Abstract
A combustion facility which includes uniaxial mechanical loading was implemented that enables environmental conditions more akin to jet engine environments compared to conventional static environment tests. Two types of woven SiC/SiC ceramic matrix composites (CMCs), melt-infiltrated (MI) and chemical vapor infiltrated (CVI), were subjected to fatigue loading in the combustion facility and under isothermal furnace conditions. Some CVI test coupons were coated with a multilayer environmental barrier coating (EBC) of mullite + ytterbium monosilicate using slurry infiltration process to demonstrate the performance with a coating. Combustion conditions were applied using a high velocity oxy fuel gun on the front side of the specimen and mechanical loading was applied using a horizontal hydraulic MTS machine. All the specimens considered were subjected to tension-tension fatigue loading at 100 MPa, stress ratio of 0.1 and specimen front-side surface temperature of 1200 ± 20 °C. Nondestructive evaluation (NDE) methods, such as electrical resistance (ER), was used as an in-situ health monitoring technique. Similar fatigue tests were performed in an isothermal furnace for comparison. A much lower fatigue life was observed for the uncoated specimens tested under combustion conditions in comparison to isothermal furnace condition. This difference in fatigue life was attributed to damage associated with added thermal stress due to the thermal gradient and higher rate of oxidative embrittlement due to the presence of high velocity combustion gases in the combustion environment. EBC coating increased the fatigue life in combustion environment. However, EBC coated specimens experienced spallation in the high-velocity flame due to the presence of micro cracks in the coating surface. Fracture surfaces of the failed specimens were investigated under the scanning electron microscope (SEM) to determine the extent of oxidation and damage.
Highlights
Ceramic matrix composites (CMCs) are candidate materials for future propulsion components because of their high temperature capability, reduced cooling requirements, low weight, and high specific strength
The main objective of this study is an initial investigation toward understanding the effect of combustion environment on the mechanical properties and damage mechanisms of coated and uncoated melt-infiltrated (MI) and chemical vapor infiltrated (CVI) specimens
Uncoated MI, CVI, and environmental barrier coating (EBC) coated CVI test coupons were tested in tension-tension fatigue loading at a peak stress of 100 MPa with combustion flame impinging on front surface
Summary
Ceramic matrix composites (CMCs) are candidate materials for future propulsion components because of their high temperature capability, reduced cooling requirements, low weight, and high specific strength. An increase in turbine inlet temperature with advanced cooling and homogenous combustion increases the efficiency of the jet engine and decreases the NOX emissions [1]. Current nickel-based superalloys employed in jet engines are operating at their maximum operating temperatures with advanced cooling and protective thermal barrier coatings and cannot withstand any increase in temperature. In a water vapor-containing combustion environment, SiO2 volatizes to Si(OH) , leading to material recession and a significantly reduced life of the component [3,4]. The application of EBC coating can protect the material from surface recession, and increase the life of the material
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