T. Vasilos

Characteristics of carbon fibre-reinforced calcium phosphate composites fabricated by hot pressing

Abstracts are not published in this journal

Processing, microstructure and mechanical properties of hot-pressed SiC continuous fibre/SiC composites

SiC (SCS-6TM) continuous fibre/SiC composites were fabricated by hot-pressing at 1700°C in vacuum using an Al sintering additive. Analytical transmission electron microscopy was used to investigate the microstructure of the composites. The room-temperature mechanical and high-temperature creep properties of the composites were investigated by four-point bending. The SiC powders used were sintered at a relatively low sintering temperature to high density (97% of theoretical density) with the addition of the Al sintering additive. It is believed that the Al additive is very efficient for the densification of SiC. The SiC fibres maintained their original form and microstructure during fabrication. The SiC matrix reacted with the outermost carbon sublayer in the fibre, forming a thin (1.8–4.8μm) interfacial layer, which was composed of Al4C3, Si–Al–C, and Si–Al–O phases. The incorporation of SiC fibre into a dense SiC matrix significantly increased the room-temperature failure strain and improved the high-temperature creep properties. In addition, the incorporation of SiC fibre into a porous SiC matrix increased the room-temperature failure strain, but did not contribute to the high-temperature creep properties.

Microstructure and thermal shock resistance of Al2O3 fiber/ZrO2 and SiC fiber/ZrO2 composites fabricated by hot pressing

Al2O3 chopped fiber/ZrO2 and SiC continuous fiber/ZrO2 composites were fabricated by hot pressing at 1550°C and 15 MPa in vacuum. The mechanical properties of thermally shocked composites were measured at room temperature by four-point bending. The addition of Al2O3 fibers into ZrO2 matrix degraded the fracture strength, but improved significantly the thermal shock resistance. In addition, the mechanical properties of SiC fiber/ZrO2 composites were much lower than those of monolithic ZrO2 because of the presence of microcracks on the surface. The SiC fiber/ZrO2 composites showed an excellent thermal shock resistance.

Silicon nitride matrix composites with unidirectional silicon carbide whisker reinforcement

SiC whisker reinforced Si3N4 was fabricated by fiber extrusion and hot pressing. SiC whiskers were unidirectionally oriented in a carrier fiber. The fibers containing the oriented whiskers were hot pressed in Si3N4 powder to form a SiCw/Si3N4 composite with approximately 5 volume% whiskers. SEM micrographs were image processed to quantify whisker orientations in the extruded fiber and the composite. Oriented whiskers contributed to nominal increase in fracture strength over monolithic samples before and after thermal shock testing from 500, 600 and 700°C.

Microstructure and mechanical properties of silicon carbide fibre-reinforced aluminium nitride composite

SiC continuous fibre (15 vol%)/AlN composite was fabricated using a sintering additive of 4Ca(OH)2 · Al2O3 by hot-pressing at 1650 °C and 17.6 MPa in vacuum. Analytical transmission electron microscopy and scanning electron microscopy were used to investigate the microstructure of as-fabricated and crept SiC fibre/AlN composites. The room-temperature mechanical and high-temperature creep properties of the composite were investigated by four-point bending. The incorporation of SiC fibre into AlN matrix improved significantly the room-temperature mechanical properties. This improvement could result from the crack deflections around the SiC fibres. However, the incorporation degraded severely the high-temperature creep properties under oxidizing atmosphere. This could be attributed to the development of the pores and various oxides at the matrix grain boundary and matrix/fibre interface during creep test.

Interfaces in Silicon Carbide Whisker and Carbon Fiber Reinforced Calcium Phosphate Composites

The microstructural and chemical characteristics of silicon carbide whisker/calcium phosphate and carbon fiber/calcium phosphate composites were studied using analytical transmission electron microscopy. The interface at these calcium phosphate-based composites was studied, with particular attention to finding evidence of a chemical reaction at the interface. Structural observations made on these composites explain the fact that their mechanical properties may be strongly affected by the structural properties.

