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  • 1
    Electronic Resource
    Electronic Resource
    Carbon nanotubes as nanoscale mass conveyors (2004)
    [s.l.] : Macmillian Magazines Ltd.
    Nature 428 (2004), S. 924-927 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The development of manipulation tools that are not too ‘fat’ or too ‘sticky’ for atomic scale assembly is an important challenge facing nanotechnology. Impressive nanofabrication capabilities have been demonstrated with scanning probe manipulation of atoms and molecules ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/nature02496
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  • 2
    Electronic Resource
    Electronic Resource
    Fracture of synthetic diamond (1995)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 78 (1995), S. 3083-3088 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The fracture behavior of synthetic diamond has been investigated using indentation methods and by the tensile testing of pre-notched fracture-mechanics type samples. Specifically, the fracture toughness of free-standing diamond plates, grown by chemically-vapor deposited (CVD) methods, was measured using Vickers indentations and by the use of disk-shaped compact-tension specimens; the latter method provides an evaluation of the through-thickness fracture properties, whereas the indentation method was performed on the nucleation surface of the sample. Measured fracture toughness (Kc) values were found to be approximately 5–6 MPa(square root of)m by both methods, indicating that the fracture resistance of CVD diamond does not vary appreciably with grain size (within the certainty of the testing procedures). Complications, however, arose with the fracture-mechanics testing regarding crack initiation from a relatively blunt notch; further work is needed to develop pre-cracking methods to permit more reliable fracture toughness testing of diamond. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.360060
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  • 3
    Electronic Resource
    Electronic Resource
    Abrasive Wear Behavior of Heat-Treated ABC-Silicon Carbide (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: Hot-pressed silicon carbide, containing aluminum, boron, and carbon additives (ABC-SiC), was subjected to three-body and two-body wear testing using diamond abrasives over a range of sizes. In general, the wear resistance of ABC-SiC, with suitable heat treatment, was superior to that of commercial SiC. When the fine-scale (3 μm) diamond abrasives were used, it was found that thermal annealing at 1300°C increased the resistance to three-body wear by a factor of almost three, and two-body wear by a factor of almost two, compared with as-hot-pressed samples. Higher annealing temperatures, however, led to a decline in wear resistance from its highest value. Similar behavior was seen for 1300°C-annealed samples subjected to 15 μm diamond abrasive, although higher-temperature annealing at 1500°–1600°C enhanced the wear resistance again. When coarse abrasives (72 μm) were used, the wear resistance progressively increased with increased annealing temperature from ∼1000° to 1600°C. Corresponding transmission and scanning electron microscopy studies indicated that, whereas transgranular, conchoidal cracking was dominant in the mild abrasive wear with fine-scale (3 μm) abrasives, intergranular cracking and subsequent grain pullout was far more predominant in the more severe abrasive wear with coarse abrasives. Because the hardness and indentation toughness were barely altered during thermal treatment, the observed wear behavior was attributed mainly to the thermally induced microstructural changes, including the crystallization of glassy grain-boundary films, the possible strengthening of the boundaries due to the enhancement of the aluminum, and the formation of aluminum-rich, coherent nanoscale precipitates in the matrix grains above 1300°C.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2003.tb03478.x
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  • 4
    Electronic Resource
    Electronic Resource
    Fracture and Fatigue Behavior at Ambient and Elevated Temperatures of Alumina Bonded with Copper/Niobium/Copper Interlayers (2002)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 85 (2002), 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: Interfacial fracture toughness and cyclic fatigue-crack growth properties of joints made from 99.5% pure alumina partially transient liquid-phase bonded using copper/niobium/copper interlayers have been investigated at both room and elevated temperatures, and assessed in terms of interfacial chemistry and microstructure. The mean interfacial fracture toughness, Gc, was found to decrease from 39 to 21 J/m2 as temperature was raised from 25° to 1000°C, with failure primarily at the alumina/niobium interfaces. At room temperature, cyclic fatigue-crack propagation occurred both at the niobium/alumina interface and in the alumina adjacent to the interface, with the fatigue threshold, ΔGTH, ranging from 20 to 30 J/m2; the higher threshold values in that range resulted from a predominantly near-interfacial (alumina) crack path. During both fracture and fatigue failure, residual copper at the interface deformed and remained adhered to both sides of the fracture surface, acting as a ductile second phase, while separation of the niobium/alumina interface appeared relatively brittle in both cases. The observed fracture and fatigue behavior is considered in terms of the respective roles of the presence of ductile copper regions at the interface which provide toughening, extrinsic toughening due to grain bridging during crack propagation in the alumina, and the relative crack propagation resistance of each crack path, including the effects of segregation at the interfaces found by Auger spectroscopy.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2002.tb00491.x
