熔渗法制备SiC/ FexSiy 复合材料显微结构和性能

用Fe_xSi_y (Fe₃Si, Fe₅Si₃, FeSi ) 熔体自发浸渗SiC 粉体预制件制备出致密度高达9615 %的SiC/ Fe_xSi_y复合材料。利用XRD、OM 和SEM 等对复合材料的相组成、显微结构和力学性能进行了分析和表征。研究发现, 自发浸渗过程中SiC 在Fe_xSi_y 熔体中有溶解和析出, 导致复合材料的相组成和显微结构发生变化。Fe₃Si 渗入后, 复合材料中有碳析出, 并生成Fe₅Si₃ 和FeSi 两种新相;Fe₅Si₃ 和FeSi 的渗入无碳析出, 而是发生碳化硅烧结、粒子合并和晶粒长大;Fe₅Si₃ 熔体渗入超细碳化硅(0.5μm ) 粉体预制件后, 生成大的碳化硅晶粒和碳化硅单晶。借助CHEMSAGE 热力学数据库对1873 K 时Fe-Si-C 体系的平衡相图进行了定量分析, 研究了SiC 在Fe_xSi_y 熔体中的稳定性;解释了铁的三种硅化物浸渗后, SiC 颗粒形态的变化。通过对复合材料显微硬度、弯曲强度和可靠性(威布尔系数) 的测试, 分析了熔渗法SiC/ Fe_xSi_y 复合材料结构和性能的关系。     

Role of preplaced silicon on a TIG processed SiC incorporated microalloyed steel

ABSTRACT

Research aimed at enhancing the surface properties of carbon steels by incorporating fine silicon carbide particulates has had limited success because the dissolution of the ceramic occurred. This research considers a method of reducing SiC dissolution by generating a high Fe–Si liquid which protects the ceramic. Three particulate groups were investigated, (1) ～ 5?µm SiC, (2) ～45?µm Si +～ 5?µm SiC, and (3) ～45?µm Si, all incorporated into a microalloyed steel using a tungsten inert gas process. Detailed microhardness of the melt zones together with microstructural analysis showed that the addition of Si resulted in a cracked hard layer containing SiC. However, in Specimen 1, a thicker, hardcrack-free layer resulted from the microstructure developed by the dissolution of SiC.     

Effect of magnesium alloying additions on infiltration threshold pressure and structure of SiC powder compacts infiltrated by aluminium-based melts

Infiltration of 53% dense preforms of 23 m size SiC particulate by Al–0 to 14 wt% Mg alloy melts has been carried out at different applied pressures at 750°C, together with microstructural characterization of the resulting composites. The threshold pressure P* for infiltration decreased with increasing Mg content in the melt at twice the rate by which its surface tension decreased, the residual effect being attributable to the expected effect of Mg on the contact angle between Al and SiC. Periodic bands of entrapped porosity at lower Mg contents and infiltration pressures just above P* are thought to have arisen from periodic arrest of the infiltration front pending ventilation of the gas accumulated at the front. The formation of monolithic Mg2Si and areas of lamellar Al–Mg2Si eutectic observed at higher Mg contents is associated with the accumulation of 5 wt% Si in the melt as a result of reaction between the melt and SiC.     

Microstructural evolution in reaction-bonded silicon carbide

A detailed microstructural investigation of reaction-bonded silicon carbide has been performed using both fully-bonded and quenched samples and other specially prepared specimens containing large original single crystals of known crystallographic habit. The development of the epitaxial SiC overgrowth on the original SiC particles has been followed and found to proceed by the progressive growth and coalescence of identically-oriented nuclei. This epitaxial layer grows with a habit characteristic of (cubic) SiC and then transforms to-SiC in the high temperature region behind the reaction front. The formation of faceted grain boundaries is explained by this growth morphology. Furthermore, SiC:SiC grain boundaries, SiC:SiC epitaxial boundaries and SiC:Si interfaces have all been characterized by TEM techniques. The grain boundaries are of particular interest since they usually comprise a thin (1 nm) layer of amorphous SiC with occasional suicide and graphite inclusions. The general cleanliness of the vast majority of interfacial area is a result of the removal of the impurities insoluble in SiC by liquid silicon moving through the sample. The overall distribution of impurities is discussed. Other microstructural features have been characterized and texturing due to original particle alignment during fabrication of the green compact investigated. The control of the mechanical properties of reaction-bonded silicon carbide by these various microstructural features is discussed. A basis for an explanation of the interesting trace-impurity-controlled contrast seen in secondary electron images of these materials is also established.     

