Jung-Soo Ha

Effect of atmosphere type on gelcasting behavior of Al2O3 and evaluation of green strength

Gelcasting of Al2O3 was performed in air, vacuum (38 torr), and N2 to find the influence of atmosphere type. The strength of gelcast green bodies was determined by a flexural test and compared with those of dry-pressed and slip-cast bodies. It was found that the atmosphere types containing oxygen, such as air and vacuum (38 torr), was improper for gelcasting since the formation of strong binder networks by the in-situ polymerization of monomers failed in the surface regions, resulting in an exfoliation problem of the formed bodies. Such a problem could be avoided with N2 atmosphere. The flexural strength of the gelcast green bodies was determined to be 30.8 MPa, which was about 14 and 22 times higher than those of dry-pressed and slip-cast bodies, respectively. This high green strength was attributed to the well-developed binder networks by the polymerization of monomers, as confirmed by SEM.

Processing, structure, and properties of mullite fiber/mullite matrix composites

Abstract Oxide fiber/oxide matrix composites form an important and very attractive subpart of ceramic matrix composites because of their inherent stability in oxidizing atmospheres at high temperature. In particular, mullite fiber/mullite matrix composites have the potential of high temperature usage in oxidizing atmospheres. The interface in mullite fiber/mullite matrix was engineered by using thick BN (1 μm) or BN/SiC double coating on mullite fibers, such that deformation mechanisms conducive to toughness enhancement could be brought to play. Significant improvements in the room temperature mechanical properties of these mullite fiber/mullite matrix composites could be achieved by incorporation of these interfacial coatings and by using a colloidal processing route to make dense mullite matrix.

Effect of BN Coating on the Strength of a Mullite Type Fiber

Nextel 480 is a polycrystalline essentially mullite fiber (70 wt.-% Al2O3+28 wt.-% SiO2+2 wt.-% B2O3). Different thicknesses of BN were applied as coatings on this fiber. Optical, scanning electron, and transmission electron microscopy were used to characterize the microstructure of the coatings and fibers. The effects of coating and high temperature exposure on the fiber strength were investigated using two-parameter Weibull distribution. TEM examination showed that the BN coating has a turbostratic structure, with the basal planes lying predominantly parallel to the fiber surface. Such an orientation of coating is desirable for easy crack deflection and subsequent fiber pullout in a composite. The BN coated Nextel 480 fiber showed that Weibull mean strength increased first and then decreased with increasing coating thickness. This was due to the surface flaw healing effect of the coating (up to 0.3 μm) while in the case of thick BN coating (1 μm), the soft nature of the coating material had a more dominant effect and resulted in a decrease of the fiber strength. High temperature exposure of Nextel 480 resulted in grain growth, which led to a strength loss.

The effect of precursor characteristics on the crystallization and densification of diphasic mullite gels

Abstract The crystallization and densification behavior of diphasic mullite gels was studied with the gels prepared by using various sources of boehmite powder and silica sol to find the effect of precursor characteristics such as the pH of mixed sols, particle size, and the type of the stabilizing alkali ion in silica sol. Both low pH and fine particle size were found to promote mullite crystallization and thus result in low densification. However, even with a high pH and a large particle size, densification was low without the presence of sodium because of the crystallization of cristobalite and mullite before densification. A small amount of sodium (0·5 wt% as Na 2 O), present as a stabilizing ion in coilloidal silica or intentionally added, suppressed the crystallization of cristobalite and mullite before densification. As a result, bulk densities of more than 95% of the theoretical density were obtained by sintering at 1320°C for 1 h without a controlled heating.

Mechanical behaviour of mullite composites reinforced with mullite fibres

Abstract Mullite matrix composites uniaxially reinforced with uncoated and BN (0.3 or 1 μm thick) coated mullite fibres were consolidated to 82 to 98% of TD (theoretical density) at 1300 °C by using a colloidal processing route to make mullite matrix. The mechanical properties were characterized by flexural testing and chevron-notch tests for fracture toughness measurement. The samples with uncoated and 0.3 μm BN-coated fibres failed in a brittle manner without any toughening effect of the fibres. In contrast, the sample with 1 μm BN-coated fibres showed toughened composite behaviour (i.e. load-carrying capacity following maximum load) with a significant enhancement in toughness (e.g. about 12 MPa m 1 2 in the as-fabricated state), despite a relatively low density of about 90% TD. The room temperature ultimate strength (320 MPa) and fracture toughness of this composite decreased a little by heat treatment at 1320 °C for inducing complete transformation to mullite of the matrix, probably due to the degradation of fibre strength.

