High-Temperature Crack Growth in Monolithic and SiCw-Reinforced Silicon Nitride under Static and Cyclic Loads

Experimental results are presented on subcritical crack growth under sustained and cyclic loads in a HIPed Si₃N₄ at 1450°C and a hot–pressed Si₃N₄–10 vol% SiC_w composite in the temperature range 1300°–1400°C. Static and cyclic crack growth rates are obtained from the threshold for the onset of stable fracture with different cyclic frequencies and load ratios. Fatigue crack growth rates for both the monolithic and SiC_w-reinforced Si₃N₄ are generally higher than the crack growth velocities predicted using static crack growth data. However, the threshold stress intensity factor ranges for the onset of crack growth are always higher under cyclic loads than for sustained load fracture. Electron microscopy of crack wake contact and crack–tip damage illustrate the mechanisms of subcritical crack growth under static and cyclic loading. Critical experiments have been conducted systematically to measure the fracture initiation toughness at room temperature, after advancing the crack subcritically by a controlled amount under static or cyclic loads at elevated temperatures. Results of these experiments quantify the extent of degradation in crack–wake bridging due to cyclically varying loads. The effects of preexisting glass phase on elevated temperature fatigue and fracture are examined, and the creep crack growth behavior of Si₃N₄–based ceramics is compared with that of oxide-based ceramics.     

Crack-Healing Behavior of Si3N4/SiC Ceramics under Cyclic Stress and Resultant Fatigue Strength at the Healing Temperature

Si₃N₄/SiC composite ceramics were sintered and subjected to three-point bending. A semi-elliptical surface crack of 100 μm surface length was made on each specimen. The crack-healing behavior under cyclic stress of 5 Hz, and resultant cyclic fatigue strengths at healing temperatures of 1100° and 1200°C, were systematically investigated. The main conclusions are as follows: (1) Si₃N₄/SiC composite ceramics have an excellent ability to heal a crack at 1100° and 1200°C. (2) This sample could heal a crack even under cyclic stress at a frequency of 5 Hz. (3) The crack-healed sample exhibited quite high cyclic fatigue strength at each crack-healing temperature, 1100° and 1200°C.     

Fabrication and Mechanical Properties of Silicon Carbide-Silicon Nitride Composites with Oxynitride Glass

A microstructure that consisted of uniformly distributed, elongated β-Si₃N₄ grains, equiaxed β-SiC grains, and an amorphous grain-boundary phase was developed by using β-SiC and alpha-Si₃N₄ powders. By hot pressing, elongated β-Si₃N₄ grains were grown via alpha right arrow β phase transformation and equiaxed β-SiC grains were formed because of inhibited grain growth. The strength and fracture toughness of SiC have been improved by adding Si₃N₄ particles, because of the reduced defect size and the enhanced bridging and crack deflection by the elongated β-Si₃N₄ grains. Typical flexural-strength and fracture-toughness values of SiC-35-wt%-Si₃N₄ composites were 1020 MPa and 5.1 MPam^1/2, respectively.     

Microstructure Characterization of Gas-Pressure-Sintered β-Silicon Nitride Containing Large β-Silicon Nitride Seeds

Fine β-Si₃N₄ powders with or without the addition of 5 wt% of large β-Si₃N₄ particles (seeds) were gas-pressure sintered at 1900°C for 4 h using Y₂O₃ and Al₂O₃ as sintering aids. The microstructures were examined on polished and plasmaetched surfaces. These materials had a microstructure of in situ composites with similar small matrix grains and different elongated grains. The elongated grains in the materials with seeds had a larger diameter and a smaller aspect ratio than those in the materials without seeds. A core/rim structure was observed in the elongated grains; the core was pure β-Si₃N₄ and the rim was β-SiAION. These results show that the large β-Si₃N₄ particles acted as seeds for abnormal grain growth and the rim was formed by precipitation from the liquid containing aluminum.     

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.     