热蒸发法碳化硅纳米晶须阵列的合成与表征

在1600℃不同真空度下, 采用热蒸发硅的方法, 在石墨基板和聚丙烯腈(PAN)炭纤维两种碳源基体原位生长具有一定取向的碳化硅纳米晶须——垂直于石墨片表面森林状和试管刷状碳化硅纳米晶须阵列。通过X射线衍射及场发射扫描电镜, 发现晶须为3C-SiC, 直径约100nm, 长度约50μm。炭纤维表面的产物顶端多为针尖状, 而石墨片表面的产物多为六方棱柱状。因其纳米尺寸效应, 在380nm波长的光激发下, 所制晶须在波长为468nm 附近出现光致发光峰。透射电镜、 多点衍射电子衍射图表明, 所制得的3C-SiC晶须为单晶, 其生长方向为3C-SiC的[111]方向。基于反应过程中硅熔体与碳源分离的事实, 讨论了3C-SiC晶须阵列生长的气固反应机理。      

熔融渗透工艺制备SiC-TiSi2复相陶瓷的反应机理

熔融Si渗透过程伴随着复杂的化学反应及多组分扩散,对该过程进行研究有助于更好地理解熔渗反应机理。本工作采用熔融渗透工艺制备SiC-TiSi2复相陶瓷,在生成SiC基体的同时原位生成TiSi2。通过扫描电子显微镜(SEM)、X射线能谱分析(EDS)和微区X射线衍射(micro-beam XRD)分别对熔融硅区域、Si/SiC界面以及SiC基体的微观结构和相组成进行表征和分析,研究了熔渗工艺制备SiC-TiSi2的反应机理。结果表明:高温下液Si渗入C-TiC预制体,发生化学反应生成SiC、TiSi2以及少量副产物Ti5Si3,其中Ti5Si3主要集中于Si/SiC界面处。随着反应进行,液Si与TiSi2形成液态Ti-Si共晶。该液态共晶通过流动扩散在Si区域中析出TiSi2。而预制体中的少量固态C在液Si中溶解、扩散,并在Si区域生成均匀分布的孤立SiC颗粒。     

Methods for the reduction of the micropipe density in SiC single crystals

Micropipes are very harmful for SiC devices. Even one micropipe in the active area can destroy a high-voltage SiC device. Therefore, it is necessary to reduce the density of micropipes in SiC single crystals. In the present paper, we proposed methods for reducing micropipes. Restriction of screw dislocations and decrease of inclusions are the key factors to reduce the number of micropipes. (0 0 0 1) Si-face, and crystal faces acted as growth surface in different experiments. Active carbon was appended to act as carbon source. The crucible and active carbon were subjected to X-ray diffraction investigation before and after growth. The experimental results indicate that the activity of the graphite crucible was low, and it decreased with the progressing crystal growth, which increased the probability of micropipe formation. Appending active carbon can act as ample carbon source for crystal growth. The reduction of micropipes was achieved by the restrained formation of Si liquid phase. Using and crystal faces as the growth surfaces the generation of micropipes was restricted, as no new micropipe generated on the and crystal faces. At the same time, the density of edge dislocations is reduced considerably.     

Basic studies of the corrosion of SiC by silicate melts

Three different commercially available SiC materials (one SiSiC and two sintered SSiC) and single crystals of -SiC were corroded at 1000 °C by silicate melts based on Na₂O-2SiO₂ with different additives of Al₂O₃, CaO, Fe₂O₃ and TiO₂. During the corrosion experiments the samples were completely immersed in the melts. Parts of the samples were subsequently leached in H F for weight loss determination and surface change examinations, and other parts of them were prepared as polished cross-sections for SEM-EDXS investigations of SiC-melt interfaces. Corrosion starts by dissolution of a silicon dioxide layer in the silicate melt followed by oxidation of the free silicon phase, and silicon carbide phase, and grain boundary attack. Corrosive attack and corrosion mechanisms are dependent on the composition and structure of the silicon carbide material. An influence of Fe, Al, Ca and Ti addition on the extent of corrosion was observed. In the vicinity of SiC-melt interfaces SiO₂-rich melt regions were found. The unusually high corrosion rate of SiC samples in the case of Fe₂O₃ addition to sodium disilicate was concluded to be the consequence of Fe²⁺-Fe³⁺ transition. Some defects in materials which were introduced during manufacturing or machining corroded much more severely than the defect-free material.On leave from J. Stefan Institute, Ljubljana, Slovenia.     

