Singular Grain Boundaries in Alumina and Their Roughening Transition

The shapes and structures of grain boundaries formed between the basal (0001) surface of large alumina grains and randomly oriented small alumina grains are shown to depend on the additions of SiO₂, CaO, and MgO. If a sapphire crystal is sintered at 1620°C in contact with high-purity alumina powder, the grain boundaries formed between the (0001) sapphire surface and the small alumina grains are curved and do not show any hill-and-valley structure when observed under transmission electron microscopy (TEM). These observations indicate that the grain boundaries are atomically rough. When 100 ppm (by mole) of SiO₂ and 50 ppm of CaO are added, the (0001) surfaces of the single crystal and the elongated abnormal grains form flat grain boundaries with most of the fine matrix grains as observed at all scales including high-resolution TEM. These grain boundaries, which maintain their flat shape even at the triple junctions, are possible if and only if they are singular corresponding to cusps in the polar plots of the grain boundary energy as a function of the grain boundary normal. When MgO is added to the specimen containing SiO₂ and CaO, the flat (0001) grain boundaries become curved at all scales of observation, indicating that they are atomically rough. The grain boundaries between small matrix grains also become defaceted and hence atomically rough.     

Origin and Growth Kinetics of Platelike Abnormal Grains in Liquid-Phase-Sintered Alumina

The grain-growth behavior of Al₂O₃ compacts with small contents (≤10 wt%) of various liquid-forming dopants was studied. Equiaxed and/or elongated grains were observed for the following dopants: MgO, CaO, SiO₂, or CaO + TiO₂. The platelike grains, defined as the abnormal grains larger than 100 μm with an aspect ratio ≥5 and with flat boundaries along the long axis, were observed when the boundaries were wet with the liquid phase and the codoping satisfied two conditions of size and valence. These dopings were Na₂O + SiO₂, CaO + SiO₂, SrO + SiO₂, or BaO + SiO₂. However, an addition of MgO to the Al₂O₃ doped with CaO + SiO₂ resulted in the change of grain shape from platelike to equiaxial. Equiaxed grains were also observed for the MgO + SiO₂ doping, indicating that two conditions were necessary but not sufficient to develop the platelike grains. The fast growth rate of the platelike grains was explained by an increased interfacial reaction rate due to the codopants. AT the same time the codopants made the basal plane, which appeared as the flat boundaries, the lowest energy plane. The appearance of the platelike grains was favored in compacts with a small grain size and with a narrow size distribution at the onset of abnormal grain growth. Accordingly, the use of starting powders with a small particle size and narrow size distribution, smaller amounts of dopings, and high sintering temperature resulted in an increased number of the platelike grains.     

Microstructural Characterization of Superplastic SiO2-doped TZP with a Small Amount of Oxide Addition

The microstructures of 5 wt% SiO₂-doped TZP, 5 wt% (SiO₂+ 2 wt% MgO)-doped TZP, and 5 wt% (SiO₂+ 2 wt% Al₂O₃)-doped TZP are characterized by high-resolution electron microscopy, energy-dispersive X-ray spectroscopy, and electron energy loss spectroscopy. An amorphous phase is formed at multiple grain junctions but not along the grain-boundary faces in these three materials. A small addition of MgO and Al₂O₃ into the SiO₂ phase results in a marked reduction in tensile ductility of SiO₂-doped TZP. This reduction seems to correlate with segregation of magnesium or aluminum ions at grain boundaries and a resultant change in the chemical bonding state.     

Imaging Secondary-Ion Mass Spectroscopy Observation of the Scavenging of Siliceous Film from 8-mol%-Yttria-Stabilized Zirconia by the Addition of Alumina

The scavenging of a resistive siliceous phase via the addition of Al₂O₃ was studied, using imaging secondary-ion mass spectroscopy (SIMS), given the improved grain-boundary conductivity in 8-mol%-yttria-stabilized zirconia (8YSZ). The grain-boundary resistivity in 8YSZ decreased noticeably with the addition of 1 mol% of Al₂O₃. Strong SiO₂ segregation at the grain boundaries was observed in a SIMS map of pure 8YSZ that contained 120 ppm of SiO₂ (by weight). The addition of 1 mol% of Al₂O₃ caused the SiO₂ to gather around the Al₂O₃ particles. The present observations provided direct and visual evidence of SiO₂ segregation at the grain boundaries (which had a deleterious effect on grain-boundary conductivity) and the scavenging of SiO₂ via Al₂O₃ addition.     

