Microstructure and Mechanical Properties of Yttria-Stabilized Zirconia Coatings Produced by Eletrophoretic Deposition and Microwave Sintering
W.WANG,S.Q.QIAN,and H.SHEN
Y2O3-stabilized zirconia coatings were deposited on superalloy K17substrates at room
temperature by the eletrophoretic deposition technique followed by two different sintering
methods.Scanning electron microscopy,X-ray diffraction(XRD),and nanoindentation tech-
niques were employed to characterize morphological,structural,and mechanical properties of
the coatings.Finer and more uniform microstructures were observed in the microwave sintered
coatings.For the conventionally sintered coatings,the monoclinic phase was observed.The
microwave sintered coatings of Y2O3-stabilized zirconia contain mainly cubic/tetragonal phases
with some metastable phase present.In comparison with the hardness of3.1GPa and elastic
modulus of83.5GPa for conventional sintered coating,the hardness and elastic modulus for
microwave sintered coating rapidly increased to4.3and172.7GPa,respectively.Such coatings
have potential in being used as thermal barrier coatings(TBCs)on superalloy substrates.
DOI:10.1007/s11661-010-0390-4
按摩锤ÓThe Minerals,Metals&Materials Society and ASM International2010
I.INTRODUCTION
T HE Ni-base superalloy is widely used in the severest operating conditions in the combustor and gas turbine sections because of their creep,toughness,and low cycle fatigue properties.However,combustor and turbine gas temperatures may exceed the melting point of the Ni superalloy,leading to structural failure by melting.To increase the temperature capability of gas turbines,a coating of heat-insulating zirconia ceramics is applied on the surface of the turbines as a thermal barrier coatings(TBCs).[1–3]
At present,there are two principle technologies for fabrication of TBCs.One is plasma spraying[4]and the other is electron beam–physical vapor deposition.[5] While these techniques are applied with great success, they are cost and time intensive,and coating of complex shapes may be difficult or ev
en impossible.The ele-trophoretic deposition(EPD)technique,with range of novel applications in the processing of advanced ceramic materials and coatings,has recently gained increasing interest not only because of the high versatility of its use with different materials but also because of its cost-effectiveness requiring simple apparatus.[6–8]The use of EPD for obtaining such coatings will be investigated in this article.
The loose powder coating needs to be densified after EPD,yet the high temperatures necessary for sintering with conventional furnaces may be detrimental for the metal.Microwave sintering is a relatively new technique employed for sintering of ceramics.It is a faster method and high densities can be achieved over a shorter period of sintering time compared with conventional sintering techniques.[9,10]Microwave sintering also has the poten-tial to produce ceramics with better microstructures due to more uniform heat distribution leading to lower thermomechanical stresses.In this work,Y2O3-stabilized zirconia has been selected for the TBCS;the effect of microwave heating onfinal microstructure and grain size of these materials has been discussed.Also, mechanical properties of the Y2O3-stabilized zirconia coatings,densified by microwave and conventional sintering,have been investigated.
II.EXPERIMENTAL PROCEDURE
In this work,EPD was carried out with electricfield applied between a graphite counter electrode(anode) and a substrate(cathode).The distance between the electrodes was20mm,and they were kept in a liquid solvent,which contained a suspension of powders of the materials to be deposited.EPD was carried out at room temperature using applied voltages in the range of60to 120V and deposition times of1to5minutes.The substrate was K17superalloy.The substrate sample was of dimensions20920910mm3.Y2O3-stabilized zir-conia(5mol pct Y2O3)(YSZ,<30nm)powder suspen-sion was prepared by magnetic stirring of the mixture of ceramic powders in acetone.Suspensions used here had Y2O3-stabilized zirconia powder concentrations in the range of20to50g/L and iodine concentration of0.1to 0.8g/L.
After deposition,the coatings were dried at room temperature prior to sintering.Conventional sintering
W.WANG,Associate Professor,S.Q.QIAN,Professor,and H.SHEN,Graduate Student,are with the School of Materials Engineering, Shanghai University of Science Engineering,Shanghai201620,People’s Republic of China.Contact e-mail:wangwei200173@sina Manuscript submitted December14,2009.发动机
飞轮壳
Article published online August10,2010
involved heating the green samples at1373.15K (1100°C)for30minutes in air.Heating and cooling rates were278.15K/min(5°C/min).Microwave sinter-ing of EPD-coated samples was performed at1173.15K to1573.15K(900°C to1300°C)for10to60minutes under controlled argon atmosphere with aflow rate of 20mL/min.Heating and cooling rates were283.15K/ min(10°C/min).The phases present in the sintered coatings were determined using X-ray diffraction (XRD).The microstructures of coatings were examined using optical microscopy and scanning electron micro-scopy.Hardness and elastic moduli were calculated from the data obtained by nanoindentation tests using a Berkovich indenter.
