Yung-Jen Lin

Oxidation of SiC powders in SiC/alumina/zirconia compacts

Abstract Oxidation of SiC powder in compacts of SiC/alumina/zirconia was studied between 1000 and 1200°C in air for up to 10 h. The thermogravimetric analysis showed that the weight gain due to passive SiC powder oxidation became significant at temperatures ⩾937°C. After oxidation, the particle usually consisted of a SiC core surrounded by a layer of amorphous silica. However, the amorphous layer would crystallize into cristobalite if the powders were oxidized at 1200°C for a longer time (⩾8 h). The oxidation rate decreased if cristobalite formed. The square of weight gain fraction after oxidation was linearly proportional to the oxidation time when the weight gain fraction was less than 0.34. The activation energy of the oxidation of SiC powders was estimated to be 213.1 kJ/mol.

Fabrication of Ceramic-Metal Composites by Melt Infiltration of Moso-Bamboo-Derived Porous SiC

Copper and 2024 aluminum alloy were melt-infiltrated into porous β-SiC to form SiC/Cu and SiC/Al composites. The porous β-SiC was prepared using Moso bamboo as the bio-template and had structural characteristics of bamboo. The Cu infiltration occurred as low as 1100°C and became significant at 1200°C. After infiltration at 1300°C for 4 h, there was still ~5 % of residual porosity. For the composites with low degree of metal infiltration, the samples fractured like the bamboo-structured porous SiC. For the composites with high degree of infiltration, the sample behaved like monolithic copper. In the infiltration of Al alloy, infiltration occurred at 900°C. Higher Infiltration temperatures would result in significant formation of Al4C3, which gradually decomposed in air.

Alumina/Glass Composites Fabricated by Melt-Infiltration of Glass into Porous Alumina

Alumina/glass composites were successfully fabricated by melt-infiltration of glass into porous alumina pellets. Alumina powder was first pressed uniaxially at 100MPa to form disc-shaped pellets, then, heated up to 1200°C for 2 h to form porous pellets with moderate strength for subsequent infiltration. A mixture of calcium aluminosilicate and magnesium borosilicate glass powders were melt-infiltrated into porous alumina at 1200°C ~1250°C by capillary pressure to form composites. The infiltration depths varied with the square root of infiltration time. And the activation energy of the infiltration process was estimated to be 621 KJ/mole. After complete infiltration, the composite had bulk density approaching 3.3 g/cm3 (~ 96% of theoretical density) and open porosity reaching zero, with slight expansion of 0.5% in diameter. Its flexural strength was 150MPa and its Vickers microhardness was about 1000 Kg/mm2.

Fabrication of SiC and SiC/ Aluminum-Silicon Composites from Rattan Charcoal

Bio-structured carbon was obtained by carbonization of rattan in Ar. Then, the charcoal was reacted with Si at 1500 °C to form SiC. It was also reacted with Si and 2024 aluminum alloy at 1400°C for various times to form composites. The results showed that the SiC was beta phase and had the structure of rattan. SiC whiskers were also found on the inner surfaces of the pore channels (vessels) of rattan structure. The bulk density of the porous SiC was 1.5 g/cm3 and its open porosity was 44%. On the other hand, the charcoal embedded in Si powder and 2024 alloy was converted into SiC/aluminum-silicon composite after heat treatment. The charcoal was reacted to form SiC first. Then, Al-Si alloy infiltrated into the pores of the porous SiC to form composites. When the alloy composition was Al-20 at% Si, the composite obtained after 5 hours of reaction had bulk density of 2.3 g/cm3, open porosity of 19%, compressive strength of 316 MPa and bending strength of 138 MPa.

