Influence of thermal shock on insulation effect of nano-multilayer

Influence of thermal shock on insulation effect of nano-multilayer

thermal barrier coatings

Chunxia Zhang,Chungen Zhou,Hui Peng,Shengkai Gong ?,Huibin Xu

School of Materials Science and Engineering,Beijing University of Aeronautics and Astronautics,Beijing,100083,China

Received 15May 2006;accepted in revised form 29November 2006

Available online 11January 2007

Abstract

Thermal barrier coatings (TBCs)with nano-multilayer structure were investigated by thermal shock test.The change of insulation effect during thermal shock test was studied by in-situ temperature monitor with a thermal couple set into the substrate.Microstructure and electrical properties of TBCs were characterized by SEM and Impedance Spectroscopy,respectively.Initial increase in insulation effect was observed and related to the formation and growth of perpendicular microcracks in top coat and transversal microcracks in TGO.With thermal shock,the insulation effect decreased due to the further growth of microcracks in top coat and TGO which induced the failure of TBCs.?2006Elsevier B.V .All rights reserved.

Keywords:Insulation effect;Thermal barrier coatings;Thermal shock;Nano-multilayer

1.Introduction

Increasing attention has been paid to ceramic thermal barrier coatings (TBCs)for advanced gas turbine engine applications.Because of its higher strain tolerance and longer liftetime,EB-PVD (Electron beam physical vapor deposition)TBCs with columnar microstructure have been widely applied in turbine blades [1–6].In order to fully take advantage of the TBC capability,reduction of the thermal conductivity of TBCs is essential.

To lower the thermal conductivity of EB-PVD TBCs,Zhu et al.[7]developed an advanced multi-component,low conductivity oxide thermal barrier coatings by adding rare earth oxides such as Nd 2O 3,Gd 2O 3,Sm 2O 3,and Sc 2O 3.Hass et al.[8]prepared low thermal conductivity yttria stabilized zirconia coatings by a low vacuum electron beam directed vapor deposition process.That top coat has lower thermal conductivity because of its zig-zag pore microstructure which elongated the thermal diffusion path.Nicholls and his colleague Rickerby et al.[1,9]proposed two methods,one is a concept to reduce the thermal conductivity of EB-PVD TBCs by introducing addi-tional phonon scattering centers into the coatings,that is to

introduce interfaces into each column structure which are essentially parallel to the ceramic/bond coat interface,but without disrupting the columnar structure that provides the EB-PVD coating with good strain tolerance and erosion resistance.The other is nano-structure coating because grain boundaries have a significant effect on thermal conductivity.

In the present work,yttria stabilized zirconia coatings with nano-multilayer structure are prepared by electron beam physical vapor deposition.Its insulation effect after thermal shock (carried out in an infra-red heating instrument)is studied by in-situ temperature monitoring.The correlation of the insulation effect with the microstructural evolution and Impedance Spectroscopy is investigated.2.Experimental details

The TBCs system used in this study consists of an alumina-Yttria stabilized zirconia composite top coat with nano-multilayer structure and NiCoCrAlY bond coat prepared onto hollow cylindrical superalloy substrate by EB-PVD.Cross-section morphology of the nano-multilayer ceramic top coat is shown in Fig.3(a).

Thermal shock tests were performed in infra-red heating system (Model1:10K-HR-I,Crystal System Inc.,Japan).The system consists of four high-power lamps whose light can

be

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6344

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?Corresponding author.Tel.:+861082317117;fax:+861082338200.E-mail address:gongsk@https://www.360docs.net/doc/324987003.html, (S.Gong).

0257-8972/$-see front matter ?2006Elsevier B.V .All rights reserved.doi:10.1016/j.surfcoat.2006.11.042

focused on a narrow area (about 20mm)by four concave mirrors to implement fast heating and high temperature.Heating temperature is controlled by operating the heating power of lamp.A thermocouple on the surface of a reference sample is used to measure the surface temperature,then a calibration curve for the heating power of lamp and the surface temperature of the specimen is figured out.Thermal shock consists of 20s heating specimen to the surface temperature be about 1100°C and dwelling at that power for 5min and ramp power from heating power to zero in 20s and dwelling at zero for 3min and forced air through the hollow during cooling and dwelling at zero.During thermal shock,a thermocouple set in the substrate (see Fig.1)is connected with a computer through a digital volt gauge to record the temperature in situ.The temperature difference (the delta T )of the surface and the substrate is measured by using the surface temperature calibrated according to the heating power of the lamps and the substrate temperature measured by thermocouple.In order to evaluate insulation effect,heating speed of substrate during heating ramp is calculated as a function of thermal shock.

