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  • 1
    Electronic Resource
    Electronic Resource
    Intermediate-Temperature Environmental Effects on Boron Nitride-Coated Silicon Carbide-Fiber-Reinforced Glass-Ceramic Composites (1997)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 80 (1997), S. 0 
    Details
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The environmental effects on the mechanical properties of fiber-reinforced composites at intermediate temperatures were investigated by conducting flexural static-fatigue experiments in air at 600° and 950°C. The material that was studied was a silicon carbide/boron nitride (SiC/BN) dual-coated Nicalon-fiber-reinforced barium magnesium aluminosilicate glass-ceramic. Comparable time-dependent failure responses were found at 600° and 950°C when the maximum tensile stress applied in the bend bar was 60% of the room-temperature ultimate flexural strength of as-received materials. At both temperatures, the materials survived 500 h fatigue tests at lower stress levels. Among the samples that survived the 500 h fatigue tests, a 20% degradation in the room-temperature flexural strength was measured in samples tested at 600°C, whereas no degradation was observed for the samples tested at 950°C. Microstructure and chemistry studies revealed interfacial oxidation in the samples that were fatigued at 600°C. The growth rate of the Si-C-O fiber oxidation product at 600°C was not sufficient to seal the stress-induced cracks, so that the interior of the material was oxidized and resulted in a strength degradation and less fibrous fracture. In contrast, the interior of the material remained intact at 950°C because of crack sealing by rapid silicate formation, and strength/toughness of the composite was maintained. Also, at 600°C, BN oxidized via volatilization, because no borosilicate was formed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.1997.tb02875.x
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  • 2
    Electronic Resource
    Electronic Resource
    Desintering Process in the Gas-Pressure Sintering of Silicon Nitride (1997)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 80 (1997), S. 0 
    Details
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: A unique desintering phenomenon has been observed in gas-pressure sintering of silicon nitrides with additives of yttria and alumina. The desintering phenomenon occurred, simultaneous with weight increase, only when a boron nitride crucible was used in combination with the application of high nitrogen pressure (5 MPa). When the nitrogen pressure was low (0.5 MPa), or when the boron nitride crucible was replaced by a graphite crucible, this desintering phenomenon was not observed. These results could be rationalized by the chemical dissolution of nitrogen into the oxynitride melts and the resultant evolution of carbon monoxide. This indicates that the high nitrogen overpressure employed in gas-pressure sintering of silicon nitride ceramics is not always beneficial.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.1997.tb02834.x
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  • 3
    Electronic Resource
    Electronic Resource
    Elementary Mechanisms behind the High-Temperature Deformation Behavior of Lutetium-Doped Silicon Nitride (2003)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 86 (2003), S. 0 
    Details
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Intergranular sliding and diffusive mechanisms behind the deformation behavior of a commercially available lutetium-doped silicon nitride were investigated and discussed. A method of locating and separating phenomena critical for mechanical relaxation at elevated temperatures was applied; the method was based on low-frequency forced-vibration damping measurements. The potentiality of lutetium addition for improving the deformation resistance of silicon nitride was clearly reflected in the high-temperature damping behavior of the investigated polycrystal. Softening of intergranular lutetium silicate phases located at multigrain junctions, which resulted in a grain-boundary sliding peak, occurred at remarkably high temperatures (〉1725 K). This phenomenon, partly overlapping diffusional flow, was followed by further damping relaxation with the melting of the lutetium silicates. Subsequent grain growth was also detected at temperatures 〉2100 K. Torsional creep results, collected up to 2100 K, consistently proved the presence of a “locking” effect by lutetium silicates with the sliding of silicon nitride grain boundaries below 1873 K.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2003.tb03323.x
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  • 4
    Electronic Resource
    Electronic Resource
    Microstructural Design of Silicon Nitride with Improved Fracture Toughness: I, Effects of Grain Shape and Size (1998)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 81 (1998), S. 0 
    Details
