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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Relationships between toughness and microstructure of reaction bonded Si3N4

Fracture toughnesses of nominally identical batches of reaction bonded silicon nitride (RBSN) differed significantly (about 2.0 and about 2.7 MPa sq rt m). Detailed fractographic and microstructural characterizations investigated underlying factors. Subtile differences between high and low toughness RBSN and between constituent Si powders have been revealed through SEM/FEG, TEM, BET, Hg-porosimetry, and XRD. The results illustrate the need for behavioral models to guide microstructural design and to interpret properties of brittle materials with intermediate levels of porosity.

Lightfoot, Annamarie↗
NASA NTRS1993

Tensile creep and creep-recovery behavior of a SiC-fiber-Si3N4-matrix composite

The tensile creep and creep-recovery behavior of a hot-pressed unidirectional SiC-fiber/Si3N4-matrix composite was investigated at 1200 C in air, in order to determine how various sustained and cyclic creep loading histories would influence the creep rate, accumulated creep strain, and the amount of strain recovered upon specimen unloading. The data accumulated indicate that the fundamental damage mode for sustained tensile creep at stresses of 200 and 250 MPa was periodic fiber fracture and that the creep life and the failure mode at 250 MPa were strongly influenced by the rate at which the initial creep stress was applied. Cyclic loading significantly lowered the duration of primary creep and the overall creep-strain accumulation. The implications of the results for microstructural and component design are discussed.

Holmes, John W.↗
NASA NTRS1995

High Temperature Slow Crack Growth of Si3N4 Specimens Subjected to Uniaxial and Biaxial Dynamic Fatigue Loading Conditions

The slow crack growth of a hot-pressed silicon nitride was determined at 1300 C in air using dynamic fatigue testing under both uniaxial and biaxial stress states. Good agreement in fatigue parameter exists between the data obtained from uniaxial and biaxial loading conditions. A reasonable prediction of dynamic fatigue from one stress state to another was made using the recently developed CARES/LIFE computer code.

Choi, Sung R.↗
NASA NTRS2002

Pest resistant MoSi2-based materials containing in-situ grown .beta.-Si3N4 whiskers

A MoSi.sub.2 pest resistant material includes in-situ grown .beta.-Si.sub.3 N.sub.4 whiskers. In addition to excellent pest resistance, the material provides a lower coefficient of thermal expansion for better match with continuous reinforcing fibers such as SiC fibers. A two stage heating and pressing production technique enables lower temperature processing with substantially full densification.

Hebsur, Mohan G.↗
NASA NTRS1988

Molten-salt corrosion of silicon nitride. I - Sodium carbonate. II - Sodium sulfate

An experimental study of the corrosion of Si3N4 under thin films of Na2CO3 at 1000 C has been conducted using both pure Si3N4 and Si3N4 with various additives. The reaction mechanism is shown to consist of: (1) the decomposition of Na2CO3 and the formation of Na2SiO3; (2) rapid oxidation; and (3) the formation of a protective silica layer below the silicate. In the second part, the corrosion mechanism of Si3N4 + Na2SO4/O2 at 1000 C was studied for both pure and additive-containing Si3N4. The reaction of Si3N4 + Na2SO4 was found to involve an initial period of slow weight loss (due to Na2SO4 vaporization and oxidation-dissolution) followed by further oxidation or the near termination of the reaction, depending on the Si3N4 additive.

