Papers by Author: Young Ze Lee

Abstract: Due to the environmental concerns, especially the greenhouse effect and GWP (Global Warming Potential), the carbon dioxide was investigated as an alternative natural refrigerant to replace HFCs (HydroFluoroCarbons) in refrigerator or air conditioning systems. To develop new compressor, especially rotary type, the friction and wear characteristics of sliding surfaces between vane and flange in the rotary compressor were evaluated in this paper. Several hard coatings, such as TiN, CrN and WC/C, were applied on vane surfaces in order to improve the tribological characteristics, and their performances were evaluated experimentally. The vane-on-flange type lubricated sliding tests were performed with a high pressure wear tester using HFC (R410A) and carbon dioxide. From the tests, wear volumes of vane surfaces applied with various coatings were compared each other.

Abstract: The tribological behaviors of Diamond-like carbon (DLC) films, prepared by the radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) method, were studied in the room temperature and the elevated temperature. The ball-on-disk tests with DLC films on steel specimens were conducted at a sliding speed of 60 rpm, a load of 10 N, and surrounding temperatures of 25°C and 75°C. The results show that the coefficients of friction and the amounts of wear of DLC films were decreased at higher temperature. After tests the wear tracks of hydrogenated DLC film were analyzed by Raman spectroscopy.

Abstract: In order to protect the global environment, especially on the point of the greenhouse effect, carbon dioxide should be used as a natural refrigerant. Because new compressors with CO2 are going to be operated under the high pressure, the interaction between sliding surfaces in the compressor becomes very important. To develop new compressor, especially rotary type, the friction and wear characteristics of sliding surfaces were very important. In this paper, the surface interactions between a shaft and a flange, which were one of the contacting parts in compressor, were evaluated. The frictional forces and wear amounts in boundary lubricated sliding condition were measured using the pin-on-disk tester. Two types of refrigerants were used, namely R410A and CO2 to compare the tribological characteristics of conventionally used one with new natural refrigerant. It was found that CO2 lead to a bad lubricity as compared with R410A or pure oil due to the formation of relatively thin protective layer on the sliding surface. Smooth surface of shaft produced lower values of friction and wear than rough surfaces. The smooth surfaces represented much more spheroidal graphite, which had very favorable tribological characteristics, on the sliding surfaces.

Abstract: Wear and scuffing tests were conducted using friction and wear measurement of piston rings and cylinder blocks for the low friction diesel engine. Scuffing, described as sudden catastrophic failure of lubricated sliding surfaces, usually characterized by a sudden rapid increase in friction, temperature and noise, is an important failure mode on sliding surfaces. In this study, the frictional forces, wear amounts and cycles to scuffing in boundary lubricated sliding condition were measured using the reciprocating wear tester. The cylinder blocks with several values of surface roughness were used as reciprocating specimens, and a piece of piston ring was used as fixed pin. As increasing load by several steps in lubricated sliding, the friction signals indicated the state of surface interactions, such as friction forces, changes of lubricating films and scuffing. There were some rapid increases in friction forces just before the scuffing would occur. It was found that there was the optimum value of initial surface roughness to prolong the wear life of sliding surfaces. As decreasing the surface roughness of cylinder blocks, the wear amounts were decreased due to increasing the contact area. There was also the optimum surface roughness to reduce the friction and to prolong the scuffing life.

Abstract: Fuel cladding tubes in nuclear fuel assembly are held up by supporting grids because the tubes are long and slender. Fluid flows of high-pressure and high-temperature in the tubes cause oscillating motions between tubes and supports. This is called as FIV (flow induced vibration), which causes fretting wear in contact parts of tube-support. The fretting wear of tube-support can threaten the safety of nuclear power plant. Therefore, a research about the fretting wear characteristics of tube-support is required. The fretting wear tests were performed with supporting grids and cladding tubes, especially after corrosion treatment on tubes, in water. The tests were done using various applied loads with fixed amplitude. From the results of fretting tests, the wear amounts of tube materials can be predictable by obtaining the wear coefficient using the work rate model. Due to stick phenomena the wear depth was changed as increasing load and temperature. The maximum wear depth was decreased as increasing the water temperatures. At high temperatures there are the regions of some severe adhesion due to stick phenomena.

