It is very difficult to obtain mechanical properties of oxide films formed on a
material in high temperature environments despite its importance of estimating material
degradation caused by such as thermal stress. Corrosion/oxidation tests were conducted for
pure titanium and titanium alloy in high temperature corrosive environments of wet air and
water vapor with hydrogen chloride at temperatures from 673 K to 973 K to look into basic
behavior of degradation and the growth of titanium oxide films. It was found that oxide films
were usually formed on the specimen surface and the growth was accelerated by the
corrosiveness of the environment. In order to examine mechanical properties and exfoliation of
corrosion products or oxide films formed on titanium and its alloy, tests of single particle
impact on the specimen surface with a glass bead were performed in high temperature
corrosive environments. The piling-up surfaces around impact craters were formed and
plastically strained. The oxide film formed on the metal surface was detached in a wide range
of the circumference and fractured a little far from the rim of the crater. Then fracture and
exfoliation stress of the oxide film were estimated by the calculation of impact energy and
fractured and detached areas. It was found that both the fracture and exfoliation stress of the
oxide films were different depending on the corrosive environment and chemical composition
of titanium alloy.
The aim of this paper is to investigate fundamental damage behaviour of metallic materials such as aluminum, carbon steel and titanium, which have typically different crystal structures, by the single impact from low velocity to hypervelocity. Impact tests by a WC ball were performed on the three metallic materials at various impact velocities and at normal angle. Both two-stage light gas gun and ordinary air gas gun were used in this study. Indentation shapes on a cross-sectional surface and the ratios of crater diameter and depth to the diameter of the WC ball were examined. Three modes were recognized in indentation configurations independent of the type of materials. Average contact pressure, which was obtained by the impact energy divided by indentation volume, showed not only a mechanical property of the target material during the impact process but also fracture strength of the projectile, A single particle impact test at a shallow impact angle and at a hypervelocity showed huge material removal even if its single impact and suggested the difference in mechanism of the material removal between subsonic and hypervelocity impacts.
The surface degradation of metals in boiler tubes and turbines in high-temperature corrosive environments causes severe problems in fuel combustion power plant systems. High-temperature resistant materials have been recently developed using a thermal barrier coating (TBC) and high-chromium alloys. Oxide films or coatings formed on metal surfaces at high temperatures can sometimes decrease the corrosion rate. However, the damage to the material is often accelerated by the mechanical removal of corrosion products from the material surface. It is therefore very important to investigate the mechanical and adhesive properties of the oxide films or coatings on metal surfaces used in high-temperature environments. This paper introduces a tribological method that uses a single spherical projectile impact at high temperature to measure the mechanical and adhesive properties of oxide films formed on various metal surfaces. Impact tests were performed on the surfaces of oxide films after their growth in a hightemperature furnace, and the deformed or fractured surfaces were observed in order to measure the mechanical and adhesive properties. The mechanical and adhesive properties of an elastic modulus, fracture, and exfoliation stresses were measured using the impact method, and the results depended on the type of metal oxide films and on the high-temperature environment.
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