For impedance measurement,the specimen surface is coated with platinum gel (5×5mm 2)that served as an electrode and the substrate is polished as another electrode.Impedance measurements were carried out at 400°C using a Solartron SI 1260Impedance/gain-phase analyzer coupled with a Solartron 1296Dielectric Interface,which is computer controlled.Spectra analysis was performed using Zview impedance analysis software (Scribner Associates,Inc.,Southern Pines,NC).In the measurements,alternating current (AC)amplitude of 100mV was employed and the AC frequency was in the range of 0.01Hz to 1×106Hz.Microstructure and composition of specimen is examined by scanning electron microscopy (SEM)coupled with energy dispersive X-ray spectroscopy

(EDS).Specimen is embedded into the epoxy resin and cut by a diamond saw to obtain cross-section and then polished with a diamond paste and etched with FeSO 4,H 2SO 4,and alcohol solution prior to SEM observation.Surface morphology of specimen is observed by multiple focus OLYMPUS optical microscope.3.Results

3.1.Insulation effect

Fig.2shows the reverse of temperature difference between surface and substrate as a function of thermal shock number.Before 220times,the curves can be divided into three parts,stage I,II,and III,respectively.For stage I (from 0to 25times),at the initial stage of thermal shock,the value decreases sharply.With the increase of thermal shock,the curve decreases slowly as fitted by stage II (from 25to220times),and then the value increases slowly in stage III (above 220).

3.2.Microstructure as a function of thermal shock time

Fig.3(a)–(b)show the cross-section SEM image and the optical photo of surface for as received specimen.Al 2O 3

/YSZ

Fig.2.The reverse of temperature difference between surface and substrate as function of thermal shock

number.

Fig. 3.Morphology of as deposited top coating with nano-multilayer microstructure,(a)cross-section,and (b)

surface.

Fig.1.Schematic diagram for temperature measurement.

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composite ceramic nano-multilayer top coating is prepared by EB-PVD to improve its thermal resistance.During deposition,two sources,one is YSZ target,another is Al 2O 3target,are alternately evaporated.As shown in Fig.3(a),the analysis results by EDS show that the dark layers are Al 2O 3layers and the grey layers are YSZ layers.Al 2O 3layers and YSZ layers are alternative and continuous,and their thickness is less than 200nm.From Fig.3(b),columnar crystal druse and their grain boundary can be seen from surface morphology of the top coating,and the surface is smooth and no crack.On the other hand,due to the roughness of the substrate and relatively fast deposition speed,the ceramic layers show the fluctuations in thickness.

Fig.4(a)–(b)show the cross-section SEM image and optical photo of surface for specimen after 30thermal shocks.As shown in Fig.4(a)and (b),there are many cracks along the columnar crystal.There are some holes localized at the bond coat caused by the etching for SEM observation.And almost all cracks end before reaching the interface of top coat and bond coat.

The growth of TGO and cracks during thermal shock is presented in Fig.5(a)and (b)for specimens after 200and 300thermal shocks,respectively.Obvious TGO can be seen in Fig.5(a),and the cracks along the columnar crystals are

prolonged.After 300thermal shocks,there are cracks along the interface of TGO and top layer.The cracks along the columnar crystals are broadening.

3.3.Impedance Spectroscopy of thermal barrier coatings as a function of thermal shock number

Fig.6(a)and (b)show the Impedance Spectroscopy of thermal barrier coatings as function of thermal shock number for the low frequency range from 0.01Hz to 1000Hz,and for high frequency range from 1000Hz to 1,000,000Hz,respectively.For the low frequency range (Fig.6(a)),the impedance of thermal barrier coatings increases with increasing thermal shock number,especially for the very low frequency such as from 0.01to 1Hz.But for the system after 150thermal shocks,its impedance is even a little larger than system 200thermal shocks.For system after 300thermal shocks,its impedance increases continually.

For the higher frequency range (Fig.6(b)),the trend of impedance with the thermal shock shows different results from the lower frequency.From 0to 30thermal shocks,firstly the impedance increases,then it decreases (from 30to 150thermal shocks),and finally increase again (from 150to 300thermal

shocks).

Fig.5.Morphology of cross-section for the specimen.(a)After 200thermal shocks,and (b)after 300thermal

shocks.

Fig.4.Morphology of the specimen after 30thermal shocks,(a)cross-section,and (b)surface.

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4.Discussion

The thermal gradient tested in the experiment depends on the microstructural features such as microcracks,defects,and micro composition of the thermal barrier coatings such as TGO formed during thermal shock.The effect of microcracks (which are perpendicular to the plane of the film)on the thermal conductivity of a film system can be expressed as [10],k net

?1teN slab ?1Te1?a Tb g

?1:

e1T

Where k is the thermal conductivity of bulk material,k net is the thermal conductivity of film-microcrack system,N slab is the number of slabs divided by microcracks,αis the thermal conductivity of the cracks,βis the size of cracks,and γis the fraction of cross-section of microcracks that they cover.

