Seong Jai Cho

Micro-indentation fracture behavior of human enamel

OBJECTIVE The purpose of this study was to determine the crack resistance behavior (K(R)) of human enamel in relation to its microstructure. METHODS Human molar teeth were precision cut, polished and tested using Vickers micro-indentation at different loads ranging from 0.98 to 9.8 N. Five indentation load levels were considered, 20 indentation cracks for each load level were introduced on the surface of the test specimen (10 indentations per tooth) and their variability was evaluated using Weibull statistics and an empirical model. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to analyze the crack morphology and propagation mechanisms involved. RESULTS The results showed that enamel exhibited increasing cracking resistance (K(R)) with increasing load. It was found that the crack propagation mainly depended on the location and the microstructure it encountered. SEM showed the formation of crack bridges and crack deflection near the indentation crack tip. The crack mode was of Palmqvist type even at larger loads of 9.8 N. This was mainly attributed to the large process zone created by the interwoven lamellar rod like microstructure exhibited by the enamel surface. SIGNIFICANCE This study shows that there are still considerable prospects for improving dental ceramics and for mimicking the enamel structure developed by nature.

Surface Strengthening of Zirconia Toughened Alumina (ZTA) Using Glass Infiltration Technique

Molten Soda lime glass (SLG) was penetrated into the surface of ZTA at 1500°C for the holding time of 1 to 10 h. The depth of the glass penetration increased with increasing holding time. The thermal expansion mismatch and elastic property mismatch between the penetrated glass and ZTA produced residual compression in the surface region during cooling. This residual compression enhanced the flexural strength and fracture toughness remarkably.

Synthesis and Characterization of Yttrium-doped Core-Shell SiO 2 Nanoparticles by Reverse Micelle and Sol-gel Processing

In this study, yttrium-doped SiO₂ nanoparticles are synthesized using a reverse micelle technique combined with metal alkoxide hydrolysis and condensation. Spherical Y-doped SiO2 nanoparticles with a uniform size distribution are prepared using selfassembly molecules in conjunction with the hydrolysis and condensation of organometallic precursors. The water/surfactant molar ratio influenced the Y-doped SiO₂ particles distribution of the core-shell composite particles and the distribution of Y doped SiO₂ particles was broadened as the water to surfactant ratio increased. The particle size of Y increase linearly as the Y(NO₃)₃ solution concentration increased. The average size of the cluster was found to depend on the micelle size, the nature of the solvent, and the concentration of the reagent. The effects of synthesis parameters, such as the molar ratio of water to surfactant and the molar ratio of water to TEOS, are discussed.

Surface Modification of SiAlON Ceramics

We investigated in situ surface modification of SiAlON ceramics by sintering powder compacts in packing powder with chemical compositions different from those of th compacts. For β-SiAlON, it was possible to make an αor O’-SiAlON layer on its surface. The mechanism of this in situ surface modification was attributed to a liquid flow from the packi ng powder to the compact, and reactions between the compact and the infiltrated liqui . It appeared that the new phase formation at the compact surface was determined by the a mount and composition of the infiltrated liquid. The surface modified β-SiAlON ceramics showed improved wear and oxidation resistance, suggesting that the surface modification technique can pr ovide opportunities for improving the mechanical properties of monolithic SiAlON ceramics.

Synthesis and Characterization of Ga Doped SiO2 Nanoparticles by a Reverse Micelle and Sol–Gel Processing

Ga doped SiO2 nanosized particles have been synthesized using a reverse micelle technique combined with metal alkoxide hydrolysis and condensation. The size of the particles and the thickness of the coating can be controlled by manipulating the relative rates of the hydrolysis and condensation reactions of tetraethyl orthosilicate(TEOS) within the micro-emulsion. The average size of synthesized Ga doped SiO2 nanoparticles were about in the size range of 10-15 nm and Ga particles 2-5 nm. The effects of synthesis parameters, such as the molar ratio of water to TEOS, and the molar ratio of water to surfactant, are discussed.

Normal and Abnormal Grain Growth in Ceramics - NbC-Co and Alumina

Normal and abnormal grain growth has been observed in 70NbC-30Co with varying B concentrations at 1450°C and in alumina with varying impurity and additive concentrations at 1600°C -1650°C as typical systems with and without liquid matrix. The grain growth behavior depends on the roughening of the interfaces as indicated by the grain and grain boundary shapes. When 4% B is added to 70NbC-30Co, the NbC grains in Co-rich liquid matrix are spherical and undergo diffusion controlled normal growth, because the grain-liquid interface is rough. As the B concentration is decreased to 3, 2, 1, and 0%, the NbC grains become more cubic and the tendency for abnormal grain growth increases because of the step growth mechanism of the flat singular surface segments. When compacts of high purity alumina powder are sintered at 1650°C, the grain boundaries are smoothly curved, indicating their atomically rough structures. With increasing impurity content—in particular SiO2—in the alumina powder, abnormal grain growth becomes more pronounced with increasing number of flat grain boundaries. These singular grain boundaries are expected to move by a step mechanism and thus cause the abnormal grain growth. These results show that the interface roughening and hence the grain growth mode changes gradually with the additive or impurity concentrations. Therefore, the abnormal grain growth cannot be sharply distinguished from the normal grain growth as has been previously suggested in general and for alumina in particular.

Contact Fracture Behaviors of Ti3SiC2 Synthesized by Hot Pressing Using TiCx and Si Powder Mixture

The contact fracture behaviors of fine-grained Ti3SiC2 and coarse-grained high purity Ti3SiC2 are examined by the Hertzian indentation and Vickers indentation technique. The Vickers hardness of bulk Ti3SiC2 is as low as 5.3~6.3 Gpa, and the Hertzian contact stress-strain curves for Ti3SiC2 deviate much from linearity, which resembles the fracture behavior of a ductile metal rather than a brittle ceramic. The contact damages by both Vickers indentation and Hertzian indentation reveal a fairly good plastic deformation nature of Ti3SiC2. Un-reacted TiCx in fine-grained Ti3SiC2 may impede the plastic deformation by slip along basal plan inside Ti3SiC2 grain, making Ti3SiC2 less plastic under loading.