Effect of Interfacial Reaction on the Thermal Conductivity of Al–SiC Composites with SiC Dispersions

The effect of interfacial reactions between Al and SiC on the thermal conductivity of SiC-particle-dispersed Al-matrix composites was investigated by X-ray diffraction and transmission electron microscopy (TEM), and the thermal barrier conductance ( h _c) of the interface in the Al–SiC composites was quantified using a rule of mixture regarding thermal conductivity. Al–SiC composites with a composition of Al (pure Al or Al–11 vol% Si alloy)–66.3 vol% SiC and a variety of SiC particle sizes were used as specimens. The addition of Si to an Al matrix increased the thermal barrier conductance although it decreased overall thermal conductivity. X-ray diffraction showed the formation of Al₄C₃ and Si as byproducts in addition to Al and SiC in some specimens. TEM observation indicated that whiskerlike products, possibly Al₄C₃, were formed at the interface between the SiC particles and the Al matrix. The thermal barrier conductance and the thermal conductivity of the Al–SiC composites decreased with increasing Al₄C₃ content. The role of Si addition to an Al matrix was concluded to be restraining an excessive progress of the interfacial reaction between Al and SiC.     

Role of Interfacial Carbon layer in the Thermal Diffusivity/Conductivity of Silicon Carbide Fiber-Reinforced Reaction-Bonded Silicon Nitride Matrix Composites

The role of an interfacial carbon coating in the heat conduction behavior of a uniaxial silicon carbide nitride was investigated. For such a composite without an interfacial carbon coating the values for the thermal conductivity transverse to the fiber direction agreed very well with the values calculated from composite theory using experimental data parallel to the fiber direction, regardless of the ambient atmosphere. However, for a composite made with carbon-coated fibers the experimental values for the thermal conductivity transverse to the fiber direction under vacuum at room temperature were about a factor of 2 lower than those calculated from composite theory assuming perfect interfacial thermal contact. This discrepancy was attributed to the formation of an interfacial gap, resulting from the thermal expansion mismatch between the fibers and the matrix in combination with the low adhesive strength of the carbon coating. In nitrogen or helium the thermal conductivity was found to be higher because of the contribution of gaseous conduction across the interfacial gap. On switching from vacuum to nitrogen a transient effect in the thermal diffusivity was observed, attributed to the diffusion-limited entry of the gas phase into the interfacial gap. These effects decreased with increasing temperature, due to gap closure, to be virtually absent at 1000°C.     

Thermal Stability of Low-Oxygen Silicon Carbide Fiber (Hi-Nicalon) Subjected to Selected Oxidation Treatment

Low-oxygen silicon carbide fibers (Hi-Nicalon) were oxidized at temperatures from 1073 to 1773 K under an oxygen partial pressure of 0.25 atm. The strength of the unoxidized core was practically unaffected by the oxidation temperatures. The strength of the as-oxidized fibers with an SiO₂ film decreased markedly with increasing oxidation temperature. When exposed subsequently to 1773 K in argon, the fibers with a SiO₂ film of 0.3–0.5 μm thickness had the best thermal stability.     

Mechanical Properties, Thermal Shock Resistance, and Thermal Stability of Zirconia-Toughened Alumina-10 vol% Silicon Carbide Whisker Ceramic Matrix Composite

Zirconia-toughened alumina (Al₂O₃–15 vol% Y-PSZ (3 mol% Y₂O₃)) reinforced with 10 vol% silicon carbide whiskers (ZTA-10SiC _w ) ceramic matrix composite has been characterized with respect to its room-temperature mechanical properties, thermal shock resistance, and thermal stability at temperatures above 1073 K. The current ceramic composite has a flexural strength of ∽550 to 610 MPa and a fracture toughness, K _IC , of ∽5.6 to 5.9 MPa·m^1/2 at room temperature. Increases in surface fracture toughness, ∽30%, of thermally shocked samples were observed because of thermal-stress-induced tetragonal-to-monoclinic phase transformation of tetragonal ZrO₂ grains dispersed in the matrix. The residual flexural strength of ZTA–10 SiC _w ceramic composite, after single thermal shock quenches from 1373–1573 to 373 K, was ∽10% higher than that of the unshocked material. The composite retained ∽80% of its original flexural strength after 10 thermal shock quenches from 1373–1573 to 373K. Surface degradation was observed after thermal shock and isothermal heat treatments as a result of SiC whisker oxidation and surface blistering and swelling due to the release of CO gas bubbles. The oxidation rate of SiC whiskers in ZTA-10SiC _w composite was found to increase with temperature, with calculated rates of ∽8.3×10⁻⁸ and ∽3.3×10⁻⁷ kg/(m²·s) at 1373 and 1573 K, respectively. It is concluded that this ZTA-10SiC _w composite is not suitable for high-temperature applications above 1300 K in oxidizing atmosphere because of severe surface degradation.     

