摘要:

Structural ceramics for high‐temperature applications should embody the following properties: oxidation resistance, chemical stability, low volatility, resistance to creep deformation, resistance to creep cavitation at interfaces, sufficient toughness at ambient temperature, and thermalshock resistance. These criteria lead to five themes for fundamental research and design of high‐temperature structural ceramics: chemical and environmental stability, grain‐boundary sliding and cavitation, single‐crystal microstructure design, room‐temperature mechanical properties, and thermal shock. It is recommended that research that is confined to any one of these five areas takes into consideration the broader implications of research results. For example, microstructure designs that require weak interfaces for obtaining toughness at room temperature directly or indirectly conflict with creep and cavitation resistance needed for long‐term service at high temperatures. New research should be directed at mechanisms that can simultaneously achieve good mechanical properties over a wide range of temperatures. This paper addresses the following recommendations: (i) although non‐oxide systems can be viable for structural applications below 1500°C, oxidebased ceramics are necessary for service above 1500°C; (ii) microstructure designs based on acicular grain morphologies and/or single‐crystal fiber reinforcements have the potential for meeting the mechanical property requirements from room temperature up to very high temperatures; (iii) for fundamental studies of mechanical properties at high temperatures, simple uniaxial tension experiments should be used in tandem with four‐point bending and uniaxial compression experiments; (iv) the study of the reinforcement phase should center on very pure, highly stoichiometric materials in the case of non‐oxides, and on mixed and alloyed single crystals of cubic symmetry, or crystals having isotropic properties, and large unit cells in the case of oxides; (v) the study of interfaces in non‐oxides should focus on the chemistry of the intergranular glass phase, particularly the control of the oxygen content and the crystallization of this phase for improvement of high‐temperature properties; (vi) the study of interfaces in oxides is best directed at the relationship between interface structure, defect chemistry, and interfacial mechanical properties over a wide range of temperature; (vii) the understanding of the micromechanisms of thermal‐shock failure and the application of this understanding for designing graded interfaces that may be able to cope with thermal‐expansion stresses without leading to microfracture and cavitation is important in all classes of ceramic materials, and is of critical importance in the development of oxides for very‐high‐temperature applications; and (viii) research in processing science should emphasize the study of basic mechanisms that lead toin‐situgrowth of a

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb07750.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

The unusual stress–strain hysteresis loop shape exhibited by ceramic‐matrix composites under cyclic loading has previously been explained as a result of either strain rate dependence of the frictional shear stress or crack closure. This investigation has determined that the response is due to neither mechanism. Instead, it is proposed that a variation of interfacial shear strength occurs during each cycle. A static coefficient of friction dominates immediately after loading or unloading. A much lower dynamic coefficient of friction operates once fiber sliding begins. This dynamic coefficient appears to be very dependent on surface roughn

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb07751.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

A study was conducted of the temperature dependence of the thermal diffusivity and thermal expansion of an alumina–aluminum titanate composite heated to a range of maximum temperatures followed by cooling to room temperature. Heating to temperatures above about 600°C resulted in a hysteresis behavior in which the data on cooling differed from the data obtained during heating. For both the thermal diffusivity and thermal expansion, the degree of hysteresis increased with increasing maximum temperature. On return to room temperature, following heating to a temperature of about 1200°C, the thermal diffusivity exhibited a significant decrease, with a corresponding increase in specimen size. This effect was attributed to an increase in microcrack density over the corresponding value prior to heating. On subsequent cycles of heating and cooling for a maximum temperature of 1200°C this decrease in thermal diffusivity was partially recovered, indicative of the structural integrity of the alumina–aluminum titanate composite of this study in practical applications involving temperatures of at least

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb07752.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Two types of brittle reticulated materials were evaluated under uniaxial tensile and compressive loading and analyzed in terms of the Gibson and Ashby model for brittle open‐cell solids. The samples consisted of an open‐cell alumina–mullite material which was tested as a function of density at a constant cell size and a reticulated vitreous carbon tested at one density and two cell sizes. The samples were mounted such that only the loading direction was varied in the tests. A combination of video photography and acoustic emission was critical to interpreting the results. The model assumes that identical deformation modes, bending failure of the struts, are responsible for failure of the bulk foam in tension and compression. The results of this work indicate a significant difference between the density dependence in tension and compression. Tensile failure in both materials appeared to be characterized by the catastrophic propagation of a single crack. Compressive failure was significantly different between the alumina and glassy carbon foams. The alumina foam failed by a damage accumulation process, whereas the carbon foam failed by the catastrophic collapse of a band of cells perpendicular to the loading dire

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb07753.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

