The resistance of silicon carbide to static and impact local loading

The physical nature of the resistance of SiC crystals to static and local impact loading has been examined. Investigation of the temperature dependence of hardness for SiC crystals allows the determination of the characteristic deformation temperature (T* ≈ 1600 K), the parameter that characterizes the degree of covalence in interatomic bonds ( ≈ 6) and the temperature range in which a phase transition under pressure during indentation is possible (T < 800 K). Indentation technique gives possibility to construct stress-strain curves for brittle materials and to determine Hugoniot Elastic Limit. During dynamic penetration of a kinetic projectile into a SiC target the phase transition takes place.     

Nanosecond laser-induced thermal evaporation of silicon carbide

Excimer (XeCl) laser pulses, 15 ns in duration and with fluences up to 10 J · cm^–2, have been employed to induce melting and evaporation of 6H-SiC thin layers in vacuum. Sample surface modification in the nanosecond time scale have been tnonitorizedin situ by optical probing. Eventually, the ablation product was collected on silicon single-crystal substrates placed in front of the SiC target. Modeling of the heating and the thermal evaporation processes resulted in estimation of surface temperatures as high as 10,000 K, evaporation rates of the order of 10²⁵ molecules · cm^–2 · s^–1 and recoil pressures of the order of 1 GPa. Comparison with experiments showed that the simple mechanism of purely thermal evaporation is able to describe the process of particle removal from a surface by short laser pulses only in the low-energy density range. Above a certain threshold the model breaks down and other mechanisms have to be considered.Paper presented at the Fourth International Workshop on Subsecond Thermophysics, June 27–29, 1995, Köln, Germany.     

Nonlinear thermal characteristics of silicon carbide devices

In the paper, the dynamic nonlinear model of SiC devices is proposed, where the dependencies of the thermal parameters on the temperature are included. The proposed model is applicable and useful in the simulations of electro-thermal transients in the devices working with high power density and in the wide range of temperature.     

Preparation of aligned silicon carbide whiskers from porous carbon foam by silicon thermal evaporation

Aligned silicon carbide whiskers were prepared from porous carbon foams by thermal evaporation of silicon. High-density silicon carbide whiskers were vertically deposited on the surface of siliconizing carbon foam. The whiskers were straight and hexagon-shaped with diameter of 1-2 μm and length of about 40 μm. They consisted of a single-crystalline zinc blende structure crystal in the [111] growth direction. The pore structure of carbon foam played an important role in determining distribution of the whiskers on the surface of siliconizing carbon foam. When carbon foam with higher porosity and larger pore size was employed, distributions of the whiskers were more ordered and more intensive. The whiskers were grown by the vapor-solid (VS) mechanism.     

Crack propagation in notched specimens of porous silicon carbide under cyclic loading

Notched specimens of porous silicon carbide with porosity 37% were fatigued under four‐point bending at frequencies of 30 and 0.3 Hz. The fatigue life expressed in terms of time was rather insensitive to the test frequency, while that expressed in terms of cycles was much shorter for the case of 0.3 Hz than for 30 Hz. A time‐dependent mechanism of stress corrosion cracking was mainly responsible for crack propagation, and stress cycling enhanced the crack‐propagation mechanism. The crack length was estimated from the change in compliance of the specimen. The crack‐propagation curve was divided into stages I and II. In stage I, the crack‐propagation rate decreased even though the applied stress intensity factor became larger with crack extension, and then turned to increase in stage II. The transition from stage I to II took place at a crack extension of around 0.8 mm. This anomalous behaviour is caused by crack‐tip shielding due to microcracking and asperity contact. Fractographic observations showed that the fracture path was along the binder phase between silicon carbide particles, or more precisely along the interface between particles and binders.     

