Studies on the transient stage of oxidation of VPS and HVOF sprayed MCrAlY coatings

Surface and Coatings Technology 120–121 (1999) 8–15www.elsevier.nl/locate/surfcoat

Studies on the transient stage of oxidation of VPS and HVOFsprayed MCrAlY coatings

Diana Toma a,*, Waltraut Brandl a, Uwe Koster ba University of Applied Science, Department of Materials Science, Neidenburger Str. 10, 45879 Gelsenkirchen, Germany

b University of Dortmund, Emil-Figge Str. 66, 4421 Dortmund, Germany

Abstract

Thermal barrier coating (TBC ) systems protect turbine blades against high-temperature corrosion. They consist of a MCrAlYbond coating and a ceramic top layer. The oxidation resistance of MCrAlY coating is based on the formation of a-Al2O3 in thesteady-state stage of oxidation. The a-Al2O3 grows very slowly between the metallic bond coating and the ceramic top coatingand is thermodynamically stable. High-temperature oxidation experiments with vacuum-plasma-sprayed (VPS) and high-velocity-oxygen-fuel (HVOF )-sprayed MCrAlY coatings showed that the transient stage of oxidation for HVOF coatings is very short.Moreover, in contrast to the VPS coatings, where also Cr2O3, NiO or CoO are identified, on HVOF coatings, only metastablemodifications of alumina were observed. The transformation of metastable alumina modifications to a-Al2O3 is very fast.

Oxidation experiments in a high-temperature chamber attached to a X-ray diffractometer were carried out. The morphologyand the structure of the formed oxide scale were characterized by X-ray diffraction and scanning electron microscopy (SEM).© 1999 Published by Elsevier Science S.A. All rights reserved.

Keywords: a-Al2O3; High-temperature oxidation; HVOF-sprayed coating; MCrAlY

1. Introduction neously nucleation and transformation sequences areoccurring. Brumm et al. [1] studied the formation ofalumina on NiAl over a wide range of temperatures andMCrAlY coatings (M=Ni, Fe, Co or their combina-

tion) as overlay coatings for turbine components determined values of the parabolic rate constants, kp.The Arrhenius-type plot of the data shows three lines,improve their oxidation resistance and provide a longer

lifetime for turbines even under severe environmental which correspond to the growth of c-, h- and a-Al2O3.The lines overlap in the temperature range where twoconditions. It is important that during operation at high

temperatures, a continuous, slow growing oxide scale modifications are formed. For the oxidation of Ni-baseis formed. alloys and MCrAlY alloys, the reported parabolic rate

In the case of MCrAlY coatings, the main component constants are somewhat different [2], and the formationof the oxide scale is Al2O3. Aluminum oxide has several of metastable alumina modifications has been observeddifferent crystal structures, including c-, d-, h- and only by a few authors for short oxidation times [3].a-Al2O3. It has been observed that in the range of lower From kinetics data and from the morphology of thetemperature metastable fast growing Al2O3 modifica- scale, it is possible (but difficult in some cases, especiallytions are formed. At higher temperatures (>900°C), the for MCrAlY ) to estimate which alumina modificationformation of slow-growing protective a-Al2O3 is will form. The alumina modifications usually have typi-expected. During oxidation at high temperature, a phase cal morphologies: h-Al2O3 is often found to have atransformation from metastable alumina modification needle-like structure, whereas a-Al2O3 usually formsto stable a-Al2O3 takes place. It is to be considered that equiaxed or elongated, fine-grained scales.not only one Al2O3 modification grows, and simulta- The formation of a protective oxide scale depends

not only on the chemical composition of the coating(Al and Y content) and the oxidation conditions (tem-* Corresponding author. Tel.: +49-290-9596-169;perature range, environment), but also on the manufac-fax: +49-290-9596-170.

E-mail address: [email protected] (D. Toma) turing process and the structure of the coating. MCrAlY

0257-8972/99/$ – see front matter © 1999 Published by Elsevier Science S.A. All rights reserved.PII: S0257-8972 ( 99 ) 00332-1

9D. Toma et al. / Surface and Coatings Technology 120–121 (1999) 8–15

coatings can be produced by thermal spraying (vacuumplasma spraying: VPS; high-velocity oxygen fuel spray-ing: HVOF), by sputtering or by evaporation (physicalvapor deposition: PVD, electron-beam physical vapordeposition: EB-PVD), but, generally, for industrialapplications, thermal sprayed coatings are preferred. Anumber of investigations were carried out to understandthe oxidation mechanism of thermal sprayed MCrAlYcoatings [4–17]. Most of them concentrate on theMCrAlY coatings produced by VPS or LPPS ( low-pressure plasma spraying). The MCrAlY coatings were

Fig. 1. Oxidation behavior of thermal sprayed MCrAlY coatings inexamined as a part of a TBC system, and the growth of synthetic air at 1050°C.the oxide scale at the ceramic/MCrAlY interface wasinvestigated. No comparison between TBC systems con-

sprayed on a steel substrate. The HVOF-spraying condi-sisting of differently manufactured MCrAlY coatingstions are summarized elsewhere [16 ]. After spraying,was done. Recently, studies on the oxidation behaviorthe coatings were separated from the substrate and thenof free standing VPS and HVOF sprayed MCrAlYvacuum-heat-treated and aged [15,16 ]. The annealedcoatings [16,17] have shown that the last coatings oxi-MCrAlY coatings both have a similar structuredize more slowly than the VPS coatings. The lower[Fig. 2(a) and (b)]. They consist of four metallic phasesoxidation rate corresponds to the fast formation ofc, c∞-Ni3Al, b-NiAl, s-(Co,Cr), which are uniformlythermodynamically stable a-Al2O3 in the initial stagesdistributed in the coatings. The VPS coating has a denseof oxidation, and the authors suppose that the

Al-containing oxide particles, which were formed duringspraying, favor the formation of a-Al2O3 [17]. In orderto clarify this assumption, VPS and HVOF-sprayedcoatings were oxidized in air in the temperature range950–1200°C for short times.

Usually, the oxidation behavior of MCrAlY coatingsis examined over very long periods, in order to determinewhether the oxide scale consists of thermodynamicallystable a-Al2O3 and guarantees long-time protection. Foran industrially applicable alloy, it is generally of minorinterest to study the transient stages of oxidation withmetastable phase formation and transformation. Theaim of the present study on the transient stage ofoxidation was to find an explanation for the slowoxidation rate of HVOF-sprayed coatings. The specificmorphology of the HVOF coatings (namely a porosityof ~2% and the presence of oxides in the coating) wasexpected to facilitate oxidation with the formation oflow adherent oxide scales and perhaps lead to internaloxidation. On the contrary, the oxidation rates of theHVOF sprayed coatings are twice lower than the oxida-tion rate of corresponding VPS MCrAlY coatings [17].The main difference between the oxidation courses is inthe transient stage of oxidation (Fig. 1). Even aftershort oxidation times (~167 h), the oxide scale alreadyconsists of a-Al2O3, and it is supposed that the oxidedispersion in the HVOF coating has an effect on thenucleation of a-Al2O3.

2. Experimental

Fig. 2. Microstructure of the VPS (a) and HVOF-sprayed (b) MCrAlYA NiCoCrAlYRe powder, which contains 12 wt% Al coatings: b-NiAl (a), c/c∞-Ni3Al (b), s-(Co,Cr) (c), Al2O3–Al

xYyOz

(d).and 3 wt% Re [18,19] was vacuum-plasma- and HVOF-

10 D. Toma et al. / Surface and Coatings Technology 120–121 (1999) 8–15

microstructure and does not contain oxides. In contrast,as a result of the spraying conditions (contact of meltedpowder with the atmosphere, incompletely meltedpowder particles), the HVOF-sprayed coating showsporosity in the order of 2%, and oxides such asa-Al2O3 and Al

xYyOz

are identified in the coating [17].The annealed coatings were metallographically pol-

ished, cut into rectangular pieces (6×7×0.4 mm), ultra-sonically cleaned in water and ethanol and isothermallyoxidized in air in the temperature range 950–1200°C ina high-temperature chamber (Anton Paar HTK 6)attached to a Phillips Xpert X-ray diffractometer. Thesamples were heated up very rapidly from room temper-ature to the selected temperature (~100 K/min; temper-ature accuracy=±5 K). After about 5 min at theoxidation temperature, the first X-ray scan (h–2h meas-urement) was started. The scanning time was about2.5 h, and a new scan was started 10 min after theprevious scan was finished. Depending on the type ofcoating (VPS or HVOF) and the oxidation temperature,the MCrAlY coatings were oxidized between 12.5 and32.5 h. The oxidized specimens were characterized byscanning electron microscopy with X-ray energy-disper-sive spectroscopy (SEM/EDS).

3. ResultsFig. 3. Diffraction patterns of VPS MCrAlY coating oxidized at 950°C.

3.1. VPS MCrAlY coating

For the VPS MCrAlY coating oxidized at 950°C for12.5 h, the X-ray measurements show no significantchanges in the microstructure of the coating (Fig. 3).The peaks of b-NiAl are smaller, but they can still beidentified. The formation of an oxide scale is confirmedonly by two peaks, which correspond to c-Al2O3. Oneof the strong-intensity peaks of a-Al2O3 (d=0.2379 nm)can be identified, but its height is too small to concludethat a-Al2O3 is really formed. The morphology of theoxidized surface (Fig. 4) is fairly flat without any indica-tion of needle-like h-Al2O3 or a-Al2O3. The EDS analysisidentifies a higher content of Al on the surface incomparison with a non-oxidized sample.

Further oxidation experiments were carried out at1050°C. The formation of c-alumina is already observed

Fig. 4. SEM micrograph of the surface of VPS MCrAlY coating oxi-during the first scan. After 5 h oxidation, the X-raydized at 950°C.patterns of h- and a-Al2O3 are identified. c-Al2O3 is still

present, but its intensity becomes weaker. After 12.5 hoxidation time, c-Al2O3 is no longer present and the no X-ray patterns of h-Al2O3 are present, and a-Al2O3

is the only component of the oxide scale (Fig. 5).presence of a-Al2O3 on the VPS coating becomes moreevident (Fig. 5). The SEM micrograph of the oxidized Extended oxidation of the sample (32.5 h) revealed no

further change in phase composition in the oxide scale.coating surface shows a typical morphology fora-Al2O3, a structure with smoothed ridges, but a The oxide scale is denser in comparison with the scale

formed after 12.5 h and seems to have a good adherencewhisker-type structure can also be observed (Fig. 6).A new VPS sample was oxidized at 1050°C for a to the bulk material (Fig. 7).

For the VPS coating oxidized at 1200°C, X-raylonger time. After about 18 h of isothermal oxidation,


Fig. 7. SEM micrograph of the surface of VPS MCrAlY coating oxi-dized at 1050°C for 32.5 h.

Fig. 5. Diffraction patterns of VPS MCrAlY coating oxidized at1050°C.

Fig. 8. Diffraction patterns of VPS MCrAlY coating oxidized atFig. 6. SEM micrograph of the surface of VPS MCrAlY coating oxi-1200°C.dized at 1050°C for 12.5 h.

diffraction indicated very fast changes in phase com- consists of Cr2O3 (Fig. 8). The SEM micrograph of thescale shows a compact oxide scale (Fig. 9).position from scan to scan. After 2.5 h oxidation,

c-Al2O3 is formed, and the peaks of b-NiAl becomesmall. After 5 h, the a modification of Al2O3 is identified. 3.2. HVOF-sprayed coatingAdditionally, the peaks of b-NiAl disappear completely.After extended oxidation (12.5 h), X-ray measurements In contrast to the VPS coating, the X-ray measure-

ments of the oxide scale formed on the HVOF sprayedindicated that in addition to a-Al2O3, the oxide scale


Fig. 9. SEM micrograph of the surface of VPS MCrAlY coating oxi- Fig. 11. SEM micrograph of the surface of HVOF-sprayed MCrAlYdized at 1200°C. coating oxidized at 950°C.

coating prove only the existence of a-Al2O3. On the formed on VPS coating at 950°C, the morphology ofHVOF-sprayed coating oxidized at 950°C, the X-ray the scale formed on the HVOF coating is a structurepatterns of a-Al2O3 are already identified after 2.5 h typical of a-Al2O3 (Fig. 11).oxidation time (Fig. 10). In contrast to the oxide scale The same observation is valid for the HVOF MCrAlY

coating oxidized at 1050°C (Fig. 12). No peak corre-sponding to c- or h-alumina modification is observed.With increasing oxidation time, the intensity of the

Fig. 10. Diffraction patterns of HVOF-sprayed MCrAlY coating oxi- Fig. 12. Diffraction patterns of HVOF-sprayed MCrAlY coating oxi-dized at 1050°C.dized at 950°C.


