High-Temperature Strength of Fluorine-Doped Silicon Nitride

High-purity Si₃N₄ (with 2.5 wt% glassy SiO₂) doped with F was prepared by immersion of the starting powder into dilute HF and hot isostatic pressing without sintering additives, using a glass encapsulation method. Oxygen content and cation impurity content were almost the same for the F-doped and undoped materials. However, X-ray fluorescence analysis revealed the order of 100 ppm of F in the doped material, and a considerable amount of F was detected from the amorphous SiO₂ phase at grain-boundary triple points by analytical transmission electron microscopy. High-resolution electron microscopy found that an amorphous intergranular film was omnipresent in both of the materials, with an equilibrium thickness of 10 ± 1 å. Subcritical crack-growth resistance and creep resistance at 1400°C were degraded significantly by the presence of F. Internal friction of doped materials exhibited a dear grain-boundary relaxation peak, which suggested that F was present in the intergranular film at the two-grain junctions; this decreased the grain-boundary viscosity considerably. The film thickness of the doped material showed no apparent chemical effects and was explained by taking into account competing repulsive forces acting normal to the film.     

High-Temperature Strength of Sinter-Forged Silicon Nitride with Lutetia Additive

A sinter-forging technique was successfully applied to fabricate a silicon nitride with a lutetia (Lu₂O₃) additive. The sinter-forged specimen had a strongly anisotropic microstructure where rodlike silicon nitride grains preferentially aligned perpendicular to the forging direction. The specimen exhibited superior strength of ∼700 MPa at 1500°C. This strength was highest when compared with previous silicon nitrides at temperatures >1400°C. Such superior high-temperature strength was attributed to grain alignment as well as to the refractory grain-boundary glassy phase and the existence of glass-free grain boundaries.     

Whisker Toughening: A Comparison Between Aluminum Oxide and Silicon Nitride Toughened with Silicon Carbide

Two whisker-toughened materials have been studied, with the objective of identifying the mechanisms that provide the major contribution to toughness. It is concluded that, for composites with randomly oriented whiskers, bending failure of the whiskers obviates pullout, whereupon the major toughening mechanisms are the fracture energy consumed in creating the debonded interface and the stored strain energy in the whiskers, at failure, which is dissipated as acoustic waves. The toughening potential is thus limited. High toughness requires extensive pullout and, hence, aligned whiskers with low fracture energy interfaces.     

Anisotropy of Silicon Nitride with Aligned Silicon Nitride Whiskers

A model based on anisotropic sintering shrinkage of silicon nitride with aligned silicon nitride whisker seeds was built in order to provide an easy way to obtain information on how the large elongated grains were aligned. The method requires a simple measuring device for the information. XRD analysis showed a good correlation with predictions of the model. Both predictions of the model and experimental results indicated that the fraction of aligned large elongated grains increased as the whisker content increased.     

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     

High-Temperature Toughness and Tensile Strength of Whisker-Reinforced Silicon Nitride

The R -curve behavior of hot-pressed silicon nitride reinforced with silicon carbide whiskers is investigated from room temperature to 1300°C using the chevron-notch bend test. The bridging stress, estimated from increment of fracture resistance in the rising R -curve, is discussed in relation to tensile strength measured with various displacement rates at 1300°C. The reinforcing whiskers provide most of the tensile strength in the creep-deformation range at 1300°C. The whiskers appear to bear a great deal of the applied tensile stress during slow crack growth.     

Spherical-Impact Damage and Strength Degradation in Silicon Carbide Whisker/Silicon Nitride Composites

The silicon carbide (SiC) whisker reinforcement of silicon nitride (Si₃N₄) improves fracture strength and toughness, hardness, and Young's modulus, resulting in higher resistance of the composites to sphere penetration and crack initiation at spherical impact. Sintered Si₃N₄ shows an elastic/plastic response and initiates median/radial cracks at 100 m/s impact velocity. SiC-whisker/Si₃N₄ composites, on the other hand, demonstrate an elastic response, with Hertzian cone crack initiation, only when impact velocity exceeds 280 m/s. The SiC-whisker/Si₃N₄ composites thus exhibit improved strength degradation versus critical impact velocity characteristics because of improved mechanical properties provided by the SiC whiskers.     

