Review on Gas Turbine Hot Section Materials and Technology

Review on Gas Turbine Hot Section Materials and

Technology Developments

Andrew S. Bakir∗

University of Colorado, Boulder, Colorado, 80309, USA

This paper has reviews the history and developments of ceramic matrix composites.Beginning with early developments in the gas turbine, new materials and manufacturingtechniques were established. The ground work on how CMCs became introduced intothe turbine industry is discussed. The drive for ceramics in turbines is based on thepursuit to enable higher turbine inlet temperatures and higher thermal efficiencies.Structural requirements are presented in a similar fashion to the modeling plan usedby manufacturers. In order for a CMC to be suitable for turbines it must satisfythe three structural requirements, to include withstanding degradation from foreignobjects and environmental factors. A discussion on the benefits of EBCs on ceramiccomponents is included in this report. A brief summary on some of the key steps isreviewed on how CMCs components are manufactured. The final discussion brieflydiscusses some of the current research and work that is being performed to continuedeveloping more suitable materials. Additionally, a glimpse into what GE has plannedto begin manufacturing silicon carbide ceramics on a large scale.

Nomenclature

BSAS = BaO0.75SrO0.25Al2O32SiO2◦C = CelsiusCMC = Ceramic Matrix CompositeCV I = Chemical Vapor InfiltrationDS = Directional SolidificationEBC = Environmental Barrier CoatingGE = General Electric CompanyHIP = Hot Isostatic PressingHPT = High Pressure TurbineNOx = Nitrogen OxidePR = Pressure RatioSEP = Societe Europeenne De PropulsionSiN = Silicon Nitride (Si3N4)SiC = Silicon CarbideTIT = Turbine Inlet Temperature

I. Introduction

The evolution of gas turbines has been restricted tothe material and techniques available for manufac-turing components. Each time a team of scientistsand engineers solve one challenge, there is anotherwaiting in the relentless pursuit to improve gas tur-bine performance.

Scientists have been developing gas turbinessince the 1940s and 1950s. In a span of 30 years,from the 1950s to the 1980s, teams have tripledthe thrust-to-weight ratios, doubled fuel efficien-cies, and increased overhaul times to exceed 10,000hours.1 Around the 1980s is also when material sci-entists began experimenting further with ceramicsand fiber matrices. They found the thermal proper-ties of these materials to be quite promising for thefuture of gas turbines. Yet, after 35 years ceramicshave barely broken into commercial use. Why sucha stagnation?

This paper aims to answer that question with abrief review on the history of hot section materialsand the corresponding challenges. Followed by adiscussion on current development and the futureplans. For the purpose of this report, the hot sec-tion refers to the burner and turbine sections of theengine.

II. Gas Turbine History of MaterialDevelopment

In the early stages of gas turbine development, theengines’ thermal efficiency was much lower than to-

∗Graduate Research Asst, Mechanical Engineering, Boulder, CO, Student Member.

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American Institute of Aeronautics and Astronautics

day’s standards, not exceeding 30%. For this rea-son, research was performed on ways to increaseperformance. This would be accomplished by rais-ing operating temperatures in the hot section. Inorder to accomplish this with the available materi-als of that time, cooling air would be routed fromthe compressor to prevent thermal related failuresin components. An impressive hurdle in the 1950s,was the development of nickel- and cobalt-based al-loys with improved heat-resistant properties, andtitanium alloys with higher mechanical strength.The nickel alloys were employed for the hot sec-tions of the engine, and titanium alloys were moresuited for the cooler compressor section.1 Figure 1shows how this affected the use of steels by shiftingto superalloys and titanium.

Fig. 1 Weight percent of materials used inaircraft gas turbine1

Ni-base alloys have good strength propertiesaround 700 - 1000 [◦C] made possible by strength-ening with a solid hardening solution. Cobalt-basedalloys performed better at slightly higher tempera-tures, around 1050 [◦C]. For these reasons, ni-alloyswere used in the disk or turbine blades a few stagesbehind the leading stages of the high pressure tur-bine (HPT) and cobalt-alloys were popular in vanesexperiencing higher temperatures.

By the 1960s, material processing was advanc-ing to keep up with material development. This al-lowed for two important processing developments:powder metallurgy techniques such as hot isostaticpressing; and directional solidification processing ofturbine components.

