Showing papers in "Advanced Ceramic Materials in 1988"
TL;DR: In this article, the superplastic behavior of the composite Y-TZP/Al{sub 2}O{sub 3} (20 wt%) in uniaxial tension has been evaluated.
Abstract: The superplastic behavior of the composite Y-TZP/Al{sub 2}O{sub 3} (20 wt%) in uniaxial tension has been evaluated. Large elongation (>200%) indicated superplasticity. The stress exponent and activation energy of thee composite were found to be the same order with those of Y-TZP. The flow behavior of the composite containing Al{sub 2}O{sub 3} grains could be described by a rheological model as a non-Newtonian flow modified by the second-phase grains. The cavitation damage and the creep crack growth could be reduced by keeping the strain rate low enough that a specimen elongated 100% at elevated temperature maintained a strength of 1,800 MPa at room temperature.
165 citations
TL;DR: In this article, homogeneous and stoichiometric LiNbO{sub 3} films were prepared on substrates from double alkoxide solutions by a dip-coating method.
Abstract: Homogeneous and stoichiometric LiNbO{sub 3} films were prepared on substrates from double alkoxide solutions by a dip-coating method. Structural control of the double alkoxide solution is critical in synthesizing crystalline stoichiometric LiNbO{sub 3} films. The coordination states of alkoxides in solution were monitored by Ir and NMR spectroscopy. A viscosity of the double alkoxide solution was adjusted to a value of 6 {times} 10{sup {minus}3} to 7 {times} 10{sup {minus}3} N{center dot}s/m{sup 2} (6 to 7 cP) by partial hydrolysis and the concentration of the solution. Amorphous LiNbO{sub 3} films prepared on Si- or sapphire-single-crystal substrates from controlled alkoxide solutions began to crystallize at temperatures as low as 250{degree}C. Films on sapphire substrates crystallized with the preferred orientation along each orientation, such as (001), (110), and (012) but not on Si(100) and Si(111) substrates. The crystallinity of the initial thin-film coating of LiNbO{sub 3} significantly influenced the crystallization of films deposited subsequently.
90 citations
TL;DR: In this article, the influence of fiber-matrix interactions for uncoated and BN-coated fibers on fracture morphologies was studied, related to the measured values of interfacial shear strengths.
Abstract: Coatings can modify fiber-matrix reactions and, hence, interfacial bond strengths. Commercial mullite, SiC, and carbon fibers or fabrics were coated, with boron nitride via low-pressure chemical vapor deposition, and incorporated into a mullite matrix by hot-pressing. The influence of fiber-matrix interactions for uncoated and BN-coated fibers on fracture morphologies was studied. These observations were related to the measured values of interfacial shear strengths.
87 citations
83 citations
80 citations
TL;DR: In this paper, a modified indentation technique has been used to measure the interfacial shear stress in a number of ceramic matrices containing silicon carbide fibers, and it has been shown that the frictional component of interfacial stress was essentially zero when matrix thermal expansion was lower than that of the fiber and increased linearly with thermal expansion mismatch.
Abstract: A modified indentation technique has been used to measure the interfacial shear stress in a number of ceramic matrices containing silicon carbide fibers. It was shown that the frictional component of interfacial stress was essentially zero when matrix thermal expansion was lower than that of the fiber and increased linearly with thermal expansion mismatch when matrix thermal expansion was higher. The interfacial shear stress was lowered when the fibers were coated with BN. Lower matrix shear stresses resulted in a more extensive fiber pullout during the composite fracture.
78 citations
TL;DR: In this paper, a silicon carbide powder was synthesized from mixtures of silane and hydrocarbons with one to four carbon atoms irradiated with an unfocused, high-power (1 kW), continuous wave industrial CO{sub 2} laser.
