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Showing papers on "Ceramic published in 1984"


Book
06 Nov 1984
TL;DR: In this article, the authors present an overview of the history of the use and behavior of aluminomethane and discuss its application in a variety of applications, including medical applications.
Abstract: 1 Introduction.- 1.1 Scope of the Book.- 1.2 General Remarks on the Use and the Behavior of Ceramic Materials.- 1.3 The History of Alumina.- 1.4 Preceding Summarizing Literature on Alumina.- 2 Physical Properties.- 2.1 Structure.- 2.2 Thermal Properties.- 2.3 Diffusion.- 2.3.1 General Remarks on Diffusion Phenomena in Ceramics.- 2.3.2 Intrinsic Diffusion and Disorder Mechanism.- 2.3.3 Extrinsic Diffusion.- 2.3.4 Problems in Determining the Diffusion Coefficient.- 2.3.5 Diffusion Data.- 2.4 Electrical Conductivity.- 2.5 Sintering and Grain Growth.- 2.5.1 Fundamental Sintering Mechanisms in Al2O3.- 2.5.2 Intermediate and Final-Stage Sintering.- 2.5.3 Influence of Additives.- 2.5.4 Effect of MgO.- 2.5.5 Influence of Atmospheres.- 2.5.6 Grain Growth.- 2.6 Hot Pressing.- 2.7 Segregation.- 3 Mechanical Properties.- 3.1 Elastic Properties.- 3.2 Fracture Strength.- 3.2.1 The Strength of Ceramics.- 3.2.2 Fracture Energy.- 3.2.3 Types of Flaws.- 3.2.4 Strength Data.- 3.2.5 Strength-Grain Size Relationships.- 3.3 Time-Dependent Strength and Subcritical Crack Growth.- 3.3.1 The Model of Time-Dependent Strength.- 3.3.2 Subcritical Crack Growth Data.- 3.3.3 Fatigue Data.- 3.3.4 Mechanisms of Slow Crack Growth in Alumina.- 3.4 Thermal and Mechanical Shock Resistance.- 3.4.1 Thermal Shock Properties.- 3.4.2 Mechanical Shock Properties.- 3.4.3 Crack Healing.- 3.5 Plastic Deformation.- 3.5.1 Slip.- 3.5.2 Twinning.- 3.5.3 Hardness.- 3.5.4 Abrasive Wear.- 3.6 Creep.- 3.6.1 Basic Creep Mechanisms.- 3.6.2 Creep of Pure and MgO-Doped Alumina.- 3.6.3 Effect of Other Dopants.- 3.6.4 High Temperature Failure Mechanisms.- 3.7 Strengthening Mechanisms.- 3.7.1 Second Phase Dispersions.- 3.7.2 Compressive Surface Stresses.- 3.7.3 Transformation Thoughening.- 4 Fabrication.- 4.1 Preparation of Powders.- 4.2 Forming.- 4.2.1 Dry Pressing.- 4.2.2 Hydrostatic Molding.- 4.2.3 Extrusion.- 4.2.4 Injection Molding.- 4.2.5 Hot Pressing.- 4.3 Sintering.- 4.4 Hard Machining.- 4.5 Quality Control.- 4.6 Manufacturing Tolerances.- 4.7 Principles of Design.- 5 Applications.- 5.1 Electronic Applications.- 5.1.1 Metal-to-Ceramic Bonding.- 5.1.2 Spark-Plug Insulators.- 5.1.3 Components and Housings for Electron Tubes.- 5.1.4 Discharge Lamps.- 5.1.5 Microelectronics.- 5.2 Mechanical Engineering Applications.- 5.2.1 Thread Guides for Textile Machines.- 5.2.2 Wire Drawing Step Cones.- 5.2.3 Paper Machine Covers.- 5.2.4 Bearings.- 5.2.5 Cutting Tools.- 5.3 Medical Applications.- 5.3.1 Artificial Joints.- 5.3.2 Dental Implants.- 5.3.3 Maxillary Reconstructions.- 5.4 Armor Applications.- 5.5 Other Applications.- References.

