Dense Silicon Carbide Research Articles

Formation of dense silicon carbide by liquid silicon infiltration of carbon with engineered structure

Fully dense and net-shaped silicon carbide monoliths were produced by liquid silicon infiltration of carbon preforms with engineered bulk density, median pore diameter, and chemical reactivity derived from carbonization of crystalline cellulose and phenolic resin blends. The ideal carbon bulk density and minimum median pore diameter for successful formation of fully dense silicon carbide by liquid silicon infiltration are 0.964 g cm−3and approximately 1 μm. By blending crystalline cellulose and phenolic resin in various mass ratios as carbon precursors, we were able to adjust the bulk density, median pore diameter, and overall chemical reactivity of the carbon preforms produced. The liquid silicon infiltration reactions were performed in a graphite element furnace at temperatures between 1414 and 1900 °C and under argon pressures of 1550, 760, and 0.5 Torr for periods of 10, 15, 30, 60, 120, and 300 min. Examination of the results indicated that the ideal carbon preform was produced from the crystalline cellulose and phenolic resin blend of 6:4 mass ratio. This carbon preform has a bulk density of 0.7910 g cm−3, an actual density of 2.1911 g cm−3, median pore diameter of 1.45 μm, and specific surface area of 644.75 m2g−1. The ideal liquid silicon infiltration reaction conditions were identified as 1800 °C, 0.5 Torr, and 120 min. The optimum reaction product has a bulk density of 2.9566 g cm−3, greater than 91% of that of pure β–SiC, with a β–SiC volume fraction of approximately 82.5%.

Formation Mechanism of Interior Microflaws in the Preparation of Polymer-Derived Silicon Carbide Fibers

Microflaws were detected on the cross-section of polymer-derived silicon carbide fibers and their formation mechanism was studied by varying the curing degree and the firing rate. The results show that microflaws decrease in size with increase of the curing degree due to an increased ceramic residue. The results also show that microflaws decrease in size with decrease of the firing rate. No microflaws are detectable when the firing rate is as low as 10K/h. This indicates that the microflaws are the main channels of evolution gases and the pressure of these gases leads to their formation and propagation. So a high curing degree and a low firing rate are both preferred in the preparation of dense silicon carbide fibers.

The use of MgO-coated SiC powders as low temperature densification materials

The densification of silicon carbide powders at relatively low temperatures (i.e. 1900 °C and below) has been investigated as a means of lowering the cost of the production route for dense silicon carbide components. The work has concentrated on the use of MgO, as a coating on α-SiC powder deposited by chemical methods to allow densification by a liquid phase mechanism. Samples of SiC containing additions of MgO, both alone and as a coating on the surface of the SiC powder have been either hot pressed at temperatures between 1500 and 1750 °C or pressureless sintered at temperatures up to 1750 °C. Densification of the MgO-coated powder was observed to take place below 1650 °C and in certain cases a considerable amount of densification had taken place at temperature as low as ∼1450 °C.

Silicon carbide fiber-shaped microtools by extrusion and sintering SiC with and without carbon powder sintering additive

Silicon carbide powder was mixed with a thermoplastic polymer binder system consisting of a wax and a polyethylene. Thermal gravimetric analysis under argon showed that a carbon residue remains in the powder after binder decomposition. Because of the possibility of utilizing this residue as a sintering aid, two different feedstock series were tested: one series with and one without an additional carbon black sintering additive. Strength measurements on sintered fibers showed that the residue from the binder cannot be used as a sintering additive. Fibers made from the feedstock series without additional carbon black powder were characterized by a lower sinterability and a bigger grain size in the sintered fiber microstructure. Bigger grain size is caused by the absence of free carbon during the heating ramp up to the sintering temperature. To achieve dense silicon carbide fibers, a SiC content above 52 vol% in the feedstock is a prerequisite.

Preparation of Dense Nanocrystalline Silicon Carbide Ceramics by Sinter Forging in the Presence of a Liquid Phase

Dense SiC materials from ultrafine starting powders may be obtained with very limited grain growth if uniaxial hot pressing with concurrent lateral flow of the sample is applied. This process, termed sinter forging, allows to decouple the kinetics of densification and grain growth owing to the elimination of the largest and most persistent population of pores by viscous deformation under the action of shear forces.

Dispersion of SiC in Aqueous Media with Al2O3 and Y2O3 as Sintering Additives

It has been well accepted that polyethylene imine (PEI) is an effective dispersant for silicon carbide (SiC) in aqueous media. However, after the addition of sintering additives (Al2O3 and Y2O3), this dispersing effect is reduced significantly. In this work, a second dispersant, citric acid, was used to resolve this problem. It was found that citric acid could decrease the slurry viscosity (without sintering additives) and enhance the PEI adsorption on SiC particle surface. The optimal amount of citric acid required to achieve a minimum viscosity for 55 vol% SiC suspensions was equal to ∼0.87 wt% (at pH ∼6.8). With the aid of citric acid, well‐stabilized SiC suspensions (containing sintering additives) were realized, which exhibited slight shear thinning rheologies. After tape casting and SPS sintering, dense SiC samples were obtained with a homogeneous fine‐crystalline microstructure. Results showed that citric acid was an effective dispersant for improving the dispersion of SiC particles containing sintering additives.

