Anisotropic Abnormal Grain Growth in TiO2/SiO2-Doped Alumina

The effect of TiO₂/SiO₂ addition on the grain growth of alumina was reinvestigated. TiO₂ promoted the grain growth, but there was no abnormal grain growth. However, codoping of TiO₂ and SiO₂ resulted in a duplex microstructure consisting of large platelike grains, ∼800 μm long and ∼100 μm thick, and fine matrix grains. The observed anisotropic abnormal grain growth was explained in terms of liquid formation during heat treatment.     

Reactive Synthesis of a Porous Calcium Zirconate/Spinel Composite with Idiomorphic Spinel Grains

Porous CaZrO₃/MgAl₂O₄ composites were synthesized in air by pressureless reactive sintering of an equimolar mixture of dolomite (CaMg(CO₃)₂), monoclinic zirconia ( m -ZrO₂), and α-alumina powders, with a 0.5 wt% lithium fluoride additive. The reaction behavior of the mixed powders (with/without LiF additive) was studied using high-temperature X-ray diffraction. A bulk porous composite resulted from sintering at 1300°C for 2 h (in a nearly closed container, so as to increase the LiF-doping effect), which consisted of fine grains (CaZrO₃ and MgAl₂O₄, ∼0.5–1 μm) and well-grown idiomorphic ones (MgAl₂O₄ octahedra ∼ 2–4 μm). The idiomorphic spinel grains were located around the inner walls of relatively large pores. The composite showed appreciably high bending strength (σ_f= 110 ± 8 MPa for a porosity of 31%). The porous CaZrO₃/MgAl₂O₄ composites can be applied as high-temperature filters and lightweight structural components.     

Preparation of Porous Cr3C2 Grains with Cr2O3

Porous Cr₃C₂ grains (∼300 to 500 μm) with ∼10 wt% of Cr₂O₃ were prepared by heating a mixture of MgCr₂O₄ grains and graphite powder at 1450° to 1650°C for 2 h in an Al₂O₃ crucible covered by an Al₂O₃ lid with a hole in the center. The porous Cr₃C₂ grains exhibited a three-dimensional network skeleton structure. The mean open pore diameter and the specific surface area of the porous grains formed at 1600°C for 2 h were ∼3.5 (μm and ∼6.7 m²/g, respectively. The present work investigated the morphology and the formation conditions of the porous Cr₃C₂ grains, and this paper will discuss the formation mechanism of those grains in terms of chemical thermodynamics.     

Fracture Strength–Fracture Resistance Response and Damage Resistance of Sintered Al2O3–ZrO2 (12 mol% CeO2) Composites

The fracture strengths of sintered Al₂O₃ containing 20 and 40 vol% ZrO₂(12 mol% CeO₂)—zirconia-toughened alumina (ZTA)—composites along with the fracture resistance can be increased (e.g., to ∼900 MPa and >12 Mpa·m^1/2, respectively), by increasing the mean grain size of the t -ZrO₂ (and the Al₂O₃) from ∼0.5 μm to ∼3 μm. At these lower t -ZrO₂ contents, the fracture strength-fracture resistance curves show a continuous rise as opposed to the strength maxima observed in polycrystalline t -ZrO₂(12 mol% CeO₂), CeTZP, and ZrO₂(12 mol% CeO₂) ceramics containing ≤20 vol% Al₂O₃. The toughened composites also exhibit excellent damage resistance with fracture strengths of 500 MPa retained with surfaces containing ∼150- N Vickers indentations which produce cracks of ∼160-μm radius. Greater damage resistance correlates with an increase in the apparent R -curve response of these composites.     

Synthesis of Magnesium Aluminate Spinel Platelets from α-Alumina Platelet and Magnesium Sulfate Precursors

Spinel platelets were formed from a powder mixture of 3–5 μm wide and 0.2–0.5 μm thick α-Al₂O₃ and 1–8 μm (average 3 μm) MgSO₄ heated 2 h at 1200°C. The hexagonal platelet shape of the original α-Al₂O₃ platelet was maintained in the spinel, although their size was slightly increased and their surface roughened. When a mixture of α-Al₂O₃ platelets and MgO powder was heated 3 h at 1400°C, the spinel formed lost the platelet morphology of the alumina.     

