Surface & Coatings Technology 218 (2013) 30–38

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Surface & Coatings Technology

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Preparation and oxidation behavior of SiO2–Al2O3–glass composite coating onTi–47Al–2Cr–2Nb alloy

Wenbo Li ⁎, Minghui Chen, Cheng Wang, Shenglong Zhu, Fuhui WangState Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62Wencui Road, Shenyang 110016, China

⁎ Corresponding author. Tel.: +86 24 23904856; fax:E-mail address: liwb@imr.ac.cn (W. Li).

0257-8972/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.surfcoat.2012.12.022

a b s t r a c t

a r t i c l e i n f o

Article history:Received 12 August 2012Accepted in revised form 17 December 2012Available online 29 December 2012

Keywords:Ti–47Al–2Cr–2Nb alloySiO2–Al2O3–glass composite coatingOxidationInterfacial reaction

A SiO2–Al2O3–glass composite coating was prepared on Ti–47Al–2Cr–2Nb alloy by air spray technique and subse-quent firing. The Ti–47Al–2Cr–2Nb alloy with and without composite coating were oxidized in air for 100 h at800 °C and 900 °C, respectively. Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS)and electron probemicroanalysis (EPMA) technique have been employed to study themicrostructures of the spec-imens before and after oxidation tests, X-ray diffraction (XRD) was used to analyze the oxide phase. For the bareTi–47Al–2Cr–2Nb alloy, a typical layered structure oxide scale formed, the outer layer is TiO2, the intermediate oneis an Al2O3-rich layer, the inner one is amixture of TiO2 and Al2O3, a small amount of dissolved Cr andNb ion beingpreferentially segregated in the inner layer. The composite coating provides good oxidation resistance for the Ti–47Al–2Cr–2Nb alloy. An interfacial reaction of the composite coating and the substrate occurred, forming anAl2O3/Ti5Si3/Al2O3/Z-phase Ti5Al3O2 layered structure in the interfacial zone. The composite coating/Al2O3 bilay-er act as oxygen diffusion barrier and benefit oxidation resistance of the Ti–47Al–2Cr–2Nb alloy. Al2O3 and quartzwere instable in the glass phase; Al2O3 and quartz inclusions partially dissolved into the glass phase, and somequartz particles transformed into cristobalite.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

TiAl intermetallic compounds have received considerable attentionas high-temperature structural materials due to their higher specificstrength than Ni-base superalloys. Multi-element TiAl based alloys,such as Ti–48Al–2(Cr orMn)–2Nb, have been developed for applicationsat high temperatures, e.g. 800–850 °C [1–3]. The applications of TiAlbased alloys, however, were limited because of their poor oxidation re-sistance [4–6].

Because glass–ceramic materials are chemically inert and stable athigh temperatures, they are extensively used as sealant in solid oxidefuel cells (SOFC) [7,8] and oxidation, corrosion, wear resistant coatingsfor steels [9,10], Ni-base alloys [11–13], titanium alloy and TiAl interme-tallic compounds [14–20]. One important advantage of glass–ceramiccoatings is the availability of many glass–ceramic compositions; there-fore it is possible to produce coatings that possess thermo-mechanicalbehaviors compatible to the substrate by carefully choosing the glasscomposition [8]. The conventional glass–ceramic materials are preparedby melt-quenched methods [12–16]. Well-designed heat treatments fornucleation and growth of crystalline phases in the glass are essential for agood compromise between thermal and mechanical properties [7].Therefore, it is more convenient to tailor the thermal and mechanicalproperties by adding ceramic inclusions into the glass matrix and con-trolling the species and amount of the inclusions.

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α-Al2O3 or SiO2 scales that form on alloy surfaces during oxidationprovide good oxidation resistance due to the low diffusion coefficientof oxygen in these materials [21–24]. So α-Al2O3 and SiO2 powdersmight be good candidates for the inclusions of a glass–ceramic coat-ing with good oxidation resistance. Aqueous solution of binary alkalisilicates, or the colloidal suspensions in an aqueous molecular sodiumsilicate solution [25], has been known for a long time and widely usedas inorganic binder in high temperature paint and lightweight refrac-tory material. When losing water in the solution, the small colloidsaggregate and gel with amorphous structure forms [26].

In this study, aqueous solution of potassium silicate and powders ofα-Al2O3 and SiO2 was selected as the starting glass and the inclusionsfor a glass–ceramic composite coating, respectively. The oxidation resis-tance of this composite coating on Ti–47Al–2Cr–2Nb alloy was evaluat-ed, and both the interfacial reaction between the substrate and thecoating and the dissolution of Al2O3 and quartz particles in the glassma-trix were characterized.

2. Experimental procedure

The Ti–47Al–2Cr–2Nb alloy (at.%) was used as substrate material;the alloy was composed mainly of γ-TiAl phase with a smallamount of α2-Ti3Al phase. Specimens with nominal dimensions of20×10×2.5 mm were cut from the alloy ingot and ground with600-grit SiC paper, and then ultrasonically cleaned in ethanol. The spec-imens to be coated were sand blasted and ultrasonically cleaned in

Fig. 1. XRD patterns of the as-prepared SiO2–Al2O3–glass composite coating on Ti–47Al–2Cr–2Nb alloy.

