Electrochemical corrosion of liquid phase sintered silicon carbide ceramics

Liquid‐phase sintered silicon carbide (LPS‐SiC) is silicon carbide ceramic which contains sintering additives forming a liquid phase during sintering. These additives segregate in the grain boundary phase during cooling. The usually used Al₂O₃ dissolves partially in the SiC‐grains and therefore changing the conductivity of the SiC (core rim structure). This study is focused on the electrochemical properties of LPS‐SiC with yttria and alumina as sintering additives. Electrochemical corrosion behaviour of LPS‐SiC has been determined by linear and cyclic voltammetry in acidic and alkaline solution. The effect of anodic oxidation on the material has been monitored by field emission scanning electron microscopy (FESEM) and energy dispersive X‐ray spectroscopy (EDX) as well as by atomic force microscopy (AFM). The core‐rim structure of the investigated materials plays a decisive role in the vulnerability towards corrosion. If oxidative attack was found to occur under anodic polarization, it happened preferentially in the rim region of the SiC‐grains, while the core of the SiC‐grains remained basically unaffected.     

High-temperature strength of silicon carbide ceramics sintered with rare-earth oxide and aluminum nitride

The effect sintering additives comprising AlN and Re₂O₃ (Re = Sc, Lu, Yb, Er and Y) in a 2:3 molar ratio on the high-temperature strength of liquid phase-sintered and subsequently annealed SiC ceramics was investigated. Clean SiC–SiC boundaries and clean SiC-junction phase boundaries without the amorphous inter-granular phase (IGP) were observed in Sc₂O₃-, Lu₂O₃- and Yb₂O₃-doped SiC ceramics. Clean SiC–SiC boundaries and SiC-junction phase boundaries with the amorphous IGP were also observed in Er₂O₃-doped SiC ceramics. The amorphous IGP was present in both SiC–SiC and SiC-junction phase boundaries in Y₂O₃-doped SiC ceramics. The high-temperature strength was not dependent on the crystalline nature of the IGP and junction phases, but was dependent on the chemistry of these phases. Lu₂O₃-, Er₂O₃-, Sc₂O₃- and Y₂O₃-doped SiC maintained their room-temperature strengths up to 1600, 1500, 1400 and 1400 °C, respectively. However, Yb₂O₃-doped SiC manifested a drastic degradation in strength at 1400 °C. Thus, clean boundaries and/or crystallization of junction phases did not always lead to the hoped-for increase in strength at temperatures above 1400 °C.     

High temperature oxidation of silicon carbide based materials

The process of high temperature oxidation of two silicon carbide based materials differing by methods of their production and properties has been studied up to 1500°C in air. The oxidation was performed under the isothermal conditions and at the programmed heat rate of 10° per minute. It was found that the oxidation resistance of the material was the function of the presence of extrinsic metals having close affinity for oxygen. It was also found that under heating up to 1500°C in air phase transitions occurred in the SiC surface layer.     

Effect of aluminum nitride-scandia content on the microstructural and mechanical properties of sintered silicon carbide ceramics

Two different SiC ceramics, one with 10 vol.% AlN-Sc₂O₃ in a 2:3 molar ratio, and the other with 20 vol.% of the same additives with the same ratio, were fabricated by hot-pressing at 1900 °C for 1 h and subsequent annealing at 2000 °C for 6 h in nitrogen. The grain boundary structures of both materials were observed by using high-resolution transmission electron microscopy. The results showed that both materials had clean boundaries without any amorphous films. Although both samples exhibited the same boundary structure, the sample with the higher AlN-Sc₂O₃ content contained more junction phases. The SiC ceramic with 10 vol.% AlN-Sc₂O₃ maintained its room-temperature strength up to 1400°C, whereas the SiC ceramic with 20 vol.% AlN-Sc₂O₃ showed a gradual decrease in strength at above 900 °C. The present results suggest that the high-temperature strength is dependent on the amount of junction phase, as well as the characteristics of the intergranular phase.     

