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Performance of MCrAlX coatings: Oxidation, Hot corrosion and Interdiffusion
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering.
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

MCrAlY coatings (M=Ni and/or Co) are widely used for the protection of superalloy components against oxidation and hot corrosion in the hot sections of gas turbines. The drive for coating systems to bestow adequate oxidation and corrosion resistance upon the components becomes urgent as an inevitable result of the necessary improvement in engine combustion efficiency and service lifetime. Through the careful design of the composition, MCrAlY coating performance can be optimized to meet the needs under different service conditions and component materials, therefore, “MCrAlX”, with “X” stands for the minor alloying elements, is used to highlight the effect. In the present thesis, the performance of new MCrAlX coatings is investigated with respect to oxidation, hot corrosion and interactions between coating-superalloy substrates.

Oxidation of MCrAlX coatings can be generally categorized into initial, steady and close-to-end stages. Coating performance can be affected by various factors at different stages, therefore, experiments were designed by targeting the oxidation stages. Investigation on the initial stage oxidation behavior of MCrAlY coatings with post-deposition surface treatments reveals the different growth mechanisms of alumina scales. Surface treatments significantly reduce the alumina growth rate by suppressing transient alumina development and aiding the early formation of α-Al2O3, which improves the long-term oxidation performance of the coating. Similarly, the modification of minor alloy elements in MCrAlX coatings also serves the purpose. The oxidation behavior of new MCrAlX coatings was investigated at the steady oxidation stage, followed by the microstructure observation, thermodynamic and kinetic simulations. As an alternative reactive element addition of Y, Ce shows a negative effect on the formation of columnar alumina scales of high strain tolerance. In comparison, Fe or Ru addition shows no influence on alumina growth, rather than strengthening the phase stability in the coating and reducing the interdiffusion between coating-substrate through different mechanisms. As the oxidation proceeds to the close-to-end stage, a reliable criterion to estimate the capability of coating to form α-Al2O3 is of great importance to accurately evaluate coating lifetime. A temperature-dependent critical Al-activity criterion was proposed to better predict the formation of a continuous α-Al2O3 scale based on correction of elemental activity using thermodynamic database to replace the empirical Al-concentration based criterion.

Severe interdiffusion occurs between coating-substrate during high temperature oxidation, accelerating the degradation of the system. Interdiffusion behavior of diffusion couples of superalloys-MCrAlX coatings were examined. It is highlighted that the recrystallization of superficial layer of the substrate contributes to the secondary reaction zone formation and element interdiffusion controls subsequent zone thickening.

Study on Type I hot corrosion behavior of new MCrAlX coatings shows that the addition of Fe has no influence on basic fluxing reactions before severe Al depletion from the coating occurs. Instead, it boosts the “effective” Al supply of coating by shifting the equilibrium concentration of Al in the γ phase to a low Al level. Besides, the pre-mature coating degradation at the coating-substrate interface was due to the fast growth of corrosion products from substrate induced large local volume expansions, resulting in early coating spallation.

Abstract [sv]

MCrAlY ytbeläggningar (M=Ni och/eller Co) används ofta för att skydda komponenter tillverkade av superlegeringar mot oxidation samt högtemperaturskorrosion i de heta gasturbindelarna. Förbättrad förbränningseffektivitet och livslängd hos gasturbiner, gör att ytbeläggningssystemen måste besitta adekvata oxidations- och korrosionsmotstånd. Genom att omsorgsfullt utforma den kemiska sammansättningen hos MCrAlY ytbeläggningar kan deras prestanda optimeras för att möta kraven från olika driftförhållanden samt olika substratmaterial, därför används beteckningen "MCrAlX" för att belysa förändringar av den kemiska kompositionen, där "X" står för reaktiva legeringsämnen som tillsätts i mindre mängder. I denna avhandling undersöks prestandan hos en ny MCrAlX ytbeläggning med hänsyn till oxidation, högtemperaturskorrosion och interaktionen mellan ytbeläggningen och superlegeringssubstratet.

