氮化物结合碳化硅耐火材料的研究现状

分别概述了以氮化硅、赛隆和氧氮化硅作为结合相 的SiC材料的结构特点、理化性能、生产工艺和应用情况,详细 介绍了国内这3种材料的研究现状,并对今后氮化物结合SiC 材料的研究内容提出了自己的观点。     

Molecular-level composition design for efficient synthesis of SiAlON ceramics

SiAlONs are a class of liquid-phase sintered ceramics with excellent room-temperature strength and toughness, but whose residual grain boundary glass softens at high temperatures, limiting use in extreme environments. For this reason, efforts are made to minimize the volume of the grain boundary glass while still facilitating full densification. This work describes a potential route for the densification of SiAlONs with very low concentrations of liquid-phase sintering additive (e.g., rare-earth oxides such as yttria) by using an organometallic precursor. Solid solution of Al and O in the Si₃N₄ lattice was accomplished through the incorporation of solute atoms via liquid organic precursor aluminum sec-butoxide (ASB). Al₂O₃ powder is conventionally used for this purpose, and the subsequent lattice softening associated with the solid solution helps to facilitate densification. However, a liquid-phase additive is still essential for the full densification of SiAlONs. Higher densities were obtained from SiAlON powder blends utilizing organometallic ASB than those utilizing alumina powder, allowing for greater densification at very low Y₂O₃ concentrations. The thermal decomposition of the organic precursor was investigated by high-temperature scanning electron microscopy, thermogravimetric analysis, and various X-ray diffraction experiments. Immersion density measurements and lattice parameter refinements were performed for samples sintered with varying Y₂O₃ concentrations and/or dwell times. Results indicate that ASB-containing powder blends favor SiAlON formation more strongly than Al₂O₃-containing powder blends and favor densification at very low Y₂O₃ concentration.     

Fabrication and mechanical properties of boron nitride nanotube reinforced silicon nitride ceramics

In this study, silicon nitride (Si₃N₄) ceramics added with and without boron nitride nanotubes (BNNTs) were fabricated by hot-pressing method. The influence of sintering temperature and BNNTs content on the microstructures and mechanical properties of Si₃N₄ ceramics were investigated. It was found that both flexural strength and fracture toughness of Si₃N₄ were improved when sintering temperature increases. Moreover, α-Si₃N₄ phase could transform into β-Si₃N₄ phase completely when sintering temperature rises to 1800 °C and above. BNNTs can enhance the fracture toughness of Si₃N₄ dramatically, which increases from 7.2 MPa m^1/2 (no BNNTs) to 10.4 MPa m^1/2 (0.8 wt% BNNTs). However, excessive addition of BNNTs would reduce the fracture toughness of Si₃N₄. Meanwhile, the flexural strength and relative density of Si₃N₄ decreased slightly when BNNTs were added. The related toughening mechanism was also discussed.     

高铝矾土-硅粉氮化合成SiAlON的过程研究

分别以w(Al2O3)为68.08%和45.56%的两种高铝矾土及硅粉为原料,按合成SiAlON的理论配比配料(Si粉过量5%),成型后在流动N2(流量为0.06~0.1m3.h-1)中进行热重分析,同时测定试样在不同温度(900~1500℃)保温6h后的质量变化,并分析氮化后试样的物相变化,从而探讨该试样的氮化过程及其机理。结果表明,高铝矾土-硅粉试样在流动N2中的氮化反应过程可大致分为3个阶段:1)Si粉氮化阶段(900~1200℃),Si粉氮化生成Si3N4和Si2N2O;2)SiAlON形成阶段(1300~1400℃),生成O’-SiAlON和β-SiAlON;3)β-SiAlON的生长发育阶段(1450~1500℃),部分O’-SiAlON转化为β-SiAlON,Al2O3在β-SiAlON中的固溶度增加。     

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.     

Effect of SiC addition on the thermal diffusivity of SiAlON ceramics

Despite the fact that thermal conductivity is a crucial parameter for SiAlON ceramics with respect to their suitability in various applications, including high-temperature structural components, wear parts, and cutting tools, studies on SiAlON ceramics reported thus far mainly focus on the improvement of their mechanical properties. In view of the lack of sufficient studies on the thermal conductivity of SiAlON ceramics, this study investigates the improvement in the thermal diffusivity behaviour of SiAlON ceramics by the addition of highly conductive SiC particles. As solid-solution SiAlON ceramics exhibit complex crystal structures typically composed of defects, the phonon scattering increases, subsequently decreasing diffusivity. In particular, the improvement in the thermal diffusivity of both α- and β-SiAlONs was investigated by the addition of 0.25 wt% SiC. In addition, the effect of the SiC particle size on the thermal diffusivity of β-SiAlON was examined. Using inverse diffusivity data, intrinsic and extrinsic scattering parameters were determined, and compared to intrinsic scattering, extrinsic scattering was a dominant factor. Furthermore, transmission electron microscopy (TEM) images of SiC_p-reinforced α and -β-SiAlON ceramics were recorded to examine the SiC particle distribution.     

