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.     

Preparation and Mechanical Properties of SiC Whisker and Nanosized Mullite Particulate Reinforced TZP Composites

The influence of the addition of nanometer mullite particulates and SiC whiskers coated with alumina on the mechanical properties of tetragonal zirconia polycrystals (TZP) was studied. With increasing mullite( p ) content the high-temperature flexural strength increased, and a maximum value of 360 MPa at 1000°C was reached at 15 vol% mullite( p . Furthermore, 10 vol% SiC( w ) reinforced 15 vol% mullite/TZP composites improved the high-temperature strength up to 490 MPa at 1000°C, 2.7 times that of pure TZP matrix. This high-temperature strengthening is attributed to load transfer from TZP matrix to SiC( w ) and mullite particulates. Significant whisker pull-out and interface debonding were also observed on the fractured surfaces when SiC( w ) was coated with Al₂O₃ film.     

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.     

Mechanisms and Kinetics of Reaction-Bonded Aluminum Oxide Ceramics

Reaction-bonded Al₂O₃ (RBAO) ceramics were fabricated starting from mechanically alloyed Al₂O₃/Al, Al₂O₃/ Al/ZrO₂, and Al₂O₃/Al/ZrO₂/Zr mixtures. Isopressed compacts were heat-treated in air up to 1550°C. Reaction-bonding mechanisms, kinetics, and the influence of ZrO₂ and Zr additions are investigated. Independent of additive, oxidation of Al proceeds both as solid/gas and liquid/gas reaction, and the reaction kinetics follow a parabolic rate law. The reaction rate depends strongly on the particle size of Al. The activation energy of the reaction depends essentially on green density. Below the melting temperature of Al, in samples containing 45 vol% Al and 55 vol% Al₂O₃, it is 112 and 152 kJ/mol at ∼64% and ∼74% TD, respectively, while above the melting temperature, it lies in the range ∼ 26–33 kJ/mol. Zr additions reduce the activation energy to some extent. Samples with only ZrO₂ additions exhibit nearly the same activation energies as ZrO₂-free samples, though ZrO₂ has a very positive effect on the microstructural development in RBAO ceramics. Microstructure evolution and some strength data of RBAO bodies are also reported.     

Microstructural Studies in Alkoxide-Derived Mullite/Zirconia/Silicon Carbide-Whisker Composites

Mullite composites toughened with ZrO₂ (with or without a MgO or Y₂O₃ stabilizer) and/or SiC whiskers (SiC( w )) were fabricated by hot-pressing powders prepared from Al, Si, Zr, and Mg(Y) alkoxide precursors by a sol–gel process. Micro-structures were studied by using XRD. SEM, and analytical STEM. Pure mullite samples contained prismatic, preferentially oriented mullite grains. However, the addition of ZrO₂, as well as the hot-pressing temperature, affected the morphology and grain size in the composites; a fine, uniform, equiaxed microstructure was obtained. The effect of SiC( W ) was less pronounced than that of ZrO₂. Glassy phases were present in mullite and mullite/SiC( W ) composites, but were rarely observed in Al₂O₃-rich or ZrO₂-containing samples. The formation of zircon due to the reaction between ZrO₂ and SiO₂ and the considerable solid solution of SiO₂ in ZrO₂ prevented the formation of the glassy phase, whereas the reaction between Al₂O₃ and MgO in MgO-containing samples formed a spinel phase and also deprived the ZrO₂ phase of the stabilizer. Intergranular ZrO₂ particles were either monoclinic or tetragonal, depending on size and stabilizer content; small intragranular ZrO₂ inclusions were usually tetragonal in structure.     

Reaction-Bonded Mullite/Zirconia Composites

The feasibility of fabricating dense, low-shrinkage, mullite/ ZrO₂ composites based on the reaction bonding of alumina (RBAO) process and the reaction sintering of zircon is examined. Compacts pressed from an attrition-milled powder mixture of Al, A1₂O₃ and zircon were heated in air according to a two-step heating cycle. The phase evolution and microstructural development during reaction bonding were traced by X-ray diffraction, nuclear magnetic resonance, and scanning electron microscopy on samples extracted from various points along the heating cycle. It is seen that, as in conventional RBAO, AI oxidizes to γ-Al₂O₃ which then transforms to α-AI₂O₃ between 1100° and 1200°C. The zircon dissociation commences at ∼1400°C and is practically complete by 1500°C. Mullite enriched in Al₂O₃ forms initially, but 3:2 stoichiometry is attained in the final product which consists of mullite, t - and m-ZrO₂, and residual α-AI₂O₃. The flexure strength of the composite is superior to that of pure mullite, and ∼80% of the strength is retained up to 1200°C. Although there was no toughness enhancement relative to mullite, this should be achievable by optimizing the fabrication procedure.     

