The investigation combines both experimental studies and numerical modeling to predict coating life in an oxidizing environment. The experimental work provides both input to and verification of two numerical models. The coatings being examined are an aluminide coating on Udimet 700 (U-700), a low-pressure plasma spray (LPPS) Ni-18Co-17Cr-24Al-0.2Y overlay coating also on U- 700, and bulk deposits of the LPPS NiCoCrAlY coating

Gedwill, Michael A.

Nesbitt, James A.

English

NASA Technical Reports Server

N88-11181
COATING LIFE PREDICTION
James A. Nesbitt and Michael A. Gedwill
NASA Lewis Research Center
Cleveland, Ohio
Hot-section gas-turbine components typically require some forml of coating for
oxidation and hot-corrosion protection. These coatings are generally either aluminide
coatings or MCrAIY overlay coatings, where M represents nickel, cobalt, or a combina-
tion of these two elements. Both coating types are protective as a result of the
selective oxidation of aluminum to form an external, continuous AI203 scale. The
coatings act as a reservoir of aluminum since the aluminum content of the coating is
always greater than that of the substrate. Any mechanism which reduces the aluminum
content of the coating degrades the coating. Two significant forms of degradation
which occur in aero gas-turbine engines are oxidation and coating-substrate inter-
diffusion. Coating-substrate interdiffusion involves not only the loss of the alumi-
num from the coating into the substrate but also the diffusion of less desirable
elements to the coating surface where they may oxidize and hinder or prohibit the
fommation of the AI203 scale. In addition, cycling of a coated component results
in cracking and spalling of the AI203 scale, which further accelerates coating
degradation. -"
Efficient use of coatings requires reliable and accurate predictions of the pro-
tective life of the coating. Currently, engine inspections and component replacements
are often made on a conservative basis. Consequently, there is a continuing need to
improve and develop the life-prediction capability of metallic coatings in various
service environments. The purpose of the present work is to develop an improved
methodology for predicting metallic coating lives in an oxidizing environment and in
a corrosive environment.
APPROACH
The present investigation combines both experimental studies and numerical
modeling to predict coating life in an oxidizing environment. The experimental work
provides both input to and verification of two numerical models. The coatings being
examined are an aluminide coating on Udimet 700 (U-700), a low-pressure plasma spray
(LPPS) Ni-18Co-17Cr-24AI-O.2Y overlay coating also on U-700, and bulk deposits of the
LPPS NiCoCrAIY coating. The approach taken in this study is shown schematically in
figure i.
Experimental Testing
The experimental testing involves isothermal and cyclic furnace oxidation at
1050, II00, and 1150 "C. In addition, Mach 0.3 cyclic burner rig testing of the
aluminide and LPPS NiCoCrAIY coating (125 and 625 pm thicknesses) on U-700 is also
being undertaken at II00 and 1150 °C. Isothermal oxidation of the coated U-700 and
bulk coating yields the growth rate of the AI203 scales which form on these coat-
ings. Cyclic furnace and burner-rig oxidation yield the weight changes of the coated
specimens reflecting the oxide growth and spallation which occurs during thermal
397
https://ntrs.nasa.gov/search.jsp?R=19880001799 2020-03-20T08:33:48+00:00Z
ORIGINAE PAGE IS
OF POOR QUALITY
cycling. Analysis of the coating after an oxidation exposure includes x-ray diffrac-
tion and polarized light metallography of the retained surface oxides, observation of
