Papers by Author: Eun Young Choi

Abstract: In this work, the Ti-Al-Si-C-N coatings were synthesized on Stainless steel and Si wafer by a hybrid coating system, where arc ion plating (AIP) technique was combined with a magnetron sputtering technique. Also, the effect of Si content on the microstructure and mechanical properties of Ti-Al-C-N coatings were systematically investigated. The Ti-Al-Si-C-N coatings characterized by a nanocomposite comprising nano-sized Ti-Al-C-N crystallites embedded in amorphous Si3N4 phase. The micro-hardness values of Ti-Al-Si-C-N coatings were largely depended on Si content and the micro-hardness value of Ti-Al-Si-C-N coatings significantly increased from 38Gpa of Ti-Al-C-N coatings to approximately 56 GPa with the addition Si content of 9.8 at.%. The enhanced hardness values of Ti-Al-Si-C-N coatings were explained with the refinement of Ti-Al-C-N crystallites and composite microstructure characteristics by the percolation of amorphous Si3N4/SiC phase. The average friction coefficient of Ti-Al-Si-C-N decreased from 0.6 of Ti-Al-C-N to 0.23 with increasing the Si content up to 15.3 at. %. The decreased friction coefficients of Ti-Al-Si-C-N coatings was elucidated by the formation of SiO2 or Si(OH)2 layer known as self-lubricant materials.

Abstract: Ternary Ti-Cx-N1-x coatings were deposited on stainless steel substrates by arc ion plating (AIP) technique using Ti target at the temperature of 300
with a negative substrate bias voltage of -25 V. The carbon content in Ti-Cx-N1-x coatings linearly increased with increasing CH4/(CH4+N2) gas flow ratio at a constant arc current of 60 A. The microstructure and mechanical properties such as micro-hardness and average friction coefficient of Ti-Cx-N1-x coatings were investigated as a function of carbon content. As the carbon content in Ti-Cx-N1-x coatings increased, the microhardness values of Ti-Cx-N1-x coatings increased from 20 GPa for TiN coatings and reached the maximum value of approximately 32 GPa at x=0.52 in Ti-Cx-N1-x coatings. The variation of microhardness of Ti-Cx-N1-x coatings had a relationship with the change of residual stress. The average friction coefficient of Ti-Cx-N1-x coatings largely decreased with increasing carbon content.

Abstract: In order to improve adhesion strength between TiN coatings and WC-Co substrate, two kinds of interlayer of Co and Ti were pre-deposited before TiN coating process. Thickness and postannealing effects of each interlayer on the adhesion property were investigated through scratch test in this work. Introduction of thin Ti interlayer largely enhanced the adhesion strength between TiN coatings and WC-Co substrate in scratch test. The critical load, Lc2 increased from 64 N without Ti interlayer up to 130N with Ti one of ~ 0.15
thickness. However, post-annealing of TiN/Ti/WCCo system at high temperature of 600 reduced the critical load again. The Ti interlayer caused a deficit of Co content in WC-Co substrate during annealing through diffusion of Co element into Ti interlayer. The reduction of critical load after post-annealing was believed due to diminution in mechanical properties of the substrate derived from the Co deficit in WC-Co substrate. On the other hand, introduction of thin Co interlayer of ~ 0.027
thickness also increased the critical load up to 84 N and improved failure mode, but did not reduce the critical load even after annealing, rather increased it. And, any Co deficit of the substrate was not found after annealing for TiN/Co/WC-Co system.

Abstract: Nickel powders were synthesized by the reduction of hydrazine of nickel salts from diethanolamine (DEA) solution, and the morphological characteristics of nickel powders were investigated with various process conditions such as the addition of hydrazine, reaction temperature, the composition of mixed solvents. The addition of hydrazine in DEA solution largely affected the size control of nickel powders. Under N2H4/Ni2+molar ratio= 1.5 and 2.0 conditions, spherical nickel powders in the submicron range were obtained owing to higher the reduction rate. An increase of temperature increased the size of nickel particles. At 220°C for 40min, the nickel powders were composed of polyhedral particles with a high crystalline in the submicron range. The mixed volume ratio of TEA to DEA solvent improved the inhibition of agglomerate between particles.

Abstract: Nickel powders were prepared by the reduction of hydrazine of Ni(OH)2 reactant slurries from non-aqueous media. The morphological characteristics of nickel powders were investigated under various processing conditions. Nickel powders were characterized by using scanning electron microscope (SEM), X-ray diffractometer (XRD), thermal gravitational analysis (TGA) and X-ray photoelectron spectroscopy (XPS). The size of nickel powders was increased as the reaction temperature and time increased because of the thermal agglomeration of nuclei and/or increased growth rate. The surface of nickel powders composed of both Ni and Ni(OH)2 was oxidized in the temperature range of 400-600°C.