SiAION Ceramics

The phase relations in and physical properties of SiAlON ceramics prepared by the hot isostatic pressing technique or by pressureless sintering with a sintering aid are reviewed. The sintering aid used is an AIN and Al₂O₃ mixture either pure or in combination with Y₂O₃ and/or various rare-earth oxides. Special attention is paid to the amount and type of phase(s) formed and to how properties such as hardness (HV10), fracture toughness ( K _IC), and oxidation resistance vary with the sintering aid used for different types of SiAlON materials, i.e., O'-, almost monophasic α-, pure β-, and mixed (α+β)-SiAION Ceramics.     

Microstructural Development of Calcium alpha-SiAlON Ceramics with Elongated Grains

Silicon nitride (Si₃N₄) and SiAlONs can be self-toughened through the growth of elongated β-Si₃N₄/β-SiAlON grains in sintering. α-SiAlONs usually retain an equiaxed grain morphology and have a higher hardness but lower toughness than β-SiAlONs. The present work has demonstrated that elongated alpha-SiAlON grains can also be developed through pressureless sintering. alpha-SiAlONs with high-aspect-ratio grains in the calcium SiAlON system have exhibited significant grain debonding and pull-out effects during fracture, which offers promise for in-situ -toughened α-SiAlON ceramics.     

Rheological Behavior of Dilute SiAlON with or without Intergranular X-Phase

Dense nearly single-phase β'-SiAlON materials (with substitutional level z ∼ 1) have been prepared by hot isostatic pressing and their high-temperature deformation behavior has been investigated using low-frequency damping and torsional creep experiments. Addition of a small fraction of AlN (∼0.5 wt%) to the starting (nominally z = 1) SiAlON powder enabled us to "balance" the excess SiO₂ which likely arises from surface contamination of the starting SiAlON powder upon exposure to atmosphere. As a result, a fine-grained β'-SiAlON polycrystal free of residual (glassy) X-phase segregated to grain boundaries could be prepared. This microstructure is in contrast with that found for an "unbalanced" composition prepared from the same raw β'-SiAlON powder but without the corrective AlN addition. In this latter case, residual glass (X-phase), consisting of Al-rich SiO₂, was entrapped at multiple grain junctions. The presence of such a low-melting intergranular glass dominates the high-temperature deformation behavior of the dilute SiAlON material, involving marked degradation of creep resistance and significant damping relaxation due to grain-boundary sliding. "Balancing" the SiAlON microstructure with a small addition of AlN enabled us to suppress anelastic relaxation by grain-boundary sliding and to increase the creep resistance of the material by more than 1 order of magnitude.     

Sr2+–Mg2+-Doped SiAlON Ceramics

In the present work, multi-cation-doped (Sr²⁺–Mg²⁺) SiAlON ceramics were investigated. MgO and SrO were used in 100:0 and 50:50 molar ratios. The mixture was sintered at 1800° and 1830°C for 1 h in a gas pressure-sintering furnace. The results showed that sintered samples were composed of mainly α- and β-SiAlON phases and small amounts of some Sr-containing phases and SiAlON polytypes. According to Rietveld analysis of X-ray diffraction patterns, Mg is incorporated into the α-SiAlON structure. However, the incorporation of Sr is limited.     

Nd2O3-Doped Sialons with ZrO2/ZrN Additions Formed by Sintering and Hot Isostatic Pressing

β-sialon and Nd₂O₃-doped α-sialon materials of varying composition were prepared by sintering at 1775° and 1825°C and by glass-encapsulated hot isostatic pressing at 1700°C. Composites were also prepared by adding 2–20 wt% ZrO₂ (3 mol% Nd₂O₃) or 2–20 wt% ZrN to the β-sialon and α-sialon matrix, respectively. Neodymium was found to be a fairly poor α-sialon stabilizer even within the α-phase solid solution area, and addition of ZrN further inhibited the formation of the α-sialon phase. A decrease in Vickers hardness and an increase in toughness with increasing content of ZrO₂(Nd₂O₃) or ZrN were seen in both the HIPed β-sialon/ZrO₂(Nd₂O₃) composites and the HIPed Nd₂O₃-stabiIized α-sialons with ZrN additions.     