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ethanol again. The surface roughness of the sand-blasted Ti–47Al–2Cr–2Nb alloy Ra=0.59 μm.

The potassium silicate concentration in the aqueous solution in thisstudy was 40 wt.%, and the ratio of SiO2 to K2O was about 3:1 (DayangChemical Plant). Both the sizes of the α-Al2O3 (Shenyang Laishengplant) and SiO2 (Sinopharm chemical reagent Co., Ltd) powders wereabout 1–10 μm.

The process to prepare SiO2–Al2O3–glass composite coating is asfollows:

1. A slurrywas prepared bymethod of ballmill using ZrO2 ballswith di-ameters about 2 mm. The ball milling had performed for 10 min at1500 rpm. The weight ratio of the ZrO2 ball to the slurry was about1:3. The formula of the slurry was 15 g quartz, 5 g α-Al2O3, 2 gwater, and 50 g aqueous solution of potassium silicate.

2. A green coating with thickness of about 25–30 μm was formed onthe surface of the Ti–47Al–2Cr–2Nb alloy by method of air sprayingof the slurry using a manual painting gun. The air pressure ofspraying is about 0.4 MPa. All the surfaces of the samplewere coated.

3. The green coating was solidified in air at room temperature for 24 h,followed by the following sequence: baking in the oven for 12 h at80 °C, 12 h at 120 °C and then 12 h at 260 °C, and finally firing at900 °C for 90 min to form a dense SiO2–Al2O3–glass compositecoating.

In order to analyze the chemical composition of the potassium silicateglass, a bulk specimen was prepared by the same process as for prepara-tion the coating, using the aqueous solution of potassium silicate withnone of alumina and quartz inclusions.

The oxidation properties of the Ti–47Al–2Cr–2Nb alloy with andwithout coatingwere examined by themethods of interrupted oxidationand cyclic oxidation. The interrupted oxidation tests were conducted at800 °C and 900 °C in a muffle furnace up to 100 h, respectively. Beforethe tests, the corundum crucibleswere fired at 1000 °C for 48 h to insureconstant weight of the crucibles. The corundum crucibles with speci-mens inside were put into a furnace where the temperature has alreadybeen elevated to the test temperature. After certain oxidation time, theywere removedquickly from the furnace and cooled in the air down to theroom temperature. Then the mass change of each crucible with a speci-men inside wasmeasured using a balancewith the sensitivity of 10−5 g.

Cyclic oxidation tests were conducted at 900 °C in static air. Thespecimens were kept at 900 °C for 1 h and cooled down to room tem-perature for 15 min as a cycle. Specimenswere weighed with a balanceafter 20 cycles to measure the mass change, and the cyclic oxidationwas conducted for 120 cycles.

The surfacemorphologies and cross-sectionalmicrostructures of thespecimens before and after oxidation were characterized by SEM (FEIINSPECT F 50, FEI, Hillsboro, OR) with an EDS (OXFORD X-Max, OxfordInstruments, Oxford, U.K.). Electron probe microanalysis (EPMA-1610,Shimadzu, Kyoto, Japan) technique was employed to study the detailsof interfacial reaction between composite coating and substrate afteroxidation tests, X-ray diffraction (PANalytical X' Pert PRO, Cu Kα radia-tion at 40 kV, PA Analytical, Almelo, Holland) was used to analyze thephase of the oxide.

The glass zones in the composite coating were picked out to analyzethe chemical composition of the glass phase by EDS. In order to obtainreliable statistical data, analysis spots were spaced so as to eliminatethe error brought by the composition fluctuation of the glass phase,and the chemical composition was evaluated by taking five spots andcalculating the average for each specimen.

3. Results

3.1. Microstructure of the as-prepared composite coating

The XRD patterns of the as-prepared SiO2–Al2O3–glass compositecoating on Ti–47Al–2Cr–2Nb alloy are shown in Fig. 1. A very broad

peak near 25° is present in the XRD patterns, indicating that there wasa silicate glass phase in the coating. Quartz, α-Al2O3, TiAl, Ti3Al andweak peaks of Z-phase (Ti5Al3O2) are detected. Quartz and α-Al2O3

were the inclusions in the composite coating, and TiAl and Ti3Al werethe phase of the substrate. Z-phase (Ti5Al3O2) might be a new phaseformed during preparation of the coating.

