Hot Isostatic Pressing of Sintered Silicon Nitride

Pressureless-sintered silicon nitride with varying additives was hot isostatic pressed under 150 MPa of nitrogen at 1800°C. Moderate increases in densities were observed when sintered densities exceeded 90% of theoretical. However, density changes became insignificant as the amount of additives exceeded 12 wt%; moreover, density reduction was occasionally observed. Microstructural analysis, after the silicon nitride was reannealed at 1650°C under 0.1 MPa of nitrogen, revealed that intergranular glass was supersaturated with nitrogen and "bloated" as a result of nitrogen evolution. This result suggested that the effectiveness of container-free hot isostatic pressing of silicon nitride was severely limited by enhanced solubility of nitrogen gas in the glassy phase under high pressure.     

Farbication of Optically Transparent Lead Magnesium Niobate Polycrystalline Ceramics Using Hot Isostatic Pressing

The physical, dielectric, and optical properties of hot isostatically pressed lead magnesium niobate polycrystalline ceramics modified with 1/2 mol% La₂O₃, Pb_{1–3/2 x} La _x □ _{x /2}-(Mg_1/3Nb_2/3)O₃, have been investigated. Methods used to characterize the ceramics included determination of the dielectric permittivity, optical transmittance, and refractive index dispersion. The materials exhibited relaxor ferroelectric type behavior with a peak dielectric constant K > 14000 and average T _c ∼−35°C. Various sintering, hot isostatic pressing, and annealing conditions were examined to produce highly dense and optically transparent materials. Through the use of hot isostatic pressing, densities more than 99.5% theoretical and transmittance greater than 50% at 633-nm wavelength were obtained. Hot isostatic pressing technique appears to be a good alternative to hot uniaxial pressing without the associated problem of PbO volatility, reactivity with the pressure vessel, and geometrical constraints.     

Fabrication, Mechanical Properties, and Electrical Conductivity of Co3O4 Ceramics

Well-densified Co₃O₄ ceramics (98.3% of theoretical) have been fabricated by the combined use of hot pressing (800°C/I h/30 MPa) and hot isostatic pressing (880°C/2 h/196 MPa). Their Vickers hardness and fracture toughness are 10.3 GPa and 4.2 MPa·m^1/2, respectively. They exhibit a high electrical conductivity of 3.35 × 10' S·cm⁻¹ at 800°C.     

Calcination and Sintering Study of Thoria

Data on the effect of calcination temperature on surface area, apparent crystallite size, compactibility, sintered density, and volume shrinkage are presented for thoria prepared from the oxy-carbonate, chloride, nitrate, and oxalate. Surface area and volume shrinkage decreased with rising calcination temperature. Thoria obtained from the oxycarbonate exhibited the greatest sinterability; material derived from the nitrate showed the least. Maximum bulk densities were achieved using material resulting from the calcination of the oxycarbonate between 600° and 100°C. Densities of 95 to 98% of the theoretical value of thoria were attained by compacting these powders at pressures above 20 tsi and firing at 1500°C for 24 hours. Urania-thoria solid solutions incorporating thoria obtained by calcination of the chloride exhibited the highest fired densities.     

Effect of Some Lithium Compounds on Sintering of MgO

The addition of 0.25% or more of certain lithium compounds, particularly the halides, facilitates the pressing and sintering of reactive forms of high-purity magnesium oxide. Densities up to 3.46 gm. per cm.³ have been obtained at 1400°C. with reagent-grade magnesium oxide containing 0.5% lithium chloride as an additive. The effectiveness of lithium salts as sintering catalysts for magnesium oxide depends on the original surface area and particle size of the MgO. After autoclaving at 150 lb. per sq. in. gauge for 2 hours, samples of MgO sintered at 1400°C. with 0.5%of LiCl showed a water absorption comparable to that of fused magnesium oxide. Similar samples of lithium-treated periclase had about the same resistance to attack by a NaOH-FeO melt as pure MgO fired at 1800° to 1850°C.     