Fabrication of dense silicon carbide through aqueous slurries containing well-dispersed carbon as a sintering aid

Tetramethyl ammonium hydroxide (TMAOH) has no fluidizing effect on carbon particles, because of their hydrophobic surface properties. On the other hand, styrene/maleic copolymer (SM) enhanced the fluidity of two kinds of carbon slurries in the basic pH range, where the particle diameters of carbon {#}2650 and MA100 were 13 and 22 nm, respectively. High-density sintered bodies of silicon carbide containing carbon and boron carbide as sintering aids were obtained by controlling the pH of ∼ 10–11 with TMAOH and by addition of SM: i.e., 9.5 and 5.5 wt% of SM were added to #2650 and MA100, respectively, in the dry weight base of the carbon. The highest sintered density product with more than 98% was obtained by intimate dispersion of the finer carbon particles in the silicon carbide slurries.

Influence of Molecular Structure of Anionic Polymer Dispersants on Dense Silicon Carbide Suspension Behavior and Microstructures of Green Bodies Prepared by Slip Casting

This paper reports on the relationship between the molecular structure of polymer dispersants, the electrosteric interaction of dispersants on a solid surface, and the viscosity of a ceramic suspension. Ammonium polycarboxylate with a different hydrophilic to hydrophobic group ratio (m : n) was prepared and added to a dense silicon carbide slurry (35 vol%). The steric interaction and adsorbed structure of the dispersant on silicon carbide powder were examined by means of an atomic force microscope (AFM). An optimum hydrophilic to hydrophobic group ratio, which was obtained from the maximum repulsive force and the minimum viscosity of the suspension, was determined to be m : n = 10 : 90. Furthermore, the optimum suspension behavior for obtaining an uniform green body structure by slip casting was evaluated and discussed.

Effect of structure of interfacial coating layer on mechanical properties of continuous fiber reinforced reaction sintered silicon carbide matrix composite

A dense silicon carbide matrix composite reinforced by Hi-Nicalon fibers CVD coated with boron nitride and silicon carbide was fabricated by slurry impregnation and subsequent reaction sintering with molten silicon. The effect of the structure and the thickness of the silicon carbide layer of the fiber coating on the mechanical properties of the composite were investigated. That is, three types of silicon carbide layers, namely a dense structure with a thickness of 0.15 μm and two porous structures with a thickness of 0.15 μm and 0.48 μm, respectively, were investigated. As a result, excellent strength property of ceramic matrix composite (CMC) was obtained in the case of the dense silicon carbide (SiC) layer. The thickness effect of the SiC layer on the strength was smaller than that of the structure.

Analysis of Anionic Polymer Dispersant Behavior in Dense Silicon Nitride and Carbide Suspensions Using an AFM

The paper focuses on the interaction mechanism caused by anionic polymer dispersants in dense silicon nitride and silicon carbide suspensions. An atomic force microscope (AFM) was used to determine the relationship between the macroscopic suspension viscosity and the microscopic structure adsorbing of a polymer dispersant at the solid/liquid interface. The surface interactions within the suspensions were analyzed under various dispersant pH values and additive conditions. The addition of an anionic polymer dispersant decreased the viscosity of silicon nitride and silicon carbide suspension and increased the electrosteric repulsive force on the non-oxide surface in solution at pH > 6, which was the isoelectric point of the materials. Based on the above results, we estimated the adsorption mechanism of anionic polymer dispersants on each solid surface in solution under relatively high pH conditions.

Mechanical and thermal properties of silicon-carbide composites fabricated with short Tyranno® Si-Zr-C-O fibre

