Multilayer thermal and environment barrier coating (EBC) for high temperature applications such as for gas turbine engines and related methods thereof.
The maturation of thermal protection concepts for metallic components in the most advanced gas turbine engines has stimulated efforts to develop turbine components from ceramic materials with much higher maximum use temperatures. The focus has been directed upon more damage tolerant fiber reinforced ceramic matrix composites (CMCs) with weak fiber/matrix interfaces.
Unfortunately, silicon containing ceramics react with oxygen and water vapor in combustion environments to form SiO2 scales, and these then react with water vapor to form gaseous silicon hydroxide by the reactions:
SiO2(s)+2H2O(g)=Si(OH)4(g)
The rate of SiC volatilization depends upon the temperature, the incident water vapor flux (engine pressure), and the effectiveness with which the silicon hydroxide reactions occur, as shown in
The presently described subject matter is directed to an air plasma method for the deposition of an advanced EBC coating system.
The presently described subject matter is directed to an air plasma method for the deposition of an advanced EBC coating system comprising or consisting of applying a silicone bond coat to a SiC substrate; exposing the silicone bond coat to oxygen to form a proactive SiO2 thermally grown oxide (TGO) layer to avoid decomposition of the SiC substrate to SiO2 and gaseous CO; applying a layer of mullite over the silicon bond coat to impede diffusion of oxygen to the silicon bond coat; and applying a ytterbium disilicate topcoat over the layer of mullite, the ytterbium disilicate having a very low silica volatility and protecting the layer of mullite and silicon layer from volatilization by water vapor.
The presently described subject matter is directed to an air plasma method for the deposition of an advanced EBC coating system comprising or consisting of applying a silicone bond coat to a SiC substrate; exposing the silicone bond coat to oxygen to form a proactive SiO2 thermally grown oxide (TGO) layer to avoid decomposition of the SiC substrate to SiO2 and gaseous CO; applying a layer of mullite over the silicon bond coat to impede diffusion of oxygen to the silicon bond coat; and applying a ytterbium disilicate topcoat over the layer of mullite, the ytterbium disilicate having a very low silica volatility and protecting the layer of mullite and silicon layer from volatilization by water vapor, wherein the silicon bond coat is applied directly to the SiC substrate.
The presently described subject matter is directed to an air plasma method for the deposition of an advanced EBC coating system comprising or consisting of applying a silicone bond coat to a SiC substrate; exposing the silicone bond coat to oxygen to form a proactive SiO2 thermally grown oxide (TGO) layer to avoid decomposition of the SiC substrate to SiO2 and gaseous CO; applying a layer of mullite over the silicon bond coat to impede diffusion of oxygen to the silicon bond coat; and applying a ytterbium disilicate topcoat over the layer of mullite, the ytterbium disilicate having a very low silica volatility and protecting the layer of mullite and silicon layer from volatilization by water vapor, wherein the ytterbium disilicate topcoat is applied directly to the silicon bond coat.
The presently described subject matter is directed to an air plasma method for the deposition of an advanced EBC coating system comprising or consisting of applying a silicone bond coat to a SiC substrate; exposing the silicone bond coat to oxygen to form a proactive SiO2 thermally grown oxide (TGO) layer to avoid decomposition of the SiC substrate to SiO2 and gaseous CO; applying a layer of mullite over the silicon bond coat to impede diffusion of oxygen to the silicon bond coat; and applying a ytterbium disilicate topcoat over the layer of mullite, the ytterbium disilicate having a very low silica volatility and protecting the layer of mullite and silicon layer from volatilization by water vapor, wherein the ytterbium disilcate topcoat is applied as a two layer coating system.
