Papers by Author: Jae Hong Yoon

Abstract: μWC-metal powder (powder) was coated onto the surface of Inconel 718 (IN718) using HVOF spraying for the improvement of surface properties. The coating was heat-treated by CO₂(g) laser for further improvement of the properties. Optimal coating process was obtained from surface properties of 9 coatings prepared by Taguchi program. Surface property improvement by HVOF coating and laser-heating were investigated. The surface hardness of IN718 was increased by coating from 399 Hv to 1260±30 Hv, and further increased to 1860±100 Hv by laser-heating. Porosity of the coating was decreased by laser-heating from 2.2±0.3% to 0.4±0.1%. The friction coefficient of the surface was decreased by the coating from 0.48±0.06 to 0.33±0.02 at room temperature. Friction coefficients of IN718 and the coating were decreased with increasing sliding surface temperatures from 25°C to 450°C. According to reciprocating sliding wear tests, wear depth and trace of the coating were smaller than IN718 at both 25°C and 450°C. The coating had better wear resistance compared to IN718, creating smaller wear traces. At 450°C, wear depth of coating was much smaller than IN718: 50-µm-deep groove compared to 75 µm for IN718.

Abstract: High velocity oxygen fuel (HVOF) thermal spray coating of micron size (μ) T800 powder has been studied for the durability improvement of sliding machine components. The optimal coating process (OCP), surface properties, friction, wear behavior and adhesion of HVOF T800 coating have been investigated. The temperature dependence of friction coefficient and wear behavior have been studied at 25°C and at an elevated temperature 538°C (1,000°F) for the study of the temperature effects on FC and wear behaviors of the coating and for the application on high speed air bearing spindle which operates with no lubricants. The OCP was determined from the best surface properties of the 16 OCP searching coatings designed by the Taguchi experimental program of four spray parameters with three levels: a hydrogen flow rate (FR) of 38-42 FMR (1 FMR=12scfh=9.44×10-5 m/s), oxygen FR of 65-70 FMR and feed rate of 30 g/min, and a spray distance of 5 inch. Hardness, roughness and porosity observed from the 16 coatings were 560-640 Hv (5488-6272 MPa), 2.2-3.0 μm and 0.01-0.04% respectively. Friction coefficient (FC) decreased from 5.5-7.0 to 4.8-6.0 with increasing the sliding surface temperature from 25°C to 538°C because of the higher lubrication effect of Co oxide debris at the higher temperature. Wear trace of the coating and counter sliding SUS 304 surface decreased to more than a half with increasing the sliding surface temperature from 25°C to 538°C. Tensile bond strength (TBS) and tensile fracture location (TFL) of Ti64 / T800 were 8,770 psi (60.5 MPa) and near the middle of the coating respectively. Bond coat NiCr did not influence on the TBS of the coating. The adhesion between Ti64 substrate and T800 coating (Ti64/T800) was stronger than the cohesion strength 8,770 psi (60.5 MPa) of T800 coating. These showed that Ti64/T800 coating was recommendable for durability improvement coating on high speed spindle of Ti64.

Abstract: High velocity oxy-fuel (HVOF) spray coating of WC base cermet has been studied as a promising candidate in some hard coating area for a replacement of the long traditional electrolytic hard chrome (EHC) plating, which has been raising public health and environmental problems. Micron size WC-Co (mWC-Co) has been coated on Inconel 718 substrate by HVOF thermal spray coating process. Surface properties of coating layer and friction behavior have been investigated for durability improvement coating on the sliding machine component surface.

Abstract: The mechanical properties of Al–5%Cu–1.3%Li–0.4%Mg–0.4%Ag–1.16%Zr alloys without Mn and with 0.3%, 0.6% and 1.2% Mn have been investigated after the aging at the temperatures of 90, 150, 180, and 230 oC. With Mn addition the alloys show a good work-hardening property, and the elongation of alloys increases. With the 0.6% Mn the best elongation can be obtained. The strength of alloys with 0.3% or 1.2% Mn is lower than that of the alloy without Mn, whereas the strength of alloy with 0.6% Mn is almost same as that of the alloy without Mn. In the alloy with 0.6% Mn aged at 180 oC for 12 hours the optimum properties – combination of tensile strength and elongation, 620 MPa and over 12 %, respectively – are obtained. These favorable effects by a proper Mn addition are considered to come mainly from the Mn-dispersoid to prevent strain localization normally associated with the shearable precipitates.

Abstract: Silicon carbide (SiC) nano-powders are successfully synthesized by a reaction between carbon nano-powders (carbon black) and SiO gas at 1300°C for 9 hrs in dynamic argon atmosphere (flow rate; 400 cm3 min-1), using the solid-vapor reaction method. The particle size of synthesized SiC nano-powders is below 40 nm and the shape is uniform. Unexpectedly, SiC nano-fibers are also coexisted in the SiC nano-powders. The quantitative and qualitative properties of the SiC nanopowders and nano-fibers are analyzed by scanning electron microscopy (SEM), transmission electron microscope (TEM), and X-ray diffraction (XRD). Carbon residuals removed by heating over 700°C in air are estimated by thermogravimetry analysis (TGA). It is found that the SiC nanopowders are easily produced by direct synthesis via the solid-vapor reaction method. The morphological characteristics of the resulting SiC nano-powders are dependent upon the morphology of carbon black used as precursor.

Abstract: There is a need to destroy both military and civilian hazardous waste and urgency, mandated by public concern over traditional waste handling methodologies, to safe and efficient alternative technologies. One very effective process for the destruction of such waste is supercritical water oxidation (SCWO). Nevertheless, corrosion of the materials of fabrication is a serious concern. This work intends to obtain the fundamental data for developing the corrosion resistant steel for the construction of SCWO system. The effects of various factors on the corrosion resistance of flat, welded, and U-bend 316L stainless steels in Trimsol solution were studied. Corroded product on surface was composed of multi-layer with oxides and salts, and dealloying was observed. Major corrosion phenomena of 316L stainless steel under SCWO condition were intergranular corrosion, pitting corrosion, SCC, and erosion corrosion. This work focused on the elucidation of corrosion mechanism of 316L stainless steel in SCWO environment.

Abstract: Super stainless steel has been used to solve corrosion problems of biomaterials because it shows the excellent corrosion resistance as like Ti and Ti alloys and has better mechanical properties than Ti and Ti alloys. We designed high Mo and Ni bearing super austenitic stainless steel. To obtain desirable microstructure, Cr, Ni, Mo, N contents were controlled. This work focused on the role of alloying elements on cytotoxic behavior and corrosion of stainless steel. In acidic chloride solution, when the alloys had high PRE values, the alloys showed high pitting resistance and low critical current density. However, in Hanks’ solution, the higher PRE’s alloys showed high critical passive current density. Namely, it was revealed that EDTA among Hanks’ solution played an important role to increase the critical passive current density of high Mo and Ni bearing super stainless steels, regardless of PRE’s value of the alloys. Therefore, even if the PRE values of the alloys were higher, high Ni and Mo bearing alloys would reveal more cytotoxic and high metal ion release rate than 316L stainless steel.