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The use of self-reinforcement by larger elongated grains in silicon nitride ceramics requires judicious control of the microstructure to achieve high steady-state toughness and high fracture strength. With a distinct bimodal distribution of grain diameters, such as that achieved by the addition of 2% rodlike seeds, the fracture resistance rapidly rises with crack extension to steady-state values of up to 10 MPam1/2 and is accompanied by fracture strengths in excess of 1 GPa. When the generation of elongated reinforcing grains is not regulated, a broad grain diameter distribution is typically generated. While some toughening is achieved, both the plateau (steady-state) toughness and the R-curve response suffer, and the fracture strength undergoes a substantial reduction. Unreinforced equiaxed silicon nitride exhibits the least R-curve response with a steady-state toughness of only 3.5 MPam1/2 coupled with a reduced fracture strength.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.1998.tb02702.x
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  • 5
    Electronic Resource
    Electronic Resource
    Lifetime-Applied Stress Response in Air of a SiC-Based Nicalon-Fiber-Reinforced Composite with a Carbon Interfacial Layer: Effects of Temperature (300° to 1150°C) (1998)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 81 (1998), S. 0 
    Details
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The lifetimes in air as a function of applied flexure stress and temperature (300–1150°C) are described for a Si–O–C based (Nicalon) fiber plain-weave cloth reinforced SiC-matrix composite (∼7% closed porosity) with an ∼0.3 µm thick carbon interfacial layer. The measured lifetimes of both samples with and without an external SiC seal coating were similar and decreased with applied flexural stress (for stresses greater than ∼90 MPa) and with temperature. At temperatures of ≥600°C, the external CVD SiC coating had negligible effect on the lifetimes; however, at 425°C, a detectable improvement in the lifetime was observed with an external SiC coating. When the applied stress was decreased below an apparent “threshold stress” (e.g., ∼90 MPa) for tests conducted at temperatures ≤950°C, no failures were observed for times of ≥1000 h. Electron microscopy observations show that the interfacial carbon layer is progressively removed during tests at 425° and 600°C. In these cases, failure is associated with fiber failure and pullout. At 950° and 1150°C, the carbon interface layer is eliminated and replaced by a thick silica layer due to the oxidation of the Nicalon fiber and the SiC matrix. This results in embrittling the composite.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.1998.tb02567.x
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  • 6
    Electronic Resource
    Electronic Resource
    Grain Boundary Sliding Measurements during Tensile Creep of a Single-Phase Alumina (1998)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 81 (1998), S. 0 
    Details
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The grain boundary sliding (GBS) behavior of a single-phase (relatively coarse-grained) alumina material was studied after tensile creep experiments were performed at 1500°C at stress levels of 20 and 35 MPa. Specimens tested at 35 MPa exhibited a number of modes of GBS, including Mode II (shear) displacements, Mode I (opening) displacements, out-of-plane sliding displacements, and in-plane grain rotation. Strains in the grain boundaries due to Mode II GBS ranged from 940% to 4400%. Average Mode II GBS displacements ranged from 0.08 to 0.29 µm in samples tested for 120 and 480 min, respectively, at 35 MPa. The GBS displacements were shown to fit a Weibull distribution. Tensile creep under a 35 MPa stress yielded a GBS rate of 9.5 10-6µm/s, while the 20 MPa stress resulted in a GBS rate of 2.2 10-6µm/s. The average Mode II GBS displacements increased linearly with specimen strain, suggesting that GBS may play an important role in creep cavitation during tensile creep. The data also revealed that compatibility and constraint rules appear to govern GBS behavior during tensile creep. GBS behavior during compressive creep will be compared to the tensile creep GBS measurements presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.1998.tb02500.x
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  • 7
    Electronic Resource
    Electronic Resource
    Sintering Silicon Nitride Ceramics in Air (2005)
    Malden, USA : Blackwell Science Inc
    Journal of the American Ceramic Society 88 (2005), S. 0 
    Details
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Silicon nitride (Si3N4) ceramics, prepared with Y2O3 and Al2O3 sintering additives, have been densified in air at temperatures of up to 1750°C using a conventional MoSi2 element furnace. At the highest sintering temperatures, densities in excess of 98% of theoretical have been achieved for materials prepared with a combined sintering addition of 12 wt% Y2O3 and 3 wt% Al2O3. Densification is accompanied by a small weight gain (typically 〈1–2 wt%), because of limited passive oxidation of the sample. Complete α- to β-Si3N4 transformation can be achieved at temperatures above 1650°C, although a low volume fraction of Si2N2O is also observed to form below 1750°C. Partial crystallization of the residual grain-boundary glassy phase was also apparent, with β-Y2Si2O7 being noted in the majority of samples. The microstructures of the sintered materials exhibited typical β-Si3N4 elongated grain morphologies, indicating potential for low-cost processing of in situ toughened Si3N4-based ceramics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1551-2916.2005.00631.x