Therefore,the ratio of the thermal conductivities can be quantitatively obtained by considering the factors of micro-cracks such as the number of slabs,the size of the cracks and the fraction of the cross-sectional area that they cover.For the top coat of thermal barrier coatings in the case of this paper,at the initial stage of thermal shock,the factors including the

number of slabs divided by microcracks (N slab ),the size of cracks (β),and the fraction of cross-section of microcracks that cover (γ)are increasing.That may be the reason that there is a sharp decline at stage I shown in Fig.2.And also it can be confirmed by the cross-section and surface morphol-ogies of the system after 30thermal shock shown in Fig.4(a)and (b).

After prolonging thermal shock,there is a slow decline as stage II shown in Fig.2.This may be due to the slow growth of perpendicular microcracks and the formation of TGO as shown in Fig.5(a).The thermal conductivity of Al 2O 3which is the main composition of TGO is 0.02394J/cm·s·k,that is larger than YSZ but less than bond coat.For the number of the perpendicular microcracks,it should not increase unlimitedly.Therefore,there is a slow decline of thermal conductivity of thermal barrier coatings.

After 220thermal shocks,the parameter 1/ΔT increases as stage III shown in Fig.2.At the same time,cracks with larger size which are perpendicular to the film plane and along the interface of top coat/bond coat are observed for system after 300thermal shock,and some cracks even penetrate through the top coat layer as shown in Fig.5(b).In this situation,the thermal conductivity of system would not follow the micro-crack model any more.Heat flux would be easy to go through the top coat layer to reach the substrate,and then the thermal conductivity of TBCs is increased.On the other hand,the sintering of the top coat during thermal shock is another possible mechanism that can lead to an increase in thermal conductivity.

According to Wiedemann-Franz (WF)law [11],about the relationship of thermal conductivity (ê)and electrical conduc-tivity (σ)for bulk materials,there is j r T ?C ?k 23j 2B

e :e2T

Where k is thermal conductivity,σis electric conductivity,T is the measurement temperature,Γis Lorenz number,k B is Boltzmann's constant,and e is electron

charge.

Fig.6.Impedance Spectroscopy of thermal barrier coatings as function of

thermal shock number,(a)low frequency range from 0.01Hz to 1000Hz,(b)high frequency range from 1000Hz to 1,000,000

Hz.

Fig.7.Changes in reverses of impedance and ΔT for TBCs as function of thermal shock.

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According to the assumption proposed by Tien[12], compared with bulk materials,the thermal conductivity and electric conductivity for thin film can be expressed as,

j film j bulk ?

r film

r bulk

:e3T

Where k film is thermal conductivity of the film,k bulk is the thermal conductivity of the bulk material,σfilm is the electric conductivity of the film,andσbulk is the electric conductivity of the bulk material.Then,the Eq.(2)can also be applied to the thin film materials and can be rewritten as,

k film

r film

?CT C:e4T

Therefore,at the temperature for the measurement,the ratio of the k film to theσfilm is a constant.Then measurement results of electric conductivity can be related with the thermal con-ductivity of thermal barrier coatings system.Impedance Spec-troscopy is used in thermal barrier coatings system to evaluate the failure process and monitor the chemical and physical variation during service[13–15].All the research results showed that the impedance at high frequency part gives the information about YSZ layer,meanwhile,the low frequency part gives the information about TGO layer.The impedance value at low frequency such as0.01Hz contained the YSZ and TGO contributions totally.Therefore,the total impedance value,|Z|at0.01Hz was used to evaluate the total conductivity of TGO and YSZ.As shown in Fig.6(a),the impedance values at0.01Hz for samples after different shock number increase with the increasing of shock number,and that the trend is opposite to the1/ΔT showed in Fig.2.The factors including the perpendicular cracks in ceramic layer and the formation and growth of TGO that can influence the thermal conductivity also affect the impedance of TBCs system.The plots of the reverse impedance at0.01Hz and the parameter1/ΔT versus the thermal shock number are shown in Fig.7.The two plots show similar trends.Therefore,the variation in thermal conductivity of thermal barrier coatings system as function of thermal shock can be characterized in terms of the impedance change to some degree.Meanwhile,the impedance at higher frequency may show the crack growth rate for top coat.At initial thermal shock, the growth of microcracks is very fast,but after that,it gets slow,and then some macrocracks formed.That may be the reason that the impedance increases after30thermal shocks and then decreases for system after150thermal shocks and then increases go on.

5.Summary

Thermal conductivity of thermal barrier coatings with nano-multilayer ceramic top layer is studied in this paper by in-situ substrate temperature monitor and Impedance Spectroscopy during thermal shock test.As function of thermal shock,change of thermal conductivity and impedance is related to the micro-structural changes such as the formation and growth of per-pendicular microcracks,TGO growth.A decrease and then an increase in the thermal conductivity and impedance at low frequency are observed and discussed in terms of cracks and relationship of thermal conductivity and electrical conductivity. The cracks existed in TBCs can be characterized by system thermal conductivity,and thermal conductivity can be evaluated by using impedance value.

Acknowledgement

This work is supported by the National Natural Science Foundation of China under the contact50546030and 50420130032.

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