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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Si3N4-Based Ceramic With Greater Hot Strength

Zyttrite-doped material outperforms MgO-doped material above 1,200 degrees C. New ceramic material produced by addition of 10 weight percent zyttrite (yttria-stabilized zirconia) to (silicon nitride) offers significantly-improved high-temperature properties (those of MgO-doped Si3N4 ceramic). Work also showed that controlled Si3N4 powder with 10 weight percent zyttrite, significant improvement in room-temperature strength achieved. Variety of high-temperature structural applications are silicon nitride and silicon carbide. Potential for use in aircraft and automobile engines and in electric-power generating systems. Improved properties strongly suggest that the 10-weight percent zyttrite/Si3N4 material has strong potential for high-temperature applications.

Dutta, S.↗
NASA NTRS1985

Factors influencing the ball milling of Si3N4 in water

A statistical study of the ball milling of Si3N4 powder in Si3N4 hardware was undertaken to understand how the resulting increase in specific surface area is related to solids loading and mill speed. An attempt was made to optimize milling conditions. The degree of communication was more dependent upon solids loading than mill speed. A practical grinding limit between 0.5 and 0.75 microns was achieved in 144 hr independent of solids loading. Ball mill wear and media wear were independent of both solids loading and mill speed.

Freedman, M. R.↗
NASA NTRS1986

Improved processing of Si3N4

A sintered Si3N4-SiO2-Y2O3 composition, NASA 6Y, was developed that reached four-point flexural average strength/standard deviation values of 857/36, 544/33, and 462/59 MPa at room temperature, 1200, and 1370 C, respectively. These strengths represented improvements of 56, 38, and 21 percent over baseline properties at the three test temperatures. At room temperature the standard deviation was reduced by more than a factor of three. These accomplishments were realized by the iterative utilization of conventional X-radiography to characterize structural (density) uniformity as affected by systematic changes in powder processing and sintering parameters. Accompanying the improvement in mechanical properties was a change in the type of flaw causing failure from a pore to a large columnar beta-Si3N4 grain typically 40 to 80 microns, 10 to 30 microns wide, and with an aspect ratio of 5:1.

Sanders, William A.↗
NASA NTRS1986

SiC fiber reinforced reaction-bonded Si3N4 composites

A technique for fabricating strong and tough SiC fiber reinforced reaction bonded Si3N4 matrix composites (SiC/RBSN) was developed. Using this technique, composites containing approximately 23, 30, and 40 volume fractions of aligned 140 micron diameter, chemically vapor deposited SiC fibers were fabricated. The room temperature physical and mechanical properties were evaluated. The results for composite tensile strength, bend strength, and fracture strain indicate that the composite displays excellent properties when compared with the unreinforced matrix of comparable porosity. The composite stress at which the matrix first cracks and the ultimate composite fracture strength increase with increasing volume fraction of fibers, and the composite fails gracefully. The mechanical property data of this ceramic composite are compared with similar data for unreinforced commercially available Si3N4 materials and for SEP SiC/SiC composites.

Bhatt, Ramakrishna T.↗
NASA NTRS1988

Microstructural evolution on crystallizing the glassy phase in a 6 weight percent Y2O3-Si3N4 ceramic

X-ray diffraction and analytical electron microscopy have been used to study the crystallization of the grain-boundary glass in a 6 wt pct Y2O3-Si3N4 ceramic. Upon crystallization, high densities of dislocations formed in the Si3N4 grains and remained after 5 h at temperature. However, prolonged holds at the crystallization temperature effectively annealed out the dislocations. Other features present in the microstructure are characterized.

Lee, W. E.↗
NASA NTRS1990

High-temperature deformation and microstructural analysis for Si3N4-Sc2O3

It was indicated that Si3N4 doped with Sc2O3 may exhibit high temperature mechanical properties superior to Si3N4 systems with various other oxide sintered additives. High temperature deformation of samples was studied by characterizing the microstructures before and after deformation. It was found that elements of the additive, such as Sc and O, exist in small amounts at very thin grain boundary layers and most of them stay in secondary phases at triple and multiple grain boundary junctions. These secondary phases are devitrified as crystalline Sc2Si2O7. Deformation of the samples was dominated by cavitational processes rather than movements of dislocations. Thus the excellent deformation resistance of the samples at high temperature can be attributed to the very small thickness of the grain boundary layers and the crystalline secondary phase.

