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SiO2–C precursors with various surface areas were derived from tetraethyl orthosilicate and phenolic resin as Si and C sources, respectively, by a modified sol–gel process using the in situ precipitation of phenol resin in a prepared wet gel. The surface area of the SiO2–C precursors was varied from 20 to 175 m2/g by changing the C/Si molar ratio in the preform. β-SiC powders were synthesized using carbothermal reduction in vacuum at the temperature range of 1200–1600 °C. The effects of the temperature and heat treatment time as well as that of the surface area of the preform on the formation of β-SiC powders were studied. It was determined that the formation of β-SiC started at 1200 °C and was considerably promoted as the heat treatment temperature and time further increased during the carbothermal reduction of SiO2–C preforms with high surface area. When high surface area SiO2–C preforms were used, highly crystalline SiC powders were synthesized at 1600 °C in vacuum with a high yield of 85%.

In this study, Si-SiC composites were fabricated using a Si melt infiltration method using β-SiC/C composite powders synthesized by the carbothermal reduction of SiO₂-C precursors made from a TEOS and a phenol resin. The purity of the synthesized SiC-C composite powders was higher than 99.9993 wt% and the average particle size varied from 4 to 6 μm with increasing carbon contents of the SiO₂-C precursors. It was found that the Si-SiC composites fabricated in this study consist of β-SiC and residual Si, without any trace of α-SiC. The 3-point bending strengths of the fabricated Si-SiC composites were measured and found to be higher than 550 MPa, although the density of the fabricated Si-SiC composite was less than 2.9 g/cm3. The bending strengths and the densities of the fabricated Si-SiC composites were found to decrease with increasing C/Si mole ratios in the SiC-C composite powders. The specific resistivities of the Si-SiC composites fabricated using the SiC-C composite powders were less than 0.018 Ωcm. With increasing C content in the SiC-C composite powders used for the fabrication of Si-SiC composites, the specific resistivity of the Si-SiC composites was found to slightly increase from 0.0157 to 0.018 Ωcm.

The objective of this study was to develop a synthesis process for β-SiC powders to reduce the synthesis temperature and to control the particle size and to prevent particle agglomeration of the synthesized β-SiC powders. A phenol resin and TEOS were used as the starting materials for the carbon and Si sources, respectively. SiO₂-C hybrid precursors with various C/Si mole ratios were fabricated using a conventional sol-gel process. β-SiC powders were synthesized by a carbothermal reduction process using SiO₂-C hybrid precursors with various C/Si mole ratios (1.6 ~ 2.5) fabricated using a sol-gel process. In this study, the effects of excess carbon and the addition of Si powders to the SiO₂-C hybrid precursor on the synthesis temperature and particle size of β-SiC were examined. It was found that the addition of metallic Si powders to the SiO₂/C hybrid precursor with excess carbon reduced the synthesis temperature of the β-SiC powders to as low as 1300℃. The synthesis temperature for β-SiC appeared to be reduced with an increase of the C/Si mole ratio in the SiO₂-C hybrid precursor by a direct carburization reaction between Si and excess carbon.

β-SiC powders were synthesized by a carbothermal reduction process using SiO₂-C precursors fabricated by a sol-gel process using phenol resin and TEOS as starting materials for carbon and Si sources, respectively. The C/Si molar ratio was selected as an important parameter for synthesizing SiC powders using a sol-gel process, and the effects of the C/Si molar ratio (1.4-3.0) on the particle size, particle size distribution, and yield of the synthesized β-SiC powders were investigated. It was found that (1) the particle size of the synthesized β-SiC powders decreased with an increase in the C/Si molar ratio in the SiO₂-C hybrid precursors, (2) the particle size distribution widened with an increase in the C/Si molar ratio, and (3) the yield of the β-SiC powder production increased with an increase in the C/Si molar ratio.

Ti 3 AlC 2 was corroded between 800 and 1100 °C in an Ar–0.2% SO 2 gas atmosphere according to the equation: Ti 3 AlC 2 + O 2 → rutile-TiO 2 + α-Al 2 O 3 + (CO or CO 2 ). The scales that formed on the Ti 3 AlC 2 were thin and rich in α-Al 2 O 3 , whose growth rate was exceedingly slow. The TiO 2 was present either as the outermost surface scale or a mixture inside the α-Al 2 O 3 -rich scale. In the Ti 3 AlC 2 , the activity and diffusivity of Ti were low, whereas those of Al were high. This was the main reason for the superior corrosion resistance of Ti 3 AlC 2 over TiAl.

In this study, we carried out a sol-gel processing to synthesize two types of phenol resin and TEOS sol with fluoric acid as the catalysts, and fabricated the β-SiC powder by carbothermal reduction of the obtained precursor gels. The gels were observed to be cross linked between TEOS and phenol resins by FT-IR and TG-DTA. After pyrolyzed at 1000oC in Ar to obtain the precursor, to confirm the phase transition and morphologies by XRD, and SEM were measured. Finally, the light green colored SiC powder was obtained after carbothermal reduction process up to 1800oC. The residual condensed SiO(v) phase and free carbons as a role of impurities for final products using the solid type phenol resin was observed in the surface of powder by BSE.

