摘要:

Phase equilibria in the system HfO2–Y2O3–CaO were studied in the temperature range 1250° to 2850°C by both experimental methods (X‐ray phase analysis at 20° to 2000°C, petrography, annealing and quenching, differential thermal analysis in He at temperatures to 2500°C, thermal analysis in air using a solar furnace at temperatures to 3000°C, and electron microprobe X‐ray analysis) and theoretical means (development of a mathematical model for the liquidus surface by means of the reduced polynomial method). Phase equilibria were determined by the structure of the restricting binary systems. No ternary compounds were found. The liquidus was characterized by the presence of six four‐phase, invariant equilibria. Solid solutions were based on monoclinic (M), tetragonal (T), and cubic (F) modifications of HfO2; C and H forms of Y2O3; CaO; and CaHfO3that crystallized in two polymorphous modifications, namely, the cubic and rhombic perovskite

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04382.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Friction coefficients and wear rates of hot‐pressed Si3N4have been measured in a ball‐on‐flat linear‐sliding geometry at temperatures ranging from room temperature to 1273 K in Ar and air containing up to 63% H2O. In inert environments, the friction coefficients (ranging between 0.6 and 0.7) are independent of temperature, and wear rates increase with temperature. In oxidizing environments, the friction and wear are reduced by the presence of tribochemical reaction products. Friction coefficients and wear rates are lowered when the wear debris spontaneously forms into rolls, oriented perpendicular to the direction of relative motion, at high H2O levels in the temperature range of 873 to

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04383.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

An optimal thermodynamic data set for the zirconia–calcia system is evaluated by the CALPHAD (CALculation of PHAse Diagrams) method applied to experimental phase diagram and thermodynamic data. The liquid andCss(cubic ZrO2solid solution) are described by a subregular solution model. Three compounds, CaZr4O9, Ca6Zr19O44, and CaZrO3, are modeled as stoichiometric compounds. A regular solution model is applied to the other phases. Comparisons between calculated and measured phase diagrams and thermodynamic quantities show that most of experimental information is satisfactorily accounted for by thermodynamic calculations. The calculations also reasonably predict the measured electromotive force concerning the metastableCs

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04384.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

An objective methodology is developed for evaluating toughness curves (T‐curves) of ceramics using indentation flaws. Two experimental routes are considered: (i) conventional measurement of inert strength as a function of indentation load; (ii) in situ measurement of crack size as a function of applied stress. Central to the procedure is a proper calibration of the indentation coefficients that determine theK‐field of indentation cracks in combined residual‐contact and applied‐stress loading, using data on an appropriate base material with single‐valued toughness. Tests on a fine‐grain alumina serve to demonstrate the approach. A key constraint in the coefficient evaluation is an observed satisfaction of the classical indentation strength–(load)−1/3relation for such materials, implying an essential geometrical similarity in the crack configurations at failure.T‐curves for any alumina‐based ceramicwithoutsingle‐valued toughness can then be generated objectively from inert‐strength or in situ crack‐size data. The methodology thereby circumvents the need for any preconceived model of toughening, or for any prescribed analytical representation of theT‐curve function. Data on coarse‐grained aluminas and alumina‐matrix material with aluminum titanate second‐phase particles are u

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04385.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The stress–strain behavior for Al2O3of different grain size, for three different Mg‐PSZ grades, and for various differently composed duplex structures is investigated and compared with their flaw resistance,KR‐curve behavior, and thermal shock behavior measured in previous works. The experimental results seem to reveal that, for most materials, quasi ductility increases with increasing flaw resistance, increasingly pronouncedKR‐curve behavior, and increasing thermal shock retained strength. However, brittle ceramics can exhibit risingKR‐curves, whereas pronounced quasiductile materials can exhibit flatKR‐curves. An explanation for the apparent pseudo relationship between quasi ductility andKR‐curve behavior may be that, apart from genuine transformation ductility, most quasi‐ductile effects such as microcracking have only a minor contribution to risingR‐curve behavior, but require the existence of strong residual stresses, which are, on the other hand, responsible for the occurrence of most toughening mechanisms. Also discussed is the influence of microcracking on flaw resistance and thermal shock str

