Enabling Technologies for Ceramic Hot Section Components

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.

This final report summarizes the developments in field assisted sintering technology (FAST) for rapid fabrication of ceramic matrix composite (CMC) materials with integrated environmental barrier coatings (EBCs), as well as their performance in a representative gas turbine combustor environment including stagnant flame zones, and high velocity product zones.CMCs are a possible high temperature tolerant material system for gas turbine engines, which are currently the dominant source of electric power production and the primary source of aircraft propulsion.However, CMCs are costly and time-consuming (on the order of weeks to months) to fabricate, and CMCs based on silicon carbide (SiC) ceramics are susceptible to water vapor degradation requiring the use of specialized EBCs.The manufacturing technique known as FAST can create consolidated parts in as little as a few hours using high pressure and temperature, and may be useful to integrate EBCs into CMCs directly during consolidation.The project consisted of five major tasks, consisting of baseline and optimized CMC and EBC development using FAST; design and fabrication of a combustion test rig; operability of the test rig with various types of fuels including high hydrogen content; exposure of CMCs to combustion conditions; and evaluation of degradation mechanisms of the CMCs.On the materials development front, the team developed layup processes that would result in high densification of the CMC and identified time/temperature requirements for the FAST process to optimize density of the final parts without causing degradation of the matrix and fibers.The team also identified the process requirements to get optimal conventional (mullite-based) and next-generation oxide EBC materials (hafnia/ytterbium disilicate based), and steps required to integrate these coatings directly with the CMC material.From the materials development, several 1" diameter and 1/8" thick test coupons for combustion testing were created: monolithic silicon carbide (SiC), uncoated SiC/SiC CMCs, and EBC-coated SiC/SiC CMCs.The other major development in this project was a high temperature exposure facility that mimicked conditions that would be found in gas turbine engine combustors and would be highly fuel flexible, including up to 100% hydrogen fuel.To ensure flame stability, the facility was designed as a trapped vortex combustor, where a flame sits in a cavity in the flowpath.CMC test articles were attached to the bottom of the cavity where they were exposed to low velocity active combustion, as well as downstream of the cavity where they were subjected to high velocity mixed combustion products.These regions simulate the range of flow conditions in a typical combustor.Novel instrumentation was designed for the material test articles, including an in-situ heat flux measurement and through-flame infrared thermography for top-side surface temperatures of the articles.After subjecting the material test coupons to combustion temperatures for over 100 hours, they were analyzed using a variety of techniques, including micro X-ray computed tomography (CT), optical profilometry for recession/surface roughness, X-ray diffractometry (XRD), and crosssectional microscope imaging.From this, it was determined that the uncoated CMCs had somewhat minor recession, likely requiring a longer test time or higher water vapor content.The thin EBC-coated CMCs also had limited recession and robustly protected the underlying SiC/SiC substrate from exposure.However, for the thicker modified coating architecture, extensive spallation was observed due to higher thermomechanical stresses that built up during repeated on/off cycles from the combustion testing and led to increased defect evolution (e.g., cracks).Overall, this project successfully created EBC-coated CMCs using the FAST process and showed that they would survive representative combustor conditions for the time subjected.Future work should increase the residence time of the materials to map longer-term degradation mechanisms and identify pathways to reduce themomechanical stress between thick coatings and the base CMC.Generating more complex shapes for the CMC using FAST, as well as methods of providing cooling to CMCs to enable even higher temperature operation, would benefit the maturation of this technology to improve gas turbine engine efficiency.

Three different oxide/oxide ceramic matrix composite (CMC) materials are described. Design concepts for the attachment of the CMC component to the metal structure of the gas turbine are developed in a first work stream focused on the combustion chamber and the turbine seal segment. Issues like environmental barrier coating (EBC)/thermal barrier coatings (TBC), application and volatilization, allowance for the different thermal expansion and the mechanical fixation are addressed. The design work is accompanied by CFD and FEM simulations. A variation of the microstructural design of the three oxide/oxide CMC materials in terms of different fiber architecture and processing of matrix are considered. Also, mechanical properties of these variations are evaluated. The material concepts are developed further in a second work stream. The CMCs are tested in various loading modes (tension, compression, shear, off-axis loading) from room temperature to maximum application temperature focusing on tensile creep behavior. By modification of the matrix and the fiber-matrix interface as well as EBC coatings, the high temperature stability and the insulation performance are enhanced. An outline of the “High Performance Oxide Ceramic”-program HiPOC for the following years is given, including manufacturing of a high-pressure tubular combustor and turbine seal segments from the improved materials as technology samples, for which validation testing up to technology readiness level 4 is scheduled for 2011.

