Microstructure and Mechanical Properties of Hot-Pressed Silicon Carbide-Aluminum Nitride Compositions

A flexural strength of up to 1 GPa was achieved in SiC-AIN materials and is attributed to a dense, equiaxial grain structure of the 2H(δ) SiC-AIN solid solution, with a relatively uniform grain size of ∼ 1 μm. The strength was found to decrease with increasing grain size. While the β→α phase transformation and the formation of various metastable polytypes make microstructural control difficult in SiC materials, excellent control is facilitated in SiC-AIN materials as a result of the stable 2H solid solution. Several mechanisms of grain refinement during the β→ 2H transition were observed, most notably the direct formation of several 2H grains from a single β grain. In addition, grain growth is limited by the diffusion-controlled nature of the transition. These mechanisms could be utilized to achieve even higher strength values, with potentially higher reliability of the materials in structural applications.     

Mechanical Properties of Hot-Pressed Silicon Nitride with Different Grain Structures

During hot-pressing of α-Si₃N₄ powders, the equiaxed α micro-structure gradually transforms into a β structure characterized by needle-shaped prismatic grains which are closely entangled and linked together. With increasing amounts of the β fraction, the bend strength, fracture toughness, and work of fracture increase significantly, then decrease as grain growth occurs. The K _lc, improves by a factor of >2 and the change in γ_F by a factor of >4. The crack resistance to achieve the same crack velocity in materials of different β contents shows a similar trend. The dependence of the mechanical properties on the microstructure is explained by linking and pullout of the β crystals and by grain coarsening.     

Oxidation of Silicon Nitride Hot-Pressed with Ceria

Short-term (<30 h) oxidation of CeO₂-doped hot-pressed Si₃N₄, studied at 773 to 1623 K in flowing dry air, resulted in parabolic-weight gain curves and in oxidation-rate constants nearly independent of the amount of grain-boundary second phase. Exolution of CeO₂ crystals occurs from the oxide layer; their morphology depends on oxidation temperature. Either a direct redox reaction between CeO₂ and Si₃N₄ or solution of Si₃N₄ in the glassy silica-rich oxide layer, followed by its oxidation by dissolved oxygen, have been proposed as possible oxidation mechanisms, both appearing strongly dependent on the low solubility of cerium oxides hi silicate melts. The value of the activation energy for oxidation of 385 kj·mol⁻¹ suggests additive and impurity migration from the bulk to the Si₃N₄/oxide reaction interface as the most probable rate-limiting step.     

High-Temperature Microstructure of a Hot-Pressed Silicon Nitride

The outstanding question as to the microstructure of silicon nitride at temperatures associated with potential high-temperature applications of the material is addressed experimentally by quenching thin (transmission electron microscopy) samples from 1450°C and examining them in the microscope. The morphology of the microstructure is qualitatively unchanged compared to the materials slowly cooled, for example, after hot-pressing, to room temperature. The most significant difference is that the thickness of the intergranular phase is larger, typically 2 to 10 nm, as compared to the ∼ 1 nm observed in the hot-pressed material. In addition there is an apparent increase in the volume fraction of the intergranular phase at the three-grain junctions. On the basis of a number of supporting experiments including both hot-stage transmission electron microscopy (up to 1000°C) and Auger electron spectroscopy of material fractured and examined at 850°C, the change in microstructure is concluded to occur at temperatures above about 1000°C.     

Static Fatigue of Preoxidized Hot-Pressed Silicon Nitride

Hot-pressed Si₃N₄ samples were preoxidized and tested in stress rupture. The lifetime of preoxidized samples was compared to samples that were not pretreated; for the test conditions chosen, lifetime was shorter for the preoxidized samples.     

Strength of Hot-Pressed Silicon Nitride After High-Temperature Exposure

Short-term exposure of hot-pressed silicon nitride to temperatures greater than 800°C in an oxidizing atmosphere causes an increase in the room-temperature strength and eliminates the truncated strength distribution produced by room-temperature proof-testing. Acid-etching the proof-tested samples restores the original truncated distribution. These strength changes are shown to be related to the formation of a glassy phase on the surface that smooths out the preexisting machining flaws. More extensive, long-term oxidation produces surface pits that lead to an irreversible change in strength     

Strength and Life Prediction for Hot-Pressed Silicon Nitride

The strength of hot-pressed Si₃N, was evaluated for constant stress rate, linear cyclic stress, and constant-stress loading. A stress-rupture rig was used to simultaneously test 10 samples under constant-stress loading. Exponential crack-growth-rate expressions were numerically integrated for constant stress rate and cyclic-stress loading and an approximate expression for the time-to-failure for these loading cases was developed. For either the power law or the exponential crack-growth-rate formulation, linear cyclic-stress and constant-stress-rate loading can be treated with the same fracture stress-time-to-failure representation. This correspondence was demonstrated for hot-pressed Si₃N₄. The exponential crack-growth formulation provided a consistent interpretation of the strength-degradation results, whereas the power-law formulation was inadequate.     