microstructural changes, scanning-electron microscopy, and electron microprobe analy-
sis to measure concentration profiles across the coating and substrate. Only the
experimental results for the furnace testing of the LPPS coating on U-700 will be
discussed in the remainder of this paper.
Numerical Modeling
Two computer models are being used to predict the oxidation-limited life of the
metallic coatings. A spalling model predicts the oxide growth and amount of oxide
spallation which occurs during cyclic oxidation. The isothermal oxide growth rate and
spall fraction Qo (the ratio of the oxide which spalls on cooling to the total
oxide present before cooling) are input to the spalling model. The spalling model
predicts both the weight change of a coated specimen undergoing cyclic oxidation and
the rate and total weight of metal consumption during cyclic oxidation. A diffusion
model simulates the diffusional transport associated with both oxidation of the coat-
ing and coating-substrate intecdiffusion. Diffusion coefficients and the rate of
metal consumption predicted by the spalling model are inputs to the diffusion model.
The diffusion model predicts aluminum and chromium concentration profiles in the
coating and substrate and the time for which the coating is able to supply sufficient
aluminum to continue formim_ an AI203 scale. The diffusion model, therefore,
predicts coating failure when there is insufficient transport of aluminum to the oxide
scale.
CURRENT STATUS AND RESULTS
Experimental
Isothermal furnace oxidation testin_ for I00 hr at 1050, II00, and 1150 °C is
complete. Cyclic furnace testing _f t_e coated U-700 was carried out to failure of
the coating at the three test temperatures (1050, ii00, and 1150 °C). Each of the
coatings on _he U-700 failed prematurely due to massive spallation of the coating
(fig. 2). This premature coating failure was due to excessive porosity formation at
the coating*-subs%rate interface. The coating_ significantly detached from the sub-
strate, spalled first from the cylinder ends, permitting extensive oxidation at the
coating-substrate interface wi_ the interconnected porosity.
Numerical Modeling
lhe s_ali_ng model wa_ _sed t_ predict the rate of aluminum consumption at 1050,
I!00, an4 I_50 oC. The spn]l i_act_on Qo_ input to the spalling model, was chosen
so as fo _eproduce the spe_3_en weight cha_Lge during cyclic oxidation before the onset
of massive co_t[n_ spallation (fi_. _). The weight of aluminum consumed, as predicted
by the _a[i_ng mod_l_ for esch test temperature is shown in figure 5. (Parameters
input t_ the _psll_ng an4 4iff_,_Jo_ modsls, except as indicated otherwise in a figure,
are stown _* tabl_ I.)
Yhe _i[fusion model predi_t_d the aluminum and chromium concentration profiles
after _ycli,_ oxidstion at _050, i_I00_ snd II50 °C. Good agreement was found between
the me_uve6 and predicted p_ofile_ (fi_s_ 6 and 7) Following this verification of
its _-sd[ci_e ability, t_e _i_,_3o_ _odel wa_ then used to examine the effect of
various parameters (e.g., coating thickness, spall fraction Qo, and substrate
composition) on coating life. The model was also used to predict the life of the
NiCoCrAIY coating on U-700 for the case where massive coating spallation had not
caused premature failure (fig. 8).
FUTURE WORK
Current burner-rig testing should provide oxidized specimens which, although
containing porosity at the coating-substrate interface, eliminate oxide formation
within the porosity and the resultant premature coating failure. An accurate test
of the life predictive ability of the diffusion model could then be conducted. A
second diffusion model is being developed to simulate T' and 8 depletion dur-