Fabrication of alumina and silicon carbide fibers from carbon fibers

Carbon fibers of ~9 μ m in diameter were used as templates to fabricate alumina and silicon carbide fibers. The carbon fibers were placed in a vacuum furnace with aluminum and heated at 1100°C for 8 h to form aluminum carbide. Then, the aluminum carbide fibers were oxidized in air at 1500°C. The resulted fibers were hollow and the alumina layer was porous in the interior. To fabricate silicon carbide fiber, carbon fibers were reacted with Si at 1300°C -1500°C in Ar. The thickness of silicon carbide layers increased with reaction temperature and reaction time. Solid fibers could be obtained after reaction at 1400°C for 4 h. In contrast to porous alumina layer, the silicon carbide layer/fibers were dense. The porous alumina hollow fibers were fragile while the solid silicon carbide fibers were flexible. BET surface area measurements revealed that the porous alumina had surface area as high as ~100 m2/g.

Pressureless Infiltration of Porous Al2O3/Ni Compacts with 6061 Al Alloy

Alumina powder compacts containing up to 30 vol% of Ni particles can be i filtrated pressurelessly with 6061 Al alloy to form ceramic/metal compos ites. The infiltration is facilitated by the presence of Ni. The more the Ni content was, the lower the threshold temperature for infiltration, and the higher infiltration rate at a specific te mperature. Other things being equal, the infiltration rate increased with the infiltration temperatures and was in the order of a few millimeters per hour. As the infiltration temperature increased, the bulk densities of the infiltrated samples remained about the same while the open porosities increased. T he shrinkage of the samples could be limited to 1% when the infiltration temperature was 1100 °C or lower. The shrinkage at higher infiltration temperatures was believed to derive from the pa rtial sintering of alumina compacts. XRD analyses show that samples after infiltration c ontain alumina, Al alloy and Al 3Ni intermetallic. Based on phase and microstructural analyses, a model of infi ltrat on is proposed. Introduction Composites of ceramic/metal or metal/ceramic are expected to have properties superior to their constituents alone [1-4]. However, the fundamental differences in atomic bonding between metals and ceramics result in quite different physical and chemical prope rties, such as surface energy, thermal expansion, chemical activity, etc. These differences pose restrictions in the fabrication of ceramic/metal composites. For example, non-wetting between oxide c erami surfaces and molten metals requires applied pressure or interfacial chemical rea ctions to ensure good bonding between metal and ceramics [5,6]. Nevertheless, many conventional and non-conventional techniques have been developed to fabricate ceramic/metal composites. Non-conventional reactive te chniques include pressureless reactive infiltration [7-9], directed metal oxidation process (D imox) [10,11] and reaction bonding of aluminum oxide [12-14]. Although the reactive techniques are proved to be s ucc s ful to have good interfacial bonding between metal and ceramics, they are li mit d to specific systems in which reactions of ceramics with metals (reactive infiltration) r metals with gas (oxidation or nitridation) are possible and can be controlled. The compositions and microstructures of th c mposites are not easy to design in the beginning of the processing. The conventional techniques include extrusion, forging, casting [15], hot-pres sing [16] and pressure infiltration [3,5]. In these conventional methods, the ceramic and metals do not react such that the non-wetting between metals and ceramics needs to be conside red. In some techniques such as extrusion, forging, casting, the volume percentage of ceramic re inforc ment is limited to less than 30 and the non-wetting problem is not severe. In infiltration techniques for fabricating composites with higher volume percentage of ceramics, pressure or vacuum is requir ed to overcome the non-wettability [3,5,17-19]. Alternatively, the surface of cerami c should be coated with metal (e.g. electroless nickel), or the composition of the metal is modi fied to improve the wetting between ceramic and metals [20-23]. In either case, spontaneous (pressureles s) infiltration becomes possible for fabricating near net-shaped ceramic/metal composites. In this research, we pressurelessly infiltrated alumina/Ni powde r compacts with molten 6061 aluminum alloy. Instead of coating the alumina powder or modifying t he infiltrating metals, we dispersed different amounts of nickel powder among alumina powder and compa cted into discs. The effects of Ni addition on the infiltration and the resulted composites were cha racterized. Key Engineering Materials Online: 2003-09-15 ISSN: 1662-9795, Vol. 249, pp 37-44 doi:10.4028/www.scientific.net/KEM.249.37