A temperature (T)−oxygen partial pressure (pO2) topologic analysis of phase relations in the system CaO–CrO–Cr2O3–SiO2, involving divalent and trivalent chromium, was undertaken to determine the relative positions of isobaric invariant points and univariant lines inT‐pO2space. Liquidus phase relationships in equilibrium with metallic chromium, CO2–H2gas mixtures, and air were used to identify the univariant and invariant assemblages involving Cr2+and Cr3+. TheT‐pO2grid was constructed using Schreine‐makers rules for the disposition of univariant lines about an invariant point. The topological relationship between univariant lines and invariant points assists in the interpretation of phase relationships over theT‐pO2regions that were not experimentally investigated. For example, the effect ofpO2on the liquidus phase relationships involving CaSiO3(wollastonite), Cr2O3(eskolaite), and Ca3Cr23+Si3O12(uvarovite) is clearly illustrated by the grid; these details are not borne out by the experimentally determined phase diagrams. The grid furthermore illustrates the stability fields of the Cr2+‐bearing phases, (Ca0.4Cr0.62+)Cr23+O4and CaCr2+Si4O10as a function of temperature andpO2and demonstrates the observed increase in liquidus temperatures

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb07754.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Chemical vapor deposition (CVD) and electrochemical vapor deposition (EVD) have been applied to deposit yttria‐stabilized‐zirconia (YSZ) on porous ceramic media. The experimental results indicate that the location of YSZ deposition can be varied from the surface of the substrates to the inside of the substrates by changing the CVD/EVD experimental conditions, i.e., the concentration ratio of the reactant vapors. The deposition width is strongly dependent on the deposition temperature used. The deposition of YSZ inside the pores resulted in pore narrowing and eventually pore closure, which was measured by using permpor‐ometry. However, deposition of YSZ on top of porous ceramic substrates (outside the pores) did not result in a reduction of the average pore size. Ultrathin, dense YSZ layers on porous ceramic substrates can be obtained by suppressing the EVD layer growth process after pore cl

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb07755.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Monolithic ceramics have been fabricated from coated green fibers to create fibrous microstructures. Thefibrous monolithsconsist of high aspect ratio polycrystalline regions (cells) of a primary phase regions (cell boundaries) designed to improve fracture resistance. The cells are the remnants of the green fiber which consists of ceramic powder and a polymer binder. The coating applied on the green fiber forms the cell boundaries. Fabrication and microstructure are described for fibrous monoliths in the SiC/graphite, silicon nitride/BN, alumina/alumina–zirconia, alumina/aluminum titanate, alumina/nickel and Ce‐TZP/alumina–Ce–zirconia systems. The SiC/graphite fibrous monolith displays noncatastrophic failure in flexure, with shear delamination along the weak graphite layers. Indentations in SiC/graphite cause cells to spall, with crack arrest and extrusion of graphite from the cell boundaries. Crack deflection and spalling of cells are also observed in alumina/alumina–zirconia fibrous monoliths. In the Ce‐TZP/alumina system, transformed regions around indentations are significantly modified by the alumina‐containing ce

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb07756.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

The fibrous monolith microstructure consists of high aspect ratio polycrystalline cells of SiC separated by thin cell boundaries containing graphite. The SiC/100% graphite fibrous monolith has noncatastrophic fracture behavior, is damage tolerant, and is notch insensitive. The failure process is characterized by fracture along weak graphite cell boundaries. The room‐temperature flexural strength is 300–350 MPa. The estimated shear strength along the graphite cell boundaries is ∼ 15 MPa. Increasing the strength of the cell boundary by additions of SiC (40–60 vol%) results in a monolithic SiC material showing brittle fracture behavior but retaining damage tolerance. Strength and fracture behavior are also influenced by cell texture and orie

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb07757.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Controlled‐geometry voids were introduced into unim‐planted and Ti‐implanted high‐purityc‐axis sapphire substrates using microfabrication techniques and ion beam etching, and subsequently transferred to an internal interface by hot‐pressing. The morphological evolution of crack‐like and channellike defects oriented parallel to the basal plane in response to anneals at 1700°C was studied. The healing behavior of defects in the unimplanted and Ti‐ion‐implanted samples differs significantly. Ti additions appear to reverse the directional dependence of the healing characteristics in undoped sapphire, greatly stabilizing defects with edges aligned along the [1100] direction, while reducing the stability of defects with edges aligned parallel to the [1120]direction relative to unimplanted sapphire. The healing characteristics of unimplanted and Ti‐implanted sapphire substrates are compared and contrasted with those observed previously in Ca‐ and Mg

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb07758.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

A fracture mechanics model is presented for the toughening of ceramics by bridging from second‐phase particles, resulting in toughness curve (T‐curve) behavior. It is assumed that the second‐phase particles are in a state of residual thermal expansion dilatational mismatch relative to the matrix. In the long‐crack region, these stresses augment frictional sliding stresses at the interphase boundaries, enhancing the crack resistance; in the short‐crack region, the same stresses drive the crack, diminishing the crack resistance. The principal manifestation of these countervailing influences is a reduced sensitivity of strength to initial flaw size, i.e., an increased flaw tolerance. In seeking to incorporate these key physical elements, our model opts for mathematical simplicity by assuming uniformly distributed stresses in two bridging domains: in the first, at small crack‐wall separations, a constant opening stress; in the second, at larger separations, a constant closing stress. The uniform crack‐plane distributions allow for simple closed‐form solutions of the crackK‐field equations, and thence an analytical formulation for theT‐curve. Indentation‐strength data on a “reference” Al2O3/Al2TiO5ceramic composite are used to demonstrate the main theoretical predictions and to calibrate essential parameters in theT‐curve formulation. The utility of the model as a route to microstructural des

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb07759.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY