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.     

Relationships between Hysteresis Measurements and the Constituent Properties of Ceramic Matrix Composites: II, Experimental Studies on Unidirectional Materials

The use of hysteresis loop measurements for assessing the constituent properties of unidirectional CMCs is evaluated, using basic theory described in a companion paper. Results are obtained on SiC/CAS and SiC/SiC composites. These materials exhibit very different hysteresis characteristics, reflected in differences in sliding stress, τ, and debond energy, T_i. These interface properties are manifest in the respective tensile stress/strain curves.     

Chemical Stability of the Fiber Coating Matrix Interface in Silicon-Based Ceramic Matrix Composites

Carbon and boron nitride are used as fiber coatings in silicon-based composites. In order to assess the long-term stability of these materials, reactions of carbon/Si₃N₄ and BN/SiC were studied at high temperatures with Knudsen effusion, coupon tests, and by microstructural examination. In the carbon/Si₃N₄ system, carbon reacted with Si₃N₄ to form gaseous N₂ and SiC. The formation of SiC limited further reaction by physically separating the carbon and Si₃N₄. Consequently, the development of high p (N₂) at the interface, predicted from thermochemical calculations, did not occur, thus limiting the potential deleterious effects of the reaction on the composite. Strong indications of a reaction between BN and SiC were shown by TEM and SIMS analysis of the BN/SiC interface. In long-term exposures, this reaction can lead to a depletion of a BN coating and/or an unfavorable change of the interfacial properties, limiting the beneficial effects of the coating.     

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.     

Comparison between SEM and TEM Imaging Techniques to Determine Grain-Boundary Wetting in Ceramic Polycrystals

Two different non-oxide ceramics, Si₃N₄ and SiC, were characterized with respect to their grain-boundary structure employing both scanning and transmission electron microscopy. The latter method, which enables one to gain direct insight of the atomistic interface structure, was utilized to verify whether grain-boundary wetting occurred. SEM imaging of plasma-etched surfaces revealed a characteristic bright contrast along interfaces for both ceramics, Si₃N₄ as well as SiC, suggesting the presence of an intergranular glass film. High-resolution TEM studies of the Si₃N₄ sample confirmed that these fine bright lines along grain boundaries represent intergranular glass films separating Si₃N₄ matrix grains. However, when high-resolution TEM was employed on SiC samples, which showed a similar contrast variation across SiC grain boundaries in the SEM, the presence of residual glass films was not detected. The SiC materials showed clean grain boundaries with no indication of residual glass even at triple pockets. Chemical analysis monitored yttrium and aluminum segregation at interfaces, which creates a potential barrier (space charges) and therefore affects both the inner mean potential at the interface (Fresnel fringes) and the plasma-etching response. Although SEM imaging showed a similar interface contrast for both Si₃N₄ and SiC ceramics, HRTEM studies clearly revealed grain-boundary wetting in the former and clean interfaces in the latter material, respectively. Hence, SEM imaging and Fresnel fringe TEM imaging alone are not conclusive when characterizing interface wetting in ceramic polycrystals.     

Silicon Carbide Monofilament-Reinforced Silicon Nitride or Silicon Carbide Matrix Composites

SiC-monofilament-reinforced SiC or Si₃N₄ matrix composites were fabricated by hot-pressing, and their mechanical properties and effects of filaments and filament coating layers were studied. Relationships between frictional stress of filament/matrix interface and fracture toughness of SiC monofilament/Si₃N₄ matrix composites were also investigated. As a result, it was confirmed experimentally that in the case of composites fractured with filament pullout, the fracture toughness increased as the frictional stress increased. On the other hand, when frictional stress was too large (>about 80 MPa) for the filament to be pulled out, fracture toughnesses of the composites were almost the same and not so much improved over that of Si₃N₄ monolithic ceramics. The filament coating layers were found to have a significant effect on the frictional stress of the SiC monofilament/Si₃N₄ matrix interface and consequently the fracture toughness of the composites. Also the crack propagation behavior in the SiC monofilament/Si₃N₄ matrix composites was observed during flexural loading and cyclic loading tests by an in situ observation apparatus consisting of an SEM and a bending machine. The filament effect which obstructed crack propagation was clearly observed. Fatigue crack growth was not detected after 300 cyclic load applications.     