Oxidation av MCrAlX ytbeläggningar kan generellt kategoriseras i tre faser; initiala, stabila och nära-slutet fasen. Ytbeläggningens prestanda kan påverkas av olika faktorer vid de olika faserna, därför utformades olika experiment för att undersöka de olika oxidationsfaserna. Undersökningen av den initiala fasen av oxidationsbeteendet hos MCrAlX ytbeläggningar som genomgått ytbehandlingar efter ytbeläggningsdeponeringen avslöjade olika tillväxtmekanismer hos aluminiumoxidskikten. Aluminiumoxidens tillväxthastighet reducerades signifikant av ytbehandlingarna, detta genom att undertrycka utvecklingen av övergående aluminiumoxid och bistå den tidiga tillväxten av α-Al2O3, vilket förbättrar ytbeläggningens oxidationsprestanda långsiktigt. De reaktiva legeringsämnena som tillsätts i mindre mänger påverkar ytbeläggningens oxidationsprestanda på liknande sätt. Oxidationsbeteendet hos de nya MCrAlX ytbeläggningarna i den stabila fasen följdes av mikrostrukturundersökning, termodynamiska och kinetiska simuleringar. Det framkom att Ce visar en negativ effekt på bildandet av kolumnära aluminiumoxidskikt med hög töjningstolerans som alternativt reaktivt legeringsämne till Y. Jämförelsevis ger Fe- eller Ru-tillsatser ingen påverkan på aluminiumoxidtillväxten, förutom att det förstärker fasstabiliteten i ytbeläggningen samt genom olika mekanismer reducerar interdiffusionen mellan ytbeläggningen och substratet. När oxidationsprocessen kommit till nära-slutet fasen, är det viktigt att uppskatta kapaciteten hos en ytbeläggning att bilda α-Al2O3, detta då det är ett tillförlitligt kriterium för att noggrant kunna utvärdera ytbeläggningens livslängd. Därför föreslogs ett temperaturberoende kriterium för kritisk Al-aktivitet för att bättre kunna förutsäga bildandet av ett kontinuerligt α-Al2O3-skikt. Kriteriet baserades på korrigering av legeringsämnens aktivitet genom att använda en termodynamisk databas, detta för att ersätta det empiriska Al-koncentrations baserade kriteriet.

Vid högtemperatursoxidation sker en omfattande interdiffusion mellan ytbeläggningen och substratet, vilket accelererar degraderingen av ytbeläggningssystemet. Därför har interdiffusionsbeteendet mellan superlegeringssubstratet och MCrAlX ytbeläggningen undersökts i denna avhandling. Det framkom att rekristallisationen av ytliga skikt av substratet bidrar till formationen av den sekundära reaktionszonen och att interdiffusion kontrollerar zonens efterföljande tjocklektillväxt.

Undersökningen av Typ I högtemperaturskorrosionsbeteendet hos en ny MCrAlX ytbeläggning visar att legeringstillägg av Fe inte påverkar de grundläggande flödesreaktionerna innan en kritisk Al utarmning sker i ytbeläggningen. Istället stimulerar det tillförseln av Al genom att skifta jämviktskoncentrationen av Al i γ fasen till en låg nivå av Al. Det framkom också att den tidiga degraderingen av ytbeläggningen vid gränsskiktet mellan ytbeläggningen och substratet kommer av att den snabba tillväxten av korrosionsprodukter från substratet inducerade en stor lokal volymsutvidgning, vilket ledde till tidig ytbeläggningsspallation.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2019. , p. 61
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 2015
National Category
Manufacturing, Surface and Joining Technology
Identifiers
URN: urn:nbn:se:liu:diva-161511DOI: 10.3384/diss.diva-161511ISBN: 9789175190051 (print)OAI: oai:DiVA.org:liu-161511DiVA, id: diva2:1367472
Public defence
2019-11-22, ACAS, A Building, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2019-11-04 Created: 2019-11-04 Last updated: 2019-11-04Bibliographically approved
List of papers
1. Long-term oxidation of MCrAlY coatings at 1000 degrees C and an Al-activity based coating life criterion
Open this publication in new window or tab >>Long-term oxidation of MCrAlY coatings at 1000 degrees C and an Al-activity based coating life criterion
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2017 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 332, p. 12-21Article in journal (Refereed) Published
Abstract [en]