Influence of ZrO2 Grain Size and Content on the Transformation Response in the Al2O3─ZrO2 (12 mol% CeO2) System

Based on experimental and modeling studies, the rate of increase in the martensite start temperature M _s for the tetragonal-to-monoclinic transformation with increase in zirconia grain size is found to rise with decrease in ZrO₂ content in the zirconia-toughened alumina ZTA system. The observed grain size dependence of M _s can be related to the thermal expansion mismatch tensile (internal) stresses which increase with decrease in zirconia content. The result is that finer zirconia grain sizes are required to retain the tetragonal phase as less zirconia is incorporated into the alumina, in agreement with the experimental observations. At the same time, both the predicted and observed applied stress required to induce the transformation are reduced with increase in the ZrO₂ grain size. In addition, the transformation-toughening contribution at temperature T increases with increase in the M _s temperature brought about by the increase in the ZrO₂ grain size, when T > M _s. In alumina containing 20 vol% ZrO₂ (12 mol% CeO₂), a toughness of ∼10 MPa. √m can be achieved for a ZrO₂ grain size of ∼2 μm ( M _s∼ 225 K). However, at a grain size of ∼2 μm, the alumina–40 vol% ZrO₂ (12 mol% CeO₂) has a toughness of only 8.5 MPa. √m ( M _s∼ 150 K) but reaches 12.3 MPa. ∼m ( M _s∼ 260 K) at a grain size of ∼3 μm. These findings show that composition (and matrix properties) play critical roles in determining the ZrO₂ grain size to optimize the transformation toughening in ZrO₂-toughened ceramics.     

Formation and Densification Behavior of Magnesium Aluminate Spinel: The Influence of CaO and Moisture in the Precursors

Different grades of stoichiometric and non-stoichiometric dense magnesium aluminate spinel (MgAl₂O₄) grains were prepared by a conventional double-stage firing process using two types of alumina and four types of magnesia raw materials. The MgAl₂O₄ spinel formation was found to be highly influenced by CaO and moisture present in the precursor oxides as confirmed by thermogravimetry (TG), differential thermal analysis (DTA), and X-ray diffraction (XRD) techniques. The Fourier transform-infrared spectroscopy (FTIR) study of the precursor oxides revealed the presence of moisture. Influence of alumina and magnesia composition on the densification behavior of MgAl₂O₄ spinels was assessed by characterizing bulk density (BD), apparent porosity (AP), water absorption (WA) capacity, and the microstructures of the stoichiometric, the magnesia-rich, and the alumina-rich spinels sintered at 1650°C for 1 h. Sintering studies indicate that to obtain dense stoichiometric spinel grains with >3.35 g/mL BD, <2.0% AP, and <0.5% WA, the spinel powder should possess a median particle size of <2 μm, CaO content of >0.9%, compact (green) density of >1.95 g/mL, and spinel content of >90%. Among various spinels synthesized, the magnesia-rich spinels exhibited superior properties in terms of high BD, low percentage of AP, and low WA capacity, whereas alumina-rich spinels showed inferior properties. Stoichiometric spinels exhibited an average grain size of 10 μm whereas alumina-rich spinels with 90% alumina had an average grain size of 20 μm. The increase in holding time at higher temperatures enhanced the sintering properties of the spinels, particularly the magnesia-rich spinels. Further, raw mixtures having >0.9% CaO exhibited better sintered properties as compared with others.     

Reactions between Synthetic Mica and Simple Oxide Compounds with Application to Oxidation-Resistant Ceramic Composites

Reaction-couple experiments have been pursued in order to evaluate the potential of a phyllosiiicate to act as a chemically protective, fracture-deflecting, oxidation-resistant interphase for oxide fiber–oxide matrix composites. The synthetic mica fluorophiogopite (KMg₃[AlSi₃]O₁₀F₂) was reacted with single-phase substrates of alumina (Al₂O₃), mullite (3Al₂O₃·2SiO₂), forsterite (Mg₂SiO₄), or enstatite (MgSiO₃). X-ray spectroscopy, X-ray diffraction, and scanning electron and transmitted polarized light microscopy were applied to the analysis of the reaction couples. Fluorophlogopite reacts strongly with alumina, mullite, and enstatite, resulting in substantial damage to the substrate as well as the breakdown of the mica. The chemical reactions between mica–alumina and mica–mullite are examined critically. In the case of alumina, the reaction results in the formation of a planar spinel (MgAl₂O₄) layer separating the substrate from the breakdown products of the mica. This unvarying result suggests, therefore, that a spinel diffusion barrier would prove effective in protecting alumina from fluorophlogopite. Experiments revealed such effectiveness: local equilibrium is established in the layer sequence alumina–spinel–fluorophlogopite; i.e., planar interfaces are established amongst these phases that are stable under conditions of high temperature and high oxygen fugacity. A similar chemical approach for protection of mullite is not obvious. Based on an understanding of its intrinsic fracture energy, the fluoromica interphase is expected to be effective in mechanically protecting adjacent oxides from propagating cracks, a behavior qualitatively demonstrated by indentation experiments on the kinetically persistent alumina–spinel–fluorophlogopite–spinel–alumina laminates.     

