Elevated-Temperature Fracture Resistance of a Sintered α-Silicon Carbide

The fracture toughness of a dense, sintered commercial α-silicon carbide was determined for temperatures from 20° to 1400°C using both straight- and chevron-notched test specimens and also controlled-surface-microflaw specimens, all in three-point bending. The flexural strengths were also measured for the same range of temperatures and the trend is compared with that of the toughness. Measurements from this study are discussed and also compared with other results in the literature. Analysis reveals the importance of contrasting sharp crack and blunt crack techniques and also the need for addressing the microhardness indentation method separately. It is concluded that the fracture toughness of this silicon carbide is about 3 MPa · m½ and that the crack growth resistance is characterized by a flat R -curve behavior, both of which are independent of temperature from 20° to 1400°C.     

Influence of Fracture Toughness on the Wear Resistance of Yttria-Doped Zirconium Oxide

Yttria-doped zirconium oxide was prepared in the fully tetragonal, mixed tetragonal and cubic, and the fully cubic phases by varying the yttria conent. The fracture toughnesses of the materials were 11.6, 8.7, and 2.5 Mpa · m½, respectively. The wear resistance, measured in air at room temperature in slow sliding (1 mm/s speed and 9.8 N load), increases by a factor of 1200 from the brittle to the toughest material; it is proportional to the fourth power of toughness. Wear occurs predominantly by fracture in the brittle (cubic) material; plastic deformation is observed in the tougher zirconium oxide.     

Adhesion of Wax Droplets to Porous Polymer Surfaces

An experimental study was done to measure the force of adhesion of molten wax droplets, 3.1 mm in diameter, dropped from heights ranging from 20 to 50 mm onto porous polyethylene and Teflon surfaces. The Teflon surface had 0.25-mm holes drilled in it and the three polyethylene surfaces had random pores with mean diameters of 35, 70, and 125 μm, respectively. The force required to remove the solidified ink from the surface was measured using a pull test. Wax splats were attached to the substrate by both adhesive and cohesive forces. The cohesive force was calculated by multiplying the ultimate tensile strength of the wax (2.2 MPa) by the cross-sectional area of the wax penetrating into surface pores. The adhesive force was obtained by multiplying the contact area between the wax and substrate by the adhesion strength per unit area, estimated to be 0.2 MPa for polyethylene and 0.1 MPa for Teflon surfaces. The contact area between splats and the substrate was typically about 60–70% of the splat area. The edges of splats lifted up, preventing complete contact.     

Effects of CaO on the Strength and Toughness of AIN

Fully dense, undoped AIN is found to have the following average mechanical properties: hardness, 12.2 GPa; strength, 377 MPa; and fracture toughness (based on the fracture load sustained by indented disks), 3.6 MPa·m½. The addition of 2 wt% CaO to AIN changes the fracture mode from transgranular to intergranular while concurrently decreasing the hardness, strenght, and toughness. Increasing the grain size of CaO-doped AlN decreases strength and increases toughness. Toughness estimates based on the fracture load sustained by indented disks are about twice the value determined for crack extension after indentation.     

An Attachment Theory for Microsphere Adhesion

A theory of the mechanics of adhesion between a microsphere and substrate is presented. When a force is applied to an elastic body, the deformation depends not only on the magnitude of the force but also its location and distribution. Molecular adhesion between bodies is a surface force localized to the contact area. In contrast, applied forces such as from gravity, flow fields, inertia, etc., are distributed over the volume (body forces) and/or surface areas. Effects of different types of force systems on deformation, particularly when these forces are combined, can influence adhesion. The Hertzian structural stiffness parameter K does not reflect the effects of differently distributed multiple forces. A theory is developed that takes into account simultaneous application of the adhesion force and applied forces through the development of a reduced stiffness, K_R. The paper also develops an equivalent Hertzian process for the condition of adhesion forces alone so that the mechanics of adhesion can be modeled completely by Hertzian theory. Illustrations of how adhesion alone is handled and how the reduced stiffness behaves are provided using experimental data from compressed, crossed rods and from hard particles in static equilibrium with both relatively hard and soft substrates.     

