High temperature bending creep behavior of a multi-cation doped α/β-SiAlON composite

SiAlON ceramics with high hardness and high toughness can be made through designing α/β-SiAlON composites. An important advantage of α-SiAlON phase is that the amount of intergranular phase is reduced by the transient liquid phase being absorbed into the matrix of α-SiAlON phase during sintering. But, the thermal stability of the α-SiAlON phase is an important concern for α/β-SiAlON composites especially at high temperatures. The use of different types of single or multiple cations during fabrication directly affects resultant microstructures and mechanical behavior of α/β-SiAlON composites. In this study, the creep behavior of a multi-cation (Y, Sm and Ca) doped α/β-SiAlON composite, in which aluminum-containing nitrogen melilite solid solution phase was designed as intergranular phase, was investigated by four-point bending creep tests under stresses from 50 to 150 MPa and at temperatures from 1300 °C to 1400 °C in air. The stress exponent was determined to be 1.6 ± 0.13 at 1400 °C and the creep activation energy was calculated to be 692 ± 37 kJ/mol⁻¹. Grain boundary sliding coupled with diffusion was identified as the rate-controlling creep mechanism for the α/β-SiAlON composite.     

Diffusion phenomena in the α-Si3N4-solid solution and 304L stainless steel static interaction couple

Compositional varieties of α-SiAlON a yttrium stabilized solid solution of α-Si₃N₄, were studied against 304L stainless steel through interacting couples prepared under hot pressing (1100-1300 °C, 1.8 MPa, 5 h) with respect to their chemical interactions and interface bonding formation. Interdiffusion of elements in both ceramic and steel side followed by the series of reactions those produced the interfaces between the joints were seen. Chemical behaviour of α′ vis-à-vis its compositional nitrogen content has been discussed.     

Synthesis of β-SiAlON: A combined method using sol–gel and SHS processes

β-SiAlON has been prepared by a SHS reaction performed under high nitrogen pressure on a compacted mixture of silicon and mullite. In order to optimise this reaction, the solid mixture was prepared from a liquid solution in which the mullite (2SiO₂-3Al₂O₃) was formed by a sol–gel process. The SHS samples were characterized by using Scanning Electron Microscopy (SEM) and X-ray diffraction. A Rietveld refinement performed on the recorded diffractograms revealed a very high formation rate of β-SiAlON (more than 80 wt%) when the reaction was carried out under low nitrogen pressure. From the whole of our results, a mechanism of β-SiAlON formation has been suggested.     

Effects of Z-value on physicochemical and biological properties of β-SiAlONs ceramics

This study evaluated the effects of different Z-values on the physical, chemical, and biological properties of β-SiAlON ceramics. Increasing the Z-value of the β-Si₃N₄ solid solution's main phase resulted in the replacement of Si–N bonds with Al–O bonds. The number of columnar crystals decreased, bulk density increased, and porosity decreased, thus transforming the fine-particle microstructure of β-Si₃N₄ into the columnar structure of β-SiAlON. The compressive strength increased, which facilitated sintering at 1500 °C without sintering auxiliaries. H⁺ and OH⁻ ions in deionized water broke the covalent bonds on the β-SiAlON surface, thereby forming new Si–OH, Al–OH, and N–H bonds on the β-SiAlON surface and producing SiO₄⁴⁻, AlO₂⁻, and NH₄⁺ groups in the solution. Increasing the soaking time changed the compositions of ionized H⁺ and OH⁻ ions, thus increasing the pH. MC3T3-E1 cells were cultured on the β-SiAlON surface, and it was observed that the increase in the Z-value of β-SiAlON had no influence on cell adhesion and spreading, but it may slightly suppress cell proliferation at high Z-values. At low Z-values, the low AlO₂⁻ concentration helps promote osteogenic differentiation and mineralized nodule formation. Thus, β-SiAlON ceramics possess excellent physical, chemical, and biological properties and are considered excellent bone-repairing materials.     

Evolution of core–rim structures and phase transformations in infra-red transmitting Y-α-SiAlON ceramics

Core–rim structures were observed as common features in Y-α-SiAlON ceramics hot-pressed between 1550?1950 °C. We found most dopants were taken into α’-rims, and a transition layer grown first on α-cores from liquid-phase over-saturated with metal solutes. Elongated β’-grain were formed as minor phase with α’- or AlN-cores thus only after the α’ matrix had consumed up all Y solutes, revealing that the α’ → β’ transformation is controlled by the transient liquid-phase and similar defects and dangling bonds could be detected in both SiAlON phases by cathodoluminescence. Quantitative assessment of Y_m/3Si_12?(m+n)Al_m+nO_nN_16?n demonstrates the multiphase evolution, initiated by over-saturation of Y solutes at low temperatures thus retaining α-phase as cores to lower the infra-red transmittance, dictated by homogenization of Al solutes at higher temperature. The elimination of those phase boundaries leads to better dopant and sintering design for achieving transparent and high-performance SiAlON ceramics.     

