-
Autoren: S. Siegmann, N. Margadant, A. Zagorski and M.
Arana-AnteloPubliziert in: ITSC 2003 International Thermal Spray
Conference - Advancing the Science and Applying the Technology,
Orlando, FL, 2003,ISBN/ISSN: 0-87170-785-3Seite: 1591-1598
Comparison of Particle In-Flight Characteristics andCoating
Properties
St. Siegmann, N. MargadantSwiss Federal Laboratories for
Materials Testing and Research, EMPA Thun, Switzerland
A. Zagorski, M. Arana-AnteloAlstom Power Gas Turbines, Baden,
Switzerland
Abstract
In thermal spraying, the knowledge of process basic
inputparameters and their influence on the final coating
propertiesis crucial. The optimization and reliability
ofatmospherically sprayed (APS) coatings, especially highlyporous
thermal barrier coatings (TBC), is closely linked toprocess
understanding.
This investigation aims to elucidate the correlation betweenthe
basic plasma spraying parameters (like current, plasmagas,
stand-off distance, etc.) and the deduced physicaldimensions like
particle temperatures and velocities and thecoating mechanical
properties. Within this project, a largerange of parameter field
was chosen as long as stable plasmaworking conditions permitted.
The influence of preparationon the microstructure analysis and data
of mechanicalcoating measurements are shown.
Introduction
In 1980, one of the first “study of the influence of
particletemperature and velocity distribution within a plasma
jetcoating formation” was published by A. Vardelle et al. [1].Since
that time, it is widely accepted that particle velocities(v) and
temperatures (T) are the main factors (besidesubstrate topography
[2]) responsible for the splashing,wetting and anchoring of the
particles during impact. In thisearly time, simultaneous
measurement of both properties (v,T) was difficult and required
mainly separate measurementdevices for velocities (e.g. laser
Doppler velocimetry) andtemperatures (high speed pyrometers)
aligned to a singleprobe volume. Nowadays, many sophisticated
equipmentsare available providing single particle information as
well asstatistical information covering the application
fromscientific based research to on-line process control in
sprayshops.
Diagnostics of the particles in the plasma jet is still the
mostforward method relating the process based input parameters(like
plasma gases, current, etc.) to the coating propertieslike
porosity, hardness, toughness, etc.
For developing coatings with predictable microstructure
andtechnological behavior, the knowledge of the dimensions Tand v
and of their influences in coating structure is crucial.
Experimental
Yttria Stabilized Zirconia PowderFor the spraying study, a
commercially available 8 wt%Yttria stabilized ZrO2 powder (YSZ) was
used. The sizefraction was -136 +30 µm and the scanning
electronmicroscopy (SEM) images show the typical morphology(Figure
1).
Figure 1: SEM pictures showing the typical morphology ofthe YSZ
powder used for this investigation (magnificationleft: ~37.5 x,
right: ~375 x).
Plasma Spray EquipmentThe coating application was done using a
vacuum plasmaspray facility type Medicoat M60 with a modified F4
torch.The spraying tests were carried out at atmospheric
pressureconditions using different plasma gas compositions and
-
Autoren: S. Siegmann, N. Margadant, A. Zagorski and M.
Arana-AnteloPubliziert in: ITSC 2003 International Thermal Spray
Conference - Advancing the Science and Applying the Technology,
Orlando, FL, 2003,ISBN/ISSN: 0-87170-785-3Seite: 1591-1598
amounts (Ar, H2). The plasma current as well as the
nozzlediameter and stand-off distance between nozzle exit
andsamples were varied and the resulting particle speed
andtemperatures were monitored by a Tecnar DPV 2000particle
monitoring system. The parameter field covered awide range of
temperatures (1800 to 2900 0C) and particlespeed (50 to 300 m/s).
The variation of the input parametersfor this study were given by
the criteria of stable plasmaconditions and are shown in
Table 1.
Table 1: Plasma parameter variations.
Plasma Gas (Argon) 22 ... 40 l/minAdditional Gas (Hydrogen) 0
... 9 l/minPlasma Current: 600 ... 800 AElectrical Input Power
(EIP): 20 ... 40 kWStand-off Distance: 75 ... 200 mmNozzle
Diameter: 6 ... 8 mmRobot Speed: 200 ... 400 mm/sPowder Feed Rate:
10 ... 40 g/min
For most of the parameter settings, samples were sprayed
ondifferent substrate materials like stainless steel andHastelloy
and afterwards metallographically andmechanically inspected.
