Effect of Hydrogen-Water Atmospheres on Corrosion and Flexural Strength of Sintered α-Silicon Carbide

Sintered α-SiC was exposed for 10 h to H₂ containing various partial pressures of H₂O ( P _H2O from 5×10⁻⁶ to 2×10⁻² atm; 1 atm≅10⁵ Pa) at 1300° and 1400°C. Weight loss, surface morphology, and room-temperature flexural strength were strongly dependent on P _H2O. The strength of the SiC was not significantly affected by exposure to dry H₂ at a P _H2O of 5×10⁻⁶ atm; and following exposure at P _H2O >5×10⁻³ atm, the strength was even higher than that of the as-received material. The increase in strength is thought to be the result of crack blunting associated with SiO₂ formation at crack tips. However, after exposure in an intermediate range of water vapor pressures (1×10⁻⁵< P _H2O <1×10⁻³ atm), significant decreases in strength were observed. At a P _H2O of about 1×10⁻⁴ atm, the flexural strength decreased approximately 30% and 50% after exposure at 1300° and 1400°C, respectively. The decrease in strength is attributed to surface defects caused by corrosion in the form of grain-boundary attack and the formation of pits. The rates of weight loss and microstructural changes on the exposed surfaces correlated well with the observed strength changes.     

Oxidation of HIPed TiC Ceramics in Dry O2, Wet O2, and H2O Atmospheres

Isothermal oxidation of dense TiC ceramics, fabricated by hot-isostatic pressing at 1630°C and 195 MPa, was performed in Ar/O₂ (dry oxidation), Ar/O₂/H₂O (wet oxidation), and Ar/H₂O (H₂O oxidation) at 900°–1200°C. The weight change measurements of the TiC specimen showed that the dry, wet, and H₂O oxidation at 850°–1000°C is represented by a one-dimensional parabolic rate equation, while the oxidation in the three atmospheres at 1100° and 1200°C proceeds linearly. Cross-sectional observation showed that the dry oxidation produces a lamellar TiO₂ scale consisting of many thin layers, about 5 μm thick, containing many pores and large cracks, while H₂O-containing oxidation decreases pores in number and diminishes cracks in scales. Gas evolution of CO₂ and H₂ with weight change measurement was simultaneously followed by heating the TiC to 1400°C in the three atmospheres. Cracking in the TiO₂ scale accompanied CO₂ evolution, and the H₂O-containing oxidation produced a small amount of H₂. A piece of single crystal TiC was oxidized in ¹⁶O₂/H₂¹⁸O to reveal the contribution of O from H₂O to the oxidation of TiC by secondary ion mass spectrometry.     

Strength of Hot Isostatically Pressed Si3N4 After Heat Treatment in Ar-O2 and H2-H2O

The effects of heat treatment in Ar-O₂ and H₂-H₂O atmospheres on the flexural strength of hot isostatically pressed Si₃N₄ were investigated. Increases in room-temperature strength, to values significantly above that of the aspolished material, were observed when the Si₃N₄ was exposed at 1400°C to (1) H₂ with water vapor pressure ( P _H2O) greater than 1 × 10⁻⁴ MPa or (2) Ar with oxygen partial pressure ( P _O2) of between 7 × 10⁻⁶ and 1.5 × 10⁻⁵ MPa. However, the strength of the material was degraded when the P _H2O in H₂ was lower than 1 × 10⁻⁴ MPa, and essentially unaffected when the P _O2 in Ar was higher than 1.5 × 10⁻⁵ MPa. We suggest that the observed strength increases are the result of strength-limiting surface flaws being healed by a Y₂Si₂O₇ layer formed during exposure.     

Steam Sintering of Uranium Dioxide

The following variables were considered in an investigation of the sintering of uranium dioxide: (1) sintering aids, i.e., small additions of substances that might aid the densification process, (2) soaking temperature, (3) soaking time, (4) furnace atmosphere, and (5) particle size of the uranium dioxide. The incipient sintering temperature, the effect of binders, and the effect of fabricating pressure also were determined. The data obtained in the investigation led to the following optimum conditions for obtaining dense UO₂ by cold pressing and sintering: (1) preparing the UO₂ as a submicron powder but not fine enough to be pyrophoric, (2) cold pressing at 20 to 30 tons per sq. in. without the use of organic binders, (3) using the following sintering schedule: ( a ) in H₂ to 1400°C., (b) in steam or a wet inert gas from 1400° to 1500°C., ( c ) in steam or a wet inert gas at 1500°C. for a 1-hour soak, and (d) cooling in H₂ from 1500°C. to room temperature. By following this procedure, pellets of UO₂ (3/8 in. in diameter by 1/4 in. high) were obtained that were 97 to 98% of theoretical density.     

