Laminar Ceramics Utilizing the Zirconia Tetragonal-to-Monoclinic Phase Transformation to Obtain a Threshold Strength

Ceramic laminates have been fabricated with thin layers, containing a mixture of unstabilized zirconia (MZ-ZrO₂) and alumina (Al₂O₃), sandwiched between thicker layers of alumina that contain a small fraction of Y₂O₃-stabilized tetragonal ZrO₂ to inhibit grain growth. The MZ-ZrO₂ undergoes a tetragonal-to-monoclinic phase transformation during cooling to produce biaxial compressive stresses in the thin layers. Cracks that extend within the thicker alumina layers can be arrested by the compressive layers to produce a threshold strength, i.e., a strength below which the probability of failure is zero. Laminates composed of Al₂O₃ layers 315 ± 15 μm thick and Al₂O₃/MZ-ZrO₂ layers 29 ± 3 μm thick exhibit a threshold strength of 507 ± 36 MPa, regardless of the MZ-ZrO₂ content, for volume fractions ≥0.35. These results, piezospectroscopic stress measurements, and microstructural observations suggest that microcracking produced during the transformation reduces the magnitude of the compressive stresses achieved, which in turn limits the magnitude of the threshold strength.     

Microstructure and Mechanical Properties of Porous Alumina Ceramics Fabricated by the Decomposition of Aluminum Hydroxide

The mechanical properties of Al₂O₃-based porous ceramics fabricated from pure Al₂O₃ powder and the mixtures with Al(OH)₃ were investigated. The fracture strength of the porous Al₂O₃ specimens sintered from the mixture was substantially higher than that of the pure Al₂O₃ sintered specimens because of strong grain bonding that resulted from the fine Al₂O₃ grains produced by the decomposition of Al(OH)₃. However, the elastic modulus of the porous Al₂O₃ specimens did not increase with the incorporation of Al(OH)₃, so that the strain to failure of the porous Al₂O₃ ceramics increased considerably, especially in the specimens with high porosity, because of the unique pore structures related to the large original Al(OH)₃ particles. Fracture toughness also increased with the addition of Al(OH)₃ in the specimens with higher porosity. However, fracture toughness did not improve in the specimens with lower porosity because of the fracture-mode transition from intergranular, at higher porosity, to transgranular, at lower porosity.     

Factors Affecting Threshold Strength in Laminar Ceramics Containing Thin Compressive Layers

Compressive layers placed within a laminate can arrest cracks. With increasing applied stress, the arrested crack can propagate through the compressive layer. These phenomena produce a material with threshold strength, i.e., failure cannot occur below a critical applied stress. A previously reported stress intensity function describes different variables, e.g., magnitude of compressive stress, thickness of compressive layer, and distance between compressive layers, which govern threshold strength. Laminar composites composed of thicker Al₂O₃ layers separated by thinner Al₂O₃/mullite layers were fabricated to test the different variables that are predicted to govern threshold strength. The data agreed well with the predicted values only when the magnitude of compressive stress and/or the thickness of the compressive layer were low. For these conditions, the crack extended straight through the compressive layers, as assumed by the model used to predict threshold strength. On the other hand, when the compressive stress and/or layer thickness were large, threshold strength was larger than the predicted value. In addition, for these conditions, the crack bifurcated through the compressive layer. The angle between the bifurcated cracks increased with increasing compressive stress.     

Microcellular Al2O3 Ceramics from Wood for Filter Applications

Microcellular biomorphous Al₂O₃ was produced by Al-vapor infiltration in pyrolyzed rattan and pine wood-derived biocarbon preforms. At 1600°C the biocarbon preforms reacted with gaseous aluminum to form Al₄C₃. After oxidation in air at temperatures between 1550° and 1650°C, for 3 h, the biocarbon preforms were fully converted into α-Al₂O₃. Owing to the high anisotropy of biomorphous Al₂O₃, the compressive strength behavior was strongly dependent on the loading direction. The compressive strength of the specimens (0.1–11 MPa) is strongly dependent on their overall porosity and their behavior could be explained using the Gibson–Ashby model. The Darcian permeability ( k ₁), as well as the non-Darcian permeability ( k ₂), increased with an increase of the total porosity. The Darcian permeability of biomorphous Al₂O₃ was found to be in the range of 1–8 × 10⁻⁹ m², which is in the order of magnitude of gas filters, and, therefore, suitable for several technological applications.     

Strengthening of Oxide Ceramics by Transformation-Induced Stress

Alumina-zirconia composites were fabricated by isostatic pressing and sintering of powder mixtures in such a way that bar-shaped specimens consisted of three layers. The outer layers contained A1₂O₃ and unstabilized ZrO₂ while the central layer contained A1₂O₃ and partially stabilized ZrO₂ (with 2 mol% Y₂O₃). When cooled from the sintering temperature, some of the zirconia in the outer layers transformed to the monoclinic form while zirconia in the central layer was retained in the tetragonal form. The transformation of zirconia in the outer layers led to the establishment of surface compressive stresses and balancing tensile stresses in the bulk. The existence of surface compressive stresses was verified by a strain gauge technique and bending strength measurements on samples with varying thickness of the outer layers. The layered composites exhibited greater strength in comparison with monolithic Al₂O₃-ZrO₂ specimens. Further, variation of strength in bending with outer layer thickness (for a fixed total thickness) indicated that failure occurred from internal flaws. Scanning electron microscopy of fracture surfaces revealed that strength-limiting flaws were voids located in the central layer near the interface separating the central and the outer layers.     

Crack Bifurcation in Laminar Ceramic Composites

Crack bifurcation was observed in laminar ceramic composites when cracks entered thin Al₂O₃ layers sandwiched between thicker layers of Zr(12Ce)O₂. The Al₂O₃ layers contained a biaxial, residual, compressive stress of ∼2 GPa developed due to differential contraction upon cooling from the processing temperature. The Zr(12Ce)O₂ layers were nearly free of residual, tensile stresses because they were much thicker than the Al₂O₃ layers. The ceramic composites were fabricated by a green tape and codensification method. Different specimens were fabricated to examine the effect of the thickness of the Al₂O₃ layer on the bifurcation phenomena. Bar specimens were fractured in four-point bending. When the propagating crack encountered the Al₂O₃ layer, it bifurcated as it approached the Zr(12Ce)O₂/ Al₂O₃ interface. After the crack bifurcated, it continued to propagate close to the center line of the Al₂O₃ layer. Fracture of the laminate continued after the primary crack reinitiated to propagate through the next Zr(12Ce)O₂ layer, where it bifurcated again as it entered the next Al₂O₃ layer. If the loading was stopped during bifurcation, the specimen could be unloaded prior to complete fracture. Although the residual stresses were nearly identical in all Al₂O₃ layers, crack bifurcation was observed only when the layer thickness was greater than ∼70 μm.     

Machining-Induced Surface Residual Stress Behavior in Al2O3–SiC Nanocomposites

The machining and subsequent annealing behavior of an Al₂O₃-SiC nanocomposite (A1₂O₃+ 5 vol% 0.2 μm SiC particles) was compared to that of single-phase A1₂O₃. The machining-induced residual line force was determined by measuring the extent of elastic bending in thin disk specimens, and the surface roughness was evaluated by profilometry. The results showed that, when the two materials were subjected to the same grinding conditions, they developed compressive residual stresses and surface roughness values of similar magnitude. The maximum thickness of the residual stress layers was estimated to be ∼ 10 μm for the A1₂O₃ and ∼12 μm for the nanocomposite. A direct linear correlation was observed between the residual force and the surface roughness for different machining treatments. Annealing of the machined samples produced complete relaxation of residual stresses in the single-phase Al₂O₃, whereas only partial stress relaxation occurred for the nanocomposite.     

Computation of Residual Stresses in Quenched Al2O3

A method of computing the residual stress profile in quenched A1₂O₃ rods was developed. For these calculations, certain material parameters must be determined. Thus, strain rate was measured as a function of stress for 96% A1₂O₃ at 1300° to 1500°C, and the heat-transfer rates of cylindrical samples quenched in several media were determined. Using calculated temperature distributions and measured strain rates, plastic strains were computed for the entire quenching period, and these strains were converted to a profile of residual room-temperature stresses. The substantial increases in flexural strength observed in Al₂O₃ after it is quenched (thermally conditioned) are considered to originate in the residual compressive surface layers.     

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.     

"Alumina" Surface Modification of Silicon Nitride for Colloidal Processing

Two different methods are used to coat silicon nitride particles with an alumina precursor to make Si₃N₄ behave like Al₂O₃ in aqueous slurries. The first method involves the precipitation of an aluminum hydroxycarbonate from dissolved Al(NO₃)₃ during the decomposition of urea. In the second method, dry silicon nitride powder is reacted with aluminum tri- sec -butoxide in hexane at room temperature. Both methods produce a coated powder in which the electrophoretic and rheological properties of aqueous slurries mimic those of alumina. When salt is added to slurries consisting of coated Si₃N₄ powder, all rheological evidence suggests the presence of a short-range repulsive potential that produces a weakly attractive particle network similar to that previously reported for Al₂O₃ powder. Although electrophoretic and rheological data showed that the coated powder behaved like Al₂O₃, consolidation data indicated that slurries of coated powder with added salt did not pack to high density. In addition, these bodies were not plastic as found for bodies consolidated from dispersed and salt-added Al₂O₃ slurries.     