For the metallographic preparation different epoxies weretested
to see the influence of embedding media on thestatistical results
of apparent porosity measured by imageanalysis. Furthermore
mechanical properties were measuredby a three-point bending
test.
Three-Point Bending TestsFor investigating the mechanical
coating properties, threepoint bending tests using
Dynamic-Mechanical Analysis(DMA) were performed on a PerkinElmer
DMA 7e device(PerkinElmer Inc., Norwalk, Connecticut, USA).
Themaximum applicable force was 8,5 N and an increment of500 mN/min
was applied until the sample broke. Thegeometry of the free
standing coatings were approximately1.8 x 11 x D mm, where D was
the coating thicknessranging from approx. 0.3 to 0.6 mm. The
elastic moduluswas extracted from the slope of the stress-strain
curvemeasured at room temperature.It could be shown by Bürkle et
al. [3], that even for stronglyporous and oxide rich metal
coatings, there exists a goodagreement between the three-point
bending tests andstatistical nanoindentation results.
Results / Discussion
Influence of Spray Parameters on Particle Speed
andTemperatureFor different positions from the nozzle exit (75 ...
250 mm),particle velocities and temperatures were measured using
the
DPV 2000 and afterwards samples were sprayed onHastelloy and
stainless steel substrates using the sameconditions. Figure 2 shows
as an example the typicalparticle speed and temperature
distributions in the cross-section (horizontal- and vertical-axe)
of the plasma jet at100mm from the nozzle exit ensuring homogeneous
andsymmetrical particle spreading.
-10 -5 0 5 10-10
010
80
90
100
110
120
Spee
d (m
/s)
Horizontal Axe (mm)
Vertical Axe (mm)
110-120100-11090-10080-90
-10 -5 0 5 10-10
010
2800
2820
2840
2860
2880
2900
Tem
pera
ture
(oC
)
Horizontal Axe (mm)
Vertical Axe (mm)
2880-29002860-28802840-28602820-28402800-2820
Figure 2: Examples of velocity (upper) and temperature(lower)
distributions of YSZ-particles in cross section of theplasma jet
measured 100 mm from the nozzle exit(coordinate system (0/0) =
torch center line at 26 kW power.
The experiments were done for different plasma gascompositions
and amounts, different current levels andfinally nozzle diameters.
Figure 3 shows the distinctregimes of the cumulative results of the
following variationswith stable plasma conditions (current: 600
...800 A, plasmagases Ar: 24 ...37 l/min, H2: 3 ...7 l/min and
stand-offdistances: 75 ...200 mm) for the two different
nozzlediameters, namely 6 and 8 mm.
-
Autoren: S. Siegmann, N. Margadant, A. Zagorski and M.
Arana-AnteloPubliziert in: ITSC 2003 International Thermal Spray
Conference - Advancing the Science and Applying the Technology,
Orlando, FL, 2003,ISBN/ISSN: 0-87170-785-3Seite: 1591-1598
2100
2200
2300
2400
2500
2600
2700
2800
2900
3000
0 50 100 150 200 250 300 350
Speed (m/s)
Tem
pera
ture
(oC
)
8mm Nozzle
6mm Nozzle
Figure 3: Influence of nozzle diameter on the distribution
ofparticle speed and temperatures for all parameter
variationstested.
There are distinct v-T-regimes for the two nozzle types. Themain
influence caused by the different nozzle diameters canbe seen for
the particle speed, whereas the meantemperatures of the particles
do not differ much. The slopebetween temperature and speed is
steeper for the 8 mmnozzle compared to the 6 mm nozzle.
Influence of Robot Speed on Coating MicrostructureFor one plasma
parameter setting (26.5 kW, 100 mm stand-off), samples were sprayed
using two different robot speeds,namely 200 mm/s and 400 mm/s. This
test was done forstudying the influence of thermal load on the
constitutingcoating as well as on the substrate [4]. In both cases,
theparticle temperatures were in the range of (mean /maximum) 2800
/ 2900 oC and the speed was in the range of86 / 105 m/s.
