WANG Zhenbo, ZHOU Chun gen, XU Hui bin, GONG Shengkai. ( Department of Materials Science and Engineering, Beijing University of Aeronautics.
V ol. 17
No. 2
CH INES E JO U RN A L O F AER ON A U TICS
M ay 2004
Effect of Thermal Treatment on the Grain Growth of Nanostructured YSZ Thermal Barrier Coating Prepared by Air Plasma Spraying WANG Zhen bo, ZHOU Chun g en, XU H ui bin, GONG Sheng kai ( Dep ar t ment of Mat erial s Science and E ngi neeri ng , Beij i ng Univer si ty of A eronauti cs and A st ronaut ics, Beij ing Abstract:
100083 , Chi na)
A nanostructured thermal barrier coating is prepared by air plasma spraying using the 8w t%
Y 2O3 part ially stabilized zir conia nano powder with an aver ag e grain size of 40 nm. T he microstructure and phase composition of feedstock nano powder and coating are investigated using SEM , T EM and X RD. It is found that the as sprayed zirco nia coating has an average gr ain size of 67 nm and mainly co n sistes o f metastable tetragonal phase, together w ith some monoclinic phase and tetragonal phase. T her mal treatment results show that the grains of the nanostructured coating g row slightly below 900 , w hereas over 1000 Key words:
the g ains g row rapidly and monoclinic phase no ticeably appeares.
nanostr ucture; thermal barrier coating; air plasma spr aying
热处理对 大气等离子喷涂纳米结构 YSZ 热障涂层晶粒长大的影 响. 王 振波, 周春根, 徐惠彬, 宫 声凯. 中国航空学报( 英文版) , 2004, 17( 2) : 119- 123. 摘
要: 采用大气等离子喷涂制备了纳米结构氧化钇 稳定的氧化锆热障涂层。运用 SEM , T EM 和
X RD 等方法研究了涂层和原料粉末的微观结构和相组成。结果表明, 喷涂过程中平均晶粒大 小由 40 nm 变为 67 nm, 涂层主要由亚稳四方 相组成。涂 层热处 理结果 显示, 900
以下晶粒 长大速 度
缓慢, 然而 1000 以上晶粒长大速度迅速增加, 而且出现较多的单斜相。 关键词: 纳米结构; 热障涂层; 大气等离子喷涂 文章编号: 1000 9361( 2004) 02 0119 05
中图分类号: T B383
T hermal barrier coat ing s ( T BCs) have been
文献标识码: A
used to provide t hermal protect ion for hot sect ions
not have t he inertia required to cross t he st ream lines in t he spray jet and w ould be seg regat ed to it s
in the t urbine eng ine for several decades[ 1, 2] . T he
periphery w ithout deposit ing on t he subst rat e. T he
development of high efficient eng ine requires t he
ot her is how t o retain the pre ex ist ing nanostruc
T BCs t o have low er thermal conductivity and high er t emperat ure reliability. It is reported that t he
t ure of the f eedstock. During t hermal spray ing pro cess, the heat of the plasma gas w ill damage t he
development of yt tria stabilized nanostructured zir
pre ex ist ing nanost ruct ure.
conia coat ing s may enhance t he performance of t he T BCs due to it s low t hermal conduct ivit y, high co
T herefore, it is very necessary to use agglom erat e part icles. At the sam e t ime, in order to ret ain
eff icient of t hermal expansion and ex cellent me
the pre ex ist ing nanost ruct ure, a low er power
chanical propert ies of this kind of coat ing
[ 3 6]
.
T here are t wo great challenges in t hermal spraying nanomat erials. One is how t o spray t he nanomat erials ont o t he substrate successfully. Indi vidual nanopart icles cannot be successf ully t hermal sprayed because of t heir low mass. T hus, t hey do
should be employed. In addit ion, larger feeding rate is benef icial t o gett ing nanostructured T BCs. How ever, t he nanost ruct ure w ill develop at high temperature in service, and t his w ill dam age it s ex cellent propert ies, so t he invest ig at ion on the st a bility of t he nanost ruct ured coatings at high t em
R eceived dat e: 2003 10 27; R evision received dat e: 2004 01 09 Foundation it em: N at ional N at ural S cience Foundat ion of China( 50176005)
! 120 !
W ANG Zhen bo, ZHOU Chun gen, XU Hui bin, GON G Sheng kai
2
perat ure becom es necessary. T he object ive of t his w ork is t o invest ig at e t he thermal t reatment ef fect on t he grain grow th. It w ill be beneficial t o furt her understanding of t he st abilit y of the nanostructured coat ings in service.