The presently described subject matter is directed to an air plasma method for the deposition of an advanced EBC coating system comprising or consisting of applying a silicone bond coat to a SiC substrate; exposing the silicone bond coat to oxygen to form a proactive SiO2 thermally grown oxide (TGO) layer to avoid decomposition of the SiC substrate to SiO2 and gaseous CO; applying a layer of mullite over the silicon bond coat to impede diffusion of oxygen to the silicon bond coat; and applying a ytterbium disilicate topcoat over the layer of mullite, the ytterbium disilicate having a very low silica volatility and protecting the layer of mullite and silicon layer from volatilization by water vapor, wherein the ytterbium disilcate topcoat is applied as a two layer coating system, wherein a first layer of the two layer coating system is deposited on the silicone bond coat at 1200° C. in a reducing (Ar/H2) atmosphere.
The presently described subject matter is directed to an air plasma method for the deposition of an advanced EBC coating system comprising or consisting of applying a silicone bond coat to a SiC substrate; exposing the silicone bond coat to oxygen to form a proactive SiO2 thermally grown oxide (TGO) layer to avoid decomposition of the SiC substrate to SiO2 and gaseous CO; applying a layer of mullite over the silicon bond coat to impede diffusion of oxygen to the silicon bond coat; and applying a ytterbium disilicate topcoat over the layer of mullite, the ytterbium disilicate having a very low silica volatility and protecting the layer of mullite and silicon layer from volatilization by water vapor, wherein the ytterbium disilcate topcoat is applied as a two layer coating system, wherein the first layer is 50 μm thick.
The presently described subject matter is directed to an air plasma method for the deposition of an advanced EBC coating system comprising or consisting of applying a silicone bond coat to a SiC substrate; exposing the silicone bond coat to oxygen to form a proactive SiO2 thermally grown oxide (TGO) layer to avoid decomposition of the SiC substrate to SiO2 and gaseous CO; applying a layer of mullite over the silicon bond coat to impede diffusion of oxygen to the silicon bond coat; and applying a ytterbium disilicate topcoat over the layer of mullite, the ytterbium disilicate having a very low silica volatility and protecting the layer of mullite and silicon layer from volatilization by water vapor, wherein the ytterbium disilcate topcoat is applied as a two layer coating system, wherein the first layer has a porosity of about 5%.
The presently described subject matter is directed to an air plasma method for the deposition of an advanced EBC coating system comprising or consisting of applying a silicone bond coat to a SiC substrate; exposing the silicone bond coat to oxygen to form a proactive SiO2 thermally grown oxide (TGO) layer to avoid decomposition of the SiC substrate to SiO2 and gaseous CO; applying a layer of mullite over the silicon bond coat to impede diffusion of oxygen to the silicon bond coat; and applying a ytterbium disilicate topcoat over the layer of mullite, the ytterbium disilicate having a very low silica volatility and protecting the layer of mullite and silicon layer from volatilization by water vapor, wherein the ytterbium disilcate topcoat is applied as a two layer coating system, wherein a second layer of the two layer coating system is deposited on the first layer at 1200° C., without a protective reducing gas atmosphere.
The presently described subject matter is directed to an air plasma method for the deposition of an advanced EBC coating system comprising or consisting of applying a silicone bond coat to a SiC substrate; exposing the silicone bond coat to oxygen to form a proactive SiO2 thermally grown oxide (TGO) layer to avoid decomposition of the SiC substrate to SiO2 and gaseous CO; applying a layer of mullite over the silicon bond coat to impede diffusion of oxygen to the silicon bond coat; and applying a ytterbium disilicate topcoat over the layer of mullite, the ytterbium disilicate having a very low silica volatility and protecting the layer of mullite and silicon layer from volatilization by water vapor, wherein a surface of the SiC substrate is roughened before applying the silicone bond coat to the surface of the SiC substrate
The presently described subject matter is directed to an EBC system comprising or consisting of a SiC substrate; a silicon bond coat applied directly to the SiC substrate; a proactive SiO2 thermally grown oxide (TGO) layer applied over the silicone bond coat; a layer of mullite applied over the silicon bond coat; and a ytterbium disilicate topcoat applied over the layer of mullite.
The presently described subject matter is directed to an EBC system comprising or consisting of a SiC substrate; a silicon bond coat applied directly to the SiC substrate; a proactive SiO2 thermally grown oxide (TGO) layer applied over the silicone bond coat; a layer of mullite applied over the silicon bond coat; and a ytterbium disilicate topcoat applied over the layer of mullite, wherein the silicon bond coat is applied directly to the SiC substrate.