Fox, Dennis S.↗
NASA NTRS1979

Development of silicon nitride of improved toughness

The application of reaction sintered Si2N4 energy absorbing surface layers to hot-pressed Si3N4 was investigated. The surface layer was formed by in-place nitridation of silicon powder. It was found that reaction sintered Si3N4 layers of 1 mm thickness, fabricated from either -100, +200, -200, or -325 mesh Si powder and nitrided in 96% N2/4% H2 so that approximately 20-25 vol % unnitrided Si remained in the layer, resulted in a sevenfold increase in ballistic impact resistance of a 0.64 cm thick hot-pressed SI3N4 substrate from RT 1370 C. Both NC-132 SI3N4, with MgO additive, and NCX-34 Si3N4, with Y2O3 additive, were evaluated as substrate material. The finer grain size -200 and -325 mesh nitrided Si layers were for their smoothness and relatively high density. It was found that nitriding in N2/H2 mixtures, rather than pure N2, resulted in a microstructure that did not substantially degrade the strength of the hot-pressed Si3N4 substrate. Thermal cycling tests on the RSSN/HPSN combinations from 200 C to 1370 C for 75 cycles in air did not degrade the impact resistance nor the interfacial bonding, although a large amount of internal silica formation occurred within the RSSN layer. Mach 0.8, 5 hr, hot gas erosion tests showed no surface recession of RSSN layers at 1200 C and slight surface recession at 1370 C.

Brennan, J. J.↗
NASA NTRS1992

Plasma etching a ceramic composite

Plasma etching is found to be a superior metallographic technique for evaluating the microstructure of a ceramic matrix composite. The ceramic composite studied is composed of silicon carbide whiskers (SiC(sub W)) in a matrix of silicon nitride (Si3N4), glass, and pores. All four constituents are important in evaluating the microstructure of the composite. Conventionally prepared samples, both as-polished or polished and etched with molten salt, do not allow all four constituents to be observed in one specimen. As-polished specimens allow examination of the glass phase and porosity, while molten salt etching reveals the Si3N4 grain size by removing the glass phase. However, the latter obscures the porosity. Neither technique allows the SiC(sub W) to be distinguished from the Si3N4. Plasma etching with CF4 + 4 percent O2 selectively attacks the Si3N4 grains, leaving SiC(sub W) and glass in relief, while not disturbing the pores. An artifact of the plasma etching reaction is the deposition of a thin layer of carbon on Si3N4, allowing Si3N4 grains to be distinguished from SiC(sub W) by back scattered electron imaging.

Hull, David R.↗
DOE OSTI2024

Preparation of a membrane-sealed cell for studying catalyst nanoparticles in flowing gas with high vacuum x-ray photoelectron spectrometer

Here a sealing-style x-ray photoelectron spectroscopy study of the surface of a 1.0 wt. %Ni/TiO2 nanoparticle catalyst in a flowing mixture of CO and O2 at 1 bar was performed with a graphene membrane-sealed Si3N4 window-based miniature cell. We report the details on how a commercial Si3N4 window is modified before assembling a graphene membrane, how single-layer graphene membranes are transferred from their metal supports to the modified Si3N4 window, how a modified Si3N4 window covered with a double-layer graphene membrane is assembled onto a blank cell cap, how a nanoparticle catalyst is introduced to the cell cap and then the cell cap is installed onto a cell body to form a complete reaction cell, and how a complete cell is interfaced with a high vacuum chamber of an XPS system before an XPS study of 1.0 wt. %Ni/TiO2 catalyst surface in a flowing mixture for 0.2 bar CO and 0.8 bar O2 is performed. How the characterization of a catalyst using this type of graphene membrane-sealed Si3N4 window-based miniature cell is relevant to the finding of the actual surface chemistry of a catalyst during catalysis is discussed.

Instruments & Instrumentation↗
NASA NTRS1972

High temperature compounds for turbine vanes

Fabrication and microstructure control studies were conducted on SiC, Si3N4, and composites based on these compounds. Charpy mode impact testing to 2400 F established that beta-spodumene, lithium aluminum silicate, coated Si3N4, Si3N4 derived from alpha-Si3N4 powder, and SiC containing 5-25 v/o chopped C fibers had the most promising strengths. Several other composite systems had excellent microstructures and could prove interesting materials in the future. Stress-rupture testing on Si3N4 established that increasing 2000 F - 100 hour strengths were obtained for increasing grain size to at least 5 micrometers, increasing density and possibly increasing phase purity. These parameters became less important at 2400 F where it is thought a grain boundary phase controls strength.

