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).     

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).     

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.     

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.     

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.     

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.     

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.     

Lower-Temperature Hot-Pressed Dy-α-Sialon Ceramics with an LiF Additive

Hot-pressed Dy-α-sialon ceramics, using LiF as a sintering additive, were fabricated at lower temperatures (≤1650°C). Some of the densified samples possessed higher hardness and fracture toughness up to 19.00–20.00 GPa and 4.00–6.00 MPa·m^1/2, respectively. The amount of LiF had a strong effect on the densification behavior in sialon preparation. As one of the experimental results, the sample with 0.1 wt% of LiF additive sintered at 1600°C produced an optical translucence of about 50% in the range of 1.5–5.0 μm wavelengths. The maximum infrared transmission reached ∼60% at a wavelength of 2.4 μm. It is inferred that these more easily sintered materials would be practical for optical applications in certain fields.     

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.     

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.     

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 heating rate on microstructure and properties of spark plasma sintered Li-α-sialon

Li-α-sialon ceramics with low oxygen content were prepared by spark plasma sintering at 1750 °C, using three heating rates of 100 °C/min, 200 °C/min and 300 °C/min. In all cases, the densification of Li-α-sialon ceramics is effectively promoted. The rapidly anisotropic growth of grains, either α-sialon or β-sialon, is significantly enhanced with a heating rate above 200 °C/min. This could be attributed to a large amount of low viscosity oxygen-rich liquid formed at elevated temperatures, which gives rise to a so-called dynamic ripening mechanism at the early stage of sintering. Furthermore, for the composition near α-sialon boundary, the oxygen-rich liquid results in the formation of β-sialon together with a large amount of intergranular glassy phase. Only the Li-doped sialon with low lithium and oxygen content possesses relatively high infrared transmittance when sintered at a rate of 100 °C/min.     

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.     

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.     

Microstructural Control for Strengthening of Silicon Carbide Ceramics

Fine-grained (<1 μm) silicon carbide ceramics with high strength were obtained by using ultrafine (∼90 nm) β-SiC starting powders and a seeding technique for microstructural control. The microstructures of the as-hot-pressed and annealed ceramics without α-SiC seeds consisted of fine, uniform, and equiaxed grains. In contrast, the annealed material with seeds had a uniform, anisotropic microstructure consisting of elongated grains, owing to the overgrowth of β-phase on α-seeds. The strength, the Weibull modulus, and the fracture toughness of fine-grained SiC ceramics increased with increasing grain size up to ∼1 μm. Such results suggested that a small amount of grain growth in the fine grained region (<1 μm) was beneficial for mechanical properties. The flexural strength and the fracture toughness of the annealed seeded materials were 835 MPa and 4.3 MPa·m^1/2, respectively.     

Preparation and study of the mechanical and optical properties of infrared transparent Y2O3–MgO composite ceramics

In this study, fine Y₂O₃–MgO composite nanopowders were synthesized via the sol–gel method. Dense Y₂O₃–MgO composite ceramics were fabricated by pre-sintering the green body in air at different temperatures for 1 h and then subjecting the sintered bodies to hot isostatic pressing at 1300°C for 1 h. The effects of pre-sintering temperature on the microstructural, mechanical, and optical properties of the resulting ceramics were studied. The average grain size of the ceramics was increased, whereas their hardness and fracture toughness were decreased with increasing pre-sintering temperature. A maximum fracture toughness of 1.42 MPa·m^1/2 and Vickers hardness of 10.4 GPa were obtained. The average flexural strength of the ceramics was 411 MPa at room temperature and reached 361 MPa at 600°C. A transmittance of 84% in the 3–5 µm region was obtained when the composite ceramics were sintered at 1400°C. Moreover, a transmittance of 76% in the 3–5 µm region was obtained at 500°C.     

Wear properties of Y–α/β composite sialon ceramics

The tribological properties of yttrium containing α/β composite sialon ceramics have been studied under non-lubricated conditions by means of block-on-ring and ball-on-disk type experiments against a commercial silicon nitride material. The sialon ceramics were produced by hot pressing powder mixtures of Si₃N₄, AlN, Al₂O₃ and Y₂O₃, resulting in composite ceramics containing different amounts of the α/β phases. The effects of microstructural differences on the mechanical properties of the ceramics, and their wear characteristics under a range of testing conditions have been assessed. It was found that Vickers hardness decreased whilst both fracture toughness and bending strength increased with increasing amount of β phase in the composite. Under mild testing conditions, material removal was considered to occur by polishing of the surface, and in this case the high α-sialon composites exhibited the highest wear resistance, reflecting their higher hardness. Under severe testing conditions, the wear behaviour was characterised as microcracking caused by the higher Hertzian stress levels, and resulted in grain removal or “dropping” from the surface of the materials. Under these conditions, the elongated microstructure and higher fracture toughness of the low α-sialon composites hinder the crack propagation and result in better wear characteristics when compared to the fine equiaxed α-sialon materials.     

Fabrication of Hard and Strong α/β-Si3N4 Composites With MgSiN2 as Addivitives

α/β-Si₃N₄ composites with various α/β phase ratios were prepared by hot pressing at 1600°–1650°C with MgSiN₂ as sintering additives. An excellent combination of mechanical properties (Vickers indentation hardness of 23.1 GPa, fracture strength of about 1000MPa, and toughness of 6.3 MPa·m^1/2) could be obtained. Compared with conventional Si₃N₄-based ceramics, this new material has obvious advantages. It is as hard as typical in-situ-reinforced α-Sialon, but much stronger than the latter (700 MPa). It has comparable fracture strength and toughness, but is much harder than β-Si₃N₄ ceramics (16 GPa). The microstructures and mechanical properties can be tailored by choosing the additive and controlling the heating schedule.     

Effect of composition on phase assemblage,microstructure, mechanical and optical properties of Mg-doped sialon

Mg-doped sialon (Mg_m/2Si_12−m−nAl_m+nO_nN_16−n) ceramics with different compositions of m = 2n = 0.6, 0.84, 1.0, 1.2, 1.6 were hot pressed at 1850 °C for 1 h. Phase assemblage, microstructure, mechanical and optical properties of these samples were investigated. All samples achieved/approached full densification. However, the densification of Mg-doped sialon ceramics with higher MgO/AlN content becomes more difficult. Additionally, the anisotropic growth of β-sialon grains was significantly inhibited. The unique characteristics of Mg-doped sialon ceramics intrinsically derive from the formation of Mg-containing AlN polytypoids, which consumed most of the high-temperature liquid. Furthermore, their high stability at high temperatures accounts for the difficulty in preparing single-phase Mg-α-sialon., The hardness of these samples gradually increases while indentation fracture toughness gradually decreases with increasing m = 2n value. Due to little residual glassy phase, high infrared transparency/translucency was more readily achieved in Mg-doped sialon. The m = 1.2 sample possesses the maximum transmittance of ∼50% at ∼2 μm.     

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.