In Situ Processing and Properties of SiC/MoSi2 Nanocomposites

A novel concept for in situ processing of SiC/MoSi₂ nanocomposites has been developed that combines the pyrolysis of MoSi₂ particles coated with polycarbosilane and subsequent densification by hot pressing. After densification, a uniform dispersion of SiC particles is obtained in the MoSi₂ matrix. The strength at both room and elevated temperature is dramatically improved by the processing protocol employed. The average room-temperature flexural strength measured for the SiC/MoSi₂ nanocomposite was 760 versus 150 MPa for unreinforced MoSi₂. The average 1250°C flexural strength measured for the SiC/MoSi₂ nanocomposite was 606 versus 77 MPa for unreinforced MoSi₂.     

Reactive Hot Pressing of SiC/MoSi2 Nanocomposites

Dense SiC/MoSi₂ nanocomposites were fabricated by reactive hot pressing the mixed powders of Mo, Si, and nano-SiC particles coated homogeneously on the surface of Si powder by polymer processing. Phase composition and microstructure were determined by methods of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and energy-dispersive spectrometry. The nanocomposites obtained consisted of MoSi₂, β-SiC, less Mo₅Si₃, and SiO₂. A uniform dispersion of nano-SiC particles was obtained in the MoSi₂ matrix. The relative densities of the monolithic material and nanocomposite were above 98%. The room-temperature flexural strength of 15 vol% SiC/MoSi₂ nanocomposite was 610 MPa, which increased 141% compared with that of the monolithic MoSi₂. The fracture toughness of the nanocomposite exceeded that of pure MoSi₂, and the 1200°C yield strength measured for the nanocomposite reached 720 MPa.     

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.     

Effect of Addition of Silicon on the Microstructures and Bending Strength of Continuous Porous SiC–Si3N4 Composites

The microstructures and mechanical properties of continuous porous SiC–Si₃N₄ composites fabricated by multi-pass extrusion were investigated, depending on the amount of Si powder added. Si powder with different weight percentages (0%, 5%, 10%, 15%, 20%) was added to SiC powder to make raw mixture powders, with 6 wt% Y₂O₃–2 wt% Al₂O₃ as sintering additives, carbon (10–15 μm) as a pore-forming agent, ethylene vinyl acetate as a binder, and stearic acid (CH₃(CH₂)₁₆COOH) as a lubricant. In the continuous porous SiC–Si₃N₄ composites, Si₃N₄ whiskers like the hairs of nostrils were frequently observed on the wall of the pores. In this study, the morphology of Si₃N₄ whiskers was investigated with the nitridation condition and silicon addition content. In composites containing an addition of 10 wt% Si, a large number of Si₃N₄ whiskers were found at the continuous pore regions. In the sample to which 15 wt% Si powder was added, a maximum value of about 101 MPa bending strength and 57.5% relative density were obtained.     

Effects of Heat Treatment on the Surface Microstructure and Mechanical Properties of MoSi2-TiC0.7N0.3 Composites

MoSi₂-TiC_0.7N_0.3 composites were prepared by hot-pressing under vacuum, and MoSi₂–TiC_0.7N_0.3 composites were heat-treated in air at various temperatures. SEM analysis showed that the surface microstructure of the MoSi₂-TiC_0.7N_0.3 composite changed because of the oxidation of MoSi₂ and TiC_0.7N_0.3 and that many TiO₂ whiskers appeared on the surfaces of the composites. Compared with the non-heat-treated MoSi₂-TiC_0.7N_0.3, the bending strength of the heat-treated MoSi₂-TiC_0.7N_0.3 was significantly improved.     