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.     

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.     

Fractography of silicon nitride based ceramics to guide process improvements

Fractography is an important tool to understand and identify the cause of the failure in materials. This understanding can be used to make changes in raw materials selection and processing to increase the strength of brittle materials. This study reports the fracture behavior of hot-pressed silicon nitride based ceramics, with focus on dominant flaw identification with respect to material and process parameters. Silicon nitride is an important material for structural applications which require high strength and wear resistance, such as bearings, nozzles, and cutting tools. Silicon nitride with a target base composition of Si_6-zAl_zO_zN_8-z (z = 0.5), along with varying boron dopant levels, was explored in this work. Detailed fractographic analyses revealed that the majority of fracture origins were internal flaws due to the foreign impurities introduced at various stages of processing. All materials were found to have reasonably high strength (800−1100 MPa). Strength was inversely proportional to the square root of the flaw size, however no correlation was found between measured flexural strength and fracture origin types. Mirror constants calculated from fracture mirror measurements ranged between 5.8 and 9.8 MPa.m^1/2.     

A comparative study of SiAlON ceramics

Two different varieties of Si₃N₄ powders were used to prepare SiAlON ceramics. 100% β-Si₃N₄ was used from refractory grade powders (B1) and another purer 98% α-Si₃N₄ (50A) powder was used to prepare the SiAlON samples. Since SiC + SiAlON composites reportedly perform better, batches were prepared with 15% SiC addition to the refractory powders (B1) and 17.5% SiC was added to the other SiAlON composition (50A). The samples were gas pressure sintered at 1840 °C and at 22 bar with 1 h dwelling time. Thereby, we could achieve 97–98% theoretical density. The hardness was recorded 14–17 GPa while fracture toughness varied from 4.3 to 5 MPa m^1/2. Fretting experiments showed initial running-in period of 300 cycles for all the tribo-couples. After which, the steady state coefficient of frictions (COF) were achieved. Steel ball of 10 mm diameter, fretting against 50A composition, showed 0.6 average steady state COF while the same composition while fretting against alumina ball of the same diameter, showed 0.57 average steady state COF. Results have been compared with SiAlON composition derived from refractory powder (B1) and found that the 50A composition performs better under identical test conditions. Moreover, cytocompatibility study also suggests that the investigated 50A composition can be used as substrate to support cell adhesion and proliferation of L929 mouse fibroblast cell lines whereas B1 composition derived from refractory powders are toxic in nature.     

烧结助剂对反应烧结氮化硅陶瓷的影响

以Si粉和C粉为主要原料 ,在氮气流量为1.2L·min- 1,氮化温度为 1380℃ ,保温时间为 2 0h的条件下 ,研究了分别以 10wt%的MgO、Al、Al2 O3和Al2 O3+Y2 O3粉为烧结助剂对反应烧结氮化硅陶瓷的影响。结果表明 :以MgO粉作烧结助剂时 ,试样的主要成分是MgSiO3,另外还有Si2 N2 O ,但没有Si3N4 生成 ;以Al粉作烧结助剂时 ,试样的主要成分是SiO2 ,仅有少量Si3N4 存在 ;以Al2 O3作烧结助剂时 ,试样的主要成分是β Si3N4 和α Si3N4 ;以 2wt%Al2 O3+8wt%Y2 O3作烧结助剂时 ,试样的主要成分为 β Si3N4 ,同时含有少量α Si3N4 。     

The fabrication of tungsten reinforced silicon nitride ceramics by altering nitrogen pressure

Due to high ductility, high-temperature melting, low thermal expansion coefficient, etc., tungsten (W) might be considered to be an ideal reinforcement in toughening or strengthening Si₃N₄ ceramics. However, it is difficult to fabricate W/Si₃N₄ composites due to the possible reactions between W and Si₃N₄ during sintering process at the high temperature. In this work, a novel way to avoid the reactions and fabricate the W/Si₃N₄ composites was proposed by thermodynamic analysis and verified by experiment. Firstly, the phase equilibrium between W and Si₃N₄ as a function of temperature and nitrogen pressure was thermodynamically calculated, which indicates that one critical nitrogen pressure exists for reactions between W and Si₃N₄ at a certain temperature. As the nitrogen pressure is higher than the critical value, the reactions would be inhibited or adversely proceeded. Based on the results, W was innovatively in-situ introduced in the form of WSi₂ after sintering at 1750?°C under 50?bar nitrogen pressure. Moreover, the fracture toughness of Si₃N₄ ceramics was enhanced from 7.1?±?0.2 to 8.0?±?0.4?MPa?m^1/2, which proposes a new reinforcement or method in toughening Si₃N₄ ceramics.     

