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.     

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.     

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.     

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.     

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.     

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.     

Thermomechanical Properties of β-Sialon Synthesized from Kaolin

β-Sialon powder was synthesized by the simultaneous reduction and nitridation of Hadong kaolin at 1350°C in an N₂–H₂ atmosphere, using graphite as a reducing agent. The average particle size of β-sialon powder was about 4.5 μm. The synthesized β-sialon powder was pressureless sintered from 1450° to 1850°C under a N₂ atmosphere. The relative density, modulus of rupture, fracture toughness, and microhardness of β-sialon ceramics sintered at 1800°C for 1 h were 92%, 248 MPa, 2.8 MN/m^3/2, and 13.3 GN/m², respectively. The critical temperature difference (ΔT_c) in water-quench thermal-shock behavior was about 375°C for the synthesized β-sialon ceramics.     

CEramics based on α-sialon

Ceramics containing pure α-sialon and α-sialon in combination with β-Si₃N₄, produced from two types of silicon nitride powder, are described. It is shown that the high-temperature strength of the ceramics increases with increase in the content of α-sialon in the composition. Data on the microstructure, phase composition, and mechanical characteristics of the synthesized materials are presented.     

Yttrium α-Sialon Ceramics by Hot Isostatic Pressing and Post-Hot Isostatic Pressing

Dense α-sialon materials were produced by hot isostatic pressing (HIP) and post-hot isostatic pressing (post-HIP) using compositions with the formula Y _x (Si_{12–4.5 x} , Al_{4.5 x} )-(O_{1.5 x} ,N_{16–1.5 x} ) with 0.1 ≤ x ≤ 0.9 and with the same compositions with extra additions of yttria and aluminum nitride. X-ray diffraction analyses show how the phase content changes from large amounts of β-sialon ( x = 0.1) to large amounts of α-sialon ( x = 0.4) and increasing amounts of mellilite and sialon polytypoids ( x = 0.8). Samples HIPed at 1600°C for 2 h contained unreacted α-silicon nitride, while those HIPed at 1750°C for 1 h did not. This could be due to the fact that the time is to short to achieve equilibrium or that the high pressure (200 MPa) prohibits α-sialon formation. Sintering at atmospheric pressure leads to open porosity for all compositions except those with excess yttria. Therefore, only samples with excess yttria were post-HIPed. Microstructrual analyses showed that the post-HIPed samples had the highest α-sialon content. A higher amount of α-sialon and subsequently a lower amount of intergranular phase were detected at x = 0.3 and x = 0.4 in the post-HIPed samples in comparison to the HIPed. The hardness (HV10) and fracture toughness ( K _IC) did not differ significantly between HIPed and post-HIPed materials but vary with different x values due to different phase contents. Measurements of cell parameters for all compositions show a continuous increase with increasing x value which is enhanced by high pressure at high x values.     

Comparison of the Luminescence Properties of Dy3+ in α-Sialon and Oxynitride Glass

Dy-α-sialon and β-Si₃N₄ materials containing Dy-oxynitride glass were hot pressed at 1800°C for 1 h. The luminescence spectra of Dy³⁺ in these samples were compared when excited at 350 nm. The results showed that two strong emission bands in the region 470–500 nm and 570-600 nm, associated with the ⁴F_9/2→⁶H_15/2 and ⁴F_9/2→⁶H_13/2 transitions of Dy³⁺ ions, were observed in Dy-α-sialon. However, no emission peak was detected from the β-Si₃N₄ sample, despite it containing the same amount of Dy³⁺ cations. This proved that only the Dy³⁺ cations in the α-sialon structure, not those in the oxynitride glass, produce the luminescence spectrum.     

Early-Stage Thermal Oxidation of Carbothermal β-Sialon Powder

Early-stage thermal oxidation (below 1100°C) of carbothermally synthesized β-sialon powder was monitored by X-ray powder diffraction, solid-state ²⁹Si and ²⁷Al MAS NMR spectroscopy, and thermogravimetry. No crystalline oxidation products were detected by XRD but ²⁹Si and ²⁷Al MAS NMR indicated the early formation of amorphous silica, followed by the formation of an amorphous aluminosilicate with an atomic environment similar to that of mullite. The initial oxidation was described by a linear kinetic law with an activation energy of 170 kJmol⁻¹, suggesting the rate-limiting step to be due to dissolution of O₂ in an amorphous silica surface layer on the β-sialon particles.     

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.     

Homogeneity Region and Thermal Stability of Neodymium-Doped α-Sialon Ceramics

Dense sialon ceramics along the tie line between Si₃N₄ and Nd₂O₃·9AlN were prepared by hot-pressing at 1800°C. The materials were subsequently heat-treated in the temperature range 1300–1750°C and cooled either by turning off the furnace (yielding a cooling rate (T_cool) of ∼50°C/min) or quenching (T_cool≥ 400°C/min). It was found necessary to use the quenching technique to reveal the true phase relationships at high temperature, and it was established that single-phase α-sialon forms for 0.30 x 0. 51 in the formula Nd_xSi_{12–4S x} Al_{4.5 x} O_{1. 5 x} , N_{16–1.5 x} . The α-sialon is stable only at temperatures above 1650°C, and it transforms at lower temperatures by two slightly different diffusion-controlled processes. Firstly, an α-sialon phase with lower Nd content is formed together with an Al-containing Nd-melilite phase, and upon prolonged heat treatment thus-formed α-sialon decomposes to the more stable β-sialon and either the melilite phase or a new phase of the composition NdAl(Si_6-zAl_z)N_10-zO_z. Nd-doped α-sialon ceramics containing no crystalline intergranular phase show very high hardness (HV10 = 22. 5 GPa) and a fracture toughness ( K _lc= 4.4 MPa·m^1/2) at room temperature. The presence of the melilite phase, which easily formed when slow cooling rates were applied or by post-heat-treatment, reduced both the fracture toughness and hardness of the materials.     

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.     

Ionic Conductivity of the System Si-Al-O-N from 850° to 1400°C

Electrical conductivity was measured from 850° to 1400°C for β-sialon and pure X phase as well as for the sintered system Si₃N₄-Al₂O₃, containing β-sialon, X phase, β-Si₃N₄, and glassy phase. Ionic conductivity was measured at >1000°C. The charge carriers were identified by electrolysis. The results showed that pure β-sialon is ionically conducting because of Si⁴⁺ migration for the temperature range studied. Pure X phase shows ionic conduction by Si⁴⁺ above 1000°; below 1000°C, it shows electronic conduction because of impurities. The conductivity of the sintered system Si₃N₄-Al₂O₃ containing β-sialon, β-Si₃N₄ X phase, and glassy phase changes as the relative quantities of β -sialon and X phase change. The apparent activation energies for the ionic and electronic conductivities are 45 and 20 kcal/mol, respectively.     

Control of Microstructures in α-SiAlON Ceramics

The effects of sintering cycles and doping elements on the microstructures of Ln-α-sialon were studied. The results showed that microstructures with an elongated α-sialon morphology could be obtained through high-temperature post-heat treatment (1800–1900°C) or by prolonging soaking times during sintering. Different rare-earth elements had a profound effect on the microstructure of the resulting α-sialon. The Ln-α-sialon doped with low- Z -value elements could easily develop elongated grains with higher aspect ratio.     

Melilite Formation in a Samarium-Stabilized α-Sialon Ceramic during Postsintering Heat Treatments

The formation of the melilite solid solution phase (M'), Sm₂Si_3−xAl_xO_3+xN_4−x, in an α-sialon sample of overall composition Sm_0.6Si_9.28Al_2.69O_1.36N_14.76, was studied as a function of time in the temperature interval 1375–1525°C. The alpha-sialon ceramic contained only minor amounts of the 21R sialon polytype and some residual grain-boundary glass before heat treatment. In situ studies by high-temperature X-ray diffraction were combined with postsintering heat treatment followed by quenching. The M'-phase was found to be formed by two different mechanisms: either crystallization of the residual grain-boundary liquid or a direct decomposition of the α-sialon phase. The liquid crystallized during the first 10–15 min of heat treatment, yielding a rapid M'-phase formation, and further formation of M'-phase continued at a much slower rate, related to the decomposition of α-sialon.     

Synthesis of β-Sialon from a Montmorillonite-Polyacrylonitrile Intercalation Compound by Carbothermal Reduction

To develop a new type of carbothermal reduction for the preparation of nitrides, a montmorillonite-polyacrylonitrile intercalation compound was heated inflowing N2 using intercalated polyacrylonitrile as a reducing agent. Nitridation occurred at >1150°C, and β-sialon was obtained, with the formation of β-Sic and AlN.