Volume 98, Issue 12

A number of recent studies claim that silicate glasses fracture by the formation, growth and coalescence of cavities at crack tips, in the same way as metals, but at a much smaller scale. Evidence for cavity formation comes from the examination of side surfaces of fracture mechanics specimens, at the point where the crack tip intersects the free surface. Such measurements exhibit small depressions in regions that are supposedly located in front of moving crack tips. These depressions were interpreted as cavities. In this paper, we summarize experimental results obtained using an atomic force microscope to characterize the fracture surfaces. The experimental results demonstrate an absence of residual damage on fracture surfaces that could be interpreted as cavity formation. We also observe cracks moving in glass and show that the features reported as cavities actually occur behind and not in front of the moving crack. A simulation of an atomic force microscope probe passing over the emerging tip of a crack in glass suggests that the features identified as cavities are in fact due to the roughness of the specimen surface. Our results support the view that cracks in glass propagate by brittle fracture. We find no evidence for nanoscale ductility in silicate glasses.

The maximum operating temperature of conventional thermal barrier coatings based on yttria-stabilized zirconia is ultimately limited by de-stabilization of the “non-transformable” t phase, rendering it susceptible to the monoclinic transformation upon cooling. Investigations into alternative thermal barrier oxide compositions suggest that Yb offers superior t stability compared with Y, Sc and larger rare earth cations at the same concentration. The present study sheds light on this behavior by comparing the microstructure evolution of specimens with 7.6 and 11.4 % MO 1.5 (M = Y or Yb) heat treated at 1450 °C for times up to 512 h. X-ray diffractometry and transmission electron microscopy revealed that the onset of partitioning occurs at short times but then the compositions of the phases evolve slowly over time until sufficient stabilizer is depleted from the t phase to render it transformable. Substitution of Yb for Y delays the onset of monoclinic formation. Differences in the transformation behavior of the Y and Yb rich phases on cooling provide new insight and suggest refinements to the current thermodynamic models for the binary ZrO 2 – MO 1.5 systems are needed.

The use of silicon-based ceramics in power generation applications is limited by recession of the passivating SiO 2 when it reacts with water-vapor to form a volatile hydroxide. Environmental barrier coatings are used to prevent diffusion of reactive species to the structural ceramic substrate. Often these barriers are multilayered and/or multiphase to provide optimal thermal expansion match, microstructural stability, as well as low diffusivities. High energy X-rays are used to assess internal stress as well as phase content and stability in two coating systems: alloyed Ta 2 O 5 for Si 3 N 4 and strontium-aluminosilicates for SiC/SiC. In particular, the role of phase transitions and precipitation on the coating internal stresses is evaluated along with their ramifications for coating lifetime.

The elastodynamic response of a columnar thermal barrier coating to normal impact by a spherical particle is explored using an explicit finite element method. The transient stress state involves a tensile component at the edge of each column, and this may lead to erosion. The contact problem can be simplified to the impact of a single column by an inclined wedge. The peak tensile stress is determined as a function of geometry of column and level of friction between wedge and columns. The initiation of a crack from a pre-existing flaw within the column is explored. The implications of the various calculations are discussed for erosion of thermal barrier coatings.

A method to create networks of intricate fluidic channels formed from metals and ceramics is proposed and demonstrated. The method exploits buckle delamination of a thin compressed film bonded to a substrate. A low adhesion layer coinciding with the desired layout of the channel network is laid down prior to deposition of the film. Once triggered, the buckle delamination propagates along the low adhesion pathways driven by release of the elastic energy stored in the film, assembling the entire channel network without external intervention. Strips, tapered strips and a selection of grids are demonstrated for diamond-like carbon films bonded to Si substrates with gold providing low adhesion. Control of the film thickness (15 to 260) and the width of the low adhesion regions (200 to microns) enables the cross-sectional area of the channel to be defined precisely with height determined by the buckle amplitude (40 to 500). The channel network has been integrated with a microfluidic interface formed from polydimethylsiloxane. Pressure-driven flow of two miscible streams shows convectively enhanced mixing in these nanoscale buckled channels.

Oxide growth stresses were measured in situ at 1100 °C on commercial Pt-modified NiAl and NiCoCrAlY bond coat alloys using synchrotron X-rays. Measurements were taken on samples that had no preoxidation, as well as on samples that had experienced 24 one-hour thermal exposures at 1150 °C, a condition known to induce rumpling in the Pt-modified NiAl alloy, but not in the NiCoCrAlY alloy. The NiCoCrAlY alloy showed continuous stress relaxation under all conditions, whereas the Pt-modified NiAl alloys would typically stabilize at a fixed (often non-zero) stress suggesting a higher creep strength in the “Thermally Grown Oxide” on the latter alloy, though the precise behavior was dependent on initial surface preparation. The formation of martensite in the Pt-modified NiAl alloys was also observed upon cooling and occurred at temperatures below 200 °C for all of the samples observed. Based on existing models, this M s temperature is too low to account for the rumpling observed in these alloys.