254 citations


Journal ArticleDOI
TL;DR: In this paper, a simple method for measuring frictional stresses between the mairix and individual fibers in a ceramic composite is described, where a standard microhardness indenter is used to apply a force to the end of the fiber and depress it below the matrix surface.
Abstract: A simple method for measuring frictional stresses between the mairix and individual fibers in a ceramic composite is described. A standard microhardness indenter is used to apply a force to the end of the fiber and depress it below the matrix surface. The frictional stress is calculated from measurement of the applied force and the amount of slipping between the fiber and matrix.

207 citations


Journal ArticleDOI
TL;DR: In this article, a model for the coating microstructure involving limited regions of good contact between lamellae is proposed, where the noncontact regions are regarded as very thin planar pores which have limited thermal conductance at low temperatures.

195 citations


Journal ArticleDOI
TL;DR: In this article, an AIN ceramic has been produced with a thermal conductivity five times higher than that of alumina, which is about one half of the theoretical value estimated for defect-free single crystal AIN.
Abstract: The development of more complex integrated circuits (IC's) with larger chip sizes and greater power generation IC's, emitter-coupled logic (ECL) circuits, very large-scale integration (VLSI), and laser diodes requires new packaging schemes with improved thermal management. From the limited number of some ten nonmetallic inorganic solids known from the literature to have thermal conductivities greater than 100 W/m°K, aluminum nitride (AIN) is the most attractive substrate material. By means of a suitable power technological process, an AIN ceramic has been produced with a thermal conductivity five times higher than that of alumina, which is about one half of the theoretical value estimated for defect-free single crystal AIN. Another advantage of this newly developed material is its low coefficient of thermal expansion which, in the temperature range of interest (20-200°C) closely matches that of silicon. It also has a high electrical resistivity and a moderate dielectric loss. These and some other physical, mechanical, and chemical properties are shown and partly discussed in relation to other ceramics in use or of potential interest as substrate materials.

171 citations


Journal ArticleDOI
G. de1, H.J.A. Van Dijk1
TL;DR: In this article, the sintering of Y 3 Al 5 O 12 to full density and translucency using either SiO 2 or MgO as a dopant is described.

155 citations


Journal ArticleDOI
TL;DR: In this paper, the authors show that fine-grained ceramics are capable of high rates of deformation at quite low stresses (1 to 20 MPa) at high deformation rates, and that microstructural changes can occur during deformation which influence the flow behavior.
Abstract: Several results pertaining to large deformations at fast strain rates in a fine-grained ceramic material are described. Results for strain-rate, grain size, and temperature dependence of the flow stress are presented. They show that (a) ultrafine-grained ceramics are capable of high rates of deformation (about 10−4 to 10−4 s−1) at quite low stresses (1 to 20 MPa); (b) the mechanism of deformation is the enhanced rate of matter transport through the liquid phase segregated in the grain boundaries; (c) either uniaxial compression or tension tests may be used to determine the flow properties, except that a correction must be implemented for friction in the case of compression tests; and (d) microstructural changes can occur during deformation which influence the flow behavior. The ceramic is almost infinitely ductile in compression, whereas in tension elongations as large as W5% in one material, and more than 400% in another, were obtained. A model material, β-spodumene glass-ceramic, was used for this study but the results are likely to hold for other materials with equivalent microstructures, e.g., liquid-phase-sintered or hot-pressed materials such as the nitrogen ceramics

140 citations


Patent
15 Feb 1984
TL;DR: In this paper, a boride-based ceramics infiltrated with aluminum are used as components of electrolytic cells for the production of aluminum by molten salt electrolysis, and the reaction with the bulk reaction mixture in contact with molten metal which moderates the reaction and infiltrates the resulting ceramic body.
Abstract: A cermet is produced by providing a bulk reaction mixture of particulate reactants which react to produce a self-sustaining ceramic body, and carrying out the reaction with the bulk reaction mixture in contact with molten metal which moderates the reaction and infiltrates the resulting ceramic body. The method is particularly applicable for boride-based ceramics infiltrated with aluminum, suitable as components of electrolytic cells for the production of aluminum by molten salt electrolysis.

132 citations


Patent
Charles V. Rue1
08 Aug 1984
TL;DR: In this article, sintered sol-gel aluminous abrasive grits, comprising sub-micron sized alpha alumina crystals, in vitrified (ceramic) wheels were used for abrasive performance superior to premium grade fused alumina in grinding tool-steels.
Abstract: Bonding of sintered sol-gel aluminous abrasive grits, comprising sub-micron sized alpha alumina crystals, in vitrified (ceramic) wheels results in abrasive performance superior to premium grade fused alumina in grinding tool-steels. Viscosity of the bond and/or temperature of firing must be controlled to avoid reaction between the bond and the abrasive.