Abstract: Ultrafine silicon carbide powders have been synthesized from mixtures of silane and hydrocarbons (with one to four carbon atoms) irradiated with an unfocused, high-power (1 kW), continuous-wave industrial CO{sub 2} laser. The chemistry of the reaction has been determined by analysis of the gaseous phase, either by infrared spectroscopy or by combined-gas chromatography-mass spectrometry; reaction yields have been determined. A silicon carbide production rate of 30 g/h with a yield > 99% was obtained from silane and acetylene mixtures with a 600-W laser power. An attempt to approach a production rate of 100 g/h at laboratory scale has been successful. Powder characteristics, such as particle size (10 to 50 nm), crystallinity, and stoichiometry can be controlled through optimization of laser intensity, gas pressure, and flow rate. The powders consist of equiaxed particles which exhibit a narrow size distribution.
74 citations
TL;DR: In this article, a solgel process for low-temperature preparation of BaTiO{sub 3} powders is described, where chemical polymerization between barium acetate and titanium(IV) isopropoxide leads to the formation of barium titanate gels.
Abstract: A sol-gel process for low-temperature preparation of BaTiO{sub 3} powders is described. Chemical polymerization between barium acetate and titanium(IV) isopropoxide leads to the formation of barium titanate gels. After suitable drying and calcination treatments, the barium titanate gels were converted to BaTiO{sub 3} powders. The barium titanate gels and powders were characterized by TGA, DTA, and XRD. The results of these analyses indicate that it is possible to obtain stoichiometric (Ba/Ti = 0.99), homogeneous, high-purity BaTiO{sub 3} powders using relatively inexpensive barium acetate as a starting material.
71 citations
TL;DR: In this article, the fracture strength of alumina reinforced with 20 vol% SiC whiskers is maintained at temperatures up to 1,100°C in air, while the apparent toughness decreases with increasing exposure time.
Abstract: The high fracture toughness of alumina reinforced with 20 vol% SiC whiskers is maintained at temperatures up to {approx} 1,100{degree}C in air. The fracture strength values ({sigma}{sub f}) decrease slowly with increasing temperature to 1,100{degree}C; however, when subjected to an applied stress (= 2/3 {sigma}{sub f}) for periods of up to 1,000 h in air at 800{degree}, 1,000{degree}, or 1,100{degree}C, the subsequent fracture strength values (retained strengths) at these temperatures increase with increasing exposure times. On the other hand, above 1,100{degree}C, the fracture strength values decrease while the apparent toughness value increases. At 1,200{degree}C, samples first subjected to applied stresses of {le} 310 MPa for up to {approx} 240 h exhibit a loss in retained strengths with increasing exposure time. The temperature dependence of the mechanical behavior is shown to be related to oxidation reactions in these composites with creep phenomena contributing to the loss in strength and increased toughness above 1,100{degree}C.
61 citations
TL;DR: In this paper, various techniques used to suppress agglomerate formation, which occurs during the synthesis and processing of wet-chemically prepared powders, are reviewed and the characteristics of the green compact required to achieve a high-density material with a homogeneous microstructure at low sintering temperatures are discussed.
Abstract: Uncontrolled agglomerate formation during ceramic powder synthesis and processing results in physical and/or chemical inhomogeneities in the sintered microstructure. The various techniques used to suppress agglomerate formation, which occurs during the synthesis and processing of wet-chemically prepared powders, are reviewed. The characteristics of the green compact required to achieve a high-density material with a homogeneous microstructure at low sintering temperatures are discussed.
54 citations
TL;DR: In this paper, an experimental investigation of the machinability of the ceramic particulate composite SiC-TiB{sub 2} was conducted using an electrical discharge machine sinker cutting process.