Figure 4: Influence of robot speed on the microstructure ofthe
constituting coating (left side sprayed with robot speedof 200 mm/s
resulting in a coating thickness of about 800µm; right side sprayed
with robot speed of 400 mm/sleading to a coating thickness of about
400 µm;Magnification: ~70 x).
To see the additional influence of substrate and/or
coatingheating during the deposition process with lower robot
speed,the number of passes have been kept constant. The
resulting
coating thickness increased by approximately a factor of 2,as
expected due to longer exposition time. However, theporosity level
looked similar for both regimes, 24 % and 29% for 200 mm/s and 400
mm/s, respectively (Figure 4).However, there is a certain
difference in the distribution ofmicro-cracks.These mainly
horizontal cracks at the higher robot speed canbe caused due to not
sufficient substrate temperature whichwould be need for good splat
cohesion. It was shown earlier[25] that the coating integrity
essentially depends on thespeed of spraying gun.
Influence of Embedding Media on Apparent PorosityThe sprayed
samples were metallographically prepared forstructure and porosity
measurements. To see in advance theinfluence of preparation on the
apparent porosity level, eightdifferent epoxy media were tested
(non-vacuum infiltrated)with specimen taken from a single sample.
Table 2 showstypical properties of the different embedding media
used forthis study like curing temperatures and times. The
hardnesswas tested using the ball indentation method described in
thestandard EN ISO 2039-1 [13]. The ball diameter was 5 �0.05 mm
and the corresponding load used was 358 N.
Table 2: Properties of eight different Epoxies: Typicalcuring
temperatures and times, as well as mean hardnessmeasured by ball
indentation method (EN ISO 2039-1).
Curing Temp. (°C)
Curing Time (h)
Mean Hardness (N/mm2) SDEV
Araldit 20 < 4 110.0 1.4Demotec 10 23 < 4 160.6
3.2Epo-Color 79 1-2 118.4 0.9Epomet 150 < 1 391.6 3.8Scandiplex
21 < 4 160.4 4.6Scandiquick 22 < 4 149.2 2.0Specifix 20 20
> 4 126.0 3.2Specifix 40 40-60 < 4 107.8 0.8
The influence of the viscosity of the different epoxies andthe
degree of infiltration can be visually seen from thedifferent gray
levels in Figure 5. Three different stages ofthe preparation are
shown for each embedding media forpolishing steps ending with
diamond of 10 µm, 1 µm andthe final stage (suspension of amorphous
SiO2, pH ~10).The apparent coating qualities, as seen in Figure 5,
showtotally different amount of pores and
coating/substrateinterfaces depending on the different type of
epoxies.Obvious large break-outs as well as apparent bad bonding
ofthe coating to the substrates can easily lead to
wronginterpretation of the real structure.
-
Autoren: S. Siegmann, N. Margadant, A. Zagorski and M.
Arana-AnteloPubliziert in: ITSC 2003 International Thermal Spray
Conference - Advancing the Science and Applying the Technology,
Orlando, FL, 2003,ISBN/ISSN: 0-87170-785-3Seite: 1591-1598
10µm
1µm
OPU
10µm
1µm
OPU
Demotec10
ScandiquickEpomet Specifix 40
Epo-ColorAraldit
Scandiplex
Specifix 20
Figure 5: Influence of 8 different embedding media and steps of
preparation (10 µm, 1 µm, OPU = SiO2 suspension) on
theinterpretation of the obvious porosity of the same TBC sample
(Magnification: ~55x).
To get quantitative results, image analysis was performedafter
the last preparation step on 5 different locations foreach sample
(Table 3). The variation in apparent porosity(factor � 2) can be
attributed to the difference in embeddingmedia (shrinkage,
hardness, etc.), as known frommetallographic preparation procedures
[5-12] and others.
Table 3: Eight different embedding media sorted in order
ofincreasing mean apparent porosity from image analysis.
Mean Apparent Porosity (Area%) SDEV
Specifix 20 16.9 3.6Demotec 10 22.5 1.8Epo-Color 23.3 0.7Araldit
24.0 2.2Specifix 40 24.5 1.4Scandiquick 25.5 4.3Scandiplex 30.9
1.6Epomet 41.0 4.2
-
Autoren: S. Siegmann, N. Margadant, A. Zagorski and M.