Duplex α,β-Sialon Ceramics Stabilized by Dysprosium and Samarium

Duplex αβ,-sialon ceramics with a minimum volume fraction of residual intergranular glass have been prepared using Dy or Sm as the α-sialon stabilizing element. These microstructures contained high aspect ratio β-sialon grains homogeneously distributed in an α-sialon matrix. A number of the larger α-sialon grains contained dislocations and showed a core/shell structure. Dy gave an α-sialon which was stable over a wide temperature range (1350–1800°C) for long holding times, while the use of Sm resulted in less stable α-sialon structures at medium temperatures (1450°C) and the formation of melilite, R₂Si_3−xAl_xO_3+xN_4−x, β-sialon, and the 21R sialon polytype during prolonged heating. High α-phase contents gave a very high hardness ( H _V10 is approximately 22 GPa) but a comparatively low indentation fracture toughness (around 4.4 MPam^1/2). Duplex sialons fabricated from powder mixtures corresponding to an α-to-β sialon ratio of around 50:50 resulted in a sialon material with a favorable combination of high hardness (around 22 GPa) and increased toughness (to around 5.5 MPam^1/2).     

Residual α–Si3N4 in O' Crystals in CeO2-Doped O'+β' SiAlON Ceramics

The microstructure of a pressureless sintered (1605°C, 90 min) O'+β' SiAlON ceramic with CeO₂ doping has been investigated. It is duplex in nature, consisting of very large, slablike elongated O' grains (20–30 μm long), and a continuous matrix of small rodlike β' grains (< 1.0 μm in length). Many α-Si₃N₄ inclusions (0.1–0.5 μm in size) were found in the large O' grains. CeO₂-doping and its high doping level as well as the high Al₂O₃ concentration were thought to be the main reasons for accelerating the reaction between the α-Si₃N₄ and the Si-Al-O-N liquid to precipitate O'–SiAlON. This caused the supergrowth of O' grains. The rapid growth of O' crystals isolated the remnant α–Si₃N₄ from the reacting liquid, resulting in a delay in the α→β-Si₃N₄ transformation. The large O' grains and the α-Si₃N₄ inclusions have a pronounced effect on the strength degradation of O'+β' ceramics.     

Generalized Fluid Flow Model for Ceramic Tape Casting

The fluid mechanics associated with the flow of a ceramic slurry during the tape casting process is analyzed in this paper. The rheology of the slurry is described in generalized terms using the Ostwald-de Waele power-law equation, T = m|γ|', where the yield constant, m , and the shear rate exponent, n, are empirical functions of the particle loading in the slurry, the particle shape and size distributions in the slurry, and the slurry temperature. The effects of an imposed pressure gradient due to the height of the slurry in the casting head, as well as those of the drag due to the moving substrate on the slurry flow, are accounted for by modeling the slurry discharge as a generalized planar Couette flow. The model yields an analytical expression for the tape thickness as a function of various slurry and process parameters. The influence of the physical parameters of the slurry and the geometrical dimensions of the casting head on the tape thickness are examined in the context of a commonly used BaTiO, system. It is shown that the various parametric effects may be represented concisely in the form of a nondimensional design plot employing (a) a flow parameter, α, (b) a shrinkage parameter, β, and (c) the rheology exponent, n .     

Physicochemical properties of high-molecular-weight poly(α,β-malic acid) synthesized by direct polycondensation

Summary We studied high-molecular-weight α,β-PMA synthesized by polycondensation in order to find possible applications for biomaterials. Its solubility in different solvents, its hydrolysis and its acidity were also examined. The α,β-PMA molecular weight increased markedly up to 20 h and then decreased showing that the molecular weights for synthesized α,β-PMA depends on the reaction time. We prepared high-yield α,β-PMA with a molecular weight of 3600 by direct polycondensation using tin(II) chloride as a catalyst at 130°C for 20 hours and concluded that our method is suitable to synthesize higher molecular weight compounds of α,β-PMA. Received: 20 December 2002/Revised version: 18 February 2003/ Accepted: 22 February 2003 Correspondence to Tetsuto Kajiyama     

Hollow Beads Composed of Nanosize Ca α-SiAlON Grains

Carbothermal reduction—nitridation (CRN) of SiO₂ is an attractive method to manufacture Si₃N₄ powders with controlled grain morphology. Moreover, β-SiAlON powders could also be synthesized from either pure powder mixture or some inexpensive raw minerals by CRN and the resulting powders favored the sintering of SiAlON product. However, there have been few works on preparing α-SiAlON powders so far. In this work, Ca α-SiAlON powder was synthesized by CRN of a SiO₂—Al₂O₃—CaCO₃ mixture. An unusual morphology of hollow beads 200 to 500 nm in diameter with a great deal of nanosize α-SiAlON particles around 10 to 30 nm in diameter was observed from the resultant Ca α-SiAlON powders, which has not been reported for SiAlON ceramics before.     

