Fabrication and Properties of Ceramic Composites with a Boron Nitride Matrix

Boron nitride (BN) matrix composites reinforced by a number of different ceramic fibers have been prepared using a low-viscosity, borazine oligomer which converts in very high yield to a stable BN matrix when heated to 1200°C. Fibers including Nicalon (SiC), FP (A1₂O₃), Sumica and Nextel 440 (Al₂O₃-SiO₂) were evaluated. The Nicalon/BN and Sumica/BN composites displayed good flexural strengths of 380 and 420 MPa, respectively, and modulus values in both cases of 80 GPa. On the other hand, FP/BN and Nextel/BN composites exhibited very brittle behavior. Nicalon fiber with a carbon coating as a buffer barrier improved the strength by 30%, with a large amount of fiber pullout from the BN matrix. In all cases except for Nicalon, the composites showed low dielectric constant and loss.     

Fabrication of Silicon Carbide-Mullite Composite by Melt Infiltration

The melt infiltration method was used to fabricate a SiC-mullite composite at high temperature. Mullite was successfully obtained from a SiO₂ and Al₂O₃ powder mixture by melting above 1830°C in a BN crucible with a lid. When infiltrated into a porous SiC preform, the mullite significantly reacted with SiC to form gaseous SiO and CO, even at the lowest investigated temperature of 1830°C, consuming SiO₂ and leaving Al₂O₃ and silicon phases in the sample. The relevant reactions were studied in detail. A closed system was adopted to suppress the reaction, and a dense composite was successfully obtained.     

High-Temperature Fatigue Strength of Crack-Healed Al2O3 Toughened by SiC Whiskers

Al₂O₃ reinforced by SiC whiskers (Al₂O₃/SiC-W) was hot-pressed to investigate the fatigue strength of crack-healed specimens at high temperature. Semielliptical surface cracks of 100 μm surface length were introduced on each specimen surface. These specimens were crack-healed at 1300°C for 1 h in air, and static and cyclic fatigue strengths were systematically investigated at room temperature, 900° and 1100°C by three-point bending. The static and cyclic fatigue limits of the crack-healed specimens were more than 70% of the average bending strength at each testing temperature. Crack-healed specimens of Al₂O₃/SiC-W were not sensitive to static and cyclic fatigue at room temperature and high temperatures. Therefore, the combination of crack-healing and whisker reinforcement can play an important role in increasing static and cyclic fatigue strengths at high temperature.     

Crack-Healing Behavior of Al2O3 Toughened by SiC Whiskers

Al₂O₃ reinforced by SiC whiskers (Al₂O₃/SiC-W) was hot-pressed to investigate the crack-healing behavior. Semielliptical surface cracks of 100 μm in surface length were introduced using a Vickers indenter. The specimens containing precracks were crack-healed at temperatures between 1000° and 1300°C for 1 h in air, and their strengths were measured by three-point bending tests at room temperature and elevated temperatures between 400° and 1300°C. The results show that Al₂O₃/SiC-W possesses considerable crack-healing ability. The surface cracks with length of 2 c = 100 μm could be healed by crack-healing at 1200° or 1300°C for 1 h in air. Fracture toughness of the material was also determined. As expected, the SiC whiskers made their Al₂O₃ tougher.     

Synthesis of Al4SiC4

The conditions necessary for synthesizing Al₄SiC₄ from mixtures of aluminum, silicon, and carbon and kaolin, aluminum, and carbon, as starting materials, were examined in the present study. The standard Gibbs energy of formation for the thermodynamic reaction SiC( s ) + Al₄C₃( s ) = Al₄SiC₄( s ) changed from positive to negative at 1106°C. SiC and Al₄C₃ formed as intermediate products when the mixture of aluminum, silicon, and carbon was heated in argon gas, and Al₄SiC₄ then formed by reaction of the SiC and Al₄C₃ at >1200°C. Al₄C₃, SiO₂, Al₂O₃, SiC, and Al₄O₄C formed as intermediate products when the mixture of kaolin, aluminum, and carbon was heated under vacuum, and Al₄SiC₄ formed from a reaction of those intermediate products at >1600°C.     