Rhodes, W. H.↗
NASA NTRS1976

Consolidation of silicon nitride without additives

The use of ceramics for gas turbine engine construction might make it possible to increase engine efficiency by raising operational temperatures to values beyond those which can be tolerated by metallic alloys. The most promising ceramics being investigated in this connection are Si3N4 and SiC. A description is presented of a study which had the objective to produce dense Si3N4. The two most common methods of consolidating Si3N4 currently being used include hot pressing and reaction sintering. The feasibility was explored of producing a sound, dense Si3N4 body without additives by means of conventional gas hot isostatic pressing techniques and an uncommon hydraulic hot isostatic pressing technique. It was found that Si3N4 can be densified without additions to a density which exceeds 95% of the theoretical value

Sikora, P. F.↗
NASA NTRS1992

Environmental effects on friction and wear of diamond and diamondlike carbon coatings

Reciprocating sliding friction experiments were conducted with a natural diamond flat, diamond film, and low and high density diamondlike carbon (DLC) films in contact with pin specimens of natural diamond and silicon nitride (Si3N4) both in humid air and dry air nitrogen. The results indicated that for natural diamond pin contacts the diamond films and the natural diamond flat were not susceptible to moisture but that moisture could increase both the coefficient of friction and the wear factors of the DLC films. The coefficients of friction and wear factors of the diamond films were generally similar to those of the natural diamond flat both in humid air and dry air nitrogen. In dry nitrogen the coefficients of friction of the high density DLC films in contact with pin specimens of both diamond and Si3N4 were generally low (about 0.02) and similar to those of the natural diamond flat and the diamond films. The wear factors of the materials in contact with both natural diamond and Si3N4 were generally in the ascending order of natural diamond flat, diamond film, high density DLC film, and low density DLC film. The moisture in the environment increased the coefficients of friction for Si3N4 pins in contact with all the materials. This increase in friction is due to the silicon oxide film produced on the surface of Si3N4 pins in humid air.

Miyoshi, Kazuhisa↗
NASA NTRS1994

Occurence and implications of silicon nitride in enstatic chondrites

Silicon nitride Si3N4, has previously been observed to be a common constituent of acid residues of Qingzhen (EH3) and Indarch (EH4). Ion probe analysis of the Si, N and C isotopic compositions of individual Si3N4 grains from Qingzhen and Indarch acid residues suggest most, if not all, grains are Solar System in origin. A few grains have isotopically anomalous C but this is probably due to small presolar SiC grains adhering to them. In situ observations of the Si3N4 in Qingzhen show that it is only present within, and probably exsolved from, host phases which contain elemental Si in solid solution. Thermodynamic calculations suggest that the Si3N4 probably formed during metamorphism and not in the nebula. Thermodynamic calculations also show that sinoite (Si2N2O) and not Si3N4 should be the stable phase during metamorphism. It appears that kinetic factors must have inhibited the formation if sinoite in Qimgzhen and Indarch.

Alexander, C. M. O'D.↗
NASA NTRS2002

High Temperature Corrosion of Silicon Carbide and Silicon Nitride in Water Vapor

Silicon carbide (SiC) and silicon nitride (Si3N4) are proposed for applications in high temperature combustion environments containing water vapor. Both SiC and Si3N4 react with water vapor to form a silica (SiO2) scale. It is therefore important to understand the durability of SiC, Si3N4 and SiO2 in water vapor. Thermogravimetric analyses, furnace exposures and burner rig results were obtained for these materials in water vapor at temperatures between 1100 and 1450 C and water vapor partial pressures ranging from 0.1 to 3.1 atm. First, the oxidation of SiC and Si3N4 in water vapor is considered. The parabolic kinetic rate law, rate dependence on water vapor partial pressure, and oxidation mechanism are discussed. Second, the volatilization of silica to form Si(OH)4(g) is examined. Mass spectrometric results, the linear kinetic rate law and a volatilization model based on diffusion through a gas boundary layer are discussed. Finally, the combined oxidation and volatilization reactions, which occur when SiC or Si3N4 are exposed in a water vapor-containing environment, are presented. Both experimental evidence and a model for the paralinear kinetic rate law are shown for these simultaneous oxidation and volatilization reactions.

