Nano tribological properties of sub-micrometer polymeric channels fabricated on silicon wafer

Duc-Cuong Pham,R. Arvind Singh,Eui-Sung Yoon 대한기계학회 2008 대한기계학회 춘추학술대회 Vol.2008 No.5

This work investigates hydrophobicity, indicated by water contact angle, and nano-scale adhesion and friction properties of sub-micrometer poly (methylmethacrylate) (PMMA) channels fabricated on silicon wafers by capillary force lithography in a comparison with flat samples including the wafer and PMMA film. Results show that the channels exhibited a higher water contact angle and significant lower nano-scale adhesion and friction forces than those of the silicon wafer and PMMA film.

Tribochemical Interactions of Si-doped DLC Film Against Steel in Sliding Contact

Pham, Duc-Cuong,Ahn, Hyo-Sok,Oh, Jae-Eung,Yoon, Eui-Sung 대한기계학회 2007 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.21 No.7

This study concerns the effects of tribochemical interactions at the interface of Si-DLC (silicon-doped diamond-like carbon) film and steel ball in sliding contact on tribological properties of the film. The Si-DLC film was over-coated on pure DLC coating by radio frequency plasma-assisted chemical vapor deposition (r.f. PACVD) with different Si concentration. Friction tests against steel ball using a reciprocating type tribotester were performed in ambient environment. X-Ray photoelectron spectroscopy (XPS) and auger electron spectroscopy (AES) were used to study the chemical characteristics and elemental composition of the films and mating balls after tests. Results showed a dark-gray film consisting of carbon, oxygen and silicon on the worn steel ball surface with different thickness. On the contrary, such film was not observed on the surface of the ball slid against pure DLC coating. The oxidation of Si-DLC surface and steel ball was also found at particular regions of contact area. This demonstrates that tribochemical interactions occurred at the contact area of Si-DLC and steel ball during sliding to form a tribofilm (so called transfer film) on the ball specimen. While the pure DLC coating exhibited high coefficient of friction $(\sim0.06)$, the Si-DLC film showed a significant lower coefficient of friction $(\sim0.022)$ with the presence of tribofilm on mating ball surface. However, the Si-DLC film possesses a very high wear rate in comparison with the pure DLC. It was found that the tribochemical interactions strongly affected tribological properties of the Si-DLC film in sliding against steel.

Nanotribological properties of silicon nano-pillars coated by a Z-DOL lubricating film

Pham, Duc Cuong,Na, Kyung-Hwan,Yang, Sung-Wook,Kim, Jin-Seok,Yoon, Eui-Sung 대한기계학회 2010 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.24 No.1

This paper reports a novel approach for improving the nanotribological properties of silicon (Si) surfaces by topographically and chemically modifying the surfaces. In the first step, Si (100) wafers were topographically modified into nano-pillars by using the photolithography and reactive ion etching (RIE) techniques. Various patterns, including nano-pillars of varying diameters and pitches (distance between pillars), were fabricated. Then, the patterns were coated with a Z-DOL (perfluoropolyether (PFPE)) lubricating film using a dipcoating technique, and this process was followed by thermal treatment. These modified surfaces were tested for their nanotribological properties, namely adhesion and friction forces, using an atomic force microscope (AFM). The results showed that the topographical modification and Z-DOL coating each independently reduced the adhesion and friction forces on the Si surfaces. However, the combination of the two surface treatments was most effective in reducing these forces. This is attributed to the combined effects of the reduction in the real area of contact due to patterning and the low surface energy of the Z-DOL lubricant. Further, it was found that adhesion and friction forces of the surfaces with combined modification varied significantly depending on the diameter of the pillars and the pitch. It is proposed that such a combination of surface modifications promises to be an effective method to improve the nanotribological performance of miniaturized devices, such as MEMS, in which Si is a typical material.

