Hatsuhiko Usami

Long-Term Creep Damage Development in a Self-Reinforced Silicon Nitride

Advanced silicon nitride ceramics are candidate materials for high temperature structural parts in the future generation of gas turbines and reciprocal engines with increased efficiency [1]. The applications of silicon nitride in ceramic rotors require high endurance of intensive tensile stresses acting over prolonged periods. Consequently, detailed characterization of creep behavior and creep lifetime data are necessary to achieve longterm reliability of the material at working conditions and for the designing of such parts.

Effects of Penetrated Graphite on Tribological Properties of Copper Based Journal Bearing

Tribological properties of copper based porous alloy with penetrated micro size graphite layer were evaluated using a ring on disc testing apparatus in dry condition. A shot peening treatment was applied to ensure the dimple for the graphite penetration. Specimens with various area fraction of the graphite layer were prepared. It was found that the friction coefficient and the wear loss decreased with the increase in the area fraction. However, the decrease of the friction and the wear loss became smaller at excess graphite area fraction.

Effect of Indenter Materials on Indentation Fracture of Alumina Ceramics

The present study describes the effect of indenter materials on Hertzian cracking behavior of alumina ceramics in sphere indentation. Numerical analysis (FEM) was carried out to investigate the influence of the frictional resistance at the interface due to the elastic mismatch between the indenter and the flat specimen on the stress distribution near the contact area. Two kinds of alumina ceramics with different mechanical properties were used for the flat specimen. Materials of the sphere indenter were cemented carbide, silicon nitride, alumina and hardened steel. The indenter was penetrated into the specimen with an electro-mechanical testing apparatus at a constant cross-head-speed in air. The indentation fracture was monitored with the acoustic emission signal. The numerical analysis revealed that the frictional resistance had influenced on the stress distribution. The experimental data showed that the ring crack radius, the crack pass below the contact zone and the indentation strength were different depending on the indenter material. From the comparison between the numerical analysis and the experimental data, it was found that the minimum value of the ring crack radius of the low density alumina flat specimen corresponds to the position of the maximum tensile stress.

Applicability of Interrupted Micro Cutting Process “Tilling” as Surface Texturing

Tribological properties of textured surfaces fabricated using a discontinuous microcutting process were investigated. Aluminum cast alloy (AC8A) discs were used for the specimens. The texturing process was carried out using a CNC machining center with a cutting edge with a novel geometry. The resulting surface morphology consisted of micro dimples with a diameter of 200-300 μm and a depth of 5-10 μm, with controlled area fractions of 10 and 40% and a top region finished with a milling cut. The tribological properties were evaluated using a ring-on-disc type testing apparatus under lubricated conditions, and showed that the friction coefficient of the textured surfaces was low and stable from the beginning of the experiment. A dependence of the friction coefficient on the area fraction was also found. Further reductions in the friction coefficient were achieved on a textured surface with a polished top region. It can be concluded that the proposed discontinuous micro cutting process is an effective means of fabricating a micro texture for the reduction and stabilization of frictional resistance.

Effects of Machining History on Tribological Properties of Bronze Containing Micro-Sized Sulfide

Bronze based alloys have come to be used as bearing materials, and require low frictional properties. The present study describes the effects of surface finish machining history on the on tribological properties of sulfide-containing bronze sintered onto a steel disc. The sulfide phase consisted of copper, iron and sulfur, and acted as a solid lubricant. The relative density of the specimen was 90 % and the microstructure showed that the micro-sized sulfide dispersed into the bronze matrix and was accompanied by pores. Various processes, including cutting, burnishing, polishing were, applied to finish the surface. It was found that the area fraction of the sulfide phase depended on the cutting conditions. A further increase in the area fraction was achieved using roller burnishing after the cutting. Tribological properties were evaluated using chromium alloy steel (SUJ2) as the mating surface under dry conditions. The results showed that the friction coefficient depended on the surface finishing process. Therefore, the machining history had an effect on the frictional properties of the sulfide-containing bronze. Optimal cutting conditions for the sulfide containing bronze are also discussed.

Micro Abrasive Jet Machining of Silicon Carbide (SiC) Fiber Reinforced Ceramic Matrix Composite

Abesive jet machining (AJM) of silicon carbide fiber reinforced silicon carbide ceramic composite (SiC/SiC CMC) was carried out with various size of silicon carbide fine abresives. A micro indentation experiment was connected to evaluate of maerial removal mechanism by the particle impact. Results showed that the machine rate was different depending on the particle size and that inteface fracture (debonding) has influenced on the material removal mechanim. Relationship between structure scale of the SiC/SiC CMC and the impact media size was discussed.

Relationship between crack growth and wear mechanism of brittle materials during sliding contact

Abstract Effects of crack growth on wear mechanisms of brittle materials such as ceramics were evaluated using a testing apparatus having a reciprocal movement at a constant frequency with various normal loads in air. Soda lime glass bars and alumina spheres were mainly used for specimens. The friction and wear behavior showed that the wear of the glass depended on the crack growth near the wear scar and that the coefficient of friction was greater corresponding to the wear rate. Critical stresses for crack growth in sliding contact were determined from indentation damage technique. The crack growth below the wear scar was estimated from inert strength measurements after the friction experiment. The maximum stress at the trailing edge in sliding contact was calculated from elastic contact theory. It was confirmed that the tangential traction at the interface had great influence on the stress at the trailing edge and that crack growth occurred during sliding contact when the stress exceeded the critical value for crack growth.

Behavior of crack growth resistance in toughened silicon nitride ceramics

Testing methods for crack-growth resistance-curve (R-curve) behavior were investigated and developed to analyze the toughened mechanism in polycry stalline ceramics. These methods are a biaxial-flexure method for small-scale disc-shaped specimens with micro-indentation cracks and a single-edged notched beam flexural method with crack stabilizers. In both methods, the growing crack length is measured directly as a function of applied stress, using the system that consist of a microscope and a CCD camera. Applying these testing methods, R-curve behavior of a toughened silicon nitride with a preferred orientation of elongated grains was evaluated to characterize the toughened mechanism, comparing with the behavior in a commercially available silicon nitride. The behavior having these rising R-curves is discussed with emphasis on the effects of microstructure such as grain-growth and grain-orientation, and resultant grain-bridgings behind the crack-tip.

Evaluation of Creep Damage Development of Quasi-Plastic GPS Silicon Nitride by X-Ray CT

Silicon nitride is one of the candidate materials for high temperature applications, such as components for gas turbine and reciprocal combustion engines. In order t improve efficiency of heat engines, it is important to raise the operating temperature. The structural materials used at the elevated temperatures thus require high levels of strength, oxidation and creep resistance.