Effect of High-Temperature Exposure in Air on Strength of Hot-Pressed Silicon Nitride

Short-term exposure of hot-pressed Sl₃N₄ to high temperature (800°C) eradi cates the truncated strength distribution produced by room-temperature proof-testing, indicating that strengths after high-temperature exposure are determined by jaws different from preexisting flaws. Since acid polishing of proof-tested samples cycled to high temperature restored the truncated after-proof strength distribution, it is believed that short-term high-temperature exposure causes amor phous SiO₂ to form on the surface, thus blunting the surface flaws.     

Tribological Properties of Unidirectionally Aligned Silicon Nitride

A silicon nitride ceramic with unidirectionally aligned β-Si₃N₄ elongated grains (UA-SN) was fabricated by sintering the extruded Si₃N₄ green body with a small amount of rodlike β-Si₃N₄ seed. The effect of anisotropy in microstructure on tribological properties was investigated, compared with a fine-grained Si₃N₄ without seed. Block-on-ring tests without lubricant were conducted at sliding speeds of 0.15 and 1.5 m/s, with a normal load of 5 N and a sliding distance of 75 m, using the UA-SN and Si₃N₄ without seeds as block specimens and commercially supplied Si₃N₄ as ring specimens. For UA-SN, tribological properties were evaluated in three directions with respect to the grain alignment: the plane normal to the grain alignment, and in the direction parallel to or perpendicular to the grain alignment in the side plane. For both sliding speeds, the plane normal to the grain alignment exhibited the highest wear resistance, and the worn surface of this plane was quite smooth, in contrast to the other specimens whose surfaces were irregular owing to grain dropping. It is considered that the high wear resistance achieved in this plane is attributable to the inhibition of crack propagation along the sliding surface by the stacked elongated grains normal to the sliding surface.     

High-Temperature Environmental Strength Degradation of a Hot-Pressed Silicon Nitride: An Experimental Test

High-temperature environmental strength degradation of hot-pressed silicon nitride ceramics is postulated to occur as a consequence of the properties of the noncrystalline inter-granular phase being altered by cation diffusion from the environment. The model proposed is tested by comparing the strength before and after prolonged high-temperature exposures to a calcium-rich magnesium silicate. Two high-temperature degradation regimes are identified, one in which the intergranular phase alone is affected, and the other, at still higher temperatures, where actual corrosion of the silicon nitride grains occurs.     

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.     

High-Temperature Fracture Mechanism of Low-Ca-Doped Silicon Nitride

High-purity Si₃N₄ (with 2.5 wt% glassy SiO₂) doped with 0 to 450 at.ppm of Ca was prepared as a model system to investigate the effects of grain-boundary segregants on fracture phenomenology at 1400°C. Subcritical crack-growth (SCG) resistance as well as creep resistance was degraded significantly by the presence of a small amount of Ca. The internal friction of the doped materials exhibited the superposition of a grain-boundary relaxation peak and a high-temperature background, and the apparent viscosity of the grain-boundary film was determined from the peak. Based on these experimental data, the fracture mechanism at 1400°C was divided into three regions: "brittle," SCG, and creep failure as a function of both external strain rate and Ca concentration, C _Ca. From the investigation of the C _Ca dependence of the critical strain rate for the transition from "brittle" to SCG fractures, the SCG phenomenon is suggested to be triggered by small-scale, grain-boundary sliding. The C _Ca dependence of "steady-state" creep rate was far from the theoretical dependence of diffusional creep via a solution-precipitation mechanism. The discrepancy was interpreted to be due to the presence of an impurity-insensitive creep component. This component may correspond to the lowest limit of the tensile creep rate in Si₃N₄ polycrystalline materials containing intergranular glassy-SiO₂ film.     

Long Crack R-Curve of Aligned Porous Silicon Nitride

Long crack R -curve of a porous Si₃N₄ with aligned fibrous grains was investigated, using a chevron-notched beam technique. A crack was constrained to propagate normal to the grain alignment. The crack growth resistance of aligned porous Si₃N₄ was much larger compared with that of dense Si₃N₄ ceramics. Microstructure observations showed that pullouts of fibrous grains in aligned porous Si₃N₄ markedly increased during crack propagation relative to those of dense Si₃N₄, due to the existence of pores. The efficient grain pullouts in porous Si₃N₄ increased the bridging stress at the crack wake.     