α-SiAlON Ceramics Obtained by Slip Casting and Pressureless Sintering

Dense α-SiAlON ceramics were obtained by pressureless sintering of green compacts prepared using slip casting. The rheological properties of the reaction SiAlON suspension were optimized to achieve a high degree of dispersion with a high solids volume fraction, which resulted in homogeneous and relatively dense green bodies with high sintering ability, which could be densified by pressureless sintering at 1750°C for 2 h. The sintered samples revealed a high degree of uniformity and almost fully dense microstructures that consisted of many small, elongated grains homogeneously dispersed in the fracture surfaces, which had properties comparable with those of other SiAlONs obtained using hot pressing.     

SiAlON-Based Ceramics from Filled Preceramic Polymers

Commercial polysiloxanes filled with alumina nanoparticles and AlN and Si₃N₄ micropowders have been used for the preparation of β-SiAlON in nitrogen, at 1450°–1550°C. SiAlON was preceded by intermediate alumino-silicate phases like mullite and sillimanite. Although reduction and nitridation to β-SiAlON finally occurred, the SiAlON yield at high temperatures was lowered by corundum contaminations. The yield was also possibly affected by phase separation in the SiOC glass residue derived from silicones. A composition refinement reduced contamination and provided a promising route for SiAlON from filled preceramic polymers.     

Microstructure of an Extruded β-Silicon Nitride Whisker-Reinforced Silicon Nitride Composite

β-Si₃N₄ whisker-reinforced β'SiAlON composites were fabricated by extrusion and densified, using pressureless sintering. Whisker alignment was observed in both the green and sintered microstructures. SEM analysis of polished, sintered samples showed a microstructure consisting of the original β-Si₃N₄ whiskers in a matrix of fine SiAlON grains. SEM of plasma-etched samples and TEM analysis showed that the whiskers, as a result of grain growth, consisted of two phases, a core and a sheath layer. X-ray mapping and EDS analysis revealed that the core material contained no trace of Al, confirming the presence of original β-Si₃N₄ whiskers. The composition of the sheath was qualitatively identical to that of the fine β' SiAlON grains in the matrix. The sheath was thus formed by the precipitation of the β'SiAlON during liquid-phase sintering and led to substantial growth of the whiskers. Microdiffraction showed that the β'SiAlON grew epitaxially on the β-Si₃N₄ whiskers, resulting in a heavily faulted SiAlON layer.     

High Thermal Conductivity in Silicon Nitride with Anisotropie Microstructure

Silicon nitride was fabricated by tape casting of α-Si₃N₄ powder with 5 wt% Y₂O₃ and 5 vol% rodlike β-Si₃N₄ seed particles, followed by tape stacking, hot pressing under 40 MPa, and annealing at 1850°C for 2-66 h under a nitrogen pressure of 0.9 MPa. Silicon nitrides fabricated by this procedure exhibited a highly anisotropic microstructure with large elongated grains (developed from seed particles) uniaxially oriented parallel to the casting direction. Thermal conductivities parallel to the grain alignment were much higher than those measured in other directions and exhibited high values of up to 120 W/(m.K). The anisotropic thermal conductivity of the specimen could be explained by the rule of mixture, considering that large elongated grains developed from seeds have higher thermal conductivity than a small-grained matrix.     

α-β Sialon Ceramics Made from Different Silicon Nitride Powders

The present work is concerned with the sintering of an α-β sialon ceramic using five different silicon nitride powders from a single source. The parameters varied in the silicon nitride were the amount of "free' silicon, iron content, α:β ratio, and grain size as measured by BET surface. The sintering atmosphere was varied by use of protective powder beds with passive (boron nitride) and active (SiO-generating) properties. Five sintering temperatures between 1600° and 1800°C were used. Microstructural characterization as well as density, hardness, and fracture toughness measurements were carried out. The sintering conditions were found to be critical for obtaining fully dense materials and low weight change. The optimum sintering temperature was 1750°C. The silicon nitride powder with a high content of free silicon resulted in a material which was more susceptible to the sintering atmosphere conditions. An α-β sialon made from a silicon nitride powder with a high β-α phase ratio resulted in a higher β-α ratio in the sintered material.     

