Papers by Author: Chun Hong Chen

Abstract: Aluminum titanate ceramics (Al2TiO5) is a synthetic ceramic material of potential interest for many structural applications. A critical feature, which greatly limits the mechanical properties of polycrystalline Al2TiO5, is considerable intergranular microcracking, which occurs due to the high thermal anisotropy of individual grains. In this study, the temperature dependencies of mechanical properties were discussed along with the microstructure observation. Both of fracture strength and fracture toughness increased considerably with increasing the temperature. These phenomena were explained on the basis of the stress redistribution and unique microscopic feature on the fracture surface of aluminum titanate ceramics. The experimental results also revealed that the repeated heat treatments resulted in the change of fracture strength and fracture toughness due to the stress redistribution in the Al2TiO5 matrix.

Abstract: A novel estimation for the critical size of the frontal process zone of ceramics is proposed using a single-edge V-notched beam (SEVNB) technique. A three-point flexure test is carried out on aluminum titanate ceramics containing a sharp V-shaped notch with different depth. An exact solution of the critical local stress is analyzed at a critical distance from the notch tip. The critical frontal process zone size is determined as the distance between the notch tip and the point where the critical local stress equals the flexural strength of specimens without notches, based on the local fracture criterion and the Griffith-Irwin criterion. The critical size of the frontal process zone, the fracture toughness and the flexural strength were also estimated for several materials, such as, alumina, porous alumina, and alumina-based nanocomposites. The relationship between these mechanical properties indicated that there was an almost linear relationship between the fracture toughness and the resultant of strength and square root of the critical frontal process zone size, and that both of them must be increased to improve the fracture toughness of ceramics.

Abstract: Based on the FGM concept, laminated alumina tube with a tailored porosity gradient along the radial direction has been successfully fabricated by the lamination method in the centrifugal molding technique. Experiments were performed with colloidally processed alumina powder and pore-former agent. The powder mixture was made into slurry in water media and the tubes were cast using a stainless steel mold. The porosity profile was designed with the presence of PMMA particles as pore-former agent and the nature of porosity was investigated by microstructure observations. The bimodal pore structure of the tubes was constructed from large spherical pores about 10 μm formed by burning-out the pore-former agent and small sub-micron pores caused by the lower sintering temperature. The fracture behavior of porous tubes with tailored porosity gradient was investigated using the O-ring compression testing. The failure behavior was divided into two types depending on the pore-former agent. The air permeability and fracture behavior were independent of the lamination process. The influence of the porosity gradient on the fracture strength was investigated and the results showed there was a reduction in fracture strength with increase in the laminate number, and the minimum strength was found in continuous graded tubes.

Abstract: Although porous ceramics are materials with high potential for helping conserve the environment, the characteristics of pore-related mechanical properties have not yet been examined sufficiently. The R-curve behavior of porous ceramics was estimated using the crack stabilizer technique developed by Nojima et al. Also, the critical frontal process zone (CFPZ) size for porous ceramics was estimated from the strength and fracture toughness of the materials used. The results revealed that the R-curve behavior was almost flat in porous ceramics, in contrast with a steeply rising R-curve behavior for porous silicon carbide observed previously, and that the CFPZ size of porous ceramics was larger than that of dense ceramics. A schematic explanation for the crack extension in porous materials was presented to discuss the R-curve behavior of porous ceramics.

Abstract: The porous alumina tube with pore gradient along the radial direction was successfully fabricated, where PMMA particles were used as pore former agent. The specimen was expected as a filter subjected to high temperatures. Alumina and PMMA particle were mixed with water to form aqueous slurry, compacted using centrifugal molding technique. The green body was dried in partial vacuum atmosphere, calcinated at 273 K to remove the organic component, sintered at 1623 K to obtain sintered porous a-alumina tubes. The control on the pore gradient of porous tubes was attempted by two ways; lamination and continuous methods. The microstructure observation showed that the inner part of the tubes had higher porosity than the outer part irrespective of the method used. Porous alumina tubes were characterized by microstructure observation, porosity and air permeability. The binary pores of 10µm and submicron in diameter were visible, which were formed by the burning-out PMMA particles and lower sintering temperature, respectively. The influence of PMMA amount on the pore morphology of the tubes was investigated. Fracture strength was carried out by the O-ring diametral compression testing. An analytical formula subject to the diametral testing was introduced with considering the porosity gradient. The correlations between the fracture strength and pore gradient were investigated, where a reduction in fracture strength was observed with the increase in laminated layers, and minimum strength was found in continuous porous tubes.