For the past decade, the aviation industry has been adopting sustainable aviation fuels (SAF) for use in aircraft to reduce the impact of aviation on climate change. Also, some nations look to SAF as an option for energy security for their military fleets. Understanding the critical impact of alternative fuel sources on hardware will provide the gas turbine industry with strategic options in sustainability and maintainability of the existing and new fleets. The alternative fuels with high hydrogen/carbon ratio (H/C) (such as synthetic paraffinic kerosenes (SPK)) could produce more water vapour content than the conventional jet fuels upon combustion, and this increased water vapour level could exert a significant impact over the long-term durability on hot section components such as the substrate blades, oxidation resistant coatings, thermal barrier coatings (TBCs), environmental barrier coatings (EBCs), resulting in an accelerated degradation of the turbine components. The possible detrimental effect of high-temperature water vapour on degradation and lifespan of hot section components was examined. Examples were specifically given on degradation and spallation of thermally grown oxides (TGO), formation of non-protective oxides and ceramics topcoats in TBCs. Results show that water vapour can lead to volatilization of TGO (Al2O3), and is responsible for the formation of non-protective oxides in both Pt-modified β-NiAl and MCrAlY coatings, leading to their early spallation. However, water vapour does not appear to directly affect the ceramic topcoat of the TBC. For EBCs coated on SiC-based substrates, the substrate recession via silica (TGO) volatilization was reviewed. These EBCs were observed undergoing degradation in highly hostile environments, e.g., constantly operating under high temperatures, pressures, and velocities condition in the presence of water vapour steam. The review intends to provide a perspective of high-temperature water vapour effect on the EBCs’ topcoat properties such as durability, degradation, crack nucleation and crack growth, and possible guidance for mitigating these degradation effects.

Environmental barrier coatings (EBCs) offer protection to Silicon carbide (SiC)-based ceramic matrix composites (CMCs) against corrosion, oxidation, and water vapor for high temperature gas turbine applications. EBCs are often heat-treated to crystallize the amorphous phase, to change residual stresses, or heal cracks and pores. Properties such as elastic modulus and creep resistance at high temperatures are needed to assess EBC integrity and predict EBC response in a gas turbine environment. This paper highlights new experimental methods to measure EBC elastic moduli before, during and after heat treatment using compressive load-unload cycles. Strain from compression of cylindrical EBC material was measured by digital image correlation (DIC) through a window in a resistance heated SiC element furnace. Elastic modulus was initially measured at room temperature followed by at temperature measurements between 500°C and 1200°C at various temperature intervals for increasing as well as decreasing temperature excursions to assess the change in modulus with temperature and after crystallization. In addition, creep tests of individual layers of EBC were performed under four constant compressive stress at different temperatures from 1000°C up to 1200°C for at least 24 hours. This study found an increase in EBC elastic moduli from room temperature to final HT temperature, and no change once the EBC had been fully crystallized after HT.

Gas turbine engines undergo very harsh operating environmental conditions, and this leads to various issues related to components materials strength limitations, degradation, cracking, and other durability problems. Under such circumstances, a robust material design is required to prevent these critical components from failing in service and preventing catastrophic events from taking place. The robust design must enhance component durability, which could be degraded due to material processing defects, variability in material properties, in-service loads, and operating environment. To encounter and manage these durability issues, materials scientists and engineers that are involved in this field along with engine makers are continuously working on developing protective materials to alleviate and increase materials damage tolerance and prevent components failure. Ceramic matrix composites (CMC) are now materials of choice for gas turbine engine design and manufacturing. The CMC has a good capability in operating at high temperatures up to 1500 °C which is within the norm of gas turbine engine operation and it is much lighter compared to metals. Good impact resistance and stability at high operating temperatures make the silicon carbide (SiC) ceramic matrix composite system a desirable option for jet engines [1]. However, CMC’s when they undergo the degradation process that typically includes coating interface oxidation as opposed to a moisture-induced matrix which is generally seen at a higher temperature. Additionally, other factors such as residual stresses, coating process-related flaws, and casting conditions may influence the degradation of their mechanical properties. These durability considerations are being addressed by introducing a highly specialized form of environmental barrier coating (EBC) that is being developed and explored in particular for hightemperature applications greater than 1100 °C [2]. In this paper, a CMC substrate is being evaluated for failure under supportive protection of EBC coatings. The primary aim is to identify the crack propagation phenomenon, the sequence of failure of the EBC and assess the life the CMC substrate. An analytical simulation applying the extended finite element method (XFEM) in ABAQUS software [3] is used to perform this analyses. Analytical results obtained are discussed and checked against test data.