Crack Healing of Machining Cracks Introduced by Wheel Grinding and Resultant High-Temperature Mechanical Properties in a Si3N4/SiC Composite

The crack-healing behavior of machining cracks in a Si₃N₄/SiC composite containing Y₂O₃ and Al₂O₃ as a sintering additive was investigated. The machining cracks were introduced by a wheel grinding process, which is the most common method for finishing ceramic components. A semicircular groove was made at the center of small bending specimens by the machining. The machined specimens were healed at various temperatures and times in air. The optimized crack-healing condition of the machined specimen was found to be a temperature of 1300°C and a time of 1 h. The specimens healed by this condition exhibited almost the same strength as the smooth specimens that underwent the healing process. Moreover, the bending strength and fatigue limit of the machined and healed specimens were systematically investigated at temperatures ranging from room temperature to 1300°C. The heat-resistance temperature has been determined to be approximately 1000°C. Also, the specimens exhibited high static and cyclic fatigue limits at temperatures of 800° and 1000°C. These results demonstrate that crack healing could be an effective method for improving the structural integrity and reducing the manufacturing costs of a Si₃N₄/SiC composite ceramic.     

Effect of Grain Growth of β-Silicon Nitride on Strength, Weibull Modulus, and Fracture Toughness

β-Si₃N₄ powder containing 1 mol% of equimolar Y₂O₃–Nd₂O₃ was gas-pressure sintered at 2000°C for 2 h (SN2), 4 h (SN4), and 8 h (SN8) in 30-MPa nitrogen gas. These materials had a microstructure of " in-situ composites" as a result of exaggerated grain growth of some β Si₃N₄ grains during firing. Growth of elongated grains was controlled by the sintering time, so that the desired microstructures were obtained. SN2 had a Weibull modulus as high as 53 because of the uniform size and spatial distribution of its large grains. SN4 had a fracture toughness of 10.3 MPa-m^1/2 because of toughening provided by the bridging of elongated grains, whereas SN8 showed a lower fracture toughness, possibly caused by extensive microcracking resulting from excessively large grains. Gas-pressure sintering of β-Si₃N₄ powder was shown to be effective in fostering selective grain growth for obtaining the desired composite microstructure.     

Relation Between Strength, Fracture Energy, and Microstructure of Hot-Pressed Si3N4

A fracture mechanics approach was used to investigate the high strength of hot-pressed Si₃N₄. Room-temperature flexural strengths, fracture energies, and elastic moduli were determined for material fabricated from α- and β-phase Si₃N₄ powders. When the proper powder preparation technique was used, α-phase powder resulted in a high fracture energy (69,000 ergs/cm²), a high flexural strength (95,000 psi), and an elongated (fiberlike) grain morphology, whereas β-phase powder produced a low fracture energy (16,000 ergs/cm²), a relatively low strength (55,000 psi), and an equiaxed grain morphology. It was hypothesized that the high strength of Si₃N₄ hot-pressed from α-phase powder results from its high fracture energy, which is attributed to the elongated grains. High-strength Si₃N₄ has directional properties caused, in part, by the elongated grain structure, which is oriented preferentially with respect to the hot-pressing direction.     

Evaluation of Slow Crack Growth Resistance in Ceramics for High-Temperature Applications

The slow (subcritical) crack growth (SCG) resistance of Si₃N₄ and SiC ceramics has been evaluated by a stepwise loading test on bending bars precracked by Vickers indentation. Three highly refractory materials were selected for the evaluation: i.e., (1) high-purity Si₃N₄ sintered by hot isostatic pressing (HIP) without additives and (2,3) α - and β - SiC pressureless sintered with B and C addition. Under the hypothesis of linear elastic behavior at high temperature, which was found satisfied in the present materials, the SCG resistance was expressed in terms of initial stress intensity factor critical for SCG failure within a predetermined lifetime. The present method was found useful in shortening the testing time and consistent with other traditional fatigue tests (e.g., static-fatigue test): It is recommended as a screening test for materials under research and development. Among the materials tested in the present study, the highest SCG resistance up to 1440°C was found in the high-purity Si₃N₄ without additives.     

Crack Resistance and Fatigue of Transforming Ceramics: I, Materials in the ZrO2–Y2O3–Al2O3 System

Crack resistance characteristics and fatigue properties have been studied in four types of Y₂O₃–TZP ceramics including one containing Al₂O₃. The largely linear-elastic behavior connected with the very small transformation zone (<5 μm) explains the absence of any resistance-curve behavior and the flaw-controlled strength. The crack resistance shows high sensitivity to environment-induced subcritical crack growth. This influence is also operative in both types of fatigue experiments, i.e., under static and cyclic stresses, leading to reduced fatigue thresholds compared with K _IC. While for static conditions a benefit is observed from enhanced t-m ZrO₂ transformation, cyclic stresses provoke an additional fatigue effect. However, if the cyclic stresses are restricted to subthreshold values, cyclic stress-induced effects in the process zone provide an improvement of the materials being visible as a strengthening effect.     