Carbon Interfacial Layers Formed by Oxidation of SiC in SiC/Ba-Stuffed Cordierite Glass-Ceramic Reaction Couples

Reaction couples between α-SiC and cordierite (2MgO·2Al₂O₃·5SiO₂═ Mg₂Al₄Si₅O₁₈) were prepared by sandwiching (and enclosing) SiC single crystals between plates of Ba-stuffed magnesium aluminosilicate (Ba-MAS) glass and hot-pressing; the Ba-MAS was subsequently crystallized at 1000° to 1200°C in argon or air. No reaction occurred at the SiC/Ba-MAS interfaces during hot-pressing, but crystallization heat treatments caused formation of amorphous carbon reaction layers at the SiC/cordierite interfaces, due to concurrent oxidation via the reaction SiC + O₂→ SiO₂+ C. The thickness of the carbon of the carbon layer was variable. These results suggest that formation of C layers at SiC/silicate interfaces in other composites (containing Nicalon fibers, for example) depends more on thermochemistry and less on the details of SiC nonstoichiometry than has heretofore been supposed.     

Hot Corrosion Mechanism of Composite Alumina/Yttria-Stabilized Zirconia Coating in Molten Sulfate–Vanadate Salt

In order to improve the hot corrosion resistance of yttria-stabilized zirconia (YSZ), an Al₂O₃ overlay has been deposited on the surface of YSZ by electron-beam physical vapor deposition. Hot corrosion tests have been performed on the YSZ coatings with and without an Al₂O₃ overlay in the molten salt mixture (Na₂SO₄+0–15 wt% V₂O₅) at 950°C. The presence of V₂O₅ in the molten salt exacerbates degradation of both the monolithic YSZ coating and the composite YSZ/Al₂O₃ system. The formation of a low-melting Na₂O–V₂O₅–Al₂O₃ liquid phase is responsible for degradation of the Al₂O₃ overlay. The Al₂O₃ overlay acts as a barrier against the infiltration of the molten salt into the YSZ coating during exposure to the molten salt mixture with <5 wt% vanadate.     

Properties of Sm2O3─Al2O3─SiO2 Glasses for In Vivo Applications

The compositional range for glass formation below 1600°C in the Sm₂O₃─Al₂O₃─SiO₂ system is (9–25)Sm₂O₃─(10–35)Al₂O₃─(40–75)SiO₂ (mol%). Selected properties of the Sm₂O₃─Al₂O₃─SiO₂ (SmAS) glasses were evaluated as a function of composition. The density, refractive index, microhardness, and thermal expansion coefficient increased as the Sm₂O₃ content increased from 9 to 25 mol%, the values exceeding those for fused silica. The dissolution rate in 1 N HCl and in deionized water increased with increasing Sm₂O₃ content and with increasing temperature to 70°C. The transformation temperature ( T _g ) and dilatometric softening temperature ( T _d ) of the SmAS glasses exceeded 800° and 850°C, respectively.     

Hot Isostatic Pressing of Silicon Nitride with Boron Nitride, Boron Carbide, and Carbon Additions

Si₃ N₄ test bars containing additions of BN, B₄C, and C, were hot isostatically pressed in Ta cladding at 1900° and 2050°C to 98.9% to 99.5% theoretical density. Room-temperature strength data on specimens containing 2 wt% BN and 0.5 wt% C were comparable to data obtained for Si₃ N₄ sintered with Y₂O₃, Y₂O₃ and Al₂O₃, or ZrO₂. The 1370°C strengths were less than those obtained for additions of Y₂O₃ or ZrO₂ but greater than those obtained from a combination of Y₂O₃ and Al₂O₃. Scanning electron microscope fractography indicated that, as with other types of Si₃N₄, roomtemperature strength was controlled by processing flaws. The decrease in strength at 1370°C was typical of Si₃N₄ having an amorphous grainboundary phase. The primary advantage of non-oxide additions appears to be in facilitating specimen removal from the Ta cladding.     

Preparation and Analysis of a Silicon Carbide Composite Membrane

Silicon carbide (SiC) porous substrates, containing alumina (Al₂O₃) dopant levels of 3, 5, and 8 wt%, are prepared by slip casting and sintering in the temperature range of 1450°–1800°C. The linear shrinkage, bulk density, and pore size of the sintered substrate increase as the sintering temperature and the amount of dopant increase. A large amount of β-phase SiC is transformed to α-phase SiC if the dopant concentration is 5 or 8 wt%. The flexural strength of the substrate doped with 8 wt% Al₂O₃ is higher than that of the substrate doped with 3 wt% Al₂O₃; however, the Weibull modulus of the former is lower. SiC composite membranes of improved selectivity and strength are fabricated by coating the porous substrate with layers of lower Al₂O₃ contents at lower sintering temperatures.     