108 citations


Patent
Yasuhiro Fukaya1, Hirai Shozo1
13 Dec 1984
TL;DR: A method for bonding oxide ceramics and metals which comprises the steps of depositing, on an oxide ceramic material, a composite insert material consisting of a mixture of at least one oxide selected from the group consisting of Cu 2 O, NiO, SiO 2, FeO, AgO, Al 2 O 3, MoO, TiO 2, ZnO, AuO, Cr 2 O3, CoO, ZrO2, ZbO, TaO, Nb and MgO as discussed by the authors.
Abstract: A method for bonding oxide ceramics and metals which comprises the steps of depositing, on an oxide ceramic material, a composite insert material consisting of a mixture of at least one oxide selected from the group consisting of Cu 2 O, NiO, SiO 2 , FeO, AgO, Al 2 O 3 , MoO, TiO 2 , ZnO, AuO, Cr 2 O 3 , CoO, ZrO 2 , TaO, WO 2 , NbO, MgO, CaO and Y 2 O 3 , and at least one metal selected from the group consisting of Cu, Ni, Si, Fe, Ag, Al, Mo, Ti, Zn, Au, Cr, Co, Zr, Ta, W, Nb and Mg by ion plating or spray coating, subjecting the deposited insert material to thermal reaction-promoting treatment to permit metallurgical bond between the composite insert material and the ceramic material, and bonding a metal member to the surface of the insert material by diffusion welding or brazing.

89 citations


Patent
James W. Warren1
04 Jun 1984
TL;DR: In this paper, a composite structural ceramic article and method of fabrication thereof is described, which is specifically designed for use in high temperature, corrosive and erosive environments such as those found in heat engines, heat exchangers, stationary power equipments and industrial process equipments.
Abstract: A composite structural ceramic article and method of fabrication thereof. The article of the invention is specifically designed for use in high temperature, corrosive and erosive environments such as those found in heat engines, heat exchangers, stationary power equipments and industrial process equipments. The article comprises a porous carbon fibrous substrate or other suitable high temperature fibrous substrate which may include: a pyrolytic carbon or appropriate chemical vapor deposited sheath formed about each fiber of the substrate; a chemically vapor deposited metallic carbide, oxide, boride or nitride coating over the coated fibers of the substrate; and an impermeable metallic carbide, oxide, boride or nitride outer protective layer formed about the entire periphery of the coated substrate. In accordance with the method of the invention, the metallic coating is applied to the fibers in such a manner such that internal porosity of the article is precisely controlled and a flaw resistant, tough, non-catastrophic failing structural ceramic body is formed.

84 citations


Journal Article
TL;DR: The fine-grained varistors exhibited high breakdown fields, compared to commercial materials as discussed by the authors, and they were used to fabricate ZnO varistors having grain sizes in the 3 to 4 µm range compared to 7 to 13 µm in commercial devices.
Abstract: Advanced ceramic processing techniques were used to fabricate ZnO varistors having grain sizes in the 3 to 4 ..mu..m range, compared to 7 to 13 ..mu..m in commercial devices. The fine-grained varistors exhibited high breakdown fields, compared to commercial materials. Processing steps included synthesis of highly active powders by sol-gel techniques and hot pressing at temperatures below about 800/sup 0/C in a reducing environment to achieve high density. Heat treatment in air at temperatures up to 1000/sup 0/C fully developed the nonohmic electrical properties. 13 references, 6 figures, 4 tables.

Book
01 Jul 1984
TL;DR: In this article, defects in ceramic materials are discussed, including aqueous dispersions of ZrO/sub 2/ and Al/Sub 2/O//sub 3, and Co/sub2/laser processing of ultra-high-purity Al/ Sub 2 O/sub O/Sub 3, isostatically pressed niobium carbide, dislocations in non-metallic crystalline materials, ion beam, rapid thermal and laser mixing phenomena; ion implantation of crystalline oxides; optical properties of YAGs, mechanical properties
Abstract: These proceedings collect papers on defects in ceramic materials. Topics include: aqueous dispersions of ZrO/sub 2/ and Al/sub 2/O/sub 3/, Co/sub 2/-laser processing of ultra-high-purity Al/sub 2/O/sub 3/, isostatically pressed niobium carbide, dislocations in non-metallic crystalline materials, alkali borate classes, ion beam, rapid thermal, and laser mixing phenomena; ion implantation of crystalline oxides; optical properties of YAGs, mechanical properties of ZrO/sub 2/-containing ceramics, and yttria-stabilized zirconia polycrystals.