1
Ex periment Procedures
2. 1
CJA
Results and Discussion
Microstructure analysis F ig. 1 ( a) presents t he ex ternal morphology of
the st art ing powder agglomerated by spray dry ing. It can be seen t hat the part icles have a loose struc t ure, w hich can be separated int o lit tle parts by ex ploding in plasma spraying. T he loose structure is
An 8 w t% Y2 O 3 part ially stabilized nanostruc t ured zirconia, having grain size of 35 45 nm, is sprayed in air on the Ni base superalloy. T he sub st rat e is prepared by m ini cut t ing w ith size of 12
disadvant ageous f or heat t ransf er, so it is more likely for agglom erat ed part icles to have less molt en part t han the traditional whole body, t herefore t he
2 5 m m. In order t o enhance t he
original nanost ruct ure of som e part icles is retained in the thermal spraying process. F ig. 1 ( b) show s
adhesion of t he coating , t he substrate is blast ed
the T EM image of t he feedstock. It reveals that
w it h alumina and washed w ith supersonic w ave. During thermal spraying process, t he compressed
the feedstock part icles are composed of fine grains
mm
15 mm
wit h size ranging from 30 to 40 nm.
air is used to cool t he surface of t he coat ing. T he thickness of t he ceram ic coating is appropriately 300 m. T he spraying parameters are list ed in T able 1. T he t herm al treat ments are performed in an air open furnace at 600, 800, 900, 1000 and 1100
respectively for 15h.
T he m icrost ructure analysis is performed v ia scanning elect ron m icroscope ( SEM ) ( JSM 5800, JEOL) , f ield electron scanning elect ron m icroscope ( FESEM ) ( JSM 6301F, JEOL) and transmission
( a) the external mo rphology o f the ag glomerate entity
elect ron m icroscope ( T EM ) ( H 800, H IT ACH I) . X ray dif fraction ( XRD) ( D/ max 2200pc, Rig aku) is employed to est ablish t he phase com po sit ion of the feedstock and coatings. XRD is also used to est imat e the average grain size of t he feedstock and coatings[ 7 9] . T he ob t ained average grain size is compared to the result s of T EM . Table 1 Plasma spray parameters for nanostructured zirconia coating
( b) the T EM imag e of the feedstock particles
Pow er/ k W
28
Current / A
560
V oltage/ V
50
Primary gas ( Ar) / ( slpm) *
60
Secon dary gas ( H 2 ) / ( slpm) *
1
Carrier gas ( Ar) / ( slpm) *
5
Spraying dist ance/ mm
80 25
Feed rat e/ ( g!min- 1 ) *
slpm: st andard liter per minut e
F ig. 1 M orpholog y of the nanostructured feedstock particles
Fig. 2 ( a) and ( b) show t he typical morpholo g y for plasma sprayed nanost ruct ured zirconia coat ing. It can be seen that t here are tw o kinds of struct ure in t he coat ing. One is the cont inuous molt en phase, w hich is similar to t he t radit ional
Eff ect of T hermal Treatment on t he G rain Grow th of Nanost ructured Y SZ M ay 2004
T hermal Barrier Coat ing Prepared by A ir Plasma Spraying
! 121 !
coat ings, and t he ot her is t he loosely cont act ed
coat ing, w here large particles are packed t oget her
phase t hat ret ains t he nanost ruct ure of the starting
to form dense layers. It is noted t hat using t his
pow der. During t he plasma spraying, the inert
met hod can avoid mist aking t he nanostructure
plasma g as ex plods the agglomerat e feedst ock part i
sources. T he nanost ruct ure in the coating has tw o
cle, and t he lit tle particles t hen adhere to t he previ
sources, one is t he starting feedst ock, and t he oth
ous layers. F ig . 2( b) reveals t hat some part icles are
er is the molt en droplet. In t he t hermal spray ing
part ially melt ed, which have a smooth surface, and
process t he molten droplet experiences high rate
the molt en parts bond each other to form dense
quenching , but t he lat ter w ill appear in t he tradi
st ruct ure.
t ional T BCs. By T EM observat ion it cannot discern the source of nanost ruct ure.
( a) the exploding particle area ( a) the nanostructure o f the coating
( b) the par tially molten particle area F ig. 2
Surface morpholog y of the nanostructured
( b) the microstructure similar to the tradit ional T BCs
YSZ coatings
Fig . 3
nanostructured zirconia coating
F ig . 3 presents the FESEM im age of the coat ing surface. In Fig 3 ( a) , t here are many lit t le
FESEM micr ostructur e of the as sprayed
2. 2
Phase determination via XRD
part icles w it h t he size below 100 nm. T hese lit t le
F ig. 4 ( a) and ( b) show t he XRD pat terns of
part icles cont ain large volume fract ion of micro
the f eedstock pow der and zirconia coat ings. T he
pores, w hich can decrease t he thermal conduct ivit y
nanotructured feedstock exhibit s the presence of
of t he zirconia coating and t he thermal mism at ch.
