MoSi2-Based Sandwich Composite Made by Tape Casting

The mechanical properties of a novel laminar composite made by tape casting have been studied. The composite consists of three layers in which an inner core of pure molybdenum disilicide (MoSi₂) is sandwiched between two layers of MoSi₂ reinforced with alumina platelets. Monolithic MoSi₂ exhibits poor room-temperature strength and a brittle indentation strength response, indicative of the absence of R -curve behavior. The flexural behavior of the sandwich composite (both strength and toughness) is dominated by the properties of the outer layer, so long as the thickness of this layer exceeds a critical value. A model has been developed which successfully predicts the critical thickness required.     

Fabrication of Oriented SiC-Whisker-Reinforced Mullite Matrix Composites by Tape Casting

Dense SiC-whisker-reinforced mullite composites with up to 50 vol% whiskers can be obtained by tape casting and hot pressing. The tape casting process results in high degrees of SiC whisker orientation as determined visually and by X-ray diffraction. The ability to achieve dense composites with as much as 50 vol % whiskers is attributed to the higher percolation threshold of aligned whiskers. The factors affecting the degree of whisker orientation during tape casting are described using a fluid dynamics model derived from Jeffery's equations and show that the orientation of anisometric particles is enhanced primarily by the casting rate and particle aspect ratio.     

SiC-Whisker-Reinforced Cellular SiO2 Composites

Fabrication of lighweight, cellular ceramic composites by foaming sol–gels is presented. Foams of 20-vol%-SiC-whisker-reinforced SiO₂ can be tailored with relative densities as low as 10%, with either open or closed cell structures. In addition to reducing drying shrinkage and thus gel cracking, whisker reinforcement significantly increases the strength of the composite foams relative to pure silica foams.     

Crack Deflection as a Toughening Mechanism in SiC-Whisker-Reinforced MoSi2

Crack propagation in SiC-whisker-reinforced MoSi₂ was studied. In particular, the deflection angles of the cracks were examined to determine the degree to which they are affected by the whisker reinforcement. The composite studied was hot-pressed MoSi₂ with 20 vol% vapor-liquid-solid β-SiC whiskers. A substantial difference was found between the deflection angles of cracks formed in the reinforced MoSi₂ and those in a control sample with no whiskers, showing the process of crack deflection as an important, but not the only, toughening mechanism.     

Tape Casting of Al2O3/ZrO2 Laminated Composites

Fracture toughness of ZrO₂-toughened alumina could he increased by macroscopic interfaces, such as those existing in laminated composites. In this work, tape casting was used to produce A/A or A/B laminates, where A and B can be Al₂O₃, Al₂O₃/5 vol% ZrO₂, and Al₂O₃/l0 vol% ZrO₂. An increase of toughness is observed, even in the Al₂O₃/Al₂O₃ laminates.     

Fabrication and Microstructures of MoSi2 Reinforced–Si3N4 Matrix Composites

Details of the fabrication and microstructures of hot-pressed MoSi₂ reinforced–Si₃N₄ matrix composites were investigated as a function of MoSi₂ phase size and volume fraction, and amount of MgO densification aid. No reactions were observed between MoSi₂ and Si₃N₄ at the fabrication temperature of 1750°C. Composite microstructures varied from particle–matrix to cermet morphologies with increasing MoSi₂ phase content. The MgO densification aid was present only in the Si₃N₄ phase. An amorphous glassy phase was observed at the MoSi₂–Si₃N₄ phase boundaries, the extent of which decreased with decreased MgO level. No general microcracking was observed in the MoSi₂–Si₃N₄ composites, despite the presence of a substantial thermal expansion mismatch between the MoSi₂ and Si₃N₄ phases. The critical MoSi₂ particle diameter for microcracking was calculated to be 3 μm. MoSi₂ particles as large as 20 μm resulted in no composite microcracking; this indicated that significant stress relief occurred in these composites, probably because of plastic deformation of the MoSi₂ phase.     

Microstructural Characterization of Sintered MoSi2/SiCP Composites

A MoSi₂/SiC_P composite was synthesized by in situ reactive sintering of a mixture of molybdenum, silicon, and carbon powders. Its microstructural features were studied by X-ray energy dispersive spectroscopy (EDS), conventional transmission electron microscopy (CTEM), and high-resolution transmission electron microscopy (HREM). It was determined that the composite was composed of α-MoSi₂ and β-SiC. There were no specific orientation relationships between the MoSi₂ matrix and SiC_P, because the MoSi₂ and SiC were formed at 1450°C by the reaction of solid Mo and C and liquid Si. The abrupt change occurring in the microstructure of the composite is explained by the presence of an interface between MoSi₂ and SiC_P, where no observable SiO₂ amorphous layer or particles were found. Microtwins and stacking faults were frequently observed in {111} planes of SiC_P.     