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  • 5
    Electronic Resource
    Electronic Resource
    Light emission during fracture of a Zr–Ti–Ni–Cu–Be bulk metallic glass (1999)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 74 (1999), S. 3809-3811 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A light emission phenomenon observed during dynamic fracture of a bulk metallic glass, Zr41.2Ti13.8Cu12.5Ni10Be22.5 (at. %), has been investigated using Charpy V-notch impact specimens. Unlike more conventional crystalline metals, these Zr-based amorphous alloys emit intense flashes of visible light when ruptured. The mechanisms for this surprising behavior are unknown and the phenomenon remains uncharacterized. Here we report spectroscopic measurements of the light emitted from specimens fractured in both room air and nitrogen gas. Spectra acquired from specimens ruptured in air exhibited a single broad peak, which could be fit to a blackbody temperature of ∼3175 K. Emission from specimens fractured in nitrogen, however, was at least four orders of magnitude less intense. The spectrum was shifted to the red with an effective blackbody temperature of ∼1400 K. Fracture surfaces of specimens ruptured in both air and nitrogen exhibited local melting, providing further evidence of intense heating during fracture. Based on these observations we argue that the intense light emission in air is associated with pyrolysis of fresh material exposed during rupture. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.124187
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  • 6
    Electronic Resource
    Electronic Resource
    Fracture toughness and fatigue-crack propagation in a Zr–Ti–Ni–Cu–Be bulk metallic glass (1997)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 71 (1997), S. 476-478 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The recent development of metallic alloy systems which can be processed with an amorphous structure over large dimensions, specifically to form metallic glasses at low cooling rates (∼10 K/s), has permitted novel measurements of important mechanical properties. These include, for example, fatigue-crack growth and fracture toughness behavior, representing the conditions governing the subcritical and critical propagation of cracks in these structures. In the present study, bulk plates of a Zr41.2Ti13.8Cu12.5Ni10Be22.5 alloy, machined into 7 mm wide, 38 mm thick compact-tension specimens and fatigue precracked following standard procedures, revealed fracture toughnesses in the fully amorphous structure of KIc∼55 MPa(square root of)m, i.e., comparable with that of a high-strength steel or aluminum alloy. However, partial and full crystallization, e.g., following thermal exposure at 633 K or more, was found to result in a drastic reduction in fracture toughness to ∼1 MPa(square root of)m, i.e., comparable with silica glass. The fully amorphous alloy was also found to be susceptible to fatigue-crack growth under cyclic loading, with growth-rate properties comparable to that of ductile crystalline metallic alloys, such as high-strength steels or aluminum alloys; no such fatigue was seen in the partially or fully crystallized alloys which behaved like very brittle ceramics. Possible micromechanical mechanisms for such behavior are discussed. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.119610
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  • 7
    Electronic Resource
    Electronic Resource
    Crack-Growth Resistance-Curve Behavior in Silicon Carbide: Small versus Long Cracks (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: Crack-growth resistance-curve (R-curve) behavior for small (〈400 μm) surface cracks and long (〉3 mm) through-thickness cracks is examined in two silicon carbide (SiC) ceramics that have sharply contrasting fracture properties. The first, an in-situ toughened material designated ABC-SiC, fails by intergranular fracture, whereas the second, a commercial SiC (Hexoloy SA), fails by transgranular cleavage. In the former microstructure, hot pressing with aluminum, boron, and carbon additives yields a network of plate-shaped grains, and the presence of an amorphous grain-boundary film that is ∼1 nm thick promotes debonding and crack deflection. The resultant grain bridging generates R-curve toughening; in contrast, no evidence of crack-tip shielding is observed in Hexoloy SA. R-curve behavior has been evaluated using two techniques for the different crack-length regimes: a small-crack R-curve has been deconvoluted from indentation-strength data and a long-crack R-curve has been directly measured using fatigue-precracked, disk-shaped compact-tension specimens. Although Hexoloy SA fails catastrophically at 〈3 MPa.m1/2, ABC-SiC exhibits much-improved flaw tolerance with significant rising R-curve behavior and a steady-state fracture toughness of ∼9 MPa.m1/2 after crack extension of ∼600 μm. In ABC-SiC, however, differences in the behavior of long and small cracks exist for crack sizes of less than ∼120 μm, with the small-crack measurements demonstrating much-reduced crack-growth resistance; this effect is not observed in Hexoloy SA. Microstructural sources of this behavior are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.1997.tb03115.x
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  • 8
    Electronic Resource
    Electronic Resource
    Effects of Moisture on Grain-Boundary Strength, Fracture, and Fatigue Properties of Alumina (2005)
