碳化硅增韧氧化铝基陶瓷刀具切削Inconel718合金的刀具界面行为研究#br#

通过宏观实验发现在切削过程中刀具的Al2O3/SiC界面发生了断裂;建立了碳化硅增韧氧化铝基陶瓷刀具切削Inconel718镍基合金的分子动力学模型和单一界面的Al2O3/SiC三维界面模型,从原子尺度分析碳化硅增韧氧化铝切削过程的界面行为。在原有势函数的基础上计算了刀具与工件原子间、刀具内部基体与增韧间的Morse势函数,分别计算了单相Al2O3、单相SiC、复相Al2O3/SiC单一界面的界面结合能。研究结果表明：与单相界面相比,复相Al2O3/SiC界面结合强度较高,SiC增韧能够增强刀具的韧性和强度,但切削过程中出现了工件原子扩散到刀具中的现象。对扩散后刀具基体和增韧间形成界面的界面结合能进行了计算,发现刀具的界面结合强度有所降低,使刀具在Al2O3/SiC界面处发生断裂。     

Scanning and transmission electron microscopy study of the microstructural changes occurring in aluminium matrix composites reinforced with SiC particles during casting and welding: interface reactions

Processing of aluminium matrix composites (AMCs), especially those constituted by a reactive system such as Al–SiC, presents great difficulties which limit their potential applications. The interface reactivity between SiC and molten Al generates an aluminium carbide which degrades the composite properties. Scanning and transmission electron microscopes equipped with energy-dispersive X-ray spectroscopes are essential tools for determining the structure and chemistry of the Al–SiC interfaces in AMCs and changes occurring during casting and arc welding. In the present work, an aluminium–copper alloy (AA2014) reinforced with three different percentages of SiC particles was subjected to controlled remelting tests, at temperatures in the range 750–900 °C for 10 and 30 min. Arc welding tests using a tungsten intert gas with power inputs in the range 850–2000 W were also carried out. The results of these studies showed that during remelting there is preferential SiC particle consumption with formation of Al₄C₃ by interface reaction between the solid SiC particle and the molten aluminium matrix. The formation of Al₄C₃ by the same mechanism has also been detected in molten pools of arc welded composites. However, in this case there was formation of an almost continuous layer of Al₄C₃, which protects the particle against further consumption, and formation of aciculate aluminium carbide on the top weld. Both are formed by fusion and dissolution of the SiC in molten aluminium followed by reaction and precipitation of the Al₄C₃ during cooling.     

Tribological behaviour of W‐alloyed carbon‐based coatings in dry and lubricated sliding contact

Carbon‐based coatings with different W contents were deposited by direct current magnetron sputtering in reactive and non‐reactive atmospheres. All deposited coatings have compact morphologies with amorphous (tungsten‐free) or nanocrystalline structures (tungsten‐doped). The latter one was indicated by very broad peaks in X‐ray Diffraction spectra in the position of tungsten carbide suggesting W‐carbide nanoparticles embedded in an amorphous carbon matrix. The hardness increased from 10 to 15 GPa with increasing W content. The coatings were tribological tested at dry and lubricated conditions with increasing temperature in a coating/steel configuration. In dry sliding, the friction coefficient increases with the increase of the temperature reaching values higher than 1.0. The friction is significantly lower in lubricated contact using three different oils: poly‐alpha‐olefin, paraffin and olive oil. The olive oil shows promising lubricating properties at the temperature lower than 70°C; however, at higher temperature, the coatings were quickly worn through. Copyright © 2014 John Wiley & Sons, Ltd.     

Synthesis of silicon carbide ceramics from a polysilastyrene at low temperatures

The conventional route for preparation of silicon carbide ceramics is by the use of pressureless sintering, hot pressing, or hot isostatic pressing of silicon carbide starting powders. High sintering temperatures (2073–2473 K) and the addition of sintering additives are normally used to enhance densification. These sintering additives, however, form second phases at grain boundaries which impair the mechanical properties of the material, particularly at high temperatures. It is therefore desirable that new processing routes are developed that overcome these difficulties. A proposed route is to use a polymeric pressure which can provide a Silicon carbide matrix as binding agent for silicon carbide powders, thus making the requirement for high temperatures and sintering additives unnecessary. This paper reports observations of the direct transformation of a polymeric precursor into amorphous Si–C, and crystalline SiC at low temperatures, and the use of this precursor as a binder for the production of SiC powder/ex-precursor SiC composites.     

