SiC Platelets Research Articles

Role of triboelectrification mechanism in the wear behaviour of Al 2O 3SiC platelet composites

Wear behaviour of Al 2O 3SiC platelet composites has been analysed as a function of the content and the orientation of the platelets. The results have shown that wear volume increased with platelet content. For long sliding times (≥ 60 min) and high SiC platelet additions (≥ 12 vol.%), a mild-severe wear transition took place. A mechanism based on the triboelectrification concept has been developed to explain the wear behaviour of Al 2O 3SiC platelet composites. A strongly anisotropic wear behaviour, that has been directly associated to the anisotropy in the electrical and mechanical responses, was observed. This behaviour was a consequence of the platelet orientation during hot pressing.

Fracture behavior of Al2O3/SiC-platelet composites

Mechanical behavior of hot-pressed SiC platelet reinforced alumina composites has been analyzed as a function of SiC platelet content for two different alumina matrix powders. Fracture toughness and flexural strength at temperatures ranging from 25 to 1200 °C, R-curve behavior, and thermal shock resistance have been determined. Small differences in the impurity content of the starting Al2O3 powders strongly determine the microstructure and the mechanical behavior of Al2O3/SiC-platelet composites. Low alkali content alumina led to composites with large matrix grain size which presented spontaneous microcracking. At high temperature, a high viscosity liquid phase is formed that shields cracks enhancing mechanical properties and R-curve behavior. A small amount of impurities reduced Al2O3 matrix grain size and avoided spontaneous microcracking. Enhanced fracture toughness (up to 30%) at room temperature, R-curve behavior, and thermal shock resistance were achieved for these materials.

Defect analysis in Lely-grown 6H SiC

Two Lely-grown 6H polytype SiC platelets were investigated in terms of their crystalline quality and defect characteristics. Synchrotron X-ray topography, along with optical microscopy and high-resolution X-ray diffractometry were used as characterization techniques. In the first platelet a distored area where the nucleation has started was observed. It consists of micropipes and a complex network of the Frank-Read dislocation loops. Their Burgers vectors were completely determined by X-ray topographic analysis. Outside the distorted area the only large defects observed were dislocations with a low density (in the order of 10 2 cm −2) and few stacking faults. The quality of this platelet has improved during the growth. The second platelet was found to be of an excellent crystalline quality. It does not contain micropipes or stacking faults and its dislocation density is very low (about 30 cm −2). The rocking curve widths (FWHM) of both samples were very narrow (from 5.8″ to 8.3″). Possible relationships between the observed defects and their formation mechanisms in the growth processes are discussed.

High-temperature deformation of cordierite glass-ceramic/SiC platelet composites

Composites based on barium-containing cordierite glass-ceramic matrices reinforced with 20 and 30 vol% SiC platelets were fabricated by hot-pressing. The ceramed specimens were tested in compression at 1000, 1200 and 1300 °C in air, parallel and perpendicular to the hot-pressing direction. Compressive fracture stresses up to 460 MPa were recorded at 1000 °C. Lower stresses were observed at 1200 and 1300 °C. The compressive stress decreased with increase in the SiC platelet content and with the compressive direction being parallel to the hot-pressing axis. The mechanical behaviour at high temperatures was related to the presence of a residual glassy phase within the cordierite matrix, as well as to the mode of the crack nucleation within the composites.

Fabrication of Textured Silicon Carbide via Seeded Anisotropic Grain Growth

Highly textured silicon carbide was fabricated via anisotropic grain growth. Large, platelike grains were grown from a fine‐grained sintered matrix which had been seeded with ≤2% SiC platelets. The seeds were initially incorporated with preferred orientation during the green‐state forming process and this resulted in oriented growth of the platelike grains. The development of texture in the samples was evident from microstructure observations and X‐ray diffraction analysis.

Slow crack growth in SiC platelet reinforced Al 2O 3 composite

Ceramic matrix composites with enhanced toughness are at present projected for many structural applications such as high temperature components in gas turbine, structures for hypersonic aircraft and bioprosthetic devices. The incorporation of a SiC dispersed second phase in form of whisker or platelets into an alumina matrix has allowed to improve material toughness, thermal shock resistance and R-curve behavior. Recently, considerable interest in the acquisition of slow crack growth (SCG) data for ceramic materials has arisen in order to predict the service lifetime of brittle components. Non-oxide ceramics such as SiC and Si{sub 3}N{sub 4} are extremely resistant to crack growth at low temperatures, whereas oxide ceramics are susceptible to stress corrosion because of the chemical interaction between water and stressed cracks. Up to date, there are not many papers devoted to SCG of SiC whiskers reinforced Al{sub 2}O{sub 3} composites and none about SiC platelets used as reinforcement. The objective of the present work has been to evaluate the slow crack growth in a Al{sub 2}O{sub 3}/SiC-platelet composite by double torsion testing analysis. The results will be compared with those obtained for SiC whisker reinforced Al{sub 2}O{sub 3} composite tested using the same conditions.