Processing and Mechanical Behavior of CrN/ZrO2(2Y) Composites

CrN powder consisting of granular particles of ∼3 μm has been prepared by self-propagating high-temperature synthesis under a nitrogen pressure of 12 MPa using Cr metal. Dense pure CrN ceramics and CrN/ZrO₂(2Y) composites in the CrN-rich region have been fabricated by hot isostatic pressing for 2 h at 1300°C and 196 MPa. The former ceramics have a fracture toughness ( K _IC) of 3.3 MPa ·m^1/2 and a bending strength (σ_b) of 400 MPa. In the latter materials almost all of the ZrO₂(2Y) grains (0.36–0.41 μm) are located in the grain boundaries of CrN (∼4.6 μm). The values of K _IC (6.1 MPa · m^1/2) and σ_b (1070 MPa) are obtained in the composites containing 50 vol% ZrO₂(2Y).     

Effect of Crack-Interface Bridging on Subcritical Crack Growth in Ferrites

A transition between transgranular fracture and intergranular fracture in polycrystalline manganese zinc ferrites has been investigated by measuring subcritical crack growth in double-cantilever-beam specimens. Log velocity vs K ₁ plots are like those for other ceramic materials with three well-defined regions of growth. Unlike other ceramics, however, the K ₁ values at which crack growth became critical showed large variations with relative humidity. Crack velocities depended on prior history and, at K ₁ > 1.1 MPa · m^1/2 in dry N₂, crack growth was intermittent. All of these features were related to an increase in intergranular fracture with K ₁. A model is presented to explain the data in terms of the generation ad removal of crack-interface bridges which were produced by intergranular fracture.     

Dynamics of Formation of Self-Organized Mesoporous AlO(OH)·αH2O Structure in Al-Metal Surface Hydrolysis in Humid Air

In humid air, a nascent Al-metal surface (S) with a surface Hg²⁺ catalyst hydrolyzes in divided reaction centers (micelles) with a vigorous exothermic reaction, {Al + 2OH⁻}+αH₂O → AlO(OH)·αH₂O + 3e⁻+ H⁺. It yields amorphous AlO(OH)·αH₂O with a huge ∼90% porosity with α= 0.25. The primary driving forces of the reaction are the chemical potential μ_e between the reaction species, the mechanical stress ς induced in expansion of S, and the flow of the reaction species. They drive it in a common direction perpendicular to S. The heat released in it flows primarily along S. It disrupts and stops the directional hydrolysis if the local temperature in the micelle reaches a critical value T _c (hot spot). The hot spot cools to the operating value T ₀, and the reaction restarts and runs over to T _c in a periodic manner, at a time scale of Δ t _i∼ 5 s, per the dynamics of hot spots, forming a self-organized mesoporous structure of 15–50-nm diameter ellipsoidal shaped particles (halo) separated through 3–5-nm pores. A pore, in continuous formation of the sample, forms in disrupted reaction during the hot spot as it cools from T _c to T ₀. The result is modeled in terms of the microstructure and dynamics of the hot spots.     

Microbial Biofouling: A Mechanistic Investigation

Microbial biofouling is important in a variety of applications including reverse osmosis membranes and in-situ bioremediation. The initial microbial adhesion plays the key role in microbial biofouling on abiotic surfaces, which can be explained in terms of microbial surface properties. This research investigated the interactions of three indigenous bacteria with the porous medium of silica sand and their corresponding initial attachment. Traditional and extended DLVO forces were calculated based on independently measured bacterial and medium surface thermodynamic properties and were related to bacterial attachment observations. Lifshitz–van der Waals, electrostatic and Lewis acid/base forces were found to be strongly dependent on the separation distance between bacteria and the medium surface. It was concluded that the electrostatic force served as the barrier preventing the bacterial strains from getting close to the medium. Once the bacterial strains overcame the barrier with the aid of hydrodynamic forces, Lifshitz–van der Waals force and electrostatic force dropped while Lewis acid/base force increased with the decrease of separation distance. Consequently, Lewis acid/base force became the dominating force controlling bacterial adhesion.     