Heterogeneous fibre microstructures and their influence on failure

A detailed microscopic, experimental and micromechanical evaluation of fibre damage initiation in a unidirectional aluminium matrix-silicon carbide fibre (SCS-8^TM) composite has been performed for a monotonic load sequence. The salient fibre features include a 33-μm-diameter monofilament turbostratic carbon (C) core, a ∼ 1·5-μm pyrolytic C layer, an interior sheath of β-phase silicon carbide (SiC) crystallites imbedded in an amorphous C matrix, and an exterior sheath of radially orientated β-SiC. Quantitative microscopy shows that the interior sheath's surplus C varies smoothly from ∼ 35% by volume near the core to zero at the mid-radius. The multi-phase structure of the fibre produces an internal mechanical stiffness that increases with distance from the core, and thus peak stresses result in the exterior sheath. X-ray tomographic microscopy (XTM) reveals that cores fracture randomly, without failure of the surrounding SiC, at stress levels above half the ultimate strength of the composite. Three-dimensional XTM reconstructions show planar, non-planar and spiral cracks in the failed fibre, suggesting multiple and competing initiation mechanisms. Qualitative fracture assessments suggest that flaws near the C core grow outward in a curved manner through the SiC–C and planar beyond the mid-radius, whereas cracks originating near the fibre–matrix interface favour planar trajectories inward across the whole fibre.     

典型碳化硅光学材料的超光滑抛光试验研究

研究了三种典型的碳化硅光学材料CVD SiC、HP SiC以及RB SiC的材料去除机理与可抛光性,并对其进行了超光滑抛光试验。在分析各种材料制备方法与材料特性的基础上,通过选择合理的抛光工艺参数,均获得了表面粗糙度优于Rq=2nm（采样面积为0.71mm×0.53mm）的超光滑表面。试验结果表明：研磨过程中,三种碳化硅光学材料均以脆性断裂的方式去除材料,加工表面存在着裂纹以及材料脱落留下的缺陷;抛光过程中,CVD SiC主要以塑性划痕的方式去除材料,决定表面粗糙度的主要因素为表面微观划痕的深度;HP SiC同时以塑性划痕与晶粒脱落的形式去除材料,决定表面粗糙度的主要因素为碳化硅颗粒大小以及颗粒之间微孔的尺寸;RB SiC为多组分材料,决定其表面粗糙度的主要因素为RB SiC三种组分之间的去除率差异导致的高差。
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Analysis of SiC fibres and composites using Raman microscopy

The application of Raman spectroscopy in the analysis of the microstructure of SCS-6 silicon carbide fibres using a Renishaw Raman microscope is described. It is demonstrated that the technique allows a detailed study to be made of the point-to-point variation in microstructure across a fibre section. It has been possible to monitor the variation of the concentration of SiC and carbon in the fibre microstructure and to detect differences in the forms of carbon present. It is also shown that Raman spectroscopy can be used to follow the micromechanics of both the deformation of silicon carbide fibres and of the fibres within a model composite. Well-defined Raman spectra have been obtained from a variety of Nicalon and Tyranno fibres and the positions of the Raman bands shown to shift on the application of stress or strain. From such stress-induced Raman band shifts, the point-to-point variation of axial fibre stress or strain along an individual fibre in an epoxy matrix can be determined. An example is given of the use of the technique to map the distributions of axial fibre strain in a Nicalon/epoxy fragmentation test specimen and to model the failure processes at the fibre/matrix interface.     

