烧结助剂对氮化硅陶瓷显微结构和性能的影响

氮化硅中氮原子和硅原子的自扩散系数很低，致密化所必需的扩散速度和烧结驱动力都很小，在烧结过程中需采用烧结助剂。烧结助剂是影响氮化硅陶瓷的显微结构和性能的关键因素之一。有效的烧结助剂不但可以改善氮化硅陶瓷的显微结构，而且可以提高氮化硅陶瓷的高温性能和抗氧化性能。     

烧结助剂对氮化硅陶瓷热导率影响的研究进展

随着大规模、超大规模集成电路的发展,以及集成电路在通讯、交通等领域的运用。对于基板材料的要求日益严苛,氮化硅陶瓷因为有着优异的力学性能、介电性能和导热性,是作为基板材料的重要候选材料之一。氮化硅陶瓷的理论热导率高达200-320 W/(m·K),但是实际上高热导率的氮化硅难以制成。随着科研者将精力投入到氮化硅上,近年来氮化硅陶瓷的实际热导率得到提高,但是与理论热导率还有着不少差距。据文献记载,选择合适的烧结助剂能够有效的提高氮化硅陶瓷的热导率。本文综述了不同种类的烧结助剂对氮化硅陶瓷热导率的影响。     

烧结助剂对氮化硅陶瓷热导率和力学性能的影响

以氮化硅(α相≥95%,平均粒径0.5μm)为原料,添加MgO–Y₂O₃为复合烧结助剂,采用气压烧结技术制备了β相高导热氮化硅陶瓷。研究和讨论了烧结助剂含量和比例对氮化硅陶瓷致密化过程、热导率、力学性能和显微结构的影响。结果表明:当烧结助剂添加量为5%MgO+4%Y₂O₃时,使用气压烧结在1 890℃烧结2 h,试样的热导率可达到85.96 W·m^–1K^–1,抗弯强度达到873 MPa,断裂韧性为8.39 MPa·m^1/2;在烧结过程中Y₂O₃与Si₃N₄反应形成化合物固定在晶界处,减少了固溶进氮化硅晶格中氧的含量,起到了净化氮化硅晶格的作用,提高了烧结试样的热导率;过量的MgO或Y₂O₃在烧结过程中形成的化合物残留在晶界处,降低了材料的力学性能和热导率。     

烧结助剂对氮化硅陶瓷高温性能的影响

研究了不同系统烧结添加剂及其用量对氮化硅陶瓷高温力学性能的影响。所获结果表明，添加Ｌａ２Ｏ３和Ｙ２Ｏ３的氮化硅材料具有好的高温抗弯强度，从室温至１３７０℃高温保持不变。     

添加Mg-Al-Si体系烧结助剂的氮化硅陶瓷的无压烧结

以MgO-Al2O3-SiO2体系作为烧结助剂,研究了氮化硅陶瓷的无压烧结。着重考察了烧结温度、保温时间以及烧结助剂用量等工艺因素对氮化硅陶瓷材料力学性能和显微结构的影响,通过工艺调整来设计材料微观结构以提高材料的力学性能。在烧结助剂质量分数为3.2％的情况下,经1 780℃,3 h无压烧结,氮化硅大都呈现长柱状β-Si3N4晶粒,具有较大的长径比,显微结构均匀。样品的相对密度达99%,抗弯强度为956.8 MPa,硬度HRA为93,断裂韧性为6.1 MPa·m1/3。具有较大长径比晶粒构成的显微结构是该材料表现较高力学性能的原因。     

烧结工艺对β-氮化硅陶瓷的影响

以β-Si3N4粉末为原料,以YAG（钇铝石榴石）为烧结助剂,通过气氛压力烧结（GPS）制备出致密的β-氮化硅陶瓷材料,形成大小均匀的柱状颗粒和小球状颗粒复合显微结构,研究了烧结助剂质量分数、烧结温度以及保温时间对β-氮化硅陶瓷致密化程度及力学性能的影响.     

烧结技术与氮化硅陶瓷显微结构研究

以文以编号为ＳＰＮ氮化硅与编号为Ｍ２０，Ｍ５０的Ｓｉａｌｏｎ材料为例在无压烧结，气烧结和热等静压烧结等工艺条件下进行烧结，测试了材料性能，用ＳＥＭ，ＥＭ和Ｘ衍射等分析手段观察材料的显微结构和相组成，及其对材料性能的影响进行了讨论。     

烧结助剂及增强相对氮化硅陶瓷材料性能的影响

氮化硅陶瓷基复合材料由于其优异的性能广泛运用于市场各领域.本文概述了制备氮化硅陶瓷基复合材料常用的几种烧结助剂和增强相,综述了烧结助剂对材料烧结过程的影响以及几种常用增强相的研究现状及增韧机理,最后对氮化硅陶瓷基复合材料未来的发展趋势和研究方向进行了总结与展望.     

