Creep Behavior of Si3N4-Whisker-Reinforced Si3N4-Matrix Composites Using an In Situ Dynamic Beam Profiling Method

A four-point flexure test, capable of simultaneously measuring load-point and beam-center displacements and permitting in situ profiling of the beam throughout the test, has been developed. Compressive and tensile strains in the outer fibers, together with the neutral axis displacement, were determined in real time for Si₃N₄-whisker-reinforced Si₃N₄-ceramic-matrix composites. Combining these data with the closed form solution of Chen and Chuang gave excellent correspondence between the creep exponents determined from flexure tests with those determined independently from tensile and compressive creep tests.     

Microstructure and Creep Behavior of a Si3N4–SiC Micronanocomposite

The microstructure and its influence on the creep behaviour of carbon derived Si₃N₄-SiC micro/nanocomposite tested in bending at temperatures from 1200° to 1400°C in air has been studied. No phase and microstructure change after creep test implied that material is stable at tested temperature range. After creep test only partial crystallization of glassy intergranular phase has been observed. Creep parameters n close to 1, apparent activation energy around 350 kJ/mol together with TEM observation indicated that the main creep mechanisms is solution precipitation controlled by interface reaction in combination with grain boundary sliding caused by the amorphous intergranular phases present in microstructure. However, the grain boundary sliding is hindered by local SiC particles interlocking neighboring Si₃N₄ grains.     

Mechanical Properties of β-Si3N4-Whisker/Si3N4-Matrix Composites

Composites containing 30 vol%β-Si₃N₄ whiskers in a Si₃N₄ matrix were fabricated by hot-pressing. The composites exhibited fracture toughness values between 7.6 and 8.6 MPa · m^1/2, compared to 4.0 MPa · m^1/2 for unreinforced polycrystalline Si₃N₄. The improvements in fracture toughness were attributed to crack wake effects, i.e., whisker bridging and pullout mechanisms.     

Hot-Pressing Behavior of Si3N4

The densification behavior of Si₃N₄ containing MgO was studied in detail. It was concluded that MgO forms a liquid phase (most likely a magnesium silicate). This liquid wets and allows atomic transfer of Si₃N₄. Evidence of a second-phase material between the Si₃N₄ grains was obtained through etching studies. Transformation of α- to β-Si₃N₄ during hot-pressing is not necessary for densification.     

Failure Modes of SiC-Fiber/Si3N4-Matrix Composites at Elevated Temperatures

An investigation of composite failure modes as a function of temperature and fiber-volume fraction was carried out in SiC-fiber-reinforced Si₃N₄-matrix composites fabricated in our laboratories. Mechanical testing was carried out at temperatures from 25° to 1500°C. Matrix-cracking stress and ultimate strength of the composites were measured from load-displacement curves. They were both found to decrease with increasing temperature, but their temperature sensitivity decreased with increasing fiber-volume fraction. The tendency for noncatastrophic failure increased with fiber-volume fraction, while the tendency for catastrophic failure increased with temperature. The failure mode was demonstrated experimentally to be determined by the fiber bundle strength, S_fb, and the matrix cracking stress, σ_c. These two parameters, in turn, were shown to be controlled by the fiber-volume fraction, f , and the temperature. Failure at various temperatures was noncatastrophic when S_fb > σ_c, and catastrophic when S_fb < σ_c. Transition in composite failure mode between noncatastrophic and catastrophic failure was controlled via the variation of fibervolume fraction and testing temperature. Catastrophic failure at high temperatures was found to be mainly a result of fiber strength loss.     

High-Temperature Failure Mechanisms of Hot-Pressed Si3N4 and Si3N4/Si3N4-Whisker-Reinforced Composites

The high-temperature flexural strength of hot-pressed silicon nitride (Si₃N₄) and Si₃N₄-whisker-reinforced Si₃N₄-matrix composites has been measured at a crosshead speed of 1.27 mm/min and temperatures up to 1400°C in a nitrogen atmosphere. Load–displacement curves for whisker-reinforced composites showed nonelastic fracture behavior at 1400°C. In contrast, such behavior was not observed for monolithic Si₃N₄. Microstructures of both materials have been examined by scanning and transmission electron microscopy. The results indicate that grain-boundary sliding could be responsible for strength degradation in both monolithic Si₃N₄ and its whisker composites. The origin of the nonelastic failure behavior of Si₃N₄-whisker composite at 1400°C was not positively identified but several possibilities are discussed.     