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04386.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Preventing the incorporation of impurities in Li‐Zn ferrite grains during sintering is essential for production of ceramics withreproduciblemagnetic and electrical properties. Li‐Zn ferrites of composition Li0.3Zn0.4Mn0.05Fe2.25O4were prepared with Bi2O3and borosilicate sintering additives. The distribution of impurity ions in the sintered ferrites was investigated using transmission electron microscopy (TEM) coupled with energy dispersive spectroscopy (EDS). Ceramics prepared with Bi2O3contained Si, Ca, and S impurities, located at grain boundaries and triple point regions. The low viscosity and good wetting properties of the Bi2O3and to a lesser extent the borosilicate liquid phase allowed impurities to be selectively removed from the growing ferrite phase during sintering, thus improving sample resistivit

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04387.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The hydration characteristics of solid solutions of composition 3CaO·xGeO2·(1 −x)SiO2were investigated at 25°C by isothermal calorimetry. The compositions hydrated were forx= 0, 0.2, 0.4, 0.6, 0.8, and 1.0. Full hydration was achieved for compositions in whichx<0.2. Both the rate of hydration and the total heat evolved for complete hydration vary with composition. Hydrate compositions were determined, and these also show a compositional dependence. The hydration product for tricalcium germanate has a Ca/Ge ratio near 1.5. Although the hydration products of the solid solution are far more fibrous, their morphologies are reminiscent of that of calcium silicate hyd

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04388.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The effects of temperature, sliding speed, and contact load on the tribological behavior of an α‐alumina ceramic sliding on the same material in water were investigated in the range from room temperature to 300°C under the corresponding saturated vapor pressures. The specific wear rate increased remarkably at elevated temperatures. The primary wear mechanisms in high‐temperature water are considered to be microfracture, promoted by the solution of grain boundary layers, and stress corrosion cracking. A film containing γ‐alumina particulates appeared on the worn surface with increasing sliding speed and increasing contact load in 300

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04389.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The tie lines between (CoXMg1−X)O solid solution with rock salt structure and orthosilicate solid solution (CoYMg1−Y)‐Si0.5O2, and between orthosilicate and metasilicate (CoZMg1‐Z)SiO3crystalline solutions, have been determined experimentally at 1373 K. The compositions of coexisting phases have been determined by electron probe microanalysis (EPMA) and lattice parameter measurement on equilibrated samples. The metasilicate solid solution exists only for 0>Z>0.213. The activity of CoO in the rock salt solid solution was determined as a function of composition and temperature in the range of 1023 to 1373 K using a solid‐state galvanic cell:Pt, (CoXMg1−X)O+Co|(Y2O3)ZrO2|Co+CoO, PtThe free energy of mixing of (CoXMg1−X)O crystalline solution can be expressed by the equationΔGE=X(1 −X)[(6048 − 2.146T)X+ (8745 − 3.09T)(1 −X)] J·mol−1The thermodynamic data for the rock salt phase is combined with information on interphase partitioning of Co and Mg to generate the mixing properties for the ortho‐ and metasilicate solid solutions. For the orthosilicate solution (CoYMg1 −Y)Si0.5O2at 1373 K, the excess Gibbs free energy of mixing is given by the relationΔGE=Y(1 −Y)[2805Y+ 3261(1 −Y)] J·mol−1For the metasilicate solution (CoZMg1 −Z)SiO3at the same temperature, the excess free energy can be expressed by the relati

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04390.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

In the enameling of steel, the oxide is generally regarded as being completely dissolved by the fusing enamel, with the enamel–metal bond forming directly between oxide‐saturated glass and metal. According to this model, the adherence of the oxide layer present on the surface of the steel as the enamel begins to fuse is irrelevant, because none of the original oxide layer remains in the matured enamel–steel bond. This model has not been completely verified, however, and some researchers have presented evidence for the presence of a layer of wüstite (FeO) at the enamel–steel interface on the order of 1 to 4 μm in thickness. Whether such a layer exists has important implications regarding the mechanism of enamel–steel adherence. In the present study, a method was developed to concentrate whatever crystalline material might be present in the interfacial zone to make it more amenable to detection by X‐ray diffraction. Through the use of wüstite standards, the present technique was shown to be capable of detecting a layer of wüstite at the enamel–steel interface as thin as 0.3 μm. However, in neither one‐coat nor two‐coat enameling could a layer of wüstite be demonstrated at the enamel–steel interface. Hence, there does not appear to be a 1‐to‐4‐μm‐thick wüstite layer at the enamel–steel interface. If a layer of iron oxide is present at the inte

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04391.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY