SAÜ
Fen Bilimleri Enstitüsü Dergisi2
(1998) 165-170
COMPARISON OF THE THERMAL SHOCK RESlST ANCE IN
Alı03-SG AND Zr02-l2°/oSi+AI COATING SYSTEMS
AhmetÖZEL
University o.f Sakarya, Facu/ty of Engineering, Esentepe Kanıpüsü, Adapazari
Enıail:
ozel@esentepe.sau.edu.trABSTRACT
In this investigation� thennal and structure fınite eleınent analysis has been employed to aııalyse tlıe level of the thennal stresses developed in A1203-SG and
Zr02-12%Si+ Al coatings subjected to them1al loading. Systeıns \Vith 0.4nıın coating thickness and 4nıın substrate ınaterial thickness were nıodelled. Alumina -Ductile Ca st Iran coatings \Vith Ni Al, Ni Cr AIY, NiCoCrAIY interlayer \vere also modelled. Noıninal and shear stresses at the cıitical interface regions ( filnı /interlaycr/substrate ) \vere obtained and compared. The results showed that the Al203-SG coatings has higher tlıennal shock resistance than Zr02- 12o/oSi+Al coating systeıns. Furthern1ore� tlıe interlayer thickness and ınaterial coınbinations have a significant influence on the level of the devcloped tl1ennal stresses. It is also concluded that the finite eleınent technique can be used to optiınise the design and the processing of ceranıic
.
coatıngs.
I. INTRODUCTiON
Surfacc preparation teclu1iques such as plasına spraying, physical vapour depositian and chemical vapour depasition have been used to ınake convenient material coınbinations in usage of high technological requireınents. High teınperatıne coatings are used for t \VO ınain functions: either to protect a base ınetal against corrosion or erosion or to miniınise wear. A th.ird function is to reduce the base ınetal teınperature in the case of thennal barrier coatings.
Plasına teclu1ique is widely eınployed for iıı ceraıuic coatings. A plasına is a very high energy state and thus transfers heat very fast to the po,vder, reducing the necessary d\vell tiıne at higlı teınperature, which ıninintizes by tJ1e inert nature of the heat source. Plasına spraying is the highly ionised state of ınass, consisting of ınolecules: atoıns, ions, electrons and light quantrun 's. Plasnıa spraying equipınent consists of coınp1ex individuaJ apparatuses and devices� for example the plasına torch.. po\ver unit, cooling systeın, powder and
gas feeder units. The teınperature in the plasma are centre even attains around 30000K. In this process the po\vder is ınelted by teınperature taken up by the beaın and thrown on tl1e substrate at high velocity. Plasına spraying rarely heats tJ1e substrate over 300°C and it is required to keep the substrate teınperature using air cooling in the range of 200-250°C. Optiınisation of plasıua spraying processes has been attempted to decrease coating porosity and achieve better adherence. The -depositian efficiency is strongly influenced by both particle size and distribution. Most of the partictes ınust be ınolten before impingeınent to produce dense deposits and ınust have suffıcient velocity to splat into the irregularities of the previous splats. Interaction of the ınalten material \vith tlıe plasına beaın and surraunding atınosphere affects a plıysical and cheınical the transforınation of the partictes in the plasına beaın ınelt
[ ı ı.
Plasına coatings are used for ınany engineering applications in order to iınprove surface properties of the ınaterials. Perhaps one of the ınost lucrative applications of ceraınic coatings \vill be for high temperature applications such as gas turbines and diesel engines. Most of those applications are in tlıe fonn of thernıal barrier coatings, which allow for higher engine tcınpcratures and better efficiency [2]. The strength of the bondcd systeın is goven1ed by a ntunber of variab i es: tlıe tlıcnnaJ and elastic ınisınatch� tl1e plastic flow stress of the ınetal� tlıe relative substrate /coating tlıickness� tlıickncss of interlayer: elasticity and tlıeı ınal expansion coeffıcient of interlayer� the fracture resistance of the interface and tlıe flavv distributions in the ceraınic and at the interface [ 3-8]. Most failures in tl1e bond coatings systeın alsa depend on processing paraıneters, i. e. contact teınperature, surface coınposition� solidification of sprayed particles [9].
