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  • 1995-1999  (20)
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  • 1
    Electronic Resource
    Electronic Resource
    Evidence for Structural Changes of Amorphous Carbon Coatings on Silicon Carbide During Tribological Tests (1999)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 82 (1999), 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: During tribological testing of hydrogenated amorphous carbon coatings (a-C:H) on silicon carbide ceramics, a friction coefficient of 0.06 was observed under dry sliding conditions, which is thought to represent a lower boundary value for the given experimental setups. Based on chemical and thermal analyses of the coating material, a structural model of the as-deposited amorphous coating was modified taking simplified quantum chemical and statistical arguments into consideration. The tribological behavior of the coating could be rationalized by this modified structure model: Tribological loading leads to a partial release of internal stresses stored in the coating. This process results in an increase of the local mobility of atoms, which can be seen as the driving force for initial structural rearrangements within the coating. Proof for the occurrence of such rearrangements has been provided by electron energy-loss spectroscopy (EELS) measurements, monitoring the local sp3/sp2 hybridization ratio, of the coating before and after tribological tests.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.1999.tb02218.x
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  • 2
    Electronic Resource
    Electronic Resource
    Formation of Mullite from Filled Siloxanes (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: Monolithic mullite with low sintering shrinkage was synthesized from polymer/filler blendsMthat is, siloxane/alumina (siloxane/Al2O3) (and siloxane/aluminum (siloxane/Al)) mixtures. The synthesis was based on a reaction-bonding process of amorphous silica, which formed when the siloxane was oxidized, with Al2O3 (or oxidized aluminum filler) at temperatures 〉1250°C. Thermodynamic calculations were used to calculate the phase composition at equilibrium. Thermoanalytical, infrared-spectroscopic, and microscopic techniques were applied to reveal the microstructural evolution. The corresponding volume changes were used to evaluate the linear shrinkage, based on the quantitative phase assemblage.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.1997.tb03156.x
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  • 3
    Electronic Resource
    Electronic Resource
    Grain-Boundary Viscosity of Polycrystalline Silicon Carbides (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: Internal friction experiments were conducted on three SiC polycrystalline materials with different microstructural characteristics. Characterizations of grain-boundary structures were performed by high-resolution electron microscopy (HREM). Observations revealed a common glass-film structure at grain boundaries of two SiC materials, which contained different amounts of SiO2 glass. Additional segregation of residual graphite and SiO2 glass was found at triple pockets, whose size was strongly dependent on the amount of SiO2 in the material. The grain boundaries of a third material, processed with B and C addition, were typically directly bonded without any residual glass phase. Internal friction data of the three SiC materials were collected up to similar/congruent2200°C. The damping curves as a function of temperature of the SiO2-bonded materials revealed the presence of a relaxation peak, arising from grain-boundary sliding, superimposed on an exponential-like background. In the directly bonded SiC material, only the exponential background could be detected. The absence of a relaxation peak was related to the glass-free grain-boundary structure of this polycrystal, which inhibited sliding. Frequency-shift analysis of the internal friction peak in the SiO2-containing materials enabled the determination of the intergranular film viscosity as a function of temperature.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.1998.tb02770.x
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  • 4
    Electronic Resource
    Electronic Resource
    Decomposition-Crystallization of Polymer-Derived Si-C-N Ceramics (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: Monolithic polymer-derived Si-C-N ceramics were processed by blending an oligomeric Si-C-N precursor (liquid polysilazane) with 70 vol% of crosslinked or pyrolyzed Si-C-N powder particles, which were obtained from the same liquid precursor preheated at 300° or 1000°C, respectively. Powder compacts subsequently were annealed at 300°C to crosslink the liquid precursor acting as a binder between the powder particles, thus yielding monolithic green bodies. Heat treatment at 1540°C was performed to initiate crystallization in the various samples. Microstructure development and, in particular, crystallization behavior were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and