SiC-Whisker-Reinforced Al2O3 Composites by Free Sintering of Coated Powders

SiC whiskers were coated with a thick cladding of finegrained Al₂O₃ powder by controlled heterogeneous precipitation in a concentrated suspension of whiskers. After calcination, the coated whiskers were compacted by cold isostatic pressing and sintered at a constant heating rate of 5°C/min in a helium atmosphere. The parameters which control the coating process and the sintering characteristics of the consolidated powders are reported. Starting with an initial matrix density of 40–45% of the theoretical, composites containing up to ≅20 vol% whiskers (aspect ratio ≅15) were sintered freely to nearly theoretical density below 1800°C. By comparison, a similar composite formed by mechanical mixing of the whiskers and the precipitated Al₂O₃ powder reached a density of only 68% of the theoretical after sintering under identical conditions. For a fixed whisker content, the sinterability of the composites formed from the coated whiskers shows a fairly strong dependence on the whisker aspect ratio.     

Hardness and Fracture Toughness of SiC-Particle-Reinforced MoSi2 Composites

The indentation technique has been used to evaluate the hardness and fracture toughness of SiC-reinforced MoSi₂ composites made by hot-pressing. It is seen that the toughness increases with increasing indentation crack length (under increasing load) and a probable mechanism responsible for this behavior is described. It is also observed that there is an optimum volume fraction of SiC particles for which the maximum fracture toughness of the composite can be achieved.     

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.     

Microstructure and Mechanical Properties of In Situ Produced TiC/TiB2/MoSi2 Composites

A novel microstructure of in situ produced TiC/TiB₂/MoSi₂ composite and its mechanical properties were investigated. The results indicate that TiC/TiB₂/MoSi₂ composites can be fabricated by reactive hot pressing the mixed powders of MoSi₂, B₄C, and Ti. A novel microstructure consisting of hollow particles of TiC and TiB₂ grains in an MoSi₂ matrix was obtained. Grains of in situ produced TiC and TiB₂ were much finer, from 100 to 400 nm. During the fracture process, hollow particles relieved crack tip stress, encouraging crack branching and changing the original direction of the main crack. The highest bending strength of this composite achieved was 480 MPa, twice that of monolithic MoSi₂, and the greatest fracture toughness of the composite reached 5.2 MPa·m^1/2.     

Fracture Modes in MoSi2

The room-temperature fracture behavior of polycrystalline MoSi₂ was characterized using Vickers indentation fracture. Fracture analysis was aided by the optically active grain structure of MoSi₂ revealed under polarized light. Radial crack propagation from indentations was found to be predominantly transgranular. The approximate indentation fracture toughness of MoSi₂ was 3 MPa.m^1/2, while the measured hardness was 8.7 GPa. Fracture behavior is believed to be controlled by anisotropy and cleavage energy of the tetragonal MoSi₂ crystal structure.     

Microscopic Mechanisms of Oxidation in SiC-Whisker-Reinforced Mullite/ZrO2 Matrix Composites

The oxidation of SiC whiskers, contained in alkoxide-derived mullite-based matrices and exposed in air at 1000–1350°C for up to 1000 h, has been studied by analytical TEM, high-resolution SEM, and XRD. Silicon carbide whiskers were effectively protected from oxidation when embedded in a pure mullite matrix, but oxidized considerably when embedded in mullite/ZrO₂ matrices. The oxidation mechanisms varied with matrix composition and exposure temperature. At 1350°C the amorphous layer first crystallized as cristobalite, then gradually incorporated alumina. At later times, the mullite portion of the mullite/ZrO₂ matrix dissolved extensively into the layer. Also, the zirconia particles reacted with silica to form zircon. At 1200°C less extensive interdiffusion and chemical reaction occurred, and the silica layer devitrified into cristobalite and quartz. At 1000°C no interdiffusion or chemical reaction was seen, and the silica layer tended to devitrify into quartz. The thickness of the oxide layer around a SiC whisker in a particular matrix depended on the morphology and composition of grains abutting it or adjacent to it.     

Lubricated Rolling Wear of SiC-Whisker-Reinforced Al2O3 Composites against M2 Tool Steel

Lubricated rolling wear studies of SiC-whisker (SiC_w) reinforced A1₂O₃ composites and monolithic A1₂O₃ against M2 tool steel were conducted using a cylinder-on-cylinder apparatus. The composites wore considerably less than A1₂O₃. The wear of the tool steel against the composites was also considerably less than against A1₂O₃. Microfracture occurred on a smaller scale in the composites than in the Al₂O₃. This was attributed to the differences in microstruc-ture and fracture toughness. The worn surfaces of the steel and the composites were polished, possibly due to fine, hard wear debris circulating with the lubricant to the contact area.     