MCrAlY type (M = Ni and/or Co) coatings are widely used for the protection of components in the hot sections of gas turbines at high service temperatures by forming a continuous alpha-alumina. A reliable criterion to estimate the capability of coating to form alpha-alumina is of great importance to accurately evaluate coating lifetime. However, some coatings retain the ability to form a continuous alpha-alumina scale when the concentration of Al in coatings decreases to a critical level, therefore, the empirical Al-concentration based criterion is inadequate to properly predict the formation of a continuous alpha-alumina. Thus, a new life criterion, namely the critical Al-activity criterion, is proposed. In this work, the critical Al-activity to form a continuous a-alumina was validated by Al-activity calculation using Thermo-Calc software based on survey of research results of critical Al-concentration to form alpha-alumina on binary Ni-Al and ternary Ni-Cr-Al systems. Long-term oxidation tests were performed to support the criterion: three different MCrAlY coatings coated on IN-792 superalloy substrates were oxidized at 1000 degrees C for various periods of time up to 10,000 h. The microstructural evolution of MCrAlY coatings was investigated using Scanning Electron Microscope. The near-surface Al concentration and interdiffusion behaviour between substrate and coating were measured using Energy Dispersive X-ray Spectroscopy. The new critical Al-activity criterion has been successfully adopted in alpha-alumina formation prediction, showing a good agreement with experiment results. Therefore, it can be concluded that the extrapolation of new criterion from binary and ternary systems to multi-alloyed MCrAlY system is reasonable. Furthermore, the partial pressure of oxygen (P-O2) in atmosphere has been taken into consideration by combination with Al-activity to calculate the critical chemical reaction constant (K) of formation of a-alumina. The potential applicability of the methodology to predict MCrAlY life is also discussed.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE SA, 2017
Keywords
MCrAIY; Al-activity; alpha-Alumina; Life criterion; Long-term oxidation
National Category
Manufacturing, Surface and Joining Technology
Identifiers
urn:nbn:se:liu:diva-144267 (URN)10.1016/j.surfcoat.2017.09.086 (DOI)000418968100003 ()
Conference
44th International Conference on Metallurgical Coatings and Thin Films (ICMCTF)
Note

Funding Agencies|Siemens Industrial Turbomachinery AB (Finspang, Sweden); Swedish Energy Agency through KME consortium [KME-703]; research school of Agora Materials and AFM Strategic Faculty Grant SFO-MAT-LiU in Linkoping University [2009-00971]

Available from: 2018-01-12 Created: 2018-01-12 Last updated: 2019-11-04
2. Investigation of Element Effect on High-Temperature Oxidation of HVOF NiCoCrAlX Coatings
Open this publication in new window or tab >>Investigation of Element Effect on High-Temperature Oxidation of HVOF NiCoCrAlX Coatings
2018 (English)In: Coatings, ISSN 2079-6412, Vol. 8, no 4, p. 129-145Article in journal (Refereed) Published
Abstract [sv]

MCrAlX (M: Ni or Co or both, X: minor elements) coatings have been used widely to protect hot components in gas turbines against oxidation and heat corrosion at high temperatures. Understanding the influence of the X-elements on oxidation behavior is important in the design of durable MCrAlX coatings. In this study, NiCoCrAlX coatings doped with Y + Ru and Ce, respectively, were deposited on an Inconel-792 substrate using high velocity oxygen fuel (HVOF). The samples were subjected to isothermal oxidation tests in laboratory air at 900, 1000, and 1100 °C and a cyclic oxidation test between 100 and 1100 °C with a 1-h dwell time at 1100 °C. It was observed that the coating with Ce showed a much higher oxidation rate than the coating with Y + Ru under both isothermal and cyclic oxidation tests. In addition, the Y + Ru-doped coating showed significantly lower β phase depletion due to interdiffusion between the coating and the substrate, resulting from the addition of Ru. Simulation results using a moving phase boundary model and an established oxidation-diffusion model showed that Ru stabilized β grains, which reduced β-depletion of the coating due to substrate interdiffusion. This paper, combining experiment and simulation results, presents a comprehensive study of the influence of Ce and Ru on oxidation behavior, including an investigation of the microstructure evolution in the coating surface and the coating-substrate interface influenced by oxidation time.

Place, publisher, year, edition, pages
M D P I AG, 2018
Keywords
MCrAlX coatings, Ruthenium, Cerium, Oxidation, Simulation
National Category
Materials Engineering
Identifiers
urn:nbn:se:liu:diva-148278 (URN)10.3390/coatings8040129 (DOI)000435187300014 ()
Funder
Swedish Energy AgencyLinköpings universitet
Note

Funding agencies: Carl Tryggers Stifelsen [CTS16:207]; Swedish Research Council [VR-2014-3079]; Erling-Persson Family Foundation [2017-10-09]; Promobilia Foundation [F17603]

Available from: 2018-06-05 Created: 2018-06-05 Last updated: 2019-11-04Bibliographically approved
3. The iron effect on oxidation and interdiffusion behaviour in MCrAlX coated Ni-base superalloys
Open this publication in new window or tab >>The iron effect on oxidation and interdiffusion behaviour in MCrAlX coated Ni-base superalloys
2019 (English)In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 166, article id 107599Article in journal (Refereed) Published
Abstract [en]

