Papers by Author: Dong Kyu Park

Abstract: TiC-based cermets attract much attention because of their excellent wear-resistance, high hardness at high temperature, good chemical stability, superior thermal deformation resistance. Therefore, titanium carbide is mainly used for cutting tools, grinding wheels, coated cutting tips and coated steel tools. In this research, Fe-TiC composite materials were fabricated by spark plasma sintering (SPS) after mechanical alloying. TiH2 and graphite powders were used to synthesize TiC phase. In order to compare the properties of sintered materials using mixture powder (D’AE+TiH2+graphite), commercial TiC powder was mixed with Distaloy AE (D’AE) powder as a same mechanical alloying method. Then, the shape of each mixture powder (D’AE+TiH2+graphite, D’AE+TiC (commercial)) and sintering properties were compared. TiC phase was synthesized by self-propagating high-temperature synthesis (SHS) reaction during spark plasma sintering. It was confirmed by using X-ray diffraction (XRD). Energy dispersive spectrometry (EDS) and Scanning electron microscopy (SEM) were used to observe shape of mixture powders and also sintering properties were examined such as hardness, relative density. In case of sintered material for 10min holding time at 1373K after mechanical alloying for 1 hour with D’AE, TiH2 and graphite, it indicated higher hardness value 49HR-C than a case using D’AE and TiC powder.

Abstract: Metal sulfide (MeS) is well known and widely used an additive for enhancing machinability, but the problems is extremely sensitive to reaction with moisture and thermal decomposition during sintering. In this study, micron or nano size compound powders of FeS were produced by mechano-chemical process. The machinability was also evaluated at the metal sulfide added sintered steel. Compared with the MnS additive, addition of the FeS powders to the iron resulted the formation of the sintering materials containing a spherical shape of pore, partly remained FeS rich-phase in iron matrix. Relative machinability behavior of sintering materials was investigated by the lathe manufacturing. Mechanical properties of sintering material with 1.2%FeS powders were improved rather than its using MnS powders.

Abstract: To improve the fracture strength and wear resistance of WC-Co cemented carbide, various technologies have been developed related producing the nano crystalline. There have been extensive studied conducted to retard grain growth during liquid phase sintering. However, when this inhibitor is added by simple mixing, a micro-pores forms during sintering In this study, WC-Co nanocrystalline powders with grain growth inhibitor in the site were prepared by MTP (Mechano-Thermic carburizing Process) to minimize this formation of micro pores and to retard grain growth effectively during sintering. In addition, then the phase and grain size of WC-Co nanocrystalline powders were evaluated according to the condition of MTP.

Abstract: The SPS process is a synthetic technique which enables sinter-bonding to occur at low temperature and within a short sintering time. Furthermore, it produces finer grain size than conventional sintering methods. Fe-TiC system is a relatively light material and is one third the size of tungsten carbide and less than half weight of tool steel. It is cost effective when used in Fe-TiC system due to high mass effect with low density and relatively low cost of changing tools and bearings. Fe-TiC system can be machined easily with conventional equipment. In the present work, SPS was conducted at the temperatures of 1200, 1250 and 1300°C at the sintering time of 3, 5 and 10min. As the sintering temperature and holding time increased, the relative density decreased and the hardness reached the lowest at 1250°C. These are attributed to the more pore being generated by liquid Pool at 1250°C and 1300°C. The result of the microstracture observation showed more M6C and MC carbide by liquid Pool as sintering temperature and holding time increased.

Abstract: TiC cermet is widely used for working dies with a high hardness and tool materials. In this research, we attempted to produce submicron sized TiC powders from the ball milled TiH2 and carbon black mixture by thermal treatment. The titanium hydride and carbon composite powders were milled under argon atmosphere for 7 hours at various ball to powder ratios. At the initial stage, an increase in particle size was observed, and graphite phase disappered. The TiC phase of 300nm mean particle size was obtained by milling for 5 hours. As a result, its morphologies were excessively agglomerated. At the heat treating temperature of 500°C, TiH2 phase transformed to Ti completely and the complete TiC of lattice parameter 0.431 nm was formed when the temperature reached 1000°C. Metal matrix composites(MMCs) based on the Fe-TiC system can be synthesized by spark plasma sintering. Specimen formed sintering Fe-TiC powders display a microstructure of uniformly dispersed TiC grain in a continuous metal matrix.

Abstract: TiC particulate reinforced Fe matrix composite compacts with controlled interfacial reaction was processed by spark plasma sintering after mechanical alloying. Milled powders were fabricated for 1-5 hours by spex shaker mill with the ball to powder ratio of 25:2. Metal matrix composites (MMCs) based on the Fe-40%TiC system can be synthesized by spark plasma sintering of the D’AE powders with TiH2-graphite powders under vacuum in the temperature range 1273-1473K for 5-20 min. TiC phase was formed by self combustion reaction with temperature increase. The specimen that was formed by sintering Fe-TiC powders displayed a microstructure of uniformly dispersed TiC grain in a continuous metal matrix. The densifications of the TiC-Fe materials were increased as the heat-treatment holding time increasing. In the same time, relative density and hardness of TiC-Fe sintering materials was increased.

Abstract: As the electrodes of secondary battery are made with sulfide composite powders, excellent electrode system of environmental non-toxicity and with high specific energy density and low material cost can be obtained. In this study, the (Fe, M)S2 composite powders was synthesized by mechanochemical processes (MCP) in order to improve of the cycle life in bettery. The formation of pyrite phase appreared at the case which adds nickel, but it was not observed in the case where the transition metal was does not add but the transition metal such as cobalt, molybdenum was added in stead. From charge-discharge test results, the initial discharge capasity of (Fe, Ni)S2 electrode was 845 mAh/g. The initial discharge capasity of (Fe, Co)S2 electrode was 500mAh/g, but it showed a better cycle perfoemance than the case where the diffrent transition metal was added.

Abstract: Nano-TiC powders were fabricated for mechanical alloying (MA) by shaker mill using the TiH2 powders mixed carbon powder. For mechanical alloying, titanium hydride was easily breaking alloy and easily decomposed titanium particles and hydrogen gas. The decomposition titanium powders in TiH2 powders were very fine particle size and unstable phase. And it easily reacted to carbon ;(TiH2 + C 􀃆 TiC + H2). The effects of mechanical allyoing, morphology, phase and particle size were evaluated with X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), TG/DTA and particle size analyzer. As-milled powders for 10 hours were successfully synthesized powders of TiC phase, mean particle 300nm size. And as-milled powder for 1 hour was included unstable phase, was annealed for 1 hour at 400°C-1300°C in 1x10-3torr vacuum atmosphere. Unstable phase was changed to recrystallize phase by heat treatment. After heat treatment for 1hour using as-milled powders, it was included many types of titanium oxide at temperature below 1000°C, was formed single phase of TiC at temperature over 1000°C.