MgSiN2 Addition as a Means of Increasing the Thermal Conductivity of β-Silicon Nitride

Si₃N₄ powders with the concurrent addition of Yb₂O₃ and MgSiN₂ were sintered at 1900°C for 2–48 h under 0.9 MPa nitrogen pressure. Microstructure, lattice oxygen content, and thermal conductivity of the sintered specimens were evaluated and compared with Si₃N₄, Yb₂O₃, and MgO addition. MgSiN₂ addition was effective for improving the thermal conductivity of Si₃N₄ ceramics, and a material with high thermal conductivity over 140 W·(m·K)⁻¹ could be obtained. For both specimens, lattice oxygen content was decreased with sintering time. However, the thermal conductivity of the MgSiN₂-doped specimen was slightly higher than the MgO-doped specimen with the same oxygen content.     

Fabrication of Continuous-SiC-Fiber-Reinforced SiAlON-Based Ceramic Composites by Reactive Melt Infiltration

A technique for fabrication of β'-SiAlON-based ceramics in three-dimensional woven fabrics of BN-coated SiC (Hi-Nicalon™) fibers was developed by reactive melt infiltration in a controlled N₂ atmosphere. β'-SiAlON was produced in situ by the reaction of β-Si₃N₄, AlN, and Y-Al-Si-O molten glass. The wettability of the fibers with the molten glass was improved by infiltration and pyrolysis of perhydropolysilazane, resulting in fully dense matrix composites. The reaction between the fiber and molten glass could be depressed by increasing the N₂ partial pressure during the melt infiltration. The inhibition of the interfacial reaction may be related to the formation of carbon and oxynitride on the SiC fiber, in agreement with thermodynamic calculations as a function of N₂ partial pressure. The fabricated composites had a high ultimate flexure strength and a large work of fracture at room temperature. Degradation of the mechanical performance of the composites was small, even at 1773 K in an argon atmosphere.     

Some New Perspectives on Oxidation of Silicon Carbide and Silicon Nitride

This study provides new perspectives on why the oxidation rates of silicon carbide and silicon nitride are lower than those of silicon and on the conditions under which gas bubbles can form on them. The effects on oxidation of various rate-limiting steps are evaluated by considering the partial pressure gradients of various species, such as O₂, CO, and N₂. Also calculated are the parabolic rate constants for the situations when the rates are controlled by oxygen and/or carbon monoxide (or nitrogen) diffusion. These considerations indicate that the oxidation of silicon carbide and silicon nitride should be mixed controlled, influenced both by an interface reaction and diffusion.     

Thermal and Electrical Properties in Plasma-Activation-Sintered Silicon Carbide with Rare-Earth-Oxide Additives

This study investigates the thermal and electrical properties of SiC ceramics with a combination of Y₂O₃ and rare-earth-oxide additions as sintering additives, by comparing four types of SiC starting powders varying in particle size and chemical composition. The powder mixtures were plasma-activation sintered to full densities and then annealed at high temperatures for grain growth. The thermal conductivity and electrical resistivity of the SiC ceramics were measured at room temperature by a laser-flash technique and a current–voltage method, respectively. The results indicate that the thermal conductivity and electrical resistivity of the SiC ceramics are dependent on the chemical composition and particle size of the starting powders. The thermal conductivities observed for all of the annealed materials with a rare-earth La₂O₃ sintering additive were >160 W·(m·K)⁻¹, although low electrical resistivity was observed for all materials, in the range 3.4–450 Ω·cm. High thermal conductivity, up to 242 W·(m·K)⁻¹, was achieved in an annealed material using a commercial 270 nm SiC starting powder.     

Processing and Thermal Conductivity of Sintered Reaction-Bonded Silicon Nitride. I: Effect of Si Powder Characteristics

This paper will present sintered reaction-bonded silicon nitride (SRBSN) material with a high thermal conductivity of 121 W·(m·K)⁻¹, which has been successfully prepared from a coarse Si powder with lower levels of oxygen and aluminum impurities, using a mixture of Y₂O₃ and MgSiN₂ as sintering additives, by nitriding at 1400°C for 8 h and subsequent post-sintering at 1900°C for 12 h at a nitrogen pressure of 1 MPa N₂. This thermal conductivity value is higher than that of the materials prepared from high-purity α-Si₃N₄ powder (UBE SN-E10) with the same additive composition under the same sintering conditions. In order to study the effects of Si powder characteristics on the processing, microstructure, and thermal conductivity of SRBSN, the other type of fine powder with higher native oxygen and metallic impurity (typically Al and Fe) contents was also used. The effects of Si particle size, native oxygen, and metallic impurities on the nitriding process, post-sintering process, and thermal conductivity of the resultant SRBSN materials were discussed in detail. This work demonstrates that the improvement in thermal conductivity of SRBSN could be achieved by using higher purity coarse Si powder with lower levels of oxygen and aluminum impurities. In addition, this work also shows that the nitriding temperature has no significant effect on the microstructure and thermal conductivity of SRBSN during post-sintering, although it does affect the characteristics of RBSN formed during nitridation.     

