Solid State Phenomena Vol. 127

Abstract: Thermal barrier coatings (TBC) have been used to improve the efficiency of turbine engine by providing the capability to sustain significant temperature gradient across the coating. TBC failure occurs easily at the interface between the metallic bondcoat and topcoat. Alumina was proposed as a potential candidate as an interlayer to improve the oxidation resistance of thermal barrier coating due to its low oxygen diffusivity against the harsh environment. The mechanical properties, thermal behaviour and high temperature oxidation resistance of the coatings formed by gas tunnel type plasma spraying were investigated in this study. The results showed that this system exhibits the improvement of mechanical properties of the coating and oxidation resistance. This interlayer is preferred in order to minimize the detrimental effect of phase transformation of γ- Al2O3 to α-Al2O3.

Abstract: In magneto-plasma-dynamic (MPD) arcjet generators, plasma is accelerated by electromagnetic body forces. Silicon nitride reactive spraying was carried out using an MPD arcjet generator with crystal silicon rods and nitrogen gas. Because higher-velocity, higher-temperature and higher-density and larger-area plasmas are produced with the MPD arcjet generator than those with conventional thermal plasma torches, nitriding of silicon can be enhanced. A dense and uniform β-Si3N4 coating with 30 μm in thickness was formed after 200 shots at a repetitive frequency of 0.03 Hz with a discharge current of 9 kA and a substrate temperature of 700 °C. The Vickers hardness reached about 1300. Furthermore, silicon carbide and aluminum nitride sprayings were conducted with some spraying systems. All results showed that the MPD arcjet generator had high potentials for spraying.

Abstract: For the improvement of a surface wear resistance of aluminum alloys, iron-based alloys were clad on the surface of an aluminum alloy by laser. By investigating the effect of CO2 and diode laser irradiation conditions on the formability of Fe-Cr-C clad layers, it was confirmed that the application of diode laser made it possible to obtain stable beads in low heat input compared with CO2 laser, which has been conventionally used for laser cladding. Besides, by optimizing the laser parameters, the dilution ratio of clad layers by Al substrate was minimized less than 10%. At the clad interface, the reaction layer consisting of Fe2Al5 and FeAl3 formed which caused cracks at the clad interface. Even by controlling laser conditions, this brittle layer formation could not suppressed. In this research, by using the immiscible Fe-Cu-Cr alloys as clad materials, Fe based clad layers were formed on Al substrate without any cracks at clad interface. By the effect of Marangoni motion of decomposed duplex liquid phases, Cu segregated between Al substrate and Fe based clad layers and suppressed the reaction between them.

Abstract: The present study tried to control grain structure in polycrystalline aluminum or titanium by means of a laser spot heating method as well as by a computer simulation technique. Monte Carlo simulation of spot heating was performed utilizing two-dimensional grain structure model composed of 200 x 200 sites with hexagonal cells. Grain growth proceeded preferentially in the higher temperature region and resulted in a large grain surrounded by small grains. This large grain tended to keep on growing during homogeneous heating after the spot heating, suggesting that the spot heating technique can fabricate a peculiar grain structure such as different grain size distribution and texture that are not realized by utilizing conventional uniform heating process. Laser spot heating utilizing a fiber-laser system realized heating of small area such as 30micorn diameter to bring about peculiar grain size distribution as calculated by the computer simulation.

Abstract: Butt resistance welding of super duplex stainless steel by inserting type 316L stainless steel wires was investigated. When the base material temperature was increased from room temperature to 1100 oC at the heating rate of 550 oC /sec, base materials were jointed through the insert wires and HAZ (heat affected zone) of the joint interface were less than 80 μm. In this joining technique, the insert wires played a role of concentrating current on the wires and increasing their temperature up to melting point or near melting point. When the welding was performed at a load of 10 N, the insert wires consisted of ferrite and austenite growing along the ferrite grain boundary. When the welding was performed at a load of 70N, insert wire remained austenite. That is because the contact resistance between insert wire and base materials at 70 N was lower than that of 10 N, and consequently the insert wire were not adequately heated.