Development of Surface Compressive Stresses in Zirconia–Alumina Composites by an Ion-Exchange Process

A two-step ion-exchange technique was developed for introducing compressive stresses on the surface of ZrO₂–Al₂O₃ composites. In the first step, a thin layer (∼250 μm) of Na-β"-Al₂O₃ was formed on the surface of the composite by a vapor-phase process at ∼1400°C. In the second step, Na⁺ ions were replaced by K⁺ ions by a heat treatment at ∼385°C for 2 h in a molten KNO₃ bath. Replacement of sodium by potassium led to the creation of surface compressive stresses. The flexural strength and Weibull modulus of ZrO₂–Al₂O₃ composite were ∼915 MPa and 10, respectively, for the as-sintered samples. By contrast, the flexural strength and Weibull modulus were ∼1140 MPa and 26, respectively, for the ion-exchanged samples. A residual surface compressive stress of ∼480 MPa was measured by a strain-gauge technique in K⁺-ion-exchanged samples. The presence of surface compressive stresses also was confirmed using an indentation technique. The technique developed here can be used to introduce compressive stresses on components of virtually any shape.     

Densification and Cell Performance of Gadolinium-Doped Ceria (GDC) Electrolyte/NiO–GDC Anode Laminates

A thin film (60 μm thick) of a gadolinium-doped ceria (GDC) electrolyte was prepared by the doctor blade method. This film was laminated with freeze-dried 42 vol% NiO–58 vol% GDC mixed powder and pressed uniaxially or isostatically under a pressure of 294 MPa. This laminate was cosintered at 1100 °–1500 °C in air for 4–12 h. The laminate warped because of the difference in the shrinkage of the electrolyte and electrode during the sintering. A higher shrinkage was measured for the electrode at 1100 °–1200 °C and for the electrolyte at 1300 °–1500 °C. The increase of the thickness of anode was effective in decreasing the warp and in increasing the density of the laminated composite. The maximum electric power density with a SrRuO₃ cathode using 3 vol% H₂O-containing H₂ fuel was 100 mW/cm² at 600 °C and 380 mW/cm² at 800 °C, respectively, for the anode-supported GDC electrolyte with 30 μm thickness.     

Strengthening of Ceria-Doped Tetragonal Zirconia Polycrystals by Reduction-Induced Phase Transformation

Phase-transformation-induced compressive surface stresses were introduced into ceria-doped tetragonal zirconia polycrystals by reduction of CeO₂. Four-point-bending strength of sintered ZrO₂ containing 12 mol% CeO₂ increased from 240 to 545 MPa after it was annealed at 1400°C for 2 h in nitrogen. The strength of the same material hot isostatically pressed in oxygen increased after it was annealed in nitrogen for 2 h at 1500°C from 430 to 595 MPa.     

Thermal, Mechanical, and Chemical Properties of Sintered Xenotime-Type RPO4 (R = Y, Er, Yb, or Lu)

Xenotime-type RPO₄ (R = Y, Er, Yb, or Lu) powder was dry-pressed into disks and bars. The disks and bars could be sintered to a relative density of greaterthan equal to98% in air without cracking at 1300° (R = Yb or Lu) or 1500°C (R = Y or Er), depending on the grain size. The linear thermal expansion coefficient (at 1000°C), thermal conductivity (at 20°C), and bending strength (at 20°C) of the xenotime-type RPO₄ ceramics were 6.2 10^-6/°C, 12.02 W(mK)^-1, and 95 ± 29 MPa for R = Y; 6.0 10^-6/°C, 12.01 W(mK)^-1, and 100 ± 21 MPa for R = Er; 6.0 10^-6/°C, 11.71 W(mK)^-1, and 135 ± 34 MPa for R = Yb; and 6.2 10^-6/°C, 11.97 W(mK)^-1, and 155 ± 25 MPa for R = Lu. The xenotime-type RPO₄ ceramics did not react with SiO₂, TiO₂, Al₂O₃, ZrO₂, or ZrSiO₄, even at 1600°C for 3 h in air, and were stable in aqueous solutions of HCl, H₂SO₄, HNO₃, NaOH, and NH₄OH at 20°C.     

