Hydrothermal Corrosion of Pure, Hot Isostatically Pressed Silicon Nitride

The aqueous corrosion behavior of hot isostatically pressed Si₃N₄ (HIPed-Si₃N₄) without additives was studied under hydrothermal conditions at 300°C and 8.6 MPa (86 atm). The accelerated weight loss in the HIPed-Si₃N₄ was attributed to uniform thinning of the specimen accompanied by dislodgement of Si₃N₄ grains from the substrate due to preferential attack at grain boundaries. Enhanced attack at grain boundaries was due to the presence of amorphous SiO₂ from impurities in the starting powder.     

Grain-Boundary Relaxation in High-Purity Silicon Nitride

Internal friction, torsional creep, and shear modulus relaxation experiments were conducted on a model Si₃N₄ polycrystalline material, which contained a continuous amorphous film of pure SiO₂ at the grain boundary. Internal friction experiments were performed in the frequency range between 3 and 13 Hz, in 5 Pa of nitrogen atmosphere. Very high temperatures (up to 2000°C) could be applied for the first time by using a newly developed torsional pendulum apparatus. This apparatus was also capable of precise torsional strain measurements under static-load conditions. The internal friction curves at various frequencies were generally found to consist of a grain-boundary peak super-imposed on an exponential-like background. The peak, of anelastic diffusive origin, was centered in the temperature range of 1612–1710°C depending on the frequency of the measurement, namely within an interval of about 100°C below the nominal melting point of the pure SiO₂ phase (i.e., ∼ 1730°C). The background was instead found to be of viscoelastic nature. A common micromechanical origin between the creep plastic strain and the internal friction background curves was identified and the data could be fitted by the same Arrhenius plot. Structural and chemical characterization of internal grain boundaries was performed by high-resolution electron microscopy (HREM) in addition to electron energy-loss spectroscopy (EELS). A small amount of nitrogen was detected within the amorphous residue along grain boundaries. According to the above set of microstructural/chemical and mechanical data, the viscosity properties of the intergranular phase were assessed and the sliding mechanism between adjacent Si₃N₄ grains was modeled.     

Effect of Load on the Hardness of Hot Isostatically Pressed Silicon Nitride

The hardness values of five hot isostatic pressed silicon nitride materials, with varying densities, were measured at loads between 1 and 200 N. For the fully dense materials, the calculated hardness decreased from about 30 to 15 GPa as the load increased to about 10 N, and the hardness remained constant at higher loads. For the samples that showed indentation size effect (ISE), cracks formed at the corners of the indentation, starting at the lowest load of 1 N. Materials with lower densities had lower hardness values, displayed a very small or no ISE, and formed corner cracks only at high loads. For the samples that displayed an ISE at low loads, the formation of cracks was analyzed using the Niihara et al . criterion for Palmqvist cracks.     

Effect of Wear Damage on the Strength of Hot Isostatically Pressed Silicon Nitride

Bending tests have been used to characterize the effect of wear damage of hot isostatically pressed silicon nitride in contact with waspaloy at 600°C. The problem arises at the junction between turbine blades and disks where wear may result in premature fatigue cracking of the blade root. The tests show that wear always results in a reduction of strength, whereas the Weibull modulus may decrease or remain unchanged, depending on the amount of surface damage.     

Evaluation of the Strength and Creep–Fatigue Behavior of Hot Isostatically Pressed Silicon Nitride

The strength of a commericially available hot isostatically pressed silicon nitride was measured as a function of temperature. To evaluate long-term mechanical reliability of this material, the tensile creep and fatigue behavior was measured at 1150°, 1260°, and 1370°C. The stress and temperature sensitivities of the secondary (or minimum) creep strain rate were used to estimate the stress exponent and activation energy associated with the dominant creep mechanism. The fatigue characteristics were evaluated by allowing individual creep tests to continue until specimen failure. The applicability of the four-point load geometry to the study of strength and creep behavior was also determined by conducting a limited number of flexural creep tests. The tensile fatigue data revealed two distinct failure mechanisms. At 1150°C, failure was controlled by a slow crack growth mechanism. At 1260° and 1370°C, the accumulation of creep damage in the form of grain boundary cavities and cracks dominated the fatigue behavior. In this temperature regime, the fatigue life was controlled by the secondary (or minimum) creep strain rate in accordance with the Monkman–Grant relation.     