the monoclinic, tetragonal and/ or cubic phase. It
How ever, t he relat ively loose st ruct ure let t he cor
can be calculat ed t hat the f eedstock pow der con
rosion g as pass t hroug h easily, which may induce
t ains about 35% monoclinic phase, and there is no
unexpected early st age degradat ion. Fig 3 ( b )
met ast able t et ragonal t∀phase in t he starting pow
shows a structure t hat is alike t o t he t radit ional
der. It should be noted t hat t he t∀phase is typical of
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W ANG Zhen bo, ZHOU Chun gen, XU Hui bin, GON G Sheng kai
CJA
thermally sprayed zirconia, w hich is formed due t o quenching of droplet s at t he subst rat e[ 10] . T he te t ragonal t∀ phase can be dist ing uished f rom t he transformable tet ragonal phase w ith respect to t he mart ensit ic t ransf ormat ion to t he monoclinic phase of zirconia[ 10] . During t hermal spraying, t here are some partially molten part icles, w hich form t he sm all amount of monoclinic phase. As a conse quence, the coat ing ( F ig 4( b) ) consists of t hree phases: metastable tetrag onal, monoclinic and te t ragonal phase. 2. 3
( b) zirconia co ating
Effect of thermal treatment temperature on
Fig . 4 XR D patter ns
the growth of grain size
Fig. 5 g ives the XRD patt erns of t he as
T he average grain size of t he as sprayed coat ing and t he samples t reat ed at 600, 800, 900,
sprayed st at e coat ing and heat treat ment coat ings
1000 and 1100
at dif ferent t emperat ure. It reveals t hat there is
for 15h are det ermined f rom
some monoclinic phase and cubic phase appeared in
XRD peak broadening using t he Scherrer equa t ion
[ 7, 11]
the heat t reatment coatings. At elev ated tempera B p ( 2 ) = 0. 9 / Dcos
t ure the metastable tetragonal phase decomposes in
w here D is t he average dimension of cryst allit e; B p
t o common tetrag onal phase and cubic phase, when
( 2 ) is t he broadening of the diff ract ion line mea
the coat ing is quenched f rom high temperature t o room t emperat ure, t he common t et ragonal phase
sured half max imum int ensit y; is the w aveleng th of t he X ray radiation and is t he Brag g angle. T he Scherrer equat ion is deduced under t he assumpt ion that only a small grain size is responsi ble for peak broadening. St rain effects, w hich may influence t he peak broadening, are not t aken int o account. In t his study, the inst rument al broaden
turns into monoclinic phase accompanied w ith t he 4% volume ex pansion. Since the t ime for heat treat ing is not long, t he t ransit ion seems lit tle. But in the long t im e service, the phase transit ion w ill become signif icant. Some measures must be em ployed to avoid t his phenomenon in furt her st udies.
ing has been removed by measuring the coarse crys t alline zirconia w ith a diam eter larger t han 10-
4
cm.
Fig . 5
XRD patterns of t he as spr ay ed state coating and heat treatment coatings at different temperatur e
According to Ref. [ 10] , ( 222) at 2 ( ~ 63#) ( a) feedstock powder
is t he most appropriate peak f or averag e grain size
Eff ect of T hermal Treatment on t he G rain Grow th of Nanost ructured Y SZ M ay 2004
! 123 !
T hermal Barrier Coat ing Prepared by A ir Plasma Spraying
det ermination for all t hese sam ples. All t he data calculated by Scherrer equat ion come f rom t he peak of ( 222) . T able 2 list s t he average g rain size of as sprayed coat ings and t hat of the coatings aft er t her
m ine t he nanost ructure of t he f eedstock preserved in t he coating . ( 3) T hermal t reat ment below 900 has lit t le eff ect on t he grain grow t h, but t reat ment above 1000
can make t he grain grow rapidly.
mal t reatment at dif ferent t emperat ures. T he t her mal t reat ment below 900 has a lit tle eff ect on t he g rain g row t h, but above 1000 the g rain grow th
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3 Conclusion
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( 1) T he starting powder ex hibits the m ix ture of tet ragonal phase and monoclinic phase. Af ter thermal spraying, the coating consist s of t he met ast able tetragonal t∀, monoclinic and t et ragonal phase. ( 2) Using the FESEM technique to observe the grain size of t herm al sprayed coat ings w ith comparing the result of XRD is pow erf ul to deter
1- 8.
Biography: WANG Zhen bo Born in 1978, he re ceived M . S. degree in 2004 fr om Beijing university of A eronautics and Astronau tics, and his main research interest is nanostr uctured ther mal barrier coatings. T el: ( 010) 82316000, E mail: bozhen wang@ 163. co m
