Reactive sintering of B_4C-TaB_2 ceramics via carbide boronizing:Reaction process,microstructure and mechanical properties

Carbide boronizing is a promising approach to obtain fine grained boron carbide based ceramics with improved mechanical properties. In this work, reaction process, microstructural characteristics and mechanical properties of BxC-TaB_2(x = 3.7, 4.9, 7.1) ceramics were comprehensively investigated via this method. Dense BxC-TaB_2 ceramics with refined microstructure were obtained from submicro tantalum carbide and boron powder mixtures at 1800?C/50 MPa/5 min by spark plasma sintering. The stoichiometry of boron carbide was determined from lattice parameters and Raman shift. It was found that uniformly distributed TaB_2 grains in the BxC matrix is favor of the densification process and restricting grain growth.Besides, planar defects with high density were observed from the as-formed B7.1 C grains and transient stress was considered to contribute to the densification involved with plastic deformation. Microstructural observations indicate the dissolution of oxygen in the TaB_2 lattice and most of the B7.1 C/TaB_2 phase boundaries were clean. Owing to the highly faulted structure and finer grain size, as-obtained BxC-TaB_2 ceramics exhibit high Vickers hardness(33.3–34.4 GPa at 9.8 N) and relatively high flexural strength ranging from 440 to 502 MPa.     

Characterisation of silicon carbide films deposited by plasma-enhanced chemical vapour deposition

The paper presents a characterisation of amorphous silicon carbide films deposited in plasma-enhanced chemical vapour deposition (PECVD) reactors for MEMS applications. The main parameter was optimised in order to achieve a low stress and high deposition rate. We noticed that the high frequency mode (13.56 MHz) gives a low stress value which can be tuned from tensile to compressive by selecting the correct power. The low frequency mode (380 kHz) generates high compressive stress (around 500 MPa) due to ion bombardment and, as a result, densification of the layer achieved. Temperature can decrease the compressive value of the stress (due to annealing effect). A low etching rate of the amorphous silicon carbide layer was noticed for wet etching in KOH 30% at 80 °C (around 13 A/min) while in HF 49% the layer is practically inert. A very slow etching rate of amorphous silicon carbide layer in XeF₂ -7 A/min- was observed. The paper presents an example of this application: PECVD-amorphous silicon carbide cantilevers fabricated using surface micromachining by dry-released technique in XeF₂.     

碳化硅纳米粉体的合成、分散与烧结工艺技术研究进展

碳化硅陶瓷具有高强度、高硬度、高耐磨性等优异的机械性能以及耐化学腐蚀等,已广泛应用于各个工业领域以及航空航天领域.本文从纳米碳化硅粉体的合成、分散方法以及碳化硅基陶瓷的烧结方法与烧结助剂等方面综合评述了目前有关碳化硅纳米复相陶瓷的研究进展.     

Mechanical characteristics of microwave sintered silicon carbide

The present work deals with the sintering of SiC with a low melting additive by microwave technique. The mechanical characteristics of the products were compared with that of conventionally sintered products. The failure stress of the microwave sintered products, in biaxial flexure, was superior to that of the products made by conventional sintering route in ambient condition. In firing of products by conventionally sintered process, SiC grain gets oxidized producing SiO₂ (∼ 32 wt%) and deteriorates the quality of the product substantially. Partially sintered silicon carbide by such a method is a useful material for a varieties of applications ranging from kiln furniture to membrane material.     

Sintering of nano crystalline α silicon carbide by doping with boron carbide

Sinterable nano silicon carbide powders of mean particle size (37 nm) were prepared by attrition milling and chemical processing of an acheson type alpha silicon carbide having mean particle size of 0.39 μm (390 nm). Pressureless sintering of these powders was achieved by addition of boron carbide of 0.5 wt% together with carbon of 1 wt% at 2050° C at vacuum (3 mbar) for 15 min. Nearly 99% sintered density was obtained. The mechanism of sintering was studied by scanning electron microscopy and transmission electron microscopy. This study shows that the mechanism is a solid-state sintering process. Polytype transformation from 6H to 4H was observed.     

Numerical simulation of long rods impacting silicon carbide targets using JH-1 model

The Johnson–Holmquist model for simulating impact and penetration into ceramic and glass materials is commonly used in continuum hydrocodes. There are two forms of the Johnson–Holmquist ceramic model: JH-1 using a segmented linear approximation to the strength envelope with instantaneous failure; JH-2 using a smooth analytic approximation to the strength envelope with damage-induced strength reduction. Both these models are now implemented in the AUTODYN^® software. The validation of the JH-1 model is presented in this paper by comparing numerical predictions with experimental data. The failure parameters of the JH-1 model are also validated in the current numerical approach. Good agreement between numerical predictions and experimental measurements is shown for the behavior of silicon carbide in various impact situations. Time histories of particle velocity in compressive and tensile spall plate impact, interface dwell in confined impact, and total penetration depth in oblique impact are used in the comparison. The JH-1 model in AUTODYN is shown to be a powerful numerical tool in the design and analysis of ceramic armor systems.     

Mechanical and tribological behaviour of nano scaled silicon carbide reinforced aluminium composites

ABSTRACT

This work assesses the impact of the presence of Nano scaled silicon carbide on the Mechanical & Tribological behavior of aluminium matrix composites. Aluminium matrix composites containing 0, 0.5, 1, 1.5, 2 and 2.5 wt.%-nano scaled silicon carbide was set up by a mechanical stirrer. The trial comes about to demonstrate that the inclusion of Nano silicon carbide brings about materials with progressively high elastic modulus and likewise brings about expanded brittle behavior, fundamentally lessening failure strain. Shear modulus and flexural shear modulus likewise increases with silicon carbide increase. The presence of Nano scaled silicon carbide in the aluminium matrix diminishes subsurface fatigue wear and increases wear resistance, because of silicon carbide lubricant activity. Wear testing, microstructure & morphological, density & void testing, hardness, flexural and tensile test of the readied composites were investigated and outcomes were analyzed which demonstrated that including nano-SiC in aluminum (Al) matrix increased wear resistance, tensile strength, and 2 wt. % of nano scaled SiC for Al MMC indicated maximum wear resistance, tensile strength, and an optimum balanced mix of both Tribological and Mechanical properties. Microstructural observation uncovered uniformand homogeneous distribution of SiC particles in the Al matrix.     

Fabrication and probabilistic fracture strength prediction of high-aspect-ratio single crystal silicon carbide microspecimens with stress concentration

Single crystal silicon carbide micro-sized tensile specimens were fabricated with deep reactive ion etching (DRIE) in order to investigate the effect of stress concentration on the room-temperature fracture strength. The fracture strength was defined as the level of stress at the highest stressed location in the structure at the instant of specimen rupture. Specimens with an elliptical hole, a circular hole, and without a hole (and hence with no stress concentration) were made. The average fracture strength of specimens with a higher stress concentration was larger than the average fracture strength of specimens with a lower stress concentration. Average strength of elliptical-hole, circular-hole, and without-hole specimens was 1.53, 1.26, and 0.66 GPa, respectively. Significant scatter in strength was observed with the Weibull modulus ranging between 2 and 6. No fractographic examination was performed but it was assumed that the strength controlling flaws originated from etching grooves along the specimen side-walls. The increase of observed fracture strength with increasing stress concentration was compared to predictions made with the Weibull stress-integral formulation by using the NASA CARES/Life code. In the analysis isotropic material and fracture behavior was assumed — hence it was not a completely rigorous analysis. However, even with these assumptions good correlation was achieved for the circular-hole specimen data when using the specimen data without stress concentration as a baseline. Strength was over predicted for the elliptical-hole specimen data. Significant specimen-to-specimen dimensional variation existed in the elliptical-hole specimens due to variations in the nickel mask used in the etching. To simulate the additional effect of the dimensional variability on the probabilistic strength response for the single crystal specimens the ANSYS Probabilistic Design System (PDS) was used with CARES/Life. The ANSYS-PDS & CARES/Life simulation correlated better to the elliptical-hole specimen data than CARES/Life predictions based on average specimen dimensions.     