Hot isostatic pressing (HIP) is a forming pro-cess which uses a highly pressurized heated gas,typically inert gases such as argon, to shape com-ponents from powders of metal or of ceramics. HIPis more effective than mechanical squeezing sincethe gas is applied uniformly on all sides of theobject, which in turns maintain the componentsshape. This is quite useful for complex geometries.

Equally important, HIP does a better job eliminat-ing porosity.2 Progress in HIP, allowed designers tocreate complex components to handle multiple con-ditions. For example, engineers could manufacturedisks with high load-bearing strength at the core,and good creep strength at the rim.1

Figure 2 shows how the better-controlled HIPprocessing allowed improved material propertieswith a plot of strength versus temperature for SiNmaterials.

Fig. 2 High temperature strength of HIP SiN4

Directional solidification (DS) was proposed toimprove the creep properties of superalloys byaligning the grain boundaries to parallel the ap-plied tensile stress. This in turn eliminated grainboundaries perpendicular to the stress, which aremore likely to crack. DS accomplished this by im-proving upon casting procedures of that time. Pre-viously, casting was performed by pouring a meltinto a mold where then a spontaneous solidifica-tion occurred due to the large temperature gradientacross the mold and melt interfaces. DS changedthe solidification method by incorporating a water-cooled copper plate to the base of the ceramic mold.Unlike the previous technique, DS allows for con-trol to adjust the temperature gradient to ensuresolidification occurred in a direction normal to thechilled surface. Engineers used this technique tomanipulate the mictrostructure to form strong cu-bic structures.1 A depiction of the difference be-tween the two casting methods is shown in figure3.

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Fig. 3 Comparison of (a) conventional castingand (b)directional solidification1

With government support and advancing HIPand DS techniques, research began shifting fromalloys to monolithic ceramics such as silicon car-bide (SiC) and silicon nitride (SiN) for their attrac-tive thermal properties and lower weight comparedto traditional metals of the time. These ceram-ics would enable higher turbine inlet temperatures(TIT) and higher efficiencies while preventing fur-ther production of nitrogen oxide (NOx).3

Being fairly new materials, there was little un-derstanding of the chemistry and microstructures ofmonolithic ceramics. Additionally, the lack of cor-responding databases with the material’s proper-ties was insufficient for design analysis. Therefore,standards for testing would need to be developedand refined iteratively from the 1970s-1980s.4

The testing would prove to be quite compli-cated. Despite the breakthrough manufacturingtechniques, engine manufacturers could wait up-wards of a year before receiving quality ceramicsfor testing.4 This caused considerable delays whena part failed a test and would require evaluationsto identify the cause of failure, improve the design,establish a database, and rerun the experiments.

From the extensive testing, progress was slow inidentifying failures modes for these monolithic ce-ramics. A major concern was the failure of ceramicturbine blades during impact tests. Modificationswere made that reduced the number of blades andmade the blade more robust.

Engineers and scientists ran into another issueduring field tests. This was the oxidation degra-dation of silicon nitride and silicon carbide fromexposure to moisture. This would be remedied byapplying protective coatings to the exterior surfaceof the components, which will be discussed later.

As the data was generated and collected, therewas no evidence that qualified the monolithic ce-ramics would survive extended exposure to hightemperature environments. Although the initial ce-ramic properties were inadequate for gas turbineapplications at the time, the standards and char-acterization testing and rigs were established toexpedite the next set of ceramics. Additionally,the performance of the ceramics in their short lifecycles under high temperature conditions, provedthey could be suitable after much improvement.

In 1977, Societe Europeenne De Propulsion(SEP), a French company, was accredited as thefirst company to successfully produce ceramic ma-trix composites (CMC).5 SEP based their work onthe technology developed in the United States andbegan to fabricate ceramic silicon carbide matri-ces reinforced with carbon fibers by chemical vaporinfiltration (CVI). The results were quite promis-ing in that the matrix did not fracture in the samebrittle fashion as the monolithic ceramics, mean-ing a higher strain-to-failure performance. GeneralElectric Company (GE) would later develop uponthis research and begin focusing their own effortsto manufacture higher quality CMCs. Figure 5 il-lustrates, how quickly the creep rate and time-to-failure improved from 1981 through 1998 with theimprovements in processing and material develop-ment.

