• Korean Journal of Materials Research
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• v.32 no.4
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• pp.200-209
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• 2022

In this study, defects generated in the YSZ coating layer of the IN738LC turbine blade are investigated using an optical microscope and SEM/EDS. The blade YSZ coating layer is composed of a Y-Zr component top coat layer and a Co component bond coat layer. A large amount of Cr/Ni component that diffused from the base is also measured in the bond coat. The blade hot corrosion is concentrated on the surface of the concave part, accompanied by separation of the coating layer due to the concentration of combustion gas collisions here. In the top coating layer of the blade, cracks occur in the vertical and horizontal directions, along with pits in the top coating layer. Combustion gas components such as Na and S are contained inside the pits and cracks, so it is considered that the pits/cracks are caused by the corrosion of the combustion gases. Also, a thermally grown oxide (TGO) layer of several ㎛ thick composed of Al oxide is observed between the top coat and the bond coat, and a similar inner TGO with a thickness of several ㎛ is also observed between the bond coat and the matrix. A PFZ (precipitate free zone) deficient in γ' (Ni₃Al) forms as a band around the TGO, in which the Al component is integrated. Although TGO can resist high temperature corrosion of the top coat, it should also be considered that if its shape is irregular and contains pore defects, it may degrade the blade high temperature creep properties. Compositional and microstructural analysis results for high-temperature corrosion and TGO defects in the blade coating layer used at high temperatures are expected to be applied to sound YSZ coating and blade design technology.

• Journal of the Korean Ceramic Society
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• v.45 no.9
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• pp.549-555
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• 2008

The thermal barrier coating must withstand erosion when subjected to flowing gas and should also maintain good stability and mechanical properties while it must also protect the turbine component from high temperature, hot corrosion, creep, and oxidation during operation. In this study we investigated the influence of subsurface layer, $Al_2O_3$ or NiCrCoAIY bond coat layer, on the indentation damage behavior of YSZ thermal barrier coating layers deposited by electron beam physical vapor deposition (EB-PVD). The bond coat is deposited using different process such as air plasma spray (APS) or spray of high velocity oxygen fuel (HVOF) and the thickness is varied. Hertzian indentation technique is used to induce micro damages on the coated layer. The stress-strain behaviors are characterized by results of the indentation tests.

• Journal of Welding and Joining
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• v.13 no.2
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• pp.139-149
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• 1995

Many pressure vessels for the power plants are fabricated from low alloy ferritic steels. The inner sides of the pressure vessels are commonly weld_cladded with austenitic stainless steels to minimize problems of corrosive attack. The submerged-arc welding(SAW) process is now used in preference to other processes because of the possibilities open to automation to reduce the overaII welding times. The most reliable way to avoid underclad cracks(UCC) which are often detected at the overlap of the clad beads is to use nonsusceptible steels such as SA508 class 3. At present domestically developed forging steel of SA508 cl.S is now being cladded with single layer by using 90mm wide strip, which transfers higher heat input into the base metal compared to the conventional two layers strip cladding which has been in wide use with 30-60 mm wide strip. But the current indices for the influence of heat input on crack susceptibility are not accurate enough to express the subtle difference in crack susceptibility of the steel. Therefore, the purpose of this present study is: l) To determine UCC susceptibility on domestic forging steel, SA508 cl.S cladded with single layer by using submerged arc 90mm strip and, 2) To optimize heat input range by which the crack susceptibility could be eliminated.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2005.06a
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• pp.1349-1352
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• 2005

This paper is to investigate a defect for thermal barrier coating layers by acoustic emission method in 4-point bending test. The two-layer thermal barrier coating is composed of $150\mu{m}\;CoNiCrAlY\;bond\;coating\;by\;vacuum\;plasma\;spray(VPS)\;process\;and\;250\mu{m}\;ZrO_2-8wt%Y_2O_3$ ceramic coating layer by air plasma spray(APS) process on Inconel-718. The specimen prepared by cyclic thermal test(500, 1000, 2000cycle) at $1050^{\circ}C$ The AE monitoring system is composed of PICO type sensor, a wide band pre-amplifier(40dB), PC and AE DSP(16/32 PAC) board. The AE event, amplitude, Cumulative energy and count of coating specimens is evaluated according to cyclic thermal test.

• Structural Engineering and Mechanics
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• v.15 no.3
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• pp.331-344
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• 2003

The plane contact problem for two infinite elastic layers whose elastic constants and heights are different is considered. The layers lying on a Winkler foundation are acted upon by symmetrical distributed loads whose lengths are 2a applied to the upper layer and uniform vertical body forces due to the effect of gravity in the layers. It is assumed that the contact between two elastic layers is frictionless and that only compressive normal tractions can be transmitted through the interface. The contact along the interface will be continuous if the value of the load factor, ${\lambda}$, is less than a critical value. However, interface separation takes place if it exceeds this critical value. First, the problem of continuous contact is solved and the value of the critical load factor, ${\lambda}_{cr}$, is determined. Then, the discontinuous contact problem is formulated in terms of a singular integral equation. Numerical solutions for contact stress distribution, the size of the separation areas, critical load factor and separation distance, and vertical displacement in the separation zone are given for various dimensionless quantities and distributed loads.

