Fiber-bonded Ceramics Research Articles

Preparation and Characterization of Sintered SiC Fiber-bonded Ceramics

Preparation and Characterization of Sintered SiC Fiber-bonded Ceramics

Microstructure and thermal conductivity of Si-Al-C-O fiber bonded ceramics joined to refractory metals

We explore joining Si-Al-C-O fiber-bonded ceramics to Cu-clad-Mo using an Ag-Ti-Cu brazing alloy. A temperature of 900 °C and times in the range of 10–20 min are required to obtain sound joints irrespectively of the fiber orientation. The reaction layer is 1–2 μm thick and free of pores and defects. The thermal conductivity of the joined samples is well described considering that the metal and the ceramic are in series for thermal resistance. This implies that the joint is highly conductive and forms an almost perfect thermal interface between the two materials, confirming the quality of the obtained brazing layer.

Microstructure and mechanical properties of joints in sintered SiC fiber-bonded ceramics brazed with Ag–Cu–Ti alloy

Active metal brazing of a new high thermal conductivity sintered SiC-polycrystalline fiber-bonded ceramic (SA-Tyrannohex®) has been carried out using a Ti-containing Ag–Cu active braze alloy (Cusil-ABA®). The brazed composite joints were characterized using scanning electron microscopy coupled with energy-dispersive X-ray spectrometry (SEM–EDS). The results show that this material can be successfully joined using judiciously selected off-the shelf active braze alloys to yield metallurgically sound joints possessing high integrity. Uniform and continuous joints were obtained irrespective of differences in the fiber orientation in the substrate material. Detailed interfacial microanalysis showed that the titanium reacts with C and Si to form TiC layer and a Ti–Si compound, respectively. Furthermore, the evaluation of shear strength of the joints was also conducted at ambient and elevated temperatures in air using the single-lap offset (SLO) shear test. The perpendicular-type SA-Tyrannohex joints exhibited apparent shear strengths of about 42MPa and 25MPa at 650°C and 750°C, respectively. The fracture at the higher temperature occurred at the interface between the reaction-formed TiC layer and braze. This might be caused by generation of stress intensity when a shear stress was applied, according to μ-FEA simulation results.

Microstructure and Mechanical Properties of Joints in Sintered SiC Fiber-Bonded Ceramics

Active metal brazing of a new high thermal conductivity SiC-polycrystalline fiber-bonded ceramic (SA-Tyrannohex™) has been conducted using a Ti-containing Ag-Cu active braze alloy (Ticusil®). The brazed joints were characterized using SEM-EDS and Knoop hardness scans across the interfaces. The effects of fiber orientation in the composite on the microstructure, elemental composition, and microhardness are presented. Results show that this material can be successfully joined using judiciously selected off-the shelf active braze alloys to yield metallurgically sound joints possessing high integrity.

001 繊維結合型セラミックスの高温特性

001 繊維結合型セラミックスの高温特性

Ouantitative Analysis of Oxidation Behavior of Free Carbon in S-Ti-C-O Fiber-Bonded Ceramics.

A model for the oxidation kinetics and mechanisms of Si-Ti-C-O fiber-bonded ceramics was newly developed to evaluate the oxidation behavior quantitatively at temperatures ranging between 773 and 1473K in air. The model considers the geometry changes of pores resulting from consumption by oxidation of the free carbon layer, and the oxygen consumption due to oxidation of the pore walls. A differential equation for the oxygen concentration profile along the pore was derived and solved numerically. For materials with very thin carbon layer, the geometry change of the pore has a significant effect on the oxygen concentration profile. The relative mass changes due to oxidation of (1) the external surface, (2) the carbon layer and (3) the pore walls were respectively calculated and thermogravimetric analysis (TGA) curves of the material derived from their sum. The calculated TGA curves well agreed with the experimental data. The length of carbon layer consumed by oxidation was calculated as a function of time and temperature. At low temperatures, the rates of the carbon consumption were low, but pores took place proceeded for long time. At high temperature, in contrast, the oxidation of the carbon layer was inhibited in a short time due to the quick sealing of the pore entrance by silica layer. Thus, the oxidation damage is limited only in the vicinity of the material surface.

00/02892 Oxidation mechanisms of Si-Ti-C-O fiber-bonded ceramics

00/02892 Oxidation mechanisms of Si-Ti-C-O fiber-bonded ceramics

Influence of Matrix Structure on Mechanical Property of Si-M-C-O Fiber-Bonded Ceramics.

