The High Temperature Sodium Sulfate Interactions With Ytterbium Disilicate Environmental Barrier Coatings Systems and Silicon Carbide Ceramic Matrix Composite Materials

It has been known for decades that turbines engines operating in marine environments form molten Na2SO4 species due to an in-situ reaction between ingested NaCl species and S impurities in the fuel. This liquid species leads to a form of deposit-based degradation of Ni-base superalloy hot-section components known as hot corrosion. Currently, SiC ceramic matrix composites (CMCs) have entered service in the hot-section of GE Aviation’s LEAP engines as non-loading bearing components, with future implementation to replace load-bearing Ni-base superalloy components a current research and development focus. SiC CMC turbine engine components have many attractive properties which include a slow-forming SiO2 thermally grown oxide (TGO), excellent high temperature capability, and lower density relative to their Ni-based superalloy counterparts. Their SiC fiber reinforcement phase/BN matrix fiber interphase/SiC matrix allows for increased toughness relative to monolithic SiC. An environmental barrier coating system is required to prevent rapid recession to the underlying SiC CMC component due to reaction with steam present as a product of combustion. A Yb2Si2O7 topcoat + Si bond coat bi-layer is a highly studied EBC system due to many of its attractive properties. However, many aspects of the performance of the SiC CMCs and Yb2Si2O7 + Si bond coat EBC systems are unaddressed, including its reaction with Na2SO4 deposits. This dissertation investigated; (i) the high-temperature interaction of Na2SO4 deposits with Hi Nicalon, Hi Nicalon S, Sylramic, and Sylramic-iBN SiC fibers relevant for application in SiC CMCs; (ii) Na2SO4 reaction with the BN interphase material and their on SiC oxidation rates, surface degradation, and reaction layer crystallization; (iii) the interaction and penetration behavior of Na2SO4 deposits with state-of-the-art air plasma sprayed (APS) Yb2Si2O7 topcoat-Si bond coat EBC systems on SiC CMC substrates; and (iv) the influence of Na2SO4 on Si bond coat oxidation kinetics and thermally grown oxide (TGO) crystallization behavior. Numerous characterization techniques revealed that exposure of SiC CMC and Yb2Si2O7 topcoat and Si bond coat EBC systems to Na2SO4 results in a range of degradation mechanisms including rapid SiC fiber corrosion, accelerated SiC corrosion due to formation of low-melting sodium borosilicate phases resulting from reaction with the BN matrix fiber interphase, granulation of Yb2Si2O7 EBC topcoats and reaction to form Yb2SiO5 and Na silicates, and increased TGO growth kinetics and crystallization.