Fig. 4 Creep Rate versus Time to Failure4

III. Ceramic Matrix Composites

As the ceramic material properties improved, thetesting became more rigorous through the 1990s.Key concerns for component designs were thestresses encountered during operations, and the po-tential for oxidation and erosion from environmen-tal exposures. CMCs were proving to be the bet-ter solution than monolithic ceramics for brittlerelated failures, however like every new material,

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CMCs brought new challenges. Availability of ma-terials and the process of manufacturing the ma-trices, resulted in costly operations. Additionally,CMCs would require extensive research to under-stand the fiber properties, matrix configurations,failure testing, and the application of coatings toprotect against oxidation.

SiC based CMCs were having great success im-proving toughness but the strength was still notas good as monolithic ceramics. The toughness iswhat allowed CMCs to begin surviving impact testsand build larger components.6 CMCs toughnesswas reported around 20 [MPa m0.5] in the 1990s,much higher than monolithic ceramics between 5-9[MPa m0.5].7

In order to continue developing a ceramic ma-trix composite that would be suitable for turbineconditions, three structural requirements were putinto place. The first requirement is that the com-ponent must possess properties that will exceed themaximum stresses incurred during operations fromthe combination of mechanical, aerodynamic, andthermal gradient loads.3 The second structural re-quirement is that the composite maintain strengthunder high temperatures over lengthy time periods.The third requirement is that the material mustwithstand degradation from foreign objects and/orenvironmental factors.

Currently, most information about the proper-ties of CMCs is unavailable due to proprietary infor-mation, but many government funded projects havemade literature available with such information.It was reported that SiC/SiC CMCs componentswould have to perform under in-plane stress loadsranging 100 to 300 [MPa] and thru-thickness tensilestress of approximately 30 [MPa] with a TIT ex-ceeding 1500 [◦C]. Unfortunately the SiC/SiC sys-tems have not been able to withstand temperaturesabove 1400 [◦C], cooling airflows are still requiredfor CMC components.3

It was mentioned earlier the stresses can occurfrom a combination of forces. It is important tonote the stresses and loads can be increased withdecreasing radii-to-curvature of a shape, increas-ing thermal gradients, and increasing mechanicalloads due to pressures, centrifugal forces, and at-tachments. This understanding will help in thesteps to design components. Stationary compo-nents or components with simpler geometries suchas combustion liners may experience less aerody-namic and thermal stress than turbine blades dueto their shape and function. The installation or

attachments of liners may still result in high me-chanical loads.

Complex components, such as turbine blades,will experience higher stresses. Uncooled turbinevanes may experience hot spots due to the unevencombustion pattern of the exiting gases. The in-troduction of a SiC/SiC CMC shell around a spurwill reduce thermal shock with cooling, but thethin wall can create a suction side introducing newstresses from pressure differentials across the sur-faces.3 Stress levels on airfoils of nozzles will in-crease as the TIT increases due to the nozzle’s ex-posure to the combustion flow of gases. Rotatingparts experience high stress levels at the root ofthe blades caused primarily from centrifugal loads.Figure 5 shows the variation in stress levels acrossdifferent parts that can be comprised of ceramics.

Fig. 5 Peak stress levels for blades, nozzles,and combustor liners vs. TIT. Materials are

Si3N4 and SiC7

The degrading effects that components can en-counter during service are of equal importance.Surface flaw can be an issue in expediting degrada-tion by contact with foreign objects or general han-dling of equipment. The current design approach ischoosing ceramics with high fracture toughness orhigh resistance against crack propagation to main-tain mechanical strength. Alone, monolithic ceram-ics have little resistance to crack propagation andwill fail catastrophically under low strains. In amatrix, the reinforcing fibers deflect an advancingcrack instead of letting it pass through unimpeded,thus increasing toughness.8 This toughness is theprime interest for gas turbine applications.

Components can also undergo erosion and/orcorrosion during high-temperature operations. Amajor concern is the interaction of water vaporformed during combustion with the silica. This re-action with the oxides occurs on the surface causesthe silica to become unstable. As water vapor and

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temperature increase, the rate at which the surfaceweakens. Silicon based ceramics can be protectedwith an environmental barrier coating (EBC) toprotect against this degradation.