Opila, E. J.↗
NASA NTRS1974

High temperature compounds for turbine vanes

Fabrication and microstructure control studies were conducted on SiC, Si3N and composites based on Si3N. Charpy mode impact testing to 2400 F established that Si3N4/Mo composites have excellent potential. Attempts to fabricate composites of Si3N4 with superalloys, both by hot pressing and infiltration were largely unsuccessful in comparison to using Mo, Re, and Ta which are less reactive. Modest improvements in impact strength were realized for monolithic Si3N4; however, SiC strengths increased by a factor of six and now equal values achieved for Si3N4. Correlations of impact strength with material properties are discussed. Reduced MgO densification aid additions to Si3N4 were found to decrease densification kinetics, increase final porosity, decrease room temperature bend strength, increase high temperature bend strength, and decrease bend stress rupture properties. The decrease in bend strength at high temperature for fine grain size SiC suggested that a slightly larger grain size material with a nearly constant strength-temperature relation may prove desirable in the creep and stress rupture mode.

Rhodes, W. H.↗
NASA NTRS1974

Behavior of ceramics at 1200 C in a simulated gas turbine environment

This report summarizes programs at the NASA Lewis Research Center evaluating several classes of commercial ceramics, in a high gas velocity burner rig simulating a gas turbine engine environment. Testing of 23 ceramics in rod geometry identified SiC and Si3N4 as outstanding in resistance to oxidation and thermal stress and identified the failure modes of other ceramics. Further testing of a group of 15 types of SiC and Si3N4 in simulated vane shape geometry has identified a hot pressed SiC, a reaction sintered SiC, and hot pressed Si3N4 as the best of that group. SiC and Si3N4 test specimens were compared on the basis of weight change, dimensional reductions, metallography, fluorescent penetrant inspection, X-ray diffraction analyses, and failure mode.

Sanders, W. A.↗
NASA NTRS1975

Silicon nitride-aluminum oxide solid solution (SiAION) formation and densification by pressure sintering

Stirred-ball-mill-blended Si3N4 and Al2O3 powders were pressure sintered in order to investigate the mechanism of solid solution formation and densification in the Si3N4-Al2O3 system. Powder blends with Si3N4:Al2O3 mole ratios of 4:1, 3:2, and 2:3 were pressure sintered at 27.6-MN/sq m pressure at temperatures to 17000 C (3090 F). The compaction behavior of the powder blends during pressure sintering was determined by observing the density of the powder compact as a function of temperature and time starting from room temperature. This information, combined with the results of X-ray diffraction and metallographic analyses regarding solutioning and phase transformation phenomena in the Si3N4-Al2O3 system, was used to describe the densification behavior.

Yeh, H. C.↗
NASA NTRS1977

Consolidation of silicon nitride without additives

The feasibility of producing a sound, dense Si3N4 body without additives was explored, using conventional gas hot isostatic pressing techniques and an uncommon hydraulic hot isostatic pressing technique. These two techniques produce much higher pressure 275-413 MN/m sq (40,000 - 60,000 psi) than hot-pressing techniques. Evaluation was based on density measurement, microscopic examination, both optical and electron, and X-ray diffraction analysis. The results are summarized as follows: (1) Si3N4 can be densified to high density, greater than 95% of theoretical, without additions. (2) The higher density Si3N4 specimens appear to be associated with a greater amount of alpha to beta transformation. (3) Under high pressure, the alpha to beta transformation can occur at a temperature as low as 1150 C. (4) Grain deformation and subsequent recrystallization and grain refinement result from hot isostatic pressing of Si3N4.

Sikora, P. F.↗