Fabrication and Properties of Low-Shrinkage Reaction-Bonded Mullite

Mullite ceramics were fabricated according to the recently developed reaction-bonded Al₂O₃ (RBAO) technology. Green compacts consisting of mechanically alloyed Al, SiC, and Al₂O₃ were heat-treated in two steps. During the first hold at 1200°C, Al and SiC were oxidized to form Al₂O₃ and SiO₂. On further heating, mullite was formed which then sintered during the second hold at 1550°C. All reactions involved in the process were associated with volume expansions that almost compensated for the shrinkage on sintering. Processing details and microstructure development are discussed. Reaction-bonded mullite ceramics exhibit high fracture strength, e.g., 290 MPa at a density of 97% of theoretical density.     

Damage Tolerance of Silicon Carbide- and Alumina-Matrix Surface Composites

A method for the fabrication of a ceramic-matrix composite (CMC) layer on the surface of a monolithic substrate via chemical vapor infiltration (CVI) is described. Preforms consisted of tows of fibers wound onto the surface of monolithic cylindrical tubes. Nicalon fibers were wound onto mullite substrates and infiltrated with β-SiC from CH₃SiCl₃/H₂ gas mixtures in a cylindrical cold-wall reactor. Similarly, Nextel fibers were wound onto A1₂O₃ substrates and infiltrated with α-Al₂O₃ from AlCl₃/H₂/CO₂/N₂ gas mixtures. Composites with densities as high as 88% of the theoretical value were fabricated in 8 h. The effective fracture strength of the SiC- and Al₂O₃-matrix surface composites, as determined from diametral compression tests of C-ring specimens, was found to be insensitive to damage caused to the outer diameter by a Vickers indentation. The tolerance of the SiC-matrix surface composites to surface damage was retained in specimens subjected to oxidation at 1000°C for 6 h.     

Tensile Creep of Alumina-Silicon Carbide "Nanocomposites"

The tensile creep behavior of an (Al₂O₃-SiC) nanocomposite that contains 5 vol% of 0.15 μm SiC particles is examined in air under constant-load conditions. For a stress level of 100 MPa and in the temperature range of 1200°–1300°C, the SiC reduces the creep rate of Al2O3 by 2–3 orders of magnitude. In contrast to Al₂O₃, the nanocomposite exhibits no primary or secondary stages, with only tertiary creep being observed. Microstructural examination reveals extensive cavitation that is associated with SiC particles that are located at the Al₂O₃ grain boundaries. Failure of the nanocomposite occurs via growth of subcritical cracks that are nucleated preferentially at the gauge corners. A modified test procedure enables creep lifetimes to be estimated and compared with creep rupture data. Several possible roles of the SiC particles are considered, including (i) chemical alteration of the Al₂O₃ grain boundaries, (ii) retarded diffusion along the Al₂O₃-SiC interface, and (iii) inhibition of the accommodation process (either grain-boundary sliding or grain-boundary migration).     

Oxidation Behavior of Liquid-Phase Sintered Silicon Carbide with Aluminum Nitride and Rare-Earth Oxides (Re2O3, where Re = Y, Er, Yb)

Silicon carbide (SiC) ceramics have been fabricated by hot-pressing and subsequent annealing under pressure with aluminum nitride (AlN) and rare-earth oxides (Y₂O₃, Er₂O₃, and Yb₂O₃) as sintering additives. The oxidation behavior of the SiC ceramics in air was characterized and compared with that of the SiC ceramics with yttrium–aluminum–garnet (YAG) and Al₂O₃–Y₂O₃–CaO (AYC). All SiC ceramics investigated herein showed a parabolic weight gain with oxidation time at 1400°C. The SiC ceramics sintered with AlN and rare-earth oxides showed superior oxidation resistance to those with YAG and Al₂O₃–Y₂O₃–CaO. SiC ceramics with AlN and Yb₂O₃ showed the best oxidation resistance of 0.4748 mg/cm² after oxidation at 1400°C for 192 h. The minimization of aluminum in the sintering additives was postulated as the prime factor contributing to the superior oxidation resistance of the resulting ceramics. A small cationic radius of rare-earth oxides, dissolution of nitrogen to the intergranular glassy film, and formation of disilicate crystalline phase as an oxidation product could also contribute to the superior oxidation resistance.     

