Influence of starting material composition and carbon content on the preparation of Mg-α SiAlON powders by carbothermal reduction-nitridation

This paper investigated the synthesis of Mg–α SiAlON from Talc (Mg₃(Si₂O₅)₂(OH)₂) and halloysite clay (Al₂Si₂O₅(OH)₄). Talc, halloysite and carbon black as a reducing agent were used as the starting materials, and 3 wt.% α-Si₃N₄ was added as the seed in all cases. The mixtures were heated in flowing N₂ (gas) to synthesize Mg–α SiAlON powders. The chosen molecular ratios of talc to halloysite were 1.0, 1.5, 2.0 and 2.5. The influences of various reaction parameters such as the carbon content, temperature and holding time at the top temperature were studied. The results showed that the synthesized powders were composed of α-SiAlON, β-SiAlON, β-SiC, 15R-AlN polytypoid and AlN phases; the phases and the particle morphology greatly depended on starting material composition and synthesis parameters. A larger amount of talc was needed to compensate for Mg evaporation loss in the starting composition. Higher carbon content seemed to retard the reaction rate, resulting in coarse particle size with an irregular grain shape. The highest content of Mg–α SiAlON, 90 wt.%, was achieved at 1480 °C for 4 h at talc to halloysite ratios of 1.5 and 2.0.     

Preparation of Ca-α SiAlON hollow spheres by carbothermal reduction-nitridation of CaO-Al2O3-SiO2 glass

Ca-α SiAlON powders were prepared by carbothermal reduction-nitridation of CaO-Al₂O₃-SiO₂ glasses at 1450 °C for 2 h in nitrogen. The content of Ca-α SiAlON phase was 65-81% in the products, and the other phases were AlN and β-SiAlON. The products contained hollow spheres with the size of around 5 μm in diameter. The particle morphology was almost identical to that of Ca-α SiAlON prepared from CaCO₃-Al₂O₃-SiO₂ powder mixtures.     

Susceptor-assisted fast microwave sintering of TiN reinforced SiAlON composites in a single mode cavity

In the present study, 17 wt % TiN reinforced α-β SiAlON composites were sintered at low temperature by susceptor-assisted microwave heating. The effect of TiN addition on dielectrical properties of starting powders, as well as the influence of sintering temperature on phase evolution, microstructure development and mechanical properties of α/β-SiAlON-TiN composites were investigated. The obtained results showed that TiN addition increased the microwave absorbing properties which is reflected in the peak sintering temperature. Thus, the α:β ratio decreased and mechanical properties were improved, especially the fracture toughness of the composites. Furthermore, an estimate of energy consumption during microwave assisted sintering at the laboratory scale is presented. As a result, the highest values for relative density (97.1%), Vickers hardness (13.35 ± 0.47 GPa), and fracture toughness (7.52 ± 0.54 MPa m^1/2) were obtained by microwave sintering for 30 min at 1300 °C.     

β-SiAlON-Al2O3复合材料的合成机理

探讨了由Si和Al2O3或由Al、Si和Al2O3合成β-SiAlON-Al2O3复合材料的合成机理,同时还探讨了刚玉原料中杂质铁和钛,以及氮化硅晶种的催化效应及其机理.研究表明以高铁棕刚玉、Si和金属Al为原料,添加适量的氮化硅晶种,可以合成高性能的β-SiAllON-Al2O3复合材料.     

R-Curve Behavior of In Situ Toughened α-SiAlON Ceramics

R -curves of single-phase Y- and Ca-containing α-SiAlON ceramics have been measured. They range from flat ones for fine-grain ceramics to pronounced rising ones when large elongated grains are present. The highest toughness measured reached 11.5 MPa·m^1/2 over a crack extension of about 1000 μm.     

Synthesis of α-SiAlON Seed Crystals

Single-phase seed crystals of Ca- and Y-α-SiAlONs have been synthesized for tailoring microstructure of α-SiAlON ceramics. The influence of composition, sintering temperature, and nitrogen pressure on the size and morphology of seeds has been explored. Guidelines for α-SiAlON seed preparation and morphology control are provided.     

利用海洋沙砾制备O＇-SiAlON的工艺研究

以海洋沙砾为原料，制备了具有较高纯度的O＇-SiAlON陶瓷粉体。分析了烧结温度、N2流量、成型方法等因素对烧结后试样纯度、显微结构的影响。结果表明，试样在1400～1480℃保温6h烧结得到了含有少量SiC的O＇-SiAlON粉体，升高温度有利于O＇-SiAlON的生成，1480℃时O＇-SiAlON含量最高。在0．4～0．8L／min范围内，加大N2流量能够提高O＇-SiAlON／SiC含量之比。     

Effect of Al metal precursor on the phase formation and mechanical properties of fine-grained SiAlON ceramics prepared by spark plasma sintering

This paper reports the effects of using nano-precursor powders, including α-Si₃N₄ or amorphous-Si₃N₄, and particularly the partial replacement of AlN by Al metal precursor, on the properties of a fixed Ca-α-SiAlON composition, consolidated by spark plasma sintering at 1450 °C and 1600 °C. The observed changes in mechanical properties are related to the phases present in the microstructures. In addition to allowing the consolidation of dense and fine ceramics at lower temperatures, the partial replacement of AlN by Al metal precursor has shown remarkable improvement in the mechanical properties of samples sintered at 1600 °C containing α-Si₃N₄, a result of fine α-SiAlON grains and nitrogen-rich grain boundary phase. Another key contribution of Al metal precursor is found in lowering the sintering temperature while keeping, or even improving, the mechanical properties of the sintered samples, as observed in samples sintered at 1450 °C.     

The effect of silicon nitride powder characteristics on SiAlON microstructures,densification and phase assemblage

The surface characteristics, particle size distribution and impurities of starting Si₃N₄ powders exert a very significant influence on the microstructure of sintered silicon nitride based ceramics. Even a change of the processing conditions such as milling liquid media (water or isopropyl alcohol) and milling time can have a substantial effect on particle surface groups, and hence on the microstructure of sintered samples. In this study, SEM, XRD, FTIR, BET, elemental analysis and laser diffraction techniques were used for the comprehensive characterization of Si₃N₄ powders which were produced by diimide, direct nitridation and combustion synthesis, in as received state, and after milling in different liquid media (aqueous or alcohol), for various milling durations. The correlation of the surface characteristics and properties of the Si₃N₄ powders with sintering behavior, and microstructural evolution, densification and phase assemblages of the resulting SiAlON ceramics were reported. The milling conditions affected the surface chemistry of Si₃N₄ powders and the subsequent microstructural evolution. The microstructures evolved from the coarser β-Si₃N₄ powders were coarser, but the fine β-Si₃N₄ powders yielded a bimodal microstructure. The critical particle diameter of the β-Si₃N₄ powder for the formation of needle like SiAlON grains was determined to be less than 0.5 µm.