Evolution of Stress Failure Resulting from High-Temperature Stress-Corrosion Cracking in a Hot Isostatically Pressed Silicon Nitride

Stress-corrosion cracking in a commercially available, hot isostatically pressed (HIPed), yttria-fluxed, silicon nitride was the prevalent mode of failure in specimens creepruptured at 1370°C. High-temperature diffusional processes associated with oxygen were responsible for the creation of an advancing stress-corrosion front that had formed at the specimen surface and advanced radially inward. The volume of material in the wake of the stress-corrosion front possessed a high concentration of lenticular cavities at two-grain boundaries, a high concentration of multigrain junction cavities, and large amorphous "pockets" in other multigrain junctions that were abnormally rich in oxygen and yttrium. The combination of tensile stress and the high concentration of cavities in the near-surface volume of the material resulted in microcrack coalescence or the formation of a planar, stress-corrosion crack . The concurrent growth of the stress-corrosion front and crack during the tensile creep-rupture tests ultimately led to stress-induced failure.     

Transmission Electron Microscopy Investigation of the Oxidation of Hot Isostatically Pressed Silicon Nitride with and without Sintering Aids

Analytical transmission electron microscopy of thin-foil cross sections has been used to examine the oxidation behavior of hot isostatically pressed silicon nitride (Si₃N₄) materials. The transmission electron microscopy (TEM) cross sections are prepared by a special technique that provides electron transparency through the entire oxide, interfacial, subscalar, and matrix regions simultaneously. The materials are oxidized in an alumina furnace at 1250°C for 100 h. TEM investigation indicates that oxidation of Si₃N₄ occurs in an oxidation reaction zone that is comprised of the scale, oxide/matrix interface, and subscalar regions; therefore, the silica (SiO₂)/Si₃N₄ interfacial surface area that is available for oxidation is very large. The oxidative attack on the Si₃N₄ grains is not uniform or sequential, and oxygen diffuses into the matrix before the surface grains are consumed. Gas bubbles, probably nitrogen gas, accumulate at all levels of the scale, and no evidence is found for the existence of an "oxynitride" layer. Disintegration of the secondary phase, Y₂Si₂O₇, in the subscalar region is observed to occur, indicating that secondary, oxidation-related phenomena are occurring.     

Superplasticity of Hot Isostatically Pressed Hydroxyapatite

Dense and translucent hydroxyapatite polycrystals (Ca₁₀(PO₄)₆(OH)₂ with a grain size of 0.64 μMm) were obtained by hot isostatic pressing at 203 MPa and 1000°C for 2 h in argon. The material exhibited superplastic elongation (>150%) in a tension test at temperatures from 1000° to 1100°C and at strain rates from 7.2×10⁻⁵ to 3.6 × 10⁻⁴ s⁻¹. Extensive strain hardening was observed. The stress exponent of the yield stress was larger than 3.     

Microstructure and Optical Properties of Hot Isostatically Pressed Nd:YAG Ceramics

Neodymium-doped transparent yttrium-aluminum garnet (Y₃Al₅O₁₂, YAG) (Nd:YAG) ceramics for solid-state laser material were fabricated by a solid-state reaction method using high-purity powders (Al₂O₃, Y₂O₃, and Nd₂O₃) as starting materials and capsule-free hot isostatic pressing (HIP). The mixed powder compacts were presintered at 1600°C for 3 h under vacuum, hot isostatically pressed at 1500°–1700°C for 3 h under 9.8 or 196 MPa of argon gas pressure, and then sintered again at 1750°C for 20 h under vacuum. Although the presintered specimen approached full density after HIP, its optical transmittance was quite low (∼5% at 1000 nm) because of lack of grain growth. Grain growth was observed in the specimens that were hot isostatically pressed and vacuum sintered at 1750°C for 20 h, but numerous pores occurred around the surface of these specimens. Consequently, the optical transmittance of Nd:YAG ceramics that were treated by HIP was inferior to that of the same ceramics that were sintered under vacuum only because of light scattering that was caused by the pores (at the grain boundaries) that were produced during the HIP treatment.     