Mechanism of Alumina-Enhanced Sintering of Fine Zirconia Powder: Influence of Alumina Concentration on the Initial Stage Sintering

The isothermal shrinkage behavior of 2.9 mol% Y₂O₃-doped ZrO₂ powders with 0–1 mass% Al₂O₃ was investigated to clarify the effect of Al₂O₃ concentration on the initial sintering stage. The shrinkage of the powder compact was measured at constant temperatures in the range of 950°–1050°C. The Al₂O₃ addition increased the densification rate with increasing temperature. The values of apparent activation energy ( nQ ) and apparent frequency-factor term (β₀ ⁿ ), where n is the order depending on the diffusion mechanism, were estimated at the initial sintering stage by applying a sintering-rate equation to the isothermal shrinkage data. The diffusion mechanism changed from grain-boundary diffusion (GBD) to volume diffusion (VD) by Al₂O₃ addition and both nQ and β₀ ⁿ increased with increasing Al₂O₃ concentration. The kinetic analysis of the sintering mechanism suggested that the increase of densification rate by Al₂O₃ addition largely depends on the increase of β₀ ⁿ , that is, the increases of n with GBD→VD change and β₀ with an increase in Al₂O₃ content, although the nQ also increases with Al₂O₃ addition. This enhanced sintering mechanism is reasonably interpreted by the segregated dissolution of Al₂O₃ at ZrO₂ grain boundaries.     

Constrained Densification of Alumina/Zirconia Hybrid Laminates, I: Experimental Observations of Processing Defects

Various forms of damage were observed in pressure-less-sintered Al₂O₃/ZrO₂symmetric laminates and asymmetric laminates (bilayers) fabricated by tape casting and lamination. These defects included channel cracks in the ZrO₂ layers, Al₂O₃ edge-effect cracks parallel to the layers, delamination in the Al₂O₃layers, and debonding between the Al₂O₃and ZrO₂layers. Based on detailed microscopic observations, the defects were attributed to sintering rate and thermal expansion mismatch between the layers. Cracks or cracklike defects were formed in the early stages of densification, and these cracks either opened during sintering or acted as preexisting flaws for thermal expansion mismatch cracks. Consequently, the extent of cracking could be reduced or even eliminated by decreasing mismatch stresses during the sintering and cooling stages. This can be accomplished by reducing the heating and/or cooling rates or by adding Al₂O₃in the ZrO₂layers. The sintering mismatch stresses were estimated from the degree of curling in asymmetric laminates and from layer viscosities that were obtained by cyclic loading dilatometry. The measured curvature was an indication of the mismatch in sintering strain between Al₂O₃and ZrO₂and were consistent with the dilatometric data that were obtained for the component layers.     

Ceramics With Ultra-Low Density Fabricated by Gelcasting: An Unconventional View

Gelcasting is conventionally used to acquire high-density ceramic parts; however, in this work, alumina (Al₂O₃) ceramics with ultra-low density (8%–40% theoretical density) were successfully fabricated by this method. In this research, polymerization of acrylamide was realized in tert-butyl alcohol/Al₂O₃ slurries with solid loading ranging from 5 to 15 vol%. Green bodies with ultra-low density could be dried with very small shrinkage, and relatively high green strengths (1–3 MPa) were achieved. By choosing different initial solid loadings and sintering temperatures, ceramic microstructures could be effectively controlled, with the porosity ranging from 60% to 92% and pore sizes from 0.1 to 2.2 μm. Sintered Al₂O₃ showed high open porosity (90%), high specific area (14 m²/g) and high compression strength (>10 MPa), which was attributed to the connection of Al₂O₃ particles. This technique is considered potentially useful in many applications, and introduces a new application field of gelcasting.     

Indentation Fracture Response and Damage Resistance of Al2O3-ZrO2 Composites Strengthened by Transformation-Induced Residual Stresses

Indentation fracture behavior of three-layer Al₂O₃-ZrO₂ composites with substantial compressive residual stresses was compared with the behaviors of monolithic Al₂O₃ and Al₂O₃-ZrO₂ ceramics without intentionally introduced residual stresses. The indentation cracks were smaller in the three-layer specimens relative to the monolithic specimens in agreement with the predictions of indentation fracture mechanics theory. Indentation and strength testing were used to show that a residual compressive stress of approximately 500 MPa exists in the outer layers of the three-layer composites. The three-layer specimens showed excellent damage resistance in that the strength differential between the three-layer and monolithic indented specimens was maintained at indentation loads up to 1000 N, the maximum indentation load used in the experiments.     

