摘要:

Friction and wear of sintered alumina with grain sizes between 0.4 and 3 μm were measured in comparison with Al2O3/TiC composites and with tetragonal ZrO2(3 mol% Y2O3). The dependence on the grain boundary toughness and residual microstresses is investigated, and a hierarchical order of influencing parameters is observed. In air, reduced alumina grain sizes improve the micromechanical stability of the grain boundaries and the hardness, and reduced wear is governed by microplastic deformation, with few pullout events. Humidity and water slightly reduce the friction of all of the investigated ceramics. In water, this effect reduces the wear of coarser alumina microstructures. The wear of aluminas and of the Al2O3/TiC composite is similar; it is lower than observed in zirconia, where extended surface cracking occurs at grain sizes as small as 0.3 μ

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08565.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

The microstructure of materials compacted from commercially produced TiB2powders was investigated using transmission electron microscopy. A number of impurity phases that are introduced during the various processing stages were identified. After exposure to liquid aluminum, grain boundaries and triple junctions of TiB2were found to be penetrated by aluminum. In the penetrated regions pure aluminum, two aluminum oxides, and an (Al2OC)1‐x(AlN)x. phase were identified. A SiO2glass phase, introduced during hot isostatic pressing, is believed to be responsible for the formation of alumina. None of the other impurity phases were found to react with aluminu

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08566.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

In an attempt to improve the colloidal processing of Si3N4ceramics, we studied the rheology and consolidation of colloidal suspensions of Si3N4particles (average particle size 0.7 μm) with small Al2O3particles (average particle size 20 nm). It was found that at pH>7, the viscosity of the mixtures increased and then decreased with an increasing concentration of Al2O3. λpotential measurements, optical micrographs, and visible light absorptance measurement suggest that such viscosity behavior is due to clustering of Si3N4particles bridged by the small Al2O3particles. This is also supported by the Derjaquin‐Landau‐Verwey‐Overbeek (DLVO) potential calculations that show the barrier height in the DLVO potential between Al2O3and Si3N4is small. The small barrier height under current experimental conditions stems from the small size of the Al2O3particles. The small barrier height allows the thermal motion of the two kinds of particles to overcome the barrier and attach to each other. The adsorption of small Al2O3particles on Si3N4can occur even when both Al2O3and Si3N4carry the same sign of charges. The adsorption of Al2O3on Si3N4also increases the density of consolidated c

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08567.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

A kinetic model based on generalized continuum time‐dependent Ginzburg‐Landau (TDGL) equations is proposed for studying coupled grain growth and Ostwald ripening in multiphase systems. In this model, an arbitrary multiphase microstructure is described by many orientation field variables which represent crystallographic orientations of grains in each phase and byn— 1 composition field variables which distinguish the compositional differences amongnphases. Microstructural development during simultaneous grain growth and Ostwald ripening is predicted by the temporal evolution of these field variables by numerically solving the TDGL equations. A particular example, Al2O3‐ZrO2particulate composite, was considered. The effects of the volume fraction of ZrO2on the microstructural features and their evolution were studied and compared to experimental observations and previous thermodynamic a

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08568.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Solute segregation at grain boundaries has been correlated with grain‐boundary conductivity in high‐purity 15‐mol%‐CaO‐stabilized ZrO2. STEM measurements of solute coverage show that the segregation of impurity silicon (present at bulk levels103at 500°C. At the lowest levels of segregation achieved,<0.1 monolayer, σspgbremains ∼102less, and possibly represents an “intrinsic” limiting value for the grain boundary. Comparison with Y2O3‐doped ZrO2suggests similar behavior in this system. The control of grain‐boundary segregation through purity, microstructure, and thermal history is discussed from the objective of engineering the grain‐boundary impedance of po

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08569.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

In fiber‐reinforced brittle laminates, crack growth under monotonic tension generally consists of crack tunneling along the weaker ply (usually the 90° ply) followed by plane strain crack growth through the adjacent, more resistant plies (the 0° plies). In this paper, the details of this transition in crack mode are examined. The tunneling crack configuration is generalized to allow the crack to penetrate the 0° ply during tunneling. The effects of crack bridging in the 0° plies on the energetics of tunneling are computed numerically for general cases and combined with analytical results for certain limits. The nature of the transition from tunneling to plane strain cracking is found to depend on the ratio of the toughnesses of the 90° and 0° plies. Implications for laminate design are di

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08570.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Both fine grain sized and coarse grain sized alumina has been used as a model material to correlate short crackR‐ curves by the surface‐flaw‐in‐flexure method with long crackR‐curves from a compact tension specimen. The analysis has been made using the crack opening displacement from long cracks combined with the appropriate weight function to compute the crack closure stresses by crack bridging as a function of crack opening displacement. Then the long crackR‐ curve, the short crack curve, as well as the crack opening displacement for short cracks are computed. The agreement with the experimental data is within an absolute value of about 0.3 MPa and attributed to uncertainties in determining the crack tip toughness and effects due to subcritical c

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08571.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

SiC particles have been synthesized in the α‐phase from a mixture of boiler‐burnt rice husk and graphite powder in a dc extended arc plasma reactor on a 200–250 g scale. A SiC yield as high as 72% was achieved at a 3.03:1 carbon‐to‐silica ratio. 21R polytypism, which is rare to grow, was observed in the SiC. About 90% of the SiC particles produced were found to lie under 163 μm, 50% under 90 μm, and 10% under 28 μm size. Very fine particles under 10 μm size occurred to the extent of 3%. XRD and SEM characterizations were carried out to study the phase and morphology of the particles. Heterogeneous gas‐phase reactions seem to be responsible for intermediate‐size SiC particle growth in the multitemperature zone dc extended

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08572.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Using small‐angle scattering, together with density measurements, X‐ray diffraction, nitrogen adsorption, and both transmission and scanning electron microscopy, the microstructural evolution during sintering in nanostructured ceramic oxides has been studied. The availability of samples with sufficient size and uniformity has permitted absolute calibration of the scattering data and made possible determinations of microstructural parameters and surface areas that are statistically representative of the undisturbed nanophase morphologies. A fully quantitative microstructural model has been developed, and the effects of varying processing parameters during powder synthesis and sintering have been assessed. Predictions are made regarding the achievable sintered full‐density microstruc

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08573.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

The chemical states of powder surfaces depend on the manufacturing processes of the powders. The surface chemistry of three different commercial AIN powders, which are processed by carbothermal nitridation of Al2O3, chemical vapor deposition (CVD), and direct nitridation of aluminum, were evaluated by using Auger electron spectroscopy (AES). In order to obtain reference AES spectra of aluminum compounds, α‐, γ‐, θ‐Al2O3, γ‐AIOOH, γ‐AION, and sintered AIN were also examined. Line shapes of aluminumLVV; Al(LVV), nitrogenKLL; N(KLL) and oxygenKLL; O(KLL) are discussed for the AIN powders and all the other aluminum compounds. The differential Auger electron spectra, i.e.,E dn/AE were obtained directly, wherenis the number of Auger electrons, andEis the kinetic energy of the electron. Their integrated spectra, i.e.,n(E) are also employed for analysis. The results confirm the conclusions of our previous temperature‐programmed desorption work. The AES line shape analysis implies the presence of an oxide‐likeθ‐Al2O3containing AION phase on the carbothermal nitride AIN powder surfaces. The surfaces of CVD and direct‐nitrided AIN powders are covered by an oxide–like γ‐Al2O3with an oxygen diffusion layer

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08574.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY