• Tribology and Lubricants
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• v.30 no.6
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• pp.330-336
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• 2014

Sintered metallic brake pads and low alloy heat resistance steel disks are applied to mechanical brake systems in high energy moving machines that are associated with recently developed 200km/h trains. This has led to the speed-up of conventional urban rapid transit. In this study, we use a lab-scale dynamometer to investigate the effects of the composition of friction materials on the tribological characteristics of sintered metallic brake pads and low alloy heat resistance steel under dry sliding conditions. We conduct test under a continuous pressure of 5.5 MPa at various speeds. To determine the optimal composition of friction materials for 200 km/h train, we test and the evaluate frictional characteristics such as friction coefficients, friction stability, wear rate, and the temperature of friction material, which depend on the relative composition of the Cu-Sn and Fe components. The results clearly demonstrate that the average friction coefficient is lower for all speed conditions, when a large quantity of iron power is added. The specimen of 25 wt% iron powder that was added decreased the wear of the friction materials and the roughness of the disc surface. However when 35 wt% iron powder was added, the disc roughness and the wear rate of friction materials increased By increasing the amount of iron powder, the surface roughness, and temperature of the friction materials increased, so the average friction coefficients decreased. An oxidation layer of $Fe_2O_3$ was formed on both friction surfaces.

• Journal of Dental Rehabilitation and Applied Science
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• v.32 no.1
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• pp.16-23
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• 2016

Purpose: The aim of this study was to evaluate whether fluorides at various pH cause changes in the surface roughness of titanium implants that alter the adherence of bacterial biofilms. Materials and Methods: The titanium disks were assigned randomly to the following seven groups according to the fluoride agents and application time (1 minute or 30 minute) used: control (no treatment); group 1 (1.23% acidulated phosphate fluoride [APF] at pH 3.5 for 1 minute); group 2 (1.23% APF at pH 3.5 for 30 minute); group 3 (1.23% APF at pH 4.0 for 1 minute); group 4 (1.23% APF at pH 4.0 for 30 minute); group 5 (2% NaF gel at pH 7.0 for 1 minute); group 6 (2% NaF gel at pH 7.0 for 30 minute). The surface roughness of the titanium disks and the amount of adherent bacteria were measured. Results: Group 2 showed a significantly greater surface roughness than the control group (P < 0.0001). No significant differences in the amount of surface bacteria were observed between the treated samples and the controls. In addition, there were no significant differences in bacterial adherence relative to the incubation period between the treated samples and the controls. Conclusion: The surface roughness of the titanium disks was significantly greater after treatment with APF at pH 3.5 for 30 min compared with that of the controls. In addition, we found that the amount of Porphyromonas gingivalis, Fusobacterium nucleatum, and Aggregatibactor actinomycetemcomitans was similar among all groups

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.44 no.1
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• pp.12-22
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• 2016

Lidar-based hazard avoidance landing system for lunar lander calculates hazard cost with respect to the desired local landing area in order to identify hazard and designate safe landing point where the cost is minimum basically using slope and roughness of the landing area. In this case, if the parameters are only considered, chosen landing target can be designated near hazard threatening the lander. In order to solve this problem and select optimal safe landing point, hazard cost based on relative distance to hazard should not be considered as well as cost based on terrain parameters. In this paper, the effect of hazard cost based on relative distance to hazard on safe landing performance was analyzed and it was confirmed that landing site designation with two relative distances to hazard results in the best safe landing performance by an experiment using three-dimensional depth camera.

• Tribology and Lubricants
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• v.32 no.2
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• pp.39-43
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• 2016

Magnetorheological elastomers (MREs) are a type of “smart” material, and their properties can be controlled rapidly and reversibly under the influence of an external stimulus. The application of an external magnetic field can change the shear modulus, hardness, and friction coefficient of MREs. The friction can cause vibration; moreover, the vibration can affect friction. The change of friction depends on the relative motion, normal force, roughness of the rubbing surfaces, material type, temperature, lubrication, relative humidity, and vibration condition. As MREs are a type of “smart material,” their friction coefficient can be reduced by applying an external magnetic field—the applications of this feature in engineering have been widely studied. However, the friction properties of MREs under vibration have not been tested to date. In this study, MRE samples and a reciprocating friction tester were fabricated. The friction coefficient was measured to evaluate the friction properties under various vibration conditions; subsequently, the wear depth and wear surface profile of the MRE were observed in order to evaluate the wear properties. The results show that the friction coefficient of the MREs decreased when a magnetic field was applied. Moreover, the friction coefficient decreased when the vibrational amplitudes increased. The wear depth of the MRE also decreased as the vibrational amplitudes increased.