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  • 8
    Electronic Resource
    Electronic Resource
    Evaluation of Mechanical Stability of a Commercial Sn88 Silicon Nitride at Intermediate Temperatures (2003)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 86 (2003), S. 0 
    Details
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Dynamic fatigue and stress rupture tests in four-point bending were conducted on a commercially available SN88 silicon nitride ceramic at temperatures in the range 700°–1000°C in air. The objective of the present study was to elucidate the failure of SN88 silicon nitride ceramic nozzles arising from a critical crack initiated at the intermediate temperature airfoil region during an engine field test. Results of dynamic fatigue tests indicated that SN88 silicon nitride tested at a stressing rate of 30 MPa/s exhibited little change in characteristic strength at the various test temperatures. However, SN88 silicon nitride exhibited a significant degradation in mechanical strength when tested at 0.003 MPa/s at temperatures indicative of a great susceptibility to slow crack growth, especially at 850°C. SEM and XRD analyses indicated that the mechanical instability of SN88 silicon nitride at intermediate temperatures resulted from the transformation of secondary phase(s) from oxidation. These phase transformations were accompanied by a large volume change, which led to the generation of large local residual tensile stresses. As a result, extensive damage zones were formed, which led to a substantial degradation of mechanical strength and reliability. Microstructural examination of failed SN88 airfoils indicated that a similar damage zone was formed in the regions exposed to intermediate temperatures during engine testing. Consequently, the ultimate failure of these vanes was attributed to the loss in mechanical strength from the damage zone formation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2003.tb03444.x
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  • 9
    Electronic Resource
    Electronic Resource
    Grain-Size Effect on Compressive Creep of Silicon-Carbide-Whisker-Reinforced Aluminum Oxide (2001)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 84 (2001), S. 0 
    Details
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The steady-state compressive creep of Al2O3 with 10 vol% SiC whiskers having grain sizes of 1.2, 2.3, and 4.0 μm has been measured at 1400°C in argon. The creep rate is related to the free volume within the whisker network, not the nominal grain size. The results are consistent with diffusional-controlled creep with different contributions from grain-boundary sliding. Under low stresses, only Liftshitz sliding is possible and the diffusional process controls deformation, while at stresses over a threshold, Rachinger sliding is the mechanism controlling deformation. The evolution between Liftshitz and Rachinger sliding is marked by a significant increase in the value of the stress exponent.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2001.tb00892.x
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  • 10
    Electronic Resource
    Electronic Resource
    Macro- to Atomic-Scale Tailoring of Si〈sub〉3〈/sub〉N〈sub〉4〈/sub〉 Ceramics to Enhance Properties (2005)
    s.l. ; Stafa-Zurich, Switzerland
    Key engineering materials Vol. 287 (June 2005), p. 233-241 
    Details
    ISSN: 1013-9826
    Source: Scientific.Net: Materials Science & Technology / Trans Tech Publications Archiv 1984-2008
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Silicon nitride ceramics are finding uses in numerous engineering applicationsbecause of their tendency to form whisker-like microstructures that can overcome the inherent brittle nature of ceramics. Studies now establish the underlying microscopic and atomic-scale principles for engineering a tough, strong ceramic. The theoretical predictions are confirmed by macroscopic observations and atomic level characterization of preferential segregation at the interfaces between the grains and the continuous nanometer thick amorphous intergranular film (IGF). Two interrelated factors must be controlled for this to occur including the generation of the elongated reinforcing grains during sintering and debonding of the interfaces between the reinforcing grains and the matrix. The reinforcing grains can be controlled by (1) seeding with beta particles and (2) the chemistry of the additives, which also can influence the interfacial debonding conditions.In addition to modifying the morphology of the reinforcing grains, it now appears thatthe combination of preferential segregation and strong bonding of the additives (e.g., the rare earths, RE) to the prism planes can also result in sufficiently weakens the bond of the interface with the IGF to promote debonding. Thus atomic-scale engineering may allow us to gain further enhancements in fracture properties. This new knowledge will enable true atomic-level engineering to be joined with microscale tailoring to develop the advanced ceramics that will be required for moreefficient engines, new electronic device architectures and composites
    Type of Medium: Electronic Resource
    URL: http://www.tib-hannover.de/fulltexts/2011/0528/01/49/transtech_doi~10.4028%252Fwww.scientific.net%252FKEM.287.233.pdf
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