Combustion Synthesis of MoSi2–SiC Composites

MoSi₂, SiC, and MoSi₂–SiC composites were prepared by the thermal explosion mode of self-propagating, high-temperature synthesis (SHS), from elemental powders Mo, Si, and carbon. The products were characterized using chemical analysis, X-ray diffraction, and scanning electron microscopy. The morphology of MoSi₂ in the product points out that it is in the molten state at the combustion temperature. SiC in the composite shows a very fine particle morphology. These results are supported by the earlier thermochemical calculation carried out on this system.     

Mechanical Behavior of MoSi2 Reinforced–Si3N4Matrix Composites

The mechanical behavior of MoSi₂ reinforced–Si₃N₄ matrix composites was investigated as a function of MoSi₂ phase content, MoSi₂ phase size, and amount of MgO densification aid for the Si₃N₄ phase. Coarse-phase MoSi₂-Si₃N₄ composites exhibited higher room-temperature fracture toughness than fine-phase composites, reaching values >8 MP·am^1/2. Composite fracture toughness levels increased at elevated temperature. Fine-phase composites were stronger and more creep resistant than coarse phase composites. Room-temperature strengths >1000 MPa and impression creep rates of ∼10⁻⁸ s⁻¹ at 1200°C were observed. Increased MgO levels generally were deleterious to MoSi₂-Si₃N₄ mechanical properties. Internal stresses due to MoSi₂ and Si₃N₄ thermal expansion coefficient mismatch appeared to contribute to fracture toughening in MoSi₂-Si₃N₄ composites.     

Compressive Creep of SiC-Whisker-Reinforced Al2O3

Compressive creep of SiC-whisker-reinforced Al₂O₃ composites (0, 5, 15, and 25 wt% SiC) was measured in the temperature range of 1300° to 1500°C in air and argon. The creep resistance increased with increasing whisker concentration. The results indicated that the whiskers degraded in air, increasing strain rates compared to those in argon. Stress exponents between 1.0 and 2.0 and an activation energy of 620 ± 100 kJ/mol were measured. Transmission electron microscopy observations indicated that cavitation was minimal and that the deformed composites had the same dislocation structure as did the as-received samples.     

Hot-Pressed MoSi2-Particulate-Reinforced α-SiAlON Composites

MoSi₂-particulate-reinforced α-SiAlON ceramic composites containing 10, 20, 25, and 30 vol% were prepared by hot pressing at 1750°-1800°C. The α-SiAlON matrix was of the composition (Y_0.48Si_10.00A1_2.30O_1.17N_15.29). The hardness for the fully dense samples changed from HV10 = 22.5 to 15.3 GPa and the toughness from 3.2 to around 5.2 MPa^.m^1/2 when up to 30 vol% MoSi₂ was present. Two interesting microstructural features have been found. First, with an increasing amount of MoSi₂ a pronounced coalescence of MoSi₂ particles formed a "dual phase" material. The second effect was the growth of elongated α-SiAlON grains in the matrix with 10 vol% MoSi₂ added. The oxidation resistance has been determined to be unaffected by the addition of 2hd vol % MoSi₂ at 1250°C in oxygen gas of l atm pressure.     

High-Temperature Behavior of MoSi2 and Mo5Si3

The high-temperature stability and behavior of MoSi₂ was studied by heating dense sintered specimens under a vacuum of 10⁻⁵ mm Hg in the temperature range 1700° to 2000°C. The resulting material was examined using physical measurements, X-ray analysis, and metallographic techniques. The decomposition of MoSi₂ into Mo₅Si₃ is described. The Mo₅Si₃-MoSi₂ eutectic temperature was determined as 1900° C, and the melting points of MoSi₅ and Mo₅Si₃ were determined as 1980° and 2085° C, respectively.     

Fabrication of Glass Matrix Composites by Tape Casting

A method has been developed to fabricate borosilicate glass matrix composites reinforced with monofilament SiC fibers by tape casting. Green matrix tapes are laminated with fiber mats of a uniform fiber spacing. The resulting laminate is sintered at 710°C to >98% relative density and HIP-consolidated to full density. The final specimens contain a high volume fraction of fibers (>35 vol%) in a uniform array. A variation of this technique can be used to mount "microcomposites" (i.e., coated fibers) in a glass matrix to facilitate fabrication of push-out test specimens.