Reactive Infiltration of Si-Mo Alloyed Melt into Carbonaceous Preforms of Silicon Carbide

A MoSi₂/reaction-bonded SiC composite was prepared from a preform of petroleum coke and commercial SiC powders (in weight ratios of 0.5 and 0.6), following reactive infiltration of a Si-Mo melt (molybdenum concentration of 7–29 wt%) made from elemental powder. The resulting material had a relative density of >90% of the theoretical density and, on a microstructural scale, contained SiC and MoSi₂, in addition to unreacted carbon and silicon. The SiC and MoSi₂ boundary was smooth and sharp, with no sign of any reaction. The occasional presence of an intermediate zone between SiC and MoSi₂ was detected; this zone contained silicon, iron, and aluminum, the formation of which may be related to the presence of impurities in the silicon and SiC.     

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.     

Feasibility of a Composite of Sic Whiskers in an MoSi2 Matrix

A composite consisting of 20 vol% Sic whiskers in a hot-pressed MoSi₂ matrix was investigated. The composite displayed an ∼100% increase inflexural strength and a 54% improvement in fracture toughness, compared to the values measured for the matrix material. The improvements are attributed to the change in the micro-structure of the MoSi₂ afforded by the presence of the whiskers.     

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.     

Thermal Oxidation Kinetics of MoSi2-Based Powders

The oxidation process of MoSi₂ is very complex, and controversial results have been reported, especially for the early-stage oxidation before the formation of passive SiO₂ film. Most oxidation studies have been carried out on bulk consolidated samples, and the early stage of oxidation has not been studied. In this investigation, very fine MoSi₂ powder with an average particle size of 1.6 μm was used. Such a fine particle size makes it easier to study the early stages of oxidation since a significant portion of the powder is oxidized before the formation of passive SiO₂ film. The oxidation kinetics of commercial MoSi₂-SiC and MoSi₂-Si₃N₄ powder mixtures were also studied for comparison. Weight changes were measured at discrete time intervals at 500° to 1100°C in 0.14 atm of oxygen. X-ray diffraction was used to identify the phases formed during oxidation. Our results show the formation of MoO₃ phase and an associated weight gain at low temperatures (500° and 600°C). At temperatures higher than 900°C, Mo₅Si₃ phase formed first and was subsequently oxidized to solid SiO₂ and volatile MoO₃, resulting in an initial weight gain followed by subsequent weight loss. A model based on the assumption that oxidation kinetics of both MoSi₂ and Mo₅Si₃ are proportional to their fractions in the system describes the experimental data well.     

Chemical Reactions in the Processing of MoSi2 Carbon Compacts

Hot-pressing of MoSi₂ powders with carbon at high temperatures reduces the siliceous grain boundary phase in the resultant compact. The chemical reactions in this process were examined using the Knudsen cell technique. A 2.3 wt% oxygen MoSi₂ powder and a 0.59 wt% oxygen MoSi₂ powder, both with additions of 2 wt% carbon, were examined. The reduction of the siliceous grain boundary phase was examined at 1350 K and the resultant P(SiO)/P(CO) rations interpreted in terms of the SiO (g) and CO (g) isobars on the Si-C-O predominance diagram. The MoSi₂+ carbon mixtures were then heated at the hot-pressing temperature of 2100 K. Large weigh losses were observed and could be correlated with the formation of a low-melting eutectic and the formation and vaporization of SiC.     

Sintering Mechanisms of Zirconium and Hafnium Carbides Doped with MoSi2

The microstructure of two pressureless-sintered ultra-high-temperature ceramics, namely ZrC+20 vol% MoSi₂ and HfC+20 vol% MoSi₂, was characterized by scanning and transmission electron microscopy. With regard to the ZrC–MoSi₂ system, Zr _x Si _y compounds and SiC were detected. In the HfC–MoSi₂ system, a mixed phase was detected at the triple points and identified as (Mo,Hf)₅Si₃. For both the systems investigated, the high wettability of the silicide-based phases on the matrix grains suggests that sintering is assisted by a liquid phase. This contribution reports for the first time on the sintering mechanisms of early transition metal carbides doped with MoSi₂ as a sinter additive, on the basis of the microstructural evolution observed upon sintering and in the light of phase diagrams and thermodynamical calculations.     