Si3N4陶瓷烧结中烧结助剂的研究进展

研究了多种烧结助剂对氮化硅烧结性能和烧结过程的影响.研究表明,多组分助剂比单一组分助剂对氮化硅的助烧效果好,其中稀土氧化物和MgO-Al2O3-SiO2体系比较受重视.     

Dielectric properties of silicon nitride ceramics produced by free sintering

The silicon nitride ceramics with a beneficial combination of low dielectric losses and improved physical properties was fabricated by cold isostatic pressing and pressureless sintering. The fine grain microstructure, three-phase composition based on the β-SiAlON, the small amount of the glass phase and relatively small porosity promote a unique combination of a low thermal conductivity 14.51 W m⁻¹ K⁻¹ and low dielectric loss 1.4·10⁻³. A novel method is proposed to overcome the main drawbacks of the commercial and high-cost technologies.     

硅复合刚玉-氮化硅材料的性能、组成及显微结构

将由50% (质量分数,下同)氧化铝粉、37. 5%氮化硅粉和12. 5%硅粉组成的混合料和由62. 5%氧化铝粉和37. 5%氮化硅粉组成的混合料分别压制成试样,分别在空气和埋炭气氛中于1600℃保温2h烧成,然后检测试样的体积密度、显气孔率和常温耐压强度,并采用XRD、SEM、EDS等手段分析试样的物相组成和显微结构。结果表明:与未加硅粉的试样相比,加入硅粉的试样在两种气氛中烧成后,显气孔率较低,强度较大,说明硅对刚玉-氮化硅材料具有助烧结作用;在空气中烧成后,试样中残留有较多的单质硅,这些单质硅均匀分布于刚玉和氮化硅颗粒的空隙间;在埋炭气氛中烧成后,单质硅原位反应生成了O’SiAlON和SiC。     

Ultrafast high-temperature sintering of silicon nitride: A comparison with the state-of-the-art techniques

Ultrafast High-temperature Sintering (UHS) has been successfully applied to fabricate the silicon nitride (Si₃N₄) bulks, as the first attempt of ultra-rapid consolidation of a non-oxide ceramics. At a heating rate of 875 °C/min, the bulk Si₃N₄ ceramic with a relative density greater than 96 % and an α-β phase transformation degree above 80 % could be obtained within 300 s. The effects of ultrafast heating on the liquid phase sintering (LPS) were also comparatively studied. Results showed that, the ultrafast heating rate and high temperature under UHS might promote the LPS system evolving to a nonequilibrium state. By comparing with other pressureless sintering processes with much lower heating rates, UHS apart from reducing the processing time, and it is also an effective method to form a bimodal microstructure composed of interlocked rod-like β-Si₃N₄ grains.     

New method of free silicon determination in pressureless sintered silicon nitride by Raman spectroscopy and XRD

Formation of silicon affects different physical properties of silicon nitride ceramics. Decomposition of Si₃N₄ and formation of free Si are highly important processes and depend on many factors. The proposed method of combined nano-Raman spectroscopy and X-ray diffraction (XRD) allows quantitative analysis of Si in silicon nitride. Raman spectroscopy enables the determination of atomic bonds and rapid and easy identification of free silicon. Further analysis of the crystalline phases by XRD enables the calculation of the amount of free silicon. The proposed complex method allows the characterization of such complicated processes as silicon nitride decomposition, microstructure formation and, in particular, the formation of the nanoscale grain boundary phase because Si nanosized precipitates are the nucleants of secondary phases during crystallization. Strong 522.8 cm⁻¹ mode and 943.1–984.3 cm⁻¹ transverse optical modes of free Si were clearly observed in the investigated silicon nitride that was subjected to pressureless sintering at 1800 °C. Reported ceramics demonstrated typical microstructures with elongated grains and relatively high microhardness and Young's modulus. It was shown that the aspect ratio depended linearly on the microhardness and Young's modulus. High values of the Young's modulus (more than 290 GPa) and microhardness (more than 1800 HV) were shown for reported silicon nitride produced by hot pressing and pressureless sintering via cold isostatic pressing with a higher quantity of sintering agent. The features of molecular structure of the reported Si₃N₄ ceramics were clearly described and discussed in detail and were found to be in good agreement with the microstructure and phase composition of these ceramics.     