Carbothermal reduction process for synthesis of nanosized chromium carbide via metal-organic vapor deposition

Nanosized chromium carbide was synthesized by metal-organic chemical vapor deposition method in a fluidized bed using mixtures of methane/hydrogen ambient as carburization source in the temperature range 700-850 °C. The microstructure and the phase evolution were deciphered by XRD, TEM and XPS analysis. The carburization process involved the sequential deposition of carbon on the outer surface of the Cr₂O₃ powder followed by carbon diffusion into the powder, leading to the formation of metastable Cr₃C_2 − x phase and stable Cr₃C₂. STEM line scan mode was utilized to delineate the resultant composition gradient within the interlayer of the metastable intermediates and the final stable powder species, that were generated during the course of the carburization process. The formation of carbon nanofilms surrounding the carbide crystallites provides the stress and assists the phase transformation from metastable Cr₃C_2 − x to stable Cr₃C₂. XPS spectral analysis revealed that, the chromium ion in stable chromium carbide carries higher valance than that in metastable chromium carbide.     

氮掺杂导电碳化硅陶瓷研究进展EI北大核心CSCD

可电火花加工的导电碳化硅(SiC)陶瓷不仅可以克服传统高电阻率SiC陶瓷难加工的突出缺点,而且能够保留传统高电阻率SiC陶瓷的其他优异性能,在结构陶瓷领域取代传统的高电阻率SiC陶瓷具有突出优势。本文阐述了粉末烧结制备氮掺杂导电SiC陶瓷的原理,归纳总结分析了其粉末烧结制备方法、烧结助剂的种类及其所获得SiC陶瓷的热电和力学性能。同时,探讨了SiC陶瓷的电性能影响因素,为调控SiC陶瓷的电性能提供了参考依据。最后,指出了氮掺杂导电SiC陶瓷面临的主要挑战,在未来研究中,应聚焦于发展新烧结技术与烧结添加剂体系以及澄清电性能调控机制,为制备电阻率可控的高性能导电SiC陶瓷奠定技术基础。     

Gas permeability behavior of mullite-bonded porous silicon carbide ceramics

An apparatus was developed to evaluate the gas permeability behavior of mullite (3Al₂O₃·2SiO₂)-bonded porous silicon carbide (SiC) ceramics at room temperature. The permeability was calculated according to Forchheimer’s equation for the compressible gas. It was found that the sintering temperature and graphite (pore former) addition during the fabrication of the porous ceramics affect the permeability extremely by varying the texture of porous ceramics such as the open porosity, pore size distribution and tortuosity of pore channels. The increased sintering temperature results in a decreased Darcian (viscous) permeability but an increased non-Darcian (inertial) permeability. However, more graphite additions lead to the larger Darcian and non-Darcian permeability.     

Synthesis of boron carbide powder from polyvinyl borate precursor

Polyvinyl borate (PVBO) was prepared by the condensation of poly(vinyl alcohol) (PVA) and boric acid, and used as a precursor for boron carbide. Boron carbide powder was synthesized by the pyrolysis of the PVBO precursor in air at 600 °C for 2 h, followed by heat treatment in Ar flow at 1300 °C for 5 h, which is a relatively low temperature compared with conventional carbothermal methods. Pyrolysis of the PVBO precursor resulted in submicron-size particles of B₂O₃ dispersed in a carbon matrix. In addition, the pyrolysis temperature governed the carbon content in the pyrolyzed product of the PVBO precursor, which led to the synthesis of crystalline boron carbide powder with little free carbon.     