氮化硅结合碳化硅树料的抗渣侵蚀性

借助光学显微镜、扫描电子显微镜和Ｘ射线衍射分析等手段，研究了矿渣侵蚀后的氮化硅结合碳化硅材料的显微结构和物相组成．探讨了氮化硅结合碳化硅材料的损坏机理。     

氮化硅结合碳化硅耐火材料的氧化

氮化硅结合碳化硅耐火材料高温氧化后，其抗折强度有所提高，但经扫描电镜观察，材料断面结构已发生了明显的变化。该材料长时间在氧化气氛中使用，可靠性将下降。     

Grinding performance improvement of silicon nitride ceramics by utilizing nanofluids

Silicon nitride ceramics are widely used as advanced structural components because of their excellent thermal and mechanical properties at ambient and elevated temperatures. In manufacturing industries, grinding is an efficient and productive technique for finishing ceramic workpieces. However, high wheel-workpiece friction and the extreme hardness associated with silicon nitride cause large heat generation during grinding. The heat produced during grinding impairs the workpiece quality by inducing surface and sub-surface damages, tensile residual stresses etc. The damages can critically limit the applications of ground ceramic components. Extensive experimental studies have been carried out to find the effect of dry and nano MQL (Graphite, WS₂ and MoS₂) grinding conditions on silicon nitride using resin bonded diamond wheel at different parametric (wheel speed, depth of cut and table speed) combinations. Results indicate that the use of nanofluids considerably improve the process performance in terms of grinding forces, surface finish and sub-surface damage. The ground surface is characterized by optical microscopy, SEM/EDX and XRD.     

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.     

Effect of rare earth additives on properties of silicon nitride ceramics

Abstract

Silicon nitride ceramics with rare earth (Re) compound (5 wt-%) and MgO (3 wt-%) additives were fabricated by spark plasma sintering and following heat treatment. The Re compounds included two groups: ReF₃ ((Re?=?La,Nd,Gd) and Re₂O₃ (Re?=?La,Nd,Gd). Specimens show the same tendency in the sintering shrinkage rate, relative density, grain size and bending strength with the increasing Re cation (Re³⁺) radius both in ReF₃ and Re₂O₃ added samples. However, as to aspect ratios and thermal conductivity, the change rules are completely opposite between the two groups of specimens.     

Fabrication of high performance silicon nitride ceramics with TiO2 additive by annealing process

The high performance Si₃N₄ ceramic was prepared firstly for TiO₂, Y₂O₃ and MgO as pressureless sintering additives. Si₃N₄ ceramic with relative density of 99.6% and flexural strength of 785 ± 23.3 MPa could be obtained with 3 mol% TiO₂ and sintered at 1800 °C for 2 h. After annealing at 1700 °C, the facture toughness of sample of 1 mol% TiO₂ increased from 8.31 ± 0.28 MPa m^1/2 to 9.84 ± 0.16 MPa m^1/2. The flexural strength of sample of 2 mol% TiO₂ increased from 707 ± 26 MPa to 981 ± 16 MPa, thermal conductivity increased from 57.8 W/(m·K) to 68.49 W/(m·K). The XRD results showed that the ratio of I₁₀₁/I₂₁₀ and grain height reached to 1.84 and 5 μm of the sample of 3 mol% TiO₂, respectively. The present investigation revealed that the three-dimensional array of highly oriented crystalline Si₃N₄ micro rods could be prepared which array on the homogeneous substrates by using TiO₂ as agent. This phenomenon may propose a method that the mechanical properties the Si₃N₄ ceramics added TiO₂ can be improved significantly after annealing process.     

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.     

Fracture behavior of silicon nitride ceramics under combined compression-torsion stresses analyzed by multiaxial fracture statistics

Combined compression-torsion tests were performed on the thermal-treated and as-machined silicon nitride ceramics to investigate their fracture behavior under multiaxial stress states. The thermal-treated samples showed considerable high strength and low anisotropy to the grinding direction in flexure tests compared to the as-machined samples. Under combined compression and torsion stress states, the thermal-treated samples showed considerably higher tensile strength than that of as-machined samples at low compressive stress states and weakening with increasing compression stress. The as-machined samples showed little decrease in tensile strength with increasing compression stress and comparable tensile strength with the thermal-treated samples under a highly compressive stress state. The behavior of thermal-treated samples were well described by the statistical theory of multiaxial fracture for volume-distributed flaws combined with a mixed-mode fracture criterion with the shear sensitivity constant of 1.75 and 1.65 for Shetty’s criterion and the ellipsoidal criterion, respectively.