Abstract: An experimental investigation of the machinability of the ceramic particulate composite SiC-TiB{sub 2} was conducted using an electrical discharge machine sinker cutting process. Machinability was evaluated in terms of material removal rates, tool wear, wear ratio, and surface finish, using copper and brass electrodes under rapid, medium, and very slow cutting conditions. Copper was found to machine SiC-TiB{sub 2} material faster with less tool wear but with a poorer surface finish than brass. A comparison of the percent of electrode wear in machining of steel to that of SiC-TiB{sub 2} under very slow cutting conditions showed that machining of SiC-TiB{sub 2} is much more efficient than of mild steel.
Abstract: It was shown that infrared transparent, dense mullite can be prepared by using sol-gel processing techniques at temperatures as low as 1,250{degree}C. The molecular structure of the gel and the packing of the particles and particle domains are the key processing parameters that control viscous phase sintering. Heat treatment above 1,280{degree}C depleted the viscous phase through mullite formation, leading to fully dense, crack-free bodies.
TL;DR: In this paper, the optimum fabrication conditions for SiC-whisker-reinforced Si{sub 3}N{sub 4} composite ceramics were determined.
Abstract: Thermodynamic equilibria were calculated to determine optimum fabrication conditions for SiC-whisker-reinforced Si{sub 3}N{sub 4} composite ceramics. Results of calculations indicate that pressureless sintering in a nitrogen atmosphere of 0.1 MPa may be performed at temperatures up to 2,123 K (1,850{degree}C) without whisker or matrix degradation. In addition, gas pressure sintering at temperatures up to 2,373 K (2,100{degree}C) requires a nitrogen pressure of 1 to 10 MPa. Hot isostatic pressing (total pressure {ge} 100 MPa) at 2,273 K (2,000{degree}C) can yield undegraded composite materials only when the nitrogen partial pressure in a gas mixture with an inert gas is kept between 1 to 10 MPa.
TL;DR: In this article, the effects of the additives on the sintering behavior and microstructural alterations were examined using optical and electron microscopy, which revealed that the iron-rich grain-boundary phase(s) existed, both at B{sub 4}C/B{ sub 4} C interfaces and at B {sub 4]C/TiB{sub 2} interfaces.
Abstract: Densities >95% of theoretical were achieved by pressureless sintering of B{sub 4}C-TiB{sub 2} composites (with up to 50 wt% TiB{sub 2}) at 2,150{degree} to 2,175{degree}C using 1 wt% Fe as a sintering aid. Iron levels between 0 and 5 wt% were examined; densities >97% of theoretical were obtained with 1 wt% Fe for a B{sub 4}C-10 wt% TiB{sub 2} composite. Excessive grain growth was observed when excess liquid was present (i.e., high Fe additions), resulting in decreased flexural strength. The effects of the additives on the sintering behavior and microstructural alterations were examined. Microstructural investigations were conducted using optical and electron microscopy. Microstructures consisted of fine-grained B{sub 4}C and intergranular TiB{sub 2} (both phases generally {approx}5 {mu}m in diameter). Transmission electron microscopy revealed that the iron-rich grain-boundary phase(s) existed, both at B{sub 4}C/B{sub 4}C interfaces and at B{sub 4}C/TiB{sub 2} interfaces.
TL;DR: In this paper, a >97% dense, fine-grained composite of alumina and titanium carbide can be produced by a novel cold press and sintering process which utilizes a rapid heating rate.
Abstract: An earlier study demonstrated that a >97% dense, fine-grained composite of alumina and titanium carbide can be produced by a novel cold press and sintering process which utilizes a rapid heating rate. The barrier to pressureless densification of alumina-TiC, which is overcome by rapid rate sintering appears to be the evolution of gaseous reaction products. The heating rate required to produce densities >95% of theoretical, however, resulted in thermal fracture during rapid heating of green compacts which were designed to produce cutting tool inserts {approx} 1 cm in diameter by 0.5 cm thick. The present study describes the development of means for preventing thermal cracking of even larger specimens. Processing conditions are described for achieving near full density with zero open porosity which permits final densification by containerless hot isostatic pressing. The possibility of enhancing densification by controlling the sintering environment was also investigated. Properties and machining performance of rapid rate sintered alumina-TiC cutting tool inserts were measured and compared with those of commercially hot-pressed material of similar chemical composition.