The presently described subject matter is directed to an EBC system comprising or consisting of a SiC substrate; a silicon bond coat applied directly to the SiC substrate; a proactive SiO2 thermally grown oxide (TGO) layer applied over the silicone bond coat; a layer of mullite applied over the silicon bond coat; and a ytterbium disilicate topcoat applied over the layer of mullite, wherein the ytterbium disilicate topcoat is applied directly to the silicon bond coat.
The presently described subject matter is directed to an EBC system comprising or consisting of a SiC substrate; a silicon bond coat applied directly to the SiC substrate; a proactive SiO2 thermally grown oxide (TGO) layer applied over the silicone bond coat; a layer of mullite applied over the silicon bond coat; and a ytterbium disilicate topcoat applied over the layer of mullite, wherein the ytterbium disilcate topcoat is applied as a two layer coating system.
The presently described subject matter is directed to an EBC system comprising or consisting of a SiC substrate; a silicon bond coat applied directly to the SiC substrate; a proactive SiO2 thermally grown oxide (TGO) layer applied over the silicone bond coat; a layer of mullite applied over the silicon bond coat; and a ytterbium disilicate topcoat applied over the layer of mullite, wherein the ytterbium disilcate topcoat is applied as a two layer coating system, and wherein a first layer of the two layer coating system is deposited on the silicone bond coat at 1200° C. in a reducing (Ar/H2) atmosphere.
The presently described subject matter is directed to an EBC system comprising or consisting of a SiC substrate; a silicon bond coat applied directly to the SiC substrate; a proactive SiO2 thermally grown oxide (TGO) layer applied over the silicone bond coat; a layer of mullite applied over the silicon bond coat; and a ytterbium disilicate topcoat applied over the layer of mullite, wherein the ytterbium disilcate topcoat is applied as a two layer coating system, wherein a first layer of the two layer coating system is deposited on the silicone bond coat at 1200° C. in a reducing (Ar/H2) atmosphere, and wherein the first layer is 50 μm thick.
The presently described subject matter is directed to an EBC system comprising or consisting of a SiC substrate; a silicon bond coat applied directly to the SiC substrate; a proactive SiO2 thermally grown oxide (TGO) layer applied over the silicone bond coat; a layer of mullite applied over the silicon bond coat; and a ytterbium disilicate topcoat applied over the layer of mullite, wherein the ytterbium disilcate topcoat is applied as a two layer coating system, and wherein a first layer of the two layer coating system is deposited on the silicone bond coat at 1200° C. in a reducing (Ar/H2) atmosphere, and wherein the first layer has a porosity of about 5%.
The presently described subject matter is directed to an EBC system comprising or consisting of a SiC substrate; a silicon bond coat applied directly to the SiC substrate; a proactive SiO2 thermally grown oxide (TGO) layer applied over the silicone bond coat; a layer of mullite applied over the silicon bond coat; and a ytterbium disilicate topcoat applied over the layer of mullite, wherein the ytterbium disilcate topcoat is applied as a two layer coating system, wherein a first layer of the two layer coating system is deposited on the silicone bond coat at 1200° C. in a reducing (Ar/H2) atmosphere, and wherein a second layer of the two layer coating system is deposited on the first layer at 1200° C., without a protective reducing gas atmosphere.
The presently described subject matter is directed to an EBC system comprising or consisting of a SiC substrate; a silicon bond coat applied directly to the SiC substrate; a proactive SiO2 thermally grown oxide (TGO) layer applied over the silicone bond coat; a layer of mullite applied over the silicon bond coat; and a ytterbium disilicate topcoat applied over the layer of mullite, wherein a surface of the SiC substrate is roughened before applying the silicone bond coat to the surface of the SiC substrate.