Fig. 2 shows the surface morphology and cross sectional microstruc-ture of the as-prepared SiO2–Al2O3–glass composite coating. Some pro-trusions observed at the surface of the coating are either Al2O3 particlesor quartz particles. The composite coating was well adherent to the sub-strate and no pore or crack were observed in the coating. The quartz andalumina particles are well dispersed in the coating. High-magnificationcross-section image (Fig. 2c) shows that there was a bright thin zone be-neath the coating. EDS analysis indicated that this zone was Al-depleted,and composed mainly of Ti, Al and O. so it might be the Z-phase(Ti5Al3O2). A thin dark layer on top of the Z-phase layer was also ob-served. EDS analysis for region A in Fig. 2c indicated that it was mainlycomposed of Al and O with a small amount of Ti (2.4 at.%) and Si(2.3 at.%). Considering that this layer is so thin that some elements inthe Z-phase layer or in the coating may be detected by the EDS, so wecould conclude that this layer was mainly Al2O3. Combining the EDS re-sults and XRD patterns, it might be concluded that the Al2O3/Z-phase(Ti5Al3O2) bilayer structure was formed at the interface between thecomposite coating and the substrate during the preparation of the com-posite coating.

3.2. Interrupted oxidation

3.2.1. Oxidation kineticsFig. 3 shows the kinetics curves of Ti–47Al–2Cr–2Nb alloy with and

without the SiO2–Al2O3–glass composite coating at 800 °C and 900 °Cin static air. The total mass gains of the bare Ti–47Al–2Cr–2Nb alloy andcoated ones after 100 h of tests at 800 °C and 900 °C were 1.06, 0.19,2.58 and 0.37 mg/cm2, respectively. That is, the mass gains of bare Ti–47Al–2Cr–2Nb alloy were much higher than that of coated specimens;the composite coating therefore decreases significantly the oxidationrate of Ti–47Al–2Cr–2Nb alloy. The oxidation kinetics of bare TiAl alloysfollowed an approximately parabolic law at the two temperatures.

3.2.2. Microstructures of oxide scales on bare Ti–47Al–2Cr–2Nb alloyThe XRD patterns for the oxide on the bare Ti–47Al–2Cr–2Nb alloy

after oxidation tests at 800 °C and 900 °C are shown in Fig. 4. Rutile-TiO2, α-Al2O3 and weak diffraction peaks of Ti2AlN were detected. Thesurface morphology and cross-sectional microstructure of Ti–47Al–2Cr–2Nb alloy after 100 h oxidation at 800 °C and 900 °C are shown inFig. 5. For the bare Ti–47Al–2Cr–2Nb alloy after 100 h oxidation at800 °C, no spallation was observed (Fig. 5a). The scale structure of the

Fig. 2. Surface morphology (a), Low-magnification cross sectional image (b) and high-magnification cross sectional image (c) of the as-prepared SiO2–Al2O3–glass compositecoating on Ti–47Al–2Cr–2Nb alloy.

Fig. 3. Interrupted oxidation kinetics of Ti–47Al–2Cr–2Nb alloy with and without thecomposite coating at 800 °C and 900 °C.

Fig. 4. XRD patterns of bare Ti–47Al–2Cr–2Nb alloy after 100 h oxidation at 800 °C and900 °C.

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bare Ti–47Al–2Cr–2Nb alloy after 100 h oxidation at 800 °C was similarto that formed at 900 °C except that the former was thinner than the lat-ter (Fig. 5b).

For the bare Ti–47Al–2Cr–2Nb alloy after 100 h oxidation at 900 °C,spallation and newly formed oxidewere observed at the corner and theborder of the sample, as shown in Fig. 5c, the magnified image showsthat the surface of the oxide was composed of fine-grained oxide,voids at the grain boundaries were also observed. The cross section

image (Fig. 5d) shows that the scale formed on the Ti–47Al–2Cr–2Nballoy at 900 °C possesses a layered structure. Combining the XRD/EDS re-sults of the oxide, we conclude that the outmost layerwas rutile-TiO2, theintermediate one was an Al2O3 rich oxide layer, the inner one was themixture of TiO2 and Al2O3. These results are in agreement with those ofthe literature [27–30].

According to the EDS analysis, it was also detected that a smallamount of dissolved Cr and Nb ion in the oxide scales. They were prefer-entially segregated in the inner layer of the oxide scale. And bright phaserich in Cr and Nbwas presented beneath the oxide scale, possibly due tooutward diffusion of Ti and Al for the formation of oxide scales. Some ni-tridewas also found by EDS at the interface between oxide and substrate,similar results were reported [27,28,30].

3.2.3. Microstructures of the oxide scales on the coated Ti–47Al–2Cr–2Nballoy after oxidation

The XRD patterns of the coated Ti–47Al–2Cr–2Nb alloy after 100 hoxidation at 800 °C and 900 °C are shown in Fig. 6. Quartz, cristobalite,α-Al2O3, Ti5Si3, Z-phase (Ti5Al3O2), Ti3Al and TiAl were detected.Cristobalite, which was only detected for the specimen after oxidationat 900 °C, might be transformed from the quartz particles added inthe composite coating. The diffraction peaks of Ti5Si3 and Z-phase(Ti5Al3O2) on the specimen after oxidation at 900 °C were much stron-ger than those at 800 °C.

Surface morphology and cross-sectional microstructure of the coatedspecimens after 100 h oxidation at 800 °C and 900 °C are shown in Fig. 7.The composite coating maintained intact and no oxides of Ti could be

Fig. 5. Surface morphology (a, c) and cross sectional microstructure(b, d) of bare Ti–47Al–2Cr–2Nb alloy after 100 h oxidation at 800 °C and 900 °C, respectively.

