Homogeneous Fabrication of Al2O3–ZrO2–SiC Whisker Composite by Surface-Induced Coating

Al₂O₃–ZrO₂–SiC whisker composites were prepared by surface-induced coating of the precursor for the ZrO₂ phase on the kinetically stable colloid particles of Al₂O₃ and SiC whisker. The fabricated composites were characterized by a uniform spatial distribution of ZrO₂ and SiC whisker phases throughout the Al₂O₃ matrix. The fracture toughness values of the Al₂O₃–15 vol% ZrO₂–20 vol% SiC whisker composites (∼12 MPa.m^1/2) are substantially greater than those of comparable Al₂O₃–SiC whisker composites, indicating that both the toughening resulting from the process zone mechanism and that caused by the reinforced SiC whiskers work simultaneously in hot-pressed composites.     

High-Resolution Electron Microscopy of the Silicon Carbide/Aluminum Carbide Interface

The interface of single-crystal SiC and Al brazed at 1273 K is investigated by high-resolution electron microscopy. The orientation relationship of SiC to the Al₄C₃ reaction layer that forms between the SiC and the Al can be expressed as (0001)_SiC∥(0001)_Al4C₃ and [11∼00]_SiC∥[11∼00]_Al4C₃. Furthermore, a very thin (two tetrahedral layers thick) transition phase and misfit dislocations are observed between the SiC and Al₄C₃ lattices. The structure of the transition phase is discussed based on the high-resolution electron microscopy, the stacking of the (Al,Si)C₄ tetrahedral layers, and the charge balance. The same reaction product, with the same orientation relationship, is observed at the interface of a polycrystalline SiC and Al brazed joint.     

Preparation and Properties of Porous Aluminum Nitride–Silicon Carbide Composite Ceramics

Microporous two-phase AlN–SiC composites were prepared using Al₄C₃ and either Si (N₂ atmosphere) or Si₃N₄ (Ar atmosphere) as precursors. The reaction mechanisms of the two synthesis routes and the effect of processing conditions on reaction rate and the material microstructures were demonstrated. The exothermic reaction between Si and Al₄C₃ under N₂ atmosphere was shown to be a simple processing route for the preparation of porous two-phase AlN–SiC materials. The homogeneous two-phase AlN–SiC composites had a grain size in the range of 1–5 μm, and the porosity varied in the range of 36%–45%. The bending strength was 50–60 MPa, in accordance with the high porosity.     

Mechanical and Thermophysical Properties of Zr–Al–Si–C Ceramics

The mechanical and thermophysical properties of quaternary-layered carbides, Zr₂[Al(Si)]₄C₅ and Zr₃[Al(Si)]₄C₆ have been investigated and compared with those of Zr₂Al₃C₄ and Zr₃Al₃C₅. These four carbides are generally alike in mechanical and thermophysical properties due to their similar crystal structures that consisting of alternatively stacked ZrC layers and Al₃C₂/[Al(Si)]₄C₃ slabs. The layer thickness of zirconium carbide and aluminum carbide has effects on their properties. Thicker layer of zirconium carbide and/or thinner layer of aluminum carbides are in favor of stiffness, hardness, thermal, and electrical conductivities, but go against density, specific stiffness, Debye temperature, and coefficient of thermal expansion.     

Alumina Sol-Assisted Sintering of SiC—Al2O3 Composites

The composite sol—gel (CSG) technology has been utilized to process SiC—Al₂O₃ ceramic/ceramic particulate reinforced composites with a high content of SiC (up to 50 vol%). Alumina sol, resulting from hydrolysis of aluminum isopropoxide, has been utilized as a dispersant and sintering additive. Microstructures of the composites (investigated using TEM) show the sol-originating phase present at grain boundaries, in particular at triple junctions, irrespective of the type of grain (i.e., SiC or Al₂O₃). It is hypothesized that the alumina film originating from the alumina sol reacts with SiO₂ film on the surface of SiC grains to form mullite or alumina-rich mullite-glass mixed phase. Effectively, SiC particles interconnect through this phase, facilitating formation of a dense body even at very high SiC content. Comparative sinterability studies were performed on similar SiC—Al₂O₃ compositions free of alumina sol. It appears that in these systems the large fraction of directly contacting SiC—SiC grains prevents full densification of the composite. The microhardness of SiC—Al₂O₃ sol—gel composites has been measured as a function of the content of SiC and sintering temperature. The highest microhardness of 22.9 GPa has been obtained for the composition 50 vol% SiC—50 vol% Al₂O₃, sintered at 1850°C.     

Thermal and Electric Properties in Hot-Pressed ZrB2–MoSi2–SiC Composites

The thermal and electrical properties of MoSi₂ and/or SiC-containing ZrB₂-based composites and the effects of MoSi₂ and SiC contents were examined in hot-pressed ZrB₂–MoSi₂–SiC composites. The thermal conductivity and electrical conductivity of the ZrB₂–MoSi₂–SiC composites were measured at room temperature by a nanoflash technique and a current–voltage method, respectively. The results indicate that the thermal and electrical conductivities of ZrB₂–MoSi₂–SiC composites are dependent on the amount of MoSi₂ and SiC. The thermal conductivities observed for all of the compositions were more than 75 W·(m·K)⁻¹. A maximum conductivity of 97.55 W·(m·K)⁻¹ was measured for the 20 vol% MoSi₂-30 vol% SiC-containing ZrB₂ composite. On the other hand, the electrical conductivities observed for all of the compositions were in the range from 4.07 × 10–8.11 × 10 Ω⁻¹·cm⁻¹.     

Evidence for Bulk Residual Stress Strengthening in Al2O3/SiC Nanocomposites

The fracture behavior of Al₂O₃/SiC nanocomposites has been studied as a function of the SiC volume fraction and compared to that of the pure Al₂O₃ matrix. A pronounced strengthening effect was only observed for materials with low SiC content (i.e., ≤10 vol%) although no evidence of concurrent toughening was found. Assessment of near-tip crack opening displacement (COD) could not experimentally substantiate significant occurrence of an elastic crack-bridging mechanism, in contrast with a recently proposed literature model. Quantitative fractography analysis indicated that transgranular crack propagation in Al₂O₃/SiC nanocomposites depends on the location of the SiC dispersoids within the matrix texture; the higher the fraction of transgranularly located dispersoids, the more transgranular the fracture mode. Experimental evidence of remarkably high residual stresses arising from thermal dilatation mismatch (upon cooling) between Al₂O₃ and SiC phases were obtained by fluorescence and Raman spectroscopy. A strengthening mechanism is invoked which merely arises from residual stress through strengthening of Al₂O₃ grain boundaries.     

Preparation and Mechanical Properties of SiC Whisker and Nanosized Mullite Particulate Reinforced TZP Composites

The influence of the addition of nanometer mullite particulates and SiC whiskers coated with alumina on the mechanical properties of tetragonal zirconia polycrystals (TZP) was studied. With increasing mullite( p ) content the high-temperature flexural strength increased, and a maximum value of 360 MPa at 1000°C was reached at 15 vol% mullite( p . Furthermore, 10 vol% SiC( w ) reinforced 15 vol% mullite/TZP composites improved the high-temperature strength up to 490 MPa at 1000°C, 2.7 times that of pure TZP matrix. This high-temperature strengthening is attributed to load transfer from TZP matrix to SiC( w ) and mullite particulates. Significant whisker pull-out and interface debonding were also observed on the fractured surfaces when SiC( w ) was coated with Al₂O₃ film.     