An air plasma method for the deposition of an advanced EBC coating system, is shown in
The EBC system consists of a silicon bond coat applied directly to a SiC substrate. The purpose of this layer is to provide prime reliant protection of the SiC. If exposed to oxygen, it is intended to form a protective SiO2 thermally grown oxide (TGO) layer, thereby avoiding decomposition of the SiC to SiO2 and (gaseous) CO. This was covered by a layer of mullite to impede diffusion of oxygen to the silicon bond coat, and by a ytterbium monosilicate topcoat that has a very low silica volatility and protects the mullite and silicon layers from volatilization by water vapor.
To achieve robust performance the EBC must be designed and fabricated from materials that provide (and retain) complete aerial protection from oxygen and water vapor penetration for up to 5,000-10,000 hours of operation at gas path temperatures approaching 1500° C. Such EBC's must also not fail by coating fracture or delamination during repeated thermal cycling; must be able to survive impact by small and large particles (exhibit erosion and FOD resistance); and be able to survive exposure to molten CMAS and various salts that are present in fuel and the naval engine operating environments.
The use of Yb2SiO5 (YbMS) is based upon its very low volatility in steam environments, as shown in
The study of advanced EBCs began with the design and assembly a state of a state of the art, robotically controlled, air plasma spray system shown in
A steam-cycling furnace based upon a design provided by NASA Glenn investigated the response of the YbMS top coat EBC system to steam cycling using a 1 hr hold time at 1316° C. (2400° F.) in a 90% H2O+10% O2 flowing environment (4.4 cm/s flow rate). The coatings made by the optimized process is compare to the response of coatings deposited with plasma spray systems at NASA Glenn, which were operated at much higher spray power settings.
Delamination of the optimized coatings began after approximately 250 steam cycles, and all the samples failed within 725 cycles. The failure mode can be seen in
The top coat suffered negligible steam erosion, as expected given the very low silica activity of this topcoat material. However, the top coats propensity for mud cracking resulted in the early penetration of cracks through the mullite and into the silicon layer where they bifurcated. This allowed oxidizing species to reach the interior of the silicon bond coat rapidly, and resulted in the progressive formation of a TGO layer on the crack faces, which then laterally extended to cause failure.
Raman spectroscopy identified the silicon bond coat TGO as α-phase cristobalite which had presumably formed as β-cristobalite at 1316° C., and transformed to the α-phase at 220° C. on cooling. There is a substantial (˜4%) volumetric contraction associated with the transformation which created a substantial stored elastic strain energy to drive delamination and other cracking processes. Insight into this was gained via collaborations with Begley (UCSB) by performing residual stress calculations assuming the system remained elastic during cooling from a stress free temperature of 1300° C., as shown in
A schematic illustration of the failure mechanism during steam cycling is shown in
Detailed FEM calculations of the cracking mechanics indicate very similar energy release rates for fracture by single crack penetration to the bond coat and crack tip bifurcation in the mullite layer. Steam cycling of the two systems then resulted in crack tip localized oxidation of the Si bond coat and extension of the cracks. In one case, the cracks extended at the silicon-mullite interface while in the low spray power case, fracture progresses through the Si layer.
These findings led to a recognition that the YbMS system cannot be used as a top coat because of its propensity to mud crack. The mullite layer also provides little benefit, since its relatively high CTE, as shown in
The YbDS topcoat system is deposited directly upon the silicon bond coat.