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detected on the surface of the coating, implying that the composite coat-ing acted as an oxygen diffusion barrier and improved the oxidation resis-tance of the Ti–47Al–2Cr–2Nb alloy. It is worth noting that the interfacebetween quartz and glasswas blurry.More particles, especially small par-ticles (including alumina and quartz), could be observed in the cross sec-tion image of composite coating after oxidation at 800 °C (Fig. 7b) than at900 °C (Fig. 7e), and more protrusions, which are either Al2O3 or quartz,

Fig. 6. XRD patterns of Ti–47Al–2Cr–2Nb alloy with the composite coating after 100 hoxidation at 800 °C and 900 °C.

could be observed at the surface of the composite coating after oxidationat 800 °C (Fig. 7a) than at 900 °C (Fig. 7d).

The coating/substrate interfacial structure after oxidation was morecomplex than the as-prepared one. The interfacial reaction between thecomposite coating and substrate during oxidation at 800 °C looks simi-lar to that at 900 °C, though the latterwas stronger than the former. EDSanalysis for region A in Fig. 7f indicates that it was composed of 47%Ti, 31% Al, 20% O and 2% (Cr+Nb) (at.%). So it might be a layer ofTi5Al3O2, the dot line drawn showing the interface between the sub-strate and Ti5Al3O2. One might remember that a Ti5Al3O2 layer was ob-served at the coating/substrate interface in the as-prepared specimens(see Section 3.1). Above the Ti5Al3O2 layer (region A), a discontinuousdark layer and a continuous bright thick layer are observed (Fig. 7f).According to EDS, they might be alumina and Ti5Si3, respectively. Fig. 8shows the element's depth profile analysis by EDS. The scanning linesfor Fig. 8a and b are indicated by solid white lines in Fig. 7b and e. Twodash lines in Fig. 8b were drawn to show the interfacial reaction zonebetween the substrate and the coating. From the coating (left) to the sub-strate, one can see the first significant high peaks were Al and O, corre-sponding to either an alumina particle within the coating near theinterface or an alumina layer formed during oxidation tests. Then alayer rich in Si and Ti followed, corresponding to the bright layer ofTi5Si3 in Fig. 7. Next, a less strong Al peak is observed, corresponding tothe dark layer of alumina in Fig. 7. Finally, a strong Ti peak showed, corre-sponding to the Ti5Al3O2 layer.

The EPMAmapping techniquewas used to analyze the interfacial re-action between the composite coating and the Ti–47Al–2Cr–2Nb alloyat 900 °C, as shown in Fig. 9. EPMA analysis indicated that a continuous

Fig. 7. Surface morphology (a, d), Low-magnification cross sectional image (b, e), High-magnification cross sectional image (c, f) of Ti–47Al–2Cr–2Nb alloy with composite coatingafter 100 h oxidation at 800 °C and 900 °C, respectively.

34 W. Li et al. / Surface & Coatings Technology 218 (2013) 30–38

Al2O3 layer depleted in Si was above the Si-rich layer depleted in Al.Therefore, the first layer rich in Al and O in Fig. 8b was more probablya continuous alumina layer formed during oxidation tests.

Fig. 8. Element depth profile (analyzed by EDS) of Ti–47Al–2Cr–2Nb alloy wi

Combining the EPMA/EDS analysis about the cross sectional mi-crostructure and XRD patterns of the specimen, it could be concludedthat an interfacial reaction between composite coating and substrate

th composite coating after 100 h oxidation at 800 °C (a) and 900 °C (b).

Fig. 9. Element mapping (Al, O, Ti, Si, K) for the Ti–47Al–2Cr–2Nb alloy with composite coating after 100 h oxidation at 900 °C.

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occurred, forming an Al2O3/Ti5Si3/Al2O3/Ti5Al3O2 layered structure inthe interfacial reaction zone.

3.3. Cyclic oxidation

The cyclic oxidation kinetics of Ti–47Al–2Cr–2Nb alloy with andwithout the composite coating at 900 °C is shown in Fig. 10. For theuncoated specimens, the competition between spallation and oxidationwas clearly observed from the oxidation kinetics curve, and spallationoccurred in the early stage of test; severe spallation occurred and

Fig. 10. Cyclic oxidation kinetics of Ti–47Al–2Cr–2Nb alloy with and without the com-posite coating at 900 °C.

resulted inmass loss after 40 cycles. For the coated specimens, the cyclicoxidation kinetics is the same as the interrupted oxidation kinetics,which proved that no spallation occurred during cyclic oxidation. Themass change for the coated specimen was much smaller than that ofuncoated one.

The surface morphology of bare Ti–47Al–2Cr–2Nb alloy after cyclicoxidation at 900 °C for 120 cycles is shown in Fig. 11. Large pieces ofoxide spallation and newly formed oxide was observed on the surface,indicating that Ti–47Al–2Cr–2Nb alloy had relatively poor cyclic oxida-tion resistance at 900 °C.