As increasingly more SiC-SiC ceramic matrix composites (CMCs) are being used in the hot sections of gas-turbine engines, there is a greater need for in situ strain measurement in these harsh conditions. Thin-film strain sensors are ideally suited for this task since they have minimal mass and are nonintrusive. However, if the bulk piezoresistive properties of SiC-SiC CMCs could be used instead of the thin-film piezoresistive properties, superior resolution and stability could be realized. Toward this end, we have fabricated strain gages in which the active strain element is the SiC-SiC CMC itself. As a result of this approach, a Pt:SiC CMC strain gage that is capable of operating at 500 °C for prolonged periods was demonstrated. The large piezoresistive responses associated with the SiC fibers and SiC matrix resulted in gage factors larger than 100. The performance of the Pt:SiC CMC strain gages under both tension and compression are reported, and the I-V characteristics of the strain gages, as well as the piezoresistive response as a function of temperature, are presented. In addition, the potential for embedded strain gages in advanced CMC engine components will be discussed.

Hydrogen or hydrogen blend fuels are expected to replace natural gas in land-based industrial gas turbines (IGTs) to support a greener power economy. Silicon carbide (SiC) base ceramic matrix composites (CMCs) are considered for replacement of Ni-based superalloys to facilitate future efficiency improvements. SiC CMCs require environmental barrier coatings (EBCs) to mitigate volatilization from high-temperature steam, thus making the EBC lifetime critical information for identifying CMC component lifetimes. The goal of this project is to determine the maximum bond coating temperature underneath the EBC for achieving an IGT component lifetime goal of 25,000 h, which is far greater than current CMC component lifetime requirements for aeroturbine applications. To provide data for the lifetime model, laboratory testing used atmospheric plasma-sprayed rare-earth silicate EBCs on monolithic SiC substrates with an intermediate Si bond coating. Specimens exposed to 1-h thermal cycles in flowing air–steam environments and reaction kinetics were assessed from 700 °C to 1350 °C by measuring the thickness of the thermally grown silica scales. The silica growth and phase transformation appear critical in predicting EBC lifetime and several strategies have been explored to reduce the oxide growth rate and improve EBC durability at elevated temperatures. Advanced characterization using Raman spectroscopy has helped clarify this system.

This presentation at the 2024 FECM Spring Project Review Meeting summarizes ongoing work at NETL aimed at enabling the use of ceramic matrix composite (CMC) materials in next-generation high-efficiency power generation turbines fueled by hydrogen, natural gas, or other fuels. The work broadly involves three aspects (1) phase-field damage modeling of CMCs and the environmental barrier coatings (EBCs) used to protect them (2) computational design of new and improved EBCs and (3) performance testing of EBCs using a novel gas turbine combustion simulation rig developed at NETL. An overview on current status and next steps for each aspect of the project are presented.

The advancement of self-sealing ceramic matrix composites offers durability improvements in hot section components of gas turbine engines. These durability improvements come with no need for internal cooling and with reduced weight. Building on past material efforts, ceramic matrix composites based on either a carbon fiber or a SiC fiber with a sequenced self-sealing matrix have been developed for gas turbine applications. The specific application being pursued on this effort is an F100-PW-229 nozzle seal. Full design life ground engine testing has been accomplished with both material systems. The ground testing has demonstrated a significant durability improvement from the baseline metal design. Residual properties are being determined for both systems by extracting tensile and microstructural coupons from the ceramic matrix composite seal. Nondestructive interrogation showed no material degradation and was used as a guide in setting cutting diagrams. The results from this effort will be presented along with documentation from flight test efforts.