We assess density functional theory studies of the effects of interfacial stoichiometry, Al activity, S segregation and Hf doping on the adhesion of the γ-Ni(Al)/α-Al 2 O 3 interface. Computations of the Al activity in γ-Ni(Al) and of the interfacial phase diagram between 1300 and 1600 K suggest that the interface phase is Al-rich, but close to the boundary with the stoichiometric phase. We reveal that the Al-rich phase has significantly stronger adhesion than the stoichiometric phase and that S substantially decreases the adhesion of both. We demonstrate that doping with Hf yields a substantial improvement in adhesion, manifest in three ways: (i) It can pin S in bulk γ-Ni(Al), even up to 1600 K. (ii) It segregates and, once there, can strengthen the relatively weak stoichiometric interface (attaining a work of separation comparable to that for the strong Al-rich interface). (iii) It has the potential to displace S from interstitial interface sites.

Epitaxial relations are often described by the mismatch of lattice parameters between the two materials. When the structures forming the epitaxial interface are dissimilar, the lattice mismatch is often less meaningful because of the different geometrical arrangement of atoms in the two structures. In a previous paper, it was shown that two rules of crystal chemistry, widely known as Pauling's rules, namely cation/anion coordination and charge balance, could be applied to understand the epitaxy of (0001) ZnO (wurtzite) on (111) MgAl 2 O 4 (spinel) substrates. It was then shown that the same rules could be applied to predict and confirm the epitaxy of (101) SiO 2 (cristobalite) on (0001) Al 2 O 3 (sapphire). Here, the same rules are applied to explain the epitaxy of four other dissimilar structures that have been previously published by others, including SrTiO 3 on Ni c (2 × 2) S, TiO 2 (anatase) on LaAlO 3 , TiO 2 (rutile) on r-plane Al 2 O 3 , and Zr 1− x (Y x )O 2 on (0001) Al 2 O 3 .

The bow quality determines the ease with which an experienced violinist can produce a high-quality sound. Factors influencing its playability are partly structural (shape, point of balance) and partly material dependent (mass, stiffness, mechanical damping). Pernambuco, a tropical wood, most revered for high-quality violin bows is practically extinct in the wild. Measuring the properties of pernambuco and thirteen alternative materials by a resonant bar technique and mechanical spectroscopy in both the vacuum- and the air-dry states and comparing their performance objectively using property charts and performance indices reveals that bow-grade pernambuco has a high bending stiffness per unit mass, a high speed of sound parallel to the wood grain, and an exceptionally low loss coefficient at room temperature, and that, on purely mechanical grounds, materials exist that could make as good violin bows as pernambuco. The consequences of the use of such alternatives for bow design are described.

The wetting of low melting point metals and Si – Ca – Al – Ti – O glasses on molybdenum has been investigated. The selected metals (Au, Cu, Ag) form a simple eutectic with Mo. Metal spreading occurs under nonreactive conditions without interdiffusion or ridge formation. The metals exhibit low (non-zero) contact angles on Mo but this requires temperatures higher than 1100 °C in reducing atmospheres in order to eliminate a layer of adsorbed impurities on the molybdenum surface. By controlling the oxygen activity in the furnace, glass spreading can take place under reactive or nonreactive conditions. We have found that in the glass/Mo system the contact angle does not decrease under reactive conditions. In all cases, adsorption from the liquid seems to accelerate the diffusivity on the free molybdenum surface.

Compressive creep of La-monazite is investigated in the temperature range 1100 °C to 1500 °C. The study includes both high-purity single-phase material and material with excess phosphorus located in amorphous grain boundary phases. The results indicate that the presence of small amounts of excess P in polycrystalline LaPO 4 has a large effect on microstructural stability and creep at high temperature. Materials with La/P ratio close to unity (within ∼500 ppm) show little grain growth at temperatures up to 1400 °C and deform by creep at rates similar to those of alumina and zirconia, with stress exponent ∼1. Materials containing excess P (as little as ∼1 %) show more rapid grain growth, higher creep rates, and cavitation during creep. The results are compared with creep rates of other refractory oxides and oxide fibers. Implications for the behavior of oxide composites containing La-monazite are considered.