The microstructure of the as-deposited two layer (YbDS/Si) coating system is shown in
Steam cycling of the YbDS/Si system does not result in spallation (other than at less protected edges of the samples). Instead, a TGO layer slowly develops on the Si bond coat. An example is shown in 12A. The TGO again consisted of α-cristobalite which has mud cracks upon cooling because of the very biaxial tension stress developed in the layer upon cooling, as shown in
If EBC systems are applied to rotating components in gas turbine engines, they will be required to sustain stresses of ˜100 MPa at 1316° C. (2400° F.) for many thousands of hours of operation. To investigate the dimensional stability of the YbDS top coat to such conditions, APS deposited thick blocks of the topcoat silicate are made to make up flexural creep specimens. These are then isothermally (and in some cases thermal gradient) tested to obtain preliminary estimates of their flexural creep susceptibility using test facilities located at NASA Glenn. As shown in
Molten silicate (CMAS) degradation of thermal and environmental barrier coatings is viewed as a fundamental obstacle to achieving higher operating temperatures and improved efficiency in gas turbine engine. These deposits are derived from siliceous minerals (dust, sand, volcanic ash, debris) that enter into the gas turbine with the intake air and deposit on the surface of EBCs. They form glassy melts which react rapidly with silicate EBC materials, leading to degradation and loss of the EBC system. The CAS problem becomes serious only after the gas flow temperature at the high pressure turbine inlet begins to exceed the melting temperature of CMAS (˜1200° C.), and is increasingly a concern for aircraft engines that operate in desert or volcanic ash-containing environments. The introduction of SiC components into engines leads to further increases in the gas flow temperatures that can make this CMAS issue even more challenging.
The reaction rate between YbMS and YbDS coatings with CMAS is summarized in
The reaction mechanism in the APS deposited coatings is more complex than that reported for monolithic materials. EDS measurements reveal a significant dissolution of Yb into the CMAS melt, and a differential rate of reaction between the YbDS and YbMS rich regions of the coatings. The situation is schematically illustrated in
The current method identifies the existence of a viable solution for the environmental protection of nonrotating SiC composite components in gas turbine engines. Using appropriately optimized APS deposition processes, a silicon bond coat that is covered by an approximately 150 μm thick YbDS top coat has the potential to provide several thousand hours of protection at 1316° C. (2400° F.), provided surface flow velocity is low and exposure to CMAS deposits is minimized.
However, it is noted that thicker or more protective coatings are needed as the local flow velocity is increased (e.g. 1 mm thick YbDS layer for a flow velocity of 100 ms−1). The next advance is the implementation of an analogous EBC protection concept for use on rotating components subjected to the same temperature. These rotated components are in the high-speed flow path, and are subjected to intense thermal cycling requiring improved steam erosion and delamination resistance. They are also subjected to a variety of mechanical loads and must therefore have significant resistance to creep. Ideally, the chosen approach provides a pathway towards the eventual development of environmental protection concepts for rotated components operating at 1482° C. (2700° F.).
One of the key objectives is to exploit the sophisticated coating deposition expertise to explore deformation resistant EBC coating systems capable of operation at 2400° F. on rotated structures. The use of air plasma spray and vapor deposition methods for deposition of advanced environmental barrier coating systems on SiC test coupons that have compositions that can survive the steam rich engine environment can be studied. The study can investigate, among other things, the fundamental phenomena governing the resistance of the coatings to imposed (centripetal) stresses, and the stress relaxation processes active in coatings subjected to severe temperature gradients. The insights gained from the study will culminate in the identification of new materials and coating architectures that enable EBC protection to be extended to rotated components operating at 2400° F. These insights can be used to suggest silicon bond coat replacements (melting temperature of silicon is 1410° C.) that might extend the use temperature of future CMCs to 2700° F. target application.
The discovery and development of affordable and reliable manufacturing methods for applying EBC systems to CMC components can benefit the performance and life cycle (operating) costs of several advanced turbine engines within the Navy that specifically require improved durability. This includes future variants of the Joint Strike Fighter (JSF) engine which will have higher engine operating temperatures over time, future engines for the joint unmanned combat air system (J-UCAS) where current approaches use existing legacy engines (such as the F404) in extreme environments (high thermal loads or long duration missions), and engines for the F-18 replacement that will require long range and super cruise capability. This will be achieved through reduced fuel consumption due to increased operating temperatures as well as reduced maintenance in comparison to systems utilizing TBCs which require numerous reapplications of the thermal protection system over the engine lifetime.