热震作用对硬质合金力学性能的影响及其失效机理

研究热震作用对3540和3475硬质合金抗弯强度、断裂韧性以及硬度的影响.结果表明,硬质合金抗弯强度和断裂韧性随热震次数呈先升后降变化,硬度随热震次数增加略有下降.这是由于淬火抑制了fcc γ→hcp γ相变,粘结相的固溶强化,γ相平均自由程λγ增大以及热震热应力的共同作用的结果.3540和3475合金热震后断面的粗糙度降低,孔洞增多,沿晶断裂比例增加.热震后合金表面发生氧化,形成疏松的氧化层,3540合金抗氧化能力比3475合金高.同时,还观察到游离碳聚集区的碳元素发生扩散,留下大量孔洞,在WC/Co界面处形成大量小裂纹.     

烧结陶瓷结合剂碳化硅磨具显微结构研究

通过电子探针微区分析研究了黏土-长石-石英系烧结陶瓷结合剂碳化硅磨具的显微结构特征,重点研究了结合剂的显微结构。结果表明:结合剂呈典型的烧结状态,有大量的相界和微细裂纹,结合剂和磨粒的结合紧密程度不一;结合剂中有尺寸较大的气孔,最大可达50μm。能谱分析和元素面分布分析表明:结合剂物相主要由玻璃相和残留石英组成,在较大颗粒的残留石英内部及其周围发育有裂纹。此外,本文对磨具显微结构与磨具性能之间的关系作了理论探讨。     

Microstructure and mechanical properties of silicon nitride with tape cast tri-layer structures

Four kinds of silicon nitride samples with tri-laminate structures were prepared by stacking tapes with aligned β-Si₃N₄ whisker seeds. Three-point flexural strengths of the samples were measured at both room temperature and 1673 K. As the total thickness of the layers with the whisker seeds aligned parallel to tension was increased, the flexural strength was increased at both room temperature and at 1673 K. The flexural strength at 1673 K was more sensitive to orientations of the grains in the surface layer than that at room temperature. Observation of the fracture surface revealed that the grains growing from the neighboring whisker seeds were often broken such that they formed a single one grain. Extensive brittle fracture of the grains without much grain pull-out occurred even at 1673 K for the samples.     

Effect of carbide and nitride addition on the strength of sintered TiC-Mo2C-Ni carbides

The effect of TaC, VC, Cr₃C₂, NbC, and TiN additives on the strength and hardness of sintered TiC-Mo₂C-Ni hard metals was studied. The additive powder was mixed separately with the TiC-Mo₂C-Ni base powder. The powder mixtures were then compacted and sintered in a vacuum. Experimental results showed that the transverse rupture strength of sintered hard metals was effectively promoted by adding the VC and TiN powders, respectively. However, no additives increased the hardness value of the base materials.     

Preparation and properties of continuous Al-containing silicon carbide fibers

Continuous SiC(OA1) fibers, named KD-A fibers, were prepared by the melt-spinning of ceramic precursor polyaluminocarbosilane, air-curing, and pyrolizing at 1 300℃. These fibers contained small amount of aluminum and 7%-9% oxygen. The KD-A fibers were converted into sintered SiC(A1) fibers, named KD-SA, by sintering at 1 800℃. The fibers were characterized by chemical analysis, tensile strength test, SEM and XRD. The tensile strength, elastic modulus and diameter of the KD-A fibers are 2.6 GPa, 210 GPa, 12- 14μm, respectively. The KD-A fibers have higher thermal stability, more excellent oxidation resistance than the Nicalon fibers. The properties of the KD-A fibers have reached the level of Hi-Nicalon fibers. The tensile strength, elastic modulus and diameter of the KD-A fibers are 2.1 GPa, 405 GPa, 10 - 12μm, respectively. The KD-SA fibers with nearly stoichiometric component have stable performance at high temperature, and better creep resistance than the Tyranno SA fibers.     

Electrochemical corrosion of silicon carbide ceramics

Recently, SiC‐based ceramics have been found to exhibit corrosion damage patterns, which can only be explained by electrochemical processes. Therefore, the current work focusses on the electrochemical test procedures to determine the corrosion behaviour of solid state sintered silicon carbide (SSiC) ceramics in acidic and alkaline media. The corrosion current densities have been determined from linear voltammetric scans. At anodic polarization potentials, electrochemically induced etching patterns were observed in alkaline solution. The formation of pores and crevices during electrochemical oxidation in acidic solution could be monitored by field emission scanning electron microscopy (FESEM) in addition to transmission electron microscopy (TEM). Impedance spectra measured after anodic polarization could be described by the assumption of a pore model equivalent circuit.     