Edge supported metallic sandwich panels subject to underwater blast respond in a manner dependent on the relative timescales for water cavitation, core crushing, and structural response in bending. This article examines the response to impulses representative of an (especially relevant) domain: wherein the water cavitates before the core crushes. In some cases, core crushing ceases before the overall structural deformation is complete, leading to what is termed strong-core response. In other situations, core crushing continues beyond the stage at which the dry-face of the sandwich panel ceases to deform, a behavior that is known as weak-core response. It is known that weak-core response provides superior panel performance in terms of three performance metrics: the back-face deflection, the tearing susceptibility of the faces and the loads transmitted to the supports. The relevant regime of behavior is determined by the magnitude of the transmitted impulse from the water, the relative strength of the core, and the aspect ratio of the panel, so that a panel must be designed carefully against a known threat to achieve optimal performance. This paper initiates the development of a map of response for sandwich panels having I-core geometry, delineating strong, weak and other behavior in terms of some of the design parameters mentioned above. The effect of fluid – structure interaction is included in the simulations, and these results are contrasted with those in which fluid – structure interaction is omitted and an initial impulse is instead given to the wet-face of the panel.

Two-dimensional, microstructure-based finite element simulations were used to elucidate the influence of grain orientation configuration on the thermal-elastic response of polycrystalline ceramic materials. The two main constituent minerals of marbles, calcite and dolomite, were considered. The crystallographic configuration of the grains is described in terms of the distribution of grain orientations and grain-boundary misorientations. To probe the influence of grain orientation configuration, we first generated the geometry of a hypothetical microstructure. Next, crystallographic orientations were assigned to each grain in the microstructure such that the grain orientations and grain-boundary misorientations matched predefined distributions. By varying the predefined distributions, we generated 45 unique microstructures covering a wide range of crystallographic configurations. After assigning thermal-elastic properties to each structure, corresponding to either calcite or dolomite, finite-element simulations were performed. The simulations demonstrated that both the orientation and misorientation distributions have a substantial impact on the thermal-elastic response of microstructures with the same material properties: varying the crystallographic configuration results in variations of stored elastic strain energy density from 10 kJ m −3 to 39 kJ m −3 for a 100 °C temperature change. Further, the ratio of the bulk coefficient of thermal expansion in the two principal directions varies from − 0.17 to 1.46. Surprisingly, the grain-boundary misorientations alone had a substantial impact on the thermal-elastic response of the system. By simply rearranging a fixed set of crystallographic orientations to obtain different nearest-neighbor misorientation configurations, we observed variations in strain energy density from 10 kJ m −3 to 39 kJ m −3 and variations in thermal expansion ratio from 0.06 to 0.90. The results suggest that to predict accurately the thermal-elastic response of polycrystalline ceramics, it is critical to consider both the distribution of grain-boundary misorientations as well as the distribution of grain orientations.

Pyramidal truss sandwich cores with relative densities in the range 1 – 10 % have been made from carbon fiber reinforced polymer laminates using a snap-fitting method. The measured quasi-static uniaxial compressive strength increased with increasing from 1 to 11 MPa over the relative density range investigated here. A robust face-sheet/truss joint design was developed to suppress truss – face sheet node fracture. Core failure then occurred by either (i) Euler buckling () or (ii) delamination failure () of the struts. Micro-buckling failure of the struts was not observed in the experiments reported here. Analytical models for the collapse of the composite cores by Euler bucking, delamination failure and micro-buckling of the struts have been developed. Good agreement between the measurements and predictions based on the Euler buckling and delamination failure of the struts is obtained. The micro-buckling analysis indicates this mechanism of failure is not activated until delamination is suppressed. The measurements and predictions reported here indicate that composite cellular materials with a pyramidal micro-structure reside in a gap in the strength versus density material property space, providing new opportunities for lightweight, high strength structural design.

Reactions between MgAl 2 O 4 and LiF during low-temperature sintering have been examined by transmission electron microscopy. It is shown that the presence of LiF at elevated temperatures leads to the formation of a defect MgAl 2 O 4 spinel phase containing oxygen vacancies due to the incorporation of both Li and F ions into the crystal lattice. Consequently, densification is promoted via the diffusion of oxygen vacancies. Note that neither of the lithium aluminate phases, LiAlO 2 and Li 2 Al 4 O 7 , were detected.

The objective of this investigation was to determine the notch-tip strain fields in Type-I collagen biomaterials. Experiments were performed in-situ in a fixture mounted on the stage of an optical stereozoom microscope with edge-notched Type-I collagen tensile specimens. Image markers were added around the notch, and a tensile strain was applied. High-resolution images were collected at different levels of far-field strain. Image analysis was performed to measure the displacements of individual surface markers. The resultant displacement fields were analyzed to yield in-plane surface strain profiles along trajectories perpendicular to the crack plane. The results revealed significant local strain amplifications near the notch tip of nearly twenty times. These results are discussed in light of existing constitutive models for crack-tip strain fields.