Silicon Carbide Specimens Research Articles

High Temperature Thermal Analysis of Graphite and Silicon Carbide with Millimeter-Wave Radiometry

ABSTRACTMillimeter-wave thermal analysis instrumentation is being developed for characterization of high temperature materials required for diverse fuel and structural needs in extreme high temperature reactor environments. A two-receiver 137 GHz system with orthogonal polarizations for anisotropic properties resolution has been implemented at MIT and is being tested with graphite and silicon carbide specimens at temperatures up to 1300ºC. Real time measurement sensitivity to submillimeter surface displacement and simulated anisotropic surface emissivity is demonstrated.

Thermal shock behaviour of ceramics under different testing conditions

Beside the presentation of interesting parts of the experimental equipment the thermal shock behaviour of two ceramics will be discussed. Specimens of silicon carbide and zirconia were investigated in air and vacuum. The influence of the testing conditions on the strength will be addressed.

Suppression of free Si formation during liquid phase sintering of polysiloxane-derived, porous silicon carbide ceramics

Porous silicon carbide ceramics were prepared from a polysiloxane, carbon black, SiC filler, sacrificial templates (co-polymer microbeads) and Al2O3-Y2O3 additives by a carbothermal reduction and subsequent sintering process. The effect of the sintering temperature on the microstructural development and structural characteristics of the porous ceramics was examined by scanning electron microscopy (SEM), X-ray diffractometry (XRD), and Raman spectroscopy. The polysiloxane-derived silicon carbide (PDSC) specimens showed a more homogeneous pore distribution than the powder-processed ones. Both the PDSC and powder-processed specimens contained only β-SiC when sintered at 1700°C. On the other hand, the PDSC specimens sintered at 2000°C revealed the formation of free Si clusters, detected by XRD and Raman spectroscopy, whereas the powder-processed ones showed only the SiC phases. The formation of such Si clusters was effectively suppressed by adding an excess of carbon during the synthetic process. Raman spectroscopy revealed the existence of carbon layers in the 1700°C-sintered specimens, which were hardly detectable in the 2000°C-sintered ones.

Effect of additive composition on microstructure and mechanical properties of SiC ceramics sintered with small amount of RE2O3 (RE: Sc, Lu, Y) and AlN

Silicon carbide is a potential material for various structural applications because of its unique combination of properties such as high hardness, high temperature strength, and excellent resistance to wear and corrosion [1–4]. In order to improve the fracture toughness, several attempts have been made to fabricate SiC ceramics consisting of heterogeneous microstructure with weak grain boundary phases [5–7]. The liquid-phase sintering using metal oxides, Al–B–C, AlN– metal oxides as sintering additives is recommended for obtaining SiC ceramics with tailored microstructure [8, 9]. Among various additive systems, AlN and RE2O3 (RE: rareearth elements) systems are preferred due to relatively less weight loss [10], easy control of oxynitride decomposition, and absence of gas-phase reactions below 2000 C [6]. The use of RE2O3 renders SiC ceramics with excellent high temperature strength and toughness properties [6, 11]. The platelet-reinforced microstructures exhibit enhanced toughness due to combination of intergranular crack mode, introduced by the glassy grain boundary phase, and energy dissipating processes in the crack wake [12]. Though most of the studies reported amorphous grain boundary or ternary junction phase, it has been recently identified that sintering additive composition plays a dominant role in affecting the crystallinity of grain boundary phase. Amorphous intergranular phase was obtained using AlN–Y2O3 [13, 14] and AlN–Er2O3 [9, 11] additives systems, whereas AlN– Sc2O3 render clean grain boundary [15, 16] and AlN–Lu2O3 systems render clean [17] or crystallized grain boundary [18]. It is to note that most of the published research has been conducted toward characterization and property evaluation of SiC ceramics prepared with relatively large amounts of additives (C5 wt%), while its performance with small additive contents is poorly understood. Therefore, an attempt has been made in this study, for the first time, to sinter SiC ceramics using small amount (3 wt%) of AlN–RE2O3 (RE: Sc, Lu and Y) additives. The major purpose is to investigate the influence of additive composition on microstructural and mechanical characteristics of SiC ceramics sintered with small additive amounts. Commercially available b-SiC (Ultrafine grade, Betarundum, Ibiden Co. Ltd., Ogaki, Japan), AlN (Grade F, Tokuyama Soda Co., Tokyo, Japan), and metal oxides (Sc2O3, Lu2O3, and Y2O3, 99.9% pure, Shin-Etsu Chemical Co., Tokyo, Japan) were used as the starting powders. The mean particle size and the specific surface area of the b-SiC powders were 0.27 lm and 17.5 m/g, respectively. A powder mixture of SiC and 3 wt% additives AlN and RE2O3 (RE: Sc, Lu, Y) was prepared. The relative amount of RE2O3 in the total content of additives was 20 mol.%. The respective powder batches were milled in ethanol for 24 h using SiC grinding balls. The milled slurry was dried, sieved, and hot-pressed at 2050 C for 6 h under a pressure of 25 MPa in N2 atmosphere. The details of specimen designation, batch composition, and additive systems are given in Table 1. The designation SCRE represents silicon carbide specimen prepared using 3 wt% RE–AlN (RE: Sc, Lu, Y) additive. The bulk density of the sintered specimen was measured using the Archimedes method in water. The theoretical B. V. Manoj Kumar M.-H. Roh Y.-W. Kim (&) W. Kim Department of Materials Science and Engineering, The University of Seoul, Seoul 130-743, Korea e-mail: ywkim@uos.ac.kr

