allbatros advanced long life blade turbine

8/14/2019 ALLBATROS Advanced Long Life Blade Turbine

1/9

ALLBATROS advanced long life blade turbine

coating systems

M.-P. Bacos a,*, P. Josso a, N. Vialas b, D. Poquillon b, B. Pieraggi b,D. Monceau b, J.R. Nicholls c, N. Simms c, A. Encinas-Oropesa c, T. Ericsson d,

S. Stekovic d

a Office National dEtudes et de Recherches Aerospatiales, 29 Avde la Div, Leclerc, B.P. 72,

92322 Chatillon Cedex, Franceb CIRIMATENSIACET-INPT 118 route de Narbonne, 31077 Toulouse, France

c Cranfield University, Bedford MK43 0AL, United Kingdomd University of Linkoping, S-58183 Linkoping, Sweden

Received 12 July 2003; accepted 8 November 2003

Available online 15 January 2004

Abstract

The scientific and technological objectives of this program are to increase the efficiency, reliability and

maintainability of industrial gas turbine blades and vanes by

developing coatings that can warrant a 50 000 h life, i.e. twice that of the usual life, of the hot compo-

nents (8001100 C) even with the use of renewable fuels such as biomass gas or recovery incinerator gas

i.e. low-grade fuels with high pollutant levels,

characterising advanced existing coatings to assess lifetime and performance of coatings and coated

materials,

providing material coating data and design criteria to use coating as a design element,

increasing the fundamental understanding of the behaviour of coated materials, their degradation, fracture

mechanisms and engineering because of the strong need for a mechanism-based modelling of durability.

These programmes permitted the selection of two reference coatings and the development of two

innovative coatings. Concurrently work has been done in order to develop corrosion, oxidation and

thermo-mechanical property models. Correlations between coatings development, experimental results and

calculations will be discussed.

European Communities, 2004. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. Tel.: +33-1-46734513; fax: +33-1-46734164.

E-mail address: [email protected] (M.-P. Bacos).

1359-4311/$ - see front matter

European Communities, 2004. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.applthermaleng.2003.11.018

Applied Thermal Engineering 24 (2004) 17451753

www.elsevier.com/locate/apthermeng
http://mail%20to:%[email protected]/http://mail%20to:%[email protected]/


2/9

Keywords: MCrAlY; Aluminium; Oxidation; Corrosion; Thermo-mechanics; Turbine

1. Introduction

The objective of this project, ALLBATROS, is to increase the efficiency, reliability and

maintainability of industrial gas turbine blades and vanes by developing new high-resistantcoatings and by increasing fundamental understanding of the behaviour of these coatings. The

innovative coatings will lead to an increase of turbine efficiency and to a decrease of emissionlevels (through increased efficiency and by the possibility of using recovery gas as combustion gasin the turbine). The aim is also to use predictive life models to design long life turbines in order to

lower maintenance and energy operating costs. These models will be based on experimental data

gained on innovative coatings as well as reference coatings (RT22-NiAlPt; MCrAlY AM-DRY997) deposited on industrial nickel-based superalloys (CMSX-4, SC2 and IN792). Thepartners of this project are ONERA (France), TURBOMECA (France), NUOVO PIGNONE

(Italy), ALSTOM POWER UK (UK), ALSTOM POWER Sweden (Sweden), CHROMALLOYFrance (France), INPT/CNRS (France), CRANFIELD UNIVERSITY (UK), UNIVERSITY

OF LINKOPING (Sweden).

2. Innovative coating development

For both conventional aluminide and MCrAlY coatings, the life of the coatings is controlled bymaintaining a reservoir of aluminium that forms a protective alumina scale on the surface. Loss of

aluminium occurs by spallation of the alumina scale due to thermal cycling and by interdiffusioninto the substrate. To warrant a high durability life, even with the use of low-grade fuels with highpollutant levels, innovative coatings must ensure the aluminium concentration and activity in the

coating is as high as possible, together with a high chromium concentration. Such coatingsare usually of MCrAlY type where M is equal to nickel, cobalt or a nickelcobalt alloy. ONERAhas developed a new MCrAlY coating via an innovative electroless-like process [1,2] and the

aim of the work was to improve it in order to reach ALLBATROS coatings specifications.Indeed the aluminium concentration of this type of electroless-like coating has been modi-

fied with the use of a modified aluminization and/or by interposing a diffusion barrier andthe aluminium activity has also been changed by use of platinum coating. Concerning chro-mium, its solubility has been altered by using a modified aluminide, such as platinum modifiedaluminide.

