J. R. Price

Ceramic Matrix Composite Combustor Liners: A Summary of Field Evaluations

Solar Turbines Incorporated, under U.S. government sponsored programs, has been evaluating ceramic matrix composite combustor liners in test rigs and Solars 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. NO X and CO emissions have been consistently <15 ppmv and <10 ppmv, respectively. Maximum test durations of 15,144 h and 13,937 h 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 12,582 h. 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 Ml and CVI CMCs. Results to date indicate that oxide/oxide CMCs with protective FG1 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.

Ceramic Gas Turbine Materials Impact Evaluation

Impact of foreign or domestic material on components in the hot section of gas turbines with ceramic components is a common cause of catastrophic failure. Several such occurrences were observed during engine testing under the Ceramic Stationary Gas Turbine program sponsored by the U.S. Department of Energy. A limited analysis was carried out at Solar Turbines Incorporated (Solar), which involved modeling of the impact in the hot section. Based on the results of this study an experimental investigation was carried out at the University of Dayton Research Institute Impact Physics Laboratory to establish the conditions leading to significant impact damage in silicon-based ceramics. The experimental set up involved impacting ceramic flexure bars with spherical metal particulates under conditions of elevated temperature and controlled velocity. The results of the study showed a better correlation of impact damage with momentum than with kinetic energy. Increased test specimen mass and fracture toughness were found to improve impact resistance. Continuous fiber-reinforced ceramic composite (CFCC) materials have better impact resistance than monolithics. A threshold velocity was established for impacting particles of a defined mass. Post-impact metallography was carried out at Oak Ridge National Laboratory to further establish the impact mechanism.© 2002 ASME

Evaluation of ceramic coatings on silicon carbide

Abstract Ten ceramic coatings were plasma sprayed onto a sintered alpha silicon carbide (SiC) tube to determine if the coatings would protect SiC from high-temperature corrosion. Several coatings consisted of mullite or mullite-zircon. Others had base layers of mullite or mullite-zircon, transition layers with increasing amounts of alumina (Al 2 O 3 ), zirconia (ZrO 2 ) or yttria (Y 2 O 3 ), and outer layers of these last three oxides. Mullite and mullite-zircon coatings, which have coefficients of thermal expansion (CTE) near to that of SiC, survived 60 thermal cycles between 300 and 1300°C without apparent damage. Coatings having Al 2 O 3 and Y 2 O 3 layers cracked, while those with ZrO 2 layers cracked and spalled. Survivors were corrosion tested at 1200°C in an oxidizing atmosphere containing Na 2 CO 3 , which corroded uncoated SiC at a rate of about 21 mm year -1 . The coatings protected SiC for up to 500 h, but degraded because the atmosphere penetrated and reacted with the coatings and SiC to produce a disruptive silicate reaction product. Denser coatings are needed for extended protection.

POTENTIAL APPLICATIONS OF STRUCTURAL CERAMIC COMPOSITES IN GAS TURBINES

A Babcock and Wilcox solar turbines team has completed a program to assess the potential for structural ceramic composites in turbines for direct coal-fired or coal gasification environments. A review was made of the existing processes in direct coal firing, pressurized fluid bed combustors, and coal gasification combined cycle systems. Material requirements in these areas were also discussed. The program examined the state of the art in ceramic composite materials. It was shown that utilization of ceramic composites in the turbine rotor blades and nozzle vanes would provide the most benefit. A research program designed to introduce ceramic composite components to these turbines was recommended.

Ceramic Oxide Coatings for the Corrosion Protection of Silicon Carbide

Silicon carbide is currently used as a structural material for heat exchanger tubes and related applications because of its excellent thermal properties and oxidation resistance. Silicon carbide suffers corrosion degradation, however, in the aggressive furnace environments of industrial processes for aluminum remelting, advanced glass melting, and waste incineration. Adherent ceramic oxide coatings developed at Solar Turbines Incorporated, with the support of the Gas Research Institute, have been shown to afford corrosion protection to silicon carbide in a simulated aluminum remelt furnace environment as well as in laboratory-type corrosion testing. The coatings are also protective to silicon carbide-based ceramic matrix composites.

25,000-Hour Hybrid Oxide CMC Field Test Summary

A Hybrid Oxide CMC Liner was evaluated in field testing of a Solar Turbines Incorporated Centaur® 50S engine for 25,404 hours and 109 starts between 2003 and 2006. The Hybrid Oxide CMC configuration consisted of a Nextel 720 aluminosilicate fiber/alumina matrix CMC and overlaying Friable Graded Insulation (FGI). The liner remained functional throughout the engine test. Borescoping during the first half of the field testing revealed little visual damage to the CMC liner, but erosion, affecting about 1–2 mm of the initial ∼ 5mm FGI thickness was noticed upon inspection of the liner after the midpoint of the field test. The erosion correlated with the hottest areas of the liner surface. Field testing was continued after minor liner repair. Damage to one area of the liner surface was observed in borescoping during the second half of the field test. Visual inspection after completion of the engine test indicated large areas of considerable CMC porosity/delamination within a few plies from the OD surface. Residual thickness measurements following the engine field testing indicated that of the original thickness, 2.0 – 2.6 mm of FGI had been lost. The surface recession is in the range expected based on a degradation model for alumina-based CMCs.Copyright

Recommended Direction of the Solar/DOE Ceramic Stationary Gas Turbine Program

Since the advent of the gas turbine more than 60 years ago, performance improvements have followed advances in the mechanical properties of hot section materials. Higher firing temperatures allow increases in cycle efficiency and specific power, becoming key differentiators in the performance of these products. The development of single crystal superalloys and the use of increasingly sophisticated active cooling techniques have been the enabling technologies in this continuing quest. As these metal-based technologies approach their practical limits, the possibilities offered by ceramic materials are attracting more and more attention.Copyright

Potential Applications of Structural Ceramic Composites in Gas Turbines

A Babcock and Wilcox - Solar Turbines Team has completed a program to assess the potential for structural ceramic composites in turbines for direct coal-fired or coal gasification environments. A review is made of the existing processes in direct coal firing, pressurized fluid bed combustors, and coal gasification combined cycle systems. Material requirements in these areas were also discussed.The program examined the state-of-the-art in ceramic composite materials. Utilization of ceramic composites in the turbine rotor blades and nozzle vanes would provide the most benefit. A research program designed to introduce ceramic composite components to these turbines was recommended.Copyright