Ionically Conducting Composite Membranes from the Li2O–Al2O3–TiO2–P2O5 Glass–Ceramic

This paper reports processing of lithium ion-conducting, composite membranes comprised of 14Li₂O·9Al₂O₃·38 TiO₂·39P₂O₅ glass–ceramic and polyethylene. The processing involved tape casting of 14Li₂O·9Al₂O₃·38TiO₂·39P₂O₅ glass powder with organic additives into tapes, subjecting the green tape to binder burnout and thermal soaking in the temperature range of 950°–1100°C, and finally infiltrating the porous tape with polyethylene solution. The ionic conductivity and microstructure of 150–350 μm thick membranes were characterized and are discussed in this paper. The crystallites of the glass–ceramic show liquid-like conductivity at ambient temperature, whereas the grain boundary conductivity is lower by a factor of five. The lower grain boundary conductivity is explained on the basis of crystallographic mismatch and the existence of AlPO₄ at the grain boundary. The polyethylene infiltration in the porous membrane improved mechanical resilience with a minor adverse effect on conductivity.     

Thermochemical Evaluation of Combustion Synthesis of MoSi2─SiC Composites

Thermochemical calculations were carried out for the selfpropagating high-temperature synthesis (SHS) of SiC-particulate-reinforced MoSi₂ composites. These composites can be prepared using the elemental powders of Mo, Si, and C by SHS. Adiabatic temperatures were calculated at different initial temperatures for the reactions forming MoSi₂, SiC, and their composites. Amounts of liquid phase of MoSi₂ formed at various temperatures and mole ratios were computed.     

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.     

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.     

Reaction-Bonding Preparation of Si3N4/MoSi2 and Si3N4/WSi2 Composites from Elemental Powders

Si₃N₄/MoSi₂ and Si₃N₄/WSi₂ composites were prepared by reaction-bonding processes using as starting materials powder mixtures of Si-Mo and Si-W, respectively. A presintering step in an At-base atmosphere was used before nitriding for the formation of MoSi₂ and WSi₂; the nitridation in a N₂-base atmosphere was followed after presintering with the total stepwise cycle of 1350°C × 20 h +1400°C × 20 h +1450°C × 2 h. The final phases obtained in the two different composites were Si₃N₄ and MoSi₂ or WSi₂; no free elemental Si and Mo or W were detected by X-ray diffraction.     

Processing and Thermal Shock Resistance of a Polymer-Derived MoSi2/SiCO Ceramic Composite

In this paper, we report a study on the thermal shock resistance (TSR) of MoSi₂/SiCO ceramic composites obtained through controlled pyrolysis of a gel-derived precursor. MoSi₂-filled gel is prepared by casting a sol obtained from MoSi₂ powder dispersed in methyltriethoxysilane. The pyrolysis product can be described as a porous ceramic composite formed by a SiCO matrix with a dispersion of MoSi₂ particles. Mechanical characterization is performed on bar samples by four-point bending. The TSR is investigated either by evaluating the R parameter (associated with strength, elastic modulus, and thermal expansion coefficient), or with the conventional water quenching technique. In both cases, the results suggest that the studied ceramic material displays a good TSR, which makes it a candidate for high-temperature application.     

Dislocations and Plastic Deformation In Molybdenum Disilicide

A composite of MoSi₂ (reinforced with SiC whiskers) has been deformed at high temperatures and the resulting dislocation substructure studied by means of transmission electron microscopy (TEM). Classical g.b analysis of the dislocations in the MoSi₂ matrix has shown that the Burgers vectors are (100), (110), and 1/2(111) which are the shortest lattice vectors in the body-centered tetragonal lattice. Trace analysis showed that there are numerous possible slip planes for each of the determined slip directions. This result suggests that various operative slip systems can contribute to the deformation, depending on the orientation of the particular grain with respect to the stress state.     

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.