An extended and integrated digital image correlation technique applied to the analysis of fractured samples

To reduce the measurement uncertainty, a measurement technique is proposed for estimating full displacement fields by complementing digital image correlation with an additional penalization on the distance between the estimated displacement field and its projection onto the space of elastic solutions. The extended finite element method is used for inserting discontinuities independently of the underlying mesh. An application to the brittle fracture of a silicon carbide specimen is used to illustrate the application. To complete the analysis, the crack tip location and the stress intensity factors are estimated. This allows for a characterization of the measurement and identification procedure in terms of uncertainty.

Dielectric-Parameter Measurements of SiC at Millimeter and Submillimeter Wavelengths

High-precision continuous spectra of the absorption coefficient, refractive index, complex dielectric permittivity, and loss tangent for several silicon carbide (SiC) specimens are reported in this paper over a broad millimeter- and submillimeter-wave range for the first time. Measurements have been successfully performed using three different types of specially constructed spectrometer systems: a dispersive Fourier transform spectroscopy, an automated 60-GHz open resonator, and a free-space quasi-optical millimeter-wave spectrometer equipped with high-power backward-wave oscillator sources and their associated state-of-the-art dielectric-measurement techniques. Data are presented as continuous functions of frequency from 30 to 600 GHz. The employment of various measurement systems and techniques ensured the measurement of polycrystalline SiC specimens with various degrees of absorption and dispersion characteristics over an extended frequency range with high precision. Results presented here provide comprehensive information of SiC on its optical and dielectric behavior as a function of frequency and purity for its potential application in semiconductor and radio-frequency devices and circuits. An error analysis of measured dielectric-parameter results is also provided.

Fracture toughness characterisation of electronic materials and interfaces

We present a technique for measuring the fracture toughness of nanoscale specimens of electronic materials and interfaces, in-situ inside electron microscopes. The technique employs a custom micro-machined mechanical testing device on which an edge-cracked tension specimen is integrated. The specimen is fabricated using a focused ion beam, which allows for testing of both single phase and interfacial materials, and virtually eliminates any restrictions on the type of materials that can be tested. We demonstrate the technique with single crystal 6H silicon carbide specimens, which are 200?500nm thick, 2?5?m wide and approximately 10?m long. The measured values of Young's modulus and fracture toughness are 590GPa and 4.82MPa-m1/2 respectively, which compare favourably to the experimental results on bulk and thin film silicon carbide in literature.

