Toru Akiba

Damage behaviour of silicon nitride for automotive gas turbine use when impacted by several types of spherical particles

Spherical particle impact damage and strength degradation phenomena of silicon nitride by several types of spheres were analysed in comparison with chipping fracture behaviour reported in the literature. It was found that steel and partially stabilized zirconia (PSZ) particles caused Hertzian cone cracks, resulting from the elastic response of the material in accordance with the Hertzian cone crack theory. In contrast, alumina and sialon particles induced both median-radial crack systems at low impact velocity range and Hertzian cone cracks at high impact velocity range. The critical impact velocity for strength degradation,Vc, correlated with the hardness of the spheres and target ceramics, andVc, for Hertzian cracks and median cracks were higher than that for chipping fracture.

Impact damage behaviour of CVD-coated silicon nitride for gas turbines

Impact tests were conducted on the silicon nitride substrates coated with Si3N4 and SiC by chemical vapour deposition (CVD). For both 100- and 200-μm-thick Si3N4-coated silicon nitride, Hertzian crack extension was reduced by debonding at the interface. Although Hertzian crack extension was not reduced for 100-μm-thick SiC-coated silicon nitride, it was reduced for 200-μm-thick SiC-coated silicon nitride. Theoretical calculations suggest that debonding at the interface consumed the fracture energy of Hertzian crack extension in the case of Si3N4 coatings, but it was observed that Hertzian cracks were not arrested at the interface.

Thickness dependence of impact damage behavior in silicon nitride ceramic

Abstract The thickness dependence of the impact damage behavior of a silicon nitride ceramic was investigated. For specimens 1·0 mm in thickness, sudden strength degradation occurred over an impact velocity of approximately 250 m/s with Hertzian cone crack or bending fracture. Hertzian cone cracks in 1·0 mm specimens were thought to be enlarged by the scabbing effect. For 1·5–3·0 mm specimens, strength degradation occurred above a 380 m/s impact velocity with Hertzian cone crack. Therefore, the specimen thickness was related to crack initiation behavior resulting in the strength degradation.

Thickness dependence of impact damage behavior in sialon ceramic

Publisher Summary The aim of this chapter is to study impact damage behavior of ceramics for turbine use, and this is analyzed by various researchers using the Hertzian conefracture theory. Although recent studies have analyzed stress caused by foreign particle impact, the thickness dependence of impact damage and fracture is still not fully understood. It discusses the thickness dependence of impact damage shown by turbine ceramics that was examined experimentally. Computer simulation results indicated that the stress level generated at the target surface became the strength level of the materials and it coincided with surface ring crack formation at impact. This calculation also exhibited that the stress level at the back face became higher than the materials strength. The stress calculations support the explanation of fracture behavior.

Impact damage on non-oxide ceramics for automotive turbine use

An investigation was conducted into spherical impact damage and strength degradation of two types of silicon nitrides, SiC, and SiC-f/CVI-SiC composite intended for automobile Gas Turbine rotor use. The surface damage morphology and strength degradation phenomena were different for each ceramic, resulting from differences in their mechanical property-related response behaviors. Fracture behavior upon impact in SiC-f/CVI-SiC was different from that of other monolithic ceramics because there is no typical crack but spalling after impact.