Patent
07 Sep 1984
TL;DR: In this paper, a chemical vapor deposition (CVD) process for preparing fiber-reinforced ceramic composites is described, where a specially designed apparatus provides a steep thermal gradient across the thickness of a fibrous preform.
Abstract: A chemical vapor deposition (CVD) process for preparing fiber-reinforced ceramic composites. A specially designed apparatus provides a steep thermal gradient across the thickness of a fibrous preform. A flow of gaseous ceramic matrix material is directed into the fibrous preform at the cold surface. The deposition of the matrix occurs progressively from the hot surface of the fibrous preform toward the cold surface. Such deposition prevents the surface of the fibrous preform from becoming plugged. As a result thereof, the flow of reactant matrix gases into the uninfiltrated (undeposited) portion of the fibrous preform occurs throughout the deposition process. The progressive and continuous deposition of ceramic matrix within the fibrous preform provides for a significant reduction in process time over known chemical vapor deposition processes.

Patent
26 Dec 1984
TL;DR: In this paper, a honeycomb is constructed from a mixture of raw materials comprising per 100 parts by weight of the mixture 40 to 90 parts of cordierite powder and 10 to 60 parts of crystalline glass powder having a solid solution of β-spodumene.
Abstract: A ceramic material for a honeycomb structure is prepared from a mixture of raw materials comprising per 100 parts by weight of the mixture 40 to 90 parts by weight of cordierite powder and 10 to 60 parts by weight of crystalline glass powder having a crystalline phase consisting mainly of a solid solution of β-spodumene. The mixture consists essentially of 51.5 to 64.4% by weight of siO 2 , 24.8 to 33.7% by weight of Al 2 O 3 , 5.5 to 12.4% by weight of MgO and 0.4 to 2.7% by weight of Li 2 O. It may further contain up to 5.1% by weight of TiO 2 and up to 1.8% by weight of ZrO 2 .

Journal ArticleDOI
TL;DR: In this paper, the optimal spraying parameters of various wear-resistant ceramic and cermet coatings are briefly described and the relationship between wear damage and their microstructure, physical properties and especially their thermal diffusivity is discussed.

Patent
January James R1
17 Dec 1984
TL;DR: In this paper, a method of preparing carbon-containing monolithic glassy ceramics from organosilsesquioxanes, metal oxides and metal alkoxides through pyrolysis of their gels is presented.
Abstract: What is disclosed is a method of preparing carbon-containing monolithic glassy ceramics from organosilsesquioxanes, metal oxides and metal alkoxides through pyrolysis of their gels. Also disclosed are certain gel compositions used in the method and the glassy ceramics.


Journal ArticleDOI
TL;DR: Hexagonal ferrites with M, Y and W structures were prepared from powders obtained by the decomposition of organometallic salt (citrate) solidified solutions, due to the improved reactivity of these powders with respect to the classical ceramic ones, a number of advantages were obtained: pure hexaferrites were produced at lower temperature; they exhibited improved magnetization and coercitivity; a finer and homogeneous granulometry was achieved in the final products as discussed by the authors.
Abstract: Hexagonal ferrites with M, Y and W structures were prepared from powders obtained by the decomposition of organometallic salt (citrate) solidified solutions, Due to the improved reactivity of these powders with respect to the classical ceramic ones, a number of advantages were obtained: pure hexaferrites were produced at lower temperature; they exhibited improved magnetization and coercitivity; a finer and more homogeneous granulometry was achieved in the final products. X-Ray diffractometry, magnetic analysis and SEM microscope observations were used for characterizing the obtained materials.

