Air plasma sprayed yttria-stabilized zirconia thermal barrier coatings (TBCs) have been successfully used to extend life of superalloy components in utility gas turbines. GE Power Generation has over ten years of field experience with TBCs on combustor hardware, and over 20,000 hours of field experience with TBCs on turbine nozzles. Despite this promising experience, the full advantage of TBCs can be achieved only when the reliability of the coating approaches that of the superalloy component substrate. Recent work at GE has emphasized characterization of mechanical properties and physical attributes of TBCs to understand better the causes of delamination crack growth and coating spallation. In addition, unique tests to examine the TBC response under conditions simulating severe gas turbine service environments have been developed. Through evaluation of the results from comparative TBC ranking tests, pre-production application experience and field test results, gas turbine design engineers and materials process engineers are rapidly gaining the practical knowledge needed to integrate the TBC into the component design.

A ceramic gas seal for a utility gas turbine was designed and analyzed using ANSYS and CARES/LIFE. SN-88 silicon nitride was selected as the candidate material. The objective was to validate the failure prediction methodology using rectangular plates which were thermally shocked in a fluidized bed. The failure prediction methodology would then be applied to the representative component geometry. Refined ANSYS finite element modeling of both the plate and component geometries was undertaken. The CARES/LIFE reliability analysis of the component geometry for fast fracture was performed for two cases: I) steady-state thermo-mechanical loads during normal gas turbine operation and II) transient thermal shock loading during a turbine trip. Thermal shock testing of alumina disks were performed in order to gain confidence in the testing and analysis procedures. Both notched and unnotched SN88 plates were then tested. Failure modes were identified through flexure tests and data censoring was performed using SAS. Weibull modulus was assumed to be invariant with temperature and the scale parameter was assumed to vary through a scaling variable such that multiple data could be pooled.

A three year program to evaluate the feasibility of using monolithic silicon nitride ceramic components in gas turbines was conducted. The use of ceramic materials may enable design of turbine components which operate at higher gas temperatures and/or require less cooling air than their metal counterparts. The feasibility evaluation consisted of three tasks: 1) Expand the material properties database for candidate silicon nitride materials, 2) Demonstrate the ability to predict ceramic reliability and life using a conceptual component model and 3) Evaluate the effect of proof testing on conceptual component reliability. The overall feasibility goal was to determine whether established life and reliability targets could be satisfied for the conceptual ceramic component having properties of an available material. Fast and delayed fracture reliability models were developed and validated via thermal shock and tensile experiments. A creep model was developed using tensile creep data. The effect of oxidation was empirically evaluated using four-point flexure samples exposed to flowing natural gas combustion products. The reliability- and life-limiting failure mechanisms were characterized in terms of temperature, stress and probability of component failure. Conservative limits for design of silicon nitride gas turbine components were established.

This paper presents initial results in the development and testing of SiC-based Continuous Fiber Ceramic Composites (CFCC) materials for combustor and stage 1 shroud components of large utility-class gas turbines. Use of CFCC’s for these components has the potential for increasing output power and thermal efficiency and reducing emissions. First stage turbine shroud components were fabricated using five material systems including three SiC/SiC-Si systems made by silicon melt infiltration (MI), chemical vapor infiltrated (CVI) enhanced SiC-SiC and directed metal oxidation (DIMOX) Al 2 O 3 -SiC composite. A combustor liner was made of MI CFCC. Before and after testing the components were inspected by several NDE techniques including IR thermography, resonance testing and visual examination. A novel, high pressure test rig was used to test four shroud components and a combustor liner simultaneously. Components were exposed to hot gas temperature of 1200°C at 12.5 bar in cyclic and steady-state tests. Cyclic testing simulated engine trip conditions with 200 flame-on, flame-off cycles. Steady state testing involved 100 hours of exposure at high temperature and pressure with hot combustion gases. At the conclusion of the first phase of testing there was visible damage to two pieces of one of the material systems. Destructive testing of the components following rig exposure showed little degradation to the MI composite materials. In summary, high pressure combustion rig testing of these components demonstrated excellent performance with little degradation among the material systems.