preliminary nuclear magnetic resonance (NMR) spectroscopy. The material containing 300°C polymer powder (with oligomeric binder, also crosslinked at 300°C) revealed a homogeneous amorphous microstructure after exposure to temperatures of 1540°C. In contrast, the specimen containing powder particles preheated at 1000°C exhibited a high volume fraction of SiC crystallites within regions that were previously filled by the binder; however, the Si-C-N powder particles themselves remained amorphous. SEM observations as well as XRD studies showed the formation of idiomorphic SiC and Si3N4 crystallites on specimen surfaces as well as along internal crack walls. This finding suggested that vapor-phase reactions at the surface were involved in the formation of crystalline phases at temperatures 〉1250°C. Moreover, NMR spectroscopy data indicated a phase separation process, implying structural rearrangement prior to crystallization.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.1998.tb02722.x
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  • 5
    Electronic Resource
    Electronic Resource
    High-Resolution Electron Microscopy and Convergent-Beam Electron Diffraction of Sintered Undoped Hydroxyapatite (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: Transmission electron microscopy (TEM) was used to characterize the microstructure of sintered undoped hydroxyapatite (OHAp). Conventional TEM observations were accom-panied by high-resolution electron microscopy (HREM) and convergent-beam electron diffraction (CBED) studies. CBED analysis enabled the determination of the space group of the OHAp, P63/m. Densification of the material was performed by pressureless sintering at 1250°C for 30 min. The undoped sample was comprised of apatite as the only crystalline phase, in addition to a small volume fraction (4%) of closed porosity. In general, the observed microstructure was homogeneous and consisted of equiaxed apatite grains with an average grain diameter of }1–2μm. No indication of a residual amorphous phase, which may have formed during sintering, was observed at multigrain junctions. HREM studies on grain boundaries also revealed that no intergranular glass film was present along two-grain junctions, indicating that densification proceeded without a liquid phase. A slightly disordered region at the interfaces was observed, suggesting an extended grain-boundary structure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.1997.tb02788.x
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  • 6
    Electronic Resource
    Electronic Resource
    Microstructure and Fracture Toughness of Si3N4 Ceramics: Combined Roles of Grain Morphology and Secondary Phase Chemistry (1999)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 82 (1999), 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 materials that contained different mixtures of sintering aids were investigated with respect to microstructure development and resulting fracture toughness. Postsintering annealing at 1850°C for various times was adopted in order to coarsen the respective microstructures. Although constant processing conditions were used, a marked variation in fracture toughness of the Si3N4 materials was evaluated. With a larger grain diameter of the Si3N4 grains, an increase in fracture resistance was generally observed. However, a correlation between fracture toughness and apparent aspect ratio could not be established. The observed changes in microstructure were in fact caused by the difference in secondary-phase chemistry. Si3N4 grain growth was dominated by diffusion-controlled Ostwald ripening and was hence affected by the viscosity of the liquid at processing temperature. In addition, crystallization at triple pockets also depends on the sintering additives employed and was found to influence fracture toughness by altering the crack-propagation mode as a consequence of local residual microstresses at grain boundaries. The stress character (compressive vs tensile) is governed by the type of crystalline secondary phase formed. Moreover, a variation in interface chemistry changes the glass network structure on the atomic level, which can promote transgranular fracture, i.e., can result in a low fracture resistance even in the presence of favorable large Si3N4 matrix grains. Therefore, secondary-phase chemistry plays a dominant role with respect to the mechanical behavior of liquid-phase-sintered Si3N4. Fracture toughness is, in particular, influenced by (i) altering the residual glass network structure, (ii) affecting the secondary-phase crystallization at triple pockets, and (iii) changing the Si3N4 grain size/morphology by affecting the diffusion rate in the liquid. The first two effects of secondary-phase chemistry are superimposed on the merely structural parameters such as grain diameter and apparent aspect ratio.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.1999.tb02009.x
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  • 7
    Electronic Resource
    Electronic Resource