Highly densified Silicon Carbide (SiC) ceramics with 10 vol.% sintering additives, RE2O3 (RE: Sc, Lu, Y) and AlN, were prepared via hot pressing in N2 atmosphere. The influence of sintering additive composition on the mechanical properties of the sintered specimens is discussed. The Sc2O3-AlN system consisted of equi-axed grain structure, while the Lu2O3-AlN and Y2O3-AlN systems rendered largely grown platelet grains. While fracture toughness was enhanced with regards to the contribution of crack deflection and crack bridging mechanisms for intergranular fracture, strength remained almost the same for all specimens.

High-purity, dense nano-laminated (Cr0.95Ti0.05)2AlC compounds were synthesized via a powder metallurgical route. Their oxidation characteristics were investigated by exposing them to temperatures between 900 and 1200 degrees C in air. The alloying Ti in the Cr layer of Cr2AlC did not significantly change the crystal structure and microstructure of Cr2AlC. But, it increased the oxidation rate of the Cr,AlC. The scale morphology of the (Cr0.95Ti0.05)2AlC was basically similar to that of the pure Cr2AlC. The main difference was that the (Cr0.95Ti0.05)2AlC contained oxide nodules. These formed because the titanium oxidized to TiO2, making the Cr2AlC susceptible to nodular oxidation. However, alpha-Al2O3 was still the major oxidation product in both the (Cr0.95Ti0.05)2AlC and pure Cr2AlC.

Reaction Bonded Silicon Carbide(RBSC) has been used for engineering ceramics due to low-temperature fabrication and near-net shape products with excellent structural properties such as thermal shock resistance, corrosion resistance and mechanical strength. Recently, attempts have been made to develop hot gas filter with gradient pore structure by RBSC to overcome weakness of commercial clay-bonded SiC filter such as low fracture toughness and low reliability. In this study a fabrication process of porous RBSC with multi-layer pore structure with gradient pore size was developed. The support layer of the RBSC with multi-layer pore structure was fabricated by conventional Si infiltration process. The intermediate and filter layers consisted of phenolic resin and fine SiC powder were prepared by dip-coating of the support RBSC in slurry of SiC and phenol resin. The temperature of 1550℃ to make Si left in RBSC support layer infiltrate into dip-coated layer to produce SiC by reacting with pyro-carbon from phenol resin.

High purity β-SiC powders were synthesized using sol-gel processing. TEOS and phenol resin were used as the starting material for the silicon source and carbon source, respectively. The process turned out to be capable of producing high purity SiC powder purity degree with 99.98 %. However, it was difficult to control the shape and size of β-SiC powders synthesized by sol-gel process. In this study, β-SiC powder with size of 1~5 ㎛ an 30 ㎚ were used as the seeds for β-SiC to control the β-SiC powder morphology. It was found that β-SiC powder seeds was effective to increase the powder average size of synthesized β-SiC using sol-gel process by acting as the preferred growing sites for β-SiC.

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

Dense, monolithic Ti3Al0.7Si0.3C2 compounds were cyclically oxidized between 900 and 1100 °C in air for 100 h. Ti3Al0.7Si0.3C2 oxidized into rutile-TiO2, α-Al2O3, and amorphous SiO2, accompanied by the evaporation of carbon. Ti3Al0.7Si0.3C2 had good thermal shock resistance, so that adherent oxide scales formed, regardless of their thickness. The cyclic oxidation resistance of Ti3Al0.7Si0.3C2 was poorer than that of Ti3SiC2, Ti3AlC2, and Cr2AlC. The thermal cycling did not affect the scale morphology or the oxidation mechanism that was identified in the isothermal oxidation tests, due mainly to the formation of the adherent oxide scales.

The layered, machinable quaternary compound, Ti 3 Al 0.5 Si 0.5 C 2 , was synthesized via the powder metallurgical route, and oxidized between 900 and 1200 °C in air for up to 80 h. The oxidation of Ti 3 Al 0.5 Si 0.5 C 2 produced rutile-TiO 2 , α-Al 2 O 3 , and amorphous SiO 2 , together with the evolution of CO or CO 2 gases. The oxide scale consisted primarily of an outer TiO 2 -rich layer, an intermediate Al 2 O 3 layer, and an inner TiO 2 -rich layer. Relatively thick scales formed because of the formation of the semi-protective TiO 2 and the gaseous escape of carbon.

High purity, dense Cr2AlC compounds were synthesized via a powder metallurgical route, and their oxidation behavior was investigated at 1300 °C in air for up to 336 h. A thin external oxide layer formed, which consisted primarily of not Cr2O3 but Al2O3. Since Al was consumed to produce the Al2O3, Al-depletion and Cr-enrichment occurred underneath the Al2O3 layer. This led to the formation of a Cr7C3 layer containing voids. These grew during oxidation, eventually destroying the Cr7C3 layer formed on the unoxidized Cr2AlC matrix.