ing degradation of aluminide coatings. This aluminide diffusion model should be
capable of predicting coating life. Measured concentration profiles after cyclic
oxidation of the aluminide coated U-700 will be compared with those predicted by the
diffusion model to determine the accuracy and usefulness of the aluminide diffusion
model. Predicted and measured coating lives will also be compared. The conclusion
of this work should result in an improved methodology for predicting the oxidation
life of both overlay and aluminide coatings.
DUAL CYCLE ATTACK
An experimental study to investigate the effect of aging in a corrosive environ-
ment (900 °C, 0.5 ppm Na) on the oxidation life of the two coatings discussed in this
paper has begun. An attempt will be made to develop an empirical model to relate
coating life to combined oxidation and hot-corrosion cyclic exposure.
399
INPUT PARAMETERSFORMODELS
SPALLING MODEL DIFFUSION MODEL
TEMPER- WEIGHT SPALLING COATING COATING SUBSTRATE
ATURE, CHANGE, FRACTION, COMPOSITION THICKNESS, COMPOSITION
oC Aw 0o pm
lO._
11O0
1150
O.263l 0" 234
•212tO.323
.2f)Sto. 337
0.0004
0 O00?5 - 0.0015
0.011
Ni-16.TCr-24.0A1 120 Ni-15.6Cr-8.4AI
OXIDATION-LIMiTEDCOATINGLIFE PREDICTION
9BJECTIVE: lO DEVELOP Ai,; ¢,_PROVED METHODOLOGY FOR PREDICTING THE OXIDATION LIFE
OF METALLIC COATINGS
I APPROACHI
i EXPERINENTAL I
i I
[ ALUMINIDE]U-IO0 IPPSNiC°CrAIY!U-7O0 I BULKNiC°CrAIYI
i
" i I CYCLIC OXIDATIONISOTHERMAL OXIDATION
' ]050, ]100. 1}50_'-" i '750,]100, I150°C
i i00 hi ] ] l-hr CYCLES RUN
i i I UNTIL FAILURE
' E
MACH 0.3 BURNER
RIG OXIDATION
!-hr CYCLES RUN
UNTIL FAILURE;
BLOCK TIME EFFECTS
' i; AI'!ALYTIC MODELING
I
I
| PREDICT !_IrlGHT ',.,lANCE 1
I FOR CYCLIC ;XIDA,iON i
t PRI!P, iCT RAH! 0!: ',_ETAL lCONSUMPTION
I DIFFUSION MODEL
PREDICT CONCENTRATION /
DISTANCE PROFILES IN
i COATING AND SUBSTRATEPREDiCI COATING LIFE
CS-85-3377
Figure !
400
OP, IGINAL PAGE Ig
,'3F POOR QUALITY
MACROSCOPIC COATING FAILURE 
lMHl 1-hr CYCLES AT 1100 *C 
0.635 cm 
0.25 in 
100 1-hr CYCLES AT 1150 OC 250 1-hr CYCLES AT 1150 O C  
Figure 2 
MICROSCOPIC COATI NG FA1 LURE 
1150 o c  
. . C-... . ., .". . ' d '  - .  rr 
6 8  
AS-SPRAYED PLUS 4-nr AKbUN ANNtAL THIRTY 1-hr CYCLES 
AT 1080 O C  
100 pm 
250 1-hr CYCLES 
Figure 3 
401 
ALUMINUMCONSUMPTIONPREDICTEDBY SPALLINGMODEL
ALUMINUM
CONSUMED,4
(mglcm)/cm
oC
-- 1150
_11 I I I I°5° I
i00 200 300 400 500
TIME,1-hrCYCLES
Figure 4
WEIGHT
CHANGE,
mglcm 2
WEIGHTCHANGEDURINGCYCLICOXIDATION
LPPS COATINGON U-700; 1100°C
2
1
' '-%._0015
t 0 EXPERIMENT _,-I -- SPALLINGMODEL "0,,XO_ '
-2 I I I I I "¢P
0 I00 200 300 400 500 60(}
TIME,1-hrCYCLES
Figure 5
402
CONCEN-
TRATION,
at%
25
2O
MEASUREDAND PREDICTEDCONCENTRATIONPROFILES
FOR LPPS NiCoCrAIYON U-700
1000 l/2-hr CYCLES LSO0hr AT 1100°C)
-- [] Cr I0 AI EXPERIMENT
-- DIFFUSIONMODEL
--- INITIAL AI CONCENTRATION
--- INITIAL Cr CONCENTRATION
_I]_D_]:L _ _2_]d2_----I_-? O O O O O OCr
I AI
    o:o;o o:p Ooooo° _ 
__ COATING
THICKNESS
[]
I I I I I
100 200 300 400 500
DISTANCE,pm
I
600
Figure 6
CONCEN-
TRATION,
at %
MEASUREDAND PREDICTEDCONCENTRATIONPROFILESFOR
LPPS NiCoCrAIYCOATINGON U-700
1000 1-hr CYCLESAT 1100°C
25--
0 Cr I EXPERIMENT1 0 AI I
I -- DIFFUSIONMODEL
I -- INITIALAI CONCENTRATION
20 - I INITIAL Cr CONCENTRATION
I
00[- l r-I
_ IJ]C]O O O O
L= -- COATING I Cr
,_THICKNESS -_
5i I I I
0 100 200 300 400
DISTANCE,
[]
F'l
0_"_ 0
50O
I
600
Figure 7
403
24B
ALUMINUMCONCENTRATIONATOXIDE/METALINTERFACE
2O
16
ALUMINUM
CONCENTRATION,12
at "/o
r
[] 11O0°C_E×PER_mT \ _ \
_ _ 1150°ci \ \ o \
- _ \
I J i tl z J _ i I l i i _" J
10 100 IOOO 10000
TIME, 1-hr CYCLES
Figure 8
4O4


Coating life prediction

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server