微孔碳陶瓷化反应机理的研究

研究了微孔碳制备SiC陶瓷的反应过程及反应机理.SiC陶瓷的最终组织受C/Si质量比及渗硅时间的影响.较低C/Si质量比时试样中心未被硅化,较高C/Si质量比时制得由SiC、Si及极少量未反应C组成的复相陶瓷.随渗硅时间延长,复相陶瓷中SiC颗粒的形状由长条状向无规则状转变,颗粒分布则由局部有序向无序转变.反应过程为：液硅因毛细作用自发的渗入微孔碳孔道,同时与溶解的碳壁发生反应生成SiC.因孔道及碳壁尺寸的不同造成液硅渗入深度和碳壁被溶解厚度的差异,结果出现被SiC包裹的小颗粒碳.随后被包裹的碳通过SiC层扩散到液硅中,沉积在先形成的SiC上促使大量先形成SiC的合并以及部分硅被SiC包裹,进而改变SiC陶瓷的显微组织.     

Mechanism and kinetics of the chemical interaction between liquid aluminium and silicon-carbide single crystals

Previous investigations of phase equilibria in the ternary system Al-C-Si have shown that silicon carbide is attacked by pure aluminium at temperatures higher or equal to 923±3 K and up to about 1600 K, according to the chemical reaction: 4Al+3SiC Al₄C₃+3Si In the present work, a study has been carried out to obtain more detailed information on the mechanism and kinetics of this reaction. For that purpose, 6H silicon carbide platelets with broad Si (0 0 0 1) and C (0 0 0 ¯1) faces were isothermally heated at 1000 K in a large excess of liquid aluminium. Characterization of the resulting samples by Auger electron spectroscopy (AES) and scanning electron microscopy (SEM) revealed that the reaction proceeds in both faces via a dissolution-precipitation mechanism. However, the polarity of the substrate surface strikingly influences the rate at which silicon carbide decomposes: dissolution starts much more rapidly on the Si face than on the C face, but, while a barrier layer of aluminium carbide is formed on the Si face protecting it against further attack, the major part of the C face remains directly exposed to liquid aluminium and thus may continue to dissolve at a low but constant rate up to complete decomposition of the -SiC crystal.     

带有SiC浓度梯度炭材料的抗氧化涂层

在炭材料表面通过莫来石（３Ａｌ２Ｏ３·２ＳｉＯ２）或锆石（ＺｒＳｉＯ４）氧化物薄膜涂层与ＳｉＣ的浓度梯度薄层结合,成功地获得了在空气流中１４００℃下的高抗氧化性能。在１４５０℃时,采用将炭材料直接浸入熔融Ｓｉ金属形成ＳｉＣ浓度梯度,通过溶胶－凝胶过程形成锆石和莫来石氧化薄膜。水解速率的控制是产生均匀薄膜的关键。具有ＳｉＣ浓度梯度和莫来石涂层的各向同性高密度石墨块足以在空气中１４００℃下抗氧化和１４００℃到液氮温度下抗淬火。莫来石涂层比锆石涂层更有效。     

Evolution of 3C-SiC islands nucleated from a liquid phase on Si face α-SiC substrates

The contact between α-SiC crystals and Si-Ge based melts provokes the nucleation of 3C-SiC islands on the crystal surface. Evolution of these islands as a function of various parameters was studied. On both 4H and 6H substrates, it was found that, after nucleation, 3C-SiC islands first enlarge and may form a complete 3C layer under certain conditions. The 3C deposit can then be dissolved by the liquid phase at high temperature or for prolonged contact at relatively moderate temperature. The graphite crucible is proposed to play a central role in these enlargement and dissolution mechanisms by providing extra carbon atoms on the seed surface (enlargement) or provoking thermal induced carbon transport toward the sidewall (dissolution). Several differences between the use of 4H and 6H substrates were also observed.     