Short-Crack Mechanical Properties and Failure Mechanisms of Si3N4-Matrix/SiC-Fiber Composites

SiC-fiber-reinforced Si₃N₄ composites were fabricated by hot pressing. The indentation-strength technique was applied to study the mechanical properties of these composites. This enabled the investigation of short-crack behavior of continuous-fiber ceramic composites (CFCCs). The flaw tolerance of composite ultimate strength, matrix-cracking stress, and work-of-fracture were investigated. Scanning electron microscopy was used to examine crack–fiber interactions. The ultimate strength was found to be independent of indentation load at a fiber volume fraction f = 0.29, while at f = 0.14 it exhibited a transition from flawsensitive to flaw-independent. The work-of-fracture was found to be independent of indentation load at both fiber volume fractions. The matrix-cracking stress was found to correspond to the first load-drop on the load–displacement curve. It decreased with increasing flaw size and therefore is the steady-state matrix-cracking stress. A failure mechanism transition from catastrophic failure to non-catastrophic failure, coupled with the transition from flawsensitive to flaw-tolerant behavior, was observed by varying the preexisting flaw size and the fiber volume fraction. These transitions were explained by analyzing the relations between ultimate strength, matrix-cracking stress, fiber volume fraction, and preexisting flaw size of the composite materials. Experimental results were compared with predictions from available models.     

R-Curve Behavior of Si3N4–BN Composites

R -curve behavior of Si₃N₄–BN composites and monolithic Si₃N₄ for comparison was investigated. Si₃N₄–BN composites showed a slowly rising R -curve behavior in contrast with a steep R -curve of monolithic Si₃N₄. BN platelets in the composites seem to decrease the crack bridging effects of rod-shaped Si₃N₄ grains for small cracks, but enhanced the toughness for long cracks as they increased the crack bridging scale. Therefore, fracture toughness of the composites was relatively low for the small cracks, but it increased significantly to ∼8 MPa·m^1/2 when the crack grew longer than 700 μm, becoming even higher than that of the monolithic Si₃N₄.     

Comparison of Tribological Behavior Between α-Sialon/Si3N4 and Si3N4/Si3N4 Sliding Pairs in Water Lubrication

We report here the study on tribological behavior of α-Sialon in aqueous medium. The results derived from a wide range of test conditions are briefly discussed. A reduction in friction coefficient from 0.7 to 0.03 and a decrease in wear rate by two orders of magnitude were achieved under low load (9.8 N) and high speed (>0.54 m/s) conditions. The tribological behavior of α-Sialon/Si₃N₄ ceramics was then compared with Si₃N₄/Si₃N₄ tribopairs.     

Oxidation Behavior of Hot-Pressed Si3N4

The high-temperature chemical stability of hot-pressed Si₃N₄ was studied between 600° and 1450°C. Reactions were followed by X-ray diffraction and scanning electron microscopy. In air, this material begins to oxidize at 700° to 750°C; a distinct amorphous siO₂ surface layer results after 24 h at 750°C-Concomitant formation of cristobalite occurs, depending on exposure time, and is enhanced as temperature is Increased. Magnesium and calcium magnesium silicates form above 1000°C. The data suggest that impurities, e.g. Mg, Ca, and Fe, greatly lower the oxidation resistance of Si₃N₄ in air.     

Combustion Synthesis of Si3N4–SiC Composite Powders

Fine Si₃N₄-SiC composite powders were synthesized in various SiC compositions to 46 vol% by nitriding combustion of silicon and carbon. The powders were composed of α-Si₃N₄, β-Si₃N₄, and β-SiC. The reaction analysis suggested that the SiC formation is assisted by the high reaction heat of Si nitridation. The sintered bodies consisted of uniformly dispersed grains of β-Si₃N₄, β-SiC, and a few Si₂N₂O.     

Microstructure and Chemical Reaction in a Si3N4–TiC Composite

The effects of TiC addition to Si₃N₄ on microstructure and the chemical aspects of Si₃N₄–TiC interphase reaction were investigated in samples hot-pressed at 1800°C in Ar and N₂. Composition of a TiC_1–xN_x solid solution was predicted based on thermodynamic calculation, with titanium carbonitride taken to be an ideal solid solution. The predicted value of x = 0.7 is slightly higher than that derived from the measured lattice parameter and Vegard's law (x = 0.67). Four distinguishable areas were observed in samples hot-pressed in nitrogen atmosphere. They were identified as β-Si₃N₄, mixtures of TiC and titanium carbonitride solid solution, SiC with twins, and iron silicide. As the duration of hot-pressing increased, more titanium carbonitride was formed, while less TiC phase remained. Thermodymanic calculations indicate one source of nitrogen gas came from the decomposition of Si₃N₄. The TiC particles also became more irregular, and reactants were found inside or between TiC as the hot-pressing time was extended. Silicon carbide was not detected in samples which were hot-pressed in argon atmosphere; however, numerous pores were found around TiC.     

Thermal Expansion Behavior of the Si3N4-Whisker-Reinforced Soda-Borosilicate Glass Matrix Composite

Thermal expansion behaviors of a Si₃N₄-whisker-reinforced sodaborosilicate glass matrix composite are studied. An abrupt increase of the coefficient of thermal expansion is observed and is attributed to formation of crystobalite in the sodaborosilicate glass matrix. This thermal expansion behavior is discussed with special reference to the phase transformation of the crystobalite.     

Hot Isostatic Pressing of Si3N4 Powder Compacts and Reaction-Bonded Si3N4

Hot isostatically pressed silicon nitride was produced by densifying Si₃N₄ powder compacts and reaction-bonded Si₃N₄ (RBSN) parts with yttria as a sintering additive. The microstructure was analyzed using scanning electron microscopy, X-ray diffraction, and density measurements. The influence of the microstructure on fracture strength, creep, and oxidation behavior was investigated. It is assumed that the higher amount of oxygen in the Si₃N₄ starting powder compared with the RBSN starting material leads to an increased amount of liquid phase during densification. This results in grain growth and in a larger amount of grain boundary phase in the hot isostatically pressed material. Compared with the hot isostatically pressed RBSN samples therefore, strength decreases whereas the creep rate and the weight gain during oxidation increase.     

Mechanical Properties of Si3N4-SiC Three-Layer Composite Materials

The effects of microstructure and residual stress on the mechanical properties of Si₃N₄-based three-layer composite materials were investigated. The microstructure of each layer was controlled by the addition of two differently sized silicon carbides: fine SiC nanoparticles (∼200 nm) or relatively large SiC platelets (∼20 µm). When the SiC nanoparticles were added, the average grain size of Si₃N₄ was reduced because of the inhibition of grain growth by the particles. On the other hand, when the SiC platelets were added, the microstructure of Si₃N₄ was not much changed because of the large size of the platelets. Three-layer composites were fabricated by placing the Si₃N₄/SiC-nanoparticle layers on the surface of the Si₃N₄/SiC-platelet layer. The residual stress was controlled by varying the amount of SiC added. The mechanical properties of three-layer composites with various combinations of microstructure and residual stress level were investigated.     

Nanosilver-Coated Porous SiC–Si3N4 Composite Using Microwave-Assisted Process

Using a novel microwave-assisted process, nano-Ag-coated continuous porous SiC–Si₃N₄ substrate was fabricated from a solution containing AgNO₃ salts and ethylene glycol. The detailed microstructure of the fabricated substrate was investigated depending on the amount of AgNO₃ salts in the starting solution and the microwave irradiation time. From a solution containing 0.4 g of AgNO₃ for 60 s irradiation time, the Ag nanoparticles, ∼25 nm in diameter, were homogeneously coated on the continuous porous SiC–Si₃N₄ matrix as well as on the surface of the Si₃N₄ whiskers. However, the Ag nanoparticles (∼15 nm) deposited from a solution containing 0.6 g of AgNO₃ for 60 s irradiation time showed maximum homogeneity and narrow size distribution. The components of Si, N, and Ag were homogeneously distributed on the deposited layer. The deposited Ag nanoparticles covered with a thin (∼2 nm), amorphous layer had nanocrystallinity and adhered well to the surface of the Si₃N₄ whiskers.     

Effects of Fiber Volume Fraction on Mechanical Properties of SiC-Fiber/Si3N4-Matrix Composites

The effects of fiber volume fraction on composite mechaniacl properties were examined in SiC-fiber-reinforced Si₃N₄ composites fabricated in our laboratories. Fiber volume fraction was found to have significant effects on important composite properties including failure mode, ultimate strength, matrix-cracking stress, fiber–matrix interfacial shear stress, and work-of-fracture. The composite mechanical properties were improved with increasing fiber volume fraction. However, when the fiber volume fraction was sufficiently large, the composite ultimate strength was degraded. This was related to fiber strength loss as a result of fiber damage from contact with surrounding fibers and abrasive matrix particles during hot pressing.