For the Ti–47Al–2Cr–2Nb alloy with composite coating, no crack orspallation could be observed. The composite coating was still well adher-ent to the substrate, as shown in Fig. 12a and b. The cross sectionalmicro-structure of the coated Ti–47Al–2Cr–2Nb alloy after cyclic oxidation issimilar to that after interrupted oxidation, therefore, the composite coat-ing significantly improved the cyclic oxidation resistance of Ti–47Al–2Cr–2Nb alloy.

3.4. Chemical composition change of the glass phase

Table 1 shows the chemical composition change of the glass phase.Although the quantitative analysis for the oxygen has relatively largeerror, the trend of the composition change in the glass phase still couldbe observed. The aqueous solution of potassium silicate could be referredto as mSiO2·K2O·nH2O, and potassium silicate glass formed after H2Oevaporated during firing at high temperatures. The as-prepared potassi-um silicate glass was free of Al, composed of SiO2 and K2O with formula25SiO2·7K2O. During oxidation at 800 °C or 900 °C, the alumina inclu-sions dissolved into the glass. Besides, evaporation of K2O occurred possi-bly during oxidation.

Fig. 11. Surface morphology of bare Ti–47Al–2Cr–2Nb alloy after 120 cycles at 900 °C.

Table 1Chemical composition (at.%) change of the glass phase by EDS (the average error of allthe values is about ±0.35% [31]).

Oxygen Silicon Aluminum Potassium

As-prepared glass Balance 25.5% 0% 13.6%Glass in the coating after 100 hoxidation at 800 °C

Balance 27.0% 2.3% 7.5%

Glass in the coating after 100 hoxidation at 900 °C

Balance 28.2% 3.5% 4.5%

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4. Discussion

4.1. Interfacial reaction between composite coating and Ti–47Al–2Cr–2Nb alloy

In comparison with the poor oxidation resistance of bare Ti–47Al–2Cr–2Nb alloy, the Ti–47Al–2Cr–2Nb alloy with SiO2–AL2O3–glass com-posite coating shows much better oxidation resistance. The compositecoating acts as an oxygen barrier to reduce the high temperature oxida-tion rate of the alloy. Similar results have been reported [16–19,32].

Fig. 12. Surface morphology (a), cross sectional microstructure (b) and element depth pr

Passerone [33] reported that titanium is very reactivewith themoltenglass. Ti enters the molten glass as Ti3+ under reducing conditions, andallows metallic cations of glasses to be reduced and to enter the metalbase itself. Si diffuses easily into themetal base, so amolten Ti–Si alloy ap-pears at temperatures higher than 1350 to 1400 °C after certain time.Moskalewicz [34] also reported that a Ti5Si3 layer formed close to the sub-strate in their research of a double layered glass–ceramic coating on thealloy TIMETAL 834 (Ti–5.8Al–4Sn–3.5Zr–0.7Nb–0.5Mo–0.35Si–0.06C–0.09O, wt.%) at 800 °C. In this study, no titanium was observed in thecomposite coating after 100 h oxidation at 800 °C or 900 °C, while a lay-ered structure (Al2O3/Ti5Si3/Al2O3/Z-Ti5Al3O2) was observed in the inter-facial reaction zone. The schematic diagram of the interfacial reaction isshown in Fig. 13.

The as-prepared specimens showed a bi-layered interface of coating/alumina/Ti5Al3O2/substrate, probably thru the reaction:

5TiAlþ 5O ¼ Ti5Al3O2 þ Al2O3: ð1Þ

More detailed reactions for the bi-layer formation might refer to se-lective oxidation of Al. At the beginning, the low partial pressure of theoxygen at the interface promotes the selective oxidation of Al, the Al inthe substrate diffuses outward and an alumina layer formed (Fig. 13a).This leads to formation of an Al-depleted zone beneath the alumina

ofile (c) of Ti–47Al–2Cr–2Nb alloy with composite coating after 120 cycles at 900 °C.

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layer. In the present study, the Al-depleted zone is a continuous Z-phase(Ti5Al3O2) layer, as shown in Fig. 13b. The bi-layered structure forms anadequate gradient of the activity of O and Al, so this structure is morestable than a single alumina layer. According to the Shemet [35] andCopland [36], a continuous stable Z-phase (Ti5Al3O2) sublayer mightsustain the growth of alumina scale. The growth of the bi-layered struc-ture is possibly dominated by outward diffusion of Al and inward diffu-sion of O.