SiC platelet-reinforced Al 2O 3 composites by free sintering of coated inclusions

SiC platelets were coated with a fine-grained Al 2O 3 precursor powder by controlled heterogeneous precipitation from solution. After calcination, the coated platelets were compacted and sintered at a constant heating rate of 5 °C min −1 in a helium atmosphere. The parameters that control the coating process and the sintering behaviour of the coated powders were investigated. For given reactant and platelet concentrations, pH and temperature, the presence of a small amount of poly(vinylpyrrolidone) (PVP) produced a more homogeneous coating which, in turn, produced an improvement in the sinterability of the coated platelets. Composites formed from the coated platelets, with an initial matrix density of 40–45% of the theoretical and containing ≈ 20 vol% platelets, reached nearly full density after sintering at 1800 °C for min. By comparison, similar composites formed by mechanical mixing of the SiC platelets and freely precipitated Al 2O 3 powder reached a density of only ≈ 70% of the theoretical under identical sintering conditions. The strength and fracture toughness of the sintered composites formed from the coated platelets were measured in three-point loading at room temperature. For the composite containing 20 vol% platelets, the strength and fracture toughness values were MPa and 5.4 MPa m 1 2 , respectively. They are comparable to the highest values reported for similar composites produced by hot-pressing of mechanically mixed systems.

Cohesive Energy of Interfaces and Toughness of Fluorine-Doped Si<sub>3</sub>N<sub>4</sub>/SiC Composites

Fracture mechanics and electron microscopy studies were systematically conducted on a dense Si3N4 material reinforced with high-aspect-ratio SiC platelets. This model system contained only an amorphous SiO2 phase at the multigrain pockets and along the internal interfaces. Two different procedures (and their empirical combination) were followed to gradually weaken the grain interfaces and, hence, deliberately change the fraction of intergranular fracture within the material upon crack propagation. These procedures were: 1) the gradual addition of a fluorine impurity, which remains segregated at the grain boundaries after sintering, and 2) the adoption of a “quenching” process under high pressure. Quantitative high-resolution and analytical electron microscopy provided fundamental insight into the internal structure of the intergranular SiO2 phase doped with fluorine and the corresponding microcracking processes. Quantitative fractography allowed to estimate the apparent cohesive energy of the internal interfaces of the composite and to relate it to the macroscopic material toughness.

Morphology of an as-grown surface of Sic — determination of step flow direction

By means of optical and scanning tunnelling microscopy it is shown that there exists a distinct difference in some features of step morphology on the rounded and the flat faces of SiC platelets grown by the Acheson method. Two different distributions of steps around obstacles have been found. One distribution exhibits perturbation of the step trains in the region which is situated along the down stairway next to an obstacle while the other is situated in the region of the up stairway. As perturbation of the step trains always occurs “behind” an obstacle, this indicates whether steps advance or shrink. The latter shows, in turn, whether a surface grows or sublimes. Altogether the results of this study show that in the Acheson process the rounded faces of SiC platelets grow, but on the flat faces sublimation dominates.

Microstructure and Compressive Strength of SiC‐Platelet‐Reinforced Borosilicate Composites

The microstructure and compressive strength of SiC‐platelet‐reinforced borosilicate composites have been examined in this study. During sintering, following cold compaction, borosilicate glass crystallized into cristobalite, and the thermal expansion mismatch between the parent glass and the crystallized phase led to extensive microcracking of the matrix. Cristobalite growth (hence cracking) could be suppressed to some extent by opting for a rapid hot‐pressing cycle. Composites fabricated with various volume fractions of SiC platelets were tested in compression. A maximum compressive strength of 510 MPa was observed at 40 vol % of platelets. Further, the compressive flow behavior of these composites has been explored in the vicinity of the glass transition temperature. At temperatures above 625°C, borosilicate glass and its composites exhibited Newtonian viscous flow characteristics. Their flow stress at a given strain rate is, however, seen to increase with increasing volume fraction of SiC platelets.