Interfacial characterization of a SiC fiber-reinforced AlN composite

In this study, an attempt was made to improve the mechanical properties of AlN by the incorporation of SiC (SCS-6) fibers (TEXTRON Specialty Materials, Lowell, MA) in a unidirectional array. The SiC fibers are one of the most important reinforcements for ceramic- and metal-matrix composites due to high tensile strength (3,450 MPs), high tensile modulus (400 GPa), and low density (3.0 g/cc). The SiC fiber (15 vol %)-reinforced AlN composite was fabricated by hot-pressing in vacuum. The microstructure and chemistry of interfacial regions in as-fabricated and crept composite were characterized using analytical transmission electron microscopy, in order to investigate the nature of the reaction between the fiber and matrix during both composite fabrication and creep tests and to understand the reinforcing effects of SiC fiber in the AlN matrix. Interfacial characteristics of the composite play an important role in influencing the mechanical properties of the composite.

Microstructure and chemistry of second phases in MgO- and NiO-codoped alumina by analytical transmission electron microscopy

Small amounts of additives can greatly affect the sintering of ceramic powders. Since the addition of small amounts of MgO (~ 0.25 wt%) to A1203 enables it to sinter close to theoretical density [1], the influence of small amounts of additives on the sintering of A1203 has been widely studied [2-4]. The additives form second phases which profoundly affect the sintering behaviour, i.e. reduce grain boundary mobility and inhibit exaggerated grain growth. MgO inhibits grain growth in fully dense alumina, and the degree of inhibition depends on the purity of the starting powder [5]. Studies have also been conducted concerning the influence of NiO on grain growth in alumina [4]. NiO has been reported to behave similarly to MgO [4, 6]. Sintering in MgOand FeO-codoped alumina has been studied by Zhao and Harmer [7] in order to investigate the role of multiple solid-solution additives in sintering. They observed that MgO inhibits grain growth strongly in very pure powders and FeO promotes grain growth more than densification in alumina. Therefore, FeO was not a favourable additive. In the study reported here, MgO and NiO were used as multiple solid-solution additives. The MgO-NiO system consists of a complete solid solution extending from the higher melting point of MgO to the lower melting point of NiO [8]. The purpose of this study was to investigate the microstructure and chemistry of second phases, segregated particles and crystalline defects in alumina codoped with MgO and NiO using analytical transmission electron microscopy (TEM). As a result, it became possible to infer the location of MgOand NiO-codopants and impurities during the sintering process. Alumina codoped with 0 .15wt% MgO and 0.10 wt % NiO was fabricated by hot pressing at 1480 °C and 27.58 MPa in vacuum. TEM samples were sectioned from the alumina using a low-speed diamond saw and mechanically ground to -300 ~m. Circular discs of 3 mm in diameter were core drilled from the ground sections, mechanically ground to -120/~m, then dimpled to -30/ ira . The samples were ion milled with 5 kV Ar ÷ ions at an incident angle of 12 ° until perforation was achieved. A light carbon film was evaporated on the samples to prevent charging in the electron microscope. The microstructure and chemistry of second phases, segregated particles and crystalline defects in the alumina were investigated by bright field image, convergent beam electron diffraction (CBED),

Second Phases and Impurity Segregations in MgO- and NiO-CO-Doped Alumina

The microstructural and chemical characteristics of the segregated particles, Ni-(Mg)-Al spinel phases, and K-s’’’ alumina precipitates in fine-grained alumina co-doped with 0.15 wt % of MgO and 0.10 wt % of NiO were studied using analytical transmission electron microscopy. The segregated Ni particles and second phases were generally found at triple junctions or on grain boundaries. The K-s’’’ alumina precipitates were found to contain occasionally a high density of stacking faults. These microstructural observations point out the location of the dopants and impurities during the sintering process.