Synthesis of Al4SiC4

The conditions necessary for synthesizing Al₄SiC₄ from mixtures of aluminum, silicon, and carbon and kaolin, aluminum, and carbon, as starting materials, were examined in the present study. The standard Gibbs energy of formation for the thermodynamic reaction SiC( s ) + Al₄C₃( s ) = Al₄SiC₄( s ) changed from positive to negative at 1106°C. SiC and Al₄C₃ formed as intermediate products when the mixture of aluminum, silicon, and carbon was heated in argon gas, and Al₄SiC₄ then formed by reaction of the SiC and Al₄C₃ at >1200°C. Al₄C₃, SiO₂, Al₂O₃, SiC, and Al₄O₄C formed as intermediate products when the mixture of kaolin, aluminum, and carbon was heated under vacuum, and Al₄SiC₄ formed from a reaction of those intermediate products at >1600°C.     

Effects of Interfacial Films on Thermal Stresses in Whisker-Reinforced Ceramics

Toughening of whisker-reinforced (or fiber-reinforced) ceramics by whisker pullout requires debonding at the whisker/matrix interface. Compressive clamping stresses, which would inhibit interface debonding and/or pullout, are expected in composites where the matrix has a higher thermal expansion coefficient than the whisker. Because such mismatch in thermomechanical properties can result in brittle composites, it is important to explore approaches to modify the thermal stresses in composites. As a result, the effects of a film at the whisker/matrix interface on the stresses due to thermal contraction mismatch upon cooling are considered in this study. Analysis of various properties of the film are considered for the whisker/matrix systems, in particular for SiC/Al₂O₃, SiC/cordierite, and SiC/mullite composites. Reduction of thermomechanical stresses is shown to occur when the interfacial film has a low Young's modulus. Also, when the whisker has a lower thermal expansion coefficient than the matrix (e.g., SiC/Al₂O₃), the interfacial stresses generated during cooling decrease as the thermal expansion coefficient of the film increases.     

Reaction Bonding and Mechanical Properties of Mullite/Silicon Carbide Composites

Based on the RBAO technology, low-shrinkage mullite/SiC/ Al₂O₃/ZrO₂ composites were fabricated. A powder mixture of 40 vol% Al, 30 vol% A1₂O₃ and 30 vol% SiC was attrition milled in acetone with TZP balls which introduced a substantial ZrO₂ wear debris into the mixture. The precursor powder was isopressed at 300–900 MPa and heattreated in air by two different cycles resulting in various phase ratios in the final products. During heating, Al oxidizes to Al₂O₃ completely, while SiC oxidizes to SiO₂ only on its surface. Fast densification (at >1300°C) and mullite formation (at 1400°C) prevent further oxidation of the SiC particles. Because of the volume expansion associated with the oxidation of Al (28%), SiC (108%), and the mullitization (4.2%), sintering shrinkage is effectively compensated. The reaction-bonded composites exhibit low linear shrinkages and high strengths: shrinkages of 7.2%, 4.8%, and 3%, and strengths of 610, 580, and 490 MPa, corresponding to compaction pressure of 300, 600, and 900 MPa, respectively, were achieved in samples containing 49–55 vol% mullite. HIPing improved significantly the mechanical properties: a fracture strength of 490 MPa and a toughness of 4.1 MPa^.m^1/2 increased to 890 MPa and 6 MPa^.m^1/2, respectively.     

Fracture Behavior of SiC-Whisker-Reinforced Barium Aluminosilicate Glass-Ceramic Matrix Composites

Barium aluminosilicate (BAS) glass-ceramic composites reinforced with various volume percents (0, 10, 20, 30, 40 vol%) of SiC whiskers were fabricated by hot pressing. The microstructure, the whisker/matrix interface structure, the phase constitution, and the mechanical properties of the composites were systematically studied by means of SEM, TEM, and XRD techniques as well as by indentation crack microfracture and single-edge-notched-beam bend testing. It was demonstrated that the incorporation of SiC whiskers could significantly increase the flexural strength and fracture toughness of BAS glass-ceramic matrices. The addition of active Al₂O₃ to the BAS matrix reduced the amount of SiO₂ in the matrix, forming needlelike mullite, which further improved the mechanical properties.     

