Abstract
Gas turbine engines provide propulsion for aircraft, ships, and some ground vehicles and are also widely used for electric power generation. The materials and coatings used in these engines are undergoing rapid development to increase engine fuel efficiency. As future engine gas temperatures rise towards 1800 °C, new materials are being investigated to enable this rise in temperature and a variety of facilities have been developed to allow experimental investigation of individual or pairs of the three main variables; temperature, environment, and stress. Due to the high-temperature capability of SiC/SiC Ceramic Matrix Composites (CMCs) compared to conventional superalloys, this material has become the lead candidate material for next-generation gas turbine engines. However, SiC-based CMCs oxidize at high temperatures and form a solid silica thermally-grown oxide (TGO) layer. This subsequently reacts with water vapor present in the high-temperature combustion environment and is converted to a gaseous silicon hydroxide (Si(OH)4). To prevent this volatilization of the SiC, environmental barrier coatings (EBCs) must be applied to CMC engine components. However, no facility currently exists to allow the study of the interactions of all three variables in a manner consistent with fundamental scientific research. This Ph.D. dissertation details the process of designing, assembling and testing the only laboratory-scale testing facility that controllably simulates the mechanical loads, the spatial/temporal temperature gradients, and the chemical environments present in gas turbine engines. The facility has been designed to enable investigation of the mechanisms by which materials in engines are degraded by the environmental attack from water vapor and oxygen-containing gases at surface temperatures up to 1800 °C and pressures up to 12 atm in the presence of static and cyclic mechanical stresses that can exceed 300 MPa.
The test facility’s first use was for an assessment of the failure modes of SiC-based composites coated with environmental barrier coatings applied using an air plasma spray (APS) approach. The dissertation describes the successful use of the LSL facility to investigate the effect of thermal gradients, the interaction between thermal shock and thermal gradients, and the interaction between thermal shock, thermal gradient, and steam exposure in the form of an impinging jet on a Yb2Si2O7-Si multilayer EBC system attached to a SiC-based CMC under atmospheric pressure and 1300-1400 °C surface temperatures. The test results indicated that the in-plane thermal gradients are primarily responsible for the formation of channel cracks in the Yb2Si2O7 layer upon cooling to room temperature. As the thermal shock rates become increased (to > 800 °C/s), the possibility of channel crack formation due to cold shock in the Yb2Si2O7 also increased since the exterior Yb2Si2O7 cooled much faster than the SiC/SiC substrate. Interestingly, the channel cracks in Yb2Si2O7 and Si appeared to partially heal upon reheating. The addition of a 0.15 atm steam partial pressure to the hot air flow led to a 30% increase in silica TGO thickness compared to a dry air environment. Over the 36hr exposure time, the coating progressively debonded at the Yb2Si2O7-SiO2 interface as the SiO2 TGO progressively thickened. The presence of steam also led to the formation of a ~ 2.4 µm thick Yb2SiO5 ytterbium monosilicate volatilization layer formed by the reaction of SiO2 in the ytterbium disilicate with water vapor to form Si(OH)4 gas. Upon cooling to room temperature, the high thermal expansion coefficient of Yb2SiO5 also contributed to the development of vertical cracks. However, these cracks did not undergo healing upon reheating, and so provided a direct path for oxidants to reach underlying layers upon reheating.