Cheong, Deock-Soo↗
NASA NTRS1991

Processing and properties of SiC whisker- and particulate-reinforced reaction bonded Si3N4

The microstructure and mechanical properties of reaction bonded Si3N4 (RBSN) reinforced with SiC whiskers of particles were investigated using RBSN composites made from colloidally pressed octanol dispersions of high-purity Si powders mixed with either SiC whiskers or alpha-SiC particles. Results of investigations, revealing high conversions of Si to Si3N4, specific surface areas, and constant relative densities and strengths, showed that the uniform microstructure and small flaw size of the matrix were maintained in the composites and that no degradation of the reinforcements was taking place. Neither the monolithic nor the composite materials exhibited R-curve behavior. A modest increase in fracture toughness was observed only in the RBSN containing 33 vol pct SiC(p).

Lightfoot, A.↗
NASA NTRS1996

Mullite fiber reinforced reaction bonded Si3N4 composites

Fracture toughnesses of brittle ceramic materials have been improved by introducing reinforcements and carefully tailored interface layers. Silicon carbide and Si3N4 have been emphasized as matrices of structural composites intended for high temperature service because they combine excellent mechanical, chemical, thermal and physical properties. Both matrices have been successfully toughened with SiC fibers, whiskers and particles for ceramic matrix composite (CMC) parts made by sintering, hot pressing or reaction forming processes. These SiC reinforced CMCs have exhibited significantly improved toughnesses at low and intermediate temperature levels, as well as retention of properties at high temperatures for selected exposures; however, they are vulnerable to attack from elevated temperature dry and wet oxidizing atmospheres after the matrix has cracked. Property degradation results from oxidation of interface layers and/or reinforcements. The problem is particularly acute for small diameter (-20 tim) polymer derived SiC fibers used for weavable toes. This research explored opportunities for reinforcing Si3N4 matrices with fibers having improved environmental stability; the findings should also be applicable to SiC matrix CMCs.

Saleh, T.↗
NASA NTRS1998

The Improvement of Ion Plated Ag and Au Film Adherence to Si3N4 and SiC Surfaces for Increased Tribological Performance

A modified dc-diode plating system, utilizing a metallic screen cage as a cathode and referred as SCREEN CAGE ION PLATING (SCIP), is used to deposit Ag and Au lubricating films on Si3N4 and SiC surfaces. When deposition is performed in Ar or N2, glow discharge, the surface displays poor adhesive strength (less than 5 MPa). A dramatic increase in adhesive strength (less than 80 MPa) is achieved when plating is performed in a reactive 50% 02 + 50% Ar glow discharge. The excited/ionized oxygen species (O2(+)/O(+) in the glow discharge contribute to the oxidation of the Si3N4 or SiC surfaces as determined by X-ray Photoelectron Spectroscopy (XTS) depth profiling. The reactively sputter-oxidized S3N4 or SiC surfaces and the activated-oxidized-metastable Ag or Au species formed in the plasma cooperatively contribute to the increased adherence. As a result, the linear thermal expansion coefficient mismatch at the interface is reduced. These lubricating Ag and Au films under sliding conditions reduce the friction coefficient by a factor of 2-1/2 to 4.

