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.     

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.     

Ablation Resistance of Different Coating Structures for C/ZrB2–SiC Composites Under Oxyacetylene Torch Flame

C/ZrB₂–SiC composites were fabricated by polymer infiltration and pyrolysis combined with slurry impregnation method. Three kinds of coating structures for these composites were applied in order to improve their ablation resistance: pure silicon carbide coatings, ZrB₂–SiC mixture coatings, and ZrC–SiC alternating multilayer coatings. The ablation experiments were carried out on an oxyacetylene torch flame with a temperature of about 3000°C. The ZrC–SiC alternating multilayer showed the best ablation resistance. The linear erosion rate for ZrC–SiC alternating multilayer coatings is half of that for ZrB₂–SiC mixture and pure SiC coatings. A model was put forward to account for such a result.     

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.     

Fast Densification of Ultra-High-Temperature Ceramics by Spark Plasma Sintering

This work investigated suitability and efficacy of the sintering technique known as spark plasma sintering to produce ultra-high-temperature-based Hf and Zr borides. Ceramic–matrix composites in the systems HfB₂–SiC, ZrB₂–MoSi₂, and ZrB₂–ZrC–SiC were processed by spark plasma sintering and hot pressing. The effects of processing were evaluated comparing the materials microstructure and properties. Compared with hot-pressing technique, spark plasma sintering offers the great advantage to fabricate successfully in short time (i.e., cuts in costs) poorly sinterable powder compositions without the help of any sintering activators.     

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.     

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.     

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.     

Oxidation of ZrB2–SiC: Influence of SiC Content on Solid and Liquid Oxide Phase Formation

The effect of SiC concentration on the liquid and solid oxide phases formed during oxidation of ZrB₂–SiC composites is investigated. Oxide-scale features called convection cells are formed from liquid and solid oxide reaction products upon oxidation of the ZrB₂–SiC composites. These convection cells form in the outermost borosilicate oxide film of the oxide scale formed on the ZrB₂–SiC during oxidation at high temperatures (≥1500°C). In this study, three ZrB₂–SiC composites with different amounts of SiC were tested at 1550°C for various durations of time to study the effect of the SiC concentration particularly on the formation of the convection cell features. A calculated ternary phase diagram of a ZrO₂–SiO₂–B₂O₃ (BSZ) system was used for interpretation of the results. The convection cells formed during oxidation were fewer and less uniformly distributed for composites with a higher SiC concentration. This is because the convection cells are formed from ZrO₂ precipitates from a BSZ oxide liquid that forms upon oxidation of the composite at 1550°C. Higher SiC-containing composites will have less dissolved ZrO₂ because they have less B₂O₃, which results in a smaller amount of precipitated ZrO₂ and consequently fewer convection cells.     

Properties of Silicon Nitride–Molybdenum Disilicide Particulate Ceramic Composites

The physical and mechanical properties of hot-pressed Si₃N₄–MoSi₂ particulate composites containing 15 and 30 vol% MoSi₂ were studied. The average room-temperature four-point bend strength, fracture toughness, and electrical resistivity are 522 MPa, 3.6 MPa·√m, and 6.3 × 10⁵Χ·cm for the 15 vol% MoSi₂ composite, and 487 MPa, 5.3 MPa·√m, and 0.31 Ω·cm for the 30 vol% MoSi₂ composite. The mechanical properties of the composites are very close to those of hot-pressed Si₃N₄ ceramics. The high electrical conductivity of the 30 vol% MoSi₂ composite was attributed to the percolation effect of MoSi₂ particles. Parabolic oxidation behaviors were observed for the 30 vol% MoSi₂ composite during the 1200°C long-term oxidation experiments.     

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.     

Fabrication of SiC-Whisker-Reinforced MoSi2 Composites by Tape Casting

In this paper, we describe a procedure for the processing of SiC-whisker-reinforced MoSi₂ composites via tape casting. Based on the characteristics of SiC whiskers and MoSi₂ powder in aqueous and nonaqueous solvents, a slip formulation (solvent, dispersant, binder, etc.) has been developed. The formulation developed allows for a uniform distribution of SiC whiskers in the matrix, easy separation of the tapes from the polymeric carrier, convenient control of both tape thickness and orientation of SiC whiskers, and a low binder burnout temperature. The latter is important for the prevention of the oxidation of MoSi₂ powder during the binder burnout in an oxidizing atmosphere.     

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.     

Fabrication and Characterization of an Ultra-High-Temperature Carbon Fiber-Reinforced ZrB2–SiC Matrix Composite

A novel carbon fiber-reinforced ZrB₂–SiC matrix composite was fabricated by heaterless chemical vapor infiltration through infiltration of SiC matrix into a carbon fiber-ZrB₂ powder preform. The C/ZrB₂–SiC composite presented a flexural strength of 148 MPa, a fracture toughness of 5.6 MPa·m^1/2, and a good oxidation and ablation resistance.     

In–Flight Carburization of Molybdenum Disilicide Powders by an Induction Plasma

In-flight carburization of MoSi₂ powders has been performed in an argon–H₂–CH₄ induction plasma. CH₄ was used as the powder carrier gas, and it reacted with MoSi₂ powders under the high-temperature conditions of the plasma. Carbon and α-SiC were the major reaction products of the in-flight carburized MoSi₂ powders. The silicon carbide formed through nucleation and subsequent growth in the liquid phase. The influence of the induction plasma power level, reactor pressure, and quantity of CH₄ on the carburization efficiency was investigated through a Box–Behnken experimental design. Under the optimal conditions achieved in this investigation, ∼8.0 wt% of carbon was incorporated into the MoSi₂ powder particles.     

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.     

Preparation and Microstructure of a ZrB2–SiC Composite Fabricated by the Spark Plasma Sintering–Reactive Synthesis (SPS–RS) Method

A mixture of Zr, B₄C, and Si powders was adopted to synthesize a ZrB₂–SiC composite using the spark plasma sintering–reactive synthesis (SPS–RS) method. SPS treatments were carried out in the temperature range of 1350°–1500°C under a varying pressure of 20–65 MPa with a 3-min holding time. A dense (∼98.5%) ZrB₂–SiC composite was successfully fabricated at 1450°C for 3 min under 30 MPa. The microstructure of the composite was investigated. The in situ formed ZrB₂ and SiC phases dispersed homogeneously on the whole. The grain size of ZrB₂ and SiC was <5 and 1 μm, respectively. A number of in situ formed ultrafine SiC particles were observed entrapped in the ZrB₂ grains.     

Microstructural Evolution and Grain Growth Kinetics in ZrB2–SiC Composites During Heat Treatment

This work reported the microstructural evolution and grain growth kinetics of ZrB₂–SiC composites containing 10, 20, and 30 vol% SiC during heat treatment at 2000°C. The coarsening of ZrB₂ occurred in the three systems, whereas the obvious coarsening for SiC appeared only in the composite with 30 vol% SiC. The kinetics analysis showed the ZrB₂ grain growth rate in the ZrB₂–30 vol% SiC was 25 times lower than that for ZrB₂–10 vol% SiC during heat treatment. Furthermore, the grain growth controlling mechanisms of ZrB₂ and SiC were discussed. In addition, it was found that the heat treatment had little effect on Vickers hardness and fracture toughness of ZrB₂–SiC.     

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.