Dispersive forces of particle–surface interactions: direct AFM measurements and modelling

Fundamentals of particle–particle interaction are of great interest in agglomeration processes. Particle adhesion depends on dispersive forces (van der Waals force), local chemical bindings, Coulomb force and capillary attractions. Additionally, surface properties like roughness, adsorption layers and surface chemistry strongly affect adhesion forces. van der Waals interactions are poorly understood because popular ab initio force calculations for molecules like density functional theory (DFT) often do not lead to proper results. van der Waals forces are difficult to measure directly. We present direct measurements of particle–particle and particle–surface interactions in the gas phase carried out with an atomic force microscope (AFM). Special emphasis is given to a proper statistical treatment of the data. For modelling of particle adhesion, we use computer-assisted empirical potential methods. Parameters like adsorbed water and surface roughness are considered. We extract parameters for weak interactions from the Lifshitz theory and gas adsorption data. Adsorbing molecules can be used as probes to measure dispersive forces. Studying surface and particle properties combined with computer-assisted modelling is a basic requisite to reach the aim of predicting particle–particle interactions in industrial processes.     

柴油液滴内空化气泡的破碎研究(英文)

A modified mathematical model is used to study the effects of various forces on the stability of cavitation bubbles within a diesel droplet. The principal finding of the work is that viscous forces of fluids stabilize the cavitation bubble, while inertial force destabilizes the cavitation bubble. The droplet viscosity plays a dominant role on the stability of cavitation bubbles compared with that of air and bubble. Bubble–droplet radius ratio is a key factor to control the bubble stability, especially in the high radius ratio range. Internal hydrodynamic and surface tension forces are found to stabilize the cavitation bubble, while bubble stability has little relationship with the external hydrodynamic force. Inertia makes bubble breakup easily, however, the breakup time is only slightly changed when bubble growth speed reaches a certain value (50 m·s?1). In contrast, viscous force makes bubble hard to break. With the increasing initial bubble–droplet radius ratio, the bubble growth rate increases, the bubble breakup radius decreases, and the bubble breakup time becomes shorter.     

Effect of Processing on Mechanical Properties of Platelet-Reinforced Mullite Composites

SiC- and Al₂O₃-platelet-reinforced mullite-matrix composites were fabricated by both conventional powder processing and a pressure filtration route with constant filtration rate. Consolidation during the pressure filtration experiment was monitored as a function of time with respect to actual pressing stages. The packing density of green bodies was strongly affected by the different processing methods. Either fracture toughness (3.2 MPa · m^1/2) or bending strength (344 MPa) increased, depending on surface conditioning of the SiC platelets. Pressure filtration was shown to decrease processing flaws in the platelet-reinforced bodies leading to substantial improvement of the mechanical properties.     

β-Sialon Ceramics Prepared at 1700°C by Hot Isostatic Pressing

Dense, single-phase β-sialon ceramics were sintered at 1700°C and 200 MPa using the glass-encapsulated hot isostatic pressing technique. The materials were very hard, 1500 to 1700 kg / mm² (98 N load), but were fairly brittle, with an indention fracture toughness of about 3 MPa · m^1/2. The addition of 1 wt% Y₂O₃ before sintering had a positive effect on the toughness, especially at the low x compositions of Si_3-xAl_xO_xN_4-x, where K_IC∼4 MPa · m^1/2.     

Fundamentals of Glass-to-Metal Bonding: VIII, Nature of Wetting and Adherence

The sessile-drop method for the evaluation of wetting is discussed in terms of forces acting on the liquid drop. For acute angles, or wetting of the solid, the driving force is the lowering of the solid-gas surface energy by the liquid. Contact angles of approximately 25° and less, obtained under appropriate conditions, appear to be associated with interfacial conditions that lead to the development of a strong chemical bond (an interchange or sharing of electrons) and to good adherence. A theory is proposed whereby chemical bonding depends on the development of a balance of bond energies across the metal-glass interface. The factors that lead to this condition are discussed. A modified Dupre's equation to take into account a strain factor and a contact coefficient is developed. Bonding of Na₂Si₂O₅ molten glass to platinum, gold, and iron and oxidized iron in several atmospheres is discussed.     

Fabrication, Microstructural Characterization, and Mechanical Properties of Polycrystalline t'-Zirconia

Large-grained (100- to 200-μm), yttria-doped, polycrystalline t '-zirconia ceramics were fabricated by heat-treating presintered samples at temperatures 2100°C. Polarized light microscopy revealed the ferroelastic domain structure in the t ' samples. XRD showed that no monoclinic phase was detected on as-polished, ground and fracture surfaces, or on surfaces while under a tensile stress as high as 400 MPa. By contrast, relative changes occurred in the tetragonal peak intensities, which were attributed to ferroelasatic domain switching. The higher toughness of 3-mol%-Y₂O₃-doped t ' samples (7.7 MPa · m^1/2) compared to that of 8 mol% Y₂O₃ cubic samples (2.4 MPa · m^1/2) was explained in part by ferroelastic domain switching.     