2.1. Diffusion barrier

A NiW diffusion barrier has been developed by modifying an old bath designed by Vaaler andHolt [3] and by developing a new procedure route. This process leads to a very adherent layer,

1746 M.-P. Bacos et al. / Applied Thermal Engineering 24 (2004) 17451753


3/9

enriched in tungsten (W) up to 50% mass. Its role as a diffusion barrierpreventing aluminium to

diffuse from coating to the substratehas been checked after aluminization and ageing.

2.2. Whole innovative coating

The whole innovative coating is composed of three deposits:

(1) a NiW under layer (barrier diffusion),

(2) a MCrAlY electroless-like coating,(3) a platinum modified coating as described in Fig. 1.

The non-directionality of the process, even in the case where specimens are coated with the useof gravity assistance, allows a very good uniformity for the coatings produced.

Corrosion behaviour of these coated specimens (SC2 (superalloy)/MCrAlY electroless-like

coating/platinum modified aluminides) compared to typical coatings (SC2 (superalloy)/LPPSMCrAlY coatings) in classical corrosion tests (Na2SO4 salt) is very good (see Fig. 2). During thefirst 1000 h, the innovative coating gains mass (alumina formation) while the typical coating loosesmass (corrosion phenomenon). And after these 1000 h of corrosion the innovative coating keeps all

its integrity. Its upper layer shows some chromium precipitates (in black in Fig. 2) and a very lowaluminium depletion leading to c0 phase (in grey in Fig. 2). More aggressive tests are in progress.

Fig. 1. Innovative coating: details of deposits. (a) 5 lm thick NiW layer; (b) 75 lm thick composite NiCrAlYTa; (c) 75

lm thick composite NiCrAlYTa + 7 lm thick platinum + inward pack aluminization.

M.-P. Bacos et al. / Applied Thermal Engineering 24 (2004) 17451753 1747


4/9

3. Models development

In parallel with innovative coating experimental development, some models are under devel-opment concerning oxidation, corrosion and thermo-mechanical fatigue behaviour.

3.1. Oxidation model

At the CIRIMAT laboratory (INPT-CNRS), the high temperature oxidation and interdiffusionof the coating/superalloy systems is studied. Reference and innovative coatings are tested.Experiments combine isothermal (TGA) and cyclic oxidation conditions in air or air + 2 vol%

H2O, between 900 and 1150 C. Cyclic oxidation tests are realised on samples allowing gravi-metric measurements. These tests are either fast cycles (typically 1000 cycles of 1 h at high tem-perature) under controlled atmosphere, or long cycles (50 cycles of 300 h) under laboratory air.

Oxidised samples are analysed using SEMEDX and XRD to determine the nature of theoxides present in the oxide scale. SEM with EDX elementary maps realised on cross-sections

Fig. 2. Innovative coating after exposure at 850

C in a hot corrosion test (a) mass gain per cm

2

versus time ofexposure. Hot corrosion at 850 C, Na2SO4 contamination, exposed in air (b) general aspect of innovative coating after

1000 h at 850 C, Na2SO4 contamination, hot corrosion test (c) outer zone: black spots are Cr enriched phases, grey

lines are c0 phases and clear zone is b-NiAl(Pt) phase.



5/9

permit the microstructural evolution of the oxide/coating/superalloy systems to be followed, as a

function of time at 900 and 1050 C. Concentration profiles are measured from the oxide layer tothe centre of the samples for nine elements (see Fig. 3).

Resulting data are used to characterise transient and steady state oxidation kinetics using thelocal parabolic rate constant fitting procedure [4] and eventually to determine the occurrence of

breakaway oxidation. Microstructural investigations permit system evolution to be followed,correlations with oxidation kinetics to be drawn, and the discussion of the various degradationcriteria such as: the critical aluminium content under the oxide scale, the extent of the beta-NiAl

phase depleted zone in the coating, the occurrence of spinel phase in the oxide, the remainingcritical thickness of coating and the occurrence of internal oxidation.

Parallel to this experimental work, numerical models are being developed and tested. All

models combine elemental diffusion in the coating/superalloy system with an outward flux ofaluminium for isothermal and cyclic oxidation. The most complete model (Fig. 4) is a finite

element code (under CASTEM [5]) treating diffusion of the nine species in two dimensions, withmoving boundaries and the possibility of cross-terms in the flux equations (when data are

available). A cyclic oxidation model is used as the boundary condition of the diffusion model. Inits simplified form, this model has an analytical solution and allows easy fitting of experimentalcyclic oxidation data [6]. The implementation of phase transformations in the coating/superalloy

system, and a connection with Thermocalc is currently under study. Most simple models, such asthe one-dimensional diffusion of aluminium in a single phase medium, are also compared withexperimental data in order to access the range of possible application of each model.

Fig. 3. Electron back scattered image and concentration profiles measured by SEMEDS for the system RT22 on

CMSX-4, after heat treatment (2 h at 1140 C and 20 h at 870 C).



6/9

3.2. Corrosion model

Cranfield Universitys main activity within the ALLBATROS programme is the developmentof stochastic hot corrosion models. These models are targeted at predicting the hot corrosion lifeof a range of commercial and model coatings over a range of expected high temperature corrosion

environments including both types I and II hot corrosion. Statistically based corrosion models arerequired due to the localised nature of hot corrosion damage. The key parameters that influencethe progress of the hot corrosion reactions have been identified as deposition flux, gas compo-

sition, temperature, deposit composition, incubation time, substrate composition, coating typeand coating composition.

A matrix of 15 laboratory hot corrosion tests is being carried out to investigate quantitativelythe role of deposition rate, temperature and gas composition on the hot corrosion of coated and

uncoated high temperature alloys. The tests are being carried out using the deposit recoat testprocedure which has proved to be a most effective way of simulating in service deposition con-ditions within a laboratory test [711]. The test variables are listed in Table 1note that 360 of

the possible combinations of these variables will be investigated during the course of these tests.

Table 1

Summary of test variables for the laboratory hot corrosion testing

Test variable Range

Substrate material CMSX-4, SC2-B and IN792

Coatings RT22, AMDRY997, innovative coating composition/structures

Deposition flux 1.5, 5.0 and 15 lg/cm2/h

Deposit composition 80/20 mol% (Na/K)2SO4

Gas composition: air + 50 or 500 vpm SO20 or 500 vpm HCl

0 or 5% H2O

Temperature 650, 700, 750, 850 and 900 C

Duration 500 h

Fig. 4. Coated Ni-base superalloy oxidation/interdiffusion model.



7/9

The performance of materials during these tests is being monitored using mass change mea-surements. However, the materials performance data required for the development of the hotcorrosion models is being produced using dimensional metrology: pre-exposure contact mea-

surements and post-exposure measurements of features on polished cross-sections [79]. Thesedimensional metrology methods allow distributions of damage data to be determined which canthen be readily used in the development of statistically based hot corrosion models. Micro-

structural observations of samples are being used to confirm that the data in each model isassociated with particular degradation mechanisms. Fig. 5 illustrates the sensitivity of type II hotcorrosion damage to changes in deposition flux, when exposed at 700 C in an air-500 vpm SOxatmosphere for 500 h.

The models developed during the laboratory hot corrosion tests will be validated under selected

conditions using burner rig exposures. Details of this test rig and its operation are availableelsewhere [9]. Samples will be assessed using the same dimensional metrology methods as for thelaboratory tests to ensure that fully compatible datasets are produced in order to permit good

model validation.

3.3. Methodology of model development regarding thermo-mechanical properties

A model for thermo-mechanical properties of coated superalloy under varying conditions oftemperature and stress must have as its starting point careful studies of cycles to failure under

Fig. 5. Effect of deposition flux on corrosion damage to RT22 coated SC2-B (exposed for 500 h at 700 C in air-500

vpm SOx).



8/9

these different conditions and of crack initiation and propagation. Metallographic examinations

of coated CMSX-4 specimens tested under LCF conditions indicate that crack initiation in mostcases start at the surface of the coating. The microcracks grow from their initiation sites through

the coating towards the coatingsubstrate interface. Some cracks are arrested at the interface andother penetrate the substrate and lead to failure. Therefore, it seems that the fatigue endurance ofa coatingsubstrate system is determined mainly by the fatigue properties of the coating itself.Pores and microcracks in the surface oxide layer are potential initiation sites. Obviously the

ductile-to-brittle transition temperature of the coating is an important property. If in an out-of-phase thermo-mechanical cycle the temperature goes below the transition temperature the risk forfailure is much increased.