Silica and Magnesia Dopant Distributions in Alumina by High-Resolution Scanning Secondary Ion Mass Spectrometry

Trace SiO₂ and MgO additive distributions in sintered alumina have been studied using high-resolution scanning secondary ion mass spectrometry (SIMS). When doped with each additive individually, evidence is seen for both strong silicon segregation to grain boundaries ( C _gb/ C _grain similar/congruent 300) in SiO₂-doped alumina and strong magnesium segregation to grain boundaries ( C _gb/ C _grain similar/congruent 400) in MgO-doped alumina. When codoped with both SiO₂ and MgO, segregation of both ions to grain boundaries is reduced by a factor of 5 or more over single doping. The additive concentrations increase proportionally in the grains, and both dopants become more uniformly distributed throughout the bulk. It is concluded that codoping with these additives increases their mutual bulk solid solubility and decreases their interfacial segregation over single doping. The beneficial effect of MgO additions in controlling microstructure development in alumina and improving corrosion resistance to aqueous HF stems from its ability to redistribute silicon ions from grain boundaries into the bulk.     

Grain-Boundary Viscosity of BaO-Doped SiC

Internal friction characterization of the viscosity of a residual SiO₂/BaO glass, segregated to grain boundaries of polycrystalline SiC, is presented. The anelastic relaxation peak of internal friction, arising from viscous slip along grain boundaries wetted by a glass phase, is analyzed. Two SiC polycrystals, containing SiO₂/BaO glasses with different compositions, are studied and compared with a SiC polycrystal containing only pure SiO₂. The internal friction peak is first analyzed with respect to its shift upon frequency change. This analysis allows quantitative assessment of both the intrinsic viscosity and the activation energy for viscous flow of the grain-boundary phase. Both parameters markedly decrease with increasing amounts of BaO dopant, which is consistent with data reported in the literature on SiO₂ and SiO₂/BaO bulk glasses with the same nominal composition. Analysis of the peak morphology is also attempted, considering the evolution of peak width while varying the grain-boundary glass composition. Moreover, the role of microstructural parameters, such as the distributions of grain size and grain-boundary angles, on the broadening of the internal friction peak is addressed, and a procedure is proposed that allows quantitative evaluation of the activation energy for viscous flow of intergranular glass merely from the width of the internal friction peak.     

Silica Glass Segregation in 3 wt% LiF-Doped Hot-Pressed Y2Si2O7

Hot-pressed yttrium disilicate ceramics have been characterized using analytical transmission electron microscopy (TEM). The microstructure consists of large grains of the γ phase of stoichiometric γ-Y₂Si₂O₇ containing rounded glassy Y-doped SiO₂ inclusions; excess glassy SiO₂-rich material is also found at the grain boundaries. Two main reasons are found for the inhomogeneity: a slight SiO₂ excess is inferred from the composition measurements, and the LiF flux used in hot pressing would also promote glass formation. Improved high-temperature mechanical properties would only be possible if residual glass formation was minimized, strategies for doing so are discussed, and the importance of analytical TEM for studying such submicron scale inhomogeneity is underlined.     

Effect of CaO on the Thermal Conductivity of Aluminum Nitride

The effect of CaO on the thermal conductivity of aluminum nitride pressureless sintered with 3 wt% Y₂O₃ as a sintering aid was investigated. Over the composition range of 0 to 2.0 wt% additions, CaO decreased the thermal conductivity of the sintered parts by 10%. CaO doping rendered the secondary oxide phases more wetting and thus with a greater tendency to penetrate along the grain boundaries. Furthermore, CaO segregation to the grain boundaries was observed even on those grain boundaries apparently free of secondary phases. These microstructural changes disrupted the connectivity of the high thermal conductivity AIN grains and were the main factors contributing to the decrease in the thermal conductivity of the ceramic parts. CaO additions to samples doped with SiO₂ had the opposite effect, increasing the thermal conductivity. CaO removed SiO₂ from the AIN grains and incorporated it into the oxide second phases, most likely through charge-compensating substitutions Ca²⁺+ Si⁴⁺ for Y³⁺ and/or Al³⁺. Thus, AIN samples containing both SiO₂ and CaO had higher thermal conductivity than those containing comparable amounts of SiO₂ alone.     