Importance of Crystallization Hierarchies in Microstructural Evolution of Silicate Glass–Ceramics

Quench studies of in-house melted glasses were used to study the microstructural evolution in Macor-type and Corning Code 9606-type glass–ceramics to elucidate the microstructures of the commercial products. They reveal that phase separation initiates crystallization in both systems which then proceeds via formation of several phases at different scales of size, the first phases to form being those containing the most rapidly diffusing species. This study highlights the concept of crystallization hierarchies whereby crystals form, sometimes simultaneously, sometimes sequentially, on heating glasses, but at different length scales . This is illustrated by the simultaneous nucleation of μ-cordierite nanocrystals and ∼0.2 μm MgAl₂Ti₃O₁₀ rosettes in Corning 9606-type compositions and of ∼0.3 μm chondrodite and 3–4 μm fluorophlogopite laths in Macor-type compositions.     

Processing and Mechanical Properties of Ti3SiC2: II, Effect of Grain Size and Deformation Temperature

In this article, the second part of a two-part study, we report on the mechanical behavior of Ti₃SiC₂. In particular, we have evaluated the mechanical response of fine-grained (3–5 μm) Ti₃SiC₂ in simple compression and flexure tests, and we have compared the results with those of coarse-grained (100–200 μm) Ti₃SiC₂. These tests have been conducted in the 25°–1300°C temperature range. At ambient temperature, the fine- and coarse-grained microstructures exhibit excellent damage-tolerant properties. In both cases, failure is brittle up to ∼1200°C. At 1300°C, both microstructures exhibit plastic deformation (>20%) in flexure and compression. The fine-grained material exhibits higher strength compared with the coarse-grained material at all temperatures. Although the coarse-grained material is not susceptible to thermal shock (up to 1400°C), the fine-grained material thermally shocks gradually between 750° and 1000°C. The results presented herein provide evidence for two important aspects of the mechanical behavior of Ti₃SiC₂: (i) inelastic deformation entails basal slip and damage formation in the form of voids, grain-boundary cracks, kinking, and delamination of individual grains, and (ii) the initiation of damage does not result in catastrophic failure, because Ti₃SiC₂ can confine the spatial extent of the damage.     

A Catalytic Process for Mullite Whiskers

Mixtures of alumina and silica react with SiF₄ above 600°C to form fluorotopaz. Pyrolysis of fluorotopaz gives a stoichiometric yield of interlocked whiskers of mullite and SiF₄, which is recovered for reuse. The diameter of whiskers (aspect ratio ∼15) can be controlled from a maximum of ∼100 μm to less than 1 μm.     

Low-Temperature Sintering of Seeded Sol—Gel-Derived, ZrO2-Toughened Al2O3 Composites

Seeding a mixture of boehmite (AIOOH) and colloidal ZrO₂ with α-alumina particles and sintering at 1400°C for 100 min results in 98% density. The low sintering temperature, relative to conventional powder processing, is a result of the small alumina particle size (∼0.3 μm) obtained during the θ-to α-alumina transformation, homogeneous mixing, and the uniform structure of the sol-gel system. Complete retention of pure ZrO₂ in the tetragonal phase was obtained to 14 vol% ZTA because of the low-temperature sintering. The critical grain size for tetragonal ZrO₂ was determined to be ∼0.4 μm for the 14 vol% ZrO₂—Al₂O₃ composite. From these results it is proposed that seeded boehmite gels offer significant advantages for process control and alumina matrix composite fabrication.     

Alumina Dissolution into Silicate Slag

Dissolution of commercial white fused and tabular Al₂O₃ grains into a model silicate slag was investigated after 1 h at 1450° and 1600°C. Formation of CA₆ and hercynitic spinel layers was observed at all Al₂O₃/slag interfaces. The spinel layer was not always continuous, and so, compared with the CA₆ layer, it had a less-significant effect on the dissolution process. The CA₆ layer that formed adjacent to the tabular Al₂O₃ was incomplete at both temperatures, so that its dissolution was not a totally indirect process. These incomplete CA₆ and spinel layers meant that slag penetrated into the tabular Al₂O₃ grains, which, thus, were corroded and disintegrated by the penetrating slag. There was evidence of liquid in the CA₆ layer adjacent to the fused Al₂O₃ after 1 h at 1450°C, which also enabled direct dissolution. After 1 h at 1600°C, fused Al₂O₃ revealed a thick (∼60 μm), continuous and unpene-trated CA₆ layer, indicating fully indirect dissolution at this temperature.     