Response of boron carbide subjected to high-velocity impact

This article presents an evaluation of the response of boron carbide (B₄C) subjected to impact loading under three different conditions. Condition A is produced by plate-impact experiments where the loading condition is uniaxial strain and the stresses and pressures are high. Under plate-impact loading the material fails at the Hugoniot Elastic Limit (HEL) and the failed material undergoes high confining pressures and relatively small inelastic strains. Condition B is produced by projectile impact onto thick targets where the stresses and pressures are dependent on impact velocity, but they are generally lower than those from plate impact. Under thick-target impact/penetration most of the material fails under compression, the inelastic strains are large and the material appears to exhibit more ductility than under condition A. Lastly, condition C is produced by projectile impact and perforation of thin targets where the stresses and pressures are a combination of compression and tension. Under thin-target perforation the material fails in both tension and compression. The Johnson–Holmquist–Beissel (JHB) constitutive model is used to evaluate the material behavior for each of the three conditions, but it is not possible to accurately reproduce the experimental results of the three conditions with a single set of constants. Instead, three different sets of constants are required to accurately model the three impact conditions. These three models/constants are used to provide insight into the complex response of B₄C, and to identify possible mechanisms that are not included in the JHB model.     

An investigational analysis of W-shaped engineering hybrid fiber composites utilizing steel and boron carbide

As engineering demands increase, ordinary conventional materials cannot satisfy requirements in the 21st Century. Hybrid ceramic/metal matrix engineering composites introduce original design concepts which surpass criteria and enhance design standards, particularly when utilized within civil and thermal engineering applications. The selection of boron carbide, B₄C, in civil engineering applications displays unique property values, including flexural rigidity and stresses, and when infused with select A992 structural steel allows secondary opportunities for complex and diverse engineering applications. This article investigates an overview of boron carbide fibers and its steel matrix (SB₄C) with conventional steel, focusing on flexural rigidity and stresses. The flexural rigidity of laminated boron carbide fibers with W-shaped structural steel members is also observed. A civil engineering application based on flexural rigidity of a basic and a laminated SB₄C member is discussed as well. The investigation reveals that the SB₄C combination displays greater strength and stiffness values than conventional steel construction.     

Mechanical and tribological behaviour of tungsten carbide reinforced aluminum LM4 matrix composites

Aluminum-based metal matrix composites (AMCs) play a vital role for potential applications in aerospace and automotive industries. This paper explores the experimental analysis of a composite with aluminum LM4 alloy as the matrix and tungsten carbide (WC) as the reinforcement material. The composite specimens were fabricated by the stir casting process. The reinforced ratios of 5, 10 and 15?wt.% of WC particulates were stirred in molten aluminum LM4 alloy (AALM4). Once the composite is solidified, the specimens are prepared to the required ASTM dimensions and tested for various mechanical properties such as tensile strength, impact strength and hardness. Moreover, the tribological behavior of the composite was studied using the pin-on-disc wear test apparatus. X-ray diffraction (XRD) analysis was conducted to analyze the various elements present in the composites. Finally, the scanning electron microscope (SEM) analysis reveals the uniform distribution of WC particles in Aluminum LM4 alloy matrix. The improvement in mechanical properties – hardness, impact strength and tensile strength – was achieved for the increase in the addition of wt.% of WC particles in the LM4 matrix. The decrease in mass loss was observed for the composite containing 15?wt.% of WC during the wear test among the various composites tested.     

