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Gao, Jiachen,Kim, Hoyeon,Kim, Jaeyeol,Kwak, Jae B. Elsevier 2019 Ceramics international Vol.45 No.15
<P><B>Abstract</B></P> <P>Chemically strengthened glass (also known as ion exchanged glass) is widely used for hand-held devices due to excellent mechanical strength. However, only top and bottom planar surfaces are sufficiently protected following a series of machining processes such as drilling, cutting and milling for manufacturing and assembly. Micro cracks that remain on the machined surface of the glass edges become very vulnerable to shock impact loading. In this study we suggest a novel PEALD technique to encapsulate surface defects with a 100 nm thickness layer of high-strength ceramics such as Al<SUB>2</SUB>O<SUB>3</SUB>. In order to optimize PEALD parameters, we have examined best practices for testing chemically strengthened cover glasses. By conducting both Charpy impact test and 3-point bending test, it was found that impact resistance and flexural toughness of the PEALD-coated cover glass were 35%–40% and 21% higher compared to non-coated samples. Through the analysis of the state and elements of the machined surface with Al<SUB>2</SUB>O<SUB>3</SUB> (500 nm) coating it was verified that the coated layer uniformly covers the entire surface of the glass and fully infiltrates into the micro cracks. This study confirms that Al<SUB>2</SUB>O<SUB>3</SUB> ceramic coated with PEALD improves not only overall glass strength, but also overcomes the shortcomings of chemically strengthened glass machining.</P>
A novel evaluation method for thin films mechanical characterizations using cutting system
이정헌,Kwak Jae B. 한국물리학회 2021 Current Applied Physics Vol.26 No.-
In this study, we suggest a nano-cutting system to determine the shear strength of the thin films using fracture mechanics analysis of the diamond blade. Based on Merchant’s cutting model, we analyze the thin films cutting process with regard to shear angle and resistant forces as initiation of the yield in the chip to establish a direct correlation between the cutting forces and the shear strength. Validating the proposed method was conducted using homogenous polycarbonate disk showing similar shear strengths between different cutting directions. Next, we examined a thin copper electroplated film used in traces of printed circuit board. A thin copper film was examined and found the intrinsic shear strength (307.5 MPa) and adhesion force (44 N/m) between the film and substrate. The result was comparable with tensile strength values reported in the literatures. Finally, we used SEM to visually verify the feasibility of nano-cutting technique to determine thin film properties.
Jia-Chen Gao,Jae B. Kwak 대한기계학회 2021 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.35 No.8
With the development of the automotive industry, the number of automotive electronic components is growing rapidly. This leads to new requirements for reliability of the automotive electronics. Especially, for difficult operating environments, such as engine compartment of the car, extreme thermal cycles, and long holding time, where prediction of the fatigue life of the component is particularly important. In this study, thermal fatigue life prediction models for two packages are proposed: TSOP over BGA, and flip chip with BGA. The overall deformation of the two packages was analyzed using Moiré fringes, and was found to be consistent with simulation results. The reliability of the model was verified by comparing thermal shock results and simulation results. Then, the fatigue life of the component under thermal cycles was predicted using the model. The overall component life of the BGA with flip chip package was 818 cycles. For TSOP over BGA this was 1005 cycles, but the shortest life of the BGA was 725 cycles, which may result in the failure of the entire component. The fatigue life prediction using different models is helpful to assess and optimize components at the early stage of product development.
Mechanical Characterization of Polymeric Thin Films Using Nano Cutting Method
이정헌,Hyo Seong Lee,Jae B. Kwak 한국정밀공학회 2020 International Journal of Precision Engineering and Vol.21 No.6
In the ultra-precision industry, physical and chemical coatings are used to provide functional properties to the surface. Especially, it is very important to examine the mechanical durability of polymer thin fi lms with super hydrophobic properties. The mechanical durability of the thin fi lm includes adhesion force and shear strength, and the latter can be measured with precise cutting. A sharp blade capable of changing cutting direction is used for cutting nano-thick fi lms. Then, nano cutting method is used to calculate the shear strength by measuring the resistant forces when the blade cuts thin fi lms simultaneously in the horizontal and vertical directions (slanted cutting). In this study, we prepared two super-hydrophobic fl uoropolymers, p(PFDMA) (1H, 1H, 2H, 2H-perfl uorodecyl methacrylate) thin fi lm and p(V4D4-PFDMA-co-DVB) thin fi lm, using initiated chemical vapor deposition method. Especially, p(V4D4-PFDMA-co-DVB) thin fi lm was obtained by continuous deposition with V4D4 (1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane) and DVB (C 6 H 4 (CH=CH 2 ) 2 , divinylbenzene) in order to improve its durability, and the abrasion test was performed to quantitatively check the durability. In order to qualitatively assess the durability, the thin fi lm was cut with a customized precision-cutting tool, and the shear strength and adhesion force were analyzed. As a result, it was confi rmed that the mechanical durability of p(V4D4-PFDMA-co-DVB) thin fi lm has dramatically improved compared to p(PFDMA) thin fi lm. In addition, the nano cutting theory was successfully verifi ed to evaluate the mechanical properties of thin fi lms.
Improving the mechanical strength of the Mg alloy with Ni electroplating
Yu Rim Kang,Jae B. Kwak 대한기계학회 2021 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.35 No.8
In this study, Ni electroplating is applied to the surface of the lightweight Mg alloy specimen to form a sandwich structure, and strengths improvement is quantitatively compared with the bare Mg alloy. Specifically, the mechanical properties of three specimens were compared one with another using mechanical tensile testing. Accordingly, two Ni electroplated specimens (Ni electroplated only and Ni electroplated with hydroxypropyl cellulose additive) were prepared and compared with the bare Mg alloy. As a result, both Ni electroplated specimens exhibit drastic improvement of the mechanical strength, which is attributed to the interaction between the sandwich structure covered with stiffer Ni material and grain refinement. Consequently, the rule of mixture was used to calculate the intrinsic elastic modulus of the Ni layer from the effective modulus. Finally, the calculated moduli were implemented using FEA (finite element analysis) and compared with the experimental results to examine their accuracy and validity.
Liang, Q.,Seo, G.,Choi, Y.,Kwak, M.J.,Ge, J.,Rodgers, Mary A.,Shi, M.,Leslie, Benjamin J.,Hopfner, K.P.,Ha, T.,Oh, B.H.,Jung, Jae U. Elsevier Inc., Cell Press Imprint 2014 Cell host & microbe Vol.15 No.2
Robust immune responses are essential for eliminating pathogens but must be metered to avoid prolonged immune activation and potential host damage. Upon recognition of microbial DNA, the cytosolic DNA sensor cyclic GMP-AMP (cGAMP) synthetase (cGAS) produces the second messenger cGAMP to initiate the stimulator of interferon genes (STING) pathway and subsequent interferon (IFN) production. We report that the direct interaction between cGAS and the Beclin-1 autophagy protein not only suppresses cGAMP synthesis to halt IFN production upon double-stranded DNA (dsDNA) stimulation or herpes simplex virus-1 infection, but also enhances autophagy-mediated degradation of cytosolic pathogen DNA to prevent excessive cGAS activation and persistent immune stimulation. Specifically, this interaction releases Rubicon, a negative autophagy regulator, from the Beclin-1 complex, activating phosphatidylinositol 3-kinase class III activity and thereby inducing autophagy to remove cytosolic pathogen DNA. Thus, the cGAS-Beclin-1 interaction shapes innate immune responses by regulating both cGAMP production and autophagy, resulting in well-balanced antimicrobial immune responses.