Seock-Sam Kim

Antioxidation Synergism Between ZnDTC and ZnDDP in Mineral Oil

Antioxidation synergistic effects between ZnDTC and ZnDDP in a mineral base oil were investigated by the Rotary Bomb Oxidation Test (RBOT) and the Indiana Stirring Oxidation Test (ISOT). The results indicate that there is strong antioxidation synergism between the two additives. Fourier transmission infrared spectroscopy (FTIR) analyses show that the oxidation products in the tested oils simultaneously containing the two additives are much less than those in the tested oils containing a single additive and the base oil.

Long-Lasting Hydrophilicity on Nanostructured Si-Incorporated Diamond-Like Carbon Films

We investigated the long-lasting hydrophilic behavior of a Si-incorporated diamond-like carbon (Si-DLC) film by varying the Si fraction in DLC matrix through oxygen and nitrogen plasma surface treatments. The wetting behavior of the water droplets on the pure DLC and Si-DLC with the nitrogen or oxygen plasma treatment revealed that the Si element in the oxygen-plasma-treated Si-DLC films played a major role in maintaining a hydrophilic wetting angle of <10° for 20 days in ambient air. The nanostructured patterns with a roughness of ∼10 nm evolved because of the selective etching of the carbon matrix by the oxygen plasma in the Si-DLC film, where the chemical component of the Si-Ox bond was enriched on the top of the nanopatterns and remained for over 20 days.

Tribological characteristics of silicon nitride at elevated temperatures

A sliding friction-and-wear test for silicon nitride (Si3N4) was conducted using a ball-on-disk specimen configuration. The material used in this study was HIPed silicon nitride. The tests were carried out from room temperature to 1000 °C using self-mated silicon nitride couples in laboratory air. The worn surfaces were observed by SEM and the debris particles from the worn surfaces were analyzed for oxidation by XPS. The normal load was found to have a more significant influence on the friction coefficient of the silicon nitride than an elevated temperature. The specific wear rate was found to decrease along with the sliding distance. The specific wear rate at 29.4 N and 1000 °C was 292 times larger than that at room temperature. The main wear mechanism from room temperature to 750 °C was caused by brittle fracture, whereas from 750 to 1000 °C the wear mechanism was mainly influenced by the oxidation of silicon nitride due to the increased temperature. The oxidation of silicon nitride at a high temperature was a significant factor in the wear increase.

Sliding-Wear Behavior of TiN- and CrN-Coated 2024 Aluminum Alloy Against an Al2O3 Ball

Dry sliding friction and wear behavior of TiN and CrN deposited on 2024 aluminum alloy by arc ion plating was investigated using the ball-on-disk wear test. The effects of normal load and ceramic coating on the friction coefficient and wear-resistance of 2024 aluminum alloy were studied. The worn surfaces were observed by scanning electron microscopy (SEM). The results show that wear volume increases while the friction coefficient decreases with an increase in normal load. The wear resistance of CrN is higher than that of TiN. The wear mechanism of TiN-coated 2024 Al is related to the oxidation of TiN coating and plastic deformation of 2024 Al. Conversely, the wear mechanism of CrN-coated 2024 Al is related to the fatigue fracture of the coating, which was affected by residual stress and plastic deformation of 2024 Al.

A new parameter for assessment of ceramic wear

Abstract Friction and wear tests were carried out on a face-loaded ring-on-ring wear tester. The materials used in this study were HIPed zirconia (ZrO 2 ) and silicon nitride (Si 3 N 4 ). Friction and wear tests were carried out at room temperature between the same materials under unlubricated conditions. It was found that the coefficient of friction of zirconia was somewhat higher than that of the silicon nitride over the range of testing conditions. Wear rate of silicon nitride was higher than that of zirconia. Worn surfaces investigated by SEM have residual surface cracks and wear particles caused by brittle fracture. It was found that surface cracks were initiated at the trailing edge and propagated to generate a wear particle. It was also found that ceramics have two types of wear particles, small scale in the range of submicrometers to 3 μm and large scale in the range of 10–16 μm. Based on these results, a ceramic wear model is proposed to explain the process of wear particles generation. A theoretical analysis based on fracture mechanics has been conducted. A non-dimensional parameter, S cf , is proposed to estimate the wear rate of ceramics and is defined as S cf = P√(1+μ 2 )a K IC S cf is given as a function of normal pressure, fracture toughness of materials, crack length (the degree of surface defect of materials) and friction coefficient in the contact region. The ceramic wear rate, W s , can be expressed in term of the S cf parameter as follows: W s = α ( S cf ) β

