• Tribology and Lubricants
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• 제34권3호
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• pp.84-92
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• 2018

Bearings are essential for reducing vibration and wear, in order to achieve high durability and increase longevity. White metal treatment of tilting pads via centrifugal casting method has the possibility of increasing durability. However, this manufacturing method has drawbacks such as long processing time, high defect rate, and harmful health effects. Laser cladding deposition technique is a powerful method that can address these issues by decreasing the processing time and providing good adhesion. In this study, we suggest optimum conditions for laser cladding deposition that can be used in industrial applications. We deposited a soft white metal layer on SCM440 that is primarily used in shafts to minimize wear of bearing pads. During the laser deposition process, we controlled factors such as laser power, powder feed rate, and laser head speed to determine the optimum conditions. In addition, we measured the hardness using micro Vickers, and performed field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, and friction tests to investigate the mechanical properties and surface characteristics for different parameters. Based on the experimental results, we suggest that laser power, powder feed rate, and laser head speed of 1.3 kW, 2.5 rpm, and 10 mm/s, respectively, constitute the optimum conditions for producing white metals using laser cladding.

• 한국표면공학회지
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• 제46권2호
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• pp.68-74
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• 2013

For the coating of diamond films on WC-Co tools, a buffer interlayer is needed because Co catalyzes diamond into graphite. W and Ti were chosen as candidate interlayer materials to prevent the diffusion of Co during diamond deposition. W or Ti interlayer of $1{\mu}m$ thickness was deposited on WC-Co substrate under Ar in a DC magnetron sputter. After seeding treatment of the interlayer-deposited specimens in an ultrasonic bath containing nanometer diamond powders, $2{\mu}m$ thick nanocrystalline diamond (NCD) films were deposited at $600^{\circ}C$ over the metal layers in a 2.45 GHz microwave plasma CVD system. The cross-sectional morphology of films was observed by FESEM. X-ray diffraction and visual Raman spectroscopy were used to confirm the NCD crystal structure. Micro hardness was measured by nano-indenter. The coefficient of friction (COF) was measured by tribology test using ball on disk method. After tribology test, wear tracks were examined by optical microscope and alpha step profiler. Rockwell C indentation test was performed to characterize the adhesion between films and substrate. Ti and W were found good interlayer materials to act as Co diffusion barriers and diamond nucleation layers. The COFs on NCD films with W or Ti interlayer were measured as less than 0.1 whereas that on bare WC-Co was 0.6~1.0. However, W interlayer exhibited better results than Ti in terms of the adhesion to WC-Co substrate and to NCD film. This result is believed to be due to smaller difference in the coefficients of thermal expansion of the related films in the case of W interlayer than Ti one. By varying the thickness of W interlayer as 1, 2, and $4{\mu}m$ with a fixed $2{\mu}m$ thick NCD film, no difference in COF and wear behavior but a significant change in adhesion was observed. It was shown that the thicker the interlayer, the stronger the adhesion. It is suggested that thicker W interlayer is more effective in relieving the residual stress of NCD film during cooling after deposition and results in stronger adhesion.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2013년도 제44회 동계 정기학술대회 초록집
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• pp.703-703
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• 2013

다이아몬드상 탄소 박막(Diamond-like carbon, DLC) 박막은 낮은 마찰 계수, 높은 내마모성, 화학적 안정성, 적외선 영역에서의 높은 투과율 등의 장점을 바탕으로 MEMS (Micro-Electro Mechanical System) 소자와 MMAs (Moving Mechanical Assemblies)의 고체윤활코팅, 마그네틱 미디어와 하드디스크의 슬라이딩 표면 등 다양한 분야에 코팅소재로써 응용되어왔다 [1,2]. 현재 전기철도용 집전판은 마찰이 적고 전도성을 지니는 카본 소재로 구성되어 있다. 그러나 그 마모 비율이 너무 심하여 이를 개선할 수 있는 방안으로 고경도 저마찰력을 지니는 DLC 박막을 코팅 소재로써 제안하고자 한다. 그러나 기존에 DLC 박막은 절연특성이 매우 우수하기 때문에 기존에 전도성을 지니는 카본 집전판에 적용하기에는 어려움이 따른다. 따라서 DLC 박막 내에 실리콘(Si) 또는 금속(Metal)을 첨가시키거나, 금속 중간층을 포함시켜 전기적으로 전도특성을 향상시키는 방안이 제시되고 있으며, 본 연구에서는 DLC 박막과 유사하게 우수한 경도특성을 지니고, 낮은 마찰계수등을 지니는 비정질 탄소박막을 연구하여 카본 집전판에 코팅하고자하며, 특히 비정질 탄소박막에 금속 Ti를 도핑하여 집전판과의 접착력과 전기적 전도 특성을 향상시키고자 한다. Ti가 도핑된 탄소박막(TiC) 박막은 비대칭 마그네트론 스퍼터링(unbalanced magnetron sputtering; UBMS) 시스템을 이용하여 제작하였으며, 스퍼터링 조건 중 기판에 인가되어지는 기판온도에 따라 변화되어지는 TiC 박막의 트라이볼로지(Tribology) 특성을 고찰하고자 하였다. 증착시 기판온도의 증가는 TiC 박막의 경도, 마찰계수 특성등 트라이볼로지 특성을 향상시켰으며, 전기적 전도 특성을 향상시켰다. 이러한 결과는 스퍼터링 방법에 의해 증착되어진 TiC 박막내에 존재하는 sp2 결합과 관계가 있음을 확인할 수 있으며, 트라이 볼로지 특성은 TiC 박막내에 sp2 탄소결합의 비율 증가와 관련되어졌다. 특히 sp2 탄소결합은 TiC 박막 증착시 증가된 기판온도와 밀접한 관계가 있으며 기판온도의 증가에 따라 나노결정 클러스터의 크기와 수의 변화와 밀접한 관계가 있음을 확인하였다. 결국 기판온도는 TiC 박막의 트라이볼로지 특성을 향상시켰으며, 전기적 특성 또한 향상시켜 전기철도 집전판에 응용을 위한 소재로 평가할 수 있다.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2013년도 제44회 동계 정기학술대회 초록집
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• pp.159-159
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• 2013

Atomically thin graphene is the ideal model system for studying nanoscale friction due to its intrinsic two-dimensional anisotropy. Furthermore, modulating its tribological properties could be an important milestone for graphene-based micro and nano-mechanical devices. Here, we report that the tribological properties can be easily altered via simple chemical modifications of the graphene surface. Friction force microscopy measurements show that hydrogenated, fluorinated, and oxidized graphenes exhibit, 2-, 6-, and 7-fold enhanced nanoscale friction on their surfaces, respectively, compared to pristine graphene. The measured nanoscale friction should be associated with the adhesive and elastic properties of the chemically modified graphenes. Density functional theory calculations suggest that, while the adhesive properties of chemically modified graphenes are marginally reduced down to ~30%, the out-of-plane elastic properties are drastically increased up to 800%. Based on these findings, we propose that nanoscale friction on graphene surfaces is characteristically different from that on conventional solid surfaces; stiffer graphene exhibits higher friction, whereas a stiffer three-dimensional solid generally exhibits lower friction. The unusual friction mechanics of graphene is attributed to the intrinsic mechanical anisotropy of graphene, which is inherently stiff in plane, but remarkably flexible out of plane. The out-of-plane flexibility can be modulated up to an order of magnitude by chemical treatmentof the graphene surface. The correlation between the measured nanoscale friction and the calculated out-of-plane flexibility suggests that the frictional energy in graphene is mainly dissipated through the out-of-plane vibrations, or the flexural phonons of graphene.