Thermal Conductivity of β-Si3N4: II, Effect of Lattice Oxygen

Dense β-Si₃N₄ with various Y₂O₃/SiO₂ additive ratios were fabricated by hot pressing and subsequent annealing. The thermal conductivity of the sintered bodies increased as the Y₂O₃/SiO₂ ratio increased. The oxygen contents in the β-Si₃N₄ crystal lattice of these samples were determined using hot-gas extraction and electron spin resonance techniques. A good correlation between the lattice oxygen content and the thermal resistivity was observed. The relationship between the microstructure, grain-boundary phase, lattice oxygen content, and thermal conductivity of β-Si₃N₄ that was sintered at various Y₂O₃/SiO₂ additive ratios has been clarified.     

Kinetics and Mechanism of the Thermal Decomposition of Si3N4

The kinetics of the thermal decomposition of silicon nitride, Si₃N₄( s )→3Si( l )+2N₂( g ), were studied from 1490° to 1750°C using a static system. The reaction was followed by measuring the rate of increase of N₂ pressure in the system as a function of time. In the initial stages, the decomposition followed first-order kinetics, with the rate constant expressed as: In the later stages, the experimental data followed the Jander solid-state kinetic law, with the Jander rate constant given by: The effects of excess Si and of Ar and N₂ atmospheres on the reaction kinetics are also reported.     

Interfacial Bonding and Friction in Silicon Carbide [Filament]-Reinforced Ceramic- and Glass-Matrix Composites

Interfacial shear strength and interfacial sliding friction stress were assessed in unidirectional SiC-filament-reinforced reaction-bonded silicon nitride (RBSN) and borosilicate glass composites and 0/90 cross-ply reinforced borosilicate glass composite using a fiber pushout test technique. The interface debonding load and the maximum sliding friction load were measured for varying lengths of the embedded fibers by continuously monitoring the load during debonding and pushout of single fibers in finite-thickness specimens. The dependences of the debonding load and the maximum sliding friction load on the initial embedded lengths of the fibers were in agreement with nonlinear shear-lag models. An iterative regression procedure was used to evaluate the interfacial properties, shear debond strength (T _d ), and sliding friction stress (T _f ), from the embedded fiber length dependences of the debonding load and the maximum frictional sliding load, respectively. The shear-lag model and the analysis of sliding friction permit explicity evaluation of a coefficient of sliding friction (μ) and a residual compressive stress on the interface (σ₀). The cross-ply composite showed a significantly higher coefficient of interfacial friction as compared to the unidirectional composites.     

Microstructure Tailoring for High Thermal Conductivity of β-Si3N4 Ceramics

β-Si₃N₄ ceramics sintered with Yb₂O₃ and ZrO₂ were fabricated by gas-pressure sintering at 1950°C for 16 h changing the ratio of "fine" and "coarse" high-purity β-Si₃N₄ raw powders, and their microstructures were quantitatively evaluated. It was found that the amount of large grains (greater than a few tens of micrometers) could be drastically reduced by mixing a small amount of "coarse" powder with a "fine" one, while maintaining high thermal conductivity (>140 W·(m·K)⁻¹). Thus, this work demonstrates that it is possible for β-Si₃N₄ ceramics to achieve high thermal conductivity and high strength simultaneously by optimizing the particle size distribution of raw powder.     

Temperature Dependence of Interfacial Shear Strength in SiC-Fiber-Reinforced Reaction-Bonded Silicon Nitride

The interfacial shear strength of AVCO SCS-6 SiC-fiber-reinforced reaction-bonded Si₃N₄ (RBSN) composites was studied as a function of temperature. Fiber "push-through" experiments were conducted with a diamond indenter and a high-temperature microhardness tester. The interfacial shear strength was variable and depended mostly on interfacial bonding at low temperatures (5 to 18 MPa at room temperature) and frictional forces at high temperatures (12 to 32 MPa at 1300°C). The frictional component is attributed to the surface roughness of the fibers. The interfacial shear strength increased with temperature, because of the relief of residual stresses arising from the thermal expansion mismatch between fiber and matrix. Because of the composite nature of these fibers, a number of interfaces were tested in each experiment. The interface which debonded and slid was not always the same. Interfacial fracture took place either between the two outermost carbon layers of the SCS-6 fibers, or between the SiC core and the innermost of the two outer carbon layers. The outermost carbon layer of the fiber always stayed bonded to the Si₃N₄ matrix.     

