摘要:

This feature article explores the concept of creating functionally graded metal‐ceramic composite microstructures for thermal barrier coatings used in gas‐turbine applications. From a thermomechanical perspective, this concept offers the possibility of significantly improving the life and reliability of thermal barrier coatings. However, prior research reveals that progress has been somewhat limited because of the oxidative instability exhibited by some metal‐ceramic composite microstructures. The present study addresses some of the materials criteria and research issues associated with preparing chemically stable, yet mechanically durable, graded metal‐ceramic microstructures for realistic application envir

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08070.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

An extensive experimental study has been conducted to examine the influence of ultraflne SiC particles on grain growth in A12O3. Grain growth has been studied as a function of annealing time, particle volume fraction, and annealing temperature. The SiC particles reduce the grain‐growth rate of A12O3by>3 orders of magnitude, resulting in final grain sizes that decrease with increasing particle volume fraction. The fraction of SiC particles on grain boundaries, φ, has been measured as a function of annealing time, particle volume fraction, and annealing temperature. φ decreases during grain‐growth anneals, implying that grain boundaries are able to break free from particles and this is true to a greater extent for smaller volume fractions of particles. Further, it has been discovered that φ and average grain size,G, are strongly correlated, such that any increase inGresults in a predictable decrease in pH. This new finding has revealed a path for microstructure development in this system that proves to be critical in the equilibrium and kinetic analyses described in a companion

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08071.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

The grain‐growth data presented in a companion paper for the Al2O3‐SiC system are analyzed. Central to the analysis is the experimentally observed relationship between the fraction of particles on grain boundaries, φ, and the average grain size, G. This relationship is included in both the equilibrium‐grain‐size expression originally formulated by Zener and classical grain‐growth rate equations. The modified expression for equilibrium grain size, GL, as a function of volume fraction of pinning particles,f, contains a power‐law dependence for GLonfthat has been developed using the relationship between φ andGduring microstructure evolution. The expressions for the grain‐growth kinetics are similarly developed. The equilibrium and kinetic relationships are used to describe the experimental data from the companion paper and predict grain‐growth behavior. A new type of map is constructed to predict the time required to approach equilibrium as afunction of volume fraction and annealing temperature. This map reveals that extreme experimental conditions (very long times and high annealing temperatures) are necessary for the system to reach equilibrium, especially for smaller volume fractions, and that the attainment of equilibrium is experimentall

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08072.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Three kinds of single‐crystalline alumina plates with the crystallographic planes of C, m, and R were diffusion‐bonded with liquid‐phase sintered (98)alumina‐(2)anorthite (in wt%) plates (P) and then heat‐treated at 1600°C under a Cr203‐containing atmosphere. During the heat treatment, for all of the specimens studied, the anorthite liquid films between alumina plates migrated to grains with surface orientation corresponding to higher coherency strain energy. This result is in better agreement with a prediction based on the coherency strain theory than the previous one obtained for grain‐boundary migration. The discrepancy between the predicted and previously observed migration directions of some grain boundaries in alumina may therefore be attributed to an effect of grain‐boundary structure and stress transmission acr

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08073.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

The rheological properties of nonaqueous silicon nitride powder suspensions have been investigated using steady shear and viscoelastic measurements. The polymeric dispersant, Hypermer KD‐3, adsorbed strongly on the powder surfaces, and colloidally stable, fluid suspensions up to a volume fraction of φ= 0.50 could be prepared. The concentrated suspensions all displayed a shear thinning behavior which could be modeled using the high shear form of the Cross equation. The viscoelastic response at high concentrations was dominated by particle interactions, probably due to interpenetration of the adsorbed polymer layers, and a thickness of the adsorbed Hypermer KD‐3 layer, Δ∼10 nm, was estimated. The volume fraction dependences of the high shear viscosity of three different silicon nitride powders were compared and the differences, analyzed by using a modified Krieger‐Dougherty model, were related to effective volume effects and the physical characteristics of the powders. The significantly lower maximum volume fraction, φm= 0.47, of the SN E‐10 powder was referred to the narrow particle size distribution and the possibility of an unfavorable particl