Al2O3─SiC Composites from Aluminosilicate Precursors

Al₂O₃ and SiC composite materials have been produced from mixtures of aluminosilicates (both natural minerals and synthetic) and carbon as precursor materials. These composites are produced by heating a mixture of kaolinite (or synthetic aluminosilicates) and carbon in stoichiometric proportion above 1550°C, so that only Al₂O₃ and SiC remain as the major phases. A similar process has also been used for synthesizing other composite powders having mixtures of Al₂O₃, SiC, TiC, and ZrO₂ in different proportions (all compounds together or selective mixtures of some of them), as desired. The microstructure of hot-pressed dense compacts, produced from these powders, revealed that the SiC phase is distributed very homogeneously, even occasionally within Al₂O₃ grains on a nanosize scale. The homogeneous distribution of SiC particles within the system produced high fracture toughness of the hot-pressed material (K_IC∼ 7.0 MPa · m^1/2) and having Vicker's hardness values greater than 2000 kgf/mm².     

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.     

Monotonic and Cyclic Fatigue Behavior of High-Performance Ceramic Fibers

Monotonic and cyclic fatigue behavior of single fibers or fiber fabrics are of significant interest, since fiber assemblies or fiber-reinforced composite materials in structural applications are often subjected to cyclic loading. Studying the cyclic fatigue behavior of fibers is particularly difficult because of their small diameter (∼10 μm) and high aspect ratio. In this paper, we report results of monotonic tension and tension–tension fatigue behavior of two sol–gel-derived ceramic fibers: Al₂O₃–SiO₂–B₂O₃ (Nextel 312) and Al₂O₃ (Nextel 610). Nextel 312 exhibited a great deal of variability in tensile strength, reflected by a Weibull modulus of 4.6, versus Nextel 610, which had a Weibull modulus of 10.5. Our experiments showed clearly that cyclic loading was more damaging than static loading and, thus, resulted in a lower cyclic fatigue life compared with static loading. The fracture behavior under fatigue loading was distinctly different from that under monotonic loading. It is believed that processing-induced flaws acted as crack initiation sites, and that the cyclic loading induced subcritical cracking, followed by coalescence of cracks immediately prior to failure.     

Studies on 4CaOAl2.O3.13H2O and the Related Natural Mineral Hydrocalumite

Single-crystal X-ray and electron-diffraction studies show the existence in one polymorph of 4CaO.Al₂O₃. 13H₂O of a hexagonal structural element with α= 5.74 a.u., c = 7.92 a. u. and atomic contents Ca₂(OH)₇- 3H₂O. These structural elements are stacked in a complex way and there are probably two or more poly-types as in SiC or ZnS. Hydrocalumite is closely related to 4CaO.A1₂O₃.13H₂O, from which it is derived by substitution of CO₃^2-for 20H^-+ 3H₂O once in every eight structural elements; similar substitutions explain the existence of compounds of the types 3CaO Al₂O₃.Ca Y ₂- xH₂O and 3CaO Al₂O₃ Ca Y xH₂O. On dehydration, 4CaO.Al₂O₃.13H₂O first loses molecular water and undergoes stacking changes and shrinkage along c. At 150° to 250°C., Ca(OH)₂ and 4CaO.3Al₂O₃.3H₂O are formed and, by 1000°C., CaO and 12CaO.7Al₂O₈. The dehydration of hydrocalumite follows a similar course, but no 4CaO.3Al₂O₃.3H₂O is formed.     

Thermal Shock Resistance of Sintered Alumina/Silicon Carbide Nanocomposites Evaluated by Indentation Techniques

The thermal shock resistance of sintered Al₂O₃/1, 2.5, and 5 vol% SiC nanocomposites was studied using two indentation techniques. In the first technique, "indentation thermal shock" measurements were made of the extension of median/radial cracks around Vickers indentations after quenching from various temperatures (up to 480°C) into a bath of boiling water. This technique allowed a critical thermal shock temperature, Δ T _C^Ind, to be quantitatively evaluated. In the second technique, "indentation fatigue" tests were conducted on the thermally shocked specimens; repeated indentations were made at the same site, and the number of load cycles needed to initiate lateral fracture was measured. The results showed that nanocomposites with an addition of SiC nanophase as low as 1 vol% had a thermal shock resistance superior to that of pure Al₂O₃.     

Effect of Alumina Content on the Oxidation of Hot-Pressed Silicon Carbide

The oxidation behavior at 1370°C of dense SiC, hot-pressed with the aid of Al₂O₃, has been investigated as a function of Al₂O₃ content. Increasing amounts of the Al₂O₃ hot-pressing aid increased the oxidation rate. Observations of the oxide surface show that a glassy phase (indicating formation of a liquid at the oxidation temperature) containing Si, Al, Fe, and K forms over the residual Al₂O₃ in the hot-pressed material. It is suggested that the oxygen transport through an impure aluminosilicate liquid is faster than that through a pure SiO₂ scale, thus causing an increased oxidation rate.