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Lee, Yonghee,Kim, Somang,Kim, Hyo Seok,Shin, Jeong Bin,Choi, Wonseok,Cho, Hyunjin,Kim, Kyungmok,Lee, Taeyang,Kim, Jinwuk,Kang, In-Byeong,Choi, Kyung Cheol,Kim, Yong-Hoon,Jeon, Duk Young The Royal Society of Chemistry 2017 Journal of materials chemistry. C, Materials for o Vol.5 No.8
<P>We demonstrate for the first time highly luminescent blue-emitting CdZnS/ZnS wurtzite core/shell nanorods (NRs) that show electric-field-induced fluorescence switching properties. Uniform CdZnS NRs were rapidly synthesized by injecting sulfur powder dissolved in 1-octadecene into a flask containing phosphonic acid ligands, and subsequently ZnS shells were coated using reagents consisting of sulfur powder, zinc sulfate heptahydrate, and oleylamine. The growth of high-quality ZnS shells resulted in a dramatically increased photoluminescence (PL) quantum yield (QY) of ∼40% with a minimal red-shift of the blue PL peak, which indicates that the combination of reagents successfully controlled a large number of defects appearing on the surface of the NR cores. By pre-annealing CdZnS cores before growing ZnS shells, we could achieve an additional increase in the maximum PL QY to 50%, decreases in both the full width at half maximum (FWHM) and the red-shift of the PL peak, and improved electric-field-induced fluorescence switching performance. Density functional theory calculations reveal that the effective relaxation of strain accumulating on the NR core during shell growth is the key to our successful synthesis of blue-emitting NRs, and that the additional improvement in performance obtained through the pre-annealing process results from the elimination of sulfur vacancies appearing at the surface of the NR core.</P>
Fretting wear performance of CoNiCrAIY-BN coating for the aero-blade/disk interface
Kyungmok Kim,Alexander M. Korsunsky 한국자동차공학회 2011 한국자동차공학회 지부 학술대회 논문집 Vol.2011 No.10-1
This paper investigates fretting wear performance of CoNiCrAlY-BN coating for aerospace components. Fretting wear tests are performed under experimental conditions similar to those found at the blade/disk interface of a turbofan engine. All samples are coated by two different thermal spraying techniques: an atmospheric plasma spraying process and a high velocity oxygen fuel thermal spraying process. Friction coefficients are measured and directly compared with those of a conventional thermally sprayed coating for aerospace components. It is identified that a plasma sprayed CoNiCrAlY-BN coating maintains better fretting performance than the conventional coating. Finally, exponential evolution law of fretting damage is applied to friction coefficient evolutions for investigating the effect of thermal spraying techniques. It is demonstrated that the damage rate constant is a useful indicator for comparing friction behaviors of CoNiCrAlY-BN coatings sprayed by two thermal spraying processes.
Studies on the Influence of a Counterpart on Fretting Wear of Cold-Rolled High Strength Steel
Kim, Kyungmok,Baek, Seung Yub Korean Society for Precision Engineering 2018 International Journal of Precision Engineering and Vol.19 No.5
This article investigates the influence of a counterpart on fretting wear of cold-rolled high strength steel for automotive applications. Fretting wear tests are conducted using a ball-on-flat test apparatus. The friction forces between cold-rolled high strength steel plates and various counterparts are measured. The worn profile of a cold-rolled high strength steel specimen is determined to calculate the wear rate. Experimental results show that the ratio of the maximum tangential to the normal force at a contact surface varies with a counterpart material. Measured worn surface profiles indicate that the cross-sectional area of a wear scar is enlarged as the number of cycles increases. The effects of counterpart hardness and the maximum Hertzian contact pressure are identified on the wear rate of cold-rolled high strength steel. From measurement and analysis results, <TEX>$ZrO_2$</TEX> counterparts are found to be more adequate than the commercial ones for the cold-rolled high strength steel rails.
Influence of Counterpart Material on Fretting Wear of FDM Printed Polylactic Acid Plates
Kyungmok Kim,Seung Yub Baek 한국정밀공학회 2023 International Journal of Precision Engineering and Vol.24 No.10
This article investigates the influence of counterpart material on fretting wear of Polylactic Acid (PLA) plates printed by Fused Deposition Modeling (FDM). Fretting tests with three different spherical counterparts (high carbon chromium alloy steel, zirconium oxide, and silicon nitride) were conducted within a gross slip regime. PLA plates were printed with an infill density of 100% and then the surface of the plate was polished for obtaining an arithmetic average surface roughness of 0.1 µm. During a fretting wear test, the evolution of the kinetic friction coefficient was determined. It was identified that the evolutions for high carbon chromium alloy steel (AISI 52100) and zirconium oxide (ZrO2) counterparts increased and then became steady. Meanwhile, the evolution for silicon nitride (Si3N4) remained stable by 2000 fretting cycles. The kinetic friction coefficient for Si3N4 was lower than those found for ZrO2 and AISI52100 counterparts. After 2000 cycles, worn surface profiles were measured and then wear volumes were calculated. The wear volume of PLA against Si3N4 was found to be lower than those against other two counterparts. It was identified that the wear rate of PLA increased with increased the hardness of the counterpart. Obtained results provide useful information on the design of tribo-components subjected to fretting wear.