Microstructure and Mechanical Properties of Mullite Prepared by the Sol-Gel Method

Mullite (3Al₂O₃·2SiO₂) of stoichiometric composition was prepared by mixing boehmite sol and silica dispersion and gelling at a pH of 3. Complete mullitization takes place at or above 1300°C. Ultrafine mullite powder prepared by calcining gel at 1400°C and attrition milling could be sintered to >98% (theoretical density) at 1650°C for 1.5 h. The flexural strength of the sintered body at room temperature was 405 MPa and 350 MPa at 1300°C. Only traces of a secondary phase were observed along the grain boundary.     

Preparation of High-Purity ZrSiO4 Powder Using Sol–Gel Processing and Mechanical Properties of the Sintered Body

Effects of the concentration of ZrOCl₂, calcination temperature, heating rate, and the size of secondary particles after hydrolysis on the preparation of high-purity ZrSiO₄ fine powders from ZrOCl₂.8H ₂ O (0.2 M to 1.7 M ) and equimolar colloidal SiO₂ using sol–gel processing have been studied. Mechanical properties of the sintered ZrSiO₄ from the high-purity ZrSiO₄ powders have been also investigated. Single-phase ZrSiO₄ fine powders were synthesized at 1300°C by forming ZrSiO₄ precursors having a Zr–O–Si bond, which was found in all the hydrolysis solutions, and by controlling a secondary particle size after hydrolysis. The conversion rate of ZrSiO₄ precursor gels to ZrSiO₄ powders from concentrations other than 0.4 M ZrOCl₂.8H₂O increased when the heating rate was high, whereupon the crystallization of unreacted ZrO₂ and SiO₂ was depressed and the propagation and increase of ZrSiO₄ nuclei in the gels were accelerated. The density of the ZrSiO₄ sintered bodies, manufactured by firing the ZrSiO₄ compacts at 1600° to 1700°C, was more than 95% of the theoretical density, and the grain size ranged around 2 to 4 μm. The mechanical strength was 320 MPa (room temperature to 1400°C), and the thermal shock resistance was superior to that of mullite and alumina, with fairly high stability at higher temperatures.     

Kinetics of Silicon Monoxide Ammonolysis for Nanophase Silicon Nitride Synthesis

Silicon monoxide vapor generated from Si/SiO₂ mixed-powder compacts was used with NH₃ to synthesize silicon nitride in a tubular flow reactor operated at temperatures in the range of 1300°-1400°C. The ammonolysis of SiO with excess NH₃ was very rapid, yielding three different types of silicon nitride at different longitudinal locations in the reactor: amorphous nanophase powder of an average size of about 20 nm, amorphous whiskers of a few micrometers in diameter, and α-polycrystals. The amorphous products were heat-treated for crystallization at temperatures between 1300° and 1560°C in a stream of dissociated NH₃, N₂, or N₂/H₂ mixture gas. When dissociated NH₃ was used, nanophase powder was crystallized at 1300°C. The yield of nanophase silicon nitride from SiO varied from 13% to 43%, depending on operating conditions.     

Microstructure and Mechanical Properties of Hot-Pressed α-Al2O3-Seeded γ-Alumina Ceramics

High-quality alumina ceramics were fabricated by a hot pressing with MgO and SiO₂ as additives using α-Al₂O₃-seeded nanocrystalline γ-Al₂O₃ powders as the raw material. Densification behavior, microstructure evolution, and mechanical properties of alumina were investigated from 1250°C to 1450°C. The seeded γ-Al₂O₃ sintered to 98% relative density at 1300°C. Obvious grain growth was observed at 1400°C and plate-like grains formed at 1450°C. For the 1350°C hot-pressed alumina ceramics, the grain boundary regions were generally clean. Spinel and mullite formed in the triple-grain junction regions. The bending strength and fracture toughness were 565 MPa and 4.5 MPa·m^1/2, respectively. For the 1300°C sintered alumina ceramics, the corresponding values were 492 MPa and 4.9 MPa·m^1/2.     