Silicon Carbide Monofilament-Reinforced Silicon Nitride or Silicon Carbide Matrix Composites

SiC-monofilament-reinforced SiC or Si₃N₄ matrix composites were fabricated by hot-pressing, and their mechanical properties and effects of filaments and filament coating layers were studied. Relationships between frictional stress of filament/matrix interface and fracture toughness of SiC monofilament/Si₃N₄ matrix composites were also investigated. As a result, it was confirmed experimentally that in the case of composites fractured with filament pullout, the fracture toughness increased as the frictional stress increased. On the other hand, when frictional stress was too large (>about 80 MPa) for the filament to be pulled out, fracture toughnesses of the composites were almost the same and not so much improved over that of Si₃N₄ monolithic ceramics. The filament coating layers were found to have a significant effect on the frictional stress of the SiC monofilament/Si₃N₄ matrix interface and consequently the fracture toughness of the composites. Also the crack propagation behavior in the SiC monofilament/Si₃N₄ matrix composites was observed during flexural loading and cyclic loading tests by an in situ observation apparatus consisting of an SEM and a bending machine. The filament effect which obstructed crack propagation was clearly observed. Fatigue crack growth was not detected after 300 cyclic load applications.     

Fabrication of Si3N4/SiC Composite by Reaction-Bonding and Gas-Pressure Sintering

Si₃N₄/SiC composite materials have been fabricated by reaction-sintering and postsintering steps. The green body containing Si metal and SiC particles was reaction-sintered at 1370°C in a flowing N₂/H₂ gas mixture. The initial reaction product was dominated by alpha-Si₃N₄. However, as the reaction processed there was a gradual increase in the proportion of β-Si₃N₄. The reaction-bonded composite consisting of alpha-Si₃N₄, β-Si₃N₄, and SiC was heat-treated again at 2000°C for 150 min under 7-MPa N₂ gas pressure. The addition of SiC enhanced the reaction-sintering process and resulted in a fine microstructure, which in turn improved fracture strength to as high as 1220 MPa. The high value in flexural strength is attributed to the formation of uniformly elongated β-Si₃N₄ grains as well as small size of the grains (length = 2 μm, thickness = 0.5 μm). The reaction mechanism of the reaction sintering and the mechanical properties of the composite are discussed in terms of the development of microstructures.     

Fracture of Si3N4–SiC Whisker Composites under Cyclic Loads

This communication demonstrates the role of cyclic compressive loads in inducing mode I fatigue crack growth at room temperature in Si₃N₄ matrix–SiC whisker composite materials containing stress concentrations. The characteristics of stable, cyclic fracture are examined for several volume fractions of the SiC whisker and are compared with those of the matrix material. It is found that the composites with higher volume fractions of SiC whiskers exhibit an inferior resistance to fracture under cyclic compressive loads despite improvements in fracture toughness values.     

Microstructure and Mechanical Properties of Sinter–Post-HIPed Si3N4–SiC Composites

The microstructural evolution and mechanical properties of Si₃N₄–SiC composites obtained by the sinter–post-HIP process were investigated. SiC addition prohibited β-Si₃N₄ grain growth; however, the grain growth followed the empirical growth law, with exponents of 3 and 5 for the c - and the a -axis directions, respectively. Mechanical properties were strongly influenced by SiC addition and sintering conditions. Short-crack propagation behavior was measured and analyzed by the indentation-strength in-bending (ISB) method. The present composites had high short-crack toughness, compared with the values for monolithic Si₃N₄. The enhanced short-crack toughness was attributed to crack-tip bridging by the SiC particles.     

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.     

Superplastic Deformation in Silicon Nitride–Silicon Oxynitride In Situ Composites

Starting with a mixture of ultrafine β-Si₃N₄ and a SiO₂-containing additive, a superplastic Si₃N₄-based composite was developed, using the concept of a transient liquid phase. Significant deformation-induced phase and microstructure evolutions occurred in the nonequilibrium, fine-grained Si₃N₄ material, which led to the in situ development of a Si₃N₄–22-vol%-Si₂N₂O composite and strong texture formation. The unusual ductility of the composites with elongated Si₂N₂O grains was attributed to the fine-grained microstructure, the presence of a transient liquid phase, and the alignment of the elongated Si₂N₂O grains. The mechanical properties of the resultant composite were enhanced rather than impaired by superplastic deformation and subsequent heat treatment; the resultant composite exhibited both high strength (957 MPa) and high fracture toughness (4.8 MPa·m^1/2).     