Phenomenology of fracture and fracture mechanisms under slow-crack-growth/creep regimes in high-purity Si3N4 and its SiC-platelet composite

The mechanical behaviors and the microfracture mechanisms of a high-purity dense Si3N4 material and its 25 vol.% SiC-platelet composite have been investigated in the range of temperature 1400–1520°C by means of extensive mechanical testing coupled to transmission (TEM) and scanning (SEM) electron microscopy. Creep fracture was not observed even after several hundred hours exposure at high temperature and the slow (subcritical) crack growth (SCG) from a single, most critical pre-existing flaw was the dominant failure mechanism. Phenomenological crack-growth laws were discussed and compared with a theoretical model for diffusive crack propagation. Marked lifetime elongations and increased flaw crack tolerance were found in the composite material due to shielding mechanisms operated by the SiC platelets in the wake of the intergranular SCG crack. These positive effects, diminishing substantially with increasing the temperature, were well explained by an algorithm incorporating into the mechanical driving force acting on the crack tip, the closure-field contribution due to the rising R-curve behavior of the material.

Microstructure and Mechanical Properties of SiC Platelet/Cordierite Glass‐Ceramic Composites

Cordierite glass‐ceramics reinforced with different volume fractions (0, 10, 20, and 30%) of SiC platelets were fabricated by mechanically mixing appropriate powders and hot pressing above the glass transition temperature. Some of the specimens were subjected to postsintering crystallization treatments. The resultant sintered microstructure was textured, and within it the SiC platelets tended to be oriented with their basal planes perpendicular to the hot‐pressing direction. The bending strength of the hot‐pressed specimens was constant, while the fracture toughness increased with the SiC content. Crystallization heat treatment in argon for 30 min at 1200°C caused a significant decrease in the bending strength, but improved the fracture toughness. Fractography has confirmed the crack deflection to be a dominant mechanism in the crystallized specimens, which contributes to the improvement in the fracture toughness. The overall changes in the mechanical properties are discussed with respect to the change in the residual stresses and oxidation characteristics of the SiC platelets.

Residual stress-induced spontaneous microcracking in α-SiC platelet Al2O3 composites

The micromechanical stresses associated with hexagonal (6H) α-SiC platelets within a fine-grained alumina matrix were calculated using an Eshelby approach. The stresses within and around the interface of SiC platelets were determined. Both stresses were found to be strongly dependent on the morphology and the volume fraction of the SiC particles. The morphology effect, however, tended to be limited at aspect ratios ⩾ 10. Owing to anisotropy in the thermal and elastic properties of α-SiC, the residual stresses just outside the inclusion also depended on the position along the SiC/Al2O3 interfaces. The maximum tensile stress was found at the edges of SiC platelets. There were two principal tangential tensile stresses which differed greatly at the edges of disc-shaped inclusions. The results of the stress analysis were consistent with observed differences in microcrack morphology and the resultant reduction of the Young's modulus of the composites.

Electron Channeling Pattern of Ceramic Platelets

Electron channeling patterns (ECPs) have been generated in the scanning electron microscope from hexagonal‐close‐packed ceramic (tungsten carbide and silicon carbide) single crystals extracted from commercially available powders. The crystals, commonly employed as reinforcing phases in ceramic composites, were typically platelet‐shaped with smooth surfaces. Their average diameter was 25 μm with a thickness ranging between 3 and 7 μm. After theoretical maps for the selected crystallographic poles were constructed with a procedure similar to that used for convergent beam electron diffraction, the experimental patterns were indexed and the crystallographic orientation of the crystals was determined. Specific applications for ceramic materials are discussed in comparison with previous work dealing with metals and, as an example of the application of the ECP method, the cleavage plane of fracture has been determined for the SiC platelet embedded in a Si3N4 matrix.

Texture Measurement on Materials Containing Platelets Using Stereology

A method for determining three‐dimensional (3‐D) orientation distributions of platelike grains or inclusions using stereology is described. The method involves relating an orientation distribution of platelet cross‐section normals taken from a plane of polish, with a corresponding orientation distribution of platelet normals in 3‐D. In this investigation, SiC platelet orientation distributions were measured in Al2O3–SiC, and Cu–SiC composites by both stereological analysis and X‐ray diffraction. The stereological measurements yielded two‐dimensional (2‐D) platelet cross‐section normal orientation distributions, and the X‐ray diffraction measurements yielded 3‐D platelet normal orientation distributions (in the form of pole figures). The calculation described in this paper correlates 2‐D and 3‐D platelet normal distributions. To experimentally substantiate the calculation, a comparison between measured and calculated distributions is presented. Good agreement between calculation and measurement is shown, thereby confirming this analysis as a viable technique for orientation measurements.

