High‐temperature elastic moduli of polycrystalline silicon nitride

Oxynitirde glasses are found at triple point junctions and as intergranular films in silicon nitride based ceramics. The glass chemistry, particularly the content of modifyer,usually Y or a rare earth (RE) ion, and the volume fractions of these oxynitride glass phases within the ceramic control the properties of silicon nitride, in particular, creep at high temperature. It is known that, as nitrogen substitutes for oxygen in silicate and aluminosilicate glass networks, increases are observed in glass transition and softening temperatures, viscosities (by two to three orders of magnitude), elastic moduli and microhardness. If changes are made to the RE:Si:Al ratios or different rare earth cation are substituted, properties such as viscosity can be increased by a further two to three orders of magnitude. These effects have implications for the high temperature properties of silicon nitride based ceramics, especially creep resistance. This paper provides an overview of oxynitride glasses and outlines the effect of composition on properties such as glass transition temperature and viscosity and discusses the effects on high temperature behaviour of silicon nitride ceramics.

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High Temperature Deformation and Fracture of Polycrystalline Silicon Nitride

Key Engineering Materials ◽

10.4028/www.scientific.net/kem.89-91.667 ◽

1993 ◽

Vol 89-91 ◽

pp. 667-672

Author(s):

I. Merkel ◽

U. Messerschmidt

Keyword(s):

High Temperature ◽

Silicon Nitride ◽

Polycrystalline Silicon ◽

High Temperature Deformation ◽

Temperature Deformation ◽

Deformation And Fracture

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Observations of inelastic deformation during high-temperature fatigue and failure of a polycrystalline silicon nitride

Journal of Non-Crystalline Solids ◽

10.1016/0022-3093(94)90514-2 ◽

1994 ◽

Vol 177 ◽

pp. 54-66 ◽

Cited By ~ 1

Author(s):

Michael G. Jenkins ◽

Edgar Lara-Curzio ◽

Mattison K. Ferber ◽

Jonathon A. Salem

Keyword(s):

High Temperature ◽

Silicon Nitride ◽

Inelastic Deformation ◽

Polycrystalline Silicon ◽

High Temperature Fatigue

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Lattice Imaging of Grain Boundaries in Ceramics

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100078171 ◽

1977 ◽

Vol 35 ◽

pp. 114-115

Author(s):

D. R. Clarke ◽

G. Thomas

Keyword(s):

Mechanical Properties ◽

High Temperature ◽

Silicon Nitride ◽

Grain Boundaries ◽

High Temperature Strength ◽

Current Interest ◽

Lattice Imaging ◽

Special Significance ◽

Structural Applications ◽

Refractory Ceramics

Grain boundaries have long held a special significance to ceramicists. In part, this has been because it has been impossible until now to actually observe the boundaries themselves. Just as important, however, is the fact that the grain boundaries and their environs have a determing influence on both the mechanisms by which powder compaction occurs during fabrication, and on the overall mechanical properties of the material. One area where the grain boundary plays a particularly important role is in the high temperature strength of hot-pressed ceramics. This is a subject of current interest as extensive efforts are being made to develop ceramics, such as silicon nitride alloys, for high temperature structural applications. In this presentation we describe how the techniques of lattice fringe imaging have made it possible to study the grain boundaries in a number of refractory ceramics, and illustrate some of the findings.

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TEM Studies of Grain Boundary Films in Si3N4 Ceramics

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100088968 ◽

1991 ◽

Vol 49 ◽

pp. 930-931

Author(s):

H.-J. Kleebe ◽

J.S. Vetrano ◽

J. Bruley ◽

M. Rühle

Keyword(s):

Mechanical Properties ◽

High Temperature ◽

Silicon Nitride ◽

Hot Isostatic Pressing ◽

Amorphous Film ◽

Liquid Phase Sintering ◽

Second Phase ◽

High Temperature Mechanical Properties ◽

Triple Points ◽

Boundary Films

It is expected that silicon nitride based ceramics will be used as high-temperature structural components. Though much progress has been made in both processing techniques and microstructural control, the mechanical properties required have not yet been achieved. It is thought that the high-temperature mechanical properties of Si3N4 are limited largely by the secondary glassy phases present at triple points. These are due to various oxide additives used to promote liquid-phase sintering. Therefore, many attempts have been performed to crystallize these second phase glassy pockets in order to improve high temperature properties. In addition to the glassy or crystallized second phases at triple points a thin amorphous film exists at two-grain junctions. This thin film is found even in silicon nitride formed by hot isostatic pressing (HIPing) without additives. It has been proposed by Clarke that an amorphous film can exist at two-grain junctions with an equilibrium thickness.

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Electron Microscopy of Silicon Nitride-Based Ceramics

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100088944 ◽

1991 ◽

Vol 49 ◽

pp. 926-927

Author(s):

Gareth Thomas

Keyword(s):

Electron Microscopy ◽

High Temperature ◽

Silicon Nitride ◽

World Wide ◽

Sintering Temperature ◽

Liquid Phase Sintering ◽

High Temperature Strength ◽

Sintering Additives ◽

Glass Forming ◽

Dense Product

Silicon nitride and silicon nitride based-ceramics are now well known for their potential as hightemperature structural materials, e.g. in engines. However, as is the case for many ceramics, in order to produce a dense product, sintering additives are utilized which allow liquid-phase sintering to occur; but upon cooling from the sintering temperature residual intergranular phases are formed which can be deleterious to high-temperature strength and oxidation resistance, especially if these phases are nonviscous glasses. Many oxide sintering additives have been utilized in processing attempts world-wide to produce dense creep resistant components using Si3N4 but the problem of controlling intergranular phases requires an understanding of the glass forming and subsequent glass-crystalline transformations that can occur at the grain boundaries.

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