III.RESULTS AND DISCUSSION
For experiments using EPD,a constant voltage of 80V,Y2O3-stabilized zirconia powder concentration of 30g/L,iodine concentration of0.3g/L,and a deposi-tion time of3minutes were chosen.These parameters were shown to lead to the best Y2O3-stabilized zirconia coatings prepared on K17superalloy in terms of homogeneity of coating microstructure,as shown in Figures1and2.
The top surface of the Y2O3-stabilized zirconia coatings was smooth and dense;no obvious crack w
as found in the coatings.It can be seen that the top surface of the coatings is not very smooth,as shown in Figure1
. Fig.1—Scanning electron micrographs of(a)microwave and(b)conventional sintering
samples.
Fig.2—Cross-sectional micrographs of(a)microwave and(b)conventionally sintered samples.
The top surface structure consists of hexagonal grains and elongated grains.Between two different grains, there are some pores,which make the surface rough. The elongated grains disperse into the gaps of hexagonal grains.The samples of microwave sintering have a much smaller grain size,comp
ared with that of the conven-tional sintering(Figure1(b)),with very few and uni-formly distributed pores(Figure1(a)).
The cross-sectional micrographs of microwave and conventional sintering samples are shown in Figure2. The coatings are around10l m in thickness,and the coating structure near the surface is looser because of crystallization,which is in accord with Figure1.How-ever,the inner layer is dense and pore free.Under microwave,the K17superalloy matrix heats the coating. Therefore,the inner layer of the coating will be softened first,followed by the outer layer.In this way,the remnant air in green coating will be pushed out completely. Consequently,the internal part of Y2O3-stabilized zirconia coatings is dense and pore free.It can be seen also that the samples of microwave sintering have a much smaller grain size compared with that of the conventionally sintered samples.
The XRD patterns of the conventionally sintered sample and that of the microwave sintered sample are shown in Figure3.It can be seen that two phases are present in the coatings,tetragonal zirconia and mono-clinic zirconia,despite the quite high content of mono-clinic phase present in the conventionally sintered sample.It appears that microwave sintering had the effect of promoting the monoclinic-tetragonal transfor-mation in the zirconia phase.Preliminary thermal treat-ments to eliminate volatiles or convert monoclinic residue to tetragonal crystals were found to be necessary b
y some authors in order to obtain uncracked samples.[11] A comparable hardness and Young’s modulus were measured for microwave and conventional sintering Y2O3-stabilized zirconia coatings,as indicated in Table I.It can be seen that the hardness elastic mod-ulus and elastic modulus for microwave sintered Y2O3-stabilized zirconia coating compared with conventionally sintered coating were improved.In comparison with the hardness of3.1GPa and elastic modulus of83.5GPa for the conventionally sintered coating,the hardness and elastic modulus for microwave sintered coating rapidly increased to 4.3and172.7GPa,respectively.This variation of the hardness and elastic modulus is attrib-uted to the change in the microstructure of the coating on the top surface.The samples of microwave sintering had a much smaller grain size on the top surface(Figure1), compared with that of conventional sintering,with very few and uniformly distributed pores.Also,it is known that the hardness and elastic modulus of Y2O3-stabilized zirconia coatings are highly related to the transform-ability of tetragonal ZrO2phase.[9]According to the XRD pattern of microwave sintered sample,the phase is almost100pct tetragonal(Figure3).
IV.CONCLUSIONS
Y2O3-stabilized zirconia coatings were fully densified by means of microwave and conventional sintering. Afiner size and more uniform microstructure were obtained by microwave sintering.The mic
rowave-sintered coatings of Y2O3-stabilized zirconia contain mainly cubic/tetragonal phases with some metastable phase present.
When compared with conventional sintered coating, the hardness elastic modulus and elastic modulus for microwave-sintered Y2O3-stabilized zirconia coating improved.In comparison with the hardness of 3.1GPa and elastic modulus of83.5GPa for conven-tional sintered coating,the hardness and elastic modulus for microwave-sintered coating rapidly increased to4.3 and172.7GPa,respectively.
ACKNOWLEDGMENTS
This work is supported by the Shanghai Municipal Developing Foundation of Science and Technology under Grant No.0852nm01400.
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Table I.Mechanical Properties of Y2O3-Stabilized Zirconia Coatings,Densified by Microwave and Conventional Sintering Sintering Method
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