Hot Hardness Behavior of Yttrium Sialon Ceramics

The hot hardness of polycrystalline single-phase α- or β- sialon ceramics declines with increasing temperature, but the measured Vickers hardness (HV1) at 1100°C is still about 1550 and 1300 for the α-sialon and the low-substituted β-sialon materials, respectively. The hardness of 'composite'β- or α-β-sialon ceramics containing a high volume fraction of glassy phase is lower at all temperatures and drops significantly above about 900°C.     

Improvement of Mechanical Properties and Corrosion Resistance of Porous β-SiAlON Ceramics by Low Y2O3 Additions

Addition of Y₂O₃ as a sintering additive to porous β-SiAlON (Si_{6− z} Al _z O _z N_{8− z} , z = 0.5) ceramics has been investigated for improved mechanical properties. Porous SiAlON ceramics with 0.05–0.15 wt% (500–1500 wppm) Y₂O₃ were fabricated by pressureless sintering at temperatures of 1700°, 1800°, and 1850°C. The densification, microstructure, and mechanical properties were compared with those of Y₂O₃-free ceramics of the same chemical composition. Although this level of Y₂O₃ addition did not change the phase formation and grain size, the grain bonding appeared to be promoted, and the densification to be enhanced. There was a significant increase in the flexural strength of the SiAlON ceramics relative to the Y₂O₃-free counterpart. After exposure in 1 M hydrochloric acid solution at 70°C for 120 h, no remarkable weight loss and degradation of the mechanical properties (flexural and compression strength) was observed, which was attributed to the limited grain boundary phase, and with the minor Y₂O₃ addition the supposed formation of Y-α-SiAlON.     

A direct method for regiospecific analysis of TAG using alpha-MAG

An analytical procedure was developed for regiodistribution analysis of TAG using α-MAG prepared by an ethyl magnesium bromide deacylation. In the present communication, the deacylation procedure is shown to lead to representative α-MAG, allowing the composition of the native TAG in the α-position to be determined directly. The composition in the β-position can then be estimated from the composition of the α-MAG and TAG according to the formula, 3×TAG-2×α-MAG. The estimates are superior to those obtained using the α,β-DAG and Brockerhoff calculations as they come closer to the theoretical value and have smaller SD. The present procedure, first demonstrated on a synthetic TAG, was then successfully applied to the analysis of borage oil, milkfat, and tuna oil.     

Role of organic additives on non-aqueous tape casting of SiAlON ceramics

Suspensions consisting of precursor α/β SiAlON forming powders, azeotropic solvent mixture of 60 MEK/40E, dispersant, binder, and plasticizer were optimized for tape casting by rheological measurements and tape properties. Sodium tripolyphosphate (STPP) was introduced as a dispersant for low temperature applications of α/β-SiAlONs. Optimum STPP amount was determined as 0.012 g/m² (of the particle surface) for stable α/β-SiAlON suspensions. Different amounts of binder/plasticizer mixtures were added to the slurries and the effects on rheological and green tape properties were investigated. Green tapes with dibutyl phthalate (DBP), and plasticizer mixture, poly(ethylene glycol) (PEG) and DBP, exhibited centered cracks with high plasticity, on the other hand, polyvinyl butral (PVB) and PEG showed no crack but low plasticity. Therefore, many different parameters were found to be effective on final tape properties. In addition, tapes were prepared with 6 vol% PVB + PEG, sintered at 1800 °C for 2 h and exhibited almost 97%TD in room temperature applications of α/β SiAlONs.     

Microstructure and Creep Behavior of Silicon Nitride and SiAlONs

Microstructural evolution of silicon nitride (Si₃N₄) and SiAlON materials and its influence on creep resistance is reviewed. Grain size, grain morphology, and the ratio of α- to β-phase grains play a part in resistance to creep. The glassy, intergranular phase typically has the strongest influence on creep. Creep data are usually obtained using uniaxial tensile or compressive tests, where creep in tension is controlled by cavitation and grain boundary sliding controls creep in compression. The impression creep methodology is also reviewed. An additional creep mechanism, dilation of the SiAlON grain structure, was found to be active in impression creep.