• Applied Microscopy
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• v.48 no.3
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• pp.73-80
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• 2018

Ductile cast iron is widely used for many automotive components due to its high wear resistance and fatigue resistance in addition to the low cost of fabrication. The improvement of wear resistance and fatigue properties is key to the life time extension and performance increase of the automobile parts. Surface nanocrystallization is a very efficient way of improving the performance of materials including the wear- and fatigue-resistance. Shot peening treatment, as one of the popular and economic surface modification methods, has been widely applied to various materials. In this study, ductile cast iron specimens were ultrasonic shot peening (USP) treated for 5 to 30 min using different ball size. The microstructures were then microscopically analyzed for determination of the microstructural evolution. After the USP treatment, the hardness of pearlite and ferrite increased, in which ball size is more effective than treatment time. With USP treatment, the graphite nodule count near the surface was decreased with grain refinement. The lager balls resulted in an increased deformation, whereas the smaller balls induced more homogenously refined grains in the deformation layer. In addition, formation of nanoparticles was formed in the surface layer upon USP.

• International Journal of Railway
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• v.1 no.3
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• pp.106-112
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• 2008

Rolling-sliding, cyclic contact of wheel and rail progressively alters the microstructure of the contacting steels, eventually leading to micro-scale crack initiation, wear and macro-scale crack growth in the railhead. Relating the microstructural changes to subsequent wear and cracking is being accomplished through modelling at three spatial scales: (i) bulk material (ii) multi-grain and (iii) sub-grain. The models incorporate detailed information from metallurgical examinations of used rails and tested rail material. The initial 2-dimensional models representing the rail material are being further developed into 3-dimensional models. Modelling is taking account of thermal effects, and traffic patterns to which the rails are exposed.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 1995.06a
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• pp.11-17
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• 1995

본 연구에서는 SiC 및 SiC-TiB$_{2}$복합재료의 마모 및 마모천이기구를 비교 실험한 결과 다음과 같은 결론을 얻었다. 두 재료의 마모천이현상은 각각의 하중이 200N(SiC)과 80N(SiC-TiB$_{2}$)이상에서 나타나게 되었다. 마모기구는 임계미끄럼시간에서 두 단계로 나뉘어진다. 소성변형에 의한 grooving과정과 잔류응력에 의한 grain pull-out과정들로써 미세 구조를 통해 관찰할 수 있었다. grain pull-out과정은 초기단계에서 부터 생성된 전위가 축적되어 잔류응력을 발생시키므로 일어난 과정이다. SiC에 TiB$_{2}$를 첨가하므로 마모는 더욱 심하게 일어나게 되었다. 입계의 강도를 더 약하게 하였기 때문이다. 그러나, 파괴인성은 입계강도가 약할수록 증가하였다. 결국 파괴인성이 큰 재료가 마모는 더 심하게 일어남으로 서로 상반된 관계를 갖고 있슴을 결론지을 수 있었다.

• Journal of the Korean Society for Precision Engineering
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• v.16 no.6
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• pp.160-165
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• 1999

In surface grinding, the conditions of the grinding wheel give a significant effect on the ground workpieces comparing with other metal removal provesses. In this paper, to assist the development, a non-contacting optical method by the laser beam is introduced. The in-process measurement of scattering intensities has been made during surface grinding processes and the surface textures of wheel working surfaces are captured. Also, in order to determine the dressing time monitoring method of a grinding wheel, a three-dimensional computer simulation of the grinding operation has been attempted based on the contact mechanism and the surface-shaping system between the grinding wheel and the workpiece. The optimal dressing time is determined by the amount of the grain wear and work surface roughness.

• Journal of the Korean Society for Precision Engineering
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• v.16 no.4 s.97
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• pp.148-154
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• 1999

This study was carried out to verify grinding performance of dental diamond bur and investigate the possibility of AE application in dentistry field. Workpieces were made of acryl and bovine respectively for the experiments in this study. Grinding test was conducted to get the data of grinding resistance and specific grinding energy of four different types of diamond bur by using tool dynamometer. AE signal was acquired to verify grinding process in the AE measuring system. Tool wear was observed to find parameters about grinding characteristics of diamond bur by means of SEM picture. It was found that the wear of dental diamond bur could be detected with polishing of grinding material, removal of adhesive parts, wear of particles neighboring cutting nose, loss of material and elevation of temperature. The wear of B, C, D type diamond bur is due to wear and fracture of grain size. Abnormal state can be found through the behavior of AE signal in the grinding working. As a result, it is expected that forecast of abnormal state is possible using AE equipments under real time process.

