Surface & Coatings Technology 206 (2011) 1628–1633

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Surface & Coatings Technology

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Effect of composition on the growth and microstructure of hafnia–zirconiabased coatings

M. Noor-A-Alam, A.R. Choudhuri, C.V. Ramana ⁎Department of Mechanical Engineering, University of Texas at El Paso, USA

⁎ Corresponding author.E-mail address: rvchintalapalle@utep.edu (C.V. Ram

0257-8972/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.surfcoat.2011.06.040

a b s t r a c t

a r t i c l e i n f o

Available online 30 June 2011

Keywords:Yttria–stabilized hafnia–zirconiaThermal barrier coatingCompositionMicrostructureScanning electron microscopyX-ray diffraction

Yttria (Y2O3)–stabilized hafnia (HfO2)–zirconia (ZrO2)-based coatings were grown and studied for theirgrowth and microstructure. Composition of the coatings was varied by varying the ratio of HfO2 and ZrO2,from 1:4 to 4:1, while maintaining the Y2O3 stabilizer content fixed at 7.5 mol%. The crystal structure, surfacemorphology and thermal stability of the grown coatings were evaluated as a function of the coatings'composition. The crystal structure analysis performed by X-ray diffraction (XRD) indicates the hafnia-cubic-phase stabilization in all the coatings. The morphology of the coatings is characterized by the columnargrowth coupled with a dense structure as revealed by the scanning electron microscopy (SEM). Thermalstability evaluation performed using high temperature XRD coupled with SEM indicates the enhancedstability of these coatings to 1300 °C. No significant changes were found in the crystal structure andmorphology of the coatings.

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1. Introduction

High-temperature coatings are critical technologies for futurepower-generation systems and industries [1–12]. Thermal barriercoatings (TBCs), which are usually the ceramic materials applied asthin coatings, protect engine components and allow further increasein engine temperatures for higher efficiency [2,6]. Thus, the durabilityand reliability of the coating systems have to be more robust com-pared to current natural gas based engines.

Extensive efforts have been directed in the last two decades to-wards the development of appropriate coatings for gas turbinesystems [1–12]. Up to now, the most successful and industrial choiceof TBC material is the 6–8 wt. % yttria stabilized zirconia (YSZ), whichare applied on engine hot-section components by plasma spraying(PS) or electron-beam physical vapor deposition (EB-PVD) [1–12].However, the major drawback of YSZ is its temperature tolerance,which is only up to 1200 °C for long term operation. The phase trans-formation at temperatures higher than 1200 °C which is associatedwith the volume change and development of cracks on the surface andinterface of the TBC and substrate limits the application of YSZ attemperatures higher than 1200 °C [1,9]. Above 1200 °C, phase trans-formations from the t′-tetragonal to tetragonal and cubic (t+c) andthen to monoclinic (m) occur, leading to the failure of YSZ coatings[1,9–15]. Moreover, the sintering-induced volume shrinkages woulddegrade the columnar structure of the YSZ coatings grown from PVD

methods and increase the elasticity modulus [13,15]. These factorsresult in the reduced strain tolerance of the YSZ coatings [13,15]. Theefforts to increase the efficiency by elevating the temperatures mustmeet the following characteristics: (a) higher temperatures of op-eration, (b) lower thermal conductivity, (c) better sintering resistanceand (d) phase stability even at higher temperatures.

It has been argued that the place to look for alternate materials forhigh-temperature applications is the set of materials based on fluoritestructured oxides [1,2,11]. These oxide ceramics exhibit excellentstructural, thermal, mechanical and thermo-chemical properties fortheir ready adoption as TBCs. The obvious candidates from fluoritestructure oxides group include ZrO2, HfO2, CeO2, ThO2 and UO2 [2,11].Of these, only ZrO2 (zirconia) and HfO2 (hafnia) are the viable can-didates because of issues such as transmutation and volatilization[11]. The present work was focused on ytrria–stabilized zirconia–hafnia (YSZH) co-doped system. Recently, it has been demonstratedthat the yttria stabilized hafnia (YSH) coatings exhibit very low ther-mal conductivity at temperatures as high as 1650 °C [16]. Matsumotoet al. have grown the 7.5 mol% yttria–stabilized hafnia coatings by EB-PVD method and studied their structural and thermal characteristics.They found that the thermal expansion coefficient of YSH coatings islow compared to YSZ within the whole temperature range from100 °C to 1400 °C [17]. The growth interface morphology of theYSH coatings was similar i.e., columnar structure, to the YSZ while thesurface morphology was characterized by the laminar triangularcrystals. They found lower thermal expansion coefficient of YSHcoatings compared to YSZ within the whole temperature rangefrom 100 °C to 1400 °C [17]. Singh et al. reported that themodificationof columnar microstructure of the thermal barrier coatings with

