Spark plasma sintering behavior of combustion-synthesized (Y,Ca)-α-SiAlON

This paper describes the sintering behavior of combustion-synthesized (Y, Ca)-α-SiAlON powders when using spark plasma sintering (SPS) technology. The effects of sintering temperature, heating rate, and holding time on the densification behavior, α→β phase transformation, and Vickers hardness were investigated in detail. Fully dense Y-α-Si_12−(m+n)Al_m+nO_nN_16−n (m=1.2, n=0.6, Y-α-SiAlON) and Ca-α-Si_12−(m+n)Al_m+nO_nN_16−n (m=1.0, n=0.5, Ca-α-SiAlON) were obtained during sintering for 10 min at final temperatures as low as 1400 °C and 1500 °C, respectively. X-ray diffraction results showed that more than 50 mass% of α-phase transformed to the β-phase for Y-α-SiAlON after SPS, whereas no obvious α→β phase transformation was observed for Ca-α-SiAlON, even after sintering at a high temperature of 1600 °C. A maximum Vickers hardness of 18.56 GPa and 19.95 GPa was reached for Y-α-SiAlON and Ca-α-SiAlON, respectively.     

Spark plasma sintering of combustion-synthesized β-SiAlON powders

In this paper, the sintering behavior of β-Si_6−zAl_zO_zN_8−z (z=1) powder prepared by combustion synthesis (CS) was studied using spark plasma sintering (SPS). The CSed powder was ball milled for various durations from 0.5 to 20 h and was then sintered at different temperatures with heating rates varying from 30 °C/min to 200 °C/min. The effects of ball milling, sintering temperature, and heating rate on sinterability, final microstructure, and mechanical property were investigated. A long period of ball milling reduced the particle size and subsequently accelerated the sintering process. However, the fine powder was easily agglomerated to form secondary particles, which accordingly decreased the densification of the SPS product. The high sintering temperature accelerated the densification process, whereas the high heating rate reduced the grain growth and increased the relative density of the sintered product.     

Diffusion Bonding of Silicon Nitride Using a Superplastic β-SiAlON Interlayer

A superplastic β-SiAlON was used as an interlayer to diffusionally bond a hot-pressed silicon nitride to itself. The bonding was conducted in a graphite furnace under a constant uniaxial load of 5 MPa at temperatures varying from 1500° to 1650°C for 2 h, followed by annealing at temperatures in the range of 1600° to 1750^oC for 2 h. The bonds were evaluated using the four-point-bend method at both room temperature and high temperatures. The results indicate that strong, void-free joints can be produced with the superplastic β-SiAlON interlayer, with bond strengths ranging from 438 to 682 MPa, and that the Si₃N₄ joints are heat resistant, being able to retain their strength up to 1000°C (635 MPa), and therefore have potential for high-temperature applications.     

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.     

Influence of SiC addition on tribological properties of SiAlON

The tribological properties of gas pressure sintered SiAlON and its composite with 18 wt% silicon carbide (SiC) against two different mating materials, i.e., alumina and SiAlON are evaluated. SiAlON and SiAlON–18%SiC composite ceramics were prepared by pressure less sintering and gas pressure sintering. Fretting wear tests were carried out under dry unlubricated ambient conditions (room temperature 23–25 °C; relative humidity 50–55%) with a load of 8 N for 45,000 cycles. Friction and wear properties of SiAlON–SiC proved better than the monolithic SiAlON. The formation of silica roll like structure on the composite worn surface was observed.     

Effect of Composition and Crystallization Temperature on Microstructure of Y- and Er-SiAlON Iw-Phase Glass-Ceramics

Glass-ceramics were produced from Y- and Er-SiAlON compositions containing the I_w phase as the only detectable crystalline phase. TEM analysis showed that these materials had fine-scale glass-ceramic microstructures with crystallite sizes <1 μm and glass contents between 20% and 40%, depending on the starting powder composition: Increased yttrium or erbium content of the starting material resulted in a reduced glass content. The I_w-phase crystals in all specimens were found by EDX analysis in TEM to display a solid-solution range with a clear anticorrelation between the yttrium or erbium content and the silicon content: A possible explanation for this effect was suggested. The average crystal composition approximated a cation ratio of Y:Si:Al = 3:2:1. The composition of the residual glass was also similar in all specimens. Some small silicon-rich amorphous features were found in the microstructures, and it was believed that these had occurred because of phase separation in the glass during the crystallization process.     

First–Principles Calculations of Microdomain Models for β–Sialon Si5AlON7

The experimentally observed preferential distribution of O and N atoms in β–sialon can be obtained from a first–principles calculation if it is assumed that microdomains are formed, in accordance with NMR observations.     

Synthesis of SiAlON-type compounds by carbothermal reduction and nitridation of nanoparticulate geopolymers

Various types of SiAlON compounds were synthesized by heating of carbon-mixed, geopolymer compositions of M₂O•Al₂O₃•4.5SiO₂•12H₂O + 9C. A fixed molar ratio of carbon to silica of 2 and charge-balancing cations Na⁺, K⁺, or Cs⁺ were used to prepare the geopolymer-carbon precursor resins. After curing, the precursors were ground to powders and then fired at 1400 to 1600°C for 2 h under flowing nitrogen. In contrast to previous studies, powdered forms of the precursors and moderate carbon ratios were used in these syntheses. X-ray diffraction results indicated that phase-pure β- or O-SiAlON powders were synthesized, in the case of potassium at 1400 or 1500°C (for β-SiAlON) and sodium cations at 1400°C (for O-SiAlON), respectively. In the cases of cesium, high purity β-SiAlON with some corundum and pollucite were synthesized. Furthermore, depending on the cation type and temperatures, tailored compositions of SiAlON or other compounds (mainly Al₂O₃) were formed by other reactions between precursors in this systematic study.     

Synthesis and photoluminescence of a novel Sr-SiAlON:Eu blue-green phosphor (Sr14Si68−sAl6+sOsN106−s:Eu (s ≈ 7))

A novel Eu²⁺ activated Sr-SiAlON oxynitride phosphor, with the chemical composition of Sr₁₄Si_68−sAl_6+sO_sN_106−s:Eu²⁺ (s ≈ 7), was synthesized by firing the powder mixture of SrO, SrSi₂, α-Si₃N₄, AlN and Eu₂O₃ at 1900 °C for 6 h under 1 MPa nitrogen atmosphere. The structure has a typical feature of SiAlON consisting of the host framework which is constructed by a three-dimensional MX₄ tetrahedral (M: Si or Al; X: O or N) network, and Sr or Eu²⁺ ions as the guest ions. It has been shown that the Sr-SiAlON:Eu²⁺ phosphor has the excitation band covering the range of the ultraviolet light region to 500 nm, and exhibits an intense blue-green color with the emission band centered at about 508 nm. The temperature dependent emission intensity of the Sr-SiAlON:Eu²⁺ phosphor is better than that of a typical blue-green Ba₂SiO₄:Eu²⁺ phosphor. It is demonstrated that Sr-SiAlON:Eu²⁺ phosphor is very promising for use in white -LEDs.