河南某低品位铝土矿制备β-SiAlON

分别以复合还原剂碳硅、碳铝还原氮化低品位铝土矿(Al2O3含量为68wt％)制备β-SiAlON.利用XRD、SEM和EDS等检测手段和试样的质量变化率,研究了两种复合还原剂制备β-SiAlON的相变过程、β-SiAlON的相对生成量和微观状态.结果表明:低品位的矾土矿利用复合还原剂可以制造出优良廉价的β-SiAlON材料;碳硅试样的β-SiAlON为O'-SiAlON和Al2O3反应生成;碳铝试样的β-SiAlON为AlN、Si3N4和Al2O3反应生成以及碳直接还原氮化莫来石生成;基本结束的反应温度为1500℃,生成的β-SiAlON为柱状、z值为3左右;复合还原剂碳硅还原氮化制备β-SiAlON相对含量高,结晶形貌好,制备成本低.     

Synthesis of β-SiAlON whiskers: dependence of uniform morphology upon preparation conditions

Abstract

Abstract

β-SiAlON whiskers with uniform morphology were prepared using reaction sintering method under different conditions. The effect of preparing conditions on the morphology of β-SiAlON whiskers was systematically studied by SEM, XRD, TEM and HRTEM. The results showed that single crystalline β-SiAlON whiskers with uniform morphology were successfully fabricated at 1773 K for 6 h under flowing nitrogen atmosphere. The well synthesised whiskers were of several hundreds of nanometres in diameter and a few hundreds of micrometres in length. Although the morphology and its size distribution are mainly determined by the reaction temperature and holding time, they can also be tailored by controlling the reaction atmosphere. The ratio of staring materials has no significant influence on the morphology of β-SiAlON whiskers. The growth of β-SiAlON whiskers follows a vapour–solid mechanism, and the formation of the belt-like whiskers is attributed to an anisotropic growth at the early nucleation/growth stage.     

Effect of nano- and micro-sized Si3N4 powder on phase formation,microstructure and properties of β′-SiAlON prepared by spark plasma sintering

The phase formation behavior of β′-SiAlON with the general formula Si_6-zAl_zO_zN_8-z was studied comprehensively for z values from 1 to 3 using spark plasma sintering (SPS) as the consolidation technique at synthesis temperatures from 1400 to 1700 °C. The samples were prepared close to the β′-SiAlON composition line: Si₃N₄ ? 4/3(AlN·Al₂O₃) in the phase diagram using (A) nano-sized amorphous Si₃N₄ and (B) micro-sized β-Si₃N₄ precursors. Field-emission scanning electron microscopy (FESEM) was used for microstructural analysis.Most compositions reached almost full density at all SPS temperatures. Compared with the micro-sized β-Si₃N₄ precursor, the nano-sized amorphous Si₃N₄ precursor accelerated the reaction kinetics, promoting the formation of dense β′-SiAlON + O′-SiAlON composites after SPS at synthesis temperatures of 1400–1500 °C. This resulted in very high values of Vickers hardness (Hv₁₀) = 18.2–19.2 GPa for the z = 1 composition related to the hardness of the O′-SiAlON component phase.In general, for samples synthesized from nano-sized amorphous Si₃N₄, which were almost fully dense, containing >95% β′-SiAlON, the hardness values were 13.4–13.8 GPa with a fracture toughness of 3.5–4.6 MPa m^1/2. For equivalent samples synthesized from micro-sized β-Si₃N₄, hardness was in the range 13.9–14.4 GPa with a fracture toughness of 4.3–4.5 MPa.m^1/2. These values are comparable with fully dense β′-SiAlONs, usually containing intergranular glass phase which has been sintered by HIP and other processes at much higher temperatures for longer times.     

Oxidation behaviour and kinetics of α-SiAlON ceramics

Abstract

In this study, the oxidation behaviours of Y, Sm and Y/Sm doped α-SiAlON ceramics are investigated to examine the oxidation kinetics. The oxidation tests are carried out at 1300–1450°C for different holding times (6–72 hours). Measurements of weight gain and observations of oxidized surfaces were made. The results show that up to 1400°C, oxidation resistance of Sm-doped samples is better than dual cations (Y/Sm) doped compositions even though the former has the higher amount of grain boundary phase. Results are discussed based on grain boundary phase, oxidation morphology and thickness.     

High infrared transmission of Y-Yb-doped α-SiAlON

Dense dual Y³⁺-Yb³⁺-doped α-SiAlON ceramic containing extra addition of 2 wt.% Y₂O₃/Yb₂O₃ was fabricated by hot pressing at 1900 °C for 1 h, and its optical transmittance was investigated over the wavelength range 200-6000 nm. The results showed that the addition of 2 wt.% extra liquid phase could effectively promote Y/Yb-α-SiAlONs densification and its assemblage only consisted of single crystallized α-SiAlON phase. The obtained sample had relatively high infrared light transmittance properties. The maximum transmittance in the medium infrared region for 1.0 mm thick specimen could reach around 72% at ~ 2500 nm. It is attributed to the uniform, dense microstructure and few residual intergranular phases.