Journal ArticleDOI
TL;DR: In this article, hot-pressed Si3N4 was joined using an Mgo-A12O3-SiO2 glass composition chosen to approximate the oxide portion of the grain-boundary phase in the ceramic.
Abstract: Hot-pressed Si3N4 was joined using an Mgo-A12O3-SiO2 glass composition chosen to approximate the oxide portion of the grain-boundary phase in the ceramic. After it has been heated at 1550° to 1650°, the interface of the joined ceramic is an interlocking mixture of Si2N2O, β-Si3N4, and a residual oxy-nitride glass. The kinetics of reactions between Si3N4 and the molten joining composition were studied by X-ray diffraction analysis of the phases present in Si3N4 powder-glass mixtures quenched after varied heat treatments. Analytical transmission electron microscopy of the composition and micro-structure of the reaction zone in joined specimens, together with the X-ray diffraction results, suggests that the driving force for joining is the lowering of the Si3N4 interfacial energy when it is wet by the molten silicate, augmented by the negative Gibbs energy for the reaction SiO2(l) + Si3N4= 2Si2N2O.

Patent
24 Dec 1984
TL;DR: The direct liquid phase bonding of ceramics to metals or other materials in an inert atmosphere without prior metallization was studied in this article, where a nickel-plated aluminum-silicon brazing alloy or aluminum or aluminum alloy filler material between the ceramic and metal layers provided a joint of high strength at a low bonding temperature without affecting the ceramic microstructure or the mechanical properties of the materials.
Abstract: The direct liquid phase bonding of ceramics to metals or other ceramics in an inert atmosphere without prior metallization wherein a nickel-plated aluminum-silicon brazing alloy or aluminum or aluminum alloy filler material between the ceramic and metal layers provides a joint of high strength at a low bonding temperature without affecting the ceramic microstructure or the mechanical properties of the materials. This technique is also useful for fabricating composite laminates.

Patent
26 Apr 1984
TL;DR: In this article, a reticulated ceramic portion defined by a plurality of interconnecting webs having a pore distribution of between 5 and 125 (ppi) pores per linear inch and a ceramic coating portion sintered to the webs along a surface defined by one face of said coating.
Abstract: A ceramic structure comprises a reticulated ceramic portion defined by a plurality of interconnecting webs having a pore distribution of between 5 and 125 (ppi) pores per linear inch and a ceramic coating portion sintered to the webs along a surface defined by one face of said coating. The coating has a thickness less than about 3 millimeters. The ratio of average thickness of the coating to the thickness of the web forming the reticulated ceramic is between 1 and 10.

Patent
01 Jun 1984
TL;DR: An article comprised of metal directly and adherently bonded onto a ceramic substrate, and a process for producing same, wherein the ceramic is adhesion promoted with molten inorganic compound, treated with halide compounds that promote adsorption of catalyst for metal deposition, and plated with metal.
Abstract: An article comprised of metal directly and adherently bonded onto a ceramic substrate, and a process for producing same, wherein the ceramic is adhesion promoted with molten inorganic compound, treated with halide compounds that promote adsorption of catalyst for metal deposition, and plated with metal. Uniform surface coverage with catalyst and metal is ensured by the halide treatment.

Patent
04 Jun 1984
TL;DR: In this article, a process for producing ultrafine particles of a ceramic, which comprises heating a ceramic having substantially the same components as the final ultrafine ceramic particles, a metal constituting the metal component of the final fine ceramic particles was described.
Abstract: A process for producing ultrafine particles of a ceramic, which comprises heating a ceramic having substantially the same components as the final ultrafine ceramic particles, a metal constituting the metal component of the final ultrafine ceramic particles, a mixture of said ceramic with said metal or carbon, or a mixture of said metal with carbon by an arc plasma or a high frequency induction plasma generated in hydrogen, nitrogen, oxygen, a gaseous mixture of hydrogen and nitrogen, or a gaseous mixture of nitrogen and oxygen.

Patent
18 Jan 1984
TL;DR: In this article, a gas turbine engine containing core engine components made of silicon carbide fiber reinforced glass matrix material is described, which is a high strength, fracture-tough, high temperature oxidatively stable, core engine component.
Abstract: High strength, fracture tough, high temperature oxidatively stable, gas turbine engine core engine components are described made of silicon carbide fiber reinforced ceramic matrix or silicon carbide fiber reinforced glass matrix material. A gas turbine engine containing core engine components as above described is also disclosed.