Low-Frequency Measurements on the Grain-Boundary Internal Friction Peak in Chlorine-Doped Silicon Nitride

A procedure is shown to quantitatively analyze the morphology of the internal friction peak resulting from grain-boundary sliding. A Si₃N₄ polycrystal containing chlorine-doped SiO₂ at grain boundaries is selected as a model system for discussing chemical (e.g., anion) gradients at glassy grain boundaries. In this model material, grain boundaries lodging Cl⁻ anions show nonuniform thickness characteristics, which suggests a non-negligible dependence of the intergranular SiO₂-network structure upon grain misorientation. Both chemical and microstructural inhomogeneities existing in a polycrystalline ceramic body can result in peak broadening. The key for separating broadening contributions of chemical gradients from grain-size/morphology distributions resides in analyzing the peak-width change upon damping frequency. Groups of grain boundaries with different chemical characteristics may produce broadening because different peak components are generated that obey a spectrum of activation energies. On the other hand, microstructural inhomogeneities obey a single activation energy, but they generate a distribution of relaxation times. As a result, when a chemical gradient is present at grain boundaries, the peak may shift upon changing damping frequency with obeying a true activation energy, but its width increases with decreasing damping frequency. When peak broadening results only from microstructural inhomogeneities, the peak width is independent of damping frequency.     

Local Viscosity of Si-O-C-N Nanoscale Amorphous Phases at Ceramic Grain Boundaries

Internal friction characterization has been used to quantitatively assess the viscosity characteristics of Si-O-C-N glasses segregated to nanometer-sized grain boundaries of polycrystalline Si₃N₄ and SiC ceramics. A relaxation peak of internal friction, which arises with rising temperature from the viscous sliding of glassy grain boundaries, was systematically collected and analyzed with respect to its shift upon changing the oscillation frequency. As a result of such an analysis, both activation energy for viscous grain-boundary flow and inherent viscosity of the intergranular glass film could be quantitatively evaluated. Two main features are shown: (i) the presence of N and/or C greatly affects the viscosity characteristics of SiO₂ phases at Si₃N₄ and SiC grain boundaries; and (ii) the internal friction method has potential as a unique experimental tool for understanding the local properties of nanoscale amorphous phases in new ceramic materials.     

Deposition of Pb(Zr,Ti)O3 Thin Films by Metal-Organic Chemical Vapor Deposition Using β-diketonate Precursors at Low Temperatures

Lead zirconate titanate (PZT) thin films were deposited by metal-organic chemical vapor deposition (MOCVD) using β-diketonate precursors and 0₂ at temperatures below 500°C on variously passivated Si substrates. PZT thin films could not be deposited on bare Si substrates, owing to a serious diffusion of Pb into the Si substrate during deposition. Pt/SiO₂/Si substrates could partially block the diffusion of Pb, but a direct deposition of PZT thin films on the Pt/SiO₂/Si substrates resulted in a very inhomogeneous deposition. A TiO₂ buffer layer deposited on Pt/SiO₂/Si substrates could partially suppress the diffusion of Pb and produce homogeneous thin films. However, the crystallinity of PZT thin films deposited on the TiO₂-buffered Pt/SiO₂/Si substrate was not good enough, and the films showed random growth direction. PZT thin films deposited on the PbTiO₃-buffered Pt/SiO₂/Si substrates had good crystallinity and a- and c-axis oriented growth direction. However, the PZT thin film deposited at 350°C showed fine amorphous phases at the grain boundaries, owing to the low chemical reactivities of the constituent elements at that temperature, but they could be crystallized by rapid thermal anneaiing (RTA) at 700°C. PZT thin film deposited on a 1000-å PbTiO₃,-thin-film-buffered Pt/SiO₂/Si substrate at 350°C and rapid thermally annealed at 700°C for 6 min showed a single-phase perovskite structure with a composition near the morphotropic boundary composition.     

Grain-Boundary Viscosity of Polycrystalline Silicon Carbides

Internal friction experiments were conducted on three SiC polycrystalline materials with different microstructural characteristics. Characterizations of grain-boundary structures were performed by high-resolution electron microscopy (HREM). Observations revealed a common glass-film structure at grain boundaries of two SiC materials, which contained different amounts of SiO₂ glass. Additional segregation of residual graphite and SiO₂ glass was found at triple pockets, whose size was strongly dependent on the amount of SiO₂ in the material. The grain boundaries of a third material, processed with B and C addition, were typically directly bonded without any residual glass phase. Internal friction data of the three SiC materials were collected up to similar/congruent2200°C. The damping curves as a function of temperature of the SiO₂-bonded materials revealed the presence of a relaxation peak, arising from grain-boundary sliding, superimposed on an exponential-like background. In the directly bonded SiC material, only the exponential background could be detected. The absence of a relaxation peak was related to the glass-free grain-boundary structure of this polycrystal, which inhibited sliding. Frequency-shift analysis of the internal friction peak in the SiO₂-containing materials enabled the determination of the intergranular film viscosity as a function of temperature.     

