Yoshinori Oka

The impact angle dependence of erosion damage caused by solid particle impact

Abstract The dependence on the impact angle of erosion damage caused by solid particle impact was characterized on several kinds of material, as expressed by a trigonometric function of both impact angle and material hardness. Erosion tests were conducted on five metallic materials, a ceramic and a plastic material using silica sand particles (mean particle size 325 μm) at impact angles from 3° to 90°, up to an impact velocity of 130 m s−1. The geometry of the specimen and the nozzle tip were devised in order to obtain erosion damage at shallow impact angles less than 20°. The reproducibility of erosion damage obtained under these conditions was consistent with that of standard erosion tests. The detailed erosion damage at shallow impact angles made the dependence of impact angle clear. Although the amount of erosion damage commonly increased with the increase in impact velocity, it was found that the dependence of the erosion damage of metallic materials on impact angle was independent of the impact velocity within 50–130 m s−1 in terms of the relative erosion damage normalized by that at a normal impact angle. This allowed simplification of the equation for erosion damage curves. It was recognized that the maximum erosion damage and the shape of the damage curves shifted monotonically according to the material hardness, which is considered generally to be correlated with other mechanical properties. As a result, the normalized erosion damage curves for tested materials were basically correlated with a trigonometric function in which constants and exponents depended on the impact conditions, except for the impact velocity, if the properties of the particles were fixed. Moreover, in the case of metallic materials, the values related to the hardness. This simulation probably enables an estimation of erosion damage at a given impact angle from that at the normal impact angle.

Relationship between surface hardness and erosion damage caused by solid particle impact

Abstract A model for erosion caused by solid particles was studied and an equation based upon the relationship between dynamic indentation and the hardness of the material was proposed. This equation was established by the relationship between the surface hardness and the erosion damage to the materials. Erosion tests using a gas gun unit were conducted on five materials over a wide range of hardness values (Hv number 9–1100) up to an impact velocity of 150 m s −1 . An increase in the hardness was seen on the fully eroded surfaces obtained in the cases of iron and aluminium, but a decrease was seen in the case of quenched carbon tool steel. No change in the hardness was seen in the cases of the acrylic resin and nylon. The hardness of the eroded surfaces, as opposed to that of the non-eroded surfaces, slightly improved the correlation with erosion damage, but it was not necessarily sound. The softening observed on the eroded surface of the quenched carbon tool steel specimen suggested that heat was generated during the impact of the particles and also that the surface hardness during the course of erosion was different from that both before and after erosion. The temperature increase of an iron leaf sample owing to the impact of a 3.18 mm steel ball or silica sand particles pointed to a transiently high temperature on the impacted surface and the possibility of softening during the course of the erosion process. As a result, the surface hardness of each material estimated with respect to work hardening and softening reasonably correlated with the erosion damage. Also, numerical formulation and the soundness of this equation were discussed through many erosion tests under various conditions.

Measurements of plastic strain below an indentation and piling-up between two adjacent indentations

Abstract Analyses of plastic strain caused by a dynamic or quasi-static intrusion of a hard steel ball were carried out to compare the plastic strain distributions and to search for triggers of erosion damage to materials around the indentation. Regular square grids with 11.81 line mm −1 were made on the mating surface of a composite block by using a photoengraving method. The two blocks were tightly held in a vice and then indentation tests were conducted at the parting line. Principal shearing strain distributions were obtained for commercially pure aluminium, iron and grey cast iron. In the case of the iron specimen the maximum shearing strain at a dimensionless indentation ratio of d / D = 0.75 was observed below the rim of the dynamic indentation, but below the centre of the quasi-static indentation. The maximum shearing strain shifted from a portion just below the quasi-static indentation into that far below the dynamic indentation for the aluminium specimen. Both the size and form of the elastic-plastic boundary on the cross-sectional surface depended upon the type of the materials and the intrusion processes. Piling-up between two dynamic indentations was simulated to investigate triggers of material removal for the iron specimen. A finite element method simulated not only lip configurations but also plastic strain distributions under the same conditions of quasi-static indentation tests, and produced information about stress distributions around indentations during an intrusion of a projectile.

Exfoliation and Fracture Behavior of Oxide Films Formed on Titanium and Its Alloy in High Temperature Environments

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.

Evaluation of Mechanical Properties for Chromium Carbide Coatings at High Temperature

High-temperature erosion and erosion-corrosion are significant problems for energy conversion systems in power plants. The source is high-temperature dry steam that contains iron oxide and fly ash particles. Compared with high-temperature alloys, ceramic coatings generally have higher resistance to high temperature corrosion and erosion. However, it would be very difficult to obtain mechanical properties for ceramic coating materials in a high temperature environment. With no experimental testing, therefore, the performance of ceramic coatings in an actual plant environment is not typically evaluated before field application. The aim of this paper is to discuss dynamic hardness and fracture toughness as mechanical properties of ceramic coating materials at high temperature, compared with those at room temperature. We chose three types of chromium carbide coating materials that were coated with atmospheric plasma spray (APS), vacuum plasma spray (VPS), and high-velocity oxygen fuel (HVOF) on SUS430 stainless steel.

Dynamic Mechanical Properties of Oxide Films Formed on Metallic Surfaces as Measured Using a Tribological Approach at High Temperature

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 high-temperature 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.