macro-micro combination structured grinding (LMMCSG) method was presented. The results indicated that the grinding force ratio and surface roughness when using LMMCSG were respectively 31% and 40% lower than the grinding force ratio and surface roughness when using conventional grinding. Moreover, the LMMCSG method effectively reduced the wheel wear and workpiece subsurface damage.     

Tensile Creep in an in Situ Reinforced Silicon Nitride

The tensile creep of an in situ reinforced silicon nitride is described in terms of the rheological behavior of the thin intergranular film present in this liquid-phase sintered silicon nitride. The high stress exponents and apparent activation energies (at constant stress) can be explained assuming non-Newtonian flow behavior of the film during grain boundary sliding. Time-to-failure is related to the minimum creep rate, even for samples which fail by slow crack growth. In addition, the primary creep region and the relaxation effects observed on unloading are described in terms of grain boundary sliding modified by the presence of a grain boundary phase with a lower elastic modulus than silicon nitride.     

Development of Short-Range Repulsive Potentials in Aqueous, Silicon Nitride Slurries

The properties of aqueous, dispersed, silicon nitride slurries, with an isoelectric point of pH 5.5, can be changed with additions of NH₄CI. At pH 10 the effect of adding NH₄Cl is similar to that suggested by DLVO theory; namely, for concentrations .0.5 M , the viscosity vs shear rate behavior, the elastic modulus, and the relative packing density are identical to those for slurries prepared at the isoelectric point. On the other hand, the effect of salt on dispersed slurries prepared at pH 2 differs from the behavior implied by classic DLVO theory; i.e., measurement of the same properties showed that the attractive interparticle potential was much weaker relative to that produced at the isoelectric point. As previously reported for alumina slurries, the results suggest that a short-range, repulsive interparticle potential is developed in salt-added slurries prepared at pH 2 which prevents attractive particles in the slurry from touching and aids particle packing. The same short-range potential apparently is not developed with salt additions at pH 10. The difference between silicon nitride and alumina slurries is apparent when the slurries are consolidated. Bodies consolidated from any silicon nitride slurry are elastic (i.e., they fracture before they flow) unlike salt-added alumina slur-ries, which are plastic.     

Development of a Self-Forming Ytterbium Silicate Skin on Silicon Nitride by Controlled Oxidation

A dense and uniform polycrystalline ytterbium silicate skin on silicon nitride ceramics was developed by a controlled oxidation process to improve the hot corrosion resistance of silicon nitride. The process consists of purposely oxidizing the silicon nitride by heating it at high temperatures. It was found that the ytterbium silicate phase was formed as an oxidation product on the surface of the silicon nitride when it was exposed to air at temperatures above 1250°C. The volume fraction of ytterbium silicate compared with that of SiO₂ on the silicon nitride surface increased with increasing oxidation time and temperature. The formation and growth of ytterbium silicate on the surface of silicon nitride is attributed to a nucleation and growth mechanism. Ultimately, a dense and uniform ytterbium silicate skin with 3–4 μm of skin thickness was obtained by oxidation at 1450°C for 24 h. The ytterbium silicate layer, formed by oxidation of the silicon nitride, is associated with the reaction of SiO₂ on the surface of silicon nitride with Yb₂O₃ introduced in the silicon nitride as a sintering additive. Preliminary tests showed that the ytterbium silicate skin appears to protect silicon nitride from hot corrosion. No observable evidence of a reaction between the skin and molten Na₂SO₄ was found when it was exposed to molten Na₂SO₄ at 1000°C for 30 min.     

Fabrication and properties of in situ silicon nitride nanowires reinforced porous silicon nitride (SNNWs/SN) composites

The in situ silicon nitride nanowires reinforced porous silicon nitride (SNNWs/SN) composites were fabricated via gelcasting followed by pressureless sintering. SNNWs were well distributed in the porous silicon nitride matrix. The tip-body appearance suggested a VLS growth mechanism. The flexural strength and elastic modulus of the prepared composites can achieve 84.3?±?3.9?MPa and 23.3?±?2.0?GPa respectively (25?°C), while the corresponding porosity was 40.7?vol.%. Remarkably, the strength retention rate of the composites at 1400?°C was up to 66.1%. This is due to the excellent thermal stability of SNNWs and silicon nitride matrix. Also, the fracture toughness of the composites was improved to ～42% larger than pure porous silicon nitride ceramics because of the bridging effect of the NWs and the interlocking effect of β-Si₃N₄ crystals. In addition, a good thermal shock resistance and dielectric properties were indicated. The good overall performance made SNNWs/SN composites promising candidate for advanced high-temperature applications.     

Mechanical reliability evaluation of silicon nitride ceramic components after exposure in industrial gas turbines

Several studies have recently been undertaken to examine the mechanical reliability and thermal stability of silicon nitride ceramic components that are currently being considered for structural application in industrial gas turbines. Specifically, ceramic components evaluated included a bow-shaped silicon nitride nozzle evaluated in an engine test rig, silicon nitride vanes exposed in an engine field test, and an air-cooled silicon nitride vane that is currently under development. Despite the differences in field test conditions all of the exposed silicon nitride ceramic components exhibited a significant material recession arising from the oxidation of silicon nitride and subsequent volatilization of the oxide (i.e., silica). The fracture strength of exposed airfoils was also decreased due to the formation of a subsurface damage zone induced by the turbine environments. In addition, studies indicated that the properties of as-processed ceramic components, especially in airfoil regions, were not always comparable to those generated from the standard specimens with machined surface extracted from production billets. The component characterization efforts provided an important insight into the effect of gas turbine environments on the material recession and mechanical reliability of materials as functions of exposure time and conditions, which were very difficult to obtain from a laboratory scale test.     

Modeling for particle size prediction and mechanism of silicon nitride nanoparticle synthesis by chemical vapor deposition

Particle size is a vital characterization for silicon nitride nanoparticle as its scale determines its application area. Particle size prediction for synthesis of silicon nitride nanoparticle by chemical vapor deposition (CVD) is much needed. In this study, a model is proposed for particle growth during silicon nitride nanoparticle synthesis by CVD in order to predict particle size. Comparison between modeling and experimental results validated the model. The modeling results showed that lower pressure in the condensation room would be an effective way of obtaining silicon nitride nanoparticles with smaller particle size. An expression is established to reveal the relation between the mean particle diameter of silicon nitride nanoparticle and pressure in the condensation room based on the modeling. The modeling method is capable of predicting the mean particle size of ultrafine silicon nitride powder to within 3.6% accuracy. Corresponding manufacturing thermal parameters are recommended for silicon nitride nanoparticle production with different mean particle sizes. Modeling and analysis in this article may provide theoretical guidance for production of silicon nitride nanoparticle by CVD.

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