TL;DR: The fracture toughness, strength, and elastic moduli of hot-pressed Si{sub 3}N{sub 4} reinforced with SiC whiskers were measured at room temperature over a range of SiC content from 0 to 20 wt%.
Abstract: The fracture toughness (K{sub IC}), strength, and elastic moduli of hot-pressed Si{sub 3}N{sub 4} reinforced with SiC whiskers were measured at room temperature over a range of SiC content from 0 to 20 wt%. The K{sub IC} was determined from cracks produced by Vickers indentation, the elastic moduli were determined by an ultrasonic technique, and the strength was measured by four-point bending. Although K{sub IC} increases from 4 to 7 MN {center dot} m{sup {minus}3/2} with the addition of 20 wt% SiC, the corresponding increase in strength is relatively small, i.e., from 375 to 550 MPa. Scanning electron microscopy reveals that the small increase in strength is partly related to processing-induced critical flaws.
TL;DR: In this paper, the synthesis of high-temperature material sialon (Si{sub 6{minus}z}Al{sub z}O{sub n}N{sub 8{minus]z}) by carbothermic reduction followed by nitridation has been examined.
Abstract: The synthesis of the high-temperature material sialon (Si{sub 6{minus}z}Al{sub z}O{sub z}N{sub 8{minus}z}) by carbothermic reduction followed by nitridation has been examined. Kaolin, sillimanite, and pyrophillite can be converted to {beta}{prime}-sialon having z values of 2, 2.5, and 0.8, respectively. Other phases that appear in small quantities along with the sialon are mainly alumina and mullite, and trace amounts of AlN and the AlON spinel phase also form. The carbon content at {ge}90% theoretical is very sensitive to nitrogen uptake and phase composition. The uniform mixing of carbon with clay is equally important. Fireclay without an Fe{sub 2}O{sub 3} catalyst produces the same product that does kaolin with an Fe{sub 2}O{sub 3} catalyst.
TL;DR: In this article, the authors used x-ray photon spectroscopy (XPS) to determine the extent of oxygen on as-received and oxidized silicon carbide whiskers.
Abstract: The extent of surface oxidation controls the stability of silicon carbide aqueous suspensions. In the present study, x-ray photon spectroscopy (XPS) was used to determine the extent of oxygen on as-received and oxidized silicon carbide whiskers. The isoelectric points at pH 6.0 and pH 4.0 for the as-received and oxidized whiskers, respectively, correlated with the XPS analyses, indicating essentially a silica surface on the oxidized whiskers and both Si-O and Si-C bonds on the surface of the as-received whiskers. Suggestions are made regarding the utility of the oxidation treatment in processing whisker-reinforced ceramics.
TL;DR: In this paper, high-Tc superconducting oxide fibers were fabricated by the sol-gel method by an aqueous solution of metal acetates was concentrated to form a viscous sol, from which gel fibers were spun.
Abstract: High-{Tc} superconducting oxide fibers were fabricated by the sol-gel method. An aqueous solution of metal acetates was concentrated to form a viscous sol, from which gel fibers were spun. The gel fibers, 5 to 1,000 {mu}m in diameter, were converted to ceramic fibers by heating to 900{degree}C. The fired fibers ({Tc}(onset) of 94 K and {Tc} (end) of 62.2 K) were rough and had porous microstructures.
TL;DR: In this paper, a technique for brazing Si{sub 3}N{sub 4} with metallic alloys was evaluated, which involved vapor coating the ceramic with a 10-{mu}-thick layer of titanium before the bracing operation.