Raman Study of Grain Size Effects in the Melting Behavior of 15Na2CO3-10BaCO3-75SiO2 Batches

By means of Raman spectroscopy the melting behavior of 15Na₂CO₃−10BaCO₃−75SiO₂ batches with different grain sizes of raw materials was investigated both qualitatively and quantitatively. The results show that the reaction rate at low temperatures ( T ≤800° to 900°C) increases when finer grains of all raw materials are used; upon pelletizing the fine batch the reaction rate increases even further. At high temperatures ( T > 900°C) the grain size of SiO₂ is the main determining factor, the melting rate being increased when fine SiO₂ grains are used.     

Densification and Mechanical Properties of Titanium Diboride with Silicon Nitride as a Sintering Aid

Titanium diboride (TiB₂) was hot-pressed at a temperature of 1800°C, and silicon nitride (Si₃N₄) was added as a sintering aid. The amount of Si₃N₄ that was added had a significant influence on the sinterability and mechanical properties of the TiB₂. When a small amount (2.5 wt%) of Si₃N₄ was added, the Si₃N₄ reacted with titania (TiO₂) that was present on the surface of the TiB₂ powder to form titanium nitride (TiN), boron nitride (BN), and amorphous silica (SiO₂). The elimination of TiO₂ suppressed the grain growth effectively, which led to an improvement in the densification of TiB₂. The formation of SiO₂ also was deemed beneficial for densification. The mechanical properties-especially, the flexural strength-were enhanced remarkably through these improvements in the sinterability and microstructure. On the other hand, when a large amount (greaterthan equal to5 wt%) of Si₃N₄ was added, the mechanical properties were not improved much, presumably because of the extensive formation of a glassy Si-Ti-O-N phase at the grain boundaries.     

Hydrothermal Corrosion of Pure, Hot Isostatically Pressed Silicon Nitride

The aqueous corrosion behavior of hot isostatically pressed Si₃N₄ (HIPed-Si₃N₄) without additives was studied under hydrothermal conditions at 300°C and 8.6 MPa (86 atm). The accelerated weight loss in the HIPed-Si₃N₄ was attributed to uniform thinning of the specimen accompanied by dislodgement of Si₃N₄ grains from the substrate due to preferential attack at grain boundaries. Enhanced attack at grain boundaries was due to the presence of amorphous SiO₂ from impurities in the starting powder.     

Effect of Sintering Aids on Microstructures and PTCR Characteristics of (Sr0.2Ba0.8)TiO3 Ceramics

The effects of liquid-phase sintering aids on the microstructures and PTCR characteristics of (Sr_0.2Ba_0.8)TiO₃ materials have been studied. The grain size of sintered materials monotonically decreases with increasing content of Al₂O₃–SiO₂–TiO₂ (AST). The ultimate PTCR properties with ρ_ht/ρ_rt as great as 10^5.61 are obtained for fine-grain (10-μm) samples, which contain 12.5 mol% AST and were sintered at 1350°C for 1.5 h. The quantity of liquid phase formed due to eutectic reaction between AST and (Sr,Ba)TiO₃ is presumably the prime factor in determining the grain size of samples. The grains grow rapidly at the sintering temperature in the first stage until the liquid phase residing at the grain boundaries reaches certain critical thickness such that the liquid–solid interfacial energy dominates the mechanism of grain growth.     

