Microstructural characterization of boron-containing SiCreinforced SiC composites exposed at high temperature in high-water-vapor-pressure environments was used to determine surface recession rates and to understand the controlling degradation processes under these conditions. Results showed that composite degradation was controlled by a series of reactions involving the formation of silica, boria, borosilicate glass, and gaseous products. Comparison of results (from characterization of composites exposed at 1200°C and 1.5 atm of H 2 O in a laboratory furnace and in the combustion zone of a gas turbine) showed that these reactions were common to both exposure conditions and, consequently, there was little effect of gas velocity on degradation rates of boron-containing SiC/SiC composite materials. High-Temperature Water Vapor Effects

Solar Turbines Incorporated (Solar) under U.S. government sponsored programs has been evaluating ceramic matrix composite (CMC) combustor liners in test rigs and Solar Centaur® 50S engines since 1992. The objective was to evaluate and improve the performance and durability of CMCs as high temperature materials for advanced low emissions combustors. Field testing of CMC combustor liners started in May 1997 and by the end of 2004, over 67,000 operating hours have been accumulated on SiC/SiC and oxide/oxide CMC liners. NOx and CO emissions measured were < 15 ppmv and < 10 ppmv, respectively. Long test durations of 15,144 hrs and 13,937 hrs have been logged for SiC/SiC liners with protective environmental barrier coatings (EBCs). An oxide/oxide CMC liner with a Friable Graded Insulation (FGI) coating has been tested for 12,582 hrs. It was observed that EBCs significantly improve SiC/SiC CMC liner life. The basic three-layer EBC consists of consecutive layers of Si, mullite, and barium strontium aluminum silicate (BSAS). The durability of the baseline EBC can be improved by mixing in BSAS with mullite in the intermediate coating layer. The efficacy of replacing BSAS with SAS has not been demonstrated yet. Heavy degradation was observed for two-layer Si/BSAS and Si/SAS EBCs, indicating that the elimination of the intermediate layer is detrimental to EBC durability. Equivalent performance was observed when the Hi-Nicalon fiber reinforcement was replaced with Tyranno ZM or ZMI fiber. Melt infiltrated (MI) SiC/SiC CMCs have improved durability compared to SiC/SiC CMCs fabricated by Chemical Vapor Infiltration (CVI) of the matrix, in the absence of an EBC. However, the presence of an EBC results in roughly equivalent service life for MI and CVI CMCs. Early results indicate that oxide/oxide CMCs with protective FGI show relatively minor degradation under Centaur 50S engine operating conditions. The results of and lessons learned from CMC combustor liner engine field testing, conducted through 2004, have been summarized.

Solar Turbines Incorporated, under U.S. government sponsored programs, has been evaluating ceramic matrix composite combustor liners in test rigs and Solar’s Centaur® 50S gas turbine engines since 1992. The objective is to evaluate and improve the performance and durability of CMCs as high-temperature materials for advanced low emissions combustors. Field testing of CMC combustor liners started in May of 1997 and by the end of 2004, over 67,000 operating hours had been accumulated on SiC∕SiC and oxide∕oxide CMC liners. NOx and CO emissions have been consistently <15ppmv and <10ppmv, respectively. Maximum test durations of 15,144h and 13,937h have been logged for SiC∕SiC liners with protective environmental barrier coatings. An oxide∕oxide CMC liner with a Friable Graded Insulation coating has been tested for 12,582h. EBCs significantly improve SiC∕SiC CMC liner life. The basic three-layer EBC consists of consecutive layers of Si, mullite, and BSAS. The durability of the baseline EBC can be improved by mixing BSAS with mullite in the intermediate coating layer. The efficacy of replacing BSAS with SAS has not been demonstrated yet. Heavy degradation was observed for two-layer Si∕BSAS and Si∕SAS EBCs, indicating that the elimination of the intermediate layer is detrimental to EBC durability. Equivalent performance was observed when the Hi-Nicalon fiber reinforcement was replaced with Tyranno ZM or ZMI fiber. Melt infiltrated SiC∕SiC CMCs have improved durability compared to SiC∕SiC CMCs fabricated by Chemical Vapor Infiltration of the matrix, in the absence of an EBC. However, the presence of an EBC results in roughly equivalent service life for MI and CVI CMCs. Results to date indicate that oxide∕oxide CMCs with protective FGI show minor degradation under Centaur® 50S gas turbine engine operating conditions. The results of, and lessons learned from CMC combustor liner engine field testing, conducted through 2004, have been summarized.

Oxide/Oxide Ceramic Matrix Composites (CMCs) are an attractive class of materials for gas turbine hot section applications. The oxide fiber reinforcement and inherent matrix porosity contributes to favorable fracture toughness and thereby enhanced resistance against impact by foreign objects. Also, the oxide composition ensures superior environmental resistance against accelerated attack by corrosive species in the gas turbine hot section and resulting surface recession typically observed in silicon-based ceramic monolithic and composite materials. Under a program sponsored by the US National Institute of Standards and Technology (NIST) a hybrid oxide/oxide CMC system has been developed with potential application for stationary gas turbine hot section components. COI Ceramics, Inc. has fabricated subscale and full scale combustor liners which have been evaluated in rig and engine testing at Solar, and in field testing in a Solar Centaur® 50S engine at a commercial end user site. Following the conclusion of the NIST program in June 2003 the engine field testing is being continued under a Solar-led program sponsored by the US Dept. of Energy (DOE). As of November 2004, a hybrid oxide/oxide CMC outer combustor liner has accumulated 12,582 field test hours with 63 starts and an extensive material experience base has been developed. The paper will summarize the progress to-date for this hybrid CMC combustor liner development and demonstration, including selected fabrication approach, NDE, and rig/engine test experience.