Fig. 6 SEM image of EBC coatingSi/Mullite/BSAS9

A basic EBC that is reported on is the threelayer Si/Mullite/BSAS system deposited by airplasma spray.7 Depicted in figure 6, the EBChas a Si bond that absorbs oxygen, and theBSAS (BaO0.75SrO0.25Al2O32SiO2) coating pre-vents water vapor from penetrating the surface ofthe ceramic component. Mullite is an intermediatelayer preventing reactions between SiO2 and Si.

Fig. 7 Fracture map of SiC/SiC CMC for30,000 with no EBC applied to combustor7

Fig. 8 Fracture map of SiC/SiC CMC for30,000 with an EBC applied to combustor7

The performance without EBC and the im-provement in performance with EBC can be shownin figures 7 and 8, respectively. The addition ofEBC to the combustor liner allows the componentto survive higher operating temperatures by over-coming premature surface erosion. The pressurelines in the figures are important indicators of howthe components perform under certain pressure ra-tios (PR). With EBC applied, combustor linerscan survive up to 30,000 hours of operation withtemperatures reaching 1260 [◦C] at a 5:1 PR. Thetemperature drops with increasing pressure ratio.Around a 30:1 PR the allowable temperature isaround 1140 [◦C].7

IV. CMC Manufacturing

With a better understanding on the challenges in-volved using ceramic matrix composites, designerscould workout goals to reach certain material prop-erties. For CMCs it is important they withstandmaximum operating temperatures, maintain ther-mal stability, high toughness, and high mechani-cal strength. The CMC’s toughness and strengthcome primarily from the ceramic fibers. The ce-ramic fibers can range from 3 to 20 µm and usuallymanufactured as yarns of polycrystalline SiC fibers.The yarns are woven into two-dimensional sheets,and the individual fibers are coated with an inter-phase material, typically boron nitride. The inter-phase material creates a weak interface between thematrix and fibers. This is important when a crackis present. If the crack begins to propagate, it willtravel along and around the fibers instead of break-ing through them.10

After having the interphase material applied,the fibers in the sheets can be aligned to maxi-mize strength and stiffness depending on the typicalloads experienced for the desired component.

Next the fibers are strengthened by being ranthrough a hardening solution or slurry that createsa rigid tape. Typically silicon melts are used whichget soaked into the preform and react with carbonto form silicon carbide. This is a process knownas liquid silicon infiltration. After the preform isdried, the tape is cut into the desired shapes be-fore it passes through an oven for a heat treatmentto obtain the required strength within the ceramicmatrix. High temperatures allow the silicon to re-act with the remaining carbon to form the CMC,which is quite dense. A cross-sectional cut of thematrix can be seen in figure 9.

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Fig. 9 Cross-section of a GE CMC9

V. Current Work

Due to the demand for structural materials thatcan withstand long term applications under highthermal loads, variations in the ceramics are beingpursued to find a more suitable alternative to theSiC/SiC CMCs.

Oxide/oxide CMCs have been studied recentlywhere they exhibited strong properties and a highdamage tolerance with the inherent oxidation re-sistance.11 Additionally, oxide/oxide CMCs havedisplayed impressive high-temperature mechanicalproperties. Yet, the oxide/oxide CMCs have per-formed poorly in steam environments by degradingquicker than anticipated.

Efforts are being researched to find the causefor these mechanical failures in the fiber properties.It is proposed that the inherent material defects infiber-matrix interface regions might be causing fail-ure under interlaminar stresses. This may result ina loss of stiffness and cause the CMC structure tofail early.11

Multiscale modeling have begun to becomemore accurate to account for linear and nonlin-ear behavior of materials for desired time lengths.These physics-based models are allowing fine-tuning on the microscale to evaluate damage on themacroscale and begin building a database to under-stand how the damage is initiated and propagatingthrough the component.12

GE feels quite confident in using ceramics intheir engines that they have released statements

earlier this year announcing a $200 million invest-ment to construct two factories in Huntsville, AL.One of the plants will be the first operations in theUnited States to produce silicon carbide ceramicfibers. The other plant will take the SiC fibers andmake the CMC tape to meet the demands of spe-cific components.