During oxidation at high temperatures, a layer of Ti5Si3 is formed be-tween the alumina layer and the Ti5Al3O2 one. The probable reaction isas follows:

5Tiþ 4Alþ 3SiO2 ¼ Ti5Si3 þ 2Al2O3: ð2Þ

According to Fig. 9, the counts of Al in the alumina layer is lower thanthat in the Al2O3 particles, indicating that this alumina layermight not be

Fig. 13. Schematic diagram of the interfacial reaction zone between composite coatingand Ti–47Al–2Cr–2Nb alloy at the firing stage (a, b) and oxidation stage (c, d).

an exclusive one, but contained some SiO2 or K2O. In the next section, wewill discuss the reaction between alumina and mSiO2·K2O glass. In thiscase, the Ti and Al in the substrate or Z-phase (Ti5Al3O2) layer diffuseoutward and react with the SiO2 in the alumina layer to form Al2O3

and Ti5Si3 (Fig. 13c), with the increase of time, a continuous Ti5Si3 layerformed and some Al2O3 (reaction product) may disperse beneath thecontinuous Ti5Si3 layer and form a discontinuous alumina layer, asshown in Fig. 13d. Ti5Si3 was reported in the interfacial reaction betweenglass and Ti or titanium alloy, it is believed that the reactions betweenSiO2 in the glass and Ti in the substrate take place and form Ti5Si3 andTiO2 [37]. In the present study, no oxide of Ti was observed at the inter-face or in the coating, but a continuous alumina layer was observed atthe interface; we believed that there are two reasons for this, one isthe higher thermodynamic stability of Al2O3 compared to TiO2 [38], theother is the high Al content in Ti–47Al–2Cr–2Nb alloy: even if TiO2

formed during the interfacial reaction, it still would be reduced by Al.The glass transition temperature of the glass with composition of

3SiO2:K2O is about 495 °C [39]. That means the glass phase in thecoating behaves as a viscous liquid and fast paths for oxygen diffusionat 800 °C and 900 °C. As Al2O3 has very low oxygen permeability, theAl2O3 layer formed at the coating/substrate interface may play betterroles in preventing oxygen diffusion into the substrate than the glass,and benefit the oxidation protectiveness of the composite coating.The composite coating/Al2O3 double layer has a better effect inpreventing the inward diffusion of oxygen than the single compositecoating. It is hopeful that the composite coating/Al2O3 double layercould improve the long term oxidation resistance for the Ti–47Al–2Cr–2Nb alloy.

4.2. Thermal cycling resistance of the composite coating

The glass–ceramic coatings on metallic substrates are subjected tospallation during thermal cycling because of large TEC mismatch in ad-dition to the brittle nature of glass–ceramic coatings. Cyclic oxidationtests of the coated Ti–47Al–2Cr–2Nb alloy carried out in this studyshowed that our SiO2–Al2O3–glass composite coating exhibited excel-lent thermal cycling resistance of the coating. The coating kept intactafter 120 cycles at 900 °C; neither crack nor spallation were observed.

In our composite coating, thematrix was the potassium silicate glass,and the inclusion phases were the alumina and quartz particles. The vol-ume fractions of glass, Al2O3 and SiO2were 55 vol.%, 8 vol.% and 37 vol.%,respectively. The TEC of the composite coatingmight be estimated usingthe Kerner's model [40]:

αeff ¼ ∑αiVi þ 4μm

km∑ ki−km

4μm þ 3kiαiVi

where k, μ, α, and V represent bulk modulus, shear modulus, TEC andvolume fraction, respectively, while “i” and “m” refer to the inclusionand matrix components, respectively. The calculation using Kerner'smodel and data in Table 2 gives the estimated TEC of the composite coat-ing is ~13.4×10−6(K−1), which is close to the TEC of the TiAl, about14.4×10−6(K−1) [43]. Therefore, the excellent thermal cycling resis-tance of our composite coating might result mainly from the compositecoating/Ti–47Al–2Cr–2Nb systemdoes not exhibit largemismatch of TEC.

Another possible reason for the excellent spallation resistance of thecomposite coatingmight be the lowglass transition temperature (Tg) ofthe potassium silicate glass. Low Tg of the glassmeans that the glass canrelax thermal stresses by viscousflowuntil Tg is reached during cooling.Thus, the thermal stresses accumulated in the coatingmay decrease to alevel below the compress strength of the coating.

4.3. The dissolution of α-Al2O3 and quartz into glass

α-Al2O3 was extensively researched as ceramic inclusion in glass ma-trix because of its good thermal–mechanical property and compatibility

Table 2Material parameters used in the calculation according to Kerner's model for the matrixand inclusion phases.

TEC (10−6/K) K (GPa) μ (GPa) ρ (g/cm3) Refs.

Glass 14.5 33 18 2.4 [39,41,42]Alumina 8.3 380–400 3.97 [43,44]Quartz 11.2 37.8 2.65 [45–47]TiAl 14.4 [43]

38 W. Li et al. / Surface & Coatings Technology 218 (2013) 30–38

with the glass matrix [48–53]. Enhancement of the strength and fracturetoughness by adding Al2O3 into glass matrix was reported by Lee et al.and Bolelli et al. [48,51,52]. Al was detected in the glass phase after oxida-tion at 800 °C or 900 °C (Table 1), indicating that theAl2O3 particles in thecomposite coating partially dissolved into the glass phase. It was alsofound that more Al2O3 particles dissolved into glass at 900 °C than at800 °C. Actually, morphology details of the composite coating also gaveevidence of the dissolution of Al2O3 particles. There were many smallAl2O3 particleswith size about 1 μmin the as-prepared composite coatingand in the one after oxidation tests at 800 °C, while such particles hadbeen hardly found in the one after tests at 900 °C. The dissolution ofAl2O3 into the glass phase changes the binary silicate glass (K2O–SiO2)into a ternary glass (K2O–Al2O3–SiO2), which may improve the chemicaldurability of the glasses [54].