Fracture Energy of an Aligned Porous Silicon Nitride

The fracture energy of a porous silicon nitride with aligned fibrous grains was investigated, using a chevron-notched-beam technique. A crack was constrained to propagate normal to the grain alignment. The obtained fracture energy was ∼500 J/m², which was ∼7 times larger than that of a dense silicon nitride with randomly oriented fibrous grains. The large fracture energy was attributable primarily to the sliding resistance associated with interlocking grains.     

Silicon Carbide Whisker Stability During Processing of Silicon Nitride Matrix Composites

The effects of two different sources of SiC whiskers on the chemistry and microstructure of the SiC-whisker—Si₃N₄ composites were evaluated using scanning transmission electron microscopy. Analyses were performed after presintering in N₂ and after encapsulated hot isostatic pressing. Significant differences in the porosity, α- to β-Si₃N₄ conversion, and whisker degradation were observed after presintering. It was also noted that whiskers containing surface iron impurities were converted to Si₃N₄ during processing. Whiskers from the source having low surface iron exhibited little reaction. After hot isostatic pressing, some oxidation of the cleaner whiskers was observed.     

High-Temperature Chemical Phenomena Affecting Silicon Nitride Joints

Knudsen cell mass spectrometry was used to study the chemical processes responsible for joint degradation in joined silicon nitride ceramics. Vapor species present above two commercial hot-pressed silicon nitrides and above three joining glasses were identified, and partial pressures were estimated at 1480 K. Oxide vaporization products related to reducing conditions were observed. The implications of these results on proposed silicon nitride joining processes are discussed. It appears that oxygen potential gradients within both glazed and unglazed hot-pressed Si₃N₄ samples are responsible for the enhanced vaporization rates of the sample and the observed instability of glazed joints at high temperatures. Observed vaporization behavior of oxide additives correlates well with that predicted for the chemically reducing environment of Si₃N₄.     

High-Temperature Fracture Energy of Superplastically Forged Silicon Nitride

The fracture energy of superplastically forged silicon nitride, where rodlike silicon nitride grains are aligned in one direction, was investigated at high temperatures from 1100° to 1300°C and at room temperature. Bending tests using chevron-notched beams were conducted at two displacement rates, 0.05 and 0.005 mm/min. The superplastically forged silicon nitride showed remarkably high fracture energies, 200–630 J/m². The fracture energy was largely dependent on the temperature and the displacement rate. The high fracture energy was attributed to grain pullout enhanced by the softened grain-boundary glassy phase and the aligned rodlike grains.     

High-Temperature Behavior of Chemically Treated Silicon Nitride Powders

The mechanical properties of chemically treated, then sintered, Si₃N₄ was studied. Raw materials consisted of two types of Si₃N₄ produced by the nitridation of silicon. The chemical treatment involved leaching in different acids (HCl, HNO₃, HF, and combinations thereof). The powders were sintered by hot isostatic pressing with 2.5% yttria, and the high-temperature properties of the resultant materials were evaluated by the stepped-temperature stress rupture (STSR) method (24-h hold time at 150-MPa stress at 1000°, 1100°, 1200°, 1300°, and 1400°C). A significant decrease in high-temperature performance was observed for acidleached powders, especially when HF-containing acids were used.     

Indentation Crack Length Anisotropy in Silicon Nitride with Aligned Reinforcing Grains

Vickers indentation was performed on surfaces of silicon nitride with an aligned microstructure in order to study the interaction between cracks and the microstructure. Although there was not much evidence of crack bridging, the transverse radial cracks were very short, resulting in high fracture toughness values. The longitudinal radial cracks tended to propagate along the grain boundary of the reinforcements and were much longer than the transverse cracks. As the sintering temperature increased, the lateral cracks on the casting surface led to spalling and consumed more energy for the crack formation, making the longitudinal cracks shorter. On the surface normal to the alignment direction, there was no spalling and the indentation cracks became longer as the sintering temperature increased.     

由碳化硅及氮化硅制造的陶瓷材料的强度

列举了由碳化硅及氮化硅加入氧化物活化剂(Al2O3、Y2O3)制造的烧结陶瓷材料的高温强度、硬度及抗裂性的研究成果。其结果表明:由Si3N4制造的材料的强度在≥1000℃时开始下降,而由SiC制造的陶瓷则具有更高的高温强度。采用维克尔氏方法在压头受到的荷重为1kg至10kg的条件下,测定了材料的硬度和抗裂性。