Reaction-Forming of Mullite Ceramics Using an Aqueous Milling Medium

Mullite and mullite/ZrO₂ ceramics were fabricated starting from Si/Al₂O₃ and Si/Al₂O₃/ZrO₂ powder mixtures, which were mixed and attrition milled with TZP balls in water. Isopressed powder compacts were subjected to a heat treatment in air, during which the Si was oxidized to SiO₂. At } 1410°C, reaction between Al₂O₃ and SiO₂ occurred, resulting in mullite (3Al₂O₃·2SiO₂). Depending on the composition of the starting powders, the end product was either single-phase mullite or a mullite composite. The reaction process was monitored by thermogravimetry and dilatometry. It was found that the microstructure and mechanical properties of the reaction-formed mullite ceramics were significantly improved by ZrO₂ additions.     

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.     

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).     

Conventional and Microwave-Induced Sintering Mechanisms for Reaction-Bonded Aluminum Oxide with Zirconium Oxide Additions

Powder compacts consisting of Al, Al₂O₃, and ZrO₂ were heated by microwave radiation. Tracing the phase evolution during reaction bonding revealed the reaction mechanism. In the case of conventional heating, the compacts expanded slightly at temperatures of <700°C due to Al surface oxidation and expanded sharply at temperatures greater than 700°C as oxidation proceeded from the surface to the interior. Then, the compacts shrank at 1550°C due to sintering. For the case of microwave heating, the compacts expanded at temperatures of <550°C due to the formation of Al₃Zr. This Al₃Zr formation was caused by the preferential heating of ZrO₂ relative to Al and Al₂O₃ by microwave radiation. Then, Al₃Zr was oxidized to form Al₂O₃ and ZrO₂ at temperatures of >1000°C. Finally, the compacts shrank at 1550°C due to sintering, similarly to conventional sintering.     

Hindrance of Grain Growth in Al2O3 by ZrO2 Inclusions.

Alumina and Al₂O₃/ZrO₂ (1 to 10 vol%) composite powders were mixed and consolidated by a colloidal method, sintered to >98% theoretical density at 1550°C, and subsequently heat-treated at temperatures up to 1700°C for grain-size measurements. Within the temperature range studied, the ZrO₂ inclusions exhibited sufficient self-diffusion to move with the Al₂O₃ 4-grain junctions during grain growth. Growth of the ZrO₂, inclusions occurred by coalescence. The inclusions exerted a dragging force at the 4-grain junctions to limit grain growth. Abnormal grain growth occurred when the inclusion distribution was not sufficiently uniform to hinder the growth of all Al₂O₃ grains. This condition was observed for compositions containing ≤2.5 vol% ZrO₂, where the inclusions did not fill all 4-grain junctions. Exaggerated grains consumed both neighboring grains and ZrO₂, inclusions. Grain-growth control (no abnormal grain growth) was achieved when a majority (or all) 4-grain junctions contained a ZrO₂ inclusion, viz., for compositions containing ≥5 vol% ZrO₂. For this condition, the grain size was inversely proportional to the volume fraction of the inclusions. Since the ZrO₂ inclusions mimic voids in all ways except that they do not disappear, it is hypothesized that abnormal grain growth in single-phase materials is a result of a nonuniform distribution of voids during the last stage of sintering.     

Diffusion Bonding of Zirconia/Alumina Composites

ZrO₂/Al₂O₃ composites with from 0% to 100% Al₂O₃ content were diffusion bonded at 12.5 MPa for 30 min in the temperature range 1450° to 1500°C. Under appropriate bonding conditions, a bonding strength greater than 1000 MPa was achievable between dissimilar materials with different thermal expansion coefficients.     

Preparation and Properties of Calcium Zirconate

The preparation of CaZrO₃ by solid-state reactions at 1450° to 2000°C. is described. A very stable material was formed by the reaction of CaCO₃ and ZrO₂, in equimolar proportions, at a final temperature of 1850°C. Lattice constant values for this material agreed with data reported by previous investigators. The linear coefficient of thermal expansion for CaZrO₈ over the temperature range 25° to 1300°C. was found to be 11.5 × 10⁻⁶ per °C. Specimens of CaZrO₃ were compatible with ZrO₂, MgO, Al₂O₃, and BeO but reacted with SiC and mullite.     

Tape Casting of Al2O3/ZrO2 Laminated Composites

Fracture toughness of ZrO₂-toughened alumina could he increased by macroscopic interfaces, such as those existing in laminated composites. In this work, tape casting was used to produce A/A or A/B laminates, where A and B can be Al₂O₃, Al₂O₃/5 vol% ZrO₂, and Al₂O₃/l0 vol% ZrO₂. An increase of toughness is observed, even in the Al₂O₃/Al₂O₃ laminates.     