Role of Oxidation in the Time-Dependent Failure Behavior of Hot Isostatically Pressed Silicon Nitride at 1370°C

Dynamic fatigue studies were conducted on a hot isostatically pressed silicon nitride in ambient air and inert (argon or nitrogen) environments using four-point flexure at 1370°C. Specimens tested in ambient air exhibited a stressing rate dependence with decreased flexure strength with decreased stressing rates. All fracture surfaces of specimens tested in ambient air possessed a sweeping stress-oxidation damage zone that originated at the tensile side of each bend bar. In addition to this stress-oxidation damage, creep damage (e.g., cavitation) was concurrently observed in the specimens tested at the slower stressing rates, which appeared to further weaken the material. However, tests conducted in argon or nitrogen revealed flexure strength to be independent of the stressing rate. Creep damage was present at the slower stressing rates, but no stress-oxidation damage was evident similar to that observed on the specimens tested in ambient air. By decoupling the effects of oxidation and creep, it was evident that the former contributed to the formation of a detrimental stress-oxidation damage zone which significantly reduced the strength of this material at 1370°C.     

Hot Isostatically Pressed Alumina-Silicon Carbide-Whisker Composites

Alumina-silicon carbide-whisker composites were hot isostatically pressed at 1550°C and 200 MPa for 1 h. The silicon carbide whiskers were treated in different acid and gas environments before they were pressed. All samples exhibited linear elastic behavior with no ductility tendency. Improved strength and fracture toughness were obtained compared with unreinforced alumina. Mechanisms for the improved mechanical properties are discussed. These include grain growth control, whisker encapsulation of defects, and related stress relief at the defect.     

Variation of Width and Composition of Grain-Boundary Film in a High-Purity Silicon Nitride with Minimal Silica

Contrary to the widely accepted observation that grain-boundary amorphous films for a given Si₃N₄ composition have common (equilibrium) widths and compositions, a significant variation for both parameters from film to film was observed in an undoped high-purity Si₃N₄ prepared using a hot isostatic pressing method. This material previously has been reported to have an equilibrium film width of 0.6 nm, as measured using a high-resolution electron microscopy (HREM) method; this value is significantly different from that which is typical for other high-purity Si₃N₄ ceramics (1.0 nm). A total of four boundaries were analyzed, using spatially resolved electron energy-loss spectroscopy methods, which can give the chemical width and composition for the film. Widths of these grain-boundary films were substantially different from each other; only the thinnest matches the previous HREM observations. The nitrogen content in the film decreased concurrently as the film thickened. This material had many cavities and complicated configurations at triple pockets, because of the very low total-SiO₂ content (0.55 vol%). They created locally different equilibrium conditions for grain-boundary films, in comparison with other fully densified Si₃N₄, causing such strong variation in both film structure and chemistry. This observation reveals the importance of triple pockets in equilibrium film structures, providing new insight in evaluating the absorption and wetting models. The thinnest film may correspond to the amorphous structure that is required to bind two randomly oriented Si₃N₄ grains under greater local stress.     

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 .     