A goal of the method, among others, is the design of creep and thermal shock resistant EBC systems suitable for use on rotating components in the high velocity, steam rich combustion gas flow characteristic of advanced gas turbine engines. The approach combines, among other things, high temperature material combination selection, novel coating deposition concepts, microstructure characterization, high temperature failure mechanism investigations and chemical transport/micromechanical analysis to develop a comprehensive understanding of the factors influencing the durability of EBCs on rotating components exposed to gas flow temperatures of 1316° C.
The approach seeks to, but not limited thereto, increase the creep resistance of the YbDS/Si system whose thermomechanical behavior and oxidation resistance appear well matched to the needs of the 1316° C. application. This will also exploit multifunctional opportunities afforded by this creep reinforcement approach. These include reduction of the steam erosion rate of the protection system (which is likely to be necessary in the high gas flow speed environment of a rotating component) and avoidance of cristobalite TGO formation (which drives premature delamination).
The creep resistance of both the Si bond coat and outer YbDS layer of a two-layer EBC can be improved by the incorporation of a creep resistant reinforcement aligned with the in-plane loading direction. Since the creep rate increases with homologous temperature, the creep resistant material should have a low homologous temperature at the operating temperature. There are several candidate materials that could be explored, but the study will begin by using HfO2. Its homologous temperature at 1316° C. is 0.47, as shown in
A preliminary ternary phase diagram for the SiO2—YbO1.5—HfO2 system at 1500° C.18 is shown in
It is noted that the Si—HfO2 and YbDS-HfO2 structures in
The failure mechanisms of state of the art thermal/environmental barrier coatings can be experimentally explored, and can be related to processing variables. Specific coating failure mechanisms to be investigated include coating fracture and delamination resulting from thermal expansion differences between protection system components and the substrate, erosion and failure of the coatings due to the coupled effects of oxygen diffusion through the coatings with water vapor induced oxidation and volatilization of the resulting silica compounds, and reactive attack by calcium magnesium aluminum silicate deposits.
A first deposit of the coating systems will use vapor techniques developed for the growth of thermal barrier coating systems. The system allows four (4) source materials to be sequentially (or simultaneously) melted and evaporated by impingement of intense electron beams on their surfaces. In operation, up to four (partially consolidated) powder sources are placed in a compound water-cooled copper crucible located in the throat of an inert gas jet-forming nozzle. By maintaining a pressure ratio of at least two (2) between the pressure up and downstream of the nozzle, it is possible to use gas expansion to create a supersonic gas jet with a velocity that can be varied by changing the gas composition, temperature, the nozzle's geometry, and the up to downstream pressure ratio. The use of helium allows jets with velocities in the 1,000 m/s range to be achieved at downstream pressures in the 10-100 Pa range. These can efficiently entrain, mix and deposit coatings at high rates (many 10's of micrometers per minute).
If the substrate is held at a homologous temperature T/Tm around 0.35 (Tm is the absolute melting temperature), the atoms and molecules that are deposited low surface mobility resulting in porous, columnar deposits, ideally suited for TBC applications. The HfO2 (with gadolina doping) TBC topcoat will therefore be deposited without the use of plasma assistance. If the coating surface temperature increased to homologous temperatures in the 0.7-0.8 range, fully dense coatings (like those needed for the bond coat and volatility resistant layers) can be made. However, the materials of interest have very high melting temperatures requiring very high substrate temperatures for their deposition.
The approach proposed here utilizes plasma assistance in which ionized heavy inert gas ions (such as Ar+) are electrostatically attracted to the growth surface and their impact provides activation of atomic reassembly processes. A schematic illustration of the electron beam coaxial plasma deposition (EB-CPD) system is shown in
An air plasma spray system with two powder feeds is used to deposit the coatings above.
Control of the phases formed during deposition can be achieved by (i) controlling the residence time of the particles and the enthalpy of the plasma jet (which control the droplet temperature and heat flux) and (ii) adjustment of the temperature of the substrate as each layer is deposited. Heat treatment of the powders prior to deposition can be used to ensure that the most stable high temperature phase exists prior to spraying, though powders are generally purchased in phase pure. Partial melting of specialty powders that contain crystal-nucleating cores can then be used to avoid the formation of metastable phases including vitreous deposits.