Abstract: A technique for brazing Si{sub 3}N{sub 4} with metallic alloys was evaluated The process involved vapor coating the ceramic with a 10-{mu}-thick layer of titanium before the brazing operation The coating improved wetting of the Si{sub 3}N{sub 4} surfaces to the extent that strong bonding between the solidified braze filler metal and the ceramic occurred Braze joints of Si{sub 3}N{sub 4} were made with Ag-Cu, Au-Ni, and Au-Ni-Pd alloys at temperatures of 790{degree}, 970{degree}, and 1,130{degree}C Silicon nitride specimens were also brazed with a Ag-Cu alloy to the molybdenum alloy TZM, titanium, and A286 steel at 790{degree}C Residual stresses resulting from mismatch of thermal expansion coefficients between the Si{sub 3}N{sub 4} and the metals caused all of the ceramic-to-metal joints to spontaneously crack in the Si{sub 3}N{sub 4} upon cooling from the brazing temperature
TL;DR: In this paper, the effect of palladium oxidation on MLC electrodes is discussed, and the system Pd-Ag-O was investigated in the temperature range in which PdO forms, and a steady state phase diagram in the range of 773 to 1,173 K in air was constructed.
Abstract: Pd, Ag, and their alloys are important components of thick-film compositions used for conductors and multilayer ceramic capacitor (MLC) electrodes. The formation of palladium oxide during firing complicates the processing of these materials. The system Pd-Ag-O was investigated in the temperature range in which PdO forms, and a steady-state phase diagram in the range of 773 to 1,173 K in air was constructed. The effect of palladium oxidation on MLC electrodes is discussed.
TL;DR: Using a hybrid thermal plasma reactor for a new CVD process, thick SiC layers were successfully deposited at a rate of {approx}500 {mu}m/h on a graphite substrate from SiCl{sub 4} and CH{sub4} under soft vacuum (approximately 2.7 {times} 10{sup 4} Pa).
Abstract: Using a hybrid thermal plasma reactor for a new CVD process, thick SiC layers were successfully deposited at a rate of {approx}500 {mu}m/h on a graphite substrate from SiCl{sub 4} and CH{sub 4} under soft vacuum ({approx}2.7 {times} 10{sup 4} Pa). The process is performed typically under flow rates of SiCl{sub 4} = 1.5 g/min and CH{sub 4} = 300 cm{sup 3}/min and deposition temperatures of 1,000{degree} to 1,100{degree}C. The effects of deposition conditions on certain characteristics of the deposited layers were investigated. SEM revealed that the appearance of the deposited layers strongly depended on the substrate position. The prepared layers were dense and nearly stoichiometric {beta}-SiC with (100) preferred orientation. A new method for measuring substrate temperature is also reported, and grown characteristics are discussed.
TL;DR: In this paper, a linear dependence of the unit-cell dimensions on the compositions reveals Vegard's law behavior for the double-substitutional process in the {beta-prime}-sialons.
Abstract: Unit-cell dimensions of {beta}{prime}-sialons have been determined. EDS microanalysis has been used to determine the actual composition of the {beta}{prime}-sialon grains. A linear dependence of the unit-cell dimensions on the compositions reveals Vegard's law behavior for the double-substitutional process in the {beta}{prime}-sialons.
TL;DR: In this paper, fine-grained SiC ceramics were fabricated by a hot-pressing technique using very fine, crystalline SiC powders made from polycarbosilane.
Abstract: Dense, (> 99% of theoretical) fine-grained SiC ceramics were fabricated by a hot-pressing technique using very fine, crystalline SiC powders made from polycarbosilane. The grain sizes were {approx} 0.3 or 0.7 {mu}m when they were hot-pressed at 1,800{degree}C with Al or at 2,050{degree}C with AlN, respectively. Fabrication processes, the characteristics of the synthesized starting SiC powders, and the microstructures and properties of the SiC ceramics were investigated.
TL;DR: In this paper, the electrical conductivity and dielectric behavior of hot-pressed AlN from 300 to 1,000 K and at 60 Hz to 10 kHz is reported. But the results are interpreted in terms of closed and open porosity.