Colloidal Processing of SiC Whisker Composites

In the present study, the dispersion processing and the characterization of the dispersity of SiC whiskers for reinforced ceramic-based composites were studied. Two kinds of methods were used to determine the dispersity of SiC whiskers. First, the sedimentation density was employed to directly characterize the dispersity of the SiC whiskers. The other is an indirect characterization. The dispersity and the distribution of the SiC whisker in a ceramic matrix were analyzed and characterized indirectly with SEM observation and the determination of the relative density. By means of the colloidal processing of the whiskers in which electrostatic and steric interaction were used to counteract the attractive forces between whiskers, four dispersion processing routes were used to mix the SiC whisker into the Al₂O₃ powder. The sedimentation results indicated that, with aluminum alkoxide as a dispersant, SiC whiskers were completely deflocculated and the sedimentation density increased twofold from 8.5% to 24% of theoretical density, and both the homogeneous distribution of SiC whiskers and densification of the 30 vol% SiC whisker-reinforced Al₂O₃ (AS) were achieved by hot pressing. The relative density of the AS composites was more than 99.7%. SEM observation showed no agglomeration of SiC whiskers.     

Control of Composition and Structure in Laminated Silicon Nitride/Boron Nitride Composites

Based on a biomimetic design, Si₃N₄/BN composites with laminated structures have been prepared and investigated through composition control and structure design. To further improve the mechanical properties of the composites, Si₃N₄ matrix layers were reinforced by SiC whiskers and BN separating layers were modified by adding Si₃N₄ or Al₂O₃. The results showed that the addition of SiC whiskers in the Si₃N₄ matrix layers could greatly improve the apparent fracture toughness (reaching 28.1 MPa·m^1/2), at the same time keeping the higher bending strength (reaching 651.5 MPa) of the composites. Additions of 50 wt% Al₂O₃ or 10 wt% Si₃N₄ to BN interfacial layers had a beneficial effect on the strength and toughness of the laminated Si₃N₄/BN composites. Through observation of microstructure by SEM, multilevel toughening mechanisms contributing to high toughness of the laminated Si₃N₄/BN composites were present as the first-level toughening mechanisms from BN interfacial layers as crack deflection, bifurcation, and pull-out of matrix sheets, and the secondary toughening mechanism from whiskers in matrix layers.     

Reaction Processing of Al2O3 Composites Containing Iron and Iron Aluminides

Different Fe-Al₂O₃ and FeAl-Al₂O₃ composites with metallic contents up to 30 vol% have been fabricated via reaction processing of Al₂O₃, Fe, and Al mixtures. Low Al contents (<∼10 vol%) within the starting mixture lead to composites consisting of Fe embedded in an Al₂O₃ matrix, whereas aluminide-containing Al₂O₃ composites result from powder mixtures with higher Al contents. In both cases, densification up to 98% TD can be achieved by pressureless sintering in inert atmosphere at moderate temperatures (1450°-1500°C). The proposed reaction sintering mechanism includes the reduction of native oxide layers on the surface of the Fe particles by Al and, in the case of mixtures with high Al contents, aluminide formation followed by sintering of the composites. Density and bending strengths of the reaction-sintered composites depend strongly on the Al content of the starting mixture. In the case of samples containing elemental Fe, crack path observations indicate the potential for an increase of fracture toughness, even at room temperature, by crack bridging of the ductile Fe inclusions.     

Development of Reaction-Bonded Electroconductive Silicon Nitride-Titanium Nitride and Resistive Silicon Nitride-Aluminum Oxide Composites