Arana-AnteloPubliziert in: ITSC 2003 International Thermal Spray
Conference - Advancing the Science and Applying the Technology,
Orlando, FL, 2003,ISBN/ISSN: 0-87170-785-3Seite: 1591-1598
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
0 100 200 300 400 500
Mean Hardness (N/mm2)
App
aren
t Por
osity
(Are
a%)
Figure 6: Mean apparent porosity measured by imageanalysis of
the same TBC coating in function of the differenthardness measured
by ball indentation according to [13] ofeight varied embedding
media.
There is a tendency of increasing apparent porosity
withincreasing hardness of the embedding media as seen inFigure 6.
Unfortunately, specific data on shrinkage were notavailable for all
materials being tested. Especially for brittlecoatings like TBC the
shrinkage of the embedding mediamay play an important role and can
cause significantbreakouts which leads to overestimating the
coatingporosity. Thus, results of image analysis are expected to
bestrongly dependent on the preparation procedure as well asthe
software settings and should be used mostly forcomparison of
different coating microstructures. Stability ofthe metallographic
procedure is a critical requirement forstudying coatings of such
type.
Influence of Particle Velocity and Temperature on
Micro-structural Properties:The structural differences of the
coatings sprayed at vastlydifferent parameters were examined using
image analysisand mechanical testing. Figure 7 shows the
dependencies ofparticle temperature and speed in relation to the
totalporosity measured by image analysis. It could be seen thatfor
decreasing particle speed and temperature the porosityincreased and
the microstructures showed less cracks,compared to the dense
coatings sprayed at “hot” and “fast”conditions. In Figure 7, one
can also see that the pronouncedtrend of porosity increase with the
lowering of particle in-flight parameters vanishes when the mean
particletemperature drops below the melting point (ca. 2600 0C
forYSZ). The explanation could be that the temperature ofparticles,
which actually form the coating, stays nearlyunchanged, whereas the
deposit rate dramatically decreasesdue to bouncing-back effect.
245025002550260026502700
27502800285029002950
20 25 30 35 40 45 50
Total Porosity [%]
Parti
cle
Tem
pera
ture
s [°
C]
0
50
100
150
200
250
300
350
Parti
cle
Velo
citie
s [m
/s]
TemperatureVelocity
Figure 7: Comparison of differently sprayed TBC coatingsand
their total porosity (mean value measured at 5 differentplaces)
compared to corresponding particle temperatureand velocity.
The images reveal entirely different microstructures for
theparameter range depicted in Figure 7. Two candidates fromthe
“extreme” positions (i.e. “hot/fast” and “cold/slow”) areshown in
Figure 8 and Figure 9.
Figure 8: Coating microstructure sprayed with high
particlevelocity (Vmax = 307 m/s) and high temperature (Tmax =
2925oC) showing an overall porosity of about 26 % and
verticalcracks with a crack density of 2.7 cracks/mm in
length(magnification: ~150x).
The sample sprayed with high velocity (Vmax = 307 m/s) andhigh
temperature (Tmax = 2925 oC) shows less porosity(approximately 26
%), but a high level of vertical cracks with
-
Autoren: S. Siegmann, N. Margadant, A. Zagorski and M.
Arana-AnteloPubliziert in: ITSC 2003 International Thermal Spray
Conference - Advancing the Science and Applying the Technology,
Orlando, FL, 2003,ISBN/ISSN: 0-87170-785-3Seite: 1591-1598
a density of 2.7 cracks/mm in length (Figure 8) compared tothe
coating sprayed with low velocity (Vmax = 115 m/s) andlow
temperature ( Tmax = 2520 oC) showing approximately44 % porosity,
but no visible cracks (Figure 9).
Figure 9: Coating microstructure sprayed with low
particlevelocity (Vmax = 115 m/s) and low temperature ( Tmax = 2520
oC)showing higher porosity (about 44 %) than Figure 8, but nocracks
(magnification: ~150 x.).