    Oxford, UK : 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: The role of moisture in affecting both intrinsic and extrinsic aspects of the fracture and fatigue-crack growth resistance of a polycrystalline alumina (99.5% pure, 25 μm grain size) has been examined in both moist and dry environments at ambient temperature. The intrinsic (crack-tip) toughness, deduced from measured crack-opening profiles, is found to be less than for a single crystal and is 30% lower (∼0.6 MPa·m1/2) in moist air versus in dry N2, implying that the grain-boundary theoretical strength is higher in a dry environment. Despite this, in dry atmospheres, the R-curves (which derive from crack deflection and grain bridging) initially rose more steeply and nominal fatigue-crack growth thresholds for short crack sizes (20–60 μm) were more than 1.3 MPa·m1/2 higher. Owing to this quicker crack bridging development, strengths for natural flaws could be more than doubled in dry atmospheres, a difference that well exceeds the effect solely due to the intrinsic toughness change. After ∼2 mm of crack growth, however, the R-curve and steady-state fatigue behavior appeared similar in both environments, although altering the atmosphere for such fatigue cracks in situ induced large, abrupt changes in transient growth rates. The environment influences the nature of the bridging zones, with uncracked-ligament bridges playing a larger role in dry atmospheres, while frictional bridges are predominant in moist air. Evidently, to achieve optimal toughness in bridging ceramics, the window for the requisite grain-boundary strength may be small; whereas weak boundaries are required to induce the necessary intergranular fracture, if too weak, shallower R-curves, less strengthening, and poorer fatigue resistance all follow.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1551-2916.2005.00434.x
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  • 9
    Electronic Resource
    Electronic Resource
    On the Effect of Local Grain-Boundary Chemistry on the Macroscopic Mechanical Properties of a High-Purity Y2O3-Al2O3-Containing Silicon Nitride Ceramic: Role of Oxygen (2005)
    Oxford, UK : 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: The effects of grain-boundary chemistry on the mechanical properties of high-purity silicon nitride ceramics have been investigated, specifically involving the role of oxygen, present along the grain boundaries, in influencing the fracture behavior. To avoid complications from inadvertently introduced impurities, studies were performed on a high-purity Si3N4 processed using two-step gas-pressure-HIP sintering. Varying the grain-boundary oxygen content, which was achieved by control of oxidizing heat treatments and sintering additives, was found to result in a transition in fracture mechanism, from transgranular to intergranular fracture, with an associated increase in fracture toughness. This phenomenon is correlated to an oxygen-induced change in grain-boundary chemistry and possibly to a concomitant structural transformation along the interface. The incorporation of oxygen appears to affect fracture by “weakening” the interface, thus facilitating debonding and crack advance along the boundaries, and hence to toughening by grain bridging. It is concluded that if the oxygen content in the thin grain-boundary films exceeds a lower limit, which is ∼0.87 equiv% oxygen content, then the interfacial structure and bonding characteristics favor intergranular debonding during crack propagation; otherwise, transgranular fracture ensues.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1551-2916.2005.00412.x
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  • 10
    Electronic Resource
    Electronic Resource
    Crack-Size Effects on Cyclic and Monotonic Crack Growth in Polycrystalline Alumina: Quantification of the Role of Grain Bridging (2004)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of the American Ceramic Society 87 (2004), 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 role of grain bridging in affecting the initial rising portion of the R-curve and the transient, non-steady-state behavior of short cracks during (cyclic) fatigue-crack propagation has been quantitatively examined in a 99.5% pure alumina. Fatigue-crack growth properties for both long and short (Δaf 〈 2 mm) cracks emanating from machined notches (root radius, ∼ 15–150 μm) were investigated, where Δaf is the extension of the fatigue crack from the notch. Growth rates (da/dN) were far higher at the same applied stress-intensity range (ΔK) and fatigue thresholds, ΔKTH, were markedly lower for short cracks than for corresponding long cracks. Crack extension was measured at the lowest driving forces for short cracks emanating from razor micronotches with ∼ 15 μm. For growth rates 〈10-8 m/cycle, da/dN vs ΔK curves for short cracks merged with the demonstrably steady-state curve for long cracks after ∼2 mm of crack extension. This length corresponds well to the extent of the measured crack-bridging zone for a near-threshold steady-state fatigue crack. For da/dN 〉 10-8 m/cycle, however, non-steady-state behavior was observed at all crack sizes, indicating that achieving steady state at each ΔK level is difficult. The crack-tip shielding contribution due to such grain bridging was determined using both direct compliance and the more accurate multi-cutting/crack-opening profile techniques. Bridging stress-intensity factors were computed and subtracted from the applied stress intensities to estimate an effective (near-tip) driving force, ΔKeff These results provided (i) a lower threshold (in terms of ΔKeff) below which both long and short fatigue cracks should not propagate, and (ii) an estimate of the intrinsic toughness, K0, for the start of the R-curve. Such results quantitatively affirm that the reduced role of grain bridging is a primary source of the transient behavior of short cracks in grain-bridging alumina-based ceramics under cyclic loading.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1551-2916.2004.00093.x
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