Abstract: The electrical conductivity and dielectric behavior of hot-pressed AlN from 300 to 1,000 K and at 60 Hz to 10 kHz is reported. Temperature dependence of dc conductivity is also measured. To understand the effect of porosity on various electrical and dielectric parameters, measurements were made on four samples having different levels of porosity (0.2 to 15 vol%). The results are interpreted in terms of closed and open porosity.
TL;DR: In this paper, a synthetic route for making carbothermally reduced powders from colloidal oxide precursors trapped in a polymer matrix is described, which is used to synthesize aluminum nitride powder via trapping of colloidal alumina in poly(furfuryl alcohol) resin.
Abstract: A synthetic route is described for making carbothermally reduced powders from colloidal oxide precursors trapped in a polymer matrix. The entrapping resin, which is formed by polymerization of a monomer dissolved in the colloid, serves both to minimize particle agglomeration during reaction and as the source of carbon for reduction. Following reduction, the remaining carbon matrix is removed by oxidation. This strategy was used to synthesize aluminum nitride powder via trapping of colloidal alumina in poly(furfuryl alcohol) resin.
TL;DR: In this article, it was shown that SiC ceramics are corroded by Na{sub 2}SO{sub 4} vapor in an oxidizing atmosphere at 1,200{degree}C for 575 h.
Abstract: Previous work has shown that SiC ceramics are corroded by alkali halide vapor in an oxidizing atmosphere. The present work was conducted to determine if Na{sub 2}SO{sub 4} vapor caused similar corrosion. Specimens were exposed to Na{sub 2}SO{sub 4} vapor in an oxidizing atmosphere at 1,200{degree}C for 575 h. As a result, the specimens were covered with a silicate glass corrosion product, and surface recession had occurred. Corrosion was attributed to chemical alteration of the normally protective SiO{sub 2} film by Na{sub 2}O, which allowed oxidation of the SiC.
TL;DR: In this paper, the authors present a critical review of the literature in terms of the ability to predict fracture behavior from flaws detected by NDE and an understanding of the effect of flaws on fracture behavior.
Abstract: Ceramic materials have been recognized as potential candidates for use in structural applications by virtue of their potentially excellent mechanical integrity and chemical stability at high temperatures. These structural applications include advanced heat engines, heat exchangers, and components subjected to friction and wear. Important structural ceramics include Si{sub 3}N{sub 4}, SiC, and toughened ZrO{sub 2}. It has been observed that the mechanical properties of these ceramics, especially the strength, are often degraded by the presence of flaws. The effective service strength can be increased by eliminating the flaws or eliminating parts that contain flaws. This type of quality assurance requires nondestructive evaluation (NDE) techniques to detect flaws and an understanding of the effect of flaws on fracture behavior; the latter area has been the subject of considerable experimental and theoretical work. This paper presents a critical review of the literature in terms of the ability to predict fracture behavior from flaws detected by NDE.
TL;DR: In this article, the development of microstructure during heating has been investigated for molded Si{sub 3}N{sub 4} bodies fabricated with commercial polysilazane powder and a preceramic polysilasane binder.
Abstract: The development of microstructure during heating has been investigated for molded Si{sub 3}N{sub 4} bodies fabricated with commercial {alpha}-Si{sub 3}N{sub 4} powder and a preceramic polysilazane binder. After the N-methyl polysilazane binder is pyrolyzed at 800{degree}C under N{sub 2}, the bodies have a uniform microstructure composed of an intimate mixture of Si{sub 3}N{sub 4} particles and polysilazane-derived amorphous silicon nitride. After the particles undergo high-temperature treatment at 1,725{degree}C in an N{sub 2} overpressure of {approx} 700 kPa, significant grain growth occurs. SEM and microhardness data suggest local densification in the body and high reactivity for the polysilazane-derived silicon nitride.