Wear mechanism of TiN-coated high-speed steel during sliding

Abstract The wear mechanism of high-speed steel coated with TiN by arc ion plating during sliding against alumina and AISI 52100 steel under lubricated conditions has been investigated experimentally. The results show that a transition in wear behaviour occurs after a defined period of sliding time for both of the countermaterials. Examinations of wear samples show that the wear transition mechanism differs, however, depending on the countermaterials. When slid against alumina, which is harder than the coating, the coating wears slowly by abrasion in the initial stage until the substrate is exposed, which leads to the onset of rapid substrate abrasion in the following wear stage. The time required for this transition does not relate to substrate hardness but increases with increasing coating thickness and decreasing applied load. On the other hand, when slid against AISI 52100 steel, which is softer than the coating, the coating does not wear significantly in the initial stage, but delaminates abruptly after a definite sliding time only under heavy loads. The time required for this delamination increases with increasing substrate hardness and decreasing applied load, and there exists an optimal coating thickness for the delayed coating delamination. It is suggested that severe plastic flow in the form of twins observed in the substrate at the vicinity of the interface could play an important role in this coating delamination.

Tribological behavior of CrN coating on aluminum alloys deposited by arc ion plating

Dry sliding wear and friction tests of CrN coating on two types of aluminum alloy substrates, 6061 Al and 7075 Al, deposited by arc ion plating were performed with a ball-on-disk tribometer. The effects of normal load and the mechanical properties of the substrate on the friction coefficient and wear resistance of CrN coating were investigated. The worn surfaces were observed by scanning electron microscopy. The results show that surface microhardness of CrN-coated 7075 Al is higher than that of CrN-coated 6061 Al. With an increase in normal load, wear volume increases, while the friction coefficient decreases. The friction coefficient of CrN-coated 6061 Al is higher than that of CrN-coated 7075 Al, while the wear resistance of CrN-coated 6061 Al is lower than that of the CrN-coated 7075 Al. This indicates that the substrate mechanical properties have strong influence on the friction coefficient and wear of CrN coating. The main wear mechanism was fragments of CrN coating, caused by apparent plastic deformation of substrate during wear tests.

Interfacial reaction and joint strength of silicon nitride ceramic composites bonded with Y2O3–Al2O3–SiO2–Si3N4 mixture

Silicon nitride ceramic composite was joined to itself by heating an interlayer of Y2O3–Al2O3–SiO2–Si3N4 mixtures above their liquid temperatures in flowing nitrogen. The microstructure and chemistry of joints were characterized by scanning electron microscopy, electron probe microanalyses and X-ray diffraction, respectively. The results show that some elements of adhesive (Y, Al, O) diffuse into silicon nitride ceramic composite to form a diffusion layer. At 1600 °C, the content of Si2N2O and β-Si3N4 increases with an increase in holding time. When the bonding temperature is higher than 1600 °C, the content of β-Si3N4 increases. The joining strength of the joint depends on the interfacial reaction, and a maximum joining strength of 350 MPa measured by the four-point bending test is achieved for the Si3N4 composite–Si3N4 composite joint bonded at 1600 °C for 10 min. The destructed part is considered to start at the diffusion layer, indicating that the joining strength will be much higher than that value.

Oxidation performance of oils containing ZnDTC, ZnDDP and their mixture after oxidation test by PDSC

Abstract Pressurized differential scanning calorimetry (PDSC) and FT–IR were employed to evaluate the anti-oxidation properties of ZnDTC, ZnDDP and their mixture in lubricating oil. PDSC data were quite different between oils containing the same additive at different concentrations and among oils containing a single additive and the mixture.

TRIBOLOGICAL EVALUATION OF STRUCTURAL CERAMICS UNDER SLIDING FRICTION

Tribological experiments were conducted on a ball-on-disk, unlubricated, with a speed of V≈ 140mm/s, V≈ 70mm/s, with an applied load between 20 and 100N, and with different combinations of ceramic materials. A wear test was conducted on disk material zirconia with regard to various ceramic ball materials (zirconia, alumina, silicon carbide and silicon nitride). The results show that the properties of the counter materials cause a difference in friction and wear characteristics.