The Study on Carbon Nanofiber (CNF)-Dispersed B4C Composites

Carbon nanofiber (CNF)-dispersed B₄C composites have been synthesized and consolidated directly from mixtures of elemental raw powders by pulsed electric current pressure sintering (1800°C/10 min/30 MPa). A 15 vol% CNF/B₄C composite with ∼99% of dense homogeneous microstructures (∼0.40 μm grains) revealed excellent mechanical properties at room temperature and high temperatures: a high bending strength (σ_b) of ∼710 MPa, a Vickers hardness ( H _v) of ∼36 GPa, a fracture toughness ( K _{I C} ) of ∼7.9 MPa m^1/2, and high-temperature σ_b of 590 MPa at 1600°C in N₂. Interfaces between the CNF and the B₄C matrix were investigated using high-resolution transmission electron microscopy, EDS, and electron energy-loss spectroscopy.     

Influence of Grain Growth on Dielectric Properties of Nb-Doped BaTiO3

Effects of grain size and grain growth in Nb-doped BaTiO₃ on temperature and frequency dependencies of the dielectric constant were investigated. When 0.65 μm powder is sintered to an average grain size of 1 μm, two dielectric constant peaks indicate the presence of Nb-free BaTiO₃ and of Nb-containing material. Single peaks are observed above room temperature after additional grain growth or when 0.07 μm powder is sintered to an average grain size of 1 μm. The Curie point of pure BaTiO₃ with 1 μm grains is 4 to 6°C lower than that of material with grains >10 μm. Thermodynamically, this behavior is accounted for by a phase inversion stress ∼ the room-temperature stress.     

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.     

Toughening and Strengthening of NiAl with Al2O3 by the Addition of ZrO2(3Y)

NiAl/10-mol%-ZrO₂(3Y) composites of almost full density have been fabricated via spark plasma sintering (SPS) for 10 min at 1300°C and 30 MPa. The former intermetallic compound, which contains a trace amount of Al₂O₃, has been prepared via self-propagating high-temperature synthesis. The composite microstructures are such that tetragonal ZrO₂ (∼0.2 μm) and Al₂O₃ (∼0.5 μm) particles are located at the grain boundaries of the NiAl (∼46 μm) matrix. Improved mechanical properties are obtained: the fracture toughness and bending strength are 8.8 MPa·m^1/2 and 1045 MPa, respectively, and high strength (>800 MPa) can be retained up to 800°C.     

Processing and Mechanical Behavior of CrN/ZrO2(2Y) Composites

CrN powder consisting of granular particles of ∼3 μm has been prepared by self-propagating high-temperature synthesis under a nitrogen pressure of 12 MPa using Cr metal. Dense pure CrN ceramics and CrN/ZrO₂(2Y) composites in the CrN-rich region have been fabricated by hot isostatic pressing for 2 h at 1300°C and 196 MPa. The former ceramics have a fracture toughness ( K _IC) of 3.3 MPa ·m^1/2 and a bending strength (σ_b) of 400 MPa. In the latter materials almost all of the ZrO₂(2Y) grains (0.36–0.41 μm) are located in the grain boundaries of CrN (∼4.6 μm). The values of K _IC (6.1 MPa · m^1/2) and σ_b (1070 MPa) are obtained in the composites containing 50 vol% ZrO₂(2Y).     

Titanium Zirconate Thick Films on Rutile Substrates by Reactive Coating of Zircon

A mechanically stable thick layer (∼SO pm) of ZrTiO₄ on a rutile substrate has been obtained at 1600°C by reactive coating of zircon. This layer is constituted by ZrTiO, grains of ∼10 μm average size embedded in a silica-rich glassy matrix. The results are discussed in terms of the ZrO₂-SOi₂-TiO₂, equilibrium diagram.     

Influences of Particle Size of Alumina Filler in an LTCC System

A low temperature co-fired ceramics system consisting of a typical calcium aluminoborosilicate glass and alumina filler was used to investigate the effects of four different sizes, 13 nm, 0.5, 3, and 39 μm, of a commercially available alumina filler on the resultant densification, crystallization, and dielectric properties. There was definitely a proper range of alumina particle size, which leads to desirable densification and enhanced dielectric properties. The onset temperatures of densification and crystallization depended strongly on the filler particle size. The 3 μm sample as an optimum filler size exhibited a promising performance of k ∼8.1 and Q ∼160 at a resonant frequency of 14.8 GHz, which results from early densification and intensive crystallization of the anorthite CaAl₂Si₂O₈ phase. Particularly, the use of nano-sized alumina (13 nm) retarded both densification by ∼200°C and crystallization by ∼80°C compared with the results of the 3 μm alumina case. The dependence of the filler particle size was postulated as being related to the wetting and connectivity behavior of glass through consequent inter-reactions between glass and ceramic.