In the case of therınal loading, it is 'veli known that the thennal stresses are generated at the interface layer when dissiınilar ınateriaJs are bonded toget11er [ 1 OJ. These stresses cause tlıe debonding and spalling of the coating froın the substrate ( 11-12]. Several nıeans of reducing the
Comparison of the Thermal Shock Resistance in Ab03-SG and Zr02-12o/oSi+AI Coating System s
Table 1 Materials Properties
Prooerties Al -,03 S.G. 12o/oSi+Al
E GPa 340 168 76
V 0.23 0.3 1 0.3
a.l/°C.ıo-6 8. lxl0-6 13.7xıo-6 ı ı.ıxıo-5
p
kg/
cm3 3780 7289 2660k W/m. oc 24 48.9 150
Cp J/kg.°C
1080 4 18.4 977them1al stresses level in tlıese systeıns have been considered, including the application of interlayer and convenient geoınetry [ 6]. Therefore interlayer nıaterials were introduced to increasc tl1e bonding strength and to reduce tlıe level of the dev el oped stresses.
Finite elen1ent nınnerical analysis has been utilised to study the stresses in a nuınber of probleıns where ınaterial properties etiffer across an interface, such as in composite,
Ni Al NiCrAIY NiCoCrAIY
103 208 27 1
0. 17 0 .17 0. 17
14.6 x ıo-6 1 2 . 7xıo-6 ı ı. sxıo-6
5800 82 10 10480
5.3 95 106
593 468 665
II. MATERIALS
Table 1 presents.. Al203, l2%Si+Al, NiAl, NiCrAlY. NiCoCrAlY and Ductile Cast Iron
(S.G)
coating_ substrate and interlayer ınaterials data. The theı ııaalexpansion coefficient data for the ınaterials are between
8.lxl0-6- 2 1.5xl0-6 (/°C), thennaJ conductivity data are between 24 - 150 W/ın°C and modules of elasticİt)' are bet\-veen 76-340 GPa. !F30 Wfm:&oc ----- ---- ---• 1 ' 1 Al203 ""-.. ..._...__,�---�---...-.-,·---. ·. ' '
i
nte rle.ye r
yS.G.
XFigure 1. Finite Element Model
ceraınic points, graded ceraınic ınetal ınaterials and coatings [6]. Coatings involve various ınaterials and geometry 's, and are generally modeli ed in two diınensions .. tlıereby calculating stresses both across as along tl1e interface. Most studies of the thennal stresses in coatings have considered stcady-state excursions. More recently, t11e tlıennaı stresses in ceramic-ınetal bonds have been analysed by taking into account the transient nature of the heat transfer process [ 13- 14].
In this investigation ANSYS fınite element packet prograınn1e was employcd to analyse and to coınpare the tb e ı ınal shock resistance of Al203-SG and
ZrOı-12o/oSi+Al. Furthennore to obtain the potential of in terlay er ınaterials in controlling the level of the thennal stresses in tl1e coating systeıns. Al203-SG Coatings witlı
NiAL NiCrAlY and NiCoCrAIY interface ınaterials
\vere ınodelled. Furtlıernıore� coatings with different coınbinations of these interlayer materials \vere also ınodelled to evaluate tlıe potential advantage of systeıns reasonably. The results sho,ved the signifıcant influence 9f interlayer geometry and ınaterial combinations on the level of the developed stresses in coatings subjected to
thennal shock.
166
I II. ANALYSIS
General purpose finite element code
ANSYS�
isemployed to obtain transient thennal/structure coupled solution. The coatings wcre ınodelled using 4-node. plane strain, quadratic eleınents ( see,
Figure ı).