流体密封用碳化硅陶瓷增韧技术研究进展

综述了几种碳化硅陶瓷增韧化技术手段,通过对自增韧、表面改性和韧化、颗粒弥散相增韧以及纤维、晶须增韧等方法在碳化硅陶瓷增韧研究的对比论述,总结了不同增韧化技术的增韧机理和在实际应用中的优缺点,提出了碳化硅陶瓷增韧实用技术的发展方向。     

Removal behavior and performance analysis of defective silicon carbide in nano-grinding

The process of grinding silicon carbide(SiC) single crystals containing defects with a single diamond was simulated using molecular dynamics(MD) software. A 4H–SiC simulated workpiece—with transverse, vertical, 45°, and 135° defects—and a diamond tool were established through molecular dynamics, in addition, the models of different depth defects were established. The grinding removal behavior of 4H–SiC in different directions and positions was analyzed. The simulation results demonstrate that there is an elastic deformation interval during 4H–SiC grinding, whereby the defect within the interval will be filled and the defect outside the interval will not. Meanwhile, the resultant force curve, von Mises stress distribution, hydrostatic pressure distribution, surface appearance and temperature distribution were also analyzed. This research has theoretical significance in the optimization of semiconductor SiC process parameters.     

Effect of machining parameters on surface roughness and tool wear for 7075 Al alloy SiC composite

In the present study, an attempt has been made to investigate the influence of cutting speed, depth of cut, and feed rate on surface roughness during machining of 7075 Al alloy and 10 wt.% SiC particulate metal-matrix composites. The experiments were conducted on a CNC Turning Machine using tungsten carbide and polycrystalline diamond (PCD) inserts. Surface roughness of 7075Al alloy with 10 wt.% SiC composite during machining by tungsten carbide tool was found to be lower in the feed range of 0.1 to 0.3 mm/rev and depth of cut (DOC) range of 0.5 to 1.5 mm as compared to surface roughness at other process parameters considered. Above cutting speed of 220 m/min surface roughness of SiC composite during machining by PCD tool was less as compared to surface roughness at other values of cutting speed considered. Wear of tungsten carbide and PCD inserts was analyzed using a metallurgical microscope and scanning electron microscope. Flanks wear of carbide tool increased by a factor of 2.4 with the increase of cutting speed from 180 to 240 m/min at a feed of 0.1 mm/rev and a DOC of 0.5 mm. On the other hand, flanks wear of PCD insert increased by only a factor of 1.3 with the increase of cutting speed from 180 to 240 m/min at feed of 0.1 mm/rev and DOC 0.5 mm.     

Ceramic particulate composites in the system SiC–TiC–TiB2 sliding against SiC and Al2O3 under water

The tribological behaviour of SiC, SiC–TiC and SiC–TiC–TiB₂ was determined in oscillating sliding against SiC and α-Al₂O₃ in water at room temperature. The tribo-systems with the composite materials containing TiC and TiB₂ differ significantly from the systems with the single phase SiC: The wear is reduced and the friction is increased. The wear reduction up to a factor of 10 is mainly due to the formation of an oxide film containing titanium oxides which is soft, stable in water and well adhering to the bulk material. This oxide film is transferred to the alumina ball but not to the silicon carbide ball.     

Ti-(SiCf/Al3Ti)层状复合材料在侵彻早期阶段基于NEIM的建模与仿真研究

一种由连续SiC陶瓷纤维强化（CCFR）的新型金属间化合物基层状（MIL）复合材料Ti-（SiC_f/Al₃Ti）研制成功,其作为装甲防护材料在被侵彻过程中的微观变形机制、强化及失效机理有待研究。首先采用电镜扫描法（SEM）表征该材料的微观结构和界面特征,而后提出一种新的建模方法,基于高效的等效夹杂数值算法（NEIM）,对刚性球侵彻靶体复合材料的早期接触过程进行建模,研究准静态加载下Ti-（SiC_f/Al₃Ti）复合材料的小变形弹塑性接触力学行为,并通过压痕试验验证该模型的精确性。结果表明,在球-面加载模式下,SiC纤维对CCFR-MIL复合材料的强度提高显著,而对延展性的强化却不如面-面加载模式下明显。此外,球-面加载模式下的最大塑性应变集中（MPSC）区出现在Al₃Ti层最接近中心SiC纤维上边界的位置,并随载荷的增大沿深度方向延伸,该区域是裂纹萌生并扩展的高发区。当相邻SiC纤维的中心距为四倍纤维直径,且Ti层体积分数为40%时,CCFR-MIL复合材料的综合力学性能最佳。更高效数值方法的运用,节约了计算成本,清晰透彻地揭示了新型CCFR-MIL复合材料在被侵彻过程中的微观变形机制、强化及失效机理,并使参数化研究更为全面,便于材料的微观结构优化。     