Quartz particles were also instable in the glass. As shown in theXRD patterns of the coated specimen after 100 h oxidation at900 °C (Fig. 6), cristobalite formed as a result of phase transforma-tion of the quartz particles. Fenner [55] reported that the quartzwould not transform into cristobalite or tridymite with the presenceof a mineralizer, such as Na2O, K2O and CaO, until the heated temper-ature is higher than 870 °C. Hunger [56] reported that the formationof cristobalite could increase the coefficient of thermal expansion(CET) of the glass–ceramic, and benefit the spallation resistance ofthe composite coating.

The dissolution of quartz into the glass phase had been reported[57,58]. Stathis [58] pointed out that fine quartz grains are moreprone to dissolution than coarser grains. The dissolution of the quartzparticles might occur in this study according to two facts. One is thatthe concentration of silicon in the glass phase increased after oxidationas shown in Table 1, and another is that the quartz/glass interface wasblurry and the profile of the quartz particles in the coating was smooth(Figs. 7 and 12) while the profile of the original quartz particles isangular.

5. Conclusions

From the above study, the following conclusions can be drawn:

1. An oxidation resistance SiO2–Al2O3–glass composite coating was suc-cessfully produced on the Ti–47Al–2Cr–2Nb alloy by air spray tech-nique and subsequent firing, and it remarkably improved theoxidation resistance of the Ti–47Al–2Cr–2Nb alloy at 800 °C and900 °C.

2. Interfacial reaction between composite coating and substrate oc-curred, forming a layered structure (Al2O3/Ti5Si3/Al2O3/Z-Ti5Al3O2)in the interfacial zone; the composite coating/continuous Al2O3

layer might act as oxygen diffusion barrier and benefit the oxidationresistance of the Ti–47Al–2Cr–2Nb alloy.

3. There was a dissolution of Al2O3 and quartz inclusions into theglass matrix, as well as the phase transformation of quartz intocristobalite during oxidation tests.

Acknowledgments

This work is funded by the National Key Basic Research Program ofChina (973 program, no. 2012CB625101) and by the National NaturalScience Foundation of China, grant no. 50774074.

References

[1] K.W. Kim, J. Miner. Met. Mater. Soc. 46 (1994) 30.[2] K.W. Kim, JOM 47 (1995) 39.[3] F. Appel, U. Brossmann, U. Christoph, S. Eggert, P. Janschek, U. Lorenz, J. Müllauer,

M. Oehring, J.D.H. Paul, Adv. Eng. Mater. 2 (2000) 699.[4] Y. Umakoshi, M. Yamaguchi, T. Sakagami, T. Yamane, J. Mater. Sci. 24 (1989) 1599.[5] K.Maki,M. Shioda,M. Sayashi, T. Shimizu, S. Isobe,Mater. Sci. Eng. A 153 (1992) 591.[6] M. Yoshihara, Y.W. Kim, Intermetallics 13 (2005) 952.[7] M. Brochu, B.D. Gauntt, R. Shah, G. Miyake, R.E. Loehman, J. Eur. Ceram. Soc. 26

(2006) 3307.[8] F. Smeacetto, M. Salvo, M. Ferraris, J. Cho, A.R. Boccaccini, J. Eur. Ceram. Soc. 28 (2008)

61.[9] D.Q. Wang, Appl. Surf. Sci. 255 (2009) 4640.

[10] F.S. Shieu, M.J. Deng, K.C. Lin, J.C. Wong, J.Y. Wu, J. Mater. Sci. 34 (1999) 5265.[11] D.Y. Zheng, S.L. Zhu, F.H. Wang, Surf. Coat. Technol. 200 (2006) 5931.[12] S. Das, S. Datta, D. Basu, G.C. Das, Ceram. Int. 35 (2009) 1403.[13] S. Das, A.K. Mukhopadhyay, S. Datta, G.C. Das, D. Basu, J. Eur. Ceram. Soc. 28

(2008) 729.[14] Z.L. Tang, F.H. Wang, W.T. Wu, Mater. Sci. Eng., A 276 (2000) 70.[15] Y.M. Xiong, S.L. Zhu, F.H. Wang, C.H. Lee, J. Coat. Technol. Res. 5 (2008) 93.[16] T. Moskalewicz, F. Smeacetto, A. Czyrska-Filemonowicz, Surf. Coat. Technol. 203