Hot Isostatic Processing of Shocked Si3N4 Powder

To enhance the sinter ability of Si₃N₄, powders mixed with 0, 2, and 5 wt% Y₂O₃ were explosively shock-treated. Compacts of these powders were encapsulated in 96% silica glass containers and isostatically hot-pressed. The shocked Si₃N₄ with 5 wt% Y₂O₃ was pressed to a density of 3.09 g/cm³ (95.4% of theoretical) at 1400°C under 430 MPa for 3 h, whereas the unshocked material attained only 82.4% of theoretical density under the same hot isostatic pressing conditions.     

Microstructure-Mechanical Property Relationships in Hot Isostatically Pressed Alumina and Zirconia-Toughened Alumina

The rates of densification and the mechanical properties of pure Al₂O₃ and ZrO₂-toughened Al₂O₃ (ZTA) have been investigated as a function of the temperatures and time schedules used for hot isostatic pressing (HIP) as a postsintering heat treatment for samples which had already been pressureless sintered in air at 1460°C for 45 min. ZTA hot isostatically presed at 1400°C had a finer grain size and a narrower grain size distribution than ZTA hot isostatically pressed at 1600°C. At both HIP conditions, the density which could be obtained was almost the maximum theoretical density. The amount of grinding-induced and fracture-induced monoclinic ZrO₂ formed as a result of the tetragonal → monoclinic martensitic transformation in ZTA was higher in the samples hot isostatically pressed at 1400°C. ZTA hot isostatically pressed at 1600°C and 100 MPa had fewer flaws and higher strengths than ZTA hot isostatically pressed at 1400°C for the same time, with a gradual improvement in mechanical properties with increasing HIP time at each of these two temperatures. The best mechanical properties were obtained from ZTA hot isostatically pressed at 100 MPa and 1600°C for 1 h: these specimens had a four-point bend strength of 940 ± 15 MPa at room temperature and 540 ± 15 MPa at 1000°C and an indentation fracture toughness at room temperature of 9.4 ± 0.2 MPa·m^1/2.     

Preparation and Sintering Behavior of Fine-Grained A12O3-SiO2 Composite

A fine, uniform A1₂O₃-SiO₂ powder was prepared by heterocoagulation of narrow Al₂O₃ and SiO₂ powders. This composite powder was dispersed, compacted, and fired in air at 900° to 1580°C for 1 to 13 h. Full density was achieved at 1550°C with the formation of a mullite phase. Relative densities of 83% and 98% (0.3 μm grain size) were measured for samples sintered at 1200°C for 13 h and at 1400°C for 1 h, respectively.     

Pressure-Induced Phase Transformation of Controlled-Porosity Pb(Zr0.95Ti0.05)O3 Ceramics

Chemically prepared Pb(Zr_0.95Ti_0.05)O₃ (PZT 95/5) ceramics were fabricated with a range of different porosity levels, while grain size was held constant, by systematic additions of added organic pore former (Avicel). Use of Avicel in amounts ranging from 0 to 4.0 wt% resulted in fired ceramic densities that ranged from 97.3% to 82.3%. Hydrostatic-pressure-induced ferroelectric (FE) to antiferroelectric (AFE) phase transformations were substantially more diffuse and occurred at lower hydrostatic pressures with increasing porosity. An ∼12 MPa decrease in hydrostatic transformation pressure per volume percent added porosity was observed. The decrease in transformation pressure with decreasing density was quantitatively consistent with the calculated macroscopic stress required to achieve a specific volumetric macrostrain (0.40%). This strain was equivalent to experimentally measured macrostrain for FE-to-AFE transformation. The macroscopic stress levels were calculated using measured bulk modulus values that decreased from 84 to 46 GPa as density decreased from 97.3% to 82.3%. Good agreement between calculated and measured values of FE-to-AFE transformation stress was obtained for ceramics fired at 1275° and 1345°C.     

Fabrication of Nanograined Silicon Carbide by Ultrahigh-Pressure Hot Isostatic Pressing

Dense nanograined SiC ceramics were obtained by using hot isostatic pressing (HIP). The starting powder was ultrafine β-SiC powder, which had a mean particle size of 30 nm and contained 3.5 wt% free carbon. SiC powders-both boron-doped and undoped-were densified via HIP under an ultrahigh pressure of 980 MPa at a temperature of 1600°C. Both doped and undoped SiC attained the same density (3.12 g/cm³) (relative density of 97.1%). The average grain sizes of boron-doped and undoped SiC were 200 and 30 nm, respectively. The compressive flow stress of undoped SiC was 3 times higher than that of boron-doped SiC at temperatures of 1800° and 1700°C; however, the flow stresses of both materials were almost the same at 1600°C. The HIPed SiC that was doped with boron could be deformed at a stress that was one-third lower than that of hot-pressed boron- and carbon-doped SiC with a grain size of 0.8 µm.     