• Journal of Power System Engineering
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• v.19 no.5
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• pp.32-37
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• 2015

The present study is to investigates the convective heat transfer characteristics and pressure drop inside the rib-roughened cooling passage of gas turbine blades. The divergent rectangular channel is fabricated with V-shaped ribs on one wall only and the inlet hydraulic diameter to outlet hydraulic diameter ratio($D_{ho}/D_{hi}$) of 1.49 is used. The current investigation has covered a Reynolds number (Re) range of 22,000~75,000, relative roughness height ($e/D_h$) of 0.1~0.2, and rib angle of attack (a) of $30^{\circ}$, $45^{\circ}$, and $60^{\circ}$ for a fixed relative pitch of 10. Results show that the Nusselt numbers are the greatest in the $60^{\circ}$-angled ribs; however, the total friction factors are the highest in the $30^{\circ}$-angled ribs.

• The Journal of Korea Robotics Society
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• v.11 no.2
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• pp.100-107
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• 2016

To realize a robot hand interacting like a human hand, there are many tactile sensors sensing normal force, shear force, torque, shape, roughness and temperature. This sensing signal is essential to manipulate object accurately with robot hand. In particular, slip sensors make manipulation more accurate and breakless to object. Up to now several slip sensors were developed and applied to robot hand. Many of them used complicate algorithm and signal processing with vibration data. In this paper, we developed novel principle slip sensor using separation layer. These two layers are moved from each other when slip occur. Developed sensor can sense slip signal by measuring this relative displacement between two layers. Also our principle makes slip signal decoupled from normal force and shear force without other sensors. The sensor was fabricated using the NBR(acrylo-nitrile butadiene rubber) and the Ecoflex as substrate and a paper as dielectric. To verify our sensor, slip experiment and normal force decoupling test were conducted.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.20 no.7
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• pp.620-625
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• 2007

The effects of $Gd_2O_3$ addition and sintering condition on optical and electrical properties of MgO films as a protective layer for AC plasma display panels were investigated. Doped MgO films prepared by the e-beam evaporation have a higher ${\Upsilon}$ (secondary electron emission coefficient) than pure MgO protective layer. Relative density and grain size increased with amount of $Gd_2O_3$ up to 100 ppm and then decreased further addition. These results showed that discharge properties and optical properties of MgO protective layers seemed to be closely related with microstructure factors such as relative density and grain size. Good optical and electrical properties of ${\Upsilon}$ of 0.138, surface roughness of 5.77 nm and optical transmittance of 95.76 % were obtained for the MgO+100 ppm $Gd_2O_3$ protective layer sintered at $1700^{\circ}C$ for 5 hrs.

• Journal of the Korean Geotechnical Society
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• v.25 no.4
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• pp.105-112
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• 2009

In this study, a series of direct shear tests were performed to investigate the characteristics of friction angles for sands and interface friction angle between sands and construction materials with respect to different relative density of the Nak-dong River sands and shearing velocity. The result of the test shows that friction angles of sands are always higher than interface friction angle between sands and construction materials. As the shearing velocity get faster, the friction angles of sand became higher. With respect to the density of sand by reducing void ratio, friction angles increase linearly, and relevant equations were proposed to calculate the friction angle by changing void ratio and relative density of sand. The interface roughness of construction materials was also an important factor in interface friction angle.

