Dowon Seo

Effects of Ce and Si Additions to CoNiCrAlY Bond Coat Materials on Oxidation Behavior and Crack Propagation of Thermal Barrier Coatings

In thermal barrier coating (TBC) systems, thermally grown oxide (TGO) forms at the interface between the top coat and the bond coat (BC) during service. Delamination or spallation at the interface occurs by the TGO formation and growth. Therefore, modifications of the BC materials are one means to inhibit the TGO formation and to improve the crack resistance of TBCs. In this study, morphologies of TGO were controlled by using Ce and Si additions to conventional CoNiCrAlY BC material. The evaluation of the crack resistance was carried out using acoustic emission methods under pure bending conditions. As a result, when the BCs of TBCs with Ce added were aged at 1373 K over 10 h, the morphologies of the TGO were changed drastically. The BC materials of TBCs coated with Ce added indicated an improved crack resistance with high-temperature exposure. It is expected that the morphologies can improve the crack resistance of TBCs.

Theoretical Consideration of Nondestructive Testing by use of Vertical Magnetization and Magneto-Optical Sensor

This paper describes a new magnetization method for non-destructive testing with magneto-optical sensor (denoted as MO sensor) which have the following characteristic; high observation sensitivity, independence of the crack orientation, and precise imaging of a complex crack geometry such as multiple cracks. When a magnetic field is applied normally to the surface of a specimen which is significantly larger than its defects, approximately the same magnetic charge per unit area occurs on the surface of the specimen. If there is a crack in the specimen, magnetic charge per unit area has the same value at the bottom of the crack. The distribution of the vertical component of the magnetic flux density, Bz, is almost uniform over the no-crack area (denoted as BZ, BASE), while the magnetic flux density is smaller in the surroundings of the crack (denoted as BZ, CRACK) : If BZ, BASE is a bit larger than the saturated magnetic flux density of the MO sensor (BS), then small magnetic domains occur over the crack area and a large domain over the non-crack area because BZ, CRACK is smaller than Bs.

Repair of Turbine Blades Using Cold Spray Technique

Hot section parts of combined cycle gas turbines are susceptible to degradation due to high temperature creep, crack formation by thermal stress, and high temperature oxidation, etc. Thus, regularly repairing or replacing the hot section parts such as gas turbine blades, vanes, and combustion chambers is inevitable. For this purpose, revolutionary and advanced repair technologies for gas turbines have been developed to enhance reliability of the repaired parts and reduce the maintenance cost of the gas turbines. The cold spraying process, which has been studied as not only a new coating technology but also as a process for obtaining a thick deposition layer, is proposed as a potential repairing solution. The process results in little or no oxidation of the spray materials, so the surfaces stay clean, which in turn enables superior bonding. Since the operating temperature is relatively low, the particles do not melt and the shrinkage on cooling is very low. In addition, this technique is based on high velocity (3001200 m/s) impinging of small solid metallic particle (generally 5-50 ┤m in diameter) to the substrate. In this spray process, the particles are accelerated by the subsonic gas jet which is usually lower than melting temperature of feedstock. Consequently, this process has solved the problem of thermal spraying, i.e. oxidation and phase transformation. A cold spray system is also simpler than a than low pressure plasma spraying (LPPS) system. Therefore, it has a possibility to apply cold spray technique instead of welding to repair the cracks. In this chapter, it is described that the possibility of applying cold spray technique for repairing the Ni-base turbine blades and its characteristic.

Characterization of Thermally Grown Oxide on Cold Sprayed CoNiCrAlY Bond Coat in Thermal Barrier Coating

This paper presents the results of a study of the microstructure and oxidation behavior of thermal barrier coating (TBC) with air plasma sprayed (APS) yttria-stabilized zirconia (YSZ) top coat and CoNiCrAlY bond coat deposited using two different spraying techniques, low pressure plasma spray (LPPS) and cold spray (CS). The objective is to investigate the thermally grown oxide (TGO) thickness and oxide scale composition of TBC subjected to isothermal oxidation and creep tests at 900 °C by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectrometry (EDX) analyses in order to evaluate the reliability of the CS technique. It was found that the TGO thicknesses for TBC with CS bond coats were smaller and the TGO was composed of mainly alumina with little or no mixed oxides. TGO growth rate was also affected by the applied stress. Smaller TGO thicknesses were observed for the non-creep TBC for both CS and LPPS bond coats. Overall findings indicate that the oxidation behavior of the TBC with CS bond coat is superior compared to that of the TBC with LPPS bond coat.

Isothermal Oxidation Behavior of Plasma Sprayed MCrAlY Coatings

Thermal spray coatings are deposited in an ambient atmosphere or vacuum chamber. Although vacuum plasma spray (VPS) coating is deposited inside vacuum, oxygen can penetrate into the flame during spraying process, as in high velocity oxygen-fuel (HVOF) spraying (C.J. Li & W.Y. Li, 2003). This causes the spray materials to be exposed directly to an oxidizing atmosphere. This oxidation significantly influences the phase composition, microstructure, properties and performance of the sprayed coatings. Metal oxides are grown on the lamellar interface. The oxides are brittle and have different thermal expansion coefficients than that of the metal, the inclusion of which may cause the spalling of the coating (Neiser et al., 1998). Moreover, the inclusion of oxides in the MCrAlYs (where M is the alloy base metal; typically nickel, cobalt, or combination of these two), the coatings will degrade the resistance to sulfur and vanadium, etc., under high temperature corrosion. The presence of the oxides in steel coating also affects its mechanical properties (Volenik et al., 1997). However, some coating properties can be improved by metal oxides in sprayed coatings. A typical example is the improvment of wear resistance (Neiser et al., 1998). The deposited oxides increase also the hardness of the coating (Dobler et al., 2000). Therefore, it is important to understand the oxidizing behavior of spray materials at spraying.

Influence of water saturation on fracture toughness in woven natural fiber reinforced composites

Woven sisal textile fiber reinforced composites were used to evaluate fracture toughness, tensile and three-point bending. The water absorption testing of all specimens was repeated five times in this study. All specimens were immersed in pure water during 9 days at room temperature, and dried in 1 day at 50°C. Two kinds of polymer matrices such as epoxy and vinyl-ester were used. Fractured surfaces were taken to study the failure mechanism and fiber/matrix interfacial adhesion. It is shown that it can be enhanced to improve their mechanical performance to reveal the relationship between fracture toughness and water absorption fatigue according to different polymer matrices. Water uptake of the epoxy composites was found to increase with cycle times. Mechanical properties are dramatically affected by the water absorption cycles. Water-absorbed samples showed poor mechanical properties, such as lower values of maximum strength and extreme elongation. The K IC values demonstrated a decrease in inclination with increasing cyclic times of wetting and drying for the epoxy and vinyl-ester.

Shape Design of Adhesive Joints for Strength Improvement of Epoxy Adhesive Structures

Adhesive-bonded joints are widely used in the industry. Recently, applications of adhesive bonding joints have been increased extensively in automobile and aircraft industry. The strength of adhesive joints is influenced by the surface roughness, adhesive shape, stress distribution, and etc. However, the magnitude of the influence has not yet been clarified because of the complexity of the phenomena. In this study, as the fundamental research of adhesive bonding joints, the effects of adhesive shape and loading speed on bonding strength properties and durability of aluminum to polycarbonate single-lap joints were studied. To evaluate the effect of adhesive shape, several modified shapes were used, and loading speeds were varied from 0.05 to 5 mm/min. As a result, the load-displacement distribution was shown a brittle fracture tendency. The trigonal edged single lap and bevelled lap joints showed the higher strength than the plain single lap, trigonal single lap, joggle lap and double lap joints in same adhesive area. The fractures of trigonal single lap and trigonal edged single lap joints that had the higher strength level were shown as the mixture type of the cohesive and interfacial-failure, mostly joggle lap joints that had the lower strength level were shown as the adhesive-failure.

Effects of Water Absorption and Surface Treatment on Mechanical Properties of Sisal Textile Reinforced Composites

Woven sisal textile reinforced composites were manufactured to evaluate fracture toughness, and tensile test. All specimens were immersed in water five times. All specimens are immersed in pure water during 9 days at room temperature, and dried in 1 day at . Two kinds of polymer matrices such as epoxy and vinyl-ester were used. Fractured surface were investigated to study the failure mechanism and fiber/matrix interfacial adhesion. It is shows that it can be enhanced to improve their mechanical performance to reveal the relationship between fracture toughness and water absorption fatigue according to different polymer matrices. Water uptake of the epoxy composites was found to increase with cycle times. Mechanical properties are dramatically affected by the water absorption cycles. Water-absorbed samples observed poor mechanical properties such as lower values of maximum strength and extreme elongation. The values demonstrate a decrease in inclination with increasing cyclic times of wetting and drying fur the epoxy and vinyl-ester.

Effect of Temperature and Water Immersion on Strength Properties of Spot Welded Zincked Steel Plates

A spot welded structures have an influence on a diverse climatic situation, for instance temperature, humidity and precipitation. In addition factors of environmental pollution such as acid rain, that courses corrosion, have the tendency to increase. But spot welded structures strength is affected by humidity and environment temperature. Therefore, it is important to evaluate effect of temperature and water immersion on strength properties of spot welded part. In this study, the strength distribution of spot welded plates is evaluated the environmental temperature of zinc coated steel plates. Test is conducted with welded part immersed in distilled and synthetic sea water. Specimens are immersed into water for 10, 100, 500 and 1000 hours to evaluate the offsets of water immersion time on tensile-shear strength under the conditions of -40, 0, 20 and 5. From this result, spot welded specimens with 5 mm clearance have lower tensile-shear strength in the distilled water or synthetic sea water than without clearance. And they have lower tensile-shear strength under -4 and over 5.