Effect of Zwitterionic Surfactants on Interparticle Forces, Rheology, and Particle Packing of Silicon Nitride Slurries

Phosphocholine (PC) zwitterionic surfactants, with different hydrocarbon chain lengths (C₆C₆PC to C₉C₉PC), were absorbed on the surface of silicon nitride near the isoelectric point (pH 6). Adsorption of the surfactants changed the lateral and normal surface forces, the rheology, and the consolidation behavior of the particles. The normal force between two silicon nitride surfaces as a function of separation and the lateral (friction) forces were measured using an atomic force microscope (AFM). These measurements indicated that surfactant adsorption reduced the magnitude of the long-range attractive van der Waals force and produced a repulsive short-range force. Although the adsorbed layers provided a barrier to particle contact, they could be ejected with a critical force that increased with the hydrocarbon chain length. The effect of an adsorbed layer on the viscosity and consolidation of slurries was also measured. The viscosity of all slurries decreased with increasing shear rate, indicative of attractive particle networks. The highest viscosity was observed for slurries formulated at the isoelectric point without added surfactant. Much lower viscosities were observed when the surfactant concentration was greater than the critical micelle concentration (cmc). A relative density of 0.46 was obtained via pressure filtration at 4 MPa without a surfactant, and between 0.46 to 0.59 (C₆C₆PC to C₉C₉PC, respectively) for surfactant concentrations greater than the cmc. Comparing force measurements with rheology and packing density provides a basis for discussing the role of interparticle forces in ceramic powder processing via colloidal routes.     

Forces Measured between Zirconia Surfaces in Poly(acrylic acid) Solutions

We have studied the forces between a sphere and a plane surface of yttria-partially-stabilized tetragonal-zirconia immersed in aqueous solutions of low-molecular-weight ( M _w= 10 000) poly(acrylic acid) (PAA) using atomic force microscopy. The measurements are performed at high pH where the adsorbed, highly charged anionic polyelectrolyte extends far into the solution, resulting in a combination of polymeric (steric) and electrostatic interactions. Analysis of the experimental data using scaling theory shows that the polymeric contribution dominates and that the electrostatic contribution is small at relatively high ionic strength (0.01 M NaCl). We find that the measured forces are highly dependent on time and interaction history of the absorbed PAA layer; consecutive compression-decompression cycles result in an increase of the surface coverage and the range of the repulsive polymeric interaction. This buildup of PAA at the interface is strongly related to attractive bridging interactions manifested as strong adhesion during decompression at less than full surface coverage. The force results are compared to rheological observations of zirconia suspensions stabilized by the same dispersant; the poor colloidal stability and high viscosity at low surface coverage of PAA are related to the attractive bridging interactions.     

Forces between Alumina Surfaces in Salt Solutions: Non-DLVO Forces and the Implications for Colloidal Processing

Direct measurements of the forces between basal (0001) surfaces of sapphire in salt solutions are presented. The measurements reveal the presence of forces in addition to those described by classical DLVO theory. At pH 7.2 in 0.01 M NaBr solution, we find an additional short-range oscillatory force with periodicity approximately equal to twice the diameter of a water molecule. At pH 3 we find an additional strong, short-range, monotonic, repulsive force and a long-range attractive force over a range of NaBr concentrations from 0.001 M to 0.1 M . Both monotonic forces are approximately exponential, with decay lengths of 0.55 and 12 nm, respectively. The short-range force is analogous to hydration forces previously measured on negatively charged surfaces. This force would provide plasticity to alumina slurries and is suggested to be the force responsible for the anomalous viscosity and consolidation behavior of alumina slurries at high salt concentrations.     

On the mechanism of bubble formation in a fluidized bed

A mechanism of bubble formation in a nonhomogeneous fluidized bed, based on instability theory, is suggested. In this mechanism it is assumed that the “bubbles” are formed at the bottom of the bed by the growth of prominences appearing because of the instability to perturbations of the lower surface of the bed. A bubble breaks off when the volume of the prominence becomes sufficiently large so that the buoyant force equals the sum of the bed resistance opposing its growth and the inertial forces. An equation for the diameter of the departing bubble is established.