Interaction of 18Cr-10Ni stainless steel with liquid aluminium

The dissolution of an 18Cr-10Ni stainless steel in liquid aluminium at 700 to 850 ° C was found by the rotating disc technique to be non-selective and diffusion controlled. Experimentally determined values of the parameters characterizing the dissolution run are presented. In the case of saturated aluminium melts two intermetallic layers were found to form between the steel and the melt material at 700 °C. The compact layer adjacent to the steel surface is probably a solid solution based upon the Fe₂Al₅ compound. Its thickness,x, tends with increasing time to the limiting valuex _max = 10m. The porous layer adjacent to the melt material is probably a solid solution based upon the FeAl₃ compound. After a certain period of non-linear growth its thickness,Y, increases with time,t, according to the linear law:Y = 1 × 10^–8 t + 6 × 10^–6 m. The time dependence of the total thickness of both layers is well described in terms of the paralinear kinetics. In the case of undersaturated aluminium melts the formation of a single-phase intermetallic layer, 3 to 11 m thick, was observed at 700 ° C for 100 to 600 sec. The steel-to-aluminium transition joints with good mechanical properties were made by interaction of a solid steel material with liquid aluminium.     

Evidence of hexagonal diamond in plasma-deposited carbon films

Hexagonal diamond grains of 30 nm diameter together with graphite and SiC are seen in predominantly amorphous carbon films deposited at low temperature on Si substrates from a CH₄ plasma vapour source. The different crystalline phases are identified by grazing-angle X-ray diffraction which allows for substrate rotation and tilting to enable the 2 peaks to be correlated with the angular displacements of specific planes. Electron energy-loss spectroscopy shows the chemical composition of the films to be predominantly carbon with traces of oxygen. Raman spectroscopy shows the peaks to be associated with amorphous carbon and graphite, together with a peak at 1170 cm^–1 which is attributed to microcrystalline hexagonal diamond.     

Oxidation reaction during laser cladding of SAE1045 carbon steel with SiC/Cu alloy powder

Laser cladding of SAE1045 carbon steel with SiC/Cu alloy powder was performed in air. The microstructures of the cross-section of the clad layer obtained were obviously divided into three laminae. Analysis results using X-ray diffraction, scanning electron microscope (SEM) and electron probe microanalysis (EPMP) indicate that there exist -Cu, SiO₂ and Cu₅Si in the top layer. However, SiC particles (SiCp) originally added almost disappeared during laser cladding. To simulate the reaction taken place in the laser pool, differential thermal analysis was carried out by using a Perkin-Elmer DSC 7 in an atmosphere of Ar and O₂. This revealed that the SiCp is stable during heating in the atmosphere of Ar. However, intensive oxidation and decomposition of the SiCp were found when heating was undertaken in an O₂ atmosphere, which reasonably explains the formation of SiO₂ and Cu₅Si in the top layer.     

Electrophoretic deposition [EPD] applied to reaction joining of silicon carbide and silicon nitride ceramics

Electrophoretic Deposition (EPD) was used to deposit a mixture of SiC or Si₃N₄ filler and reactive carbon (graphite and carbon black) particles onto various SiC or Si₃N₄ parts in preparation for reaction bonding. The particles had gained a surface charge when mixed into an organic liquid consisting of 90 w % acetone + 10 w % n-butyl amine to form a slurry. The charged particles then moved when placed under the influence of an electric field to form a green deposit on the ceramic parts. The green parts were then dried and subsequently joined using a reaction bonding method. In this reaction bonding, molten Si moves into the joint via capillary action and then dissolves carbon and precipitates additional SiC. An optimum mixture of SiC filler to C powder ratio of 0.64 was identified. Residual un-reacted or free Si was minimized as a result of selecting powders with well-characterized particle size distributions and mixing them in batch formulas generated as part of the research. Image analysis of resulting microstructures indicated residual free Si content as low as 7.0 vol % could be realized. Seven volume percent compares favorably with the lowest free Si levels available in experimental samples of bulk siliconized (reaction-bonded) SiC manufactured using conventional reaction-bonding techniques. The joints retained the residual silicon over a large number of high-temperature thermal cycles (cycling from below to above the melting point of silicon). Comparisons to commercial reaction-bonded SiC indicated the majority of residual silicon of the joint was retained in closed porosity. This infers that parts made with these joints might be successfully utilized at very high temperatures. It was demonstrated that the EPD technique could be applied to butt, lap, and scarf type joints, including the capability to fill large gaps or undercut sections between parts to be joined. The overall results indicate that EPD, combined with reaction bonding, should allow for the fabrication of large complex structures manufactured from smaller components consisting of silicon carbide or silicon nitride.     