Failure mechanisms in laminates of silicon carbide/calcium-aluminosilicate ceramic composite

A systematic investigation was conducted of failure mechanisms and their quantitative effects on the overall mechanical behaviour in laminates of a fibre-reinforced ceramic-matrix composite (silicon carbide/calcium-aluminosilicate) under static tensile loading. Three laminates were investigated: quasi-isotropic, cross-ply and angle-ply. Matrix cracking initiated in all laminates well below the proportional limit of the stress/strain relationship. This cracking grew in a random manner with increasing load; however, no continuous crack was observed across the whole width of the specimen. The measured Young's modulus and Poisson's ratio agreed well with the corresponding values from classical laminated plate theory, but this was not the case for the first ply failure and ultimate strengths.     

Structural relaxation of amorphous silicon carbide thin films in thermal annealing

Amorphous Si_0.4C_0.6 thin films were deposited by radio frequency magnetron sputtering onto non-heated single crystal Si substrates, followed by annealing at 800 °C or 1100 °C in the vacuum chamber. The chemical bond properties and atomic local ordering as a function of the annealing temperature were characterized by Auger electron spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, Infrared absorption spectroscopy, X-ray diffraction, and Raman spectroscopy measurements. We have examined the evolution of microstructure in annealing-induced relaxation process, and investigated the initial stages of thermal crystallization of amorphous Si_0.4C_0.6. Meanwhile, the structure of excess C in the films also has been studied.     

Thermal diffusivity of chemically vapour deposited silicon carbide reinforced with silicon carbide or carbon fibres

The thermal diffusivity of chemically vapour deposited silicon carbide reinforced with either Nicalon SiC yarn or PAN-precursor carbon fibres was measured by the laser-flash method during various time-temperature treatments. The diffusivity was found to depend on the degree of densification, the direction of heat flow with respect to the fibre orientation, and the thermal history. Structural modifications, confirmed by X-ray diffraction, produced large permanent changes in the thermal properties of the SiC-SiC composites when heated above 1200° C, while only minor changes were seen in C-SiC composites heated above 1500° C. On sabbatical leave of absence from the Société Européenne de Propulsion, Bordeaux, France.     

Polypropylene reinforced with silicon carbide whiskers

Thermal, crystallization and mechanical behaviour of isotactic polypropylene (iPP) reinforced with advanced silicon carbide whiskers (SiC_w) has been investigated. It is well established that the existence of chemical and physical interactions between the matrix and the reinforcement enhances the cohesive strength at the interphase thus improving the mechanical performance of the composite. In order to improve chemico-physical interactions between the components of the inorganic–organic composite system, their affinity has been enhanced in two ways: by coating the whiskers with a thin layer of acrylate-grafted polydivinylbenzene and by using as matrix a chemically modified polypropylene. The mechanical properties of the resulting composite materials have been compared and related to the dispersion grade of the whiskers within the matrices and the morphology of the samples. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of the loading rate on the strength of self-bonded silicon carbide ceramic materials

In this paper, the procedures for testing self-bonded silicon carbide ceramic materials under static and dynamic (impact) loading are described. The results of the investigations on the effect of the loading rate on the strength variation of ceramic materials used as elements of impact-resistant barriers are presented.     

Electrodeposition of silicon carbide

Thin layers of SiC were electrodeposited from $\text{Li}{}_2\text{CO}{}_3\text{SiO}{}_2$ melts onto α-SiC substrates at 1000°–1050°C using potential differences around ?0.5V versus polycrystalline SiC anodes. The layers appear to be epitaxial. Although conditions were found under which the melt exhibited good long term stability, attempts to grow bulk SiC crystals were handicapped by the delamination of the seed crystals.     

Nonequilibrium heteroepitaxy of silicon carbide on silicon

A method of silicon carbide (SiC) deposition onto silicon via a nonequilibrium disilicon carbide (Si₂C) vapor phase is proposed, theoretically described, and experimentally verified. Estimates obtained using thermochemical calculations show that a sufficiently large number of SiC molecules, which are transported by mobile Si₂C species, are obtained using this method on the silicon surface. It is established that homogeneous epitaxial SiC layers can be grown on a Si(111) substrate surface.     

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.     

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。