Spalvins, Talivaldis↗
NASA NTRS1998

Development and Characterization of SiC)/ MoSi2-Si3N4(p) Hybrid Composites

Intermetallic compound MoSi2 has long been known as a high temperature material that has excellent oxidation resistance and electrical/thermal conductivity. Also its low cost, high melting point (2023 C), relatively low density (6.2 g/cu cm versus 9 g/cu cm for current engine materials), and ease of machining, make it an attractive structural material. However, the use of MoSi2 has been hindered due to its poor toughness at low temperatures, poor creep resistance at high temperatures, and accelerated oxidation (also known as 'pest' oxidation) at temperatures between approximately 450 and 550 C. Continuous fiber reinforcing is very effective means of improving both toughness and strength. Unfortunately, MoSi2 has a relatively high coefficient of thermal expansion (CTE) compared to potential reinforcing fibers such as SiC. The large CTE mismatch between the fiber and the matrix resulted in severe matrix cracking during thermal cycling. Addition of about 30 to 50 vol % of Si3N4 particulate to MoSi2 improved resistance to low temperature accelerated oxidation by forming a Si2ON2 protective scale and thereby eliminating catastrophic 'pest failure'. The Si3N4 addition also improved the high temperature creep strength by nearly five orders of magnitude, doubled the room temperature toughness and significantly lowered the CTE of the MoSi2 and eliminated matrix cracking in SCS-6 reinforced composites even after thermal cycling. The SCS-6 fiber reinforcement improved the room temperature fracture toughness by seven times and impact resistance by five times. The composite exhibited excellent strength and toughness improvement up to 1400 C. More recently, tape casting was adopted as the preferred processing of MoSi2-base composites for improved fiber spacing, ability to use small diameter fibers, and for lower cost. Good strength and toughness values were also obtained with fine diameter Hi-Nicalon tow fibers. This hybrid composite remains competitive with ceramic matrix composites as a replacement for Ni-base superalloys in aircraft engine applications.

Hebsur, Mohan G.↗
NASA NTRS1998

Fundamentals of Passive Oxidation In SiC and Si3N4

The very slow oxidation kinetics of silicon carbide and silicon nitride, which derive from their adherent and passivating oxide films, has been explored at length in a broad series of studies utilizing thermogravimetric analysis, electron and optical micrography, energy dispersive spectrometry, x-ray diffractometry, micro-analytical depth profiling, etc. Some interesting microstructural phenomena accompanying the process of oxidation in the two materials will be presented. In Si3N4 the oxide is stratified, with an SiO2 topscale (which is relatively impervious to O2)underlain by a coherent subscale of silicon oxynitride which is even less permeable to O2- Such "defence in depth" endows Si3N4 with what is perhaps the highest oxidation resistance of any material, and results in a unique set of oxidation processes. In SiC the oxidation reactions are much simpler, yet new issues still emerge; for instance, studies involving controlled devitrification of the amorphous silica scale confirmed that the oxidation rate of SiC drops by more than an order of magnitude when the oxide scale fully crystallizes.

Thomas-Ogbuji, Linus U.↗
DOE OSTI2020

Materials Data on Si3N4 by Materials Project

Si3N4 crystallizes in the cubic I-43d space group. The structure is three-dimensional. Si4+ is bonded to four equivalent N3- atoms to form corner-sharing SiN4 tetrahedra. All Si–N bond lengths are 1.76 Å. N3- is bonded in a trigonal planar geometry to three equivalent Si4+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Si3N4 by Materials Project

Si3N4 is Hausmannite structured and crystallizes in the cubic Fd-3m space group. The structure is three-dimensional. there are two inequivalent Si4+ sites. In the first Si4+ site, Si4+ is bonded to six equivalent N3- atoms to form SiN6 octahedra that share corners with six equivalent SiN4 tetrahedra and edges with six equivalent SiN6 octahedra. All Si–N bond lengths are 1.89 Å. In the second Si4+ site, Si4+ is bonded to four equivalent N3- atoms to form corner-sharing SiN4 tetrahedra. The corner-sharing octahedral tilt angles are 57°. All Si–N bond lengths are 1.79 Å. N3- is bonded in a distorted rectangular see-saw-like geometry to four Si4+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Si3N4 by Materials Project

Si3N4 crystallizes in the trigonal P31c space group. The structure is three-dimensional. there are two inequivalent Si4+ sites. In the first Si4+ site, Si4+ is bonded to four N3- atoms to form corner-sharing SiN4 tetrahedra. There are a spread of Si–N bond distances ranging from 1.74–1.76 Å. In the second Si4+ site, Si4+ is bonded to four N3- atoms to form corner-sharing SiN4 tetrahedra. There are a spread of Si–N bond distances ranging from 1.73–1.75 Å. There are four inequivalent N3- sites. In the first N3- site, N3- is bonded in a trigonal planar geometry to three Si4+ atoms. In the second N3- site, N3- is bonded in a trigonal planar geometry to three Si4+ atoms. In the third N3- site, N3- is bonded in a trigonal planar geometry to three equivalent Si4+ atoms. In the fourth N3- site, N3- is bonded in a trigonal planar geometry to three equivalent Si4+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Si3N4 by Materials Project

