Microstructure and microanalysis of silicon nitride ceramics in the Y-Si-Al-O-N and Y-Si-O-N systems

1990 ◽  
Vol 48 (4) ◽  
pp. 1072-1073
Author(s):  
Michael K. Cinibulk
Keyword(s):  
Silicon Nitride ◽  
Grain Boundary ◽  
Grain Boundary Sliding ◽  
Liquid Phase Sintering ◽  
Boundary Phase ◽  
Full Density ◽  
High Temperature Strength ◽  
Transmission Electron ◽  
Structural Applications ◽  
Nitride Ceramics

Silicon nitride ceramics are among the leading candidate materials for use in structural applications at high temperatures. Due to the highly covalent nature of the Si-N bond and therefore low self-diffusivity, processing Si3N4 to full density requires the use of additives to provide a medium for liquid-phase sintering. When exposed to temperatures above ∼1000°C the resulting amorphous grain-boundary phases soften, leading to grain-boundary sliding and the eventual failure of the ceramic. The objectives of this work were to modify the grain-boundary phase composition and then attempt to devitrify the resulting intergranular phase to a refractory crystalline phase, producing a sintered Si3N4 with improved high-temperature strength and oxidation resistance. Transmission electron microscopy (TEM) and energy-dispersive x-ray spectroscopy (EDS) were used to characterize these materials. This paper describes these results.

Silicon Nitride Grain Boundary Glasses: Chemistry, Structure and Properties

2011 ◽  
Vol 484 ◽  
pp. 46-51 ◽  
Author(s):  
Stuart Hampshire ◽  
Michael J. Pomeroy
Keyword(s):  
Silicon Nitride ◽  
Liquid Phase ◽  
Grain Boundary ◽  
High Performance ◽  
Elastic Moduli ◽  
Liquid Phase Sintering ◽  
Structural Applications ◽  
Nitride Ceramics ◽  
Property Changes ◽  
Surface Silica

Silicon nitride is recognised as a high performance material for both wear resistant and high temperature structural applications. Oxide sintering additives such as yttrium oxide and alumina are used to provide conditions for liquid phase sintering, during which the additives react with surface silica present on the Si3N4 particles and some of the nitride to form an oxynitride liquid which allows densification and transformation of - to -Si3N4 and on cooling remains as an intergranular oxynitride glass. This paper provides an overview of liquid phase sintering of silicon nitride ceramics, grain boundary oxynitride glasses and the effects of chemistry and structure on properties. As nitrogen substitutes for oxygen in oxynitride glasses, increases are observed in glass transition and softening temperatures, viscosities, elastic moduli and microhardness. These property changes are compared with known effects of grain boundary glass chemistry in silicon nitride ceramics.

Examination of Fracture Interfaces in Silicon Nitride

1975 ◽  
Vol 33 ◽  
pp. 60-61
Author(s):  
Nancy J. Tighe
Keyword(s):  
Silicon Nitride ◽  
Grain Boundary ◽  
Crack Growth ◽  
Subcritical Crack Growth ◽  
Slow Crack Growth ◽  
Ceramic Materials ◽  
Grain Boundary Sliding ◽  
Boundary Phase ◽  
Turbine Engines ◽  
Boundary Sliding

Silicon nitride is one of the ceramic materials being considered for the components in gas turbine engines which will be exposed to temperatures of 1000 to 1400°C. Test specimens from hot-pressed billets exhibit flexural strengths of approximately 50 MN/m2 at 1000°C. However, the strength degrades rapidly to less than 20 MN/m2 at 1400°C. The strength degradition is attributed to subcritical crack growth phenomena evidenced by a stress rate dependence of the flexural strength and the stress intensity factor. This phenomena is termed slow crack growth and is associated with the onset of plastic deformation at the crack tip. Lange attributed the subcritical crack growth tb a glassy silicate grain boundary phase which decreased in viscosity with increased temperature and permitted a form of grain boundary sliding to occur.

Silicon Nitride Ceramics with Sodium Ion Conductive Grain Boundary Phase

2003 ◽  
Vol 18 (12) ◽  
pp. 2752-2755 ◽  
Author(s):  
Hirokazu Kawaoka ◽  
Tohru Sekino ◽  
Takafumi Kusunose ◽  
Koichi Niihara
Keyword(s):  
Electrical Conductivity ◽  
Silicon Nitride ◽  
Ionic Conductivity ◽  
Grain Boundary ◽  
Sodium Ion ◽  
Boundary Phase ◽  
Structural Ceramics ◽  
Silicon Nitride Ceramic ◽  
Nitride Ceramics ◽  
Conductive Particles

Sodium ion-conductive silicon nitride ceramic with Na2O–Al2O3–SiO2 glass as the grain boundary phase was fabricated by adding Na2CO3, Al2O3, and SiO2 as sintering additives. The electrical conductivity was two and four orders of magnitude higher than that of Si3N4 ceramic with Y2O3 and Al2O3 additives at 100 and 1000°C, respectively. This result clearly indicates that ionic conductivity can be provided to insulating structural ceramics by modification of the grain boundary phase without dispersion of conductive particles.