Atomistic Modeling of Spall Response in a Single Crystal Aluminum

Materials used in soldier protective structures, such as armor, vehicles and civil infrastructures, are being improved for performance in extreme dynamic environments. Accordingly, atomistic molecular dynamics simulations were performed to study the spall response in a single crystal aluminum atom system. A planar 9.6 picoseconds (ps) shock pulse was generated through impacts with a shock piston at velocities ranging from 0.6 km/s to 1.5 km/s in three <1,0,0>, <1,1,0>, and <1,1,1> crystal orientations. In addition to characterizing the transient spall region width and duration, the spall response was characterized interms of the traditional axial stress vs. axial strain response for gaining an understanding of the material failure in spall. Using an atom section averaging process, the snapshots, or the time history plots of the stress and strain axial distributions in the shock direction, were obtained from the MD simulations’ outputs of the atom level stresses and displacements. These snapshots guided the analyses to an estimation of the spall widths and spall transients. The results were interpreted to highlight the effects of crystal orientation and impact velocity on the spall width, spall duration, spall stress, strain rate, critical strain values at the void nucleation, and the void volume fraction at the void coalescence. For all the combinations of the crystal orientations and the impact velocities, the void nucleation was observed when the stress reached a peak hydrostatic state and the stress triaxiality factor reached a minimum, i.e. by the simultaneous occurring of these three conditions for the stress state: 1. pressure reaching a negative minimum, 2. axial stress reaching the magnitude value of the peak pressure, and 3. the effective stress reaching a zero value. At these conditions, void nucleation was mainly caused by atom de-bonding. In fact, the void nucleation strains were shown to have been preceded by the strains of the stress triaxiality condition in this study, thus confirming the stress triaxiality condition for the void nucleation in spall. Based on the observation that the axial stress reached a maximum value of ∼6 GPa during the void nucleation phase in spall and stayed approximately at that value for different crystal orientations and impact velocities, the value was proposed as a material spall strength.     

Evaluation of mechanical properties of aluminium alloy–alumina–boron carbide metal matrix composites

This paper deals with the fabrication and mechanical investigation of aluminium alloy, alumina (Al₂O₃) and boron carbide metal matrix composites. Aluminium is the matrix metal having properties like light weight, high strength and ease of machinability. Alumina which has better wear resistance, high strength, hardness and boron carbide which has excellent hardness and fracture toughness are added as reinforcements. Here, the fabrication is done by stir casting which involves mixing the required quantities of additives into stirred molten aluminium. After solidification, the samples are prepared and tested to find the various mechanical properties like tensile, flexural, impact and hardness. The internal structure of the composite is observed using Scanning Electron Microscope (SEM).     

Production and characterization of boron carbide and silicon carbide reinforced copper-nickel composites by powder metallurgy method

In this study, composite samples were produced by reinforcing boron carbide and silicon carbide particles in different rates by weight into copper-nickel powder mixture using powder metallurgy method. The prepared powder mixtures were cold pressed under 600 MPa pressure and pelletized. The pelletized samples were then sintered in an atmosphere-controlled furnace. Scanning electron microscopy to determine the microstructure of the produced samples and x-ray diffraction method analysis to determine the phases forming in the structure of the produced samples were used and microhardness was taken to determine the effect of boron carbide and silicon carbide on hardness. In addition to that, the mechanical properties the transverse rupture strength were investigated using three-point bending tests. The corrosion tests were performed potentiodynamic polarization curves of the samples in 3.5 % sodium chloride solution. The highest hardness value was measured as 162 HV 0.05 in the sample reinforced with 10 % boron carbide. As the amount of silicon carbide increased, the corrosion resistance of the composite increased. Moreover, as the amount of boron carbide increased, the corrosion resistance of the composite decreased. Load-contact depth values were examined, copper-nickel+10 % silicon carbide has the highest peak depth of 48.12.     

碳化硅陶瓷的活化烧结与烧结助剂

综述了作为碳化硅陶瓷烧结助剂的热力学条件及液相烧结的有效条件,介绍了碳化硅陶瓷烧结助剂的研究进展.     

Structure, mechanical and functional properties of aluminum nitride-silicon carbide ceramic material

Two-phase ceramic composites of the dielectric-semiconductor type having different semiconducting phase content (aluminum nitride ceramics with uniformly distributed inclusions of silicon carbide of a certain size) have been produced by pressureless sintering. These composites are characterized by Vickers hardness HV (150 N) 9.5–15.8 GPa, Palmqvist fracture toughness 3.0–4.2 MPa m^0.5, bending strength 132–209 MPa, thermal conductivity 37–82 W/(m K), and by a coefficient of the microwave electromagnetic energy attenuation to 36.3 dB/cm. It has been found that as the size of silicon carbide grains in aluminum nitride-based ceramics increases, the thermal conductivity increases and microwave energy attenuation decreases, which is indicative of the decisive role of grain boundaries in scattering both phonons and microwave radiation.