Abstract: The experimental investigation was performed to find the associated changes in characteristics of fretting wear with various water temperatures. Fretting can be defined as the oscillatory motion with very small amplitudes, which usually occur between two contacting surfaces. The fretting wear, which occurs between cladding tubes of nuclear fuel rod and grids, causes in damages the cladding tubes by flow induced vibration in a nuclear reactor. In this paper, the fretting wear tests were carried out using the zirconium alloy tubes and the grids with increasing the water temperature. The tube materials in water of 20
, 50
and 80
were tested with the applied load of 20N and the relative amplitude of 200. The worn surfaces were observed by SEM, EDX and 2D surface profiler. As the water temperature increased, the wear volume was decreased. However, oxide layer was increased on the worn surface. The abrasive wear mechanism was observed at water temperature of 20
and adhesive wear mechanism occurred at water temperature of 50
and 80
. As the water temperature increased, surface micro-hardness was decreased. Also, wear depth and wear width were decreased due to increasing stick phenomenon. Stick regime occurred due to the formation of oxide layer on the worn surface with increasing water temperatures.

Abstract: Tubes in nuclear steam generators are held up by supports because the tubes are long and slender. Fluid flows of high-pressure and high-temperature flows in the tubes cause oscillating motions between tubes and supports. This is called as FIV (flow induced vibration) which cause fretting wear in contact part of tube-support. The reduction of tube thickness due to fretting wear of tube-support can threaten the safety of nuclear power plant. Therefore, a research on the fretting wear characteristics of tube-support is required. This work is focused on investigations of fretting wear characteristics and wear mechanisms of tube-support. Results show that the wear rate of tube is proportional to that of support and that with increasing the water temperature the wear volume of tube-support decreases because the oxidation rate decreases due to reduction of the oxygen concentration in contact surfaces. Also, the wear mechanisms of tube-support are abrasive and oxidational wear.

Abstract: The carbon dioxide (R-744) as a natural refrigerant has been an attractive alternative refrigerant to replace HFC (hydrofluorocarbon) refrigerants currently used in air conditioning systems, due to the environmental concerns. Because new compressors with CO2 are going to be operated under the high pressure, the tribology of sliding surfaces in the compressor becomes very important. To develop new compressor, especially scroll type, the friction and wear characteristics of sliding surfaces between a fixed scroll and an orbiting scroll in the scroll compressor were investigated in this paper. The sliding tests of pin-on-disk type were carried out between scroll surfaces under various sliding speeds, normal loads, surface roughness, and refrigerant pressures. Two types of lubricants were used, namely POE (polyol ester) oil and PAG (polyalkylene glycol) oil. The friction forces, wear amounts and surface temperatures were monitored during the tests. Test results showed that the wear amounts increased with increasing normal load and rotating speed, and also decreased with decreasing surface roughness. Also, the results of the sliding tests showed that using PAG oil has an advantage over POE oil in CO2 environment. As the pressure of CO2 refrigerant became higher, the amount of wear and coefficient of friction became larger in CO2/POE mixed environment, but wear and friction remained constant in CO2/PAG mixed environment. The wear coefficients in CO2/PAG and CO2/POE mixed environment were 8.60×10-9 ㎣/N·m and 2.35×10-8 ㎣/N·m, respectively.

Abstract: Tubes in nuclear steam generators are held up by supports because the tubes are long and slender. Fluid flows of high-pressure and high-temperature in the tubes cause oscillating motions between tubes and supports. This is called as FIV (flow induced vibration), which causes fretting wear in contact parts of tube-support. The fretting wear of tube-support can threaten the safety of nuclear power plant. Therefore, a research about the fretting wear characteristics of tube-support is required. This work is focused on fretting wear transitions from mild wear to severe wear of tube-support materials by various loads and relative displacements. The transition is defined on the basis of the changes in wear amount. To investigate the transition, the fretting wear tester was contrived to prevent the reduction of relative displacement between tube and support by increasing the load. The tube and support materials were Inconel 690 and 409 SS, respectively. The results show that the transition of tube-support wear is caused by the changes of the dominant wear mechanism depending on the applied load and the relative displacement.

Abstract: Fretting can be defined as the oscillatory motion with very small amplitudes, which usually occur between two contacting surfaces. Fretting wear is the removal of material from contacting surfaces through fretting action. This fretting wear, which occurs between cladding tubes of nuclear fuel rod and grids, causes in damages the cladding tubes by flow induced vibration in a nuclear reactor. In this paper the fretting wear tests were performed with two types of cladding tubes and three types of supporting grids in water. Fretting wear tests were done using various applied loads. From the results of fretting tests, the wear amounts of tube materials can be predictable by obtaining the wear coefficient using the work rate model. Depending on various normal load, tube materials, and supporting grid shapes, distinctively different wear scar of fretting and stick-slip mechanism can occur.