Defect production in silicon carbide irradiated with Ne and Xe ions with energy of 2.3 MeV/amu

In the present work specimens of mono-crystalline silicon carbide (4H polytype) were irradiated to three successively increasing ion fluences ranging from 7.2×1014 to 6.0×1016ions/cm2 (corresponding to the peak displacement damage of 1, 4 and 13dpa) with Ne and Xe ions respectively with the energy of 2.3MeV/amu. The irradiated specimens were subsequently annealed at temperatures of 1173 and 1273K. Defect structure was investigated with transmission electron microscopy (TEM) using a cross-sectional specimen preparation technique.The typical microstructures of the annealed specimens irradiated with Ne or Xe ions to high fluences are characterized by small gas bubbles in high concentration in the peak damage region and black dots and dislocation loops (located in the basal plane) in a shallower and broader depth region. Larger dislocation loops were observed in the Xe-ion irradiated specimen than in the Ne-ion irradiated specimen at the same peak damage level. The enhanced formation of dislocation loops in the case of Xe-ion irradiation is understandable by assuming stronger inclination of heavier inert-gas atoms to occupy substitute site in the peak damage region.

Microstructure of SiC / TiAl Interface Formed by Solid-State Diffusion Bonding

The microstructure of the solid-state diffusion bonded interfaces of silicon carbide (SiC) and titanium aluminide (TiAl) were investigated. A 100-µm-thick Ti-48at%Al foil was inserted between two SiC specimens and then heat-treated in vacuum. The interfacial microstructure has been analyzed by scanning electron microscopy, electron probe microanalysis and X-ray diffractometry. Four layers of reaction products are formed at the interface by diffusion bonding: a layer of TiC adjacent to SiC followed by a diphase layer of TiC+Ti2AlC, a layer of Ti5Si3CX containing Ti2AlC particles and a layer of TiAl2. However, the TiAl2 layer is formed during cooling. The actual phase sequence at the bonding temperatures of 1573 K and 1673 K are described as SiC/TiC/(TiC+ Ti2AlC)/(Ti5Si3CX+Ti2AlC)/Ti1-XAl1+X/TiAl and SiC/TiC/(TiC+Ti2AlC)/(Ti5Si3CX+Ti2AlC)/Ti5Al11 /Ti1-XAl1+X/TiAl, respectively. The phase sequences are successfully expressed on the basis of the Ti-Al-Si-C quaternary chemical potential diagram.

Mechanical properties of epitaxial 3C silicon carbide thin films

Microscale tensile specimens of epitaxial 3C silicon carbide (3C-SiC) thin films were fabricated on Si substrates and tested to provide measurements of strength and elastic modulus. Samples were fabricated using both micromolding and reactive ion etching (RIE) processes to pattern the 3C-SiC films. All specimens were on the (100) plane with a <110> tensile direction. Testing was performed on a microsample tensile testing machine previously used on other materials. The samples had a thickness of 0.5 to 2 /spl mu/m, a gauge length of 4 mm, and a minimum width of 600 /spl mu/m. Testing results show an average strength of 1.19 /spl plusmn/ 0.53 GPa and 1.65 /spl plusmn/ 0.39 GPa for micromolded and RIE patterned specimens, respectively. The elastic modulus was measured to be 424 /spl plusmn/ 44 GPa, which was consistent with but slightly lower than the elastic modulus calculated with single crystal elastic constants found by ab initio calculations.

Notch Effect on Fatigue Strength of Porous Ceramics

Smooth specimens, semi-circular notched and V-notched specimens of porous silicon carbide with porosity of 35% were fatigued under cyclic four-point bending at a stress ratio of 0.1. The reduction of the fatigue strength due to notches can be estimated from the reduction of the crack initiation stress in static bending fracture tests. The crack propagation curve in V-notched specimens was divided into stages I, II and III. In stage I, the crack propagation rate decreased even though the applied stress intensity factor got larger with crack extension. The crack propagation rate was nearly constant in stage II, and then increased in stage III. The anomalous behavior in stages I and II was caused by crack-tip shielding due to microcracking and asperity contact, while the crack tip shielding was minimal in stage III. For the case of semi-circular notched specimens, stage I was followed by stage III without stage II. The crack length at the end of stage I took a constant value of about 0.068 mm for semi-circular notched specimens and 0.12 mm for V-notched specimens. The stress intensity factor at the onset of unstable fracture was about 1 MPa·m 1/2 for both types of notched specimens.