Microstructure control and mechanical properties of porous silicon nitride ceramics

Porous silicon nitride ceramics with a fibrous interlocking microstructure were synthesized by carbothermal nitridation of silicon dioxide. The influences of different starting powders on microstructure and mechanical properties of the samples were studied. The results showed that the microstructure and mechanical properties of porous silicon nitride ceramics depended mostly on the size of starting powders. The formation of single-phase β-Si₃N₄ and the microstructure of the samples were demonstrated by XRD and SEM, respectively. The resultant porous Si₃N₄ ceramics with a porosity of 71% showed a relative higher flexural strength of 24 MPa.     

A novel strategy for c-axis textured silicon nitride ceramics by hot extrusion

Highly c-axis textured β-silicon nitride (β-Si₃N₄) ceramic with fine grains was prepared by a new method of hot extrusion for the first time. The (002) pole figure on the section plane vertical to extruding direction showed a characteristic of center rotational symmetry. The average cline angle between elongated β-Si₃N₄ grains and hot extruding direction was about 14.4°. The degree of c-axis texturing by hot extrusion was comparable to that achieved by rotating magnetic field. The hardness and toughness anisotropy in different direction was apparent and relatively higher hardness was achieved in the present work mainly due to the finer grain size. Therefore, many different compositions of c-axis aligned Si₃N₄-based ceramics with tailored mechanical properties could be achieved by the strategy of hot extrusion.     

Response of silicon nitride ceramics subject to laser shock treatment

A comprehensive and novel investigation on multiple-layer, square-beam laser shock treatment (“laser peening”) of Si₃N₄ ceramics is reported in this work. Surface topography, hardness, fracture toughness (K_Ic), residual stresses, and microstructural changes were investigated. The evaluation of fracture toughness via the Vickers hardness indentation method revealed a reduction in crack lengths produced by the indenter after laser shock treatment (LST). Upon appropriate calculation, this revealed an increase in K_IC of 60%. This being attributed to a near-surface (50 μm depth) compressive residual stress measured at −289 MPa. Multiple layer LST also induced beneficial residual stresses to a maximum measured depth of 512 μm. Oxidation was evident, only on the top surface of the ceramic, post LST (<5 μm depth) and was postulated to be due to hydrolyzation. The surface enhancement in K_IC and flaw-size reduction was assigned to an elemental change on the surface, whereby, Si₃N₄ was transformed to SiO₂, particularly, with multiple layers of LST. Compressive residual stresses measured in the sub-surface were attributed to mechanical effects (below sub-surface elastic constraint) and corresponding shock-wave response of the Si₃N₄. This work has led to a new mechanistic understanding regarding the response of Si₃N₄ ceramics subject to the LST deployed in this resesrch. The findings are significant because inducing deep compressive residual stresses and corresponding enhancement in surface K_IC are important for the enhanced durability in many applications of this ceramic, including cutting tools, hip and knee implants, dental replacements, bullet-proof vests and rocket nozzles in automotive, aerospace, space and biomedical industries.     

Microstructural and mechanical evaluation of a Cu-based active braze alloy to join silicon nitride ceramics

Self-joining of St. Gobain Si₃N₄ (NT-154) using a ductile Cu-Al-Si-Ti active braze (Cu-ABA) was demonstrated. A reaction zone (∼2.5-3.5 μm thick) developed at the interface after 30 min brazing at 1317 K. The interface was enriched in Ti and Si. The room temperature compressive shear strengths of Si₃N₄/Si₃N₄ and Inconel/Inconel joints (the latter created to access baseline data for use with the proposed Si₃N₄/Inconel joints) were 140 ± 49 MPa and 207 ± 12 MPa, respectively. High-temperature shear tests were performed at 1023 K and 1073 K, and the strength of the Si₃N₄/Si₃N₄ and Inconel/Inconel joints were determined. The joints were metallurgically well-bonded for temperatures above 2/3 of the braze solidus. Scanning and transmission electron microscopy studies revealed a fine grain microstructure in the reaction layer, and large grains in the inner part of the joint with interfaces being crack-free. The observed formation of Ti₅Si₃ and AlN at the joint interface during brazing is discussed.