• Tribology and Lubricants
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• v.11 no.5
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• pp.128-135
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• 1995

Atomic force microscopy/friction force microscopy (AFM/FFM) techniques are increasingly used for tribological studies of engineering surfaces at scales, ranging from atomic and molecular to microscales. These techniques have been used to study surface roughness, adhesion, friction, scratching/wear, indentation, detection of material transfer, and boundary lubrication and for nanofabrication/nanomachining purposes. Micro/nanotribological studies of single-crystal silicon, natural diamond, magnetic media (magnetic tapes and disks) and magnetic heads have been conducted. Commonly measured roughness parameters are found to be scale dependent, requiring the need of scale-independent fractal parameters to characterize surface roughness. Measurements of atomic-scale friction of a freshly-cleaved highly-oriented pyrolytic graphite exhibited the same periodicity as that of corresponding topography. However, the peaks in friction and those in corresponding topography were displaced relative to each other. Variations in atomic-scale friction and the observed displacement has been explained by the variations in interatomic forces in the normal and lateral directions. Local variation in microscale friction is found to correspond to the local slope suggesting that a ratchet mechanism is responsible for this variation. Directionality in the friction is observed on both micro- and macro scales which results from the surface preparation and anisotropy in surface roughness. Microscale friction is generally found to be smaller than the macrofriction as there is less ploughing contribution in microscale measurements. Microscale friction is load dependent and friction values increase with an increase in the normal load approaching to the macrofriction at contact stresses higher than the hardness of the softer material. Wear rate for single-crystal silicon is approximately constant for various loads and test durations. However, for magnetic disks with a multilayered thin-film structure, the wear of the diamond like carbon overcoat is catastrophic. Breakdown of thin films can be detected with AFM. Evolution of the wear has also been studied using AFM. Wear is found to be initiated at nono scratches. AFM has been modified to obtain load-displacement curves and for nanoindentation hardness measurements with depth of indentation as low as 1 mm. Scratching and indentation on nanoscales are the powerful ways to screen for adhesion and resistance to deformation of ultrathin fdms. Detection of material transfer on a nanoscale is possible with AFM. Boundary lubrication studies and measurement of lubricant-film thichness with a lateral resolution on a nanoscale have been conducted using AFM. Self-assembled monolyers and chemically-bonded lubricant films with a mobile fraction are superior in wear resistance. Finally, AFM has also shown to be useful for nanofabrication/nanomachining. Friction and wear on micro-and nanoscales have been found to be generally smaller compared to that at macroscales. Therefore, micro/nanotribological studies may help def'me the regimes for ultra-low friction and near zero wear.

• Journal of Periodontal and Implant Science
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• v.36 no.2
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• pp.319-334
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• 2006

Mechanical and chemical methods are the two ways to treat the implant surfaces. By using mechanical method, it is difficult to eliminate bacteria and by-products from the rough implant surface and it can also cause the structural change to the implant surface. Therefore, chemical method is widely used in order to preserve and detoxicate the implant surface more effectively. The purpose of this study is to evaluate the effect of tetracylcline-hydrochloride(TC-HCI) on the change of implant surface microstructure according to application time. Implants with pure titanium machined surface, SLA surface and porous surface were used in this study. Implant surface was rubbed with sponge soaked in 50mg/ml TC-HCI solution for $\frac{1}{2}$ min., 1 min., $1\frac{1}{2}$ min., 2 min., and $2\frac{1}{2}$ min. respectively in the test group and with no treatment in the control group. Then, specimens were processed for scanning electron microscopic observation. 1. Both test and control group showed a few shallow grooves and ridges in pure titanium machined surface implants. There were not significant differences between two groups. 2. In the SLA surfaces, the control specimen showed that the macro roughness was achieved by large-grit sandblasting. Subsequently, the acid-etching process created the micro roughness, which thus was superimposed on the macro roughness. Irrespective of the application time of 50mg/ml TC-HCI solution, in general, test specimens were similar to control. 3. In the porous surfaces, the control specimen showed spherical particles of titanium alloy and its surface have a few shallow ridges. The roughness of surfaces conditioned with tetracycline-HCI was lessened and seen crater-like irregular surfaces relative to the application time. In conclusion, pure titanium machined surfaces and SLA surfaces weren't changed irrespective of the application time of tetracycline-HCI solution. But the porous surfaces conditioned with tetracycline-HCI solution began to be slightly changed from 2 min. This results are expected to be applied to the regenerative procedures for peri-implantitis treatment.