Crystal Chemistry and Dielectric Properties in the Systems BaTiO3-UO2 and BaTiO3-BaUO3

Polycrystalline barium titanate fired in nitrogen at 1300° to 1400°C accommodates up to 3 mole % UO₂ in solid solution; its structure is then cubic at room temperature. With BaUO₃ additions the structure becomes disordered and quasi-cubic. In air, about 1 mole % UO₂ goes into solid solution in BaTiO₃ but the structure remains tetragonal. Diffraction peaks of a new phase, possibly a ternary oxide of barium, uranium, and titanium, appear in patterns of specimens containing more than 2 mole % UO₂. The dielectric constant of BaTiO₃ ceramics fired in air, steam, or oxygen increases with up to about 0.5 mole % UO₂ but declines rapidly above this level. The dielectric constant of BaUO₃ is about two orders of magnitude lower than that of BaTiO₃, and additions of BaUO₃ invariably lower the dielectric constant of BaTiO₃.     

SiC-Whisker-Reinforced Al2O3 Composites by Free Sintering of Coated Powders

SiC whiskers were coated with a thick cladding of finegrained Al₂O₃ powder by controlled heterogeneous precipitation in a concentrated suspension of whiskers. After calcination, the coated whiskers were compacted by cold isostatic pressing and sintered at a constant heating rate of 5°C/min in a helium atmosphere. The parameters which control the coating process and the sintering characteristics of the consolidated powders are reported. Starting with an initial matrix density of 40–45% of the theoretical, composites containing up to ≅20 vol% whiskers (aspect ratio ≅15) were sintered freely to nearly theoretical density below 1800°C. By comparison, a similar composite formed by mechanical mixing of the whiskers and the precipitated Al₂O₃ powder reached a density of only 68% of the theoretical after sintering under identical conditions. For a fixed whisker content, the sinterability of the composites formed from the coated whiskers shows a fairly strong dependence on the whisker aspect ratio.     

Reaction Sintering and Properties of Silicon Oxynitride Densified by Hot Isostatic Pressing

Silicon oxynitride ceramics were reaction sintered and fully densified by hot isostatic pressing in the temperature range 1700°C to 1950°C from an equimolar mixture of silicon nitride and silica powders without additives. Conversion to Si₂N₂O increases steeply from a level around 5% of the crystalline phases at 1700°C to 80% at 1800°C, and increases a few percent further at higher temperatures. α -Si₃N₄ is the major residual crystalline phase below 1900°C. The hardness level for materials containing 85% Si₂N₂O is approximately 19 GPa, comparable with the hardness of Si₃N₄ hot isostatically pressed with 2.5 wt% Y₂O₃, while the fracture toughness level is around 3.1 MPa. m^1/2, being approximately 0.8 MPa.m^1/2 lower. The three-point bending strength increased with HIP temperature from approximately 300 to 500 MPa.     

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.     

Properties of Isostatically Hot-Pressed Silicon Nitride

Hot isostatic pressing was studied for densification of reaction-bonded Si₃N₄ containing various levels of Y₂O₃. Near-theoretical density was achieved for com positions containing 3 to 7 wt% Y₂O₃. An Si₃N₄-5 wt% Y₂O₃ composition had a 4-point flexural strength at 1375°C of 628 MPa and survived 117 h of stress rupture testing at 1400°C and 345 MPa .     

Postsintering Hot Isostatic Pressing of Ceria-Doped Tetragonal Zirconia/Alumina Composites in an Argon–Oxygen Gas Atmosphere

Ceria-doped tetragonal zirconia (Ce-TZP)/alumina (Al₂O₃) composites were fabricated by sintering at 1450° to 1600°C in air, followed by hot isostatic pressing (postsintering hot isostatic pressing) at 1450°C and 100 MPa in an 80 vol% Ar–20 vol% O₂ gas atmosphere. Dispersion of Al₂O₃ particles into Ce-TZP was useful in increasing the relative density and suppressing the grain growth of Ce-TZP before hot isostatic pressing, but improvement of the fracture strength and fracture toughness was limited. Postsintering hot isostatic pressing was useful to densify Ce-TZP/Al₂O₃ composites without grain growth and to improve the fracture strength and thermal shock resistance.     

