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  • 1
    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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  • 2
    Electronic Resource
    Electronic Resource
    Rheological Behavior of Dilute SiAlON with or without Intergranular X-Phase (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: Dense nearly single-phase β′-SiAlON materials (with substitutional level z∼ 1) have been prepared by hot isostatic pressing and their high-temperature deformation behavior has been investigated using low-frequency damping and torsional creep experiments. Addition of a small fraction of AlN (∼0.5 wt%) to the starting (nominally z= 1) SiAlON powder enabled us to “balance” the excess SiO2 which likely arises from surface contamination of the starting SiAlON powder upon exposure to atmosphere. As a result, a fine-grained β′-SiAlON polycrystal free of residual (glassy) X-phase segregated to grain boundaries could be prepared. This microstructure is in contrast with that found for an “unbalanced” composition prepared from the same raw β′-SiAlON powder but without the corrective AlN addition. In this latter case, residual glass (X-phase), consisting of Al-rich SiO2, was entrapped at multiple grain junctions. The presence of such a low-melting intergranular glass dominates the high-temperature deformation behavior of the dilute SiAlON material, involving marked degradation of creep resistance and significant damping relaxation due to grain-boundary sliding. “Balancing” the SiAlON microstructure with a small addition of AlN enabled us to suppress anelastic relaxation by grain-boundary sliding and to increase the creep resistance of the material by more than 1 order of magnitude.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2001.tb00705.x
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  • 3
    Electronic Resource
    Electronic Resource
    Grain-Boundary Viscosity of Preoxidized and Nitrogen-Annealed Silicon Carbides (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: Internal friction experiments were conducted on a model SiC polycrystal prepared from preoxidized (high-purity) SiC powder. This material contained high-purity SiO2 glass at grain boundaries in addition to a free-carbon phase, which was completely removed upon powder preoxidation. Comparative tests were conducted on a SiC polycrystal, obtained from the as-received SiC powder with the addition of 2.5 vol% of high-purity SiO2. This latter SiC material was also investigated after annealing at 1900°C for 3 h in a nitrogen atmosphere. Electron microscopy observations revealed a glass-wetted interface structure in SiC polycrystals prepared from both as-received and preoxidized powders. However, the former material also showed a large fraction of interfaces coated by turbostratic graphite. Upon high-temperature annealing in nitrogen, partial glass dewetting occurred, and voids were systematically observed at multigrain junctions. The actual presence of nitrogen could only be detected in a limited number of wetted interfaces. A common feature in the internal friction behavior of the preoxidized, SiO2-added and nitrogen-annealed SiC was a relaxation peak that resulted from grain-boundary sliding. Frequency-shift analysis revealed markedly different characteristics for this peak: both the magnitude of the intergranular glass viscosity and the activation energy for grain-boundary viscous flow were much higher in the nitrogen-annealed material. Results of torsional creep tests were consistent with these findings, with nitrogen-annealed SiC being the most creep resistant among the tested materials.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2001.tb01017.x
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  • 4
    Electronic Resource
    Electronic Resource
    Structure and Viscosity of Grain Boundary in High-Purity SiAlON Ceramics (2000)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 83 (2000), 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: Three high-purity SiAlON materials (Si6−zAlzOzN8−z, z= 1, 2, 3) were characterized with respect to both structure and viscous behavior of internal grain boundaries. Internal friction experiments provided a direct measure of the intrinsic viscosity of grain boundaries and concurrently revealed the occurrence of a grain-boundary interlocking mechanism that suppressed sliding. A residual glass phase (consisting of aluminum-rich SiO2) and nanometer-sized mullite residues were found at glassy triple-grain junctions of the z= 1 SiAlON. A low-melting intergranular phase dominated the high-temperature behavior of this material and caused grain-boundary sliding at temperatures as low as 1100°C. A quantitative analysis of the grain-boundary internal friction peak as a function of oscillation frequency indicated an intergranular film viscosity of log η∼ 7.5 Pa · s at 1100°C. Glass-free grain boundaries were a characteristic of SiAlON materials with z≥ 2, which yielded a significant improvement in refractoriness as compared to the z= 1 SiAlON material. In these materials, relaxation resulting from grain-boundary sliding was suppressed, and the internal friction curve simply experienced an exponential-like increase.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2000.tb01589.x
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  • 5
    Electronic Resource
    Electronic Resource
    Mechanical Damping Arising from Dislocation Motion in Sapphire and Ruby Crystals (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: Internal friction was measured up to the melting point (i.e., Tm≈ 2050°C) in sapphire and two ruby single crystals (chromium content of 0.1 and 0.5 wt%), as a function of strain amplitude, temperature, and composition. The frequency range of the measurements was 6.7–17 Hz. Static (torsional creep) experiments also were performed up to very high temperatures, to provide further phenomenological insight about the mechanism of crystal deformation. Results of dynamic and static experiments were analyzed and discussed in terms of dislocation motion along the basal (0001) plane, perpendicular to which the torsion axes of the crystals were oriented. Particular emphasis was placed on isolating the role of chromium concentration, in regard to impeding dislocation motion and multiplication. An equivalent spring-dashpot model, which attempts to represent the damping results quantitatively, also has been given.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.1997.tb03109.x
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  • 6
    Electronic Resource
    Electronic Resource
    Grain-Boundary Viscosity of BaO-Doped SiC (2000)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 83 (2000), 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 characterization of the viscosity of a residual SiO2/BaO glass, segregated to grain boundaries of polycrystalline SiC, is presented. The anelastic relaxation peak of internal friction, arising from viscous slip along grain boundaries wetted by a glass phase, is analyzed. Two SiC polycrystals, containing SiO2/BaO glasses with different compositions, are studied and compared with a SiC polycrystal containing only pure SiO2. The internal friction peak is first analyzed with respect to its shift upon frequency change. This analysis allows quantitative assessment of both the intrinsic viscosity and the activation energy for viscous flow of the grain-boundary phase. Both parameters markedly decrease with increasing amounts of BaO dopant, which is consistent with data reported in the literature on SiO2 and SiO2/BaO bulk glasses with the same nominal composition. Analysis of the peak morphology is also attempted, considering the evolution of peak width while varying the grain-boundary glass composition. Moreover, the role of microstructural parameters, such as the distributions of grain size and grain-boundary angles, on the broadening of the internal friction peak is addressed, and a procedure is proposed that allows quantitative evaluation of the activation energy for viscous flow of intergranular glass merely from the width of the internal friction peak.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2000.tb01234.x
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  • 7
    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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  • 8
    Electronic Resource
    Electronic Resource
    Mechanistic Analysis of Dislocation Damping in Single-Crystal Magnesia Close to Its Melting Point (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: Single-crystal magnesia (MgO) that contained 0.004 wt% of iron as a major impurity was tested in torsional geometry, with respect to its damping response, up to temperatures close to its melting point. The MgO crystal was oriented so that maximum shear stress would be applied along the (110)〈110〉 primary slip system of its rock-salt cubic structure. Damping data were also collected as a function of torsional strain amplitude and analyzed according to a proposed mechanistic procedure for metallic alloys. The amplitude-dependent decrement of the single-crystal MgO at moderate strain amplitudes (10−5−10−4) qualitatively followed the predictions of the hysteretic dislocation damping theory (i.e., Granato-Lücke theory). Using the results of this analysis, the binding energy that controlled dislocation motion was calculated from damping data. In addition, an attempt to semiquantitatively discuss the dislocation/atom binding energy in comparison with the theoretical prediction for the solute-atom pinning mechanism has also been presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.1998.tb02752.x
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  • 9
    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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  • 10
    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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