Effects of carbon content of carbon steel on its dissolution into a molten aluminum alloy

Hot dip aluminizing of carbon steels with different carbon concentration ranging 0.2–1.1 wt.% was carried out in a molten Al–9.08 wt.% Si–0.98 wt.% Fe alloy at 660 °C. The steel specimens lost weight as a result of dissolution into the melt, and an intermetallic layer was formed on the surface of them. The specimens showed varied dissolution rates depending on carbon concentration. The specimen with the highest carbon content exhibited the slowest dissolution rate. The thickness of the intermetallic layer increased with dipping time following a parabolic relationship. The growth rate of the layer decreased with increase of the carbon content. A diffusion mechanism to control the dissolution of the carbon steel into the molten aluminum alloy was suggested, and the effect of carbon content on the dissolution of the steel substrate into the melt was discussed in connection with the proposed diffusion mechanism and microstructural observations.     

Inoculation mechanism of rare earth–Mg alloy in molten cast iron

The dissolution region of RE–Mg (RE=rare earth) alloy in the reaction chambers of in-mould inoculated castings was studied by scanning electron microscopy and on electron microprobe. The dissolution region of the inoculant was divided into 7 zones according to its microstructure. The RE–Mg alloy is composed of Mg2Si, a Mg2Si–Si eutectic, FeSi, RE–Mg–Si and other silicides. The dissolution of the alloy is a process in which the low melting point phases such as Mg2Si, a Mg2Si–Si eutectic and RE–Mg–Si dissolve first, and the high melting point phases such as FeSi and FeSi2 dissolve later. In addition, some intermediate products form during the dissolution process. This causes a concentration gradient of Mg and RE in the melt which results in a transition of the graphite morphology from normal spheroid to open nodule, vermicular and flake graphite. The dissolution of the FeSi phase in the melt forms local sites of high silicon concentration which promote the nucleation and growth of the graphite. © 1998 Chapman & Hall     

Effect of doping elements on the morphology of graphite grown from Ni-C melts

The morphology of graphite crystals grown from Ni-C melts doped with various elements was studied in detail, utilizing scanning electron microscopy, secondary ion mass spectroscopy and X-ray diffraction. Graphite crystals grown from pure Ni-C melts are normally flaky. The addition of lanthanum or calcium to the melt resulted in a tendency towards a spherical morphology, which was associated with the appearance of tiny carbide crystallites on the {0 1¯1 0} graphite surfaces. The addition of lead resulted in enhanced graphite crystal dimensions. Pyramidal-shaped tiny graphite crystallites covered the (0001) surfaces. An approximately 20 nm thick layer of adsorbed Pb also appeared on the (0001) surfaces. Similar effects on the morphology were obtained by the addition of sulphur (S), bismuth (Bi), antimony (Sb), or selenium (Se). The addition of La or Ca with Pb induced the growing of graphite crystals in the form of pyramids. These were covered by very tiny carbide crystallites situated on the {0 1¯1 0} faces of the graphite. The results are discussed in terms of the dopants' effects on the undercooling and growing rates in the different graphite crystallographic directions, as well as on adsorption to or interaction between the added elements and the growing graphite crystal surfaces.     

在泡沫碳化硅载体上原位生长silicalite-1型沸石晶体

以多晶硅颗粒为硅源, 在泡沫碳化硅载体上原位水热合成silicalite--1型沸石晶体。研究了硅颗粒加入量、NaOH浓度以及合成时间等因素对沸石晶体的负载量、晶体尺寸和沸石晶体/泡沫碳化硅复合材料比表面积的影响。结果表明,
以多晶硅颗粒为硅源控制硅酸根的释放速度, 使沸石晶体在碳化硅载体表面异质界面形核, 从而实现沸石晶体在泡沫碳化硅载体表面的连续生长; 当多晶硅量过少时, 溶液中的硅酸根浓度过低, 不能在载体表面形成连续生长的沸石层;
而当多晶硅量过大时, 溶液中硅的浓度过高, 部分沸石晶体在溶液当中形核, 使沸石晶体在载体表面的负载量下降; 提高溶液中NaOH的浓度, 加快硅的溶解, 使溶液中硅的饱和浓度升高, 沸石晶体的形核率也随之升高, 使沸石晶体的负载量增加。在最优条件下制备的silicalite--1/泡沫碳化硅复合材料其沸石晶体的比表面积为81.28 m²g^-1。