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
The development of ceramic materials for incorporation into the hot section of gas turbine engines has been ongoing for about fifty years. Researchers have designed, developed, and tested ceramic gas turbine components in rigs and engines for automotive, aero-propulsion, industrial, and utility power applications. Today, primarily because of materials limitations and/or economic factors, major challenges still remain for the implementation of ceramic components in gas turbines. For example, because of low fracture toughness, monolithic ceramics continue to suffer from the risk of failure due to unknown extrinsic damage events during engine service. On the other hand, ceramic matrix composites (CMC) with their ability to display much higher damage tolerance appear to be the materials of choice for current and future engine components. The objective of this paper is to briefly review the design and property status of CMC materials for implementation within the combustor and turbine sections for gas turbine engine applications. It is shown that although CMC systems have advanced significantly in thermo-structural performance within recent years, certain challenges still exist in terms of producibility, design, and affordability for commercial CMC turbine components. Nevertheless, there exist some recent successful efforts for prototype CMC components within different engine types.
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.
Ceramic gas turbine development that started in the 1950s has slowed considerably since most of the large-scale ceramic gas turbine development programs of the 1970s–1990s ended. While component durability still does not meet expectations, the prospect of significant energy savings and emissions reductions, potentially achievable with ceramic gas turbines, continues to justify development efforts. Four gas turbine applications have been identified that could be commercially attractive: a small recuperated gas turbine (microturbine) with ∼35% electrical efficiency, a recuperated gas turbine for transportation applications with ∼40% electrical efficiency with potential applications for efficient small engine cogeneration, a ∼40% efficient mid-size industrial gas turbine and a ∼63% (combined cycle) efficient utility turbine. Key technologies have been identified to ensure performance and component durability targets can be met over the expected life cycle for these applications. These technologies include: a Si3N4 or SiC with high fracture toughness, durable EBCs for Si3N4 and SiC, an effective EBC/TBC for SiC/SiC, a durable Oxide/Oxide CMC with thermally insulating coating, and the Next Generation CMCs with high strength that can be used as structural materials for turbine components for small engines and for rotating components in engines of various sizes. The programs will require integrated partnerships between government, national laboratories, universities and industry. The overall cost of the proposed development programs is estimated at U.S. $100M over ten-years, i.e. an annual average of U.S. $10M.
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