Si3N4 crystallizes in the hexagonal P6_3/m space group. The structure is three-dimensional. Si4+ is bonded to five N3- atoms to form a mixture of edge and corner-sharing SiN5 trigonal bipyramids. There are a spread of Si–N bond distances ranging from 1.75–1.99 Å. There are two inequivalent N3- sites. In the first N3- site, N3- is bonded in a see-saw-like geometry to four equivalent Si4+ atoms. In the second N3- site, N3- is bonded in a trigonal planar geometry to three equivalent Si4+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Sr(Si3N4)2 by Materials Project

Sr(Si3N4)2 crystallizes in the orthorhombic Imm2 space group. The structure is three-dimensional. Sr2+ is bonded in a 10-coordinate geometry to ten N+2.25- atoms. There are a spread of Sr–N bond distances ranging from 2.70–3.20 Å. There are two inequivalent Si+2.67+ sites. In the first Si+2.67+ site, Si+2.67+ is bonded to four N+2.25- atoms to form corner-sharing SiN4 tetrahedra. There are a spread of Si–N bond distances ranging from 1.69–1.78 Å. In the second Si+2.67+ site, Si+2.67+ is bonded in a trigonal non-coplanar geometry to three N+2.25- atoms. There is two shorter (1.75 Å) and one longer (1.76 Å) Si–N bond length. There are three inequivalent N+2.25- sites. In the first N+2.25- site, N+2.25- is bonded in a 2-coordinate geometry to two equivalent Sr2+ and two equivalent Si+2.67+ atoms. In the second N+2.25- site, N+2.25- is bonded in a trigonal planar geometry to one Sr2+ and three Si+2.67+ atoms. In the third N+2.25- site, N+2.25- is bonded in a distorted trigonal non-coplanar geometry to one Sr2+ and three Si+2.67+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ba(Si3N4)2 by Materials Project

Ba(Si3N4)2 crystallizes in the orthorhombic Imm2 space group. The structure is three-dimensional. Ba2+ is bonded in a 10-coordinate geometry to ten N+2.25- atoms. There are a spread of Ba–N bond distances ranging from 2.82–3.26 Å. There are two inequivalent Si+2.67+ sites. In the first Si+2.67+ site, Si+2.67+ is bonded to four N+2.25- atoms to form corner-sharing SiN4 tetrahedra. There are a spread of Si–N bond distances ranging from 1.68–1.78 Å. In the second Si+2.67+ site, Si+2.67+ is bonded in a trigonal non-coplanar geometry to three N+2.25- atoms. All Si–N bond lengths are 1.76 Å. There are three inequivalent N+2.25- sites. In the first N+2.25- site, N+2.25- is bonded in a distorted trigonal non-coplanar geometry to one Ba2+ and three Si+2.67+ atoms. In the second N+2.25- site, N+2.25- is bonded in a trigonal planar geometry to one Ba2+ and three Si+2.67+ atoms. In the third N+2.25- site, N+2.25- is bonded in a 2-coordinate geometry to two equivalent Ba2+ and two equivalent Si+2.67+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Si3N4 by Materials Project

Si3N4 crystallizes in the trigonal P31c space group. The structure is three-dimensional. there are two inequivalent Si4+ sites. In the first Si4+ site, Si4+ is bonded in a distorted see-saw-like geometry to four N3- atoms. There are a spread of Si–N bond distances ranging from 1.79–2.12 Å. In the second Si4+ site, Si4+ is bonded to five N3- atoms to form a mixture of distorted edge and corner-sharing SiN5 trigonal bipyramids. There are a spread of Si–N bond distances ranging from 1.77–1.94 Å. There are four inequivalent N3- sites. In the first N3- site, N3- is bonded in a distorted trigonal planar geometry to three Si4+ atoms. In the second N3- site, N3- is bonded to four Si4+ atoms to form distorted corner-sharing NSi4 trigonal pyramids. In the third N3- site, N3- is bonded in a 4-coordinate geometry to three equivalent Si4+ and one N3- atom. The N–N bond length is 1.27 Å. In the fourth N3- site, N3- is bonded in a 4-coordinate geometry to three equivalent Si4+ and one N3- atom.

36 MATERIALS SCIENCE↗