Reaction-bonded Si₃N₄· TiN and Si₃N₄· Al₂O₃ composites were successfully fabricated by heating mixed powder compacts of Si and TiN or Si and Al₂O₃ in a nitrogen atmosphere. The former showed electrical conductivity, owing to the presence of TiN. An electrical resistivity of 2.6 × 10⁻⁵Ω· m was obtained for the Si₃N₄· TiN composite with 70 vol% TiN. The composite with 20 vol% TiN showed an electrical resistivity of 0.22 Ω· m and a bending strength of 460 MPa. On the other hand, the Si₃N₄· Al₂O₃ composite had insulating properties. The use of an appropriate amount of resin binder resulted in a higher green density and, consequently, a higher bending strength. Moreover, electroconductive Si₃N₄· TiN/resistive Si₃N₄· Al₂O₃ complex ceramics could be fabricated by heating green compacts composed of two different portions, one composed of mixed powders of Si and TiN and the other of Si and Al₂O₃. Attainment of such complex ceramics was attributed to the small dimensional change at the nitriding stage, under 0.3% and the similarity of the thermal expansion coefficients of the two composites.     

Tensile Properties of Ceramic Matrix Fiber Composites

The mechanical properties of various 2D ceramic matrix fiber composites were characterized by tension testing, using the gripping and alignment techniques development in this work. The woven fabric composites used for the test had the basic combinations of Al₂O₃ Fabric/Al₂O₃, SiC fabric/SiC, and SiC minofilament uniweave fabric/SiC. Tension testing was performed with strain gauge and acoustic emission instrumentation to identify the first-matrix cracking stress and assure a valid alignment. The peak tensile stresses of these laminate composites were about one-third of the flexural strengts. The SiC monofilament uniweave fabric (14 vol%)/SiC composites showed a relatively high peak stress of 370 MPa in tension testing.     

Zirconium Aluminum Carbides: New Precursors for Synthesizing ZrO2–Al2O3 Composites

ZrO₂–Al₂O₃ nanocrystalline powders have been synthesized by oxidizing ternary Zr₂Al₃C₄ powders. The simultaneous oxidation of Al and Zr in Zr₂Al₃C₄ results in homogeneous mixture of ZrO₂ and Al₂O₃ at nanoscale. Bulk nano- and submicro-composites were prepared by hot-pressing as-oxidized powders at 1100°–1500°C. The composition and microstructure evolution during sintering was investigated by XRD, Raman spectroscopy, SEM, and TEM. The crystallite size of ZrO₂ in the composites increased from 7.5 nm for as-oxidized powders to about 0.5 μm at 1500°C, while the tetragonal polymorph gradually converted to monolithic one with increasing crystallite size. The Al₂O₃ in the composites transformed from an amorphous phase in as oxidized powders to θ phase at 1100°C and α phase at higher temperatures. The hardness of the composite increased from 2.0 GPa at 1100°C to 13.5 GPa at 1400°C due to the increase of density.     

Contribution to the Phase Diagram Al4C3-AIN-SiC

Compositions on the join A1₄C₃·2AIN-A1₄C₃.2SiC were investigated using X-ray diffraction and thermal analysis of hot-pressed bodies prepared from materials of various purities. The quaternary phase A1₄C₃· AIN · SiC was observed in samples prepared from the high-purity elements, but the ternary phases A1₄C₃·2AIN and A1₄C₃·2SiC were observed only in samples containing significant impurities. A projection onto the base triangle at 1860°C for the pseudoternary system Al₄C₃-AlN-SiC was constructed using X-ray diffraction, thermal analysis, and existing information in the literature.     

Thermochemical Analysis of the Silicon Carbide-Alumina Reaction with Reference to Liquid-Phase Sintering of Silicon Carbide

The stabilities of different phases in the Si-Al-C-O system are calculated from thermodynamic considerations with the objective of identifying the liquid phases formed during sintering of SiC in the presence of Al₂O₃. It is shown that a liquid phase can form at the sintering temperatures by the reaction of SiC with Al₂O₃. Depending on the carbon activity, the liquid can be either of the following: Al₂O₃+ Al₄C₃, SiC + Al₄C₃, or molten aluminum. The stability of the aluminosilicate melts that can form by the reaction of Al₂O₃ with the surface silica layer on SiC powders is also evaluated. Several factors that influence liquid-phase sintering, such as the solubility of SiC in the melts and the generation of gases during sintering, are discussed. The results of the thermodynamic analysis are compared with the observed sintering behavior for SiC.