Intermediate Temperature Degradation of Turbine Cooling Holes in a SiC/SiC Composite

Date of Award

8-1-2024

Degree Name

Ph.D. in Materials Engineering

Department

Department of Chemical and Materials Engineering

Advisor/Chair

Ronald Kerans

Abstract

Ceramic matrix composites (CMC) are candidate high-temperature materials for gas turbine engines. GTE components that could utilize CMCs include exhaust diffusers, combustor shrouds, and turbine blades and vanes (1, 2). However, current gas path temperatures are high enough that superalloy metals and even CMCs require cooling air drawn from the compressor for turbomachinery that is immediately downstream of the combustor. This cooling air will subject those blade and vane interior cooling passages and cooling holes to intermediate temperature air at a water vapor partial pressure near 1 atm. Specimens from a Hi-Nicalon Type S/BN/SiC (MI) composite with multiple machined holes were subjected to a furnace dwell and then tensile tested at room temperature. The furnace temperature range was 500°C-1100°C and the atmosphere was pure steam. Pure steam at 1 atmosphere approximated the sea level partial pressure of water vapor in the cooling air going through turbomachinery cooling holes. Exposure of control specimens was done in laboratory air at the same temperatures as the steam furnace specimens. Data included tensile results along with digital image correlation, computed tomography, microscopy, and spectroscopy results. The hole array was analytically modeled to predict its resulting stress concentration factor; test results showed that the hole array caused a lower stress concentration factor than analytical modeling predicted. There was significant strength degradation in some conditions due primarily to oxidation of the boron nitride fiber coating, with coating volatilization at lower temperatures and probable fiber to matrix bonding and imposed fiber stresses at higher temperatures. The greatest strength degradation occurred in the 500°C-600°C steam range. The fiber coating oxidation/volatilization was especially evident near the holes and was exacerbated in the lower temperature tests by porosity networks from composite processing that were open to the atmosphere at fiber tow ends at the specimen edges and along the inner walls of the holes.

Keywords

boron nitride; ceramic matrix composites; oxidation; silicon carbide; volatilization

Rights Statement

Copyright © 2024, author.

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