Patent
09 Jan 1984
TL;DR: In this paper, a method for preparing the articles is also disclosed wherein the appropriate mixture in a hypoeutectic, eutectics, or hypereutectical ratio is heated to its melting point, poured into a preheated mold and cooled.
Abstract: Ceramic shaped articles are disclosed which are formed of dense, non-metallic, mechanically resistant materials and contain eutectic constituents consisting of zirconium oxide, hafnium oxide and at least one other high-melting oxide and mixtures thereof. The composition exhibits excellent hardness, wear properties and bending strength. A method for preparing the articles is also disclosed wherein the appropriate mixture in a hypoeutectic, eutectic, or hypereutectic ratio is heated to its melting point, poured into a preheated mold and cooled.

Patent
02 Jul 1984
TL;DR: In this article, a structural component which is coolable for use under high thermal load conditions, such as a turbine blade, has a metallic support core with cooling ducts separated by lands in its surface.
Abstract: A structural component which is coolable for use under high thermal load conditions, such as a turbine blade, has a metallic support core with cooling ducts separated by lands in its surface. The core and its cooling ducts and lands are enclosed by an inner layer of metal felt and an outer layer of heat insulating ceramic material which partially penetrates into the metal felt to form a bonding zone between the felt and the ceramic material. Thus, any heat passing through the ceramic layer is introduced into the large surface area of the metal felt enabling the latter to efficiently introduce the heat into a cooling medium flowing in the ducts, thereby preventing thermal loads from adversely affecting the metal core to any appreciable extent.

Patent
21 May 1984
TL;DR: In this paper, a process for forming a high density solder pad and fan-out metallurgy system in a green-ceramic substrate is described, where a pattern of indented lines is formed in the surface of a green ceramic substrate, the lines filled with a conductive metal paste, a layer of dielectric green ceramic material is deposited over at least a portion of the area of the pattern, and sintering the resultant substrate.
Abstract: A process for forming a high density solder pad and fan-out metallurgy system in a ceramic substrate wherein a pattern of indented lines is formed in the surface of a green ceramic substrate, the lines filled with a conductive metal paste, a layer of dielectric green ceramic material deposited over at least a portion of the area of the pattern of indented lines, and sintering the resultant substrate.

Patent
21 Dec 1984
TL;DR: In this article, a multi-layer ceramic wiring board consisting of alternately laminated ceramic insulating substrates and wiring conductor layers in which the inner layer of the insulating substrate contacting the inside layers of signal wiring conductor are made of a ceramic insulator having a relative dielectric constant of 6.0 or below and a relatively low density and the ceramic insulators of other layers are made with a tensile strength of 4 kgf/mm 2 or above.
Abstract: A multi-layer ceramic wiring board consisting of alternately laminated ceramic insulating substrates and wiring conductor layers in which the inner-layer ceramic insulating substrates contacting the inside layers of signal wiring conductor are made of a ceramic insulator having a relative dielectric constant of 6.0 or below and a relatively low density and the ceramic insulating substrates of other layers are made of a ceramic insulator having a tensile strength of 4 kgf/mm 2 or above and a relatively high density. In preparation of the board, each of the inner-layer ceramic insulating substrates is preferably formed from a green sheet obtained by drying a thin layer of a slurry containing 70 to 90 parts of ceramic filler particles having a particle size of 5 μm or greater and 30 to 10 parts of glass powder while each of other ceramic insulating substrates is preferably formed from a green sheet obtained by drying a thin layer of a slurry containing 70 to 90 parts of ceramic filler particles having a particle size of less than 5 μm and 30 to 10 parts of glass powder.

Journal ArticleDOI
TL;DR: In this article, a bibliografic review of the thermal sprayed ceramics is performed and it appears that plasma spraying is the most common technique, that ceramic coatings are most often made of the metal oxides and their main application is for wear resistance.

Journal ArticleDOI
TL;DR: In this paper, the Young's modulus and internal friction dependence on temperature in alumina, silicon nitride, and partially stabilized zirconia (Y-PSZ) ceramics were investigated.
Abstract: A study is conducted of the Young's modulus and internal friction dependence on temperature in alumina, silicon nitride, and partially stabilized zirconia (Y-PSZ) ceramics. While little change in internal friction was noted in the case of alumina, an increase was found for silicon nitride ceramics above 700 C. The internal friction of Y-PSZ markedly increased with temperature, up to a peak at about 200 C. 7 references.