Fig. 15. SEM micrograph of the surface of HVOF-sprayed MCrAlYFig. 13. SEM micrograph of the surface of HVOF-sprayed MCrAlYcoating oxidized at 1050°C for 25 h. coating oxidized at 1200°C.

a-Al2O3 peaks increases. The morphology of the scale micrograph of the scale shows a dense oxide scaleformed after 25 h oxidation at 1050°C is typical of the without spallation (Fig. 15).a-Al2O3 morphology with smooth ridges (Fig. 13).

For the HVOF-sprayed coating oxidized at 1200°C,high-intensity patterns of a-Al2O3 are already identified 4. Discussionafter 2.5 h (Fig. 14), and the a-Al2O3 remains the onlyoxide phase after longer oxidation times. The SEM This work confirms our previous assumption of fast

formation of the thermodynamically stable a-Al2O3 ona HVOF-sprayed MCrAlY coating. Even after a veryshort oxidation time of 2.5 h at 950 and 1050°C, theformation of a-Al2O3 on the HVOF-sprayed coating isobserved. On the VPS coating, a-Al2O3 is identified onlyafter longer oxidation times, and in the transient stageof oxidation, metastable alumina modifications are iden-tified. After 32.5 h, c- and h-alumina are still present inthe oxide scale formed on VPS MCrAlY coatings. InX-ray diffraction patterns, the formation of the aluminascale can be well correlated with the identification ofNiAl X-ray patterns. The NiAl phase is the reservoir,which provides the Al for the formation of aluminascale. The disappearance of the NiAl patterns in theearly stage of oxidation suggests fast oxidation. On theVPS coatings, the patterns of NiAl phase already disap-peared after 5 h of oxidation at 1050°C, whereas on theHVOF-sprayed coatings, the NiAl phase can still beidentified after 12.5 h of oxidation at 1200°C.

The thermogravimetrical measurements show that theoxidation rate of VPS MCrAlY coatings is higher thanthat of HVOF MCrAlY coatings and, as studies onyttria-dispersed NiCrAl-base ODS alloys have shown[20,21], this could be attributed to the formation offast-growing h-Al2O3 on VPS coatings. Similar observa-tions of fast formation of a-Al2O3 scale were reportedby Pint et al. [22] for a Al2O3-dispersed b-NiAl alloy.Unfortunately, the kinetics data [22] are inconclusive,Fig. 14. Diffraction patterns of HVOF-sprayed MCrAlY coating oxi-

dized at 1200°C. and a comparison between the oxidation behavior of


the Al2O3-dispersed alloy and the Y2O3-dispersed alloy HVOF-sprayed MCrAlY coatings appears to be analternative to the more expensive VPS coatings.is not possible.

Burtin et al. [23,24] examined the thermal stabilityof alumina, and observed that additives influence thetransformation from metastable alumina to a-alumina.They have shown that small ions such as Al3+ and 5. ConclusionMg2+ accelerate the transformation, whereas large ions

Short-time oxidation experiments on VPS and(Zr4+, Ca4+) are found to be inhibitors for the trans-HVOF-sprayed MCrAlY coatings were carried out. Forformation c�h�a. A small dopant ion seems to favorthe VPS coating, it was found that the formation of thethe transformation of the Al2O3 lattice from a close-metastable alumina leads to a fast oxidation rate in thepacked structure (metastable alumina modification) totransient stage. This fact also influences the oxidationthe hexagonal close-packed structure of a-Al2O3. Thisrate in the steady-state stage. The oxide scale formedmodel could give an explanation for the beneficial effecton the HVOF-sprayed coating already consists, after aof the Al2O3 dispersion on the fast formation ofshort oxidation time (~2.5 h), only of a-Al2O3. Thisa-Al2O3 on HVOF MCrAlY coatings. The presence ofdetermines a slow oxide growth. It seams that the finefinely dispersed a-Al2O3 in the HVOF coatings couldoxide dispersion formed in HVOF-sprayed MCrAlYfavor the nucleation and, if metastable alumina modifi-coatings has a beneficial effect on the high-temperaturecations occur, their transformation to a-Al2O3. oxidation behavior of the coatings.In contrast to VPS MCrAlY coatings, in HVOF-

sprayed coatings Y is present not only as a constituentof metallic phases (Y and M

xYy) but also in oxide

phases (AlxYyOz). This fact allows a comparison of the

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Studies on the transient stage of oxidation of VPS and HVOF sprayed MCrAlY coatings

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