Effect of the Amount of Additives and Post-Heat Treatment on the Microstructure and Mechanical Properties of Yttrium–α-Sialon Ceramics

The yttrium–sialon ceramics with the composition of Y_0.333Si₁₀Al₂ON₁₅ and an excess addition of Y₂O₃ (2 or 5 wt%) were fabricated by hot isostatic press (HIP) sintering at 1800°C for 1 h. The resulting materials were subsequently heat-treated in the temperature range 1300–1900°C to investigate its effect on the α→β-sialon phase transformation, the morphology of α-sialon grains, and mechanical properties. The results show that α-sialons stabilized by yttrium have high thermal stability. An adjustment of the α-sialon phase composition is the dominating reaction in the investigated Y–α-sialon ceramics during low-temperature annealing. Incorporation of excess Y₂O₃ could effectively promote the formation of elongated α-sialon grains during post-heat-treating at relatively higher temperature (1700° and 1900°C) and hence resulted in a high fracture toughness ( K _IC= 6.3 MPa·m^1/2) via grain debonding and pullout effects. Although the addition of 5 wt% Y₂O₃ could promote the growth of elongated α grains with a higher aspect ratio, the higher liquid-phase content increased the interfacial bonding strength and therefore hindered interface debonding and crack deflection. The heat treatment at 1500°C significantly changed the morphology of α-sialon grains from elongated to equiaxed and hence decreased its toughness.     

Optical and Mechanical Properties of α/β Composite Sialons

Two-phase α/β composites have been produced with a combination of high hardness, fracture toughness, and strength. Compared with a single-phase α-sialon, the composite showed around a twofold increase in both fracture toughness and bending strength, with only minimal reduction in hardness. Despite being a two-phase material, the optical properties of the composite were very good, showing transparency in sections of around 0.5 mm thickness. The optical properties were in fact better for the composite than for the single-phase α-sialon. Work to date on transparent sialons has focused on single-phase α-materials, which have inherently low fracture toughness unless elongated microstructures are developed. However, this microstructural development appears to adversely affect optical transparency. In this work it has been shown that good combination of mechanical properties can be achieved while maintaining optical transparency in two-phase composite sialons. The development of such materials should widen their range of application.     

Phase Relationships and Related Microstructural Observations in the Ca-Si-Al-O-N System

A systematic set of samples with compositions in the Ca alpha-SiAlON (α') system has been fabricated using pressureless sintering to evaluate the phase relationships on, and near, the Ca α' plane. A phase behavioral diagram for the Ca α' plane within the Jänecke prism has been determined and found to be similar to those for the Y and Sm α' systems. However, distinct differences were also observed; two additional regions (α'+ 27R + liquid) and (α'+ 33R + liquid) have been identified. Investigation of the microstructures revealed elongated Ca α' grains in the samples prepared by pressureless sintering. The aspect ratio of the elongated grains of α' increased as the concentration of stabilizing cation was increased. Single crystalline phase Ca α' samples of comparable density to hot-pressed material were fabricated successfully using pressureless sintering.     

Translucent α-Sialon Ceramics by Hot Pressing

The single-phase α-sialon ceramics with high optical transmittance have been prepared by hot pressing. The maximum transmittance reached 65.2% and 52.2% in the infrared wavelength region, 58.5% and 40% in the visible region for the samples 1.0 and 1.5 mm thickness, respectively. The material also exhibited good mechanical properties of high hardness (20 GPa) and better fracture toughness (5.1 MPa·m^1/2). Both high optical transmittance and improved toughness of α-sialon ceramics were attributed to the less-grain-boundary glassy phase and the homogeneous microstructure, which was obtained by a proper process and confirmed by SEM and TEM observation, compared to that prepared by ordinary sintering. It is, therefore, expected that the translucent α-sialon ceramics could be a promising optical window material.     

Optical Properties of Gd–α-Sialon Ceramics: Effect of Carbon Contamination

Thick translucent and luminescent Gd–α-sialon ceramic disks (0.7–1.06 mm in thickness) were prepared by hot pressing. The effect of carbon atmosphere on their optical properties during sintering was explored by change packing methods. The results show that the sample with a lower carbon contamination has a higher translucence in the visible band and IR band (450–3500 nm), increasing transmission around 10% even if it is thicker. When excited at 350 nm, Gd–α-sialon with the lower carbon contamination can produce a visible light at 450–500 nm bands, but the luminescence is very weak in the sample containing more carbon contamination. These indicate that carbon contamination causes a severe degradation of the optical properties of α-sialon ceramics, and reduction of carbon contamination of α-sialon ceramics is very important for the optical property improvement.     

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).     

Highly Texturing β-Sialon Via Strong Magnetic Field Alignment

This paper reports the texturing behavior of β-sialon by strong magnetic field alignment (SMFA) during slip casting, followed by reaction pressureless sintering, using either α or β-Si₃N₄, Al₂O₃, and AlN as the starting materials. It is found that the β-Si₃N₄ crystal exhibits a substantially stronger orientation ability than the α-Si₃N₄ crystal regardless of the Si₃N₄ raw powders in the magnetic field of 12 T. The β-raw powder produces a highly a , b -axis-oriented β-Si₃N₄ green body with a Lotgering orientation factor of up to 0.97. During sintering, the β-raw powder allows the a , b -axis-oriented β-sialon to retain the Lotgering orientation factor similar to and even higher than that of β-Si₃N₄ in the green body. In contrast, the α-raw powder leads to a faster transformation rate of α/β-Si₃N₄ to β-sialon but a substantially lower texture in β-sialon. The results indicate that the use of the β-raw powder is more efficient for producing highly textured β-sialon via SMFA than that of the α-raw powder as well as the prolonged sintering.     

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.     

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.     

Phase Assemblages of Pr α-sialon Derived from SHS-ed Powders and TEM Study on the Nucleation of Pr α-sialon

The Pr α-sialon powders prepared by self-propagating high-temperature synthesis (SHS), consisting of 55 wt% Pr α-sialon and 45 wt% of β-sialon (abbreviated as α' and β'), were hot-pressed at 1800°C for 1 h. The results showed that Pr α' phase would transfer to β' with the appearance of JEM phase (Pr(Si_{6− z} Al _z )(N_{10− z} O _z )) after sintering, thus resulting in the increase of β' phase to 86 wt%. The addition of Y₂O₃ into SHS-ed Pr α' powders as the starting materials restrains the transformation of α' to β' and prevents the formation of JEM phase as well. The nucleation mechanism of Pr α' grain during hot-pressing was investigated in terms of transmission electron microscope and energy-dispersive spectrometer analysis. Two nucleation modes of Pr α' grains were found, i.e., nucleating on the undissolved Pr α' grains and on the nuclei of (Pr, Y) α' grains precipitated from liquid phase.     

Corrosion Behavior of Sialon Ceramics in Supercritical Water

The corrosion behavior of sialon ceramics was investigated in supercritical water at 450°C under 45 MPa for 2–50 h. α-sialon exhibited better corrosion resistance than β-sialon and α/β-sialon. Pitting corrosion with the formation of corrosion products was observed in the case of β-sialon and α/β-sialon. By contrast, the corrosion behavior of α-sialon was characterized by uniform corrosion with the formation of corrosion products. The degree of strength deterioration was strongly dependent on the corrosion morphology. The bending strength of α-sialon after corrosion for 30 h was about 90% of its initial strength, while the strength of β-sialon decreased to 65% of its original strength.     

Translucent Mg-α-Sialon Ceramics Prepared by Spark Plasma Sintering

The translucent Mg-α-sialon ceramics have been prepared by spark plasma sintering (SPS) α-Si₃N₄ powder with AlN and MgO as the additives at 1850°C for 5 min. The sample possesses a uniform, dense microstructure under the rapid densification of SPS process. The translucent Mg-α-sialon ceramics achieve the maximum transmittance of 66.4% for the sample of 0.5 mm in thickness in the medium infrared region, which could be attributed to the equiaxed microstructure and few glassy phase confirmed by the observation of transmission electron microscopy. The material also exhibits good mechanical properties of high hardness (21.4±0.3 GPa) and fracture toughness (6.1±0.1 MPa·m^1/2).     

In S/Yu-Toughened Silicon Carbide-Titanium Carbide Composites

A process based on liquid-phase sintering and subsequent annealing for grain growth is presented to obtain in situ -toughened SiC-30 wt% TiC composites. Its microstructures consist of uniformly distributed elongated α-SiC grains, matrixlike TiC grains, and yttrium aluminum garnet (YAG) as a grain boundary phase. The composites were fabricated from β-SiC and TiC powders with the liquid forming additives of A1₂O₃ and Y₂O₃ by hot pressing. During the subsequent heat treatment, the β→α phase transformation of SiC led to the in situ growth of elongated α-SiC grains. The fracture toughness of the SiC-30 wt% TiC composites after 6-h annealing was 6.9 MPa-m^1/2, approximately 60% higher than that of as-hot-pressed composites (4.4 MPa-m^1/2). Bridging and crack deflection by the elongated α-SiC grains appear to account for the increased toughness of this new class of composites.     

Microstructure and mechanical properties of multi-cation containing α-sialons

Dense Y-, Yb- and (Y + Yb)-doped α-sialons containing 2 wt.% extra liquid phase were fabricated by hot pressing. The results show that the elongated grains morphology can be obtained by partially substituting Yb with Y as a modifying cation, and hence improve the toughness of the materials. An additional intermediate holding step during sintering has no obvious effect on the microstructures and the mechanical properties. Post-heat treatment can further facilitate the growth of elongated α-sialon grains and increase the toughness of the materials. All the materials exhibited very similar high hardness of over 20 GPa.     

Densification Behavior in Microwave-Sintered Silicon Nitride at 28 GHz

Si₃N₄ powders were sintered using a 28 GHz gyrotron source, with Y₂O₃, Al₂O₃, and MgO as sintering aids, in an attempt to investigate the effect of microwave radiation on densification behavior. The microwave-sintered samples were compared with identical samples produced by conventional pressureless sintering. The effect of sintering on the microstructural development and grain growth of the samples was assessed using scanning electron microscopy. Phase transformation behavior was assessed using X-ray diffractometry. In the microwave-sintered samples, densification and α→β transformation occurred at temperatures ∼200°C lower than those of the conventionally sintered samples. More importantly, at comparable stages of densification, the microstructures of the microwave-sintered and conventionally sintered samples were significantly different, with the microwave-sintered samples showing the development of elongated β grains at a much earlier stage of the α→β transformation. It was concluded that the effect of microwave radiation on sintering was not simply a decrease in sintering temperatures, but in possibly a different sintering mechanism, clearly related to localized heating within the grain-boundary phase.     

Inhomogeneous grain growth and elongation of Dy-α sialon ceramics at temperatures above 1800°C

A series of samples with dysprosium α-sialon compositions and different amounts of sintering additives has been fabricated from α-Si₃N₄, AlN, Al₂O3 and Dy₂O₃ starting powders, using pressureless sintering (PLS) at 1800°C plus gas pressure sintering (GPS) at the same and higher temperatures under a relatively low gas pressure of 0.9 MPa N₂. The resultant α-sialon grains show significantly different features, such as regularly fine and equiaxed, elongated, and even a few extraordinarily large with high aspect ratio grains in the fine matrix, which has rarely been observed in α′ ceramics. It is suggested that the temperature strongly influences the grain morphologies of α-sialon, playing an important role particularly in the latter stage of the “nucleation-growth” process. Such a kind of microstructural morphology as mixed with equiaxed and elongated grains toughens the α-sialon ceramics and leaves them still hard as usual. Overly high temperature treatment leads to the preferential growth of a few grains which become excessively large and long, and do not significantly improve the materials toughness.     

In Situ-Toughened Silicon Carbide

A new processing strategy based on atmospheric pressure sintering is presented for obtaining dense SiC-based materials with microstructures consisting of (i) uniformly distributed elongate-shaped α-SiC grains and (ii) relatively high amounts (20 vol%) of second-phase yttrium aluminum garnet (YAG). This strategy entails the sintering of β-SiC powder doped with α-SiC, Al₂O₃, and Y₂O₃. The Al₂O₃ and Y₂O₃ aid in the liquid-phase sintering of SiC and form in situ YAG, which has a significant thermal expansion mismatch with SiC. During a subsequent grain-growth heat treatment, it is postulated that the α-SiC "seeds" assist in controlling in situ growth of the elongated α-SiC grains. The fracture pattern in the in situ -toughened SiC is intergranular with evidence of copious crack-wake bridging, akin to toughened Si₃N₄ ceramics. The elongate nature of the α-SiC grains, together with the high thermal-residual stresses in the microstructure, enhance the observed crack-wake bridging. This bridging accounts for a measured twofold increase in the indentation toughness of this new class of in situ -toughened SiC relative to a commercial SiC.     

Plasma Etching of α-Sialon Ceramics

Plasma etching of β-Si₃N₄, α-sialon/β-Si₃N₄ and α-sialon ceramics were performed with hydrogen glow plasma at 600°C for 10 h. The preferential etching of β-Si₃N₄ grains was observed. The etching rate of α-sialon grains and of the grain-boundary glassy phase was distinctly lower than that of β-Si₃N₄ grains. The size, shape, and distribution of β-Si₃N₄ grains in the α-sialon/β-Si₃N₄ composite ceramics were revealed by the present method.     

Investigation on the microstructure of self-reinforced Nd-α-sialon ceramics

α-Sialon ceramics doped with Nd₂O₃ were prepared by hot-pressing sintering at 1900 °C holding for 1 h with a heat preservation for 1 h at 1500 °C. Microscopic observations indicate that elongated α-sialon grains appear and are embedded in the fine equiaxed-grain matrix. A small amount of β-sialon phases and secondary crystallized phases M′ (Nd₂Si_3−xAl_xO_3+xN_4−x) also exist in the Nd-α-sialon ceramic. A core/shell structure can be found in the elongated α-sialon and β-sialon grains with a high aspect ratio, in which some misfit dislocations surround the core. The 21R AlN-polytype was observed by transmission electron microscopy (TEM), which could not be detected by X-ray diffraction (XRD) due to its trace amount. Different nucleation and growth modes of sialon grains were also discussed.     

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.