Thick Protective UHTC Coatings for SiC-Based Structures: Process Establishment

A new process to form thick and dense ultra-high-temperature ceramic (UHTC) composite coatings over SiC surfaces is described. Coatings of ZrB₂/SiC/(ZrC) thicker than 100 μm are formed by a reaction-bonded SiC (RBSC) approach based on Si infiltration into ZrB₂/C preform coating. The residual Si, typically found in RBSC, can be eliminated efficiently to provide a coating material that performs at temperatures above 1500°C. The process is performed at 1500°C in Ar at ambient pressure. The interface between the in situ formed SiC and the Zr phases is very tight, as is the interface with the substrate.
The ZrB₂ particles used in this process are rearranged in their morphology and an additional new phase containing Zr–C is formed. The coatings exhibit excellent integrity, hardness, and bonding to the tested substrates. A preliminary oxidation study indicates good protection of substrates at 1500°C under both passive and active oxidation conditions, provided that the coatings have sufficient thickness.     

Mullite Coatings on SiC and C/C–Si–SiC Substrates Characterized by Infrared Spectroscopy

Mullite (3Al₂O₃·2SiO₂) coatings on SiC substrates and SiC precoated carbon/carbon composite (C/C-Si-SiC) substrates were produced by pulsed laser deposition (PLD) using pressed mullite powder targets. The layers can be characterized efficiently by IR reflection spectroscopy in the spectral range between 650 and 5000 cm⁻¹. The deposited coatings turn into mullite upon oxidation in air at temperatures between 1400° and 1600°C. Fabry-Perot interferences indicate a high quality and homogeneity of the mullite coating/SiC substrate interface. Amorphous SiO₂ gradually forms during prolonged heating or at higher temperatures.     

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.     

Tensile and Stress-Rupture Behavior of SiC/SiC Minicomposite Containing Chemically Vapor Deposited Zirconia Interphase

The tensile and stress-rupture behavior of SiC/SiC minicomposite containing a chemically vapor deposited (CVD) ZrO₂ interphase was evaluated. Fractographic analyses showed that in situ fiber strength and minicomposite failure loads were strongly dependent on the phase contents and microstructure of the ZrO₂ interphase. When the ZrO₂ interphase structure possessed a weakly bonded interface within the dense ZrO₂ interphase coating layer, the interphase sufficiently protected the fiber surface from processing degradation and promoted matrix crack deflection around the fibers. With this weakly bonded interphase, the stress-rupture properties of SiC/SiC minicomposite at 950° and 1200°C appeared to be controlled by fiber rupture properties, and compared favorably to those previously measured for state-of-the-art BN fiber coatings.     

Boron Nitride Interphase in Ceramic-Matrix Composites

A BN interphase has been deposited, by isothermal/isobaric chemical vapor infiltration (ICVI) from BF₃─NH₃, within a preform made from ex-polycarbosilane (ex-PCS) fibers, at about 1000°C. In a second step, the BN-treated preform was densified with SiC deposited from CH₃SiCl₃─H₂ at about the same temperature. From a thermodynamic standpoint, ex-PCS fibers could be regarded as unreactive vs the BF₃─NH₃ gas phase assuming they are coated with a thin layer of carbon or/and silica. The as-deposited interphase consists of turbostratic BN (N/B < 1) containing oxygen. The SiC infiltration acts as an annealing treatment: (i) the BN interphase becomes almost stoichiometric and free of oxygen; (ii) the fibers undergo a decomposition process yielding a SiO₂/C layer at the BN/fiber interface. The weaker link in the interfacial sequence seems to be the BN/SiO₂ interface. Deflection of microcracks arising from the failure of the matrix takes place at (or nearby) that particular interface.     

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.     

Indentation Method for Determining the Macroscopic Fracture Energy of Brittle Bimaterial Interfaces

Based on the mechanics analysis of crack-interface interaction, a simple and direct micro-indentation technique has been developed to evaluate the fracture energy of a bimaterial interface. The technique, when applied to a pristine planar SiC-Si₃N₄ interface at various angles of attack, is shown to provide a reasonable estimate of the interfacial fracture resistance. The experimentally obtained fracture energy has been compared favorably with a proposed atomic model of bond breaking between SiC and Si₃N₄.     