Pressureless Sintering of Titanium Diboride with Nickel, Nickel Boride, and Iron Additives

The effect of relatively small additions (1-5 wt%) of nickel, nickel boride (NiB), and iron to promote the liquid-phase sintering of titanium diboride (TiB₂) has been studied. Carbon also was added to some samples, to reduce the amount of oxygen impurities in the TiB₂ ceramics. Green bodies that were formed by uniaxial pressing were sintered in a graphite furnace at 1300°–1700°C, both under vacuum and in a 500 mbar argon atmosphere, and high densities (>94% of theoretical density) were obtained at temperatures greater than or equal to 1500°C. The weight loss of the samples during sintering was shown to be dependent on the densification rate and the final density and was not governed only by the thermodynamics of the system. Significant exaggerated grain growth was observed in samples with nickel, NiB, and iron during sintering at 1700°C. The exaggerated grain growth was observed to be closely related to the oxygen content of the samples and to temperature. The addition of carbon strongly reduced the density and the oxygen content and, thereby, inhibited grain growth. We have proposed that the exaggerated grain growth is enhanced by surface diffusion in a titanium-oxide-rich layer on the TiB₂ grains.     

Fabrication and Mechanical Properties of Continuously Graded MoSi2–ZrO2(2Y) Materials Using Wet-Molding

Continuously graded MoSi₂-ZrO₂(2Y) materials with high density (97.5% of theoretical) have been fabricated by uniaxial wet-molding, followed by hot pressing (1000°C/1 h/30 MPa) and hot isostatic pressing (1400°C/2 h/196 MPa). Their composition profiles are greatly influenced by the viscosity of mixed solutions of glycerin and ethanol used as a dispersion medium; a linear compositional gradient from MoSi₂/ZrO₂(2Y) 70/30 to 20/80 mol% is obtained from the solution (50/50 vol%) with a viscosity of 20 mPa s. Vickers hardness (Hv) and fracture toughness (KIC) increase from 9.7 to 12.4 GPa and from 5.1 to 12.5 MPa m1/2, respectively, with increasing ZrO₂(2Y) composition.     

Densification of Precursor-Derived Si-C-N Ceramics by High-Pressure Hot Isostatic Pressing

Densification behavior of precursor-derived Si-C-N ceramics by hot isostatic pressing (HIP) has been investigated to obtain dense ceramics derived from polymer precursor. An as-pyrolyzed ceramic monolith, which had a porosity of about 17%, could be deformed up to a strain of 8% in preliminary uniaxial compression tests. The flow stress of the material was much higher than 200 MPa at 1600°C; thus high stress was necessary for densification by HIP. The density of the monolith increased from 1.9 to 2.4 g/cm³ by HIP at 1600°C and 980 MPa. Although the number of pores decreased, large pores were formed in the hot isostatically pressed monolith. On the other hand, denser ceramics, in which pores were not observed by optical microscopy, were obtained by hot isostatically pressing the pyrolyzed powder compact.     

Effect of Excess PbO on the Sintering Characteristics and Dielectric Properties of Pb(Mg1/3Nb2/3)O3–PbTiO3-Based Ceramics

Additions of excess PbO to the perovskite Pb[(Mg_1/3Nb_2/3)_0.92Ti_0.08]O₃ solid solution enhanced the formation of a liquid phase at 840°C, which served as a densification aid for the ceramics. The liquid phase allowed elimination of pores and promoted grain growth during sintering. With additions of 1 to 2 wt% excess PbO, densities in excess of 97% of theoretical were obtained at a sintering temperature of 950°C. The peak dielectric constants of the resulting ceramics were over 18 000 at 30°C and dissipation factors less than 1%. Additions of PbO in excess of 2 wt% resulted in inferior dielectric properties due mainly to the dilution of the ferroelectric phase.