Silicon carbide (SiC) composites reinforced with 10–50 mass% (10.5–51.2 vol%) of short Tyranno® Si-Zr-C-O fibre (average length ∼0.5 mm) and 0–10 mol% of Al4C3as a sintering aid were fabricated using the hot-pressing technique. Firstly, the effect of Si-Zr-C-O fibre addition on the relative density (bulk density/true density) of the SiC composite hot-pressed at 1800 °C for 30 min was examined by fixing the amount of Al4C3to be 5 mol%. Although the relative density was reduced to 87.4% for 10 mass% of Si-Zr-C-O addition, further increases in the amount of Si-Zr-C-O fibre increased density to a maximum of 92.8% at 40 mass% of fibre addition. Secondly, the effect of varying the amount of Al4C3addition on the relative density was examined by fixing the amount of Si-Zr-C-O fibre to be 40 mass%. The optimum amount of Al4C3addition for the fabrication of dense SiC composite was found to be 5 mol%. The fracture toughness of the hot-pressed SiC composites with 20–40 mass% of Si-Zr-C-O fibre addition (amount of Al4C3: 5 mol%) was 3.2–3.4 MPa · m1/2and approximately 1.5 times higher than that (2.39 MPa · m1/2) of the hot-pressed SiC composite with no Si-Zr-C-O fibre addition. SEM observation showed evidence of Si-Zr-C-O fibre debonding and pull-out at the fracture surfaces. The hot-pressed SiC composite with 5 mol% of Al4C3and 40 mass% of Si-Zr-C-O fibre additions showed excellent heat-resistance at 1300 °C in air due to the formation of a SiO2layer at and near exposed surfaces.

Torsional behaviour of tungsten heavy alloys made from mechanically alloyed powders: D.K.Kim et al. (University of Science and Technology, Pohang, Korea.) J. Korea Inst. Met. Mater., vol 37, no 1, 1999, 33–41. (In Korean.)

Zirconia is the preferred material used in many applications including biomedical implant. The zirconia in its sintered form is the most preferred one to manufacture a near net shaped implant material with minimal work. But it is very difficult to shape as the material chip away during the machining or grinding process. Hence zirconia in a pre-sintered form is used to achieve the required shape and size for a specific application. Then it is sintered and used. In this work, the grindability of the presintered zirconia was evaluated using a dense vitreous bond silicon carbide wheel. The grinding parameters such as wheel speed, radial depth of cut and feed rate are varied to investigate its grindability in terms of force ratio, specific energy, surface finish, G-ratio and ground surface under both wet and MQL cooling conditions. The forces produced during the grinding of pre-sintered zirconia was observed lower in the MQL (Minimum Quantity Lubrication) technique due to the reduction of friction, when compared to the wet cooling condition. The calculated specific energy was less in MQL cooling condition, due to the reduction in heat generation and friction. The surface finish of the workpiece obtained from the wet cooling condition was better due to the reduction in wheel loading. The percentage difference of the G-ratio between both the wet and MQL cooling conditions was observed to be 24 percent. This was due to the active participation of grains and less wheel loading in wet grinding condition. The ground surfaces obtained from the wet cooling condition were smooth and regular, compared to the MQL grinding condition.

Friction and Wear Properties of Carbon/Silicon Carbide Composites.

This paper describes the friction and wear properties of carbon/silicon carbide composites in comparison with that of porous silicon carbide and dense silicon carbide. Tests were conducted in ambient air, using a pin-on-disk friction test apparatus. The friction and wear properties of these materials varied according to the combination of pin and disk materials. The carbon/silicon carbide composite containing the largest carbon volume (about 40vol%) showed the lowest values of friction coefficient and specific wear among all the investigated materials. In addition, it also showed a stable behavior with respect to sliding velocity. The friction and wear mechanisms of these materials were discussed according to the morphology of the worn surfaces observed by scanning electron microscopy (SEM).

Development of Reaction Sintered Silicon Carbide Matrix Composite and Tensile Strength Properties.

Dense silicon carbide matrix composite reinforced by Hi-Nicalon® with BN or BN/SiC coating was fabricated by slurry impregnation of carbon and SiC and subsequent reaction sintering with molten Si. The optimum sintering condition was investigated and the effect of BN layer thickness on the tensile strength at room temperature was evaluated. The composite which was uni-directionally reinforced by 30vol% fibers with BN/SiC coating of 1.0/0.3μm thickness and sintered at 1693K for 0.3ks showed Young's modulus of 242GPa, the ultimate tensile strength of 556MPa and the failure elongation of 0.9%. The ultimate tensile strength showed a good agreement with the prediction from in situ fiber strength via fracture mirror size.

Crystallographic Orientation in Bulk Polycrystalline Silicon Carbide Produced by a Chemical Vapor Deposition (CVD) Process

AbstractPolycrystalline, theoretically dense silicon carbide was deposited onto graphite substrates via the reductive pyrolysis of methyltrichlorosilane in a hot-walled chemical vapor deposition (CVD) chamber. The resulting product can be considered a bulk material with deposit thicknesses in the range of 4 to 8 millimeters. The material was characterized using powder x-ray diffraction and Laue back-reflection techniques. Under the deposition conditions investigated in this study, the crystallographic orientation varied as a function of distance from the substrate. The material exhibited a high degree of randomness in proximity to the substrate, and progressively showed a higher degree of preferred crystallographic orientation as the deposit progressed. This phenomenon is correlated with the microstructure of the material as well as such mechanical properties as hardness and fracture toughness.