Viscous Slip along Grain Boundaries in Chlorine-Doped Silicon Nitride

The effect of chlorine doping on the anelastic-relaxation and torsional-creep behavior of a silicon nitride (Si₃N₄) polycrystalline body was studied. Two model polycrystals—one undoped and the other doped with a small fraction of chlorine—were investigated. Their microstructures consisted of equiaxed and well-faceted Si₃N₄ grains whose boundaries were separated by a continuous, nanometer-sized film of silica (SiO₂) glass. The actual presence of chlorine in the doped polycrystal was ascertained by ion chromatography and is thought to be enriched at the grain boundaries. The effect of chlorine on the intergranular film structure was characterized by high-resolution electron microscopy. The micromechanical response of the SiO₂ grain boundary under shear stress was monitored up to very high temperatures (i.e., ∼2000°C) by internal-friction and torsional-creep experiments. The presence of the chlorine dopant, which is a network modifier of SiO₂ glass that also causes a widening of the grain-boundary film, significantly lowered the bulk viscosity of the residual glass. As a consequence of the change in grain-boundary chemistry, the internal-friction curve of the chlorine-doped material shifted toward lower temperatures and the torsional-creep rate markedly increased, as compared to the undoped material. According to a viscoelastic model of the Si3N4 polycrystal, the internal-friction data resulted as a superposition of two individual components: (i) a relaxation peak that is related to the anelastic slip mechanism along grain boundaries and (ii) a background component that results from an irreversible diffusional-creep mechanism.     

Grain Growth of Silica-Added Zirconia Annealed in the Cubic/Tetragonal Two-Phase Region

The grain growth in silica-doped 3-mol%-yttria-stabilized tetragonal zirconia polycrystals (SiO₂-doped 3Y-TZP) and undoped 3Y-TZP has been examined in the temperature range of 1400°-1800°C. The presence of a SiO₂ phase inhibits rather than promotes the grain growth in 3Y-TZP, particularly at high temperatures. During the grain growth in 3Y-TZP, yttrium ions are partitioned between grains, and the grain growth mechanism can be understood from Ostwald ripening dominated by lattice diffusion of cations. In SiO₂-doped 3Y-TZP, an amorphous SiO₂-rich phase exists only in the grain-boundary corners or junctions, not in the grain-boundary faces. The grain growth in SiO₂-doped 3Y-TZP is controlled by using different mechanisms below and above the eutectic temperature of the zirconia-silica (ZrO₂-SiO₂) system. The glass phase does not have a major role in grain growth below the eutectic temperature, and the grain growth is dominated by a similar mechanism in undoped 3Y-TZP. The grain growth is more effectively retarded by the presence of a SiO₂ phase above the eutectic temperature and is likely to be controlled by a solution-reprecipitation process in the amorphous phase at the grain-boundary corners or junctions.     

Microstructural Analysis of Sintered High-Conductivity Zirconia with Al203 Additions

Transmission electron microscopy (at 100 and 1000 kV potential) and analytical scanning transmission electron microscopy were used to study α-Al₂0₃ second-phase particles and their interactions with grain boundaries in two high-conductivity Y₂0₃/Yb₂0₃ stabilized zirconia ceramics containing deliberate additions of the alumina as a sintering aid. Most of the Al₂0₃ particles were intragranular and microanalysis showed that they contained inclusions rich in Zr or Si plus Zr. Al₂O₃ particles at grain boundaries were frequently associated with amorphous cusp areas rich in Si and Al. The results suggest that the Al₂0₃ acts as a scavenger for SiO₂, removing it from grain-boundary localities. A model is proposed whereby this process occurs as the boundaries meet the second-phase particles, assisted by rapid grain-boundary diffusion. Such an ZrO₂-Al₂O₃-SiO₂ interaction and partitioning is predicted thermodynamically and offers a possible explanation for the improvements in ionic conductivity brought about by Al₂O₃ additions, as reported in the literature.     

Grain-Boundary Viscosity of Calcium-Doped Silicon Nitride

Internal-friction data of calcium-doped Si₃N₄ polycrystalline materials that otherwise contain only SiO₂ at grain boundaries were examined and compared with those collected on the same polycrystal in the undoped state or doped with anions (i.e., fluorine and chlorine). Precise microstructural characterizations previously performed on these materials enabled us to quantitatively evaluate the inherent viscosity of the intergranular SiO₂ film through the analysis of the anelastic internal-friction-peak components. The intergranular glass viscosity and its scaling with increasing calcium addition followed the same trend as bulk SiO₂ glasses with the same chemical composition. Broadening of the internal-friction peak with increased calcium content in the material has also been rationalized according to the reduction of the activation energy for the viscous flow of bulk glasses. The present analysis, which is in agreement with our previous studies on undoped and anion-doped Si₃N₄, has demonstrated that the overall viscoelastic response of the polycrystal is mainly dictated by the chemistry of the intergranular glass.