Papers by Author: Seock Sam Kim

Abstract: This paper investigated the wetting and adhesion property of undulated a-C:H surfaces with surface morphology controlled for a reduced real area of contact. The nano-undulated a-C:H films were prepared by radio frequency plasma enhanced chemical vapor deposition (r.f. PECVD) using nanoscale Cu dots surface on a Si (100) substrate. FE-SEM, AFM analysis showed that the after repeat deposition and plasma induced damage with Ar ions, the surface was nanoscale undulated. This phenomenon changed the surface morphology of a-C:H surface. Raman spectra of film with changed morphology revealed that the plasma induced damage with Ar ions significantly suppressed the graphitization of a-C:H structure. Also, it was observed that while the untreated flat a-C:H surfaces had wetting angle starting ranged from 72° and adhesion force of 332.79 nN. Had wetting angle the undulated a-C:H surfaces, which resemble the surface morphology of a cylindrical shape, increased up to 103.6° and adhesion force decreased down to 11 nN. The measurements agree with Hertz and JKR models. The surface undulation was affected mainly by several factors: the surface morphology affinity to cylindrical shape, reduction of the real area of contact and air pockets trapped in cylindrical double asperities of the surface.

Abstract: The purpose of this study is to investigate the wear characteristics of yttria-stabilized zirconia coating as a function of effect on post-spray heat treatment. We concerned that the residual stress is due to during cooling from a high deposition temperature. We will focus on the tribological wear performance as a magnitude of residual stress. The effect of residual stress as post-spay heat treatment temperature will be discussed. The plasma-sprayed 8%Y2O3-Zirconia coating is studied to know the relationship between phase transformations and wear properties after post-spray heat treatment. Wear tests is carried out with ball on disk type on 50 N, 70 N, 90 N under room temperature. The transformation of phase and magnitude of residual stress are measured by x-ray diffraction method (XRD). Tribological characteristics and wear mechanism for post-spray heat treatment is observed by SEM.

Abstract: In this paper the effects of an enhanced solution and aging on the strengthening behavior of commercially available 2024Al alloy were investigated using tension testing and scanning electron microscopy. A differential scanning calorimeter (DSC) was used to measure the incipient melting behavior and then determine the technological parameter of an enhanced solution heat treatment for strengthening the 2024Al alloy. The results show that the incipient melting behavior of the commercially available 2024Al alloy was characterized by a small amount of liquid phase between the temperatures 773K to 788K. A solution treatment, appropriately enhanced by slowly increasing the solution temperature from 773K to 783K at a heating rate of 5K/h, can make the final solution temperature of the alloy higher than the conventional solution temperature of 773K. This improves the constituent phase solution without the formation of overheated microstructures. The results are the strength, especially the yield strength, and the resistance to over-aging can both be considerably improved without a large deterioration to tensile ductility.

Abstract: In contact conditions of crane sheave steel materials against a wire rope, the contact surface of the sheave generally exhibits scuff damage. In the present study, sliding wear tests using annealed-low carbon cast steel were carried out under three kinds of normal loads calculated from the actual model conditions in a pin-on-disk grease lubrication test. The sliding speed was fixed at 220mm/s. Therefore, the wear behavior and wear mechanism and the residual stress distribution of the substrate surface were examined. The results of this study indicate that the wear rate of annealedlow carbon cast steel depends on the diameter of the wires. The wear mechanism of annealed-low carbon cast steel is characterized by fatigue and abrasive wear, but the wire shows partial adhesion due to micro abrasive wear by fatigue crack propagation. The contact distribution of residual stress is independent of sliding distance. Therefore, there are constant values on the contact surface.

Abstract: A sudden failure of the bearing of the turbocharger shaft in a diesel engine, with the power of 11520KW and the total running time of 2,417h, is investigated in this paper. Based on an examination of the disassembled turbocharger and a residue analysis, it was found that the failure of the turbocharger originated from a fatigue fracture of an aluminum-alloy bearing retainer (cage) on its compressor side. The axial load was a chief cause of the fatigue fracture and the looseness defects in the aluminum alloy retainer deteriorated its fatigue strength which in turn accelerated the development of fatigue cracks. After the retainer broke, the off-centered rotation of the turbocharger shaft triggered a severe impact and led to deformation, wear and a fracture of the bearing parts, ultimately resulting in the failure of the turbocharger.

Abstract: An ingot aluminum alloy (Al-Cu-Mg-Ti-Sr) with high strength and high strain rate superplasticity has been successfully developed through a conventional manufacture route consisting of casting, heat treatment, hot extrusion with a low extrusion rate of only 10:1, hot-rolling and further cold-rolling, which is cost effective and suitable for large volume production industries. The tensile test result showed the alloy possesses not only a high ultimate strength of 513.85MPa at room temperature, but also a good high strain rate superplasticity with the tensile elongation of 174~224%, the flow stress of 17.1~33.9MPa and the strain rate sensitivity m-value of 0.174~0.293 in the initial strain rate of 3.16×10-2~1.0×10-1s-1 and at the temperature of 748K~793K. Differential scanning calorimeter (DSC) analysis showed that the superplastic deformation has no relationship with liquid phase. Scanning electron microscopy (SEM) analysis of fracture surface and surface showed that the superplastic deformation results from fine grain boundary sliding and dislocation slip.