核主泵备用机械密封材料的摩擦性能研究

采用Falex-1506摩擦磨损试验机,研究了水润滑、室温条件下,载荷和速度对核主泵用机械密封材料:无压烧结碳化硅(WNV2)和碳化硅加碳(CHV1)、反应烧结碳化硅(R)和碳化硅加碳(R2)、石墨(MSMG)在不同配副条件下摩擦学特性的影响规律。使用扫描电镜(SEM),对磨损表面进行了观察和分析。研究结果表明,碳化硅和石墨材料自身的孔隙,在高载荷下容纳了更多的润滑流体,因此,不同配副条件下的摩擦系数均随载荷的增加而减小。另外,滑动速度引起的温度改变通过影响表面层性质影响摩擦力,而碳化硅和石墨在很宽的温度范围内机械性质保持不变,所以摩擦系数随速度的增加基本不变。     

固体碳源在碳化硅浆料中的分散

碳化硅固相烧结必须添加C、B烧结助剂,才能促进碳化硅的致密。而烧结助剂在碳化硅粉体中间的分散是影响碳化硅陶瓷结构是否均匀以及能否提高其力学性能的关键。本文研究3种固体碳源材料石油焦、碳黑和石墨粉末在固含量为15%的碳化硅浆料中的分散行为。用沉降实验数据比较无分散剂、加入分散剂焦磷酸钠或Darven C3种不同分散条件下,固体碳源粉体的分散性。实验结果表明:分散剂的加入对SiC浆料分散效果有很大影响,3种碳源中碳黑分散最好;最佳分散组合为碳黑加入Darven.C。     

Effect of CNFs content on the tribological behaviour of spark plasma sintering ceramic–CNFs composites

Alumina-carbon nanofibres (CNFs) and silicon carbide–CNFs nanocomposites with different volume fraction of CNFs (0–100 vol.%) were obtained by spark plasma sintering. The effect of CNFs content on the tribological behaviour in dry sliding conditions on the ceramic–carbon nanocomposites has been investigated using the ball-on-disk technique against alumina balls. The wear rate of ceramic–CNFs nanocomposites decreases with CNFs increasing content. The friction coefficient of the Al₂O₃/CNFs and SiC/CNFs nanocomposites with high CNFs content was found to be significantly lower compared to monolithic Al₂O₃ and SiC due to the effect of CNFs and unexpectedly slightly lower than CNFs material. The main wear mechanism in the nanocomposite was abrasion of the ceramic and carbon components which act in the interface as a sort of lubricating media. The experimental results demonstrate that the addition of CNFs to the ceramic composites significantly reduces friction coefficient and wear rate, resulting in suitable materials for unlubricated tribological applications.     

SiCf/SiC陶瓷基复合材料单颗磨粒磨削试验研究

对碳化硅纤维增强碳化硅陶瓷基复合材料（SiCf/SiC）进行了单颗磨粒磨削试验,研究了脆性去除模式下磨削中侧边崩碎规律。研究结果表明：磨削SiC基体时,基体内裂纹易引发磨削中大块侧边崩碎;磨削纤维时,侧边崩碎宽度随纤维与磨削方向夹角的增大而增大;磨削90°纤维时,提高磨削速度可减小侧边崩碎程度;以50 m/s和90 m/s的磨削速度磨削造成的侧边崩碎宽度比以20 m/s磨削时的侧边崩碎宽度分别小30%和60%;在试验参数范围内,增大磨削用量不会增大磨削中侧边崩碎程度,但可以提高材料去除率。     