Hydrophobicity and Nanotribological Properties of Silicon Channels coated by Diamond-like Carbon Films

Duc Cuong Pham,Kyunghwan Na,Van Hung Pham,Eui-Sung Yoon 한국트라이볼로지학회 2009 KSTLE International Journal Vol.10 No.1/2

This paper reports an investigation on nanotribological properties of silicon nanochannels coated by a diamond-like carbon (DLC) film. The nanochannels were fabricated on Si (100) wafers by using photolithography and reactive ion etching (RIE) techniques. The channeled surfaces (Si channels) were then further modified by coating thin DLC film. Water contact angle of the modified and unmodified Si surfaces was examined by an anglemeter using the sessile-drop method. Nanotribological properties, namely friction and adhesion forces, of the Si channels coated with DLC (DLC-coated Si channels) were investigated in comparison with those of the flat Si, DLC-coated flat Si (flat DLC), and Si channels, using an atomic force microscope (AFM). Results showed that the DLC-coated Si channels greatly increased hydrophobicity of silicon surfaces. The DLC coating and Si channels themselves individually reduced adhesion and friction forces of the flat Si. Further, the DLCcoated Si channels exhibited the lowest values of these forces, owing to the combined effect of reduced contact area through the channeling and low surface energy of the DLC. This combined modification could prove a promising method for tribological applications at small scales.

Nanotribological Properties of Nano-channels Overcoated by DLC Films

Duc Cuong Pham,Kyounghwan Na,Sungwook Yang,Jinseok Kim,Van Hung Pham,Eui-Sung Yoon 한국트라이볼로지학회 2009 한국트라이볼로지학회 학술대회 Vol.2009 No.11

Factors Influencing the Choices of Accounting Policies in Small and Medium Enterprises in Vietnam

PHAM, Cuong Duc,PHI, Trong Van Korea Distribution Science Association 2020 The Journal of Asian Finance, Economics and Busine Vol.7 No.10

Accounting policies are principles and practices by which an entity uses to recognize, measure and report economic transactions. Improper application of accounting policies can lead to misrepresentation of firms' financial position and performance which consequently results in incorrect accounting information to the users. This paper aims to investigate the factors influencing the choices of accounting policies in small and medium enterprises (SMEs) in Vietnam by reviewing relevant literature to build a research model. The research model comprises of one dependent variable that is income-decreasing accounting procedures and six independent variables namely the firm size, financial leverage, incentives, auditor, accountants, and tax policies. After this, the authors collected primary data from more than 200 questionnaires sent to directors and chief accountants of the SMEs for the period 2018 to 2019. We then used Ordinary Least Squares regression method (OLS) to analyze the data. The results showed that four factors influenced selection of accounting policies in which auditors are associated with income-increasing accounting policies; and there are three factors associated with income-decreasing accounting policies which are, company size, tax and accountant. Especially, the research results indicate that company size has a significant influence on the selection of accounting policies in the SMEs. Based on the results, we propose instructive suggestions for regulators and lawmakers improve choices of accounting policies in the SMEs.

Hydrophobicity and Nanotribological Properties of Silicon Channels coated by Diamond-like Carbon Films

Pham, Duc Cuong,Na, Kyung-Hwan,Pham, Van Hung,Yoon, Eui-Sung Korean Tribology Society 2009 KSTLE International Journal Vol.10 No.1

This paper reports an investigation on nanotribological properties of silicon nanochannels coated by a diamond-like carbon (DLC) film. The nanochannels were fabricated on Si (100) wafers by using photolithography and reactive ion etching (RIE) techniques. The channeled surfaces (Si channels) were then further modified by coating thin DLC film. Water contact angle of the modified and unmodified Si surfaces was examined by an anglemeter using the sessile-drop method. Nanotribological properties, namely friction and adhesion forces, of the Si channels coated with DLC (DLC-coated Si channels) were investigated in comparison with those of the flat Si, DLC-coated flat Si (flat DLC), and Si channels, using an atomic force microscope (AFM). Results showed that the DLC-coated Si channels greatly increased hydrophobicity of silicon surfaces. The DLC coating and Si channels themselves individually reduced adhesion and friction forces of the flat Si. Further, the DLC-coated Si channels exhibited the lowest values of these forces, owing to the combined effect of reduced contact area through the channeling and low surface energy of the DLC. This combined modification could prove a promising method for tribological applications at small scales.

Dual Surface Modifications of Silicon Surfaces for Tribological Application in MEMS

Pham, Duc-Cuong,Singh, R. Arvind,Yoon, Eui-Sung Korean Tribology Society 2007 KSTLE International Journal Vol.8 No.2

Si(100) surfaces were topographically modified i.e. the surfaces were patterned at micro-scale using photolithography and DRIE (Deep Reactive Ion Etching) fabrication techniques. The patterned shapes included micro-pillars and microchannels. After the fabrication of the patterns, the patterned surfaces were chemically modified by coating a thin DLC film. The surfaces were then evaluated for their friction behavior at micro-scale in comparison with those of bare Si(100) flat, DLC coated Si(100) flat and uncoated patterned surfaces. Experimental results showed that the chemically treated (DLC coated) patterned surfaces exhibited the lowest values of coefficient of friction when compared to the rest of the surfaces. This indicates that a combination of both the topographical and chemical modification is very effective in reducing the friction property. Combined surface treatments such as these could be useful for tribological applications in miniaturized devices such as Micro-Electro-Mechanical-Systems (MEMS).

Evaluating Performance of Vietnamese Public Hospitals Based on Balanced Scorecard

PHAM, Cuong Duc,VU, Sen Thi,PHAM, Yen Thi Kim,VU, Nam Thi Korea Distribution Science Association 2020 The Journal of Asian Finance, Economics and Busine Vol.7 No.6

The study evaluates the performance of public hospitals in Vietnam by applying the Balanced Scorecard (BSC). The authors first review the literature to find the research gap of performance in public hospitals. Then, we built Likert questionnaires to collect data from more than 200 managers of public hospitals in the Northwestern provinces of Vietnam. The research uses correlation regression to evaluate the performance based on the influence of factors in the BSC model, including Strategic planning, Internal process, Finance, Mission, Customer, and Employee learning and growth. The results show that the performance of public hospitals in the research sample is influenced by the factors in the BSC model in descending order based on the regression coefficient as follows: Internal process, Finance, Mission, Strategic planning, Customer, Employee learning and growth. Based on the quantitative research findings, we continue by conducting some deep interviews with specialty to propose intensive recommendations about how to implement Mission, Internal process, Financial policies, etc. to managers in public hospitals with an aim to improve the performance of public hospitals in the Northwestern mountainous region of Vietnam. The lessons could be applied for other public hospitals in Vietnam and other jurisdictions that have similar conditions.

Tribological Properties of Sputtered Boron Carbide Coating and the Effect of ${CH}_4$ Reactive Component of Processing Gas

Cuong, Pham-Duc,Ahn, Hyo-Sok,Kim, Jong-Hee,Shin, Kyung-Ho Korean Tribology Society 2003 KSTLE International Journal Vol.4 No.2

Boron carbide thin coatings were deposited on silicon wafers by DC magnetron sputtering using a ${B}_4$C target with Ar as processing gas. Various amounts of methane gas (${CH}_4$) were added in the deposition process to better understand their influence on tribological properties of the coatings. Reciprocating wear tests employing an oscillating friction wear tester were performed to investigate the tribological behaviors of the coatings in ambient environment. The chemical characteristics of the coatings and worn surfaces were studied using X-ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES). It revealed that ${CH}_4$addition to Ar processing gas strongly affected the tribologcal properties of sputtered boron carbide coating. The coefficient of friction was reduced approximately from 0.4 to 0.1, and wear resistance was improved considerably by increasing the ratio of ${CH}_4$gas component from 0 to 1.2 vol %. By adding a sufficient amount of ${CH}_4$(1.2 %) in the deposition process, the boron carbide coating exhibited lowest friction and highest wear resistance.