AlSiC电子封装基片的制备与性能

采用模压成形制备SiC预制件和真空压力浸渗相结合的技术,成功制备出AlSiC电子封装基片。研究磷酸铝含量和成形压力对SiC预制件抗弯强度和孔隙率的影响规律,并对所制备的AlSiC电子封装基片的性能进行评价。结果表明,在磷酸铝含量为0.8%,成形压力为200MPa时,经600℃恒温2h处理的SiC预制件抗弯强度为8.46MPa,孔隙率为37%。当温度为100~500℃时,AlSiC电子封装基片的热膨胀系数介于6.88×10-6和8.14×10-6℃-1之间,热导率为170W/(m·K),抗弯强度为398MPa,气密性小于1×10-8Pa·m3/s。用钯盐活化进行化学镀镍,得到光亮、完整的镀层。镀层于450℃恒温120s后,镀层不变色,未见起皮和鼓泡。     

Dielectric properties of doped silicon carbide powder by thermal diffusion

The doped SiC powders were prepared by the thermal diffusion process in nitrogen atmosphere at 2 000℃. Graphite film with holes was used as the protective mask, The dielectric properties of the prepared SiC powders at high frequencies were investigated. The complex permittivity of the undoped and doped SiC powders was measured within the microwave frequency range from 8.2 to 12.4 GHz. The XRD patterns show that before and after heat treatment no new phase appears in the samples of undoped and nitrogen-doped, however, in the aluminum-doped sample the AIN phase appears. At the same time the Raman spectra indicate that after doping the aluminum and nitrogen atoms affect the bond of silicon and carbon. The dielectric real part （ε） and imaginary part （ε＂） of the nitrogen-doped sample are higher than those of the other samples. The reason is that in the nitrogen-doped the N atom substitutes the C position of SiC crystal and induces more carriers and in the nitrogen and aluminum-doped the concentration of carriers and the effect of dielectric relaxation will decrease because of the aluminum and nitrogen contrary dopants.     

Processing and mechanical properties of porous silica-bonded silicon carbide ceramics

A simple processing route for manufacturing highly porous, silica-bonded SiC ceramics with spherical pores has been developed. The strategy adopted for making porous silica-bonded SiC ceramics entails the following steps: (i) fabricating a formed body through a combination of SiC and polymer microbeads (employed as sacrificial templates) and (ii) sintering the formed body in air. SiC particles are bonded to each other by oxidation-derived SiO₂ glass. By controlling the microbead content and the sintering temperature, it was possible to adjust the porosity such that it ranged from 19 to 77%. The flexural and compressive strengths of the porous silica-bonded SiC ceramics with ≈40% porosity were ≈65 MPa and ≈200 MPa, respectively. The superior strengths were attributed to the homogeneous distribution of small (≤30 μm), spherical pores with dense struts in the porous silica-bonded SiC ceramics.     

Aluminum doping and dielectric properties of silicon carbide by CVD

Cubic β-SiC coating was grown onto the graphite substrate by the normal pressure chemical vapor deposition using CH3SiCl3 （MTS） as a source precursor at 1 150 ℃. But the hexagonal Al4SiC4 phase was generated in the doped process with trimethylaluminium （TMA） as the dopant. Microstructure of the deposit coating as-prepared was characterized by scanning electron microscope （SEM）, which consists of spherical particles with a very dense facet structure. The real component of permittivity ε＇ and dielectric loss tang of the coatings undoped and doped by TMA were carried out by a vector network analyzer in the microwave frequency ranges from 8.2 GHz to 12.4 GHz. The results show that both of them have low values, and doped coating has lower ε＇ and tan δ than undoped one due to the existence of Al4SiC4 impurity phase, which indicates that the desired Al/SiC solid solution at 1 150 ℃ in a normal argon atmosphere is not produced.     

Dielectric properties of sintered aluminum nitride

In the present work, the effects of sintering additives on dielectric loss tangent (tan δ) of AlN ceramics were explored. Different amounts of Y₂O₃ and Mg₃N₂ were respectively added as sintering additives to AlN powders, and pressureless-sintering was performed at 1900 °C for 2 h in a nitrogen flow atmosphere. The resulted AlN ceramics became denser due to the addition of Mg₃N₂, and nearly fully dense sample was obtained with a relative density of 0.998. tan δ decreased with increasing Mg₃N₂ amount and it was found to depend on the density of the AlN ceramic. The best tan δ value of 3.9 × 10⁻⁴ was obtained by adding 1 mol% of Y₂O₃ and Mg₃N₂ together.     