(2009) 2249.[17] Y.M. Xiong, S.L. Zhu, F.H. Wang, Surf. Coat. Technol. 190 (2005) 195.[18] Y.M. Xiong, S.L. Zhu, F.H. Wang, Corros. Sci. 50 (2008) 15.[19] Y.M. Xiong, S.L. Zhu, F.H. Wang, Surf. Coat. Technol. 197 (2005) 322.[20] S. Sarkar, S. Datta, S. Das, D. Basu, Surf. Coat. Technol. 203 (2009) 1797.[21] F.H. Wang, Oxid. Met. 48 (1997) 215.[22] Z.L. Tang, F.H. Wang, W.T. Wu, Oxid. Met. 48 (1997) 511.[23] N. Birks, G.H. Meier, F.S. Pettit, Introduction to the High-Temperature Oxidation of

Metals, second ed. Cambridge University Press, U. K., 2009[24] B.C. Valek, J.M. Hampikian, Surf. Coat. Technol. 93–95 (1997) 13.[25] R.K. Iler, The Chemistry of Silica, Solubility, Polymerization, Colloid and Surface

Properties, and Biochemistry, Wiley-Interscience Publication, New York, 1979.[26] D. Boschel, H. Roggendorf, J. Sol–Gel Sci. Technol. 25 (2002) 191.[27] J.D. Sunderkiitter, H.J. Schmutzler, V.A.C. Haanappel, R. Hofmant, W. Glatz, H.

Clemen, M.F. Stroosnijder, Intermetallics 5 (1997) 525.[28] Y.D. Jang, D.B. Lee, D.Y. Seo, Met. Mater. Int. 8 (2002) 503.[29] D.J. Kim, D.Y. Seo, H. Saari, T. Sawatzky, Y.W. Kim, Intermetallics 19 (2011) 1509.[30] F. Dettenwanger, E. Schumann, M. Ruhle, J. Rakowski, G.H. Meier, Oxid. Met. 50 (1998)

269.[31] L.Y. Xu, Z.D. Liu, Y.H. Shang, Anal. Test. Technol. Instrum. 5 (1999) 115, (In Chinese).[32] M.L. Shen, S.L. Zhu, F.H. Wang, Thin Solid Films 519 (2011) 4884.[33] A. Passerone, G. Valbusa, E. Blagini, J. Mater. Sci. 12 (1977) 2465.[34] T. Moskalewicz, F. Smeacetto, G. Cempura, L.C. Ajitdoss, M. Salvo, A. Czyrska-

Filemonowicz, Surf. Coat. Technol. 204 (2010) 3509.[35] V. Shemet, H. Hoven, W.J. Quadakkers, Intermetallics 5 (1997) 311.[36] E.H. Copland, B. Gleeson, D.J. Young, Acta Mater. 47 (1999) 2937.[37] I.W. Donald, J. Mater. Sci. 28 (1993) 2841.[38] Y.J. Liang, Y.C. Che, Handbook of Thermodynamic Data for Inorganic Compounds,

first ed. Shenyang, 1993. , (In Chinese).[39] J.E. Shelby, J. Appl. Phys. 47 (1976) 4489.[40] E.H. Kerner, Proc. Phys. Soc. 69 (1956) 808.[41] N.P. Bansal, R.H. Doremus, Handbook of Glass Properties, Academic Press, San

Diego, CA, 1986, p. 223.[42] Z.H. Jiang, L.L. Hu, Sci. China Ser. E 26 (1996) 395.[43] Z.L. Tang, F.H. Wang, W.T. Wu, Surf. Coat. Technol. 110 (1998) 57.[44] M.V. Swain, Structure and property of ceramics, Material Science and Technology,

vol. 11, VCH, New York, 1994.[45] J.H. Jean, T.K. Gupta, IEEE Trans. Comp. Packag. Manuf. Technol. B Adv. Packag. 17

(2) (1994) 228.[46] I. Ohno, J. Phys. Earth 43 (1995) 157.[47] Y.N. Xu, W.Y. Ching, Phys. Rev. B 44 (1991) 11048.[48] F.F. Lange, J. Am. Ceram. Soc. 54 (1971) 614.[49] W.D. Wolf, K.J. Vaidya, L.F. Francis, J. Am. Ceram. Soc. 79 (1996) 1769.[50] A. Ray, A.N. Tiwari, Mater. Chem. Phys. 67 (2001) 220.[51] D.Y. Lee, D.J. Kim, B.Y. Kim, Y.S. Song, Mater. Sci. Eng., A 341 (2003) 98.[52] G. Bolelli, V. Cannillo, L. Lusvargh, T. Manfredini, Surf. Coat. Technol. 201 (2006) 458.[53] R. Venkatesh, Mater. Sci. Eng., A 268 (1999) 47.[54] F. Moreau, A. Duran, F. Munoz, J. Eur. Ceram. Soc. 29 (2009) 1895.[55] C.N. Fenner, Am. J. Sci. 36 (1913) 331.[56] A. Hunger, G. Carl, A. Gebhardt, C. Rüssel, J. Non-Cryst. Solids 354 (2008) 5402.[57] M.L. Shen, S.L. Zhu, M.H. Chen, F.H. Wang, J. Am. Ceram. Soc. 94 (2011) 2436.[58] G. Stathisa, A. Ekonomakoub, C.J. Stournarasb, C. Ftikosa, J. Eur. Ceram. Soc. 24

(2004) 2357.