Long-Term Degradation of Ceramics for Gas Turbine Applications

Abstract

A study has been conducted to establish the effect of long-term (30,000+ hours) properties of monolithic ceramics (Si3N49 SiC), SiC/SiC and oxide/oxide ceramic matrix composites (CMCs), and protective coatings on component life in gas turbine engines with pressure ratios (PRs) ranging from 5:1 to 30:1. A model has been presented that shows the interaction between two major long-term degradation modes of ceramics, creep and degradation from water vapor attack in the ceramic hot section. Water vapor attack is the most severe mode overshadowing creep for long-term (∼30,000 hours) gas turbine operation, and its impact on component durability becomes more severe as PR increases. Components in the turbine hot section, downstream from the combustor (blades, integral turbine rotors, nozzles), fabricated from Si3N4 without protective coatings, have a temperature limitation of ∼800°C for gas turbines with PR ranging from 5:1 to 30:1. These ceramic components afford little, if any, advantage over metallic components for improving gas turbine performance. The application of a BSAS-type Environmental Barrier Coating (EBC) would improve temperature capability of turbine nozzles and rotating parts to ∼1100–1200°C. For small low-PR (5:1) engines, thick (∼10 mm) uncoated monolithic silicon-based combustor liners can be used up to ∼1360°C and thinner (∼3 mm) SiC/SiC CMCs up to ∼1100°C. These temperatures are reduced for higher-PR engines. The incorporation of a BSAS-type EBC improves temperature capability of silicon-based ceramic combustor liners. Oxide/oxide CMCs with protective coatings have a predicted temperature capability of ∼1220-∼1380°C over the range of PR range studied. They can be used as structural materials for combustor liners and other stationary turbine hot section components. As PR increases the durability of coated oxide/oxide CMCs improves relative to that of silicon-based monolithics and CMCs. As expected, ceramic component durability increases for shorter component design lives, making these materials more acceptable for shorter-term applications, such as automotive transportation (∼3,000 hours/150,000 km).