Microstructure and Grain-Boundary Composition of Hot-Pressed Silicon Nitride With Yttria and Alumina

The microstructure of two hot-pressed silicon nitrides containing Y₂O₃ and Al₂O₃ was examined by electron microscopy, electron diffraction, and quantitative, energy-dispersive X-ray microanalysis. A crystalline second phase was identified in the material with additives of 5 wt% Y₂O₃+2 wt% Al₂O₃, as a solid solution of nitrogen mellilite and alumina. An amorphous third phase as narrow as 2 nm is discerned at all grain boundaries of this material by high-resolution dark-field and lattice imaging. The second phase in a material with additives of S wt% Y₂O₃+5 wt% Al₂O₃ was found to be amorphous. Some of the additional alumina additive appears in solid solution with silicon nitride. In situ hot-stage experiments in a high-voltage electron microscope show that the amorphous phase volatilizes above 1200°C, leaving a skeleton of Si₃N₄ grains linked by the mellilite crystals at triple points. The results show that intergranular glassy phases cannot be eliminated by the Y₂O₃/Al₂O₃ fluxing.     

Grain-Boundary Viscosity of Calcium-Doped Silicon Nitride

Internal-friction data of calcium-doped Si₃N₄ polycrystalline materials that otherwise contain only SiO₂ at grain boundaries were examined and compared with those collected on the same polycrystal in the undoped state or doped with anions (i.e., fluorine and chlorine). Precise microstructural characterizations previously performed on these materials enabled us to quantitatively evaluate the inherent viscosity of the intergranular SiO₂ film through the analysis of the anelastic internal-friction-peak components. The intergranular glass viscosity and its scaling with increasing calcium addition followed the same trend as bulk SiO₂ glasses with the same chemical composition. Broadening of the internal-friction peak with increased calcium content in the material has also been rationalized according to the reduction of the activation energy for the viscous flow of bulk glasses. The present analysis, which is in agreement with our previous studies on undoped and anion-doped Si₃N₄, has demonstrated that the overall viscoelastic response of the polycrystal is mainly dictated by the chemistry of the intergranular glass.     

Grain-Boundary Sliding in Fluorine-Doped Silicon Nitride

A systematic study of the effect of small additions of fluorine impurity on the relaxation and creep behavior of Si₃N₄ ceramics was conducted. A model polycrystal, consisting of equiaxed grains and containing only pure (glassy) SiO₂ at the grain boundary, was selected for this investigation. The boundary SiO₂-glass film, completely surrounding the grains, was doped with increased amounts of the glass-network modifier fluorine to systematically lower its bulk viscosity. The addition of the fluorine made both the grain-boundary relaxation peak and the background of internal friction notably shift toward lower temperatures; the scaling was in accordance with the lowered viscosity of the bulk fluorine-doped SiO₂ glass. Invoking basic viscoelasticity principles, the grain-boundary relaxation phenomenon could be rationalized and the internal friction data consistently related to the macroscopic (torsional) creep behavior of the polycrystal.     

Microstructural Changes in Hot Isostatically Pressed Alumina-Glass Composites

Alumina-glass composites with a small amount of a continuous glassy phase and closed pores were prepared and isostatically hot-pressed under varied conditions. Induced macropore structure showed partiallcomplete pore filling with glass and some evidence of pore collapsing. Pore filling was achieved by viscous from of the glass from the grain boundary to the macropores, which causes a simultaneous localized rearrangement of the alumina grain structure.     

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.     

Oxidation and Fracture Strength of High-Purity Reaction-Bonded Silicon Nitride

Reaction-bonded Si₃N₄ (RBSN) made from high-purity Si powder is unusually resistant to degradation caused by exposures to air for up to 50 h at temperatures up to 1400°C. The weight gain during oxidation of this SiH₄-originating RBSN is approximately 10 times less than conventional RBSN. Contrary to normally observed strength degradations, room-temperature strengths of this high-purity, oxidized RBSN (avg = 435 MPa, max. = 668 MPa) remained at their unusually high, as-processed levels after 1000° and 1400°C oxidizing exposures. Fracture toughness values were unaffected by oxidation ( K _IC= 2.3 to 2.4 MPa · m^1/2). This superior oxidation resistance results from the high purity and the small diameter pore channels (0.01 to 0.06 μm) achieved in this SiH₄-originating RBSN.