X-ray diffraction methods, BSE mode SEM, EDS compositional analysis and FIB sectioned TEM samples can be used to characterize both the powder particles before and after heat treatment and the deposited structures after initial cooling and stabilization annealing. Particular attention should be paid to the interfacial structures that form between the HfO2—YbDS and HfO2—Si interfaces during deposition as well the reinforcing phase shape. During high temperature thermal exposure of the system, morphological changes to the reinforcement will be investigated and the formation of a hafnon TGO carefully characterized. A high-resolution μ-XCT system will be investigated as a means to nondestructively characterize the evolution of the coatings periodically during thermal testing.
Tensile creep coupon samples of the HfO2 reinforced silicon bond coat and YbDS layers with the axis of the reinforcement in the loading direction can be prepared. The creep strain dependence upon stress at several temperatures around 1316° C. can be measured, and the creep exponent and activation energy can be determined. Interrupted tests can be used to investigate microstructural changes, characterize micro-failure processes and to infer the creep mechanisms. These investigations will then be used to design coating morphologies that impede creep processes. Other experiments will investigate the basic mechanisms of coating cracking and delamination and its dependence upon layer thickness, interfacial toughness and composition.
The program will explore, among other things, the fundamental mechanisms of water vapor volatilization in the creep resistant system. The volatility of HfO2 is expected to be much less than YbDS, for it to therefore accumulate and protect the EBC surface. This can be carefully evaluated. The YbDS-HfO2 layer can be coated with a HfO2 TBC, and its effect upon reducing steam erosion of the EBC layer can be investigated. The study will also investigate the effects of CMAS exposure at surface temperatures up to 1400° C. and beyond on the HfO2+Gd2Hf2O7 TBC protected structure. In these experiments, thermal gradients will be set up by back cooling the substrates with compressed argon.
An environmentally controlled (steam) cycling furnace is assembled to measure the volatilization rates of the EBC system to be investigated in this program, as shown in
The steam-cycling furnace allows samples to be programmably dropped from the hot zone into a cold region to allow thermal cycling and assessments of coating fracture and the effects of steady state silica scale growth on delamination to be evaluated. These experimental studies will be complemented with thermomechanical modeling to compute the residual stress and stored strain energies of the EBC systems. Measurements of the interfacial toughness on specially prepared microscale test coupons will be used with these analyses to understand the factors leading to cracking and delamination of the EBC systems. Samples with systematically varied coating thicknesses will also enable systematic variation of the crack driving forces in the system and an independent means for validation of thermomechanical fracture models.
This method seeks support for three sequential tasks.
The objective of Task 1 is to develop economical plasma spray and vapor deposition methods for the controlled the morphology of creep resistant EBC/TBC systems on SiC coupons. The initial focus will be directed at a silicon+HfO2 bond coat with Yb2Si2O7+HfO2 environmental barrier and HfO2+Gd2Hf2O7 TBC top coat system. By using low power plasma spray deposition conditions that only partially melt the powder particles and variation of the substrate temperature, we will explore the opportunity to control the phases that are formed during deposition of the various coatings. The use of higher power plasma spray conditions enable us to evaporate a fraction of powder particles (the smaller diameter ones) and to deposit coatings by a combination of liquid droplet and vapor condensation methods. The directed vapor deposition approach allows us to extend this trend and deposit from just the vapor phase or from one that contains a significant mass fraction of nanoparticles to seed desired phases. We will investigate the mechanisms by which porosity is avoided in the coatings, and explore strategies for eliminating interconnected pores and splat boundaries that can provide diffusional short circuits to the easily volatized components of the system. The surfaces and interfaces of the coatings will be optimized to improve gas flow over the component surface and to better achieve longer (predictable) lifetimes.
