• Korean Journal of Materials Research
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• v.8 no.4
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• pp.293-298
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• 1998

The fracture behavior of $Si_3N_4N$, coated $Si_3N_4N$-BN composite was studied by the Hertzian indentation technique. New types of contact-induced cracks were found, and it was confirmed that these cracks have cone crack geometry. Contact damage was distributed in the substrate layer, which can absorb energy, as well as in the coating layer, so the propagation of initiated cracks in the coating layer were suppressed.

• Journal of Welding and Joining
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• v.20 no.6
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• pp.99-99
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• 2002

In order to evaluate the proper fracture strength of microstructures in a steel weldment, smaller size loading ball than used for a conventional small punch(CSP) testing is required due to regional limitation on constitutive structures. In this study, the minimized loading ball(φ 1.5mm) and bore diameter of lower die(φ 3mm) were designed for an advanced small punch(ASP) test. The results obtained from the ASP test were compared with those from a CSP testing for a X20CrMoV121 steel weldment. It was found that the ASP test clearly showed the microstructural dependance on fracture strength and ductile-brittle transition behavior of the weldment. In the ASP test, especially, the cracks tend to initiate for various directions within hemispherical indentation region of an objective structure in SP test. This indicates that the evaluation of more proper fracture strength for F.L+CGHAZ, FGHAZ and ICHAZ can be performed by means of the ASP test in a steel weldment.

• Journal of Welding and Joining
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• v.20 no.6
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• pp.823-829
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• 2002

In order to evaluate the proper fracture strength of microstructures in a steel weldment, smaller size loading ball than used for a conventional small punch(CSP) testing is required due to regional limitation on constitutive structures. In this study, the minimized loading ball(${\varphi}1.5mm$) and bore diameter of lower die(${\varphi}3mm$) were designed for an advanced small punch(ASP) test. The results obtained from the ASP test were compared with those from a CSP testing for a X20CrMoV121 steel weldment. It was found that the ASP test clearly showed the microstructural dependance on fracture strength and ductile-brittle transition behavior of the weldment. In the ASP test, especially, the cracks tend to initiate for various directions within hemispherical indentation region of an objective structure in SP test. This indicates that the evaluation of more proper fracture strength for F.L+CGHAZ, FGHAZ and ICHAZ can be performed by means of the ASP test in a steel weldment.

• Nuclear Engineering and Technology
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• v.53 no.9
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• pp.2960-2967
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• 2021

In this work, a new hardening model is proposed for the depth-dependent hardness of ion-irradiated polycrystals with obvious grain size effect. Dominant hardening mechanisms are addressed in the model, including the contribution of dislocations, irradiation-induced defects and grain boundaries. Two versions of the hardening model are compared, including the linear and square superposition models. A succinct parameter calibration method is modified to parametrize the models based on experimentally obtained hardness vs. indentation depth curves. It is noticed that both models can well characterize the experimental data of unirradiated polycrystals; whereas, the square superposition model performs better for ion-irradiated materials, therefore, the square superposition model is recommended. In addition, the new model separates the grain size effect from the dislocation hardening contribution, which makes the physical meaning of fitted parameters more rational when compared with existing hardness analysis models.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 2004.11a
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• pp.215-220
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• 2004

Molecular Dynamics(MD) method was used to investigate the change of friction force due to interaction between dislocations and a grain boundary when a Ni tip was scratched on a Cu bicrystal. The substrate comprised a Cu bicrystal containing a vertical$\Sigma=5(210)$ grain boundary. The moving tip for scratching simulation was consisted of fixed Ni atoms emulating a rigid tip. The indentation depth was $3.6\AA$ and the scratching was performed along <110>direction in the first grain. As the scratching was continued, nucleation and propagation of dislocations were observed. In the early stage, the grain boundary played as a barrier to moving dislocations and interrupting further dislocation movement with no dislocation resulting in no propagation across the grain boundary. As the Ni tip approached the grain boundary, dislocations were nucleated at the grain boundary and propagated to the second grain. However, stick-slip phenomena that were observed on a single crystal scratching were not observed in the bicrystal. And, instead, irregular oscillation of friction force was observed during the scratching due to the presence of a grain boundary.

• Tribology and Lubricants
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• v.21 no.4
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• pp.177-184
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• 2005

The molecular dynamics simulation of nanoimprint lithography (NIL) using $SiO_2$ stamp and amorphous poly-(methylmethacrylate) (PNMA) film is performed to study pattern transfer in NIL. Force fields including bond, angle, torsion, van der Waals and electrostatic potential are used to describe the intermolecular and intramolecular force of PMMA molecules and $SiO_2$ stamp. Nose-Hoover thermostat is used to control the system temperature and cell multipole method is adopted to treat long range interactions. The deformation of PMMA film is observed during pattern transfer in the NIL process. For the detail analysis of deformation characteristics, the distributions of density and stress in PMHA film are calculated. The adhesion and friction forces are obtained by dividing the PMMA film into subregions and calculating the interacting force between subregion and stamp. Their effects on the pattern transfer are also discussed as varying the indentation depth and speed.

• Macromolecular Research
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• v.14 no.5
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• pp.559-564
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• 2006

The improvement of spatial resolution is a fundamental issue in the two-photon, polymerization-based, laser writing. In this study, a voxel tuning method using a radical quencher was proposed to increase the resolution, and the quenching effect according to the amount of radical quencher was experimentally investigated. Employing the proposed method, the lateral resolution of the line patterns was improved almost to 100 nm. However, a shortcoming of the quenching effect was the low mechanical strength of polymerized structures due to their short chain lengths. Nano-indentation tests were conducted to evaluate quantitatively the relationship between mechanical strength and the mixture ratio of the radical quencher into the resins. The elastic modulus was dramatically reduced from an average value of 3.015 to 2.078 GPa when 5 wt% of radical quencher was mixed into the resin. Three-dimensional woodpile structures were fabricated to compare the strength between the resin containing radical quencher and the original resin.

• Journal of the Korean Ceramic Society
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• v.43 no.8 s.291
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• pp.492-497
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• 2006

SiC coating has been introduced as protective layer in TRISO nuclear fuel particle of High Temperature Gas cooled Reactor (HTGR) due to excellent mechanical stability at high temperature. In order to inhibit the failure of the TRISO particles, it is important to evaluate the fracture strength of the SiC coating layer. ]n present work, thin silicon carbide coating was fabricated using chemical vapor deposition process with different microstructures and thicknesses. Processing condition and surface status of substrate.affect on the microstructure of SiC coating layer. Sphere indentation method on trilayer configuration was conducted to measure the fracture strength of the SiC film. The fracture strength of SiC film with different microstructure and thickness were characterized by trilayer strength measurement method nanoindentation technique was also used to characterize the elastic modulus and th ε hardness of the SiC film. Relationships between microstructure and mechanical properties of CVD SiC thin film were discussed.

• Corrosion Science and Technology
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• v.18 no.3
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• pp.92-101
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• 2019

This study investigates how rivet head height affects the corrosion performance of carbon fiber reinforced plastic (CFRP) to aluminum alloy self-piercing riveted joints. Specimens with two different head heights were prepared. A rivet head protruding out of the top CFRP laminate forms the proud head height while a rivet head penetrating into the top CFRP generates the flush head height. The salt spray test evaluated corrosion performance. The flush head joints suffered from severe corrosion on the rivet head. Thus, the tensile shear load of flush head joints was substantially reduced. Electrochemical corrosion tests investigated the corrosion mechanisms. The deeper indentation of the flush head height damaged the CFRP around the rivet head. The exposure of damaged fibers from the matrix increased the cathodic potential of local CFRP. The increased potential of damaged CFRP accelerated the galvanic corrosion of the rivet head. After the rivet head coating material corroded, a strong galvanic couple was formed between the rivet head base metal (boron steel) and the damaged CFRP, further accelerating the flush rivet head corrosion. The results of this study suggest that rivet head flushness should be avoided to enhance the corrosion performance of CFRP to aluminum alloy self-piercing riveted joints.

• Journal of Welding and Joining
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• v.32 no.6
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• pp.1-7
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• 2014

For the resistance spot welds of CR1180 and GA1180 TRIP steels, the weld quality evaluation method using the digitalized X-ray transmission imaging apparatus was investigated in comparison with the crosssectional examination method. In the case of the resistance spot welding of CR1180, three circular regions, such as WZ(white zone), GZ(grey zone) and DZ(dark zone), appeared on X-ray image and they corresponded to the diameters of indentation mark, nugget and corona bond, respectively. The variation of X-ray transmission thickness due to the thickness variation of the weld seemed to be mainly responsible for the formation of those contrasts. The X-ray image contrast formed from the variation of transmission thickness at the outer border line of DZ could also enable the inspections of the notch shape, nonuniformity of the welding pressure and spatter from its sharpness, concentricity and the normal straight line, respectively. The X-ray image of the resistance spot weld of galvannealed GA1180 TRIP steel was very similar to that of CR1180 TRIP steel except the crown shaped outer border line of DZ which was considered to be due to the melting behavior of zinc having the boiling temperature even lower than the melting temperature of steel.