• Journal of Adhesion and Interface
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• v.4 no.2
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• pp.21-29
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• 2003

Hot AC anodising has been evaluated us pre-treatment for aluminium prior to structural adhesive bonding. Phosphoric and sulphuric acid hot AC anodising showed very promising adhesion promoter capabilities with durability comparable with the best standard DC anodising procedures. AC anodising does not required etching prior to anodising and offers u pre-treatment time down to 20 seconds. The interface/interphase between the aluminium substrate and the adhesive was investigated in order to get a better understanding of the involved adhesion mechanisms and to explain the long-tenn properties. The alkaline medium formed at the oxide layer/adhesive interface has been shown to induce a partial dissolution of the oxide layer leading to the formation of metallic ions which diffuse in the adhesive (EPMA measurements). The effect of diffusion of the Al ions on adhesion and joint durability is still uncertain but studies showed that pre-bond moisture affected the joints durability and to some extent the diffusion length. specially for DC anodised samples. So far no direct correlation could be established between the diffusion length d and the joints durability but new trials with better control over the elapsed time between bonding and adhesive curing are expected to help getting a better understanding of the involved mechanisms.

• Journal of Welding and Joining
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• v.34 no.5
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• pp.77-82
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• 2016

In this study, Microstructure and hardness of 1st layer with crystallographic orientation were investigated about solidification structure in multipass weld using high Mn-Ni flux cored wire. Microstructure of solidification consisted of austenite matrix and a little ${\varepsilon}-phase$ in grain boundaries. Orientation of grains was usually (001), (101), (111). According to crystallographic orientation, morphology of primary dendrite was different. The depletion of Fe and the segregation of Mn, C, Ni, Si, Cu, Cr, O were found along the grain boundaries. The area of segregation was wide with an order of (001), (101), (111) grains. And hardness of grains with crystallographic orientation increased with an order of (001), (101), (111) grains because of the segregation along dendrite boundary.

• Journal of the Korean Society for Precision Engineering
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• v.32 no.3
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• pp.269-277
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• 2015

Compressed natural gas (CNG) composite vessels for vehicles have been generally made of 34CrMo4 for a inner liner part and E-glass/epoxy for a composite layer part. But, there is a problem of material loss of CNG composite vessels used in vehicles due to the design of excessive thickness of the liner. And, light weight of the CNG composite vessel is required for improving fuel efficiency. In this study, optimal design for CNG composite pressure vessel was performed by using basalt fiber, which is the environment-friendly material having a good mechanical strength. The optimal thickness of each part (inner liner and composite layer) was determined by theoretical analysis and FEA for satisfying structural safety and lightweight of the vessel. Also, for improving fatigue life, optimal autofrettage pressure was derived from FEA results.

• Corrosion Science and Technology
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• v.2 no.1
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• pp.53-57
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• 2003

The MarM247 based superalloy (8wt.%Cr- 9wt.%Co- 3wt.%Ta- 1.5wt.%Hf- 5.6%wt.Al- 9.5wt.%W- Bal. Ni) specimens were diffusion aluminized by for types of pack cementation methods, and their coating structure and their high temperature oxidation resistance were investigated. The coated specimens treated at 973K in high aluminum concentration pack had a coating layer containing large hafunium rich precipitates, which were originally included in substrate alloy. After the high temperature oxidation test in air containing 30 vol.% $H_2O$ at 1273K ~ 323K, the deep localized corrosion which reached to the substrate were observed along with these hafnium rich precipitates. On the other hand, the coated specimens treated at 1323K using low aluminum concentration pack showed the coating layer without the large hafunium rich precipitates, and after the high temperature oxidation test at 1273K for 1800 ksec, it did not show the deep localized corrosion. The nickel electroplating before the aluminizing forms thick hafnium free area, and its high temperature oxidation resistance were comparable to platinum modified aluminizing coatings at 1273K.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.9 no.11
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• pp.4623-4643
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• 2015

This paper considers physical-layer security protocols in multicast cognitive radio (CR) networks. In particular, we propose dual-hop cooperative decode-and-forward (DF) and randomize-and-forward (RF) schemes using partial relay selection method to enhance secrecy performance for secondary networks. In the DF protocol, the secondary relay would use same codebook with the secondary source to forward the source's signals to the secondary destination. Hence, the secondary eavesdropper can employ either maximal-ratio combining (MRC) or selection combining (SC) to combine signals received from the source and the selected relay. In RF protocol, different codebooks are used by the source and the relay to forward the source message secretly. For each scheme, we derive exact and asymptotic closed-form expressions of secrecy outage probability (SOP), non-zero secrecy capacity probability (NzSCP) in both independent and identically distributed (i.i.d.) and independent but non-identically distributed (i.n.i.d.) networks. Moreover, we also give a unified formula in an integral form for average secrecy capacity (ASC). Finally, our derivations are then validated by Monte-Carlo simulations.