快速凝固结合粉末冶金法制备SiC_p/AZ91复合材料的组织及性能

用快速凝固结合粉末冶金法制备了SiC颗粒增强镁合金基复合材料(SiCp/AZ91)棒材,研究了SiC颗粒含量对复合材料室温力学性能及显微组织的影响.结果表明:制备的复合材料棒材中SiC颗粒在基体中分布均匀,但仍存在局部颗粒团聚现象;随SiC颗粒含量的增加,复合材料的屈服强度、抗拉强度和断后伸长率均逐渐降低;热挤压过程中,镁、SiC和SiO2之间发生了界面反应,在界面生成Mg2Si等脆性相,影响了复合材料的界面性能.     

TIG电弧辅助熔滴沉积增材制造SiCp增强铝基复合材料中的熔池动力学与颗粒迁移行为

基于计算流体力学方法,考虑电弧力、碳化硅颗粒(SiC_p)增强相与液态铝合金基体相之间的相互作用以及表面张力等因素,建立了Si C_p增强铝基复合材料钨极氩弧焊(TIG)电弧辅助熔滴沉积增材制造中的三维瞬态熔池行为数值模型,通过与堆积试样的横截面形貌、Si C_p分散状态实验结果对比,验证了熔池行为数值模型的有效性。通过数值模拟,揭示了堆积过程熔池峰值温度演变规律、熔滴冲击诱导的熔池动力学行为、熔池流态对Si C颗粒迁移行为的影响。结果表明,堆积过程涉及熔滴冲击、合并、铺展、熔池回弹4个阶段;冲击点附近出现重熔,熔滴冲击造成熔池中心区域产生明显的V字形凹陷,熔池边缘形成冠状隆起;在熔体拖拽力的作用下Si C颗粒更多的分布于堆积层两侧;熔滴冲击引起的熔池内局部高压以及熔池底部对流,抑制了Si C颗粒向熔池底部沉降。     

Fracture behavior of C/SiC composites at elevated temperature

The fracture behavior of carbon fiber-reinforced silicon carbide (C/SiC) composites used in rocket nozzles has been investigated under tension, compression, and fracture conditions at room temperature, 773 K and 1173 K. The C/SiC composites used in this study were manufactured by liquid silicon infiltration process at ~1723 K. All experiments were conducted using two types of specimens, considering fiber direction and oxidation condition. Experimental results show that temperature, fiber direction, and oxidation condition affect the behavior of C/SiC composites. Oxidation was found to be the main factor that changes the strength of C/SiC composites. By applying an anti-oxidation coating, the tensile and compressive strengths of the C/SiC composites increased with temperature. The fracture toughness of the C/SiC composites also increased with increase temperature. A fractography analysis of the fractured specimens was conducted using a scanning electron microscope.     

Tool fabrication system for micro/nano milling—function analysis and design of a six-axis Wire EDM machine

The dies or molds used for the fabrication of micro products usually are made of ultra-hard materials such as tungsten carbide or silicon carbide and have sophisticated three-dimensional geometries. Such kind of dies or molds can only be fabricated by milling operations instead of grinding processes with ultra-hard milling tools made of PCD or CBN. Electrical discharge machining (EDM) is a good choice for the fabrication of such ultra-hard tools. In this paper, a function analysis and design of a six-axis Wire EDM (WEDM) machine is introduced. Based on the typical micro/nano cutting tool geometry features, a mathematical model between the cutting tool and the electrode wire is built. Then, the mathematical model is analyzed and it turns out that six axes are needed for cutting such complicated tool geometries. According to the WEDM features, first the axes are allocated to the workpiece side and the electrode wire side. The workpiece is assigned three linear motions and one rotary motion around its center line and the wire has two rotary motions. Second, the axis sequences are defined. At last, the best concept of the mechanical structure for the six-axis WEDM machine is selected.