Influence of Thermal History on Crystal Nucleation in Silicon Carbide Amorphized with Neon Irradiation

Silicon carbide (SiC) specimens were amorphized with neon ions and annealed at 1273 K via transmission electron microscopy (TEM). Two types of annealing procedures were conducted: (i) continuous annealing for 60 min and (ii) annealing for 5 min, repeated 10 times. TEM observations revealed that crystal nucleation was more apt to occur in the SiC amorphized by repeated annealing than in the continuously annealed SiC.

Interfacial Microstructure of Silicon Carbide and Titanium Aluminide Joints Produced by Solid-State Diffusion Bonding

This paper describes the microstructure and the formation mechanism of solid-state diffusion bonded interfaces of silicon carbide (SiC) and titanium aluminide (TiAl). Two SiC specimens were diffusion bonded using a Ti-48 at%Al foil in vacuum. The interfacial microstructure has been investigated by means of scanning electron microscopy, electron probe microanalysis and X-ray diffraction. Four layers of reaction products are formed at the interface by diffusion bonding: a layer of TiC adjacent to SiC followed by a diphase layer of TiC+Ti2AlC, a layer of Ti5Si3CX containing Ti2AlC particles and a layer of TiAl2. However, the TiAl2 layer is formed during cooling. The actual phase sequence at the bonding temperatures of 1573 K and 1673 K are SiC/TiC/(TiC+Ti2AlC)/(Ti5Si3CX+Ti2AlC)/Ti1� XAl1þX/TiAl and SiC/TiC/(TiC+ Ti2AlC)/(Ti5Si3CX+Ti2AlC)/Ti5Al11/Ti1� XAl1þX/TiAl, respectively. Ti5Al11 and Ti1� XAl1þX rapidly transform to TiAl2 during cooling. The layers grow obeying the parabolic law. The growth rate of the reaction layers change sensitively depending on the bonding temperature.

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

ABSTRACTNotched 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.

Dose dependence of formation of nanoscale cavities in helium-implanted 4H–SiC

A cross-sectional transmission electron microscope investigation of dose dependence and annealing behavior of microstructures in helium-implanted silicon carbide is presented. Specimens of silicon carbide (4H–SiC) were mainly implanted with 30-keV-He ions at intermediate temperatures of 500 and 873 K to doses ranging from 5×1015 to 2×1017 cm−2, and subsequently annealed at temperatures up to 1173 K. For comparison other specimens were implanted at 293 K up to 5×1015 cm−2 and subsequently annealed. Three dose regimes of microstructural evolution were found, each of them exhibiting quite different annealing behavior. No cavities were found at the low dose range below 3.5×1016 cm−2. At intermediate doses of (3.5–5)×1016 cm−2, a low density of planar clusters of bubbles located in the basal plane was found in a well defined region around the damage peak on annealing above 973 K. After implantation to the high dose of 1017 cm−2, a high density of bubbles in alignment in the basal plane was formed, and remained stable during the subsequent annealing at temperatures up to 1173 K. The difference in annealing behavior suggests that helium is trapped with different energies for the three dose regimes. There appears to be a threshold dose for the formation of the bubble clusters of approximately 5×1015 cm−2 for implantation at room temperature. This increases to a value between 2×1016 and 5×1016 cm−2 for implantation at 500 and 873 K, respectively. Such a temperature dependence is consistent with that of effective damage production, indicating a certain level of damage is necessary for the formation of bubble clusters. The formation of planar clusters of bubbles at the intermediate doses is ascribed to an Ostwald ripening (OR) process through the dissociation of the simple He/vacancy clusters and metastable bubbles and the formation of planar cavities of helium on annealing above 973 K.

Microstructural Changes in Liquid‐Phase‐Sintered Silicon Carbide during Creep in an Oxidizing Environment

The knowledge of the microstructural evolution during exposure to high temperatures is important to understanding the mechanisms responsible for the creep resistance of silicon carbide (SiC) ceramics. This includes not only the phase transformation of the SiC grains, but also the phase transformations of the oxynitride grain‐boundary phases. For this study, a series of SiC specimens were prepared with varying molar ratios of AlN‐Y2O3 additives. Increased creep resistance was observed in specimens with an additive system containing a 2:3 molar ratio or 60 mol% Y2O3. A continuous oxide layer of Y2Si2O7 formed at the surface during elevated temperature testing in air. No blistering or cracking was observed in this oxide coating. Further increase of the creep resistance was achieved by a post‐sintering nitrogen anneal.