Structural evolution of alumina membrane prepared on an alumina support using a sol-gel method

We prepared supported and unsupported alumina membranes using a sol–gel method. The supported membrane system consisted of an alumina support, two intermediate α-Al2O3 layers, and a top alumina membrane. The θ- to α-Al2O3 transformation in supported and unsupported alumina membranes was investigated using X-ray diffraction (XRD) and scanning electron microscopy. XRD patterns showed that the supported membrane had a 100°C higher θ- to α-Al2O3 transformation temperature than the unsupported one. A similar effect was observed for microstructures of the membranes. We explained their transformation-temperature difference with a stress generated in the supported top membrane using a theoretical approach.

A new process for Al2O3/SiC nanocomposites by polycarbosilane infiltration

Abstract Conventional powder processing is commonly used to fabricate Al2O3/SiC nanocomposites. Recently a new processing route using SiC polymeric precursor such as polysilazane, polysilastyrene or polycarbosilane (PCS) has also been attempted, where a polymeric precursor solution is mixed with alumina powder. The present work investigated a new process to make the nanocomposites by modifying such a polymer precursor route. Unlike the existing process, the solution of a SiC polymer precursor was infiltrated into partially sintered (i.e. porous) alumina bodies, sintered at normal pressure. With PCS precursor, it was studied how the precursor solution concentration and sintering condition affected the SiC content, densification, phase and microstructure.

Effect of processing and fiber coating on fiber-matrix interaction in mullite fiber-mullite matrix composites

Abstract The fiber-matrix interaction in mullite fiber-mullite matrix composites was investigated with the composites fabricated using a commercial mullite powder and a mullite powder synthesized in our laboratory (via a diphasic gel route) as the matrix with uncoated and BN-coated fibers. Two different BN-coating thicknesses, 0.3 μm and 1 μm, were used. The composites made from the mullite powder synthesized in our laboratory showed less interaction between the fiber and matrix as well as no fiber deformation owing to a lower processing temperature, compared with the composite made from the commercial mullite powder. However, the fiber-matrix bonding in both the uncoated and even 0.3 μm BN-coated cases was still strong so that no fiber pullout was observed on the fracture surfaces. In the case of the 0.3 μm BN-coated fiber, the strong interfacial bonding resulted because the coating thickness was too thin to survive during processing. With 1 μm BN-coated fiber, however, extensive pullout could be obtained because the coating survived and prevented bonding between the fiber and matrix.

Properties of Porous SiC Ceramics Prepared by Wood Template Method

Porous SiC samples were prepared with three types of wood (poplar, pine, big cone pine) by simply embedding the wood charcoal in a powder mixture of Si and SiO₂ at 1600 and 1700℃. The basic engineering properties such as density, porosity, pore size and distribution, and strength were characterized. The samples showed full conversion to mostly β-SiC with good retention of the cellular structure of the original wood. More rigid SiC struts were developed for 1700℃. They showed similar bulk density (0.5~0.6 g/㎤) and porosity (81~84%) irrespective of the type of wood. The poplar sample showed three pore sizes (1, 8, 60 ㎛) with a main size of 60 ㎛. The pine sample showed a single pore size (20 ㎛). The big cone pine sample showed two pore sizes (10, 80 ㎛) with a main size of 10 ㎛. The bend strength was 2.5 ㎫ for poplar, 5.7 ㎫ for pine, 2.8 ㎫ for big cone pine, indicating higher strength with pine.

Processing of Al 2 O 3 Ceramics with a Porous Cellular Structure

Porous Al₂O₃ ceramics were prepared by the gelcasting foams method (a slurry foaming process) with acrylamide monomer. The foaming and gelation behavior was investigated with the parameters such as the type and concentration of surfactant, solid loading of slurry, and the concentrations of initiator and catalyst. Density, porosity, microstructure, and strength of the green and sintered samples were characterized. Of the four kinds of surfactants tested, Triton X-114 showed the highest foaming ability for the solid loading of 55-30 vol%. The gelation condition giving the idle time of 3 min was found to set the foamed structure without significant bubble enlargement and liquid lamella thinning. The green samples were fairly strong and machinable and showed maximum strength of 2.4㎫ in diametral compression. The sintered samples showed densities of 10-36% theoretical (i.e. porosity 90-64%) with a highly interconnected network of spherical pores with sizes ranging from 30 to 600 ㎛. The pore size and connectivity increased but the cell strut thickness decreased with decreasing the solid loading. Flexural strength of 37.8-1.7㎫ was obtained for the sintered samples.