• 한국생산제조학회지
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• 제25권5호
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• pp.380-385
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• 2016

Railway tracks are repeatedly overstressed and damaged owing to increase in passing tonnage and numerous contact cycles between wheels of train and rails. In order to ensure safe train operation, heat-treated rails are used in addition to regular inspection and maintenance of these rails. Normal rails were treated using ultrasonic nanocrystal surface modification (UNSM) to strengthen the surface of rails. A few changes in surface properties were detected with respect to hardness and compressive residual stress after UNSM treatment. Wear and rolling contact fatigue tests were performed using rails whose surfaces were hardened by UNSM and heat-treated rails. The amount of wear and fatigue life cycles were measured to estimate the effect of UNSM on the rail material. The material of the surfacehardened rail showed improved wear and rolling contact fatigue properties.

• Tribology and Lubricants
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• 제37권1호
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• pp.25-30
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• 2021

The surface texture produced via surface texturing is an important approach for controlling the tribological behavior of friction behavior of mechanical devices. The purpose of this study is to investigate the effect of grooves generated via ultrasonic nanocrystal surface modification (UNSM) technology on the tribological performance of AISI 4150 steel against stainless steel 316L. In the study, tribological tests are performed under two different regimes, namely mixed and hydrodynamic lubrication, by varying the applied normal load and reciprocating speed during the tests. According to the test results, the friction coefficient decreases as static load (10 N, 30 N, and 50 N) of UNSM technology increases in the mixed lubrication regime. Conversely, the friction coefficient increases as the static load (10 N, 30 N, and 50 N) of UNSM technology increases in the hydrodynamic lubrication regime. Hence, the results indicate that micro-grooves generate hydrodynamic pressure in the outlet, which increases the oil film thickness between the two mating surfaces. This potentially leads to a reduction in friction in the mixed lubrication regime due to the prevention of contact of asperities and debris. However, the results indicate an adverse effect in the hydrodynamic lubrication regime. In this regard, additional experiments should be performed to investigate the effect of grooves generated by UNSM technology at varying conditions on the friction behavior of AISI 4150 steel, which in turn can be controlled by the generated pressure and oil film thickness at the contact interface.

• Tribology and Lubricants
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• 제37권3호
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• pp.83-90
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• 2021

In this study, an ultrasonic nanocrystal surface modification (UNSM) was used to improve the mechanical properties, scratch resistance and tribological performance of Cu-based bimetals, which are usually used to manufacture sliding bearings and bushings for internal combustion engines (ICEs). Two different Cu-based bimetals, namely CuPb10Sn10 and CuSn10Bi7, were sintered onto a low carbon steel substrate. The mechanical properties and dry tribological performance using a tensile tester and micro-tribo tester were evaluated, respectively. The scratch resistance was assessed using a micro-scratch tester at an incremental load. The tensile test results showed that the yield strength (YS) and ultimate tensile strength (UTS) of both Cu-based bimetals increased after UNSM. Furthermore, the scratch and tribological tests results revealed that the scratch resistance and tribological performance of both Cu-based bimetals were improved by the application of UNSM. These improvements were mainly attributed to the eliminated pores, increased hardness and reduced roughness after UNSM. CuSn10Bi7 demonstrated better mechanical properties, scratch resistance and tribological performance than CuPb10Sn10. It was found that the presence of Bi in CuSn10Bi7 formed a Cu₁₁Bi₇ intermetallic phase, which is harder than Cu₃Sn. Hence, CuSn10Bi7 demonstrated higher strength and wear resistance than CuPb10Sn10. In addition, a CuSn10Bi7 formed both SnO₂ and Bi₂O₃ that prevented adhesion and improved the tribological performance. It can be expected that under dry tribological conditions, ICEs can utilize UNSM bearings and bushings made of CuSn10Bi7 instead of CuPb10Sn10 under oil-lubricated conditions.

• 한국해양공학회지
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• 제32권6호
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• pp.492-501
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• 2018

The following results were obtained from a series of studies to accumulate data to reduce the coefficient of friction for press dies by performing tribological tests before and after the UNSM treatment of SM45C. The UNSM-treated material had a nano-size surface texture, high surface hardness, and large and deep compressive residual stress formation. Even when the load was doubled, the small amount of abrasion, small weight of the abrasion, and width and depth of the abrasion did not increase as much as those for untreated materials. When loads of 5 N, 7.5 N, and 10 N were applied to the untreated material of SM45C, the coefficient of friction was approximately 0.76-0.78. With the large specimen, a value of 0.72-0.78 was maintained at a load of 50 N despite the differences in the size of the wear specimen and working load. Tribological tests of large specimens of SM45C treated with UNSM under tribological conditions of 100 N and 50 N showed that the frictional coefficient and time constant stably converged between 0.7 and 0.8. The friction coefficients of the small specimens treated with UNSM showed values between 0.78 and 0.75 under 5 N, 7.5 N, and 10 N. The friction coefficients of the SM45C treated with UNSM were comparable to each other.

• Tribology and Lubricants
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• 제34권6호
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• pp.217-225
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• 2018

In this study, we spray a Ni-Cr composite powder onto AISI 4340 steel using the high velocity oxygen fuel method. We subsequently subject the Ni-Cr coating (as-sprayed) to ultrasonic nanocrystal surface modification (UNSM) process to improve the tribological performance. This study aims at increasing the wear resistance and durability of the Ni-Cr coating by altering the surface integrity and microstructure via the UNSM process. The UNSM process reduces the surface roughness of the as-sprayed coating by about 64%, which is explained by observing the elimination of high peaks and valleys and filling up micro-pores. Furthermore, a change in the microstructure of the coating due to continuous high-frequency strikes to the surface by a tip can lead to an increase in hardness from about 48 to 60 HRC. Furthermore, we investigate the characterization of the friction and wear behavior of Ni-Cr coating by a ball-on-disc tribometer in the dry conditions. We determine that after the UNSM process, there is a significant reduction in the friction coefficient of the as-sprayed coating from approximately 1.1 to 0.75. This is owing to the increased hardness and smoothed surface roughness. In addition, we investigate the surface morphology and wear track of the coatings before and after the UNSM process using a scanning electron microscope, energy dispersive spectrometer, and three-dimensional laser scanning microscope. We observe that the wear track of the Ni-Cr coating after the UNSM process is lower than that of the as-sprayed one. Thus, we confirm that the UNSM process has a significant influence on the improvement of the tribological performance of the Ni-Cr composite coating.

• Tribology and Lubricants
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• 제34권6호
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• pp.270-278
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• 2018

STS 316L prepared by additive manufacturing (AM) exhibits deterioration of mechanical properties and wear resistance due to the presence of defects such as black-of-fusion defects, internal porosity, residual stress, and anisotropy. In addition, high surface roughness (integrity) of AM products remains an issue. This study aimed to apply ultrasonic nanocrystal surface modification (UNSM) technology to STS 316L prepared by AM to increase the surface hardness, to reduce the surface roughness, and to improve the friction and wear behavior to the level achieved by bulk material manufactured using traditional processes. Herein, the as-received and polished specimens were treated by UNSM technology and their resulting properties were compared and discussed. The results showed that UNSM technology increased the surface hardness and reduced the surface roughness of the as-received and polished specimens. These results can be attributed to grain size refinement and pore elimination from the surface. Moreover, the friction of the as-received and polished specimens after UNSM technology was lower compared to those of the as-received and polished specimens, but no significant differences in wear resistance were found.

• 한국해양공학회지
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• 제32권6호
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• pp.502-510
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• 2018

In order to accumulate data to lower the friction coefficient of a press mold, tribological tests were performed before and after coating SM45C with a PVC/PO film and plasma coating (CrN, concept). The ultrasonic nanocrystal surface modification (UNSM)-treated material had a nano-size surface texture, high surface hardness, and large and deep compressive residual stress formation. Even when the load was doubled, the small amount of abrasion, small weight of the abrasion, and width and depth of the abrasion did not increase as much as those of untreated materials. A comparison of the weight change before and after the tribological test with the CrN and the concept coating material and that of the untreated material showed that the wear loss of the concept coating material and P-UNSM treated material (that is, the UNSM treated material treated with the concept coating) showed a tendency to decrease by approximately 55-75%. Concept 100N had a lower friction coefficient of about 0.6, and P-UNSM-30-100N showed almost the same curve as concept 100N and had a low coefficient of friction of about 0.6. The concept multilayer coating had a thickness of $5.32{\mu}m$. In the beginning, the coefficient of friction decreased because of the plasma coating, but it started to increase from about 250-300 s. After about 350 s, the coefficient of friction tended to approach the friction coefficient of the SM45C base metal. The SGV-280F film-attached test specimen was slightly pushed back and forth, but the SM45C base material was not exposed due to abrasion. The friction coefficient was 0.22, which was the lowest, and the tribological property was the best in this study.

• 대한기계학회논문집A
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• 제41권11호
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• pp.1011-1020
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• 2017

본 연구는 실온에서 미처리재와 UNSM처리재, 수소취화재(100 bar, $300^{\circ}C$에서 120 h) 및 이 수소취화재의 UNSM처리재의 스테인리스강 316L 시험편의 재료특성파악을 위한 실험을 실시하였다. 여기서 수소취화재는 기존의 미처리재의 S-N곡선보다 피로강도가 약간 감소하는 경향을 나타내었으며, 수소취화효과는 적게 나타났다. 미처리재에 비해서 UNSM처리재의 피로한도는 약 43.8 % 상승하였고, 수소취화재의 UNSM처리재는 수소취화재보다 약 57.1 % 상승하는 경향을 나타내었다. EBSD에 의한 IQ, IPF 및 KAM 맵에서 UNSM처리 후, 이 처리의 영향을 받는 나노 입자 표면층의 두께는 약 $152{\mu}m$이다. 그러나 세 가지 맵에서 수소취화의 깊이 영향을 정량적으로 평가할 수 없었다. 시험편상에서 발생하는 표면균열은 수소취화의 영향으로 평균입경($35{\mu}m$)보다 작은 균열의 비율이 약 90 %를 차지하였다.