Journal of Powder Materials

Tai-Joo Chung

3 Articles

Direct Observation on the Low Temperature Degradation Due to Surface Treatment in Y-TZP: Tai-Joo Chung, Hye-Sung Kim; J Korean Powder Metall Inst. 2010;17(3):197-202.; DOI: https://doi.org/10.4150/KPMI.2010.17.3.197

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Abstract PDF: Low temperature degradation behavior in yttria stabilized tetragonal zirconia polycrystal (Y-TZP) ceramics was microscopically observed from the phase contrast between monoclinic surface and tetragonal matrix. The degradation behavior was dependent on the surface treatment of sintered Y-TZP, even if the sintering history is same. In the mirror polished specimen, the monoclinic layer appeared in a uniform thickness from the surface. On the contrary, for the specimen with coarse scratch, the thickness of degraded surface was more than double especially from the coarse scratch. Since the scratch results in local deformation, the residual stress should be induced around the scratch. With the transformation from tetragonal to monoclinic, the volume expansion exerts a stress on a neighboring grains and promotes a successive phase transformation. Such a autocatalytic effect can be triggered from the part of coarse scratch.

Microstructural Evolution and Properties in Ti(CN)-Co/Ni Cermet Depending on the Starting Material for Incorporation of WC: Tai-Joo Chung, Sun-Yong Ahn, Seung-Su Ahn, Myung-Soo Shin, Hak-Kyu Kim, Kyung-Bae Kim, Kyung-Sik Oh, Hyuk-Jae Lee; J Korean Powder Metall Inst. 2007;14(2):132-139.; DOI: https://doi.org/10.4150/KPMI.2007.14.2.132

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In the Ti(CN)-Co/Ni cermet, WC is an effective additive for increasing sinterability and mechanical properties such as toughness and hardness. In this work, WC, (WTi)C and (WTi)(CN) were used as the source of WC and their effects were investigated in the respect of microstructural evolution and mechanical properties. Regardless of the kinds of WC sources, the hard phase with dark core and bright rim structure was observed in the Ti(CN)-Co/Ni cermet under the incorporation of relatively small amount of WC. However, hard phases with bright core began to appear and their frequency increased with the increase of all kinds of WC source addition. The ratio of bright core to dark one in the (TiW)(CN)-Co/Ni cermet was greatest under the incorporation of (WTi)C compared at the same equivalent amount of WC. The mechanical properties were improved with the addition of WC irrespective of the kinds of sources, but the addition of (WTi)(CN) was less effective for the increase of fracture toughness.

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Investigation into the microstructure and cutting performance of (Ti,Ta,W)(CN)–Co/Ni cermets
Se-Woong Oh, Sun-Yong Ahn, Kyung-Sik Oh, Hyukjae Lee, Tai-Joo Chung
International Journal of Refractory Metals and Hard Materials.2015; 53: 36. CrossRef

Effect of Grain Size and Heat-treating Atmosphere on the Phase Stability of Y-TZP: Tai-Joo Chung, Seung-Su Ahn, Eun-Wha Song, Kyung-Sik Oh, Jong-Sook Lee, Young-Sik Kim; J Korean Powder Metall Inst. 2006;13(5):360-365.; DOI: https://doi.org/10.4150/KPMI.2006.13.5.360

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The phase stability of tetragonal phase in Y-TZP was investigated in terms of the distribution of grain sizes and heat-treating atmosphere. Y-TZP with various grain sizes were prepared using duration time at 1600°C as experimental parameter. Accumulated grain size distributions were built from the SEM micrographs and the amount of tetragonal phase were measured using XRD. Both results were compared to determine the critical grain size before and after heat-treatment in vacuum. The critical grain size drastically decreased compared with the small increase of average grain size due to the autocatalytic effect which critically affects the tetragonal to monoclinic phase transformation. After heat-treatment in reductive atmosphere critical grain size relatively increased due to the stabilization of tetragonal phase. The formation of oxygen vacancies during heat-treatment was ascribed to the increase of stability.

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Direct Observation on the Low Temperature Degradation Due to Surface Treatment in Y-TZP
Tai-Joo Chung, Hye-Sung Kim
Journal of Korean Powder Metallurgy Institute.2010; 17(3): 197. CrossRef

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