Processing, Fracture Toughness, and Vickers Hardness of Allylhydridopolycarbosilane‐Derived Silicon Carbide

Bulk specimens of precursor‐derived silicon carbide (SiC) suitable for mechanical‐property measurements were prepared from allylhydridopolycarbosilane (AHPCS), which is a commercially available, hyperbranched polycarbosilane. Crack‐free pellets were obtained by cold‐pressing mixtures of finely ground, 1000°C pyrolyzed, “AHPCS‐SiC” with neat AHPCS, followed by pyrolysis to 1000°C and ten subsequent reinfiltration/pyrolysis steps with the neat liquid AHPCS. Then, these pellets were heat‐treated to 1200°, 1400°, and 1600°C, followed by additional reinfiltration/pyrolysis cycles to the final respective maximum temperatures. This fabrication process simulated the production of the matrix phase for ceramic‐matrix composites via successive infiltration/pyrolysis cycles. The density of the material processed at these temperatures, measured via the Archimedes method, was 2.3, 2.5, 2.6, and 2.9 g/cm3, respectively, and the average open porosities of the samples were 2, 0.2, 1, and 9 vol%, respectively. The fracture toughness was measured using the single‐edge V‐notched‐beam method, and the hardness was measured via Vickers indentation. The samples had an average toughness of 1.40 ± 0.08, 1.65 ± 0.09, 1.67 ± 0.07, and 1.46 ± 0.08 MPa·m1/2 for the samples that were treated at 1000°, 1200°, 1400°, and 1600°C, respectively. The Vickers hardness for these samples, measured at a load of 1000 g, was 12 ± 1, 13 ± 2, 11 ± 1, and 9 ± 1 GPa, respectively.

R-curve behavior in fracture of notched porous ceramics

Notched specimens of porous silicon carbide (SiC) with porosity 37% were fractured under four-point bending. A single edge notch with six depths ranging from 0.1 to 2.8 mm was introduced to the specimen with a height of 7 mm. The fracture of specimens with a notch depth of 0.1 mm did not start from the notch, but from the intrinsic defect. The size of the non-damaging notch is about 0.1–0.2 mm and roughly equal to the size of SiC particles. When the notch depth was larger than 0.4 mm, the fracture started from the notch for all specimens. The record of the strain gage glued on the compression surface of the specimen as a function of the load showed nonlinearity before reaching the maximum load. The critical stress intensity factor was nearly constant for crack initiation from the notch. The resistance curve was constructed by estimating the crack length from the compliance change of the specimen, and was used for determining the maximum load point in bending tests. Fractographic observations showed the fracture path along the binder phase between silicon particles.

The effect of reducing and oxidizing gases and molten slag and alkalis on the properties and resistance of nitride-based refractory materials

Silicon carbide refractory materials and specimens of silicon nitride and silicon oxynitride are tested under high-temperature attack from reducing (H2 and CO) and oxidizing gases (O2, air) and from molten slag, alkalis, and iron. It is shown that nitride-based refractory materials are prone to change their properties, structure, and resistance when exposed to molten slag, iron, and alkalis. Silicon oxynitride and silicon nitride resist attack from carbon monoxide and alkali and provide protection to silicon carbide used as the refractory lining for blast furnaces.

Micro- and macro-tribological properties of SiC ceramics in sliding contact

Surface properties of single-crystalline and polycrystalline silicon carbide (SiC) specimens were measured using atomic force/friction force microscopy, Auger electron spectroscopy and nano-indentation techniques. Running-in behavior during sliding tests in vacuum was studied on self-mated SiC pairs as a function of surface quality produced by machining. Tribological mechanisms were analyzed in short-time tests using a high-resolution micro-tribometer and then were related to results obtained from macro-tribological tests in humid air and in the presence of distilled water. Information on the structure, chemical composition and properties of SiC surfaces resulting from measurements on the nanoscale can be very useful for explaining tribological performance under far different operating conditions such as in vacuum or air, with contact areas ranging from diameters of a few nanometers to one millimeter and initial applied contact pressure from about 1MPa to 3GPa. Friction and wear mechanisms are discussed as functions of surface composition and roughness, vacuum and humidity of air.