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controlled micro-porosity and diffused sub-interfaces results in lowerthermal conductivity by 20–30% depending onmicro-porosity volumefraction [18]. This modification in hafnia-based coatings increasesthermal reflectance by 15–20% and strain tolerance as comparedwith standard thermal barrier coatings [18]. Therefore, hafnia-basedmaterials warrant further investigations in order to look for high-temperature stability and lower thermal conductivity. The impetusfor the present work is towards understanding the growth andmicrostructure evolution at the reduced dimensions which, in turn,may provide opportunities to tailor the structure and stability of thecoatings for their eventual TBC application in gas turbines.

2. Experimental

2.1. Target preparation

Ceramic targets made with mixed proportions of HfO2 and ZrO2

stabilized by Y2O3 were used to fabricate the coatings by sputter-deposition. The composition of the target material is varied by varyingthe ratio of HfO2 and ZrO2 while keeping the Y2O3 stabilizer contentconstant (7.5 mol%). The ratio of HfO2 and ZrO2 was varied in theproportions of 4:1, 2:1, 1:1, 1:2 and 1:4. The composition is thenstabilized with yttria at 7.5 mol% of the total amount of materialincluding yttria. Yttria stabilized hafnia (YSH) without zirconia wasalsomade to investigate the effect of zirconia. The targetmaterials andtheir composition are listed in Table 1. Stoichiometric amounts ofHfO2, ZrO2 and Y2O3 powders were mixed thoroughly and com-pressed into a 2″ pellet in a die at a pressure of 24,000 lbs for 30 minfollowed by sintering at 1050 °C for 24 hrs. The 2″ target thus obtainedwas then fixed to a 2″ Cu backing plate in order to employ for sputter-deposition of the coatings.

2.2. Coatings fabrication

YSHZ targets of 2″ diameter, 0.125″ thick were used for sputtering.Coatings were grown onto different types of substrates, namely Ni-based super alloy Inconel-738, stainless steel-403, alumina and silicon.The respective target was placed on a 2-inch sputter gun, which wascorrespondingly placed at a distance of 8 cm from the substrate. Asputtering power of 30 W was initially applied to the target whileintroducing high purity argon (Ar) into the chamber causing plasmaignition. Once ignited, the power was increased to 100 W to depositthe coatings. The flow of the Ar was controlled using MKS mass flowmeter. Before each deposition, the targetwas pre-sputtered for 10 minusing Ar with the shutter above the gun closed. The growth tem-perature during deposition was varied from room temperature (RT)to 500 °C. Coatings were grown to obtain a thickness of ~1 μm. Thesubstrateswere heated by halogen lamps and the desired temperaturewas controlled by an Athena X25 controller.

2.3. Characterization

The grown coatings were characterized by studying their crystalstructure, morphology and structural stability employing X-ray dif-fraction (XRD), scanning electron microscopy (SEM) coupled with

Table 1Composition of the target materials.

Targets Molar ratio of HfO2 to ZrO2 Amount of Y2O3

YSH 1:0 7.5%YSHZ-1 4:1 7.5%YSHZ-2 2:1 7.5%YSHZ-3 1:2 7.5%YSHZ-4 1:4 7.5%YSHZ-5 1:1 7.5%

energy dispersive X-ray spectrometry (EDS) and high-temperatureXRD measurements. Bruker D8 Advance x-ray diffractometer wasemployed for the crystal structure and phase analysis of the coatings.All the measurements were made ex-situ as a function of the coatings'fabrication conditions. XRD patterns were recorded using Cu Kαradiation (λ=1.54056 Å) at RT.

