Papers by Author: Myung Chang Kang

Abstract: This work investigated the thermal fatigue behavior of Inconel-738LC material deposited thermal barrier coating by Vacuum Plasma Spray(VPS). The Inconel-738LC material which is widely used as a turbine parts is a CoNi-based superalloy with CoNiCrAlY bond coating and ZrO2-8wt%Y2O3 ceramic top coating. The microstructure of coating layer was analyzed by SEM/EDS and XRD. Thermal fatigue tests were performed using compact tension specimens at 550°C, 750°C, 950°C with the load frequency of 10Hz. The fatigue crack growth behavior was characterized depending on number of cycles. The crack growth rate was found to be greater at lower frequency and higher temperature.

Abstract: This study deals with the micro v-grooving of a single crystal diamond tool that is implemented on a 3-axis micro-stage. A method for monitoring the machining conditions is investigated using acoustic emission (AE) signals and cutting force signals in the micro-grooving. The AE signals and machined surface profiles are obtained under various machining conditions. The signals are acquired from an AE sensor that is attached to the tool holder and are investigated to identify the correlation with the machined surface profiles. It is found that the AE signal is an effective parameter for monitoring the texture of the machined surface.

Abstract: Ti-Al-Si-N and Ti-Al-N coatings were deposited on WC-Co substrates by a DC magnetron sputtering method. The oxidation behavior of two kinds of Ti0.75Al0.25N and Ti0.69Al0.23Si0.08N coatings were comparatively investigated by XRD patterns and GDOES depth profiles. Si addition of 8 at.% into Ti-Al-N film modified its microstructure to a fine composite comprising, Ti-Al-N crystallites and amorphous Si3N4, and to a smoother surface morphology. While the solid solution Ti0.75Al0.25N film had superior oxidation resistance up to around 700°C, the composite Ti-Al-Si-N film showed further enhanced oxidation resistance. Both Al2O3 and SiO2 layers played roles as a barrier against oxygen diffusion for the quaternary Ti-Al-Si-N film, whereas only the Al2O3 oxide layer formed at surface did a role for the Ti-Al-N film. The cutting performances of two coated carbide ball-end mills were evaluated by cutting of AISI D2 cold-worked die steel (60 HRC) under high-speed cutting condition. The tool wear and cutting temperature are discussed along with coating characteristics.

Abstract: CrN-based multi-component coatings were deposited by a hybrid coating system combining the arc ion plating (AIP) and sputtering technique. In this work, comparative studies on microstructure and mechanical properties of microhardness and wear behaviors among Cr-C-N, Cr-Si-N, and Cr-Si-C-N coatings were systematically conducted. Adding carbon and silicon atoms into CrN coatings had large effects on microstructural change and mechanical properties of CrN coatings. The hardness value of Cr-Si-C-N coatings showed about 44 GPa, while those of Cr-Si-N and Cr-C-N coatings were 34 and 23 GPa, respectively. The average friction coefficient of CrN-based coatings decreased from 0.65 to 0.4 with the incorporation of silicon and carbon content.

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.