Tlıe constraints \Vere in1posed on the left and bottom sides ofthe ınodel and heat transfer was only allowed from the top surface of the ınodel. Therınal loading was app
l
ied by cooling down tl1e ınodel froın 600 oc to roonı teınperature in very short tiıne using air flow'vith
a 30W /ın2 oc heat transfer coefficient at the top surface of the
ınodel. Alı03-SG and Zr02_ 12o/oSi+ Al systems '\•ith coatiııg 1 substrate thickness ratio 1/1 O were modelled. To study the influence of the coating geo
m
etıy
on the thennal stress levels 1/20 and 1/4 ratios of Alı03-SG \vere also ınodelled. Furtlıerınore,three
different interlayer materials coınbinations were alsomodelled. In
those ınodels the coınbincd interlayer thickness '\vere kept 0.4 ırun in total. A transient coupled thennal 1structure fınite eleınent analysis was executed for all coating model s. The thcnnal stresses crx, cry and ı:xy were
A.Özel
c m E - Q.) Cl) ... > (/) Q)� - (/') Cl) (l)C) (Jl(j) en ' �("!") U) O eN -<{ en o Q)-rn c ro o � "' u ·;:: c co ·- a. ÖE �o o u300
250
200
150
100
50
o
-50
-100
't xy Zr02 -(12Si+AI)Figure 2: Comparison of the developed thermal stresses in Zr02/12Si+AI) to AL203 -SG system
IV. RESUL TS AND DISCUSSJONS
Figure 2 rcpresents the percentage of changes in stresses
leyeJs of AI:!01-SG S) st e ın in coıuparison to the stresses dc\·elopcd in ZrO:> 1 2o/oSi+ AJ systenı. It is clear froın tJüs figure that therc are 250 %, 60% increases in ox �
and t:-..y stresses rcspcctivcly anda decrease of 70 o/o in cry
value. The increase in Zr02_ 12o/oSi+A1 systeın stress 1 levcls is due to the large nıisınatch in thennal expansion coeffıcients of the ınaterials, see Figure 3. This figure presents tlıe variation in stresses \Vith the ratio of tlıe thennal expansions of the ıuateriaJs. There is a linear variation in stress lcvels \Vİth tlıe increase in tlıennal cxpansion coeffıcicnt ratios.
• • o C/) 2,5 1 �"") o 2 N ....J <( c: ı ... 1,5 vı
l
O) C/) U'; ll) ı.... f-J V: 1 o f-J ı.r. Q) V: Vl � 0,5 ı... ... Vl c.... o o ·-o ... � cG o • ax • ay • -rxy_j
.-, --1 . - . ---- - . .. "' �-. --· �-. �-. --. --.--. - ----.. ... -... ·-. ·----2coating material 1 Al203- SG
•
_ ...
li
Figure 3. Variation of stress levels with the ratio of
coating materials thermal expansion
3
::
,.---·--·-� � ---·- - --- ----l
-+--cr�
1
o 12 ·- -� 10 � 8 � - 6 en 4 2 ��
{ • -rxy ._____1
1
ı 0+---�---·- -�-- • r ---..,.---�_] 0.05 0,10 0.15 0.20 0.25 0,30C aating to substrate th ickness ratio
Figure 4. lnfluence of coating thickness on the developed
stresses level for Al20 3- SG system
Comparison of the Thermal Shock Resistance in Ah03-SG and Zr02-12°/oSi+AI Coating Systems - 100 ,.---(1)
�
--100 200 - ·-rJ) Q) O> -300 -c ro .c o -400 -� o 1ı
T ı]
�;�;
>-:·:�
� +>-�
'2. + <t '2. -500 ..ı..____ ___________ ___ _ _ _ _ _ ____ _ __,Figure
5.lnfluence of interlayer material combination on the
stresses level
Figure 4 presents the variation in stress levels with
coating thickness for Ah03-SG system. The results showed tinıes I 5� 5 and 4 increase in crx, cry and 'txy levels
respectively \vith the increase in coating tlıickness froın 0.2 to 0.8 nu1ı. This is due to the large changes in teınperature gradients in tlıick coating layers.
Figure 5 presents the variation in stress levels with using
different interlayer ınaterials. It is clear from this figure that the lowest levels of stresses are in NiAl+ Ni Cr AIY interlayer systeıns. The drop in stresses levels are 50o/o,
66% and 100% for NiCrAIY -NiCoCrAlY, NiAI, NiAl
NiCrAIY .. interlayer respectively. We believe that this is resulted froın the non -uniforınity in ternperature
distributions because of the large nıismatch in sonıe interlayer nıaterials properties.
V. CONCLUSIONS
Under therınal loading conditions, coating, substrate and interlayer n1aterials coınbinations significantly influence the level of the developed thennal stresses. The thennal shock resistance of Alı03-SG systeın is higher than that of Zrü2. 12o/oSi+ Al systeın. For this system the ınost convenient interlayer ınaterials con1bination is NiAI .. NiCrAIY. Furthennore the tlıinner the coating the lower are the levels of the developed stresses. It is also concluded that the Fin.ite Eleınent Teclınique can be used to optin1ise the design and the processing of ceramic coatings.
ı
168
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A.Özel
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