Microstructure and fracture behavior of SiC-platelet-reinforced Si 3N 4 matrix composites

The microstructure and crack propagation behavior of 25vol.%SiC-platelet-reinforced Si 3N 4 composites, which were prepared by hot isostatic pressing (HIP) accompanied by slow (100°Ch −1) and rapid (650°Ch −1) cooling, were investigated. Most of the SiC platelets were polytype α structures (4H and 6H) and they were found to be homogeneously dispersed in the Si 3N 4 matrix. The crystal structure of Si 3N 4 in both slowly and rapidly cooled composites was β type (h.c.p.) and most of the Si 3N 4 grains had an equiaxed shape. A few large and elongated Si 3N 4 grains were also observed in the neighbourhood of SiC platelets. Most of the Si 3N 4 grain boundaries in both the composites contained a thin SiO 2 amorphous layer about 1–2 nm in thickness. In the slowly cooled composite, cracks formed by indentation mainly propagated along Si 3N 4 grain boundaries and Si 3N 4SiC interfaces with deflection and microcracking, although transgranular fracture of SiC platelets was also observed along the (001) plane. In contrast, in the rapidly cooled composite, fracture occurred mainly intragranularly both in Si 3N 4 grains and in SiC platelets.

Effects of rigid platelet additions on matrix texture and microstructural development in a model platelet-reinforced composite

Composites of lithium fluoride and silicon carbide provide a unique opportunity for exploring deformation textures in soft matrix-rigid reinforcement composites. The deformation to true strains near −1.0 of unreinforced polycrystalline LiF produces substantial 110 and 100 fibre texture components in axisymmetric forging at 400°C. Additions of SiC platelets as a reinforcement phase suppress both texture components, but not to the same extent. Mechanisms in the texture development of LiF with 0, 10, 20 and 30 vol.% SiC platelets are discussed in relation to microstructural and deformation results.

Formation of carbon films on carbides under hydrothermal conditions

CARBON films find applications in a wide range of fields, ranging from microelectronics to materials science1. In ceramic matrix composites they confer the high strength and toughness needed for applications in aerospace, nuclear and automotive engineering2. Chemical vapour deposition is traditionally used to prepare carbon films, but it is relatively expensive, and not easily adapted to coating samples in the form of whiskers, platelets or powders. Here we report that silicon carbide, the most common component of composite ceramics, can be coated with carbon films of nanometre to micrometre thickness by hydrothermal treatment at 300–800 °C. We have applied the technique to SiC fibres, powders, platelets and single crystals, as well as to other carbides. Our method should provide a general and inexpensive route to high-toughness composites and lubricating coatings.

Investigation of the formation and growth processes of silicon carbide monocrystals

The formation and growth processes of SiC monocrystals from vapour phase by sublimation method have been investigated. It is shown that the peaks of the evaporation and growth whiskers are the centres of SiC monocrystals formation. One of the mechanisms of development of the whisker into SiC platelet is established. It is shown that on the naturally mirror like crystal face either the edge of the crystal or the screw dislocation, situated on the crystal edge, is the source of the steps. On the stepped crystal face the root of the crystal is the source of the steps. [Russian Text Ignored].

Processing and mechanical properties of a silicon carbide platelet/alumina matrix composite

Abstract This paper presents an integrated study of the processing and mechanical properties of a SiC platelet/alumina composites. The major goal was to investigate the potential application of plate-like particles in toughening ceramic matrices. In the processing part of the study, the microstructures were found to possess preferred platelet orientation, with the platelets tending to lie parallel to each other after hot-pressing. In the mechanical property part of the study, Young's modulus, fracture toughness, flexural strength, and R-curve behavior were measured as a function of platelet content. It was determined that the preferred orientation did not lead to significant elastic anisotropy but it did lead to toughness anisotropy. Indeed the platelets led to a decrease in fracture toughness for crack propagating parallel to the platelet faces. In addition, it was found that the platelets also led to a strength decrease compared to the alumina. The optimum properties were a Young's modulus of 421 GPa, fracture toughness of 7·1 MPa√m, and a flexural strength of 480 MPa at SiC volume fraction of 0·3. Overall, it was concluded that with controlled processing SiC platelets are strong candidates for reinforcing ceramic matrices but that further development is needed.