    Viscous Slip along Grain Boundaries in Chlorine-Doped 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: The effect of chlorine doping on the anelastic-relaxation and torsional-creep behavior of a silicon nitride (Si3N4) polycrystalline body was studied. Two model polycrystals—one undoped and the other doped with a small fraction of chlorine—were investigated. Their microstructures consisted of equiaxed and well-faceted Si3N4 grains whose boundaries were separated by a continuous, nanometer-sized film of silica (SiO2) glass. The actual presence of chlorine in the doped polycrystal was ascertained by ion chromatography and is thought to be enriched at the grain boundaries. The effect of chlorine on the intergranular film structure was characterized by high-resolution electron microscopy. The micromechanical response of the SiO2 grain boundary under shear stress was monitored up to very high temperatures (i.e., ∼2000°C) by internal-friction and torsional-creep experiments. The presence of the chlorine dopant, which is a network modifier of SiO2 glass that also causes a widening of the grain-boundary film, significantly lowered the bulk viscosity of the residual glass. As a consequence of the change in grain-boundary chemistry, the internal-friction curve of the chlorine-doped material shifted toward lower temperatures and the torsional-creep rate markedly increased, as compared to the undoped material. According to a viscoelastic model of the Si3N4 polycrystal, the internal-friction data resulted as a superposition of two individual components: (i) a relaxation peak that is related to the anelastic slip mechanism along grain boundaries and (ii) a background component that results from an irreversible diffusional-creep mechanism.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.1997.tb03125.x
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  • 8
    Electronic Resource
    Electronic Resource
    Suppression of Viscous Grain-Boundary Sliding in a Dilute SiAION Ceramic (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: Aluminum nitride (AIN) and alumina (Al2O3) additives were blended into a high-purity silicon nitride (Si3N4) powder, and optimized processing conditions were applied for promoting a sintering process, which was shown to partly remove the amorphous silica-rich (SiO2-rich) remains from grain boundaries. Such an approach led to the formation of a dilute SiAlON structure. The microstructure of the obtained SiAlON material consisted of two distinct types of matrix phases, namely, a main network of equiaxed (dilute) ß-SiAlON grains, with a minor fraction of (oxygen-rich) acicular O'-SiAlON grains. As a consequence of the resulting solid solution of aluminum plus oxygen in the Si3N4 lattice, many grain boundaries were free from any amorphous interlayer, as revealed by high-resolution electron microscopy (HREM) observation. Other than some boundaries that were still wetted, small amounts of residual glass were noted at some triple-grain pockets. However, the average size of such glass pockets was only a few nanometers. The observed microstructural characteristics were critical for the anelastic response of the present SiAlON material, as compared with undoped Si3N4, whose grains were continuously encompassed by an amorphous SiO2 film. Internal-friction data that were collected up to very high temperatures showed that, in the present SiAlON material, no anelastic relaxation peak was monitored. Thus, viscous sliding along grain boundaries was inhibited. In addition, the partial elimination of the amorphous SiO2 film also implied that no continuous path for oxygen diffusion was available along grain boundaries. Because of this important circumstance and despite the possible softening effect due to solid solution, the viscoelastic (background) component of the internal-friction data shifted toward higher temperatures and the creep resistance of the material was improved.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.1997.tb03126.x
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  • 9
    Electronic Resource
    Electronic Resource
    A covalent micro/nano-composite resistant to high-temperature oxidation (1995)
    [s.l.] : Nature Publishing Group
    Nature 374 (1995), S. 526-528 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] ADVANCED ceramic materials that can withstand high temperatures (over 1,500 °C) without degradation or oxidation are needed for applications such as structural parts for motor engines, gas turbines, catalytic heat exchangers and combustion systems1,2. Hard, ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/374526a0
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  • 10
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    Effect of residual microstresses at crystalline multigrain junctions on the toughness of silicon nitride (1999)
    Elsevier
    Details
    Publication Date: 1999-04-01
    Print ISSN: 0955-2219
    Electronic ISSN: 1873-619X
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Published by Elsevier
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