Influence of a Ceramic Substrate on Aqueous Precipitation and Structural Evolution of Alumina Nano-Crystalline Coatings

Either boehmite (γ-AlOOH) or gibbsite (γ-Al(OH)₃) nanocrystalline thin films (h≈100 nm) can be precipitated from AlCl₃ solution at fixed pH and temperature onto different substrates. It depends on the nature of the substrate (mica flakes, SiO₂ flakes, or α-Al₂O₃ flakes), on their crystallographic properties (crystalline or amorphous), and on some experimental parameters (agitation rate, addition rate). According to the surface charge of the substrates, different alumina species are involved in the precipitation process. When negative charges are present on the substrate, the [Al₃O(OH)₃(OH₂)₉]⁴⁺ polycation is promoted, leading to the formation of the (Al₄) tetramer ([Al₄O(OH)₁₀(OH₂)₅]^o) and then to the precipitation of bohemite. When positive charges are present, a ligand bridge containing complex ([Al₃O(OH)₃(O₂H₃)₃(OH₂)₉]⁺) is likely favored, giving rise to hexagonal ring structures or amorphous solids that lead to the formation of gibbsite. Besides the surface effects, crystalline substrates can act as a template during precipitation of aluminum species as shown for the formation of gibbsite on muscovite. Finally, calcination at 850°C of boehmite samples leads to porous γ-Al₂O₃ layers, while calcination of gibbsite leads to δ-Al₂O₃ layers.     

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.     

Preparation and Properties of Alumina/Nickel-Cobalt Alloy Nanocomposites

High-density nickel-cobalt alloy-dispersed Al₂O₃ (Al₂O₃/Ni-Co alloy) composites were obtained via the hydrogen reduction and hot pressing of Al₂O₃, Ni(NO₃)₂6H₂O, and Co(NO₃)₂6H₂O powder mixtures. Microstructural investigations revealed that nanometer-sized alloy particles were dispersed homogeneously at the matrix grain boundaries, forming the intergranular-type nanocomposite. High strength (>1 GPa) was registered for the Al₂O₃/10 wt% Ni-Co alloy composite. An inverse magnetostrictive response to applied stress was observed, because of the Ni-Co alloy dispersions, which indicates promise for incorporating new functions such as stress and fracture sensing into the structural ceramics without any loss of mechanical properties.     

Preparation of Titanium Nitride/Alumina Laminate Composites

A preparation route for TiN/Al₂O₃ laminate composites has been described. A water-based process using Al₂O₃ and TiN slurries with solids contents of 40 and 35 vol%, respectively, was used to make TiN and Al₂O₃ tapes. The removal of the binder was monitored by weight-loss measurements in a thermogravimetry unit. Bodies composed of Al₂O₃ and TiN tapes were densified at temperatures of 1400° and 1500°C using the Spark Plasma Sintering^® (SPS) technique. Densities of >98% of the theoretical densities were approached. Crack-free and almost fully densified TiN/Al₂O₃ compacts were prepared by heating the burned-out green bodies to the final sintering temperature (1500°C) at a rate of 100°C/min, and with a holding time of 5–10 min, under a pressure of 75 MPa. The microstructures of the obtained compacts were studied using scanning electron microscopy. Grain sizes in the sintered Al₂O₃ and TiN compacts were similar to those of the precursor powders. Hardness and indentation fracture toughness were measured at room temperature, and the monolithic compacts as well as the laminate composites exhibited anisotropic mechanical behavior; i.e., the cracks propagated much more easily in a direction parallel to the laminas than perpendicular to them.     

High-Temperature Mechanical Properties of Ceramic Materials: IV, Sintered Mullite Bodies

An attempt was made to sinter relatively pure 3A1₂O₃-2SiO₂ and 2A1₂O₃-SiO₂ compositions to low porosities at 1710° and 1650°C, respectively, using an addition of 1% MgO to each body to facilitate the reaction. The 3A1₂O₃-2SiO₂ body sintered to a porosity of 7.1% and was practically all mullite. The 2Al₂O₃SiO₂ body sintered to a porosity of 10.9% and was composed of mullite and corundum. Strength and elastic modulus measurements were made at several temperatures up to 1200°C, and some observations of the load-bearing properties were made.     

Uniformly Porous Al2O3/LaPO4 and Al2O3/CePO4 Composites with Narrow Pore-Size Distribution

Porous Al₂O₃/20 vol% LaPO₄ and Al₂O₃/20 vol% CePO₄ composites with very narrow pore-size distribution at around 200 nm have been successfully synthesized by reactive sintering at 1100°C for 2 h from RE₂(CO₃)₃· x H₂O (RE = La or Ce), Al(H₂PO₄)₃ and Al₂O₃ with LiF additive. Similar to the previously reported UPC-3Ds (uniformly porous composites with a three-dimensional network structure, e.g. CaZrO₃/MgO system), decomposed gases in the starting materials formed a homogeneous open porous structure with a porosity of ∼40%. X-ray diffraction, ³¹P magic-angle spinning nuclear magnetic resonance, scanning electron microscopy, and mercury porosimetry revealed the structure of the porous composites.