Two MCrAlX powders with different Fe content were deposited on an IN792 superalloy using high velocity oxygen-fuel spraying (HVOF). The oxidation and interdiffusion behaviour of the coated specimens were investigated at 900, 1000 and 1100 degrees C for different exposure times. Experimental results showed that high Fe addition in the MCrAlX coating had no obvious effect on oxidation, however, a great impact on coating-substrate interdiffusion and coating degradation was observed. Although no influence on Al diffusivity in IN792, high Fe addition promotes the formation of a discontinuous sigma phase layer at the coating/substrate interface, which retards Al diffusion in the IN792 substrate at 900 degrees C. To investigate Fe influence on phase equilibrium, thermodynamic calculation was also performed. It was shown that high Fe addition in the MCrAlX coating increased the beta phase fraction and stabilized the beta phase by extending the beta + gamma phase region. Besides, high Fe addition in the MCrAlX coating modifies Cr diffusion, and alters local phase equilibrium at the coating/substrate interface. These two factors suppress inner-beta depletion of high Fe containing coating. Grain coarsening of the coating can be observed during oxidation, it was retarded by high Fe addition. (C) 2019 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

Place, publisher, year, edition, pages
ELSEVIER SCI LTD, 2019
Keywords
Coating; Iron addition; Interdiffusion; Thermodynamics; Microstructure
National Category
Manufacturing, Surface and Joining Technology
Identifiers
urn:nbn:se:liu:diva-154527 (URN)10.1016/j.matdes.2019.107599 (DOI)000458260700003 ()
Note

Funding Agencies|Siemens Industrial Turbomachinery AB (Finspang, Sweden); Swedish Energy Agency through KME consortium; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [SFO-Mat-LiU 2009-00971]

Available from: 2019-02-20 Created: 2019-02-20 Last updated: 2019-11-04
4. Failure Mechanism of MCrAlY Coating at the Coating-Substrate Interface under Type I Hot Corrosion
Open this publication in new window or tab >>Failure Mechanism of MCrAlY Coating at the Coating-Substrate Interface under Type I Hot Corrosion
2019 (English)In: Materials and corrosion - Werkstoffe und Korrosion, ISSN 0947-5117, E-ISSN 1521-4176, Vol. 70, no 9, p. 1593-1600Article in journal (Refereed) Published
Abstract [en]

MCrAlY coatings are widely used to provide protection of hot component in modern gas turbine engines against high‐temperature oxidation and hot corrosion. Coating‐substrate interface, where the substrate is only partially covered by the coatings, is vulnerable to the hot corrosion attack. The accelerated degradation at the coating‐substrate interface can cause fast spallation of the coating, leading to the early failure of the gas turbine components. In this paper, MCrAlY powder was deposited on IN792 disks by high‐velocity oxygen‐fuel spraying. The hot corrosion behavior of the coated sample was investigated using (0.8Na, 0.2K)2SO 4 salt deposition at 900°C in lab air. Results showed a minor attack in the coating center, however, an accelerated corrosion attack at the coating‐substrate interface. The fast growth of corrosion products from substrate caused large local volume expansions at the coating‐substrate interface, resulting in an early coating spallation.

Place, publisher, year, edition, pages
Wiley-VCH Verlagsgesellschaft, 2019
Keywords
Coating-subtrate interface, hot corrosion, IN792, MCrAlY coating
National Category
Materials Engineering
Identifiers
urn:nbn:se:liu:diva-155790 (URN)10.1002/maco.201810720 (DOI)000483817500006 ()2-s2.0-85063385770 (Scopus ID)
Note

Funding agencies:  KME consortium-ELFORSK; Linkoping University [SFO-Mat-LiU 2009-00971]

Available from: 2019-03-27 Created: 2019-03-27 Last updated: 2019-11-04Bibliographically approved
5. Effects of Surface Finish on the Initial Oxidation of HVAF-sprayed NiCoCrAlY Coatings
Open this publication in new window or tab >>Effects of Surface Finish on the Initial Oxidation of HVAF-sprayed NiCoCrAlY Coatings
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2019 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Journal of Surface and Coatings Technology, ISSN 0257-8972, Vol. 364, p. 43-56Article in journal (Refereed) Published
Abstract [en]

Oxide scale formed on HVAF-sprayed NiCoCrAlY coatings and the effect of surface treatment were investigated by a multi-approach study combining photo-stimulated luminescence, microstructural observation and mass gain. The initial oxidationbehaviour of as-sprayed, polished and shot-peened coatings at 1000 °C is studied. Both polished and shot-peened coatings exhibited superior performance due to rapid formation of α-Al2O3 fully covering the coating and suppressing the growth of transient alumina, assisted by a high density of α-Al2O3 nuclei on surface treatment induced defects. Moreover, the fast development of a two-layer alumina scale consisting of an inward-grown inner α-Al2O3 layer and an outer layer transformed from outward-grown transient alumina resulted in a higher oxide growth rate of the as-sprayed coating.