Change in Oxygen Content of Y2O3-Nd2O3-Doped Silicon Nitride During Firing

The oxygen content of silicon nitride with 1 mol% Y₂O₃—Nd₂O₃ additive was measured after firing to determine the compositional change during gas-pressure sintering. Oxygen content decreases from 2.5 to 0.94 wt% during firing for 4 h at 1900°C and 10-MPa pressure in N₂. This decrease in oxygen results from the release of SiO gas generated by a thermaldecomposition reaction between Si₃N₄ and SiO₂. The resultant sintered silicon nitride material contains less than 1 wt% oxygen.     

Thermal Decomposition in the System Si-Y-AI-O-N

The limited transparency of high N (13.3 at.%) glasses in the system Si-Y-AI-O-N was found to be due to Si precipitates that resulted from thermal decomposition during melting. Thermodynamic analysis supported experimental findings that decomposition in N₂ at 200 kPa resulted from oxide-nitride reactions and not from simple dissociation of Si₃N₄. The Si content of the glass was reduced much more by substitution of AIN for Si₃N₄ in the glass batch and melting at low N₂ pressure (200 kPa) than it was by merely melting in N₂ at high pressure (1.6 MPa). The thermodynamic analysis suggested that decomposition could be minimized effectively by melting under a modest partial pressure of SiO(g), but this could not be verified experimentally.     

Thermal Etching and Grain-Boundary Grooving Silicon Ceramics

Polished polycrystalline specimens of Si, Sic, and Si₃N₄ were heated to high temperatures and the rate of thermal etching was measured. Grain-boundary grooving occurred on silicon by surface diffusion, with a surface-diffusion coefficient given by Silicon carbide surfaces became extremely rough and very little grain-boundary grooving occurred. Silicon nitride decomposed in an N₂-H₂, atmosphere with an activation energy of 757 kJ/mol, which was very near the activation energy calculated from thermochemical data. The surfaces became fairly rough but grain-boundary grooves formed by an evaporation processsimilar to that for decomposition.     

Effect of Interfacial Space-Charge Polarization on the Ionic Conductivity of Composite Electrolytes

An enhanced carrier concentration within the interfacial space-charge layers has been proposed as one possible mechanism leading to the enhanced ionic conductivity observed for a variety of composite electrolytes containing a dispersed second phase of fine insulating particles such as A1₂O₃. This study tests this proposal by estimating the maximum space-charge polarization that might reasonably occur at interfaces of nominally pure AgCI, β-AgI, and LiI. The resulting interface conductances are used in effective medium calculations of the total conductivity of a randomly distributed composite of these salts with A1₂O₃ particles. Comparison of the results with published conductivities for the composites shows that the modest enhancement found for AgC1/A1₂O₃ can be accounted for by the interfacial space-charge polarization. However, the large (50- to 500-fold) enhancements found for β-AgI/A1₂O₃ and LiI/A1₂O₃ must be attributed to another mechanism.     

Microstructural Characterization of High-Thermal-Conductivity Aluminum Nitride Ceramic

An aluminum nitride (AlN) ceramic with a thermal conductivity value of 272 W·(m·K)⁻¹, which is as high as the experimentally measured thermal conductivity of an AlN single crystal, was successfully fabricated by firing at 1900°C with a sintering aid of 1 mol% Y₂O₃ under a reducing N₂ atmosphere for 100 h. Oxygen concentrations were determined to be 0.02 and 0.03 mass% in the grains and in the grain-boundary phases, respectively. Neither stacking fault in the grains nor crystalline phase in the grain-boundary regions was found by transmission electron microscopy. An amorphous phase possessing yttrium and oxygen elements was detected between the grains as thin films with a thickness of <1 nm. Because the amount of grain-boundary phase was small, the high-thermal conductivity of the ceramic was attributable to the low oxygen concentration in the AlN grains.     

Molybdenum Oxycarbide and Tungsten Oxycarbide Coatings on Silicon Carbide Fibers

The strength and toughness of fibrous composites depend on the interface properties which control the bonding between the fibers and matrices. One method of controlling the interface involves coating the fiber with an appropriate material. In a previous study, it was found that there is a definite advantage in using low coating temperatures to prevent fibers from degrading. We therefore were interested in a report that Mo₂C could be deposited from Mo(CO)₆ at temperatures as low as 300° to 475°C. Our studies indicated that the material was not Mo₂C, but an oxycarbide, which, with an analogous tungsten oxycarbide coating, was applied to SiC yarns. Both oxycarbides could be converted to the metals by heat-treating in N₂.