High-Temperature Properties of Si3N4 Ceramics

MRS Bulletin ◽  
1995 ◽  
Vol 20 (2) ◽  
pp. 28-32 ◽  
Author(s):  
M.J. Hoffmann
Keyword(s):  
High Temperature ◽  
Silicon Nitride ◽  
Grain Boundary ◽  
Volume Fraction ◽  
Liquid Phase Sintering ◽  
Coarse Grained ◽  
High Fracture Toughness ◽  
Boundary Phase ◽  
Transgranular Fracture ◽  
Sintering Additives

Silicon nitride is a highly covalent bonded compound which decomposes at 1877°C. Therefore, it is impossible to densify Si3N4 without sintering additives. Densification is achieved by liquid-phase sintering usually using metal oxides such as MgO, Y2O3, A12O3, and most of the rare-earth oxides as sintering additives. The oxides react with SiO2—always present at the surface of Si3N4 particles—to form an oxide melt and, with increasing temperature, an oxynitride melt by dissolution of Si3N4. The resulting microstructure consists of elongated Si3N4 needles embedded in a matrix of smaller equiaxed Si3N4 grains and a grain boundary phase, as shown in Figure 1. The amount and chemistry of the sintering aids determine the volume fraction of the grain boundary phase. The content required for complete densification depends on the sintering techniques: 2–5 vol% additives are sufficient if densification is supported by a high external pressure (hot pressing [HP] or hot isostatic pressing [HIP]); pressureless-sintered and gas-pressure-sintered (10-MPa nitrogen pressure) materials have additive contents of up to 15 vol%. Today, silicon nitride ceramics are regarded as a class of material comparable to steel. Different qualities depend on the size and shape of the silicon nitride grains and the amount and chemistry of the grain boundary phase. Materials with a high room-temperature strength exhibit a finegrained, elongated microstructure, while materials with a high fracture toughness are more coarse-grained. In both cases, a weak interface is required to induce transgranular fracture. (See Becher et al. in this issue.) Since all Si3N4 grains are completely wetted by the grain boundary phase, the interface strength is determined by the additive composition. Nevertheless, a contradiction arises between the development of high-strength and high-toughness Si3N4 ceramics and high-temperature resistant materials because the grain boundary phase is responsible for the excellent properties at low temperatures, but limits the properties at temperatures above its softening point.

Effect of Grain Boundary Phase on Contact Damage Resistance of Silicon Nitride Ceramics

2005 ◽  
Vol 287 ◽  
pp. 421-426 ◽  
Author(s):  
Chul Seung Lee ◽  
Kee Sung Lee ◽  
Shi Woo Lee ◽  
Do Kyung Kim
Keyword(s):  
Silicon Nitride ◽  
Grain Boundary ◽  
Grain Morphology ◽  
Boundary Phase ◽  
Contact Damage ◽  
Damage Resistance ◽  
Intergranular Phase ◽  
Nitride Ceramics ◽  
Damage Response ◽  
Different Types

Contact damage resistances of silicon nitride ceramics with various grain boundary phases are investigated in this study. The grain boundary phases are controlled by the addition of different types of sintering additives, or the crystallization of intergranular phase in a silicon nitride. We control the microstructures of materials to have similar grain sizes and the same phases to each other. Contact testing with spherical indenters is used to characterize the damage response. The implication is that the grain boundary phase can be another controllable factor against contact damage and strength degradation even though it is not critical relative to the effect of grain morphology.

Effect of Oxynitride Grain Boundary Phase on Toughening of Silicon Nitride Ceramics

2006 ◽  
Vol 317-318 ◽  
pp. 649-652 ◽  
Author(s):  
Takafumi Kusunose ◽  
Tohru Sekino ◽  
P.E.D. Mogan ◽  
Koichi Niihara
Keyword(s):  
Fracture Toughness ◽  
Silicon Nitride ◽  
Grain Boundary ◽  
Crack Propagation ◽  
Hot Pressing ◽  
Boundary Phase ◽  
Toughening Mechanism ◽  
Nitride Ceramics

The Si3N4/YSiO2N composite in which crystalline YSiO2N was formed as grain boundary phase was fabricated by hot-pressing the mixture of SiO2, Si3N4 and Y2O3. The fracture toughness of this composite was significantly improved, compared to the Si3N4 composites containing Y5Si3O12N or Y2Si3O3N4 as a grain boundary phases. To clarify the toughening mechanism, the microstructure and the crack propagation profiles were observed.

Lattice Imaging of Grain Boundaries in Ceramics

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.

Electron Microscopy of Silicon Nitride-Based Ceramics

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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