The failure mechanisms of these state of the art thermal/environmental barrier coatings can be experimentally explored. Coating failure mechanisms to be specifically investigated include coating fracture and delamination resulting from thermal expansion differences between protection system components and the substrate, environmental damage of the coatings due to the coupled effects of oxygen diffusion through the coatings and water vapor induced volatilization of the resulting silica compounds, impact damage caused by small particle impacts at velocities up to 300 m/s and reactive attack by calcium magnesium aluminum silicate deposits. Synchrotron based approaches to measure (the lattice parameter and thus) residual stress distributions in the coatings at a variety of temperatures as well as wafer curvature methods combined with layer removal can be used. These will then be used in conjunction with thermoelastic models to explore the use of processing variables (especially particle superheat and substrate temperature) during each layers deposition to control the stored strain energy in the EBC system.
Steam jets and thermogravimetric analysis (TGA) can be utilized to measure the volatilization rate of each of the EBC layer components and the EBC system. These experiments will be complimented with modeling of the gas phase transport of reactants/products to/from the steam-exposed surface. By independently varying both the steam flow conditions and substrate temperature, the kinetic factors that control silicon hydroxide volatility for the rare earth silicate system can be identified. Atomistic scale models will be used to rationalize the kinetics and identify promising strategies for reducing the volatilization rate. Some of the T/EBC systems will be exposed to CMAS at 1300-1400° C. and the mechanism of CMAS attack investigated. The transport of CMAS through inter-splat boundaries and the potential to retard this by process control of microstructure can be focused on. Very little data exists on the effect of CMAS surface reaction products on silica volatility. Some of the CMAS-reacted samples will therefore be tested using the steam jet furnace and TGA to measure volatilization rates and compare them with un-reacted samples.
The insights gained from these Tasks will provide a comprehensive understanding of the relationships between process methods and conditions used for EBC deposition and the mechanisms and rates of coating failure as surface temperatures are increased towards 1400° C. These new insights will be used to explore novel coating materials and multilayer architectures that promise to extend the life and increase the maximum use temperature of EBC systems. This will specifically include concepts to replace the silicon bond coat since its melting temperature (of 1410° C.) limits the maximum use temperature of future CMCs.
The key milestones are summarized in
Facilities to support the research include: (1) a top of the line model 7700 UPC air plasma spray (APS) system manufactured by Praxair-TAFA that utilizes fully digital closed loop control of the APS process including gun manipulation using a 6-axis ABB robot, IR pyrometer system for work-piece thermal management, multiple carrier and secondary gas abilities including Hydrogen, and dual powder feed system for spraying two powders simultaneously; (2) a state-of-the-art EB-CPD synthesis tool that uses an e-beam in a low vacuum environment (˜10−3−1 Torr) to entrain the evaporant in a carrier gas stream, (simultaneous evaporation form four sources is possible); (3) a FLIR ThermaCAM SC3000 high speed thermal imaging camera having high resolution (320×240 pixels) and ultra high sensitivity (<20 mK at 30° C.); (4) a FEI Quanta 650 electron microprobe/SEM equipped with a field emission filament and both energy-dispersive (EDS) and wavelength-dispersive (WDS) X-ray detectors for high-resolution imaging (4 nm) and microanalysis of all elements down to boron in the Periodic Table; (5) a second FEI Quanta 200 electron microprobe/SEM equipped with a tungsten filament, as well as energy-dispersive (EDS) and wavelength-dispersive (WDS) X-ray detectors for high-resolution imaging (3.0 nm) and microanalysis of all elements down to boron in the Periodic Table; and (6) a PANalytical X'Pert PRO MPD X-ray diffractometer with X'celerator CCD line and proportional point detectors will allow crystal structure determination and coating texture measurements.
The EBC protection of silicon-based CMC's is shown in
A table of material candidates for EBC systems is shown in
The layering structure or arrangement of the proactive coating is shown in
The cracking of the “as-deposited” YbSiO5 topcoat EBC is shown in
The phase precipitation in YbSiO5 is shown in
The deposition system for making the coating is shown in
A Ytterbium monosilicate deposition matrix is shown in
A steam cycling furnace is shown in
The structure of the annealed coating is shown in
The SAED patterns for the reaction product are shown in
Number | Date | Country | |
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62166789 | May 2015 | US |