Surface and interface structural analysis was performed using aHitachi S-4800 SEM. Secondary electron imaging was performed toprobe the surface morphology and grain distribution characteristics.The elemental composition was investigated using an incidentelectron beam of 20 keV and by analyzing the generated x-rays fromthe samples. Thermal stability at high temperaturewas investigated byusing the high-temperature XRD analysis in the XRD machineemploying hot-stage, where the sample is mounted on a ceramicbed. The temperature was ramped at 10°/min and kept at the desiredtemperature for 30 min. The programmed XRD scans were collectedright after reaching the temperature, after 15 min, and finally at30 min duration before proceeding to the next temperature. Approx-imately 15 min was required to finish a full scan at a scan speed of5°/min. The temperature was increased in steps of 100 °C until thetemperature of 1300 °C is reached.

3. Results and discussion

3.1. Crystal structure

The XRD patterns of the YSZH coatings grown on alumina andstainless steel substrates are shown in Figs. 1 and 2, respectively. It isevident from the XRD curves that the coatings are crystallized in thecubic structure of HfO2. All of the compositions of hafnia and zirconiacoatings on all the substrates showed the similar crystal structures(Figs. 1 and 2). The effect of temperature is not significant althoughsome peak sharpening can be seen with increasing temperature.Another important feature that can be noted in XRD data is that thepatterns of YSHZ coatings are very similar to the cubic structure of YSHcoatings as shown in Fig. 3. This observation indicates that thestabilization of cubic hafnia phase is mainly due to the presence ofyttria stabilizer. The phase stabilization is independent of the ratio ofHfO2 and ZrO2 and the type of substrates. The lattice parameter of thecoatings calculated from XRD data was found to be ~0.5 nm, which issimilar to the reported value of cubic hafnia [19]. However, there is anincreasing trend of the lattice constant with increasing ZrO2 in YSHZcoatings. The lattice constant variation with ZrO2 content of the YSZHcoatings grown on SS-403 is shown in Fig. 4. Enhancement in thelattice parameterwith a progressive increase in the ZrO2 concentration

Fig. 1. XRD curves of YSZH coatings on alumina substrates. The observed peaks can beindexed to cubic hafnia as indicated.

Fig. 2. XRD curves of YSZH coatings on SS-403 substrates. The observed peaks can beindexed to cubic hafnia as indicated.

Fig. 4. Variation of lattice constant of YSZH coatings as a function of ZrO2 content. Latticeexpansion with increasing ZrO2 can be noted.

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may be due to the interactions of Zr4+ andHf4+ ions.While the chargeand radius of the zirconium and hafnium ions are more or less similar,the lattice parameter of cubic Y2O3–ZrO2–HfO2 coatings seems to beinfluenced by the ZrO2 addition since the ionic radius of Zr4+ is slightlyhigher than that of Hf4+[20].

The XRD results and the physical mechanisms involved in thelattice parameter variation of Y2O3–HfO2–ZrO2 coatings can be un-derstood as follows. It has already been established that ZrO2 can bestabilized in cubic phase by selectively doping with Y2O3, CeO2 andMgO [21]. Since Y2O3 exhibits cubic-centered Tl2O3-type structure it isvery much possible that yttria can stabilize the hafnia in cubicstructure [22–27]. Therefore, cubic-phase stabilization in Y2O3–HfO2–

ZrO2 coatings is a result of oxygen vacancy creation similar to thecase of ZrO2. The Y+3 ions having radii (0.96 Å) larger than that ofHf+4/Zr+4 imparts lattice distortion and enforce the elongation ofbondwith oxygen in close propinquity [28].Monoclinic and tetragonalphases consist of several shorter Hf―O bonds length (2.0–2.1 Å)compared to cubic HfO2 (2.37 Å) [28]. As a consequence, the strainenergy because of the size mismatch becomes significant in mono-clinic and tetragonal phases. On the other hand, the distortion is quiteless in cubic phase with the oversized Y+3 doping, which makes cubicHfO2 stable even at low temperature. Furthermore, the structuralrelaxation plays another role in the stabilization of the cubic phase of

Fig. 3. XRD curves of YSH coatings on SS-403 substrates. The observed peaks can beindexed to cubic hafnia as indicated.