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.     

Compositional Modification of Hot-Pressed Silicon Nitride by High-Temperature Electrolysis

Impurities and additives used in the liquid-phase densifica-tiondensification of silicon nitride ceramics are preferentially partitioned to the residual intergranular glass phase remaining in these materials. On the basis of a simple phenomenological ionic migration model, it is proposed and demonstrated that a high-temperature electrolysis treatment may be used to modify the concentration of both the additive and impurities. The compositional modification has a number of consequences and, as an illustration, the improvement in high-frequency dielectric loss produced as a result of the electrolysis treatment is presented.     

热压氮化硅材料中小裂纹的稳态扩展

对一种HPSN材料中Vickers压痕裂纹的稳态扩展过程进行了研究,发现裂纹扩展阻力随着裂纹尺寸的增大而增大,由于残余应力的影响,其K_(R)～C关系曲线的形状及位置与压痕压制荷载及压痕裂纹初始尺寸有关。经退火消除残余应力后测得的K_(R)～C曲线可能反映了材料的本征性能。     

Orientation Effects on the Thermal Diffusivity of Hot-Pressed Silicon Nitride

The thermal diffusivity perpendicular to the hot-pressing direction for a variety of silicon nitrides was found to be higher than parallel to the hot-pressing direction. This effect was attributed to the preferred elongation of the β-grains perpendicular to the hot-pressing direction.     

Structural Reliability of Yttria-Doped Hot-Pressed Silicon Nitride at Elevated Temperatures

The strength of yttria-doped hot-pressed silicon nitride was investigated as a function of temperature, time, and applied load. Data collected at 1200°C are presented in the form of a strength-degradation diagram for an applied stress of 350 MPa. At this temperature, the behavior of yttria-doped hot-pressed silicon nitride is found to be superior to that of magnesia-doped hot-pressed silicon nitride, in which creep results in the formation of microcracks that lead to strength degradation. By contrast, the yttria-doped material does not suffer from microcrack formation or strength degradation at 1200°C. Strength degradation does occur at higher temperatures and, as a consequence, an upper limit of 1200°C is recommended for yttria-doped hot-pressed silicon nitride in structural applications.     

Fabrication and Characterization of Isostatically Hot-Pressed MgO

Isostatic hot-pressing was used to fabricate solid MgO bodies of high purity (with respect to metallic impurities) to 90 to 95y0 density with high translucency and a grain size of approximately 0.2μ. The densified material contained significant amounts of brucite as a result of water in the starting material which was not removed during vacuum baking. The fabricated pieces showed many cracks which were apparently introduced into the compacts before densification. When the hot-pressed pieces were reheated, the brucite decomposed, the density increased, and the porosity coalesced while grain growth occurred. The general technique appears to be usable for fabricating finely divided powders if powder degassing and predensification procedures are improved.     

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.     

Influence of Rare-Earth Additives on Wear Properties of Hot-Pressed Silicon Nitride Ceramics under Dry Sliding Conditions

The influence of different rare-earth sintering additives (Y, Yb, Lu) on the wear properties of Si₃N₄ ceramics was investigated during sliding contact without lubricant. The kind of rare-earth additives was shown to have a significant effect on the wear behavior for contact sliding under the present testing conditions. Samples sintered with Y₂O₃ as the sintering additive showed evidence of fracture type wear although this was not observed in samples sintered with Yb₂O₃ and Lu₂O₃. These smaller rare earths lead to higher grain boundary bonding strength and superior high-temperature properties and resulted in higher wear resistance. These results showed that the wear properties of Si₃N₄ ceramics could be tailored by judicious selection of the sintering additives.     

Grain-Boundary Microstructure and Chemistry of a Hot Isostatically Pressed High-Purity Silicon Nitride

Two high-purity Si₃N₄ materials were fabricated by hot isostatic pressing without the presence of sintering additives, using an amorphous laser-derived Si₃N₄ powder with different oxygen contents. High-resolution transmission electron microscopy and electron energy-loss spectroscopy (EELS) analysis of the Si₃N₄ materials showed the presence of an amorphous SiO₂ grain-boundary phase in the three-grain junctions. Spatially resolved EELS analysis indicated the presence of a chemistry similar to silicon oxynitride at the two-grain junctions, which may be due to partial dissolution of nitrogen in the grain-boundary film. The chemical composition of the grain-boundary film was SiN_xO_y, (x ∼ 0.53 and y ∼ 1.23), and the triple pocket corresponded to the amorphous SiO₂ containing ∼2 wt% nitrogen. The equilibrium grain-boundary-film thickness was measured and found to be smaller for the material with the lower oxygen content. This difference in thickness has been explained by the presence of the relatively larger calcium concentration in the material with the lower amount of SiO₂ grain-boundary phase, because the concentration of foreign ions has been shown to affect the grain-boundary thickness.