The inherent toughness of ceramic matrix composites (CMCs) in advanced gas-turbine engines must be predictable under impact from small foreign objects to lower the amount of full scale testing needed to produce robust designs. Fiber/matrix/architecture properties of the composites, and a damage evolution based progressive failure code that can be used for a full range of composite architectures (GENOA) coupled with an explicit FEM impact code (LS-DYNA) were used to simulate impact and residual 4pt flexural strength of the ceramic engine components. This approach uses physics-based mechanics coupled at the micro and macro scale boundaries. The benefit of this technique is that the root cause of damage advancement at the micromechanical level could be understood and simulations could be performed to assess better damage tolerance structures. Steel projectiles with a diameter of 1.59 mm were used to impact the composites at speeds from 100–400 m/s (Mach 0.3–1.2) and the results shown to compare to prior test data for 2-D 5H Sylramic iBN CVI MI SiC at 25°C and 1316°C and for 2-D 8H N720/AS ceramic composites at 25°C. Simulations also gave insight to the micromechanical damage progression and were comparable with test data.

Environmental barrier coatings (EBCs) greatly improve the service performance of SiC‐based ceramic matrix composites (CMCs) in high‐temperature combustion chambers. Working environments with physical ablation, high temperature, and chemical corrosion require the performance of designed EBC materials and/or structures to be properly evaluated before their real applications. In this paper, EBCs’ lifetime‐related phase stability, chemical compatibility, and microstructure retainability are discussed. And then, evaluation methods for basic and environmental properties of EBCs are thoroughly reviewed with newly proposed methods and improved techniques. Pros and cons of each method along with some potential strategies/techniques are also provided. We hope this article can give a timely and overall review for efficient and effective evaluation of EBCs and provide guidance not only for beginners but also for seasoned researchers when they design and develop high‐performance EBC systems.

A primary failure mode for environmental barrier coatings (EBCs) on SiC ceramic matrix composites (CMCs) is the oxidation of the intermediate Si‐bond coating, where the formation of SiO 2 at the bond coating–EBC interface results in debonding and spallation. This work compares the microstructure evolution and steam oxidation kinetics of the Si‐bond coating beneath yttrium/ytterbium disilicate ((Y/Yb)DS) and ytterbium disilicate/monosilicate (YbDS/YbMS) EBCs to better understand the impact of EBC composition on oxidation kinetics. After 500 1‐h cycles at 1350°C, (Y/Yb)DS displayed a decreasing concentration of the monosilicate minor phase and increasing concentration of porosity as furnace cycling time increased, whereas the YbDS/YbMS EBC displayed negligible microstructural evolution. For both EBC systems, thermally grown oxide growth rates in steam were found to increase by approximately an order magnitude compared to dry air oxidation. The (Y/Yb)DS EBC displayed a reduced steam oxidation rate compared to YbDS/YbMS.

In Japan, the development of a 1700°C-class gas turbine utilizing combustion of methane (LNG) and oxygen was carried out as a national project. A ceramic matrix composite (CMC (SiC/SiC)) was focused as one of candidate materials for hot-gas-path parts, and its development and evaluation were conducted. In order to examine a relationship between bending strength and fracture process, 3-point bending tests were carried out, and they were interrupted halfway to observe the fracture of the specinen. While the CMC showed elastic behavior in a load-displacement curve, a crack was initiated in the tensile side of the specimen. It was clarified that the elastic behavior of CMC was different from that of metals. In order to protect the CMC from receission under combustion gas flow, Y 2 SiO 5 was selected as a candidate material for an environmental barrier coating (EBC). Y 2 SiO 3 specimen, which was by prepared by sintering, and Y 2 SiO 5 -coated specimen, in which Y 2 SiO 5 was coated on CMC substrate, were prepared. Combustion gas exposure tests were performed by means of these specimens (gas temperature 1200∼1500°C, a gas pressure of 0.3 MPa, a flow velocity of 150ms/, exposure time of 5∼40hours). The utilizaon of Y 2 SiO 5 for the EBC improves the environmental resistance of the CMC. However, taking long-term use in an in-service gas turbine into consideration, the development of new materials with more resistance to recession is necessary.

Ceramic matrix composites with environmental barrier coatings (CMC/EBCs) are the most promising material solution for hot section components of aero-engines. It is necessary to access relevant information and knowledge of the physical properties of various CMC and EBCs, the characteristics of defects and damages, and relevant failure mechanisms. Then, effective failure prediction models can be established. Individually assessing the failure of CMC and EBCs is not a simple task. Models considering the synergetic effect of coating properties and substrate fibrous architecture are more reasonable and more challenging. This paper offers a review and a detailed description of the materials features, failure mechanism, and failure modeling for both CMC substrate and EBC coatings. The various methods for failure analyses and their pros and cons are discussed. General remarks on technical development for failure modeling are summarized subsequently.