HfO2 by Y2O3 doping [28]. As seen in the XRD data (Figs. 1 and 2), thecubic structure of YSHZ coatings is retained down to room temper-ature. Our results are in good agreement with those reported byChoong Ki Lee et al. [28]. The lattice parameter enhancement, ingeneral, in Y2O3–HfO2–ZrO2 coatings compared to pure HfO2 is causedby the addition of 7.5% Y2O3 and associated lattice distortion andelongation of bonds [28]. The observed further increase in latticeparameter with progressive increase of ZrO2 can be attributed to theslightly higher ionic radius of Zr4+ than that of Hf4+ [19]. The latticedistortion increases when the number of Zr4+ ions is more than theHf4+ ions in the matrix leading to overall lattice expansion to accom-modate the Y3+, Zr4+ and Hf4+ ions.

3.2. Surface morphology

The surface morphology and interface structure of the YSZHcoatings are shown in Figs. 5–7. High magnification SEM images showthe morphology consisting of very well and densely oriented grains inall of the YSHZ coatings grown on SS-403 at 500 °C (Fig. 5). However,YSHZ coatings grown at room temperature exhibit the grains, whichare smaller in size and not very clear even at high magnifications(Fig. 6). The morphology looks like a dense cauliflower structurewhen grown on Inconel-738 (Fig. 6). Whereas, YSH coatings arecharacterized by thin, elongated triangular network of grainsconnected together to form the dense surface structure when grownon either SS-403 or Inconel-738 (Fig. 6). The results obtained on themorphology of the sputtered, thin YSH coatings are similar to that ofthe EB-PVD grown coatings [17]. However, the grains are smaller inthis case which can be attributed to the differences in temperatureand thickness of the coatings. It is evident that the ZrO2 additioninduces changes in the morphology of the coatings. Based on theresults obtained and results reported for EB-PVD grown coatings, itappears that thin, elongated triangular shape morphology is thecharacteristic of the yttria–stabilized hafnia. However, as it is seen inXRD, the deviation of such morphology with progressive addition ofzirconia is believed to be due to the distortion as a result Zr4+ ionsreplacing the Hf4+ ions [29]. Most remarkable feature noted in thiswork is the columnar growth of the coatings. All the YSZH and YSHcoatings exhibit the columnar structure that is typically observed inthese ceramic coatings. A representative of the YSZH coatingsinterface structure exhibiting the columnar growth is shown in Fig. 7.

Fig. 5. Surface morphology of YSHZ coatings grown at 500 °C.

Fig. 6. Comparison of surface morphology of YSH and YSHZ on SS-403 and Inconel-738.

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Fig. 7. Typical columnar growth of the YSHZ coatings.

Fig. 9. EDS curves of YSHZ coatings before and after thermal treatment at 1300 °C.

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3.3. Thermal stability

The enhanced thermal stability of the coatings was found in high-temperature XRD measurements. No phase or compositional changewas observed in the YSH coatings by exposing to ambient temperatureup to 1300 °C. The stable cubic structurewas retained after exposure to1300 °C (Fig. 8). The thermal-induced changes in the chemicalcomposition are probed using the EDS measurements before andafter thermal stability experiments (Fig. 9). EDS indicates that thecomposition of the coatings is the same before and after thermalstability experiments. In addition, the EDS data showed the compo-sition of the coatings similar to that of the target used for the de-position which indicates the stoichiometric, uniform and efficientdeposition of the coatings. Fig. 9 shows the EDS plots for YSH coatinggrown on alumina substrate. Thermo-chemical analysis based on theEDS measurement indicates the coatings' chemical stability to highertemperatures (1300 °C).

4. Conclusions

Hafnia–zirconia based coatings were grown and their microstruc-ture and thermo-chemical–physical stability were evaluated. Thecubic-hafnia phase is stabilized in all the coatings. Increasing ZrO2

Fig. 8. High-temperature XRD curves of YSHZ coatings on alumina substrates. It is evidentthat the XRD curves did not show any significant changes to the final temperature of1300 °C.

increases the lattice parameter in the coatings due to lattice distortionas a result of Zr4+ and Hf4+ ion interaction. All the YSHZ coatingsexhibit the columnar structure. The high-temperature XRD measure-ments indicate that the cubic structure is stable to 1300 °C. Eventhough the thickness is not so high no compositional or microstruc-tural change is observed in the coatings upon exposure to hightemperature. The results indicate that YSHZ compositions showpromise to be used as high temperature TBCs.

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