Ceramic Properties of Samarium Oxide and Gadolinium Oxide; X-Ray Studies of Other Rare-Earth Oxides and Some Compounds

Samarium oxide forms a ceramic of medium strength and density (6.0) when it is fired at 1300°C. When it is fired at 1500°C., its density is increased to 7.4 but it loses its stability toward boiling water. X-ray data are given concerning the structure of the oxide as received and at 1300°C. and of the reaction product of Sm203 with water. Gadolinium oxide forms a ceramic of somewhat higher density (7.0) at 1300° than does Sm₂O₃; at 1500°C. its density is slightly higher (7.6) whereas its stability toward water is unchanged. Data concerning both oxides include density, shrinkage, moduli of elasticity and of rupture, linear thermal expansion, differential thermal analysis, and specific heat as well as X-ray data. Results are given of a brief survey of the reaction products obtained when Sm₂O₃ and Gd₂O₃ are heated at 1500°C. in equimolecular quantities with Al₂O₃, BaO, CaO, CdO, Fe₂O₃, HfO₂, MgO, SO₂, SrO, ThO₂, and ZrO₂. X-ray data for the following rare-earth oxides as received and after calcination at 1400°C. are given: ceria, praseodymia, neodymia, europia, dysprosia, holmia, erbia, thulia, and ytterbia.     

Wetting in an Electronic Packaging Ceramic System: II, Wetting of Alumina by a Silicate Glass Melt under Controlled pO2 Conditions

The wetting of polycrystalline alumina by a colored, calciamagnesia aluminosilicae glass was found to be dependent on temperatutre between 1300° and 1500°C, but independent of gas atmosphere effects. Neither the oxygen partial pressure, oveer the range of 10^-6 to 10^-10 Pa, the gas buffer system (Co/CO₂ or H₂/H₂O), nor pre-equilibration of the substrate surface with the atmosphere at tge exoperimental temperature before solid-liuid interface formation affected the stable contact angle. An initial drop in contact angle the stable contact angle. An initial drop in contact angle occurring within the first hour is attributed to repaid dissolution of alumina and the formation of a stable glass/alumina interface. The contact angle after an 8-h isothermal hold decreased from 1300° to 1500°C. The solid-liquid interfacial energy, μM_S1, controls the wetting behavior. Changes in μM_s1 are attributed to he breakup of the silica network as temperature increases.     

Effect of CaO Content on the Hot Strength of Alumina-Spinel Castables in the Temperature Range of 1000° to 1500°C

The modulus of rupture of Al₂O₃-spinel castables containing 20 wt% Al₂O₃-rich MgO-Al₂O₃ spinel and 1.36-2.04 wt% CaO generally increases with an increase in both CaO content and temperature from 1300° to 1500°C, but it remains virtually constant from 1000° to 1300°C. Microscopic observation of the castable fired at 1500°C for 3 h reveals the growth of some CA₆ crystals out of the Al₂O₃-rich spinel grains in the bonding matrix of the castable. The bond linkage between the CA₆ and spinel grains in the matrix is believed to cause both the CaO content and temperature dependence of the hot strength of the Al₂O₃-spinel castables as well as the hot strength enhancement of high-Al₂O₃ castables with addition of Al₂O₃-rich spinel.     

Phase Equilibria and Physical Properties of Oxygen-Deficient Zirconia and Thoria

The compositions of anion-deficient zirconia and thoria in equilibrium with O₂ were measured from 1 to 10⁻⁶ atm and 1400° to 1900°C; for ZrO_{2- x} (po₂ in atm, and T in °K), log x∼−0.890-[(0.400×10⁴)/ T ]-[(log p )/6]; for ThO_{2- x} , log x∼−1.870-[(0.340×10⁴)/ T ]-[(log p )/6]. The ZrO_{2- x} -Zr boundary was located at x=0.014 at 1800°C; thoria was single-phase over the entire range. Consistent results were obtained when O₂/inert gas mixtures were used, but use of H₂/H₂O and CO/CO₂ at 1000° to 1200°C gave abnormal and, in the latter case, erratic data; side reactions in these atmospheres are inferred. The monoclinic-tetragonal phase change of ZrO₂ and the lattice thermal expansion, room-temperature Young's modulus, and strength properties of ZrO₂ and ThO₂ bodies were not appreciably altered by oxygen deficiency. The lattice dimensions decreased slightly with departure from stoichiometry.     

