Papers by Author: Dong Won Joo

Abstract: In this work we have evaluated the effect of platinum (Pt) pre-coating on microstructure evolution of MCrAlY coating during isothermal oxidation, to improve high temperature oxidation and corrosion resistance of the nickel-based superalloy MAR M247-DS. Pt was deposited on the superalloy by electroplating before the MCrAlY coating by a vacuum plasma spray, which was compared to the superalloy without Pt pre-coating. The samples were subjected to isothermal oxidation in a box furnace at 1100°C for 25–150 hrs. The weight gain and loss caused by the oxidation of samples was recorded, and the variation of surface layer and interface morphology were observed before and after the oxidation tests. The length decrement of cross section is remarkably low on according to the precipitation of (Ni,Pt)Al phase by Pt pre-coating. Also, the amount of voids at interface between the coating and the superalloy, which would be introduced as a processing defect, is reduced by Pt pre-coating.

Abstract: WC nanosized powders are synthesized by the plasma arc discharge process and annealing under an inert (Ar) atmosphere. The high temperature used during discharging at local region causes the work-piece and electrode to melt and evaporate. The melted tungsten on the arc discharge electrode and carbon arising from decomposition CH4 gas forms WC1-x nanosized powders. The WC1-x phase is encapsulated in a amorphous carbon shell. The WC1-x nanosized particles are annealed at 1200~1400°C under an Ar atmosphere. The WC1-x nanosized powder is transformed to the W2C phase, and then it becomes WC above at 1400°C.

Abstract: A novel process in the synthesis of SiC microtubes is proposed, based on modified CVD process. This process was adopted to siliconize the carbon fibers that perform with relatively small surface area, either partially or completely, into the SiC microtubes with large surface area. An oxidic vapor generated from a metal reacts with a carbon solid preform, which leads to a carbide possessing with a unique morphology. The carbon fiber preforms have been unidirectionally arranged into alumina tube, and then a solid-vapor reaction between carbon fiber preforms and SiO vapor originated from the mixture powder pack of Si and SiO2 has been carried out in restoration atmosphere (argon gas/hydrogen gas = 80/20) at 1450°C. The synthesized SiC.C composites were re-sintered with the modified mixture powder pack of Si, SiO2, Al, and Al2O3 at the same condition, in which amounts of Al and Al2O3 employed as a impurity have been controlled under 1 wt% with different mole ratios (Al2O3 : Al = 1:1, 2:1, 3:1). Two kinds of SiO and AlO vapors are generated in the second step reaction process, which react with carbon residuals. The quantitative and qualitative properties of the SiC micorotubes were analyzed by using TGA, SEM, and XRD. The inner morphology of the SiC micorotubes is dependent on the impurity concentration and the gas nature, showing the grains grown with membrane type in the second reaction process because of the impurity.