Tensile Testing of Ceramics

A simple, self-aligning, uniaxial test firtiire has been developed for testing ceramics. The primary emphasis is placed on a simple specimen geometry and test procedure. The strain gage measurements on α-Sic and steel specimens indicate very good uniformity in the tensile stresses (with variation less than 1%) across the cross section. The tensile testing of SiC and SiC/TiB₂ composites exhibits 55% and 79% failures, respectively, in the gage section with possible further improvements. Average tensile strength values of 160 and 202 MPa were obtained for α-SiC and SiC/TiB₂ respectively.     

Low-Temperature Synthesis of Aluminum Silicon Carbide Using Ultrafine Aluminum Carbide and Silicon Carbide Powders

The conditions for preparing α-aluminum silicon carbide (α-Al₄SiC₄) were examined by heating stoichiometric mixtures of ultrafine A1₄C₃ and SiC powders with sizes of <0.1 μm at and below 1600°C. The starting A1₄C₃ powder was obtained by the pyrolysis of trimiethylaluminum; the starting SiC powders were obtained by the pyrolyses of triethylsilane (3ES), tetraethylsilane (4ES), and hexamethyldisilane (6MDS). The reactivity of SiC with Al₄C₃ to form α-Al₄SiC₄ varies according to the kind of starting alkylsilane: 3ES > 4ES > 6MDS. The reaction of 3ES-derived SiC with A1₄C₃ produced α-Al₄SiC₄ at temperatures as low as 1400°C for 240 min, regardless of the presence of A1₄C₃ (trace). Only α-Al₄SiC₄ was formed at and above 1500°C for 60 min; the crystal growth was appreciable.     

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.     

High-Temperature Stability of Lanthanum Orthophosphate (Monazite) on Silicon Carbide at Low Oxygen Partial Pressures

The stability of lanthanum orthophosphate (LaPO₄) on SiC was investigated using a LaPO₄-coated SiC fiber at 1200°–1400°C at low oxygen partial pressures. A critical oxygen partial pressure exists below which LaPO₄ is reduced in the presence of SiC and reacts to form La₂O₃ or La₂Si₂O₇ and SiO₂ as the solid reaction products. The critical oxygen partial pressure increases from ∼0.5 Pa at 1200°C to ∼50 Pa at 1400°C. Above the critical oxygen partial pressure, a thin SiO₂ film, which acts as a reaction barrier, exists between the SiC fiber and the LaPO₄ coating. Continuous LaPO₄ coatings and high strengths were obtained for coated fibers that were heated at or below 1300°C and just above the critical oxygen partial pressure for each temperature. At temperatures above 1300°C, the thin LaPO₄ coating becomes morphologically unstable due to free-energy minimization as the grain size reaches the coating thickness, which allows the SiO₂ oxidation product to penetrate the coating.     

Silicon Nitride Based Ceramic Nanocomposites

Nanocomposites (Si₃N₄/SiC) were studied by combined high-resolution transmission electron microscopy and electron energy-loss spectroscopic imaging (ESI) techniques. In ESI micrographs three types of crystalline grains were distinguished: Si₃N₄ matrix grains (0.5 μΩ), nanosized SiC particles (<100 nm) embedded in the Si₃N₄, and large SiC particles (100–200 nm) at grain boundary regions (intergranular particles). Amorphous films were found both at Si₃N₄ grain boundaries and at phase boundaries between Si₃N₄ and SiC. The Si₃N₄ grain boundary film thickness varied from 1 to 2. 5 nm. Two kinds of embedded SiC particles were observed: type A has a special orientation with respect to the matrix, and type B possesses a random orientation with respect to the matrix. The surfaces of type B particles are completely covered by an amorphous phase. The existence of the amorphous film between the matrix and the particles of type A depends on the lattice mismatch across the interface. The mechanisms of nucleation and growth of Ω-Si₃N₄ grains are discussed on the basis of these experimental results.     

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.     

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.