Chang-Min Suh

Effect of intrinsic properties of ceramic coatings on fatigue behavior of Cr–Mo–V steels

Abstract The effect of intrinsic properties of ceramic coatings such as TiN, TiCN and TiAlN films on fatigue behavior has been studied on the commonly used rotor steel, Cr–Mo–V steel, in which test samples were deposited with ceramic coating layers of 2.5–5 μm thick by a filtered arc ion plating. The coating layer micro-hardness, film characteristics and residual stresses of coating films by X-ray diffraction were investigated, and the high-cycle fatigue tests were conducted under rotary bending and axial constant amplitude loading. The hardness of coating layers increased approximately 5–10 times more than that of uncoated substrate dependent on the coatings, and large compressive residual stresses appeared on the coating layers. It is also shown that the fatigue strength of coated specimens is superior to those of uncoated substrate, in particular at long fatigue life. Based on detailed observation of crack initiation, growth and fracture surfaces, it has been concluded that the improved fatigue strength of ceramic-coated material is mainly attributed to the retardation of crack initiation of the substrate by hard coating layers, compared with the influence of crack growth resistance by ceramic coatings.

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.

EFFECT OF CERAMIC COATING THICKNESS ON RESIDUAL STRESS AND FATIGUE CHARACTERISTIC OF 1Cr-1Mo-0.25V STEEL

To evaluate the effect of coatings on the fatigue behaviors of turbine rotor steel, TiN and TiAlN films were deposited on the 1Cr-1Mo-0.25V steels by arc-ion plating (AIP) method with and wihtout screen ion filter. The coating thickness were varied with 2.5 μm, 3.5 μm, and 5.0 μm. A Cu-Kα beam source was used as a characteristic X-ray and the crystal plane of (422) was selected to evaluate the residual stresses. In order to clear the relationship between fatigue behavior and residual stress of specimen coated with TiN and TiAlN films, the fatigue tests of specimens with and without coating were carried out at room temperatures respectively. It is shown that the fatigue life of the coated specimen was longer than that of uncoated specimen. The compressive residual stresses on the coatings were higher, and the fatigue crack initiated at an inclusion in the substrate near bond interface. It is known that compressive residual stress caused by hard coating would retard the fatigue crack initiation on the specimen surface, and then led to fatigue strength and fatigue life increasing.

A STUDY OF THE MECHANICAL CHARACTERISTICS OF ULTRASONIC COLD FORGED SKD61

Ultrasonic Cold Forging (UCF) technology is a method that to induces intensive plastic deformation on a material surface, so that the structure of the material becomes very fine from the surface to a certain depth. It improves the mechanical properties, hardness, compressive residual stress, and the wear and fatigue characteristics of the surface. In this study, UCF technology is applied to a cutting tool on a rolling process at a steel mill. At first, the ultrasonic cold forged specimens of SKD-61 are prepared and tested to verify the effects of UCF technology with regard to the mechanical properties, UCF is applied to the trimming knives of the cold rolling process. It is found that UCF improves the mechanical properties effectively and it is a pratical method that can improve the service time needed for the trimming knives. The productivity of the cold rolling process can be increased by the application of the ultrasonic cold forged trimming knives.

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.

FATIGUE CHARACTERISTICS OF SKD-61 BY ULTRASONIC NANOCRYSTAL SURFACE MODIFICATION TECHNOLOGY UNDER STATIC LOAD VARIATION

At first, the specimens of SKD-61 are prepared and tested to verify the effects of ultrasonic nano-crystal surface modification (UNSM) technology on the variation of mechanical properties under static load variation. 20 kHz frequency was applied to the ball tip, and the applied static forces were changed three kinds of load level 40, 60, and 80 N, respectively. The grain size of SKD-61 surface treated by UNSM becomes very fine to nano-scale crystal and structure is observed till certain depth. The compressive residual stress becomes -810, -1200 and -1400 MPa to a 150 µm depth after the UNSM process according to three kinds of load level 40, 60, and 80 N, respectively. Fatigue limits of 107 cycles are increased by 8.3, 11.2, and 17.9% after UNSM at the smooth specimen according to three kinds of load level, respectively. Interior-originating fracture, fish-eye crack, occurs after UNSM because of a nano structured modification by a surface plastic deformation and compressive residual stress in the case of t...

Analysis of Traumatic Brain Injury Using a Finite Element Model

In this study, head injury by impact force was evaluated by numerical analysis with 3-dimensional finite element (FE) model. Brain deformation by frontal head impact was analyzed to evaluate traumatic brain injury (TBI). The variations of head acceleration and intra-cranial pressure (ICP) during the impact were analyzed. Relative displacement between the skull and the brain due to head impact was investigated from this simulation. In addition, pathological severity was evaluated according to head injury criterion (HIC) from simulation with FE model. The analytic result of brain damage was accorded with that of the cadaver test performed by Nahum et al. (1977) and many medical reports. The main emphasis of this study is that our FE model was valid to simulate the traumatic brain injury by head impact and the variation of the HIC value was evaluated according to various impact conditions using the FE model.

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.

Characteristics of Fatigue Crack Initiation and Fatigue Strength of Nitrided 1Cr-1Mo-0.25V Turbine Rotor Steels

To investigate the effect of nitriding layer on both fatigue crack initiation and fatigue life, turbine rotor steel (lCr-IMo-0.25V steel) specimens were nitrided by the nitemper method and then put to a rotary bending fatigue test at room and elevated temperatures. In nitriding, temperature and time were controlled to obtain a different nitrided thickness. Microstructure analysis, micro-Vickers hardness test, and scanning electron microscope observation were carried out for evaluating experiments. In results, the fatigue cracks of nitrided specimens were initiated at inclusion near the interface between nitrided layer and substrate, which showed fish-eye type appearance in fractograph. The fatigue life of nitrided specimens at every temperature was prolonged compared to that of the non-nitrided. However, there was not observable improvement in fatigue characteristics with the increase of a nitrided thickness.

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.