A porous reaction bonded silicon carbide (RBSC) was fabricated by a molten Si infiltration method. The porosity and flexural strength of porous RBSC fabricated in this study were dependent upon the amount of carbon source used in the SiC/carbon preform as well as the amount of Si infiltrated into the SiC/carbon preform. The porosity and flexural strength of porous RBSC were in the range of 20 vol.~49 vol.% and 38~61㎫, respectively. With increase of carbon contents and molten Si for infiltration, volume fraction of the pores was gradually decreased, and flexural strength was increased. The porous RBSCs fabricated with the same amount of molten Si show less residual Si around neck with increase of carbon source, as well as a new SiC was formed around neck which resulted in the decreased porosity and improvement of the flexural strength. In addition, decrease of the porosity and increase of the flexural strength were also obtained by increase of the amount of molten Si with the same amount of carbon source. However, it was found that the flexural strength of porous RBSC depends on the porosity rather than the amount of the newly formed SiC in neck phase between SiC particles used as a starting material.

The Ti3SiC2 materials were synthesized by hot pressing TiCx and Si powder mixtures. The matrix grains were lamellar, having a small amount of TiCx. The high-temperature stability was investigated by subjecting Ti3SiC2 to high-temperature oxidation up to 1200oC in air. Ti3SiC2 began to oxidize appreciably above 850oC. The oxidation resulted in the formation of the oxide layer that consisted of TiO2 and SiO2. The scales formed were adherent.

Boron carbide is a very hard material with high abrasive wear resistance. It requires a very high sintering temperature of above 2200 oC to fabricate a monolithic B4C close to the theoretical density. However, the mechanical property of monolithic B4C is not good enough to use it directly to industrial applications. In this investigation, B4C based ceramic composites were fabricated by in-situ reaction hot pressing using B4C, TiC and SiC powder as starting materials. The reaction synthesized composites by hot pressing at 1950 oC was found to posses very high relative density. The reaction synthesized B4C composites comprise B4C, TiB2, SiC and graphite by the reaction between TiC and B4C. The newly formed TiB2 and graphite was embedded both inside grain and at grain boundary of B4C. The mechanical properties of reaction synthesized B4C-TiB2- SiC-graphite composites were more enhanced compared to those of monolithic B4C. The flexural strength and fracture toughness of these in-situ B4C synthesized composites were 400-570 MPa and 6-9.5 MPam1/2, respectively.

Nanosized ITO(indium tin oxide) powders were prepared in ethylene glycol solution under mild temperature and pressure conditions by precipitation from metal nitrates with aqueous potassium hydroxide. The average size and distribution of the synthesized ITO powders were about 20 ㎚ and broad, respectively. The phase of synthesized particles was crystalline reacted at 230 0C for 6 h.. The optical properties of the synthesized ITO powders were transparent.

The oxidation kinetics of TiAl alloys with and without 3 and 5 wt.%additions of Si3N4 particles were studied at 1173 and1273 K in 1 atm of air. The Si3N4 dispersions wereunstable in the matrix phase, so that some of them reacted with titaniumduring sintering to form Ti5Si3 and dissolvednitrogen. The oxide scale formed on TiAl–Si3N4alloys consisted of an outer TiO2, an intermediate(Al2O3+TiO2), and an inner(TiO2+Al2O3) mixed layers. The enhancedalumina-forming tendency, the presence of discrete SiO2 particlesbelow the outer TiO2 layer, and the improved scale adhesion bySi3N4 dispersions were attributable mainly to theincreased oxidation resistance compared to the Si3N4-freeTiAl alloys. Marker experiments showed that, for TiAl–Si3N4 alloys, the primary mode of scale growth was the outward diffusion oftitanium ions for the outer scale and the inward transport of oxygen ionsfor the inner scale.

In order to improve the room-temperature fracture strength of the brazed silicon nitride (Si 3 N 4 )/steel joint, the use of creep-resistant and oxidation-resistant Ni-interlayer has been attempted. The present study deals with two different methods (i.e. direct brazing method, and double active metal brazing method) for joining ceramic and metal materials. The dissolution of Ni into the brazing alloy for the joint that was fabricated by direct brazing method resulted in the decrease of the fracture strength with the increase in the thickness of Ni-interlayer. The cause of the strength reduction was attributed to be (1) by the formation of Ni-Ti compound inhibiting the effective reduction of the residual stress, and/or (2) by the depletion of Ti near the interface restricting an adequate interface bonding. The double active metal brazing method enabled an effective reduction in the dissolution of Ni-interlayer into the brazing alloy, which lead to the increase of the joint fracture strength. The finite element analysis was performed to calculate the residual stress within the joint.

A new, faster process was developed for the fabrication of Nicalon-fibre-reinforced SiC composites by combining polymer solution infiltration (PSI) and chemical vapour infiltration (CVI). The process led to the near-net-shape fabrication of fibre-reinforced ceramic-matrix composites and reduced infiltration time. Typical flexural strength and fracture toughness of these composites were 296 MPa and 10.9 MPa m1/2 at room temperature (RT) and 252 MPa and 9.6 MPa m1/2 at 1000 °C, respectively. The composites exhibited load-carrying capability after crack initiation.