Self-Reinforced Nitrogen-Rich Calcium–α-SiAlON Ceramics

Nitrogen-rich Ca–α-SiAlON ceramics with nominal compositions Ca _x Si_{12−2 x} Al_{2 x} N₁₆ and 0.2≤ x ≤2.6, extending along the Si₃N₄–1/2Ca₃N₂:3AlN tie line, were prepared from Si₃N₄, AlN, and CaH₂ precursors by hot pressing at 1800°C. The x values attained were determined by energy-dispersive X-ray (EDX) microanalysis and X-ray powder diffraction (XRPD) data using the Rietveld method. The results show that Ca–α-SiAlONs form continuously within the compositional range x =0 to at least x =1.82. Phase assemblages, lattice parameters, Vickers hardness, and fracture toughness were determined and correlated to the calcium content, x . Owing to a high sintering temperature and the use of CaH₂ as a precursor, grain growth was kinetically enhanced, resulting in self-reinforced microstructures with elongated grains. The obtained Ca–α-SiAlON ceramics demonstrate a combination of both high hardness ∼21 GPa, and high fracture toughness ∼5.5 MPa·m^1/2.     

Residual α–Si3N4 in O' Crystals in CeO2-Doped O'+β' SiAlON Ceramics

The microstructure of a pressureless sintered (1605°C, 90 min) O'+β' SiAlON ceramic with CeO₂ doping has been investigated. It is duplex in nature, consisting of very large, slablike elongated O' grains (20–30 μm long), and a continuous matrix of small rodlike β' grains (< 1.0 μm in length). Many α-Si₃N₄ inclusions (0.1–0.5 μm in size) were found in the large O' grains. CeO₂-doping and its high doping level as well as the high Al₂O₃ concentration were thought to be the main reasons for accelerating the reaction between the α-Si₃N₄ and the Si-Al-O-N liquid to precipitate O'–SiAlON. This caused the supergrowth of O' grains. The rapid growth of O' crystals isolated the remnant α–Si₃N₄ from the reacting liquid, resulting in a delay in the α→β-Si₃N₄ transformation. The large O' grains and the α-Si₃N₄ inclusions have a pronounced effect on the strength degradation of O'+β' ceramics.     

Mode I Fracture Toughness of a Small-Grained Silicon Nitride: Orientation, Temperature, and Crack Length Effects

The Mode I fracture toughness ( K _{I C} ) of a small-grained Si₃N₄ was determined as a function of hot-pressing orientation, temperature, testing atmosphere, and crack length using the single-edge precracked beam method. The diameter of the Si₃N₄ grains was <0.4 µm, with aspect ratios of 2–8. K _{I C} at 25°C was 6.6 ± 0.2 and 5.9 ± 0.1 MPa·m^1/2 for the T–S and T–L orientations, respectively. This difference was attributed to the amount of elongated grains in the plane of crack growth. For both orientations, a continual decrease in K _IC was observed through 1200°C, to ∼4.1 MPa·m^1/2, before increasing rapidly to 7.5–8 MPa·m^1/2 at 1300°C. The decrease in K _IC through 1200°C was a result of grain-boundary glassy phase softening. At 1300°C, reorientation of elongated grains in the direction of the applied load was suggested to explain the large increase in K _IC. Crack healing was observed in specimens annealed in air. No R -curve behavior was observed for crack lengths as short as 300 µm at either 25° or 1000°C.     

Microstructural Development During Gas-Pressure Sintering of α-Silicon Nitride

Gas-pressure sintering of α-Si₃N₄ was carried out at 1850 ° to 2000°C in 980-kPa N₂. The diameters and aspect ratios of hexagonal grains in the sintered materials were measured on polished and etched surfaces. The materials have a bimodal distribution of grain diameters. The average aspect ratio in the materials from α-Si₃N₄ powder was similar to that in the materials from β-Si₃N₄ powder. The aspect ratio of large and elongated grains was larger than that of the average for all grains. The development of elongated grains was related to the formation of large nuclei during the α-to-β phase transformation. The fracture toughness of gaspressure-sintered materials was not related to the α content in the starting powder or the aspect ratio of the grains, but to the diameter of the large grains. Crack bridging was the main toughening mechanism in gas-pressure-sintered Si₃N₄ ceramics.