Place, publisher, year, edition, pages
Elsevier: Elsevier, 2019
Keywords
Oxidation, Transient ot alpha transformation, Surface treatment, Polishing, Shot-peening, Photo-stimulated liminescence spectroscopy
National Category
Materials Engineering
Identifiers
urn:nbn:se:liu:diva-154936 (URN)10.1016/j.surfcoat.2019.02.068 (DOI)000463302800006 ()2-s2.0-85062231529 (Scopus ID)
Note

Funding agencies: Siemens Industrial Turbomachinery AB (Finspang, Sweden) [KME-703]; Swedish Energy Agency through KME consortium [KME-703]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [SFO-Mat-LiU 2009-00

Available from: 2019-03-06 Created: 2019-03-06 Last updated: 2019-11-04Bibliographically approved
6. The iron effect on hot corrosion behaviour of MCrAlX coating in the presence of NaCl at 900 °C
Open this publication in new window or tab >>The iron effect on hot corrosion behaviour of MCrAlX coating in the presence of NaCl at 900 °C
2020 (English)In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 815, article id 152381Article in journal (Refereed) Epub ahead of print
Abstract [en]

Cyclic hot corrosion tests of two MCrAlX coatings with different Fe contents were carried out in a molten salt (75 wt% Na2SO4 + 25 wt% NaCl) environment at 900 °C under type I hot corrosion. The positive effect of high Fe addition which reduces the advancing of corrosion front in the coating was discussed. The experimental results showed that the main corrosion reaction is the basic fluxing of Al, while Fe is relatively inert. Combined with thermodynamic modelling, it was demonstrated that a high Fe addition in MCrAlX coatings shifts the equilibrium Al content of the γ phase towards a low level and also reduces the loss of Cr from coating to the substrate. Both effects contributed to a higher “effective” Al supply of the coating to resist basic fluxing and thereby enhance the coating resistance to hot corrosion by reducing the rate of advancing of the corrosion front.

Keywords
Iron effect, Type I hot corrosion, MCrAlX coating, Thermodynamic, Effective Al content
National Category
Manufacturing, Surface and Joining Technology
Identifiers
urn:nbn:se:liu:diva-161512 (URN)10.1016/j.jallcom.2019.152381 (DOI)
Available from: 2019-11-04 Created: 2019-11-04 Last updated: 2019-11-04Bibliographically approved
7. Hot gas corrosion and its influence on the thermal cycling performance of suspension plasma spray TBCs
Open this publication in new window or tab >>Hot gas corrosion and its influence on the thermal cycling performance of suspension plasma spray TBCs
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2019 (English)In: Proceedings of ASME Turbo Expo 2019: Turbomachinery Technical Conference and Exposition, New York, NY: American Society of Mechanical Engineers , 2019Conference paper, Published paper (Refereed)
Abstract [en]

Thermal barrier coatings (TBCs) manufactured with suspension plasma spray (SPS) are promising candidates for use in gas turbines due to their high strain tolerance during thermal cyclic fatigue (TCF). However, corrosion often occurs alongside thermal fatigue and coating durability under these conditions is highly desirable. The current study focuses on understanding the corrosion behavior and its influence on the thermal cyclic fatigue life of SPS TBCs. Corrosion tests were conducted at 780 OC using a mixed-gas (1SO2-0.1CO-20CO2-N2(bal.) in vol. %) for 168h. They were later thermally cycled between 100-1100 ⁰C with a 1h hold time at 1100 ⁰C. Corrosion test results indicated that the damage predominantly started from the edges and a milder damage was observed at the center. Nickel sulfide was observed on top of the top coat and also in the columnar gaps of the top coat. Chromium oxides were observed inside the top coat columnar gaps but close to the bond coat/top coat interface. They were believed to reduce the strain tolerance of SPS TBCs to an extent and also amplify the thermal mismatch stresses during TCF tests. This, together with a fast growth of alumina during the TCF, resulted in a significant drop in the TCF life compared to the standard TCF tests.

Place, publisher, year, edition, pages
New York, NY: American Society of Mechanical Engineers, 2019
National Category
Manufacturing, Surface and Joining Technology
Identifiers
urn:nbn:se:liu:diva-161516 (URN)
Conference
ASME Turbo Expo: Turbomachinery Technical Conference and Exposition, Phoenix, Arizona, USA, June 17-21, 2019
Available from: 2019-11-04 Created: 2019-11-04 Last updated: 2019-11-04Bibliographically approved

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