Development of Continuous SiC Fiber-Reinforced Reaction Sintered SiC Matrix Composite.

A dense silicon carbide matrix composite reinforced by Hi-Nicalon fibers with boron nitride coating was fabricated by slurry impregnation and subsequent reaction sintering process. Hi-Nicalon fibers were not stable in contact with molten silicon. Accordingly the effect of the fiber coating structure and the infiltrating metal composition on the mechanical properties of the composite was investigated. Selecting a boron nitride and silicon carbide dual coating structure and a boron-doped silicon as the infiltrating metal, a ceramic matrix composite with excellent fracture energy could be newly developed. The microstructure and the bending strength at room and high temperatures were also evaluated. Optimized composite showed cumulative failure mode both at room temperature and at 1573K. The failure behavior kept this cumulative fracture mode even after oxidation at 1573K for 360ks. Good oxidation resistance was thought to be caused by the presence of the dense SiC matrix.

In-situ hot pressing/solid-liquid reaction synthesis of dense titanium silicon carbide bulk ceramics

An in-situ hot pressing/solid-liquid reaction process was developed for the synthesis of dense poly-crystalline Ti3SiC2 ceramics using Ti, Si, and graphite powders as starting materials. The present work demonstrated that this process was one of the most effective and simple methods for the preparation of dense bulk Ti3SiC2 materials. Lattice constants of a=3.068 and c=17.645 are calculated for Ti3SiC2 made through this process. The synthesis temperature influenced the phase composition, microstructure and mechanical properties of Ti3SiC2 prepared at different temperatures. And bulk materials with flexural strength of 480 MPa and fracture toughness of 7.88 MPa.m1/2 were obtained at 1600°C. The high fracture toughness and strength are discussed based on microstructure analysis.

Material strength and inelastic deformation of silicon carbide under shock wave compression

In-material, lateral, manganin foil gauge measurements were obtained in dense polycrystalline silicon carbide (SiC) shocked to peak longitudinal stresses ranging from 10–24 GPa. The lateral gauge data were analyzed to determine the lateral stresses in the shocked SiC and the results were checked for self-consistency through dynamic two-dimensional computations. Over the stress range examined, the shocked SiC has an extremely high strength: the maximum shear stress supported by the material in the shocked state increases from 4.5 GPa at the Hugoniot elastic limit (HEL) of the material (11.5 GPa) to 7.0 GPa at stresses approximately twice the HEL. The latter value is 3.7% of the shear modulus of the material. The elastic–inelastic transition in the shocked SiC is nearly indistinctive. At stresses beyond twice the HEL, the data suggest a gradual softening with increasing shock compression. The post-HEL material strength evolution resembles neither catastrophic failure due to massive cracking nor classical plasticity response. Stress confinement, inherent in plane shock wave compression, contributes significantly to the observed material response. The results obtained are interpreted qualitatively in terms of an inhomogeneous deformation mechanism involving both in-grain microplasticity and highly confined microfissures.

Sliding Velocity Dependence of Friction and Wear Properties of Oil-Retaining Porous Silicon Carbide

The friction and wear properties of oil-retaining porous silicon carbide for sliding contact in air were investigated by using a pin-on-disk friction machine. The friction tests were carried out under the conditions of various sliding velocities (0.13 to 0.57m/s) and various combinations of oil-retaining porous silicon carbide and dense silicon carbide (pin/disk). In the dense SiC/oil-retaining porous SiC system, the lowest and most stable friction coefficient and the least wear were obtained independent of sliding velocity. This is due to the lubrication effect of the oil supplied from the oil-retaining porous SiC disk at the friction interface. The friction and wear mechanisms of these ceramics were classified into four modes depending on the relation between friction coefficient and specific wear amount and SEM observation of frictional surfaces.

Effect of some technological parameters on structure formation in materials based on reaction-sintered silicon carbide

Results of petrographic, x-ray, chemical, and spectral investigations of the structure and composition of materials based on SiC fabricated by reaction sintering of preforms pressed from grainy silicon carbide and its mixtures with petroleum coke in molten and volatilized silicon are presented. It is shown that the structure and composition of reaction-sintered silicon carbide materials can be controlled by changing the proportion of silicon carbide and petroleum coke in the pressed preform, the coarseness of carbide and carbon particles, the density of the pressings, and the temperature of reaction sintering. It is established experimentally that secondary silicon carbide formed as a result of the reaction between petroleum coke and silicon binds the grains of the initial carbide into a dense silicon carbide skeleton, whereas the retained pores are filled with free silicon. A single-phase material consisting entirely of silicon carbide can hardly be obtained by the method of reaction sintering. In practice, this method gives double-phase (SiC-Si) and triple-phase (SiC-Si-C) materials with a maximum content of the principal phase (SiC) equal to 94–96% (mass fractions).