These new operations will facilitate the con-struction of GE’s new LEAP engines which are tocontain SiC based CMC shrouds around the tur-bine’s rotating blades and using CMC nozzles.10

As GE works towards commercializing fibers on alarge scale, the result should be readily available sil-icon ceramics at consistent properties to further theadvancement in understanding the material prop-erties.

VI. Conclusion

This paper has reviewed the history and develop-ments of ceramic matrix composites. With advanc-ing technology and better equipped tests to performanalysis, ceramics were more difficult to improvethan originally purposed. This has caused exten-sive time to research and develop the ceramics inthe CMCs used today.

Beginning in the 1950s, it has been a persistentpursuit to improve the gas turbine engine. Workin the 1960s introduced new superalloys and man-ufacturing techniques. These material processingdevelopments allowed the scientists and engineersof the 1970s to start working on monolithic ceram-ics to enable higher turbine inlet temperatures andhigher thermal efficiencies. In order to overcomethe weaknesses of monolithic ceramics, ceramic ma-trix composites were formed which greatly improveceramics toughness. Improving the ceramics tough-ness is important, especially in protection againstimpacts.

Structural requirements were presented in asimilar fashion to the modeling plan used by man-ufacturers. In order for a CMC to be suitablefor turbines it must satisfy the three requirements:survive the maximum stress incurred during gasturbine operations, maintain strength under highthermal loads for extended period times and resistcreep, and finally withstand degradation from for-eign objects and environmental factors. A discus-sion on the benefits of EBCs on ceramic compo-nents was included in the report.

As more private companies begin to fund re-search, the information is not as readily available

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compared to government funded projects. For thisreason it was difficult to find the exact procedureto manufacture CMCs due to proprietary informa-tion. However, a brief summary on some of the keysteps was discussed on how CMCs components aremanufactured.

The final discussion briefly discussed some ofthe current research and work that is being per-formed to continue developing more suitable mate-rials. As GE begins to settle into their new SiCfiber operations, an exciting new era will begin forCMCs. With a large operations, GE expects tooptimize their processing and gain a better under-standing of how CMCs fail in order to improve uponthe current ceramics to build more efficient engines.

References

1Kear,B.H. and E.R. Thompson. ”Aircraft Gas TurbineMaterials and Processes.” Science 208 (May 1980):847-56.

2Helle, A.S., et al. ”Hot-Isostatic Pressing Diagrams:New Developments.” Acta Metallurgica 33.12 (1985): 2163-2174.

3DiCarlo, James A, and Mark van Roode. ”Ceramic

Composite Development for Gas Turbine Engine Hot Sec-tion Components.” ASME Paper GT2006-90151.

4Richerson, David W. ”Historical Review of Addresss-ing the Challenges of Use of Ceramic Components in GasTurbine Engines.” ASME Paper GT2006-9033, 2006.

5Lee, Stuart M. Handbook of Composite Materials. CA:VCH Publishers,Inc, 1993.

6van Roode, Mark, and Arun K. Bhattacharya. ”Dura-bility of Oxide/Oxide Ceramic Matrix Composites in GasTurbine Combustors.” Journal of Engineering for Gas Tur-bines and Powers 135 (May 2013).

7van Roode, Mark, and Mattison K. Ferber. ”Long-Term Degradation of Ceramics for Gas Turbine Applica-tions.” ASME Paper GT2007-27956, 2007.

8Myers, Marc, and Krishan Chawla. Mechanical Behav-ior of Material. Cambridge: Cambridge University Press,2009.

9Delvaux, John. GE CMC. United Turbine System Re-search Conference. Atlanta, GA, 2015.

10Fellet, Melissae. Ceramic-matrix composites take theheat. MRS Bulletin 40 (November 2015): 916-917.

11Ruggles-Wress, M.B., and S.R. Hilburn Creep in In-terlaminar Shear of an Oxide/Oxide Ceramic Matrix Com-posite at Elevated Temperature. ASME Turbo Expo (June15-19 2015) Paper GT2015-44034, 2015.

12Borkowsi, Luke, and Aditi Chattopadhyay. Multiscalemodel of woven ceramic matrix composites considering man-ufacturing induced damage. Composite Structure 126 (2015)62-71.

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Review on Gas Turbine Hot Section Materials and Technology