Mechanical Behavior of Alumina–Silicon Carbide „Nanocomposites”

The fracture behavior of Al₂O₃ containing 5 vol% 0.15μm SiC particles was investigated using indentation techniques. A significant increase in strength was achieved by the addition of SiC particles to the base Al₂O₃. Specifically, the strength increased from 560 MPa for Al₂O₃ to 760 MPa for the composite samples (average values for unindented hotpressed bars tested in four-point bending). After annealing for 2 h at 1300°C, the average strength of the composite samples increased to about 1000 MPa. Toughness was estimated using indentation-strength data. While there was a slight increase in toughness, the increase was not sufficient to account for the increase in the unindented strength on SiC particle addition. It is suggested that the observed strengthening and apparent toughening were due to a machining-induced compressive surface stress.     

Strength and Phase Stability of Yttria-Ceria-Doped Tetragonal Zirconia/Alumina Composites Sintered and Hot Isostatically Pressed in Argon–Oxygen Gas Atmosphere

Yttria-ceria-doped tetragonal zirconia (Y,Ce)-TZP)/alumina (Al₂O₃) composites were fabricated by hot isostatic pressing at 1400° to 1450°C and 196 MPa in an Ar–O₂ atmosphere using the fine powders prepared by hydrolysis of ZrOCl₂ solution. The composites consisting of 25 wt% Al₂O₃ and tetragonal zirconia with compositions 4 mol% YO_1.5–4 mol% CeO₂–ZrO₂ and 2.5 mol% YO_1.5–5.5 mol% CeO₂–ZrO₂ exhibited mean fracture strength as high as 2000 MPa and were resistant to phase transformation under saturated water vapor pressure at 180°C (1 MPa). Postsintering hot isostatic pressing of (4Y, 4Ce)-TZP/Al₂O₃ and (2.5Y, 5.5Ce)-TZP/Al₂O₃ composites was useful to enhance the phase stability under hydrothermal conditions and strength.     

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.     

Microstructure and High-Temperature Mechanical Behavior of Alumina/Alumina–Yttria-Stabilized Tetragonal Zirconia Multilayer Composites

Layered composites of alternate layers of pure Al₂O₃(thickness of 125 μ m) and 85 vol% Al₂O₃-15 vol% ZrO₂ that was stabilized with 3 mol% Y₂O₃(thickness of 400 μ m) were obtained by sequential slip casting and then fired at either 1550° or 1700°C. Constant-strain-rate tests were conducted on these materials in air at 1400°C at an initial strain rate of 2 × 10^-5 s^-1. The load axis was applied both parallel and perpendicular to the layer interfaces. Catastrophic failure occurred for the composite that was fired at 1700°C, because of the coalescence of cavities that had developed in grain boundaries of the Al₂O₃ layers. In comparison, the composite that was fired at 1550°C demonstrated the ductility of the Al₂O₃+YTZP layer, but at a flow stress level that was determined by the Al₂O₃ layer.     

A Strong and Damage-Tolerant Oxide Laminate

A novel laminar oxide composite was developed. This oxide laminate, which was fabricated by hot pressing, consisted of three types of layers, which were stacked in a repeating sequence of YPO4, yttria-stabilized ZrO₂, 30 vol% yttria-stabilized ZrO₂-70 vol% Al₂O₃, and yttria-stabilized ZrO₂. The behavior of the oxide laminate was evaluated by four-point flexural testing and the indentation technique. The flexural strength from one test was 358 MPa, and the load-displacement curve of this test displayed a graceful failure. Pronounced interfacial delamination contributed to a high work of fracture and damage tolerance. These properties have rarely been observed in oxide composites and are comparable to those of non-oxide composites, such as SiC/graphite, SiC/BN, and Si₃N₄/BN laminates.     

Mechanical Properties of CoAl Materials with the Combined Additions of ZrO2(3Y) and Al2O3

Intermetallic CoAl powder has been prepared via self-propagating high-temperature synthesis (SHS). Dense CoAl materials (99.6% of theoretical) with the combined additions of ZrO₂(3Y) and Al₂O₃ have been fabricated via spark plasma sintering (SPS) for 10 min at 1300°C and 30 MPa. The microstructures are such that tetragonal ZrO₂ (0.3 μm) and Al₂O₃ (0.5 μm) particles are located at the grain boundaries of the CoAl (8.5 μm) matrix. Improved mechanical properties are obtained; especially the fracture toughness and the bending strength of the materials with ZrO₂(3Y)/Al₂O₃= 16/4 mol% are 3.87 MPa·m^1/2 and 1080 MPa, respectively, and high strength (>600 MPa) can be retained up to 1000°C.