In this paper, a summary of the development of high-temperature silicon nitride (T>1200°C) is provided. The high-temperature capacity of various advanced commercial silicon nitrides and materials under development was analyzed in comparison with a silicon nitride without sintering additives produced by hot isostatic pressing. Based on this model Si 3 N 4 composed of only crystalline Si 3 N 4 grains and amorphous silica in the grain boundaries the influence of various sintering additive systems will be evaluated with focus on the high-temperature potential of the resulting materials. The specific design of the amorphous grain-boundary films is the key factor determining the properties at elevated temperatures. Advanced Si 3 N 4 with Lu 2 O 3 or Sc 2 O 3 as sintering additive are characterized by a superior elevated temperature resistance caused by effective crystallization of the grain-boundary phase. Nearly clean amorphous films between the Si 3 N 4 grains comparable to that of Si 3 N 4 without sintering additives were found to be the reason of this behavior. Benefit in the long-term stability of Si 3 N 4 at elevated temperatures was observed in composites with SiC and M o Si 2 caused by a modified oxidation mechanism. The insufficient corrosion stability in hot gas environments at elevated temperatures was found to be the main problem of Si 3 N 4 for application in advanced gas turbines. Progress has been achieved in the development of potential material systems for environmental barrier coatings (EBC) for Si 3 N 4 ; however, the long-term stability of the whole system EBC-base Si 3 N 4 has to be subject of comprehensive future studies. Besides the superior high-temperature properties, the whole application process from cost-effective industrial production, reliability and failure probability, industrial handling up to specific conditions during the application have to be focused in order to bring advanced Si 3 N 4 currently available to industrial application.

Silicon Nitride for High‐Temperature Applications

As compared to oxides, nitride-type compounds can have specific properties and applications. The outline of this review focuses on the crystal chemistry of nitrides and oxynitrides. Given the nitride-oxynitride association, this article mainly deals with compounds which are predominantly covalent in character and does not include compounds which are metallic in character and in which nitrogen can be considered as an interstitial element. The classification which is used thus highlights the two main coordination polyhedra: nitrided or oxynitrided tetrahedra or octahedra.

Nitrides and oxynitrides: Preparation, crystal chemistry and properties

The fabrication and intergranular-phase devitrification of silicon nitride densified with rare-earth (RE) oxide additives has been investigated. The additions of the oxides of Sm, Gd, Dy, Er, and Yb, having high melting points and behaving similarly to Y2O3, were compositionally controlled to tailor a microstructure with a crystalline secondary phase of RE2Si2O7. The lanthanide oxides were found to be as effective as Y2O3 in densifying Si3N4, resulting in identical microstructures and densities of 98–99% of theoretical density. The crystallization behavior of all six disilicates was similar, characterized by a limited nucleation and rapid growth mechanism resulting in large single crystals. Complete crystallization of the intergranular phase was obtained with the exception of a thin residual amorphous film which was observed at interfaces and believed to be rich in impurities, the cause of incomplete devitrification.

Fabrication and Secondary‐Phase Crystallization of Rare‐Earth Disilicate–Silicon Nitride Ceramics

Detailed microstructural analysis of a 10 mol% Y/sub 2/O/sub 3/ fluxed hot-pressed silicon nitride reveals that, in addition to the yttrium-silicon oxynitride phase located at the multiple Si/sub 3/N/sub 4/ grain junctions, there is a thin boundary phase 10 to 80 A wide separating the silicon nitride and the oxynitride grains. Also, x-ray microanalysis from regions as small as 200 A across demonstrates that the yttrium-silicon oxynitride, Y/sub 2/Si(Si/sub 2/O/sub 3/N/sub 4/), phase can accommodate appreciable quantities of Ti, W, Fe, Ni, Co, Ca, Mg, Al, and Zn in solid solution. This finding, together with observations of highly prismatic Si/sub 3/N/sub 4/ grains enveloped by Y/sub 2/Si(Si/sub 2/O/sub 3/N/sub 4/), suggests that densification occurred by a liquid-phase ''solution-reprecipitation'' process.

/pdf/microstructure-of-y2o3-fluxed-hot-pressed-silicon-nitride-28cm0ltwzq.pdf

Microstructure of Y2O3 Fluxed Hot‐Pressed Silicon Nitride

Impurity phases in commercial hot-pressed Si3N4 were investigated using transmission electron microscopy. In addition to the dominant, β-Si3N4 phase, small amounts of Si2N2O, SiC, and WC were found. Significantly, a continuous grain-boundary phase was observed in the ∼ 25 high-angle boundaries examined. This film is ∼ 10 A thick between, β-Si3N4 grains and ∼ 30 A thick between Si2N2O and β-Si3N4 grains.

Impurity Phases in Hot-Pressed Si3N4

Examination of compositions in the system Si3N4-Y2O3-SiO2 using sintered samples revealed the existence of two regions of melting and three silicon yttrium oxynitride phases. The regions of melting occur at ∼1600° C at high SiO2 concentrations (∼13 mol% Si3N4 + 19 mol% Y2O3 + 68 mol% SiO2) and at 1650° C at high Y2O3 concentrations (25 mol % Si3N4 + 75 mol % Y2O3). Two ternary phases 4Y2O3 ·SiO2 ·Si3N4 and 10Y2O3 ·9SiO2 ·Si3N4 and one binary phase Si3N4 ·Y2O3 were observed. The 4Y2O3 ·SiO2 ·Si3N4 phase has a monoclinic structure (a= 11.038 A, b=10.076 A, c=7.552 A, β=108° 40′) and appears to be isostructural with silicates of the wohlerite cuspidine series. The 10Y2O3 ·9SiO2 ·Si3N4 phase has a hexagonal unit cell (a=7.598 A c=4.908 A). Features of the Si3N4-Y2O3-SiO2 systems are discussed in terms of the role of Y2O3 in the hot-pressing of Si3N4, and it is suggested that Y2O3 promotes a liquid-phase sintering process which incorporates dissolution and precipitation of Si3N4 at the solid-liquid interface.

Phase relationships in the system Si3N4-Y2O3-SiO2

A new group of materials called the silicon lanthanide oxynitrides has been prepared by the reaction between Si3N4 and several oxides of the lanthanide series (La2O3, Sm2O3, Dy2O3, Er2O3, and Yb2O3). These oxides formed compounds of the type Si3N4 · R2O3 and R4Si2O7N2 (R being lanthanide). In addition La2O3 and Yb2O3 formed the compounds 2Si3N4 · La2O3 and Yb2Si3O5N2, respectively. Certain similarities in the unit cells of these compounds have been noted, and their structures are discussed in terms of similarity to known minerals. It is suggested that this group of materials contains a large number of compounds.

The silicon lanthanide oxynitrides

Silicon cerium oxynitride

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Phase relationships in the system Si3N4-Y2O3-SiO2

The silicon lanthanide oxynitrides

Silicon cerium oxynitride

Comments on “silicon cerium oxynitride”