In Linkoping University the first step in the modelling work will be to establish damageequations or parametric life equations to predict number of cycles to crack initiation from the

stress and strain obtained. The first choice is to use frequency-modified strain life or stress lifeapproaches. It is important to include the interaction stresses that can be generated between the

coating and the substrate due to different thermal expansions. Another factor to consider is thatalloy element depletion of the coating due to selective oxidation and also due to interdiffusionbetween coating and substrate will change the thermo-mechanical properties of the coating. This

means that a certain coating will behave differently together with different substrates. After crackinitiation, crack growth life can be taken into account by the use of a fracture mechanics ap-proach. The crack growth does not need to go much below the coatingsubstrate interface before

one can consider the component life as exhausted.It is well known that the crack growth can occur by combined oxidation and mechanical ac-

tions. A model in the literature for acceleration of the growth rate due to simultaneous oxidation,

or rather the formation of oxide spikes will be tested. In the model the local stressstrain field in

the vicinity of a spike is calculated and used to define model constants. As is pointed out abovecrack initiation sites can be pores and microcracks in the oxide layer. This opens the possibility tocombine a statistical treatment of oxidation attack with fatigue life calculations. Such a general

model would be of great value.

4. Conclusion

In the framework of this program an innovative coating has been developed. With the proposed

process a three-layer protective coating composed of a modified beta nickel aluminide, a twophase b=c coating and an NiW diffusion barrier layer can be obtained. Experimental resultsshowed that this coating has good corrosion resistance in a typical corrosion test.

Predictive models into oxidation, corrosion and fatigue mechanisms fields are under develop-ment in order to build up the predicted lives of reference and innovative coating systems. The

oxidation model would permit the simulation of long-term oxidation and the development of newexperimental procedures for an accurate determination of oxidation kinetics for very long-termexposures. The corrosion model provides a statistical model for extreme rates of corrosion attack

as a function of deposit chemistry, deposition rate, gas chemistry and temperature would permitthe prediction of hot corrosion life for a large range of high temperature corrosion environments.The model for thermo-mechanical properties of coated superalloy will take into account accel-



9/9

eration of cracks growth rate due to simultaneous oxidation and will be therefore closer to real

engine conditions than simple thermo-mechanical models.These models, based on experimental data and gathering many parameters, would permit the

estimation of long life turbine durability and the development of new coatings developments.

Acknowledgements

The authors would like to thank the European Commission for part funding of this work. The

authors would also like to thank all the partners of the project for their fruitful discussions.

References

[1] B. Girard, M.-P. Bacos, E. Berger, P. Josso, Procede pour former sur un substrat metallique un revetement

metallique protecteur exempt de soufre, French patent, 2 807 073.

[2] M.-P. Bacos, B. Girard, P. Josso, C. Rio, MCrAlY coating developed via a new electroless-like route: influence of

deposition parameters, Surf. Coat. Technol. 162 (2003) 248260.

[3] L.E. Vaaler, M.L. Holt, Codeposition of tungsten and nickel from an aqueous ammoniacal citrate bath, Trans.

Electrochem. Soc. 90 (1946) 4353.

[4] D. Monceau, B. Pieraggi, Determination of parabolic rate constants from a local analysis of massgain curves,

Oxidat. Metals 50 (1998) 477493.

[5] Castem 2000 Finite element Code: .

[6] D. Poquillon, D. Monceau:, Application of a simple statistical spalling model for the analysis of high temperature

cyclic oxidation kinetics data, Oxidat. Metals 59 (2003) 409431.

[7] N.J. Simms, J.E. Oakey, J.R. Nicholls, A methodology for the quantification of corrosion and erosion damagein laboratory, burner rig and plant environments, Proc. Electrochem. Soc. 99-38 (2000) 305316.

[8] N.J. Simms, J.E. Oakey, J.R. Nicholls, Development and application of a methodology for the measurement of

corrosion and erosion damage in laboratory, burner rig and plant environments, Mater. High Temp. 17 (2) (2000)

355362.

[9] N.J. Simms, J.R. Nicholls, J.E. Oakey, Materials for solid fuel fired gas turbines: burner rig and laboratory studies,

Mater. Sci. Forum 369372 (2001) 833840.

[10] N.J. Simms, P.J. Smith, A. Encinas-Oropesa, S. Ryder, J.R. Nicholls, J.E. Oakey, The development of type II hot

corrosion in solid fuel fired gas turbines, in: M. Schutze et al. (Eds.), Lifetime Modelling of High Temperature

Corrosion Processes, EFC No. 34, Maney Publishing, London, 2001, pp. 246260.

[11] Draft Code of Practice for Discontinuous Corrosion Testing in High Temperature Gaseous Atmospheres, EC

Project SMT4-CT95-2001, ERA Technology, UK, 2000.

http://www.castem.org:8001/http://www.castem.org:8001/

allbatros advanced long life blade turbine

Documents