Kentaro Shinoda

Jet Engine Coatings for Resisting Volcanic Ash Damage

The severe disruption in air travel and the economic loss caused by the explosive eruption of the Eyjafjallajokull volcano in Iceland reminds us of the enormous threat posed to aircraft jet engines by volcanic ash clouds over vast airspace. The nature of damage to gas-turbine jet engines flying through ash clouds depends on several factors, one of which is the temperature in the hottest parts of the engine. The maximum achievable operating temperature in modern jet engines has been increasing steadily in response to a strong demand for higher power output which scales with that temperature This is enabled by: (1) improvements in metal-alloys used to make hot-section engine components; (2) advanced air-cooling technologies that cool them; and (3) the use of ceramic thermal barrier coatings (TBCs) to insulate them. Higher operating temperatures also make modern jet engines more vulnerable to ash damage because ingested ash melts and adheres to TBC surfaces that can be as hot as 1200°C. This can result in buildup of a molten-glass deposit that penetrates into the TBCs causing them to spall-off, exposing the bare metal to dangerously hot gases. In extreme cases where the ash concentration is very high, catastrophic engine failure can occur due to heavy ash blockage of engine components. It is recognized that ingestion of some amount of ash by engines may be inevitable. A similar threat to engines is posed by airborne silicate sand particles which, although lower in concentration at flight altitudes, are more pervasive. There is a growing need to build protective measures within modern jet engines against damage from a broad range of undesirable silicate deposits. Jet engine coatings are discussed in this paper.

Effect of Deposition Rate on the Stress Evolution of Plasma-Sprayed Yttria-Stabilized Zirconia

The deposition rate plays an important role in determining the thickness, stress state, and physical properties of plasma-sprayed coatings. In this article, the effect of the deposition rate on the stress evolution during the deposition (named evolving stress) of yttria-stabilized zirconia coatings was systematically studied by varying the powder feed rate and the robot-scanning speed. The evolving stress during the deposition tends to increase with the increased deposition rate, and this tendency was less significant at a longer spray distance. In some cases, the powder feed rate had more significant influence on the evolving stress than the robot speed. This tendency can be associated with a deviation of a local deposition temperature at a place where sprayed particles are deposited from an average substrate temperature. At a further higher deposition rate, the evolving stress was relieved by introduction of macroscopic vertical cracks as well as horizontal branching cracks.

Transition From GMR to AMR at the Percolation Threshold in Ferrite-Magnetic Alloy Composites

A thermal spray process has been developed for the deposition of ferrites and highly anisotropic composites of magnetic alloys and ferrites. The critical volume fraction for conduction is at 5 to 10 volumetric percent (vol.%) metal in these thick films as compared to 33 vol.% in isotropic mixtures of spheroidal particles. The low percolation threshold and highly anisotropic transport properties are associated with the unusual microstructure of thermal spray coatings which are composed of lamellar splats stacked one upon the other. By varying the volume fraction of metal in these composites it is possible to drastically change the electrical, magnetic and mechanical properties of these coatings. In the sintered mixture the conductivity increases rapidly when the metal volume fraction reaches about 33 vol.% whereas in the thermal spray coating the conductivity increase occurs at metal fractions of about 2-6 vol.%. The magnetoresistance was measured in the parallel, perpendicular and transverse geometry in each case in fields up to 2 kilogauss. There is a clear and sharp transition from giant magnetoresistance (GMR) (negative ΔR/R) to anisotropic magnetoresistance (AMR) at the percolation threshold in both cases. These results show that the GMR comes from the scattering of weakly exchange coupled spins in the polycrystalline ferrite which can be aligned in high fields decreasing the resistance. The AMR is in the conductive metallic phase and its field dependence is mainly controlled by the shape anisotropy of the disk shaped splats as expected.