The higher the particle speed and temperatures are, thehigher
the degree of melting and spreading will be, whichresulted in a
higher coating density. However, due to largerinternal stresses,
the coatings tended to crack, as alreadyknown from literature
[14-16]. In contrast, the lower thetemperature and the particle
speed are, the more porous thecoatings have been until no more
adhesion and blasting withcold particles took place.
The gun electric input power (EIP), argon and hydrogenflow rates
varied for each nozzle diameter andcorresponding changes in the
coating porosity wereanalyzed. As far as power is concerned,
results of theexperiments followed the well known trend of
generalincrease in particle speed and temperature with the powerand
decrease of the coating porosity [17-24]. However,variations of the
total gas flow rate and of relative content ofthe gas components
when keeping the input powerapproximately constant have not
revealed any clear trends inthe particle temperatures. Nor there
was any clearcorrelation between those parameters and the
coatingmicrostructure. Generally, increase in the flow rate of one
orboth gas components led to higher particles velocities whilethe
temperature could decrease due to the reduction ofparticle dwelling
time and specific plasma enthalpy.
Mechanical Analysis by Three-Point Bending TestsThe results from
the three-point bending tests showed for allcoatings the typical
brittle behavior and a total bendingstrain of about 0.3 %. The
pseudo-plastic behavior may beattributed to the crack propagation
through the porouscoatings. Figure 10 represents a typical
stress-strain curve ofa three-point bending measurement for a TCB
coatingsprayed with reference conditions and with higher
hydrogencontent.
0
5
10
15
20
25
30
35
40
45
50
0 0.05 0.1 0.15 0.2 0.25 0.3Strain [%]
Stre
ss [M
Pa] "Higher Hydrogen"
"Reference"
Figure 10: Stress-Strain curve of two samples sprayed
with“reference” and “higher hydrogen” parameters at 100 mmfrom the
nozzle exit (porosity approximately 20 %).
Out of the DMA data from Figure 10, the tangent E-Modulus was
determined from the stress-strain slope and isshown as function of
strain in Figure 11.
0
5000
10000
15000
20000
25000
30000
0 0.05 0.1 0.15 0.2 0.25Strain [%]
Elas
tic M
odul
us [M
Pa] "Higher Hydrogen"
"Reference"
Figure 11: Tangent E-Modulus of the TBC samples as afunction of
strain at “reference” and “higher hydrogen”parameters leading to
the same porosity (about 20 %), butapprox. 65 % higher
E-Modulus.
It could be seen that for coatings sprayed with
differenthydrogen content, the resulting porosity remained
unchanged
-
Autoren: S. Siegmann, N. Margadant, A. Zagorski and M.
Arana-AnteloPubliziert in: ITSC 2003 International Thermal Spray
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Orlando, FL, 2003,ISBN/ISSN: 0-87170-785-3Seite: 1591-1598
at about 20 %, whereas the mechanical testing showed
largedifferences with about 65 % higher E-Modulus.
A typical fracture surface of the “reference” sample afterthree
point bending test is shown in Figure 12. The zones ofbrittle
fracture as well as cracks and pores can be seen.
Figure 12: SEM image of fracture surface of a free standingTBC
coating sprayed with reference condition after DMA-testing
(Magnification ~1200 x).
Conclusions
Changes in plasma input parameters and conditions stronglyaffect
the final coating microstructure. From theexperiments, the
following consequences can be drawn:
� Strong influence of nozzle diameter on v,T-regimes;� Proper
stable metallographic preparation is needed for
correct microstructure interpretation (porosity, crackdensity,
etc.);
� For this specific TBC system, the influence of robotspeed on
final coating porosity is minor, whereas somedifference in the
micro-crack pattern could be expected,especially in the
near-substrate area;
� Variations of plasma gas flow rates could haveambiguous
influence on the coating properties and canbe hardly used for the
purpose of coating qualitycontrol;
� Same level of porosity can show different E-Modulus;� Three
point bending tests can reveal mechanical coating
data rather than porosity level can show frommicroscopy
data.
Acknowledgements
The metallography team of EMPA Thun is acknowledgedfor the
stimulating discussions and support in the samples
preparation. G. Bürkle from University of Ulm (Faculty
ofEngineering, Dep. of Materials) is acknowledged for hissupport in
the Dynamic-Mechanical Analysis (DMA).
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