Jihong She

Oxidation bonding of porous silicon carbide ceramics

A oxidation-bonding technique was successfully developed to fabricate porous SiC ceramics using the powder mixtures of SiC, Al2O3 and C. The oxidation-bonding behavior, mechanical strength, open porosity and pore-size distribution were investigated as a function of Al2O3 content as well as graphite particle size and volume fraction. The pore size and porosity were observed to be strongly dependent on graphite particle size and volume fraction. In contrast, the degree of SiC oxidation was not significantly affected by graphite particle size and volume fraction. In addition, it was found that the fracture strength of oxidation-bonded SiC ceramics at a given porosity decreases with the pore size but increases with the neck size. Due to the enhancement of neck growth by the additions of Al2O3, a high strength of 39.6 MPa was achieved at a porosity of 36.4%. Moreover, such a porous ceramic exhibited an excellent oxidation resistance and a high Weibull modulus.

Reinforcement by crack-tip blunting in porous ceramics

A crack-tip blunting mechanism in porous ceramics was identified in this study. The reinforcement by crack tip blunting in porous ceramics was analyzed quantitatively, based on a grain fracture model. The investigation revealed that the crack tip blunting increases the fracture toughness of porous ceramics, depending on pore morphology and grain arrangement. The theoretical predictions were in good agreement with the experimental results.

Oxidation bonding of porous silicon carbide ceramics with synergistic performance

Abstract Porous silicon carbide (SiC) ceramics were fabricated by an oxidation-bonding process, in which the powder compacts are heated in air so that SiC particles are bonded to each other by oxidation-derived SiO2 glass. It has been shown that a high porosity can be obtained by adding a large amount of graphite into the SiC powder compacts and that the pore diameter can be controlled by the size of graphite particles and/or SiC powders. When a 0.3-μm SiC powder was used, a high strength up to 133 MPa was achieved at a porosity of 31.5%. Moreover, oxidation-bonded SiC (OBSC) ceramics were observed to exhibit an excellent resistance to oxidation and thermal shock.

Hertzian contact damage in a highly porous silicon nitride ceramic

Abstract Hertzian indentation tests were performed to evaluate the contact damage behavior of a highly porous Si 3 N 4 ceramic. Using a bonded-interface technique, the Hertzian contact damage patterns were examined. As a result of intragranular microfracture under Hertzian contact, a distributed subsurface damage region is developed beneath the indenter. It was found that the damage region extends progressively with increasing contact load. In strength tests, failures were observed to originate from Hertzian indentation sites, giving rise to a gradual strength degradation.

Thermally stable high-strength porous alumina

A two-step heating schedule involving pulse electric current sintering, a kind of pressure-assisted vacuum sintering, and a subsequent postsintering in air was used to fabricate sintered porous alumina compacts. During pressure-assisted vacuum sintering, a dense microstructure of the A1 2 0 3 -C system was obtained and in the second stage (i.e., during postsintering in air at different temperatures ranging from 800 to 1300 °C for more than 10 h) carbon particles present in the A1 2 O 3 -C system burned out to form a highly porous A1 2 O 3 compact. In this work, the porosity (30%) was successfully controlled and did not change with the postsintering temperature. The intriguing aspect of this study is that porous alumina compacts are fabricated with high strength and remain stable against the postsintering temperature and extended soaking time. This behavior merits the material fabricated here as a potential porous compact, mechanically withstanding for high-temperature applications.