硬质合金/钢钎缝中的夹杂物及其对接头力学性能的影响

以YG8硬质合金与45钢的钎焊接头为研究对象,选用BCu54ZnMnNiSn钎料钎焊。借助金相显微镜(OM)、扫描电镜(SEM)、能谱分析仪(EDS)、MTS万能力学试验机等手段分析了钎焊接头夹杂物与力学性能的关系。结果表明钎焊接头的夹杂物有氧化物、硫化物、磷化物、硅酸盐、硼化物等。剪切断口观察到大范围的层状撕裂和局部的小韧窝,说明断裂为韧-脆混合型断裂。在裂纹发源处很容易观察到Fe-Co-O,MnO,FeS-MnS,FeP,2MnO·SiO_2等非金属夹杂物。由于非金属夹杂物的熔点、沸点较高,与钎料基体的热膨胀系数差别较大,容易成为裂纹萌发并扩展的区域,宏观表现为剪切强度降低。     

退火工艺对3003铝合金板微观组织与力学性能的影响

研究了退火工艺对3003铝合金板微观组织与力学性能的影响.试验结果表明,3003铝合金板退火后第二相粒子主要为Al6(Mn,Fe)、Al6 Mn以及在位错或者亚晶界等缺陷处形成的α-Al(Fe,Mn)Si相.随退火温度升高和保温时间延长,第二相粒子发生粗化,并出现了少量的弥散第二相.当退火温度为450℃时,第二相又重新...     

Formation of a chromium-carbide conversion coating on silicon carbide

The formation of a chromium-carbide conversion coating on SiC was achieved using the pack-cementation technique. The conversion coating is intended to improve the corrosion resistance of SiC and its derivatives, such as SiC-base continuous fibers and composites, by forming a protective Cr₂O₃ scale upon exposure to high-temperature corrosive environments. Different pack chemistry and processing parameters were evaluated in the laboratory. Results indicated that the coating morphologies and compositions achieved were significantly affected by variation of these processing factors. In this paper, the conversion coating obtained from one of the systems investigated is reported. The coating consists of a multilayered structure with each of the sublayers containing a high-Cr concentration. In addition, the coating surface is relatively dense and pore free compared to the underlying SiC substrate material. A dense and pore-free morphology is highly desirable for coating applications, especially on porous substrates. The multilayered coating structure consists of the following sublayers: Cr₂₃C₆/Cr₇C₃/Cr₇C₃+Cr₃Si/Cr₅Si₃C_x/SiC substrate.     

退火温度对低成本TC4LCA钛合金板材组织和性能的影响

为提高低成本TC4LCA钛合金板材的强度和冲击性能,选取不同退火温度对典型规格板材进行热处理,研究了其显微组织和力学性能的变化规律,分析了显微组织对强度和冲击性能的影响。结果表明,随着退火温度的升高,TC4LCA钛合金中的长条状初生α相转变为等轴状,β转变组织中析出针状或片状次生α相;退火温度越高,长条状初生α相含量减少,等轴化倾向明显,直至发生粗化;针状或片状次生α相长大。合金的强度先增大后减小、断后伸长率略有降低,冲击吸收能量则呈增大趋势。综合考虑,在800～880 ℃范围进行退火可使TC4LCA钛合金板材获得强度、塑韧性的最佳匹配。     

基体梯度结构对TiN涂层硬质合金力学和切削性能的影晌

研究基体梯度结构对TiN涂层硬质合金力学和切削性能的影响;采用阴极弧蒸发涂层工艺分别在均质和梯度硬质合金基体上制备TiN涂层:运用金相观察、扫描电镜分析、三点抗弯强度测试、显微硬度测试和切削性能测试,研究基体梯度结构对TiN涂层硬质合金组织结构、力学性能和切削性能的影响.结果表明:基体结构梯度化后,TiN涂层表面形貌由平整状变为网状结构,显微硬度提高19%,抗弯强度提高6.1%;基体结构梯度化后,涂层硬质合金的结构发生变化、力学性能得到提高,涂层刀片的抗冲击性能和切削性能分别提高10%和15%左右.