• Tribology and Lubricants
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• v.16 no.6
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• pp.477-483
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• 2000

Rolling contact fatigue (RCF) tests were performed for two kinds of commercial silicon nitride balls using 4-Ball rolling contact fatigue life tester under EHL condition (Λ=8.9) and boundary lubrication condition (Λ=0.2). All the test balls were finished up to the dimensional accuracy of Grade 5 defined in KS B 2001 (Steel Balls for Ball Bearings) with a size of 8.731 mm. RCF tests were then conducted under the initial theoretical maximum contact stress 6.63 GPa and the spindle speed 10,000 rpm. All the test balls were not failed until 3.75 $\times$ 107 contact cycles and wear tracks of test balls were not conspicuous under EHL condition (Λ= 8.9). In the operations of low lambda regime (Λ= 0.2), all the test balls were surface damaged and high rolling wear resistance was achievable in fully densified using MgO 1 wt% and HIPed balls. Rolling wear of silicon nitride balls under boundary lubrication condition depend mainly on grain size and intergranular phase content of silicon nitride balls.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.9 no.4
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• pp.25-33
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• 2000

The reorientations of the atoms by frictional shear deformation at the surface induce cracks at the boundary of the grain. The cracks grow and propagate in regions where the hydrostatic component of stress is least compressive because the compressive component restores the cracks by three-dimensional crystallizing $\pi$-bondings. The materials with Lder's band have very small amount of wear at the initial state. It suggests that initial frictional shear deformation be consumed to the formation of the Lder's band. The average wear amounts of the materials increase very steeply as the øu the stress-strain ratio at the ultimate point, decreases.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 1999.10a
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• pp.204-209
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• 1999

The dressing time monitoring in cylindrical grinding is very important with respect to machining efficiency. Therefore, the purpose of this paper is to determine the wheel life by monitoring behavior of grinding power for Wa, 19A and GC. For this purpose, we investigated indirectly the attritious wear of grain edge, the loading of grinding wheel and the breakage of grain through the grinding power and the surface roughness under various grinding conditions. From obtained the results, the relationship between the wheel life and the average sectional chip area is examined to guide for the determination of dressing time.

• Transactions of Materials Processing
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• v.14 no.1 s.73
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• pp.71-77
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• 2005

Thermal stability and dry sliding wear behavior of ultra-fine grained 6061 Al alloy fabricated by an accumulative roll-bonding (ARB) process have been investigated. After 4 ARB cycles, an ultra-fine grained microstructure of the 6061 Al alloy composed of grains with average size of 500nm, and separated mostly by high-angle boundaries was obtained. Though hardness and tensile strength of the ARB processed Al alloy increased with ARB cycles up to 4 cycles, the processed alloy exhibited decreased ductility and little strain hardening. Thermal stability of the ARB-processed microstructure was studied by annealing of the severely deformed alloy at $423K{\sim}573K$. The refined microstructure of the alloy remained stable up to 473K, and the peak aging treatment of the alloy at 450K for 8 hrs increased the thermal stability of the alloy. Sliding-wear rates of the alloy increased with the number of ARB cycles in spite of the increased hardness with the cycles. Wear mechanisms of the ultra-fine grained alloy were investigated by examining worn surfaces, wear debris, and cross-sections by a scanning electron microscopy (SEM).

• Journal of the Korean Ceramic Society
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• v.40 no.6
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• pp.525-529
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• 2003

Biotribological properties, such as wear rate and friction coefficient, of 3Y-TZP and Low Temperature Degradation (LTD) free materials were investigated via a ball(SiC)-on-plate sliding wear test to evaluate the relationship between wear mechanism and phase transformation. Wear test was conducted with a sliding speed of 0.035 m/s at room temperature and at 25$0^{\circ}C$ in air under a normal load of 49 N, respectively. Although friction coefficient of 3Y-TZP was the lowest due to the fine grain size, the highest wear loss and rate were observed due to the debris of monoclinic grains introduced during sliding and their values increased drastically with raising temperature. However, the biotribological properties of LTD-free materials were insensitive to temperature due to the inertness of the phase transformation, suggesting that they may be applicable to the biomechanical parts.