Oxidation of Silicon Nitride in Wet Air and Effect of Lutetium Disilicate Coating

Oxidation behavior of silicon nitride (Si₃N₄) was investigated in flowing air (2.45 cm/s) containing 10%–50% H₂O at a total pressure of 1.8–10 atm at 1300°–1500°C for 100 h. The oxidation of Si₃N₄ progressed with volatilization of the SiO₂ scale; it was more enhanced at a high partial pressure of H₂O rather than at high temperature. The total pressure had little effect on the oxidation. In order to avoid the oxidation, Si₃N₄ substrate was coated with lutetium disilicate (Lu₂Si₂O₇) layer through the intermediate SiO₂-rich phase. While the coating layer well suppressed the oxidation in case of small amount of water vapor, it was not sufficiently effective to suppress the oxidation when the water vapor was rich. SiO₂ volatilization was observed between the layer and substrate. The flexural strength of the coated Si₃N₄ at room temperature was somewhat increased after the oxidation in wet air, while that of the uncoated one was almost unchanged. This increase was attributable to crack healing of the substrate during the oxidation.     

Threshold Stress Intensity for Crack Growth in Silicon Carbide Ceramics

Measurements of threshold stress intensities for crack growth, K _h, of three polycrystalline SiC materials were attempted using interrupted static fatigue tests at 1200°–1400°C. Weibull statistics were used to calculate conservative K_th values from test results. The K _th of a chemically vapor deposited β-SiC could not be determined, as a result of its wide variations in strength. The K_th ≥ 3.3,2.2, and 1.7 MPa·m^1/2 for an Al-doped sintered α-SiC; and K_th ≥ 3.1, 2.7, and 2.2 MPa·m^1/2 for a hot isostatically pressed α-SiC, both at 1200°, 1300°, and 1400°C, respectively. A damage process concurrent with subcritical crack growth was apparent for the sintered SiC at 1400°C. The larger K_th 's for the HIPed SiC (compared to the sintered SiC) may be a result of enhanced viscous stress relaxation caused by the higher silica content and smaller grain size of this material. Values measured at 1300° and 1400°C were in good agreement with the K_th's predicted by a diffusive crack growth model, while the measured K_th 's were greater than the predicted ones at 1200°C.     

Synthesis and Characterization of a Remarkable Ceramic: Ti3SiC2

Polycrystalline bulk samples of Ti₃SiC₂ were fabricated by reactively hot-pressing Ti, graphite, and SiC powders at 40 MPa and 1600°C for 4 h. This compound has remarkable properties. Its compressive strength, measured at room temperature, was 600 MPa, and dropped to 260 MPa at 1300°C in air. Although the room-temperature failure was brittle, the high-temperature load-displacement curve shows significant plastic behavior. The oxidation is parabolic and at 1000° and 1400°C the parabolic rate constants were, respectively, 2 × 10⁻⁸ and 2 × 10⁻⁵ kg²-m⁻⁴.s⁻¹. The activation energy for oxidation is thus =300 kJ/mol. The room-temperature electrical conductivity is 4.5 × 10⁶Ω⁻¹.m⁻¹, roughly twice that of pure Ti. The thermal expansion coefficient in the temperature range 25° to 1000°C, the room-temperature thermal conductivity, and the heat capacity are respectively, 10 × 10⁻⁶°C⁻¹, 43 W/(m.K), and 588 J/(kgK). With a hardness of 4 GPa and a Young's modulus of 320 GPa, it is relatively soft, but reasonably stiff. Furthermore, Ti₃SiC₂ does not appear to be susceptible to thermal shock; quenching from 1400°C into water does not affect the postquench bend strength. As significantly, this compound is as readily machinable as graphite. Scanning electron microscopy of polished and fractured surfaces leaves little doubt as to its layered nature.     

Strength of Nicalon Silicon Carbide Fibers Exposed to High-Temperature Gaseous Environments

The effects of exposures to high-temperature gaseous atmospheres on the strength of Nicalon SiC fibers were investigated. The exposure conditions were as follows: (1) H₂ with various P _H2O for 10 h at 1000° and 1200°C, and (2) air for 2 to 100 h at 800° to 1400°C. Individual fibers were tested in tension following each exposure. The strengths of the fibers were strongly influenced by the exposure atmosphere and temperature, but less affected by time at temperature. When exposed in air, a SiO₂ layer was formed on the surface, minimizing the degradation of strength. However, this beneficial effect was negated under conditions in which the SiO₂ layer became too thick. The most severe degradation resulted from exposure to a reducing atmosphere, presumably due to the reduction of SiO₂ inherent in the fibers.