The microstructure and mechanical properties of Si-Ti-C-O and Si-Zr-C-O fiber-bonded ceramics, produced by hot-pressing a laminated material of pre-oxidized Si-(Ti or Zr)-C-O fibers, were investigated. The Si-Ti-C-O fiber-bonded ceramic had a close-packed structure of fibers with a volume fraction of 85% and a SiO2-based oxide matrix including TiC particles. On the other hand, the Si-Zr-C-O fiber-bonded ceramic consists of a close-packed structure with a volume fraction of 83% and a SiO2-based oxide matrix without any precipitated phase. Moreover, an interfacial carbon layer is constructed around all the fiber elements in both the fiber-bonded ceramics during the hot-pressing. The high-temperature flexural strength was much higher in the Si-Ti-C-O fiber-bonded ceramic than in the Si-Zr-C-O one. This was thought to be due to the SiO2-matrix in the Si-Ti-C-O fiber-bonded ceramic, which seems to be refrained from the softening effect by dispersed TiC particles at high temperature.

514 Si-Ti-C-O繊維結合型セラミックスの引張破壊機構(OS 耐熱材料)

Tensile fracture behavior of two dimensionally reinforced Si-Ti-C-O fiber bonded ceramics (FBC) was investigated. Stress-strain curve of the FBC was linear up to 120 MPa, and showed nonlinear behavior in the higher stress level. Micro-fracture in 90° layers, such as delamination at the fiber/ matrix interface, was observed by the SEM. It was shown that evolution of the micro-fracture was closely related to the degradation in Young's modulus.

Influence of Oxide Layer on Oxidation of Free Carbon in Si-Ti-C-O Fiber-Bonded Ceramics.

Influence of a silica layer on oxidation of free carbon in Si-Ti-C-O fiber-bonded ceramics has been investigated. The rate of oxidation of the Si-Ti-C-O fibers has been measured by thermogravimetric analysis at temperatures ranging between 773 and 1573K. The relative mass of the fibers increased parabolically and the kinetics of the oxidation followed the parabolic rate characteristic of diffusion controlled processes. The thickness of the silica layer, calculated according to a simple equation, well agreed well with the experimentally observed one. Pores created by the oxidation of free carbon were observed inside the fibers after etching the surface of a heat-treated Si-Ti-C-O fiber-bonded ceramics at 773K. Using the parabolic rate constant and the volume expansion coefficient of the fiber, the time required to close the pores by the formation of silica layer was calculated. The thickness of the free carbon layer and the oxidation rate of the fibers were the key parameters for the analysis of the oxidation behavior of the materials.

Oxidation Mechanisms of Si-Ti-C-O Fiber-Bonded Ceramics.

The oxidation mechanisms of Si-Ti-C-O fiber-bonded ceramics had been investigated through thermogravimetric analysis, electron microscopy and X-ray diffraction. The influence of temperatures ranging from 773 to 1773K in air is discussed. The materials showed mass loss due to oxidation of the carbon interface and free carbon in the fibers at the beginning of the oxidation tests. Then, mass gain due to the formation of silica on the surface of the fibers was observed. When heat-treated below 1023K, most of the free carbon was oxidized and flexural strength decreased to almost zero. The free carbon contained in the materials was about 7.7mass% and oxidized along the fiber direction. On the other hand, upon heat treatment above 1073K, the oxidation of carbon was strongly inhibited due to a cristobalite coat formed on the surface of the specimen which maintained its initial strength even after oxidized for 100h. After the formation of the cristobalite coating, the materials showed an excellent oxidation resistance even if oxidized at 973K

Ｓｉ−Ｔｉ−Ｃ−Ｏ繊維結合型セラミックスの微細構造と特性

The microstructure and mechanical properties of Si-Ti-C-O fiber-bonded ceramics (Tyrannohex (R)) produced by hot-pressing pre-oxidized Si-Ti-C-O fibers with a surface oxide (SiO2 and TiO2) layer of 250, 430 and 580nm thickness were investigated. The hot-pressing temperature and pressure were 2023K and 50MPa, respectively. Tyrannohex, which was produced using the pre-oxidized fiber with a surface oxide layer of 250nm thickness, showed a close-packed structure with 90% of volume fraction (Vf) of the fiber. The interstices were packed by an oxide material, which existed on the surface of the raw fiber. It was observed that the oxide material changed to TiC-particle dispersed structure by a reaction between TiO2, which existed on the fiber's surface, and excess carbon in the fiber during hot-pressing. Moreover, an interfacial carbon layer (10-20nm) around the fiber was formed. Tyrannohex showed very high fracture energy (8900J/m2) by Chevron notched beam method. Tyrannohex (Vf=90%) showed elastic fracture behavior without any plasticity even at 1673K, because all the fibers were restricted by the close-packed structure. At all the contact points of the fibers, an interfacial carbon layer existed without oxide material. About 90% of the initial strength was maintained after heat-treatment in air for 1000 hours because of an effective formation of a protective oxide layer on the surface.

Si-Ti-C-O繊維結合型セラミックスの力学的特性に及ぼす繊維体積率の影響

Si-Ti-C-O繊維結合型セラミックスの力学的特性に及ぼす繊維体積率の影響