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08074.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Nd: YVO4powders and thin films were successfuly synthesized by the sol–gel method using metal alkoxides. A homogeneous and stable solution was prepared by the reaction of Y(OEt)3, VO(OiPr)3, and Nd(OEt)3in 2‐methoxyethanol. The precursor was a mixture of vanadium and yttrium double alkoxide. Precursor films were prepared by dip coating and crystallization to single‐phase YVO4at 500°C. Nd:YVO4films were crystallized with (200) preferred orientation on glass substrates, which showed the characteristic optical absorption of neo

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08075.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

A theoretical model is developed for the description of the compaction of granular materials exemplified by granulated ceramic powders. Its predictions compare satisfactorily to results of uniaxial compaction tests of ceramic granules of lead magnesium niobate‐lead titanate (PMN‐PT), rutile, and spray‐dried alumina. The theory uses volume‐based statistical mechanics and an activation analogy to treat, in parallel, the rearrangement of granules and their deformation. Variation of the model incorporates a distribution of barriers to deformation, which can be considered to include the effects of statistical pressure distributions within the compact. Other curve‐fitting schemes available in the literature are shown to correspond to particular cases of the theory, and a justification of the equations used in those schemes is provided on physica

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08076.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Hardness and toughness are often used to analyze the abrasive machining behavior of ceramic materials. However, toughness values of silicon nitride ceramics with microstructures containing elongated grains increase with crack extension. The present study investigates the effect of toughness on the process of abrasive machining to determine which value of toughness should be used in the analysis. The toughness curves (i.e., toughness as a function of crack length) of ten different silicon nitride materials are characterized by an indentation‐strength technique and an indentation technique. The forces in surface grinding are measured as a function of the depth of cut. Examination of ground surfaces by scanning electron microscopy indicates that the material‐removal processes in grinding follows the formation of short cracks (i.e., microcracks) and grain‐scale material dislodgement. An indentation fracture model for material removal in abrasive machining is used to correlate the grinding forces with toughness and hardness of the materials. An agreement is obtained between the experimental results and the indentation model only when the toughness associated with short cracks is used. This study shows the importance of using appropriate toughness values corresponding to the microfracture processes in analyzing abrasive machining results for materials possessing rising toughness c

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08077.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Tension‐tension cyclic‐fatigue tests of a two‐dimensional‐woven‐SiC‐fiber‐SiC‐matrix composite (SiC/SiC) prepared by chemical vapor infiltration (CVI) were conducted in air at room temperature and in argon at 1000°C. The cyclic‐fatigue limit (107cycles) at room temperature was ∼160 MPa, which was ∼80% of the monotonic tensile strength of the composite. However, the fatigue limit at 1000°C was only 75 MPa, which was 30% of the tensile strength of the composite. No difference was observed in cyclic‐fatigue life at room temperature and at 1000°C at stresses>180 MPa; however, cyclic‐fatigue life decreased at 1000°C at stresses<180 MPa. The fracture mode changed from fracture in 0° and 90° bundles at high stresses to fracture mainly in 0° bundles at low stresses. Fiber‐pullout length at 1000°C was longer than that at room temperature, and, in cyclic fatigue, it was longer than that in monotonic tension. The decrease in the fatigue limit at 1000°C was concluded to be possibly attributed to creep of fibers and the reduction of the sliding resistance of the interfa

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08078.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

The oxygen nonstoichiometry in Mn‐doped Gd2Ti2O7, Gd2(Ti0.975Mn0.025)2O7+x, was measured electrochemically, as a function of temperature and oxygen partial pressure, with the aid of an oxygen titration cell. The analysis of the data shows that the defect equilibrium can be described by considering the dominant point defects to be neutral oxygen interstitials, doubly charged oxygen vacancies, and trivalent and quadrivalent Mn ions substituted in the Ti sites. The enthalpies for the formation of neutral oxygen interstitials and trivalent Mn are determine

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08079.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY