Processing and Mechanical Properties of Dense Si3N4-SiC-Whisker Composites without Sintering Aids

Si₃N₄ with 20 vol% SiC whisker was fabricated without sintering aids by hot isostatic pressing. Density higher than 99.5% was attained after sintering at 2000°C and 170 MPa for 1 h. Careful mixing procedures and the use of an appropriate amount of a dispersant was found to be effective in avoiding whisker segregation and inhomogeneity. Mechanical properties of the composite were investigated by measurements of flexural strength, microhardness, frature toughness, and Young's modulus as a function of temperature. At room temerature, Vickers microhardness and Young's modulus increased from the matrix value about 20% and 5%, respectively. Toughness was about 30% higher, without reduction in flexural strength, up to 1400Deg;C.     

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.     

Fabrication of Textured Si3N4 Ceramics by Slip Casting in a High Magnetic Field

Developing the texture of ceramics is one of the effective ways for improving properties. Although the magnetic susceptibility of nonmagnetic materials is very small, there is a possibility to control the crystal orientation using a high magnetic field due to a magnetic anisotropy. In this study, Si₃N₄ ceramics were manufactured by a slip-casting process under high magnetic field and pressureless sintering. The texture of Si₃N₄ ceramics was studied using X-ray diffraction and scanning electron micrographs of polished and plasma-etched specimens. It has been found that most of the a,b -axes texture of β-Si₃N₄ grains aligned to the magnetic field direction.     

Oxidation Effects on the Mechanical Properties of a SiC-Fiber-Reinforced Reaction-Bonded Si3N4 Matrix Composite

The room-temperature mechanical properties of a SiC-fiberreinforced reaction-bonded silicon nitride composite were measured after 100 h treatment in nitrogen and oxygen environments to 1400°C. The composite heat-treated in nitrogen to 1400°C showed no appreciable loss in properties. In contrast, composites heat-treated in oxygen from 600° to 1000°C retained ∼65% and 35% of the matrix fracture and ultimate strength, respectively, of the as-fabricated composites, and those heat-treated from 1200° to 1400°C retained greater than 90% and 65% of the matrix fracture and ultimate strength, respectively, of the as-fabricated composites. For all nitrogen and oxygen treatments, the composite displayed strain capability beyond the matrix fracture strength. Oxidation of the fiber surface coating, which caused degradation of bond between the fiber and matrix and reduction in fiber strength, appears to be the dominant mechanism for property degradation of the composites oxidized from 600° to 1000°C. Formation of a protective silica coating at external surfaces of the composites at and above 1200°C reduced oxidation of the fiber coating and hence degrading effects of oxidation on their properties.     

Effect of Oxidation on Mechanical Properties of Fibrous Monolith Si3N4/BN at Elevated Temperatures in Air

The oxidation behavior and its effect on the mechanical properties of fibrous monolith Si₃N₄/BN after exposure to air at temperatures ranging from 1000° to 1400°C for up to 20 h were investigated. After exposure at 1000°C, only the BN cell boundary was oxidized, forming a B₂O₃ liquid phase. With increasing exposure temperature, the Si₃N₄ cells began to oxidize, forming crystalline Y₂Si₂O₇, SiO₂, and silicate glass. However, in this case, a weight loss was observed due to extensive vaporization of the B₂O₃ liquid. After exposure at 1400°C, large Y₂Si₂O₇ crystals with a glassy phase formed near the BN cell boundaries. The oxidation behavior significantly affected the mechanical properties of the fibrous monolith. The flexural strength and work-of-fracture decreased with increasing exposure temperature, while the noncatastrophic failure was maintained.     

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.     

Friction and Wear Properties of Si3N4/Carbon Fiber Composites with Aligned Microstructure

The friction and wear properties of silicon nitride/carbon fiber composites have been assessed and compared with monolithic Si₃N₄. Three different types of composites have been produced; one in which both the Si₃N₄ grains and the carbon fibers were aligned, one in which only the fibers had alignment, and a third where both the grains and fibers had random orientation. The friction coefficients of all of the composites, following running in, were around 0.2–0.3, typically less than one-third of that of the monolithic material. However there was no significant difference in friction coefficient between the three different types of composite. The specific wear rates of all the materials decreased with sliding distance and those of the composites were lower than the monolithic material. Among the composites, the wear rate of the one with aligned fibers in a randomly oriented Si₃N₄ matrix showed no dependence on sliding direction relative to the fiber alignment, and the specific wear rates of these samples were similar to that of the randomly oriented fiber composite, indicating little effect of fiber alignment alone on the wear properties under the present testing conditions. However, the specific wear rate of the composite with both fiber and grain alignment showed directional dependence. Grain cracking was observed perpendicular to the sliding direction, and the Spara specimen, in which the sliding direction was parallel to the Si₃N₄ grain alignment, showed higher wear rates than the Sperp and N samples of this composite. Such cracks are perpendicular to the major axis of the grains in the Spara sample and are thought to lead to easier removal of grains following their cracking under the tensile stresses induced particularly during the running in period.     

Mechanical Properties of Si2N2O/SiC-Whisker Composites

The mechanical and thermal properties of Si₂N₂O/SiC-whisker composites were studied with emphasis on the effect of matrix composition and of whisker content. The fracture toughness of Si₂N₂O was remarkably improved by 90% with a concomitant 70% strength improvement by addition of SiC whiskers of only 10 vol%. Optimum mechanical and thermal properties of Si₂N₂O/SiC-whisker composites were obtained at an equimolar ratio of Si₃N₄/SiO₂, which is the stoichiometric composition for Si₂N₂O. Additional investigation concerning the Si₂N₂O-matrix/SiC-whisker interface by controlling sintering additives is necessary for further improvement of mechanical and thermal properties of Si₂N₂O/SiC-whisker composites.     

Microstructure and Short-Term Oxidation of Hot-Pressed Si3N4/ZrO2(+Y2O3) Ceramics

Composite ceramic materials based on Si₃N₄ and ZrO₂ stabilized by 3 mol% Y₂O₃ have been formed using aluminum isopropoxide as a precursor for the Al₂O₃ sintering aid. Densification was carred out by hot-pressing at temperatures in the range 1650° to 1800°C, and the resulting micro-structures were related to mechanical properties as well as to oxidation behavior at 1200°C. Densification at the higher temperatures resulted in a fibrous morphology of the Si₃N₄ matrix with consequent high room-temperature toughness and strength. Decomposition of the ZrO₂ grains below the oxidized surface during oxidation introduced radial stresses in the subscalar region, and from the oxidation experiments it is suggested that the ZrO₂ incorporated some N during densification.     

Tribological Behavior of a Si3N4/Carbon Short Fiber Composite under Water Lubrication

The tribological behavior of monolithic Si₃N₄ and a Si₃N₄/carbon fiber composite has been assessed under high load and low speeds in an aqueous environment. The results showed that the friction coefficient of the Si₃N₄ was not significantly reduced when compared with dry sliding, and this was attributed to the failure to maintain a lubricating layer between the solid–solid surfaces. In the case of the composite, the initial high friction coefficient was reduced shortly after the beginning of the wear test and maintained a low value (about 0.03) throughout. This was attributed to the solid lubricating effect of the composite resulting in lower stress at the contact asperities, preventing the removal of the lubricating layer.     

Elastic Properties of Al2O3 and Si3N4 Matrix Composites with SiC Whisker Reinforcement

A study of the elastic moduli of Al₂O₃ and Si₃N₄ ceramics reinforced with 0 to 25 wt% SiC whiskers has been performed. The Young's moduli, shear moduli, and longitudinal modulus are compared with calculated predictions for aligned fiber composites by Hill and Hashin and Rosen, and for fibers randomly oriented in three dimensions by Christensen and Waal. The measured values are in excellent quantitative agreement with those derived for the random orientation of the SiC whiskers.     

Fabrication of TiN/Si3N4 Ceramics by Spark Plasma Sintering of Si3N4 Particles Coated with Nanosized TiN Prepared by Controlled Hydrolysis of Ti(O-i-C3H7)4

TiN-coated Si₃N₄ particles were prepared by depositing TiO₂ on the Si₃N₄ surfaces from Ti(O- i -C₃H₇)₄ solution, the TiO₂ being formed by controlled hydrolysis, then subsequently nitrided with NH₃ gas. A homogeneous TiO₂ coating was achieved by heating a Si₃N₄ suspension containing 1.0 vol% H₂O with the precursor at 40°C. Nitridation successfully produced Si₃N₄ particles coated with 10–20 nm TiN particles. Spark plasma sintering of these TiN/Si₃N₄ particles at 1600°C yielded composite ceramics with a relative density of 96% at 25 vol% TiN and an electrical resistivity of 10⁻³Ω·cm in compositions of 17.5 and 25 vol% TiN/Si₃N₄, making these ceramics suitable for electric discharge machining.     

Densification of Si3N4 with LiYO2 Additive

This paper deals with the densification and phase transformation during pressureless sintering of Si₃N₄ with LiYO₂ as the sintering additive. The dilatometric shrinkage data show that the first Li₂O- rich liquid forms as low as 1250°C, resulting in a significant reduction of sintering temperature. On sintering at 1500°C the bulk density increases to more than 90% of the theoretical density with only minor phase transformation from α-Si₃N₄ to β-Si₃N₄ taking place. At 1600°C the secondary phase has been completely converted into a glassy phase and total conversion of α-Si₃N₄ to β-Si₃N₄ takes place. The grain growth is anisotropic, leading to a microstructure which has potential for enhanced fracture toughness. Li₂O evaporates during sintering. Thus, the liquid phase is transient and the final material might have promising mechanical properties as well as promising high-temperature properties despite the low sintering temperature. The results show that the Li₂O−Y₂O₃ system can provide very effective low-temperature sintering additives for silicon nitride.     

Measurement of Interfacial Shear Strength in SiC-Fiber/Si3N4 Composites

An indentation method for measuring shar strength in brittle matrix composites was applied to SiC-fiber/Si₃N₄-matrix samples. Three methods were used to manufacture the composites: reaction bonding of a Si/SiC preform, hot-pressing, and nitrogen-overpressure sintering. An indentation technique developed by Marshall for thin specimens was used to measure the shear strength of the interface and the interfacial friction stresses. This was done by inverting the sample after the initial push through and retesting the pushed fibers. SEM observations showed that the shear strength was determined by the degree of reaction between the fiber and the matrix unless the fiber was pushed out of its (well-bonded) sheath.     

Slip Casting of SiC-Whisker-Reinforced Si3N4

Si₃N₄ composite materials containing up to 20 vol% SiC whiskers were slip cast and pressureless sintered at 1820°C and 0.13 MPa of N₂. Viscosimetry showed no influence of whisker loading on the rheology of the highly concentrated aqueous slips up to 15 vol% whiskers. During casting the whiskers were preferentially aligned parallel to the mold surfaces. Depending on the whisker loading, green densities of 0.64 to 0.69 fractional density could be achieved. Strong anisotropic shrinkage occurred during sintering with a maximum linear shrinkage of 21% perpendicular but only 7% parallel to the whisker plane. With increasing whisker content from 0 to 20 vol% sintered densities decreased from 0.98 to 0.88, respectively.     

Si3N4/SiC-Whisker Composites without Sintering Aids: I, Quantitative Evaluation of Microstructure

A microstructural evaluation of Si₃N₄ with 20 vol% SiC whiskers, fully densified by hot isostatic pressing (HIP) without sintering aids, is presented. The grain size and morphology of the matrix, the whisker aspect ratio after sintering, interfacial bonding, and the structural stability of reinforcement up to 2000°C are discussed. Image analysis provides quantitative information about whisker dispersion and orientation. It is pointed out that a whisker dispersion and orientation. It is pointed out that a whisker composite with a high degree of homogeneity and isotropy can be obtained by optimizing the mixing procedure and using HIP.     

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.     

Microstructure and Densification of Pressureless-Sintered Al2O3/Si3N4-Whisker Composites

Composite densification was studied by performing slip casting and sintering experiments on an Al₂O₃ matrix and Si₃N₄ whisker system. Even though all the slip-cast powder compacts exhibited high green densities (up to 70% of the theoretical) and narrow pore-size distribution (pore radius around 15 to 30 nm), significant differential densification on a microscopic scale was found due to the existence of local whisker agglomeration. The inhomogeneous whisker distribution resulted in a binary mixture of large and small pores in the sintered composites, in which whisker-associated flaws remained stable even after prolonged sintering. The sintered microstructures showed that the spatial distribution as well as the volume fraction of the Si₃N₄ affect composite densification. Inhomogeneous whisker distribution dominated the complete densification of the composites.     

Powder Processing, Rheology, and Mechanical Properties of Feedstock for Fused Deposition of Si3N4 Ceramics

Fused deposition of ceramics (FDC) is a technique in which green parts are fabricated directly from CAD designs. The feedstock for FDC is a 1.778 mm diameter filament that requires a low viscosity and high column strength. This study explores the powder processing science, as well as the rheological and mechanical properties required for a successful FDC feedstock material. GS44 Si₃N₄ powders were dispersed in RU9 binder using oleyl alcohol (OA). The viscosity of the RU9/OA/Si₃N₄ mixture was measured as a function of temperature, solids loading, and OA concentration. The mechanical properties of the filament feedstock were evaluated in compression to establish FDC process limits. The feedstock material shows a shear thinning behavior with OA acting mainly as a plasticizer. The viscosity of GS44-filled RU9 decreases with temperature, and increases with solids content. At 185°C and 55 vol% loading, the viscosity was found to be in the range of 49–7 Pa·s for a corresponding shear rate of 70–1128 s⁻¹. This was sufficiently low for FDC. Based on pressure requirements for FDC extrusion (Δ P ), and maximum sustainable stress without buckling by the filament (σ_E), it has been found that for successful FDC of RU955, 1.1Δ P < σ_E.     

Si3N4/SiC-Whisker Composites without Sintering Aids: III, High-Temperature Behavior

The fracture behavior at high temperature of a Si₃N₄-based SiC-whisker composite fabricated by hot isostatic pressing without sintering aids is compared with that of other highly refractory materials. Particular attention is directed toward evaluating the slow-crack-growth resistance of the composite up to 1440°C and relating this resistance to the microfracture behavior of Si₃N₄ grains, SiC whiskers, and the intergranular, glassy SiO₂ phase. Only thick whiskers operate to bridge the wake of the crack; these whiskers may make a positive contribution to the slow-crack-growth resistance. Impurities detected by EDX microanalysis at the grain boundary, however, apparently degrade the high-temperature properties, a finding supported by internal-friction measurements. Nevertheless, the high potential of the system without sintering aids for high-temperature structural applications has been demonstrated by the time to failure estimated from the measured slow-crack-growth resistance for a fixed flaw size.