Comparison between micrometer and millimetre sized metal and ceramic
lamellas, for a better understanding of the spray process.
S. Goutier, M. Vardelle, and P. Fauchais
SPCTS Laboratory, University of Limoges, 123 avenue Albert Thomas, 87060 Limoges Cedex
Abstract: An experimental set-up has been developed, at the SPCTS Laboratory
of the University of Limoges, to produce fully melted, millimetre-sized, ceramic
or metal drops with impact velocities up to 10 m/s. Such impact velocities allow
reaching impact Weber numbers, close to those of the plasma spray process
(We=2300) with droplets in the micrometer sized range. A fast camera (4000
image/s) combined to a fast pyrometer (4000 Hz), allows following the drop
flattening. The flattening of droplets (at the micrometer scale), has been studied
with a direct current (DC) plasma torch spraying particles about 45 µm in
diameter. The corresponding experimental set-up comprised a very fast (50ns)
two-color pyrometer and two fast exposure (at least 1 µs) CCD cameras (one
orthogonal and the other tangential to the substrate). The flattening of millimetre
and micrometer sized particles are compared. First are studied impacts of
alumina and nickel-aluminium drops (millimetre sized) with impact velocities up
to 10 m/s. Then are considered the same particles but micrometer sized (about 45
µm in diameter) sprayed with the DC plasma torch. A correlation has been found
between both flattening scales. This work shows that when comparing
phenomena at the two different scales (three orders of magnitude difference both
in size and flattening time), the Weber number is by far more important than the
Reynolds one and it is mandatory to have Weber numbers as close as possible to
make pertinent comparisons.
Keywords: plasma spray, Weber, Reynolds, particle flattening, surface
treatments, flattening velocity
1. Introduction
The flattening of plasma sprayed droplets (few tens
of micrometer in diameter) is very difficult to follow
experimentally according to the very short flattening
time (a few µs)1-4. That is why many works have
been devoted to the flattening of millimeter-sized
drops where characteristic flattening times are in the
millisecond range 5-10. Indeed, the flattening studies
for millimeter sized and micrometer sized melted
particles, called successively in this study drop and
droplet, showed similarities for the liquid material
flow onto the substrate and the different types of
liquid droplet ejections11.
To predict the maximum flattening diameter, most
studies were focused on the energy balance at
impact, and/or series of experiments with different
impacting materials, melted particle diameters and
impact velocities. These researches allowed
deducing evolutions of the maximum flattening
diameter linked to the dimensionless numbers
(mainly the Reynolds, Reo, and the Weber, Weo,
numbers at impact). These equations, established
according to the particle characteristics at impact,
however do not take into account substrate
properties and its surface chemical and physical
characteristics.
In the frame of this study, the parameters of the
incident particles were kept constant as much as it
possible. Only the substrate surface parameters were
modified and to take them into account the only
expressions available are those related to the thermal
contact resistance (Dhiman12), or the contact angle
(Pasandihed13), or the flattening velocity (Fukumoto
14
). However experiments do not allow measuring
the time evolution of the thermal contact resistance
or the contact angle of the liquid with the substrate.
The only measurable parameter is the flattening
velocity.
In a first part of this work, the evolution the
flattening velocity is presented at both scales
according to the impacting particles and the
substrate temperature. Then in a second part, in
order to take into account the viscosity and the
surface tension strength of the particle, the Reynolds
and Weber numbers are presented according to the
particle flattening conditions.
2. Experimental setup
Experimentations are carried out with two
techniques for studying impact and flattening
phenomena. The first one is a modified free-falling
set-up to study millimeter-sized drops with high
Weber numbers (up to 2000) and the second one
uses a plasma spraying set-up with micrometer-sized
drops.
Running in parallel these two studies enable to
compare particle flattening and cooling on smooth
stainless steel (304L) or titanium alloy (Ti-6242)
substrates, preheated or not, at spatial and time
scales differing by almost three orders of magnitude.
2.1. Millimeter sized particles study
To produce liquid ceramic or metal drops, a rod is
introduced and melted in an electrical arc furnace. A
suspended drop is formed. When the gravity force
overcomes that of the surface tension, the drop falls.
This set-up is disposed into an argon controlled
atmosphere chamber.
The substrate, fixed onto a pneumatic jack, can be
moved up during the drop fall and the relative
impact velocity can reach up to 10 m/s15.
A detector located at the chamber output, generates a
TTL pulse when one single drop crosses its
measuring volume. This pulse is then directed to the
measuring system composed of a fast camera
(Photron 4000 image/s) targeting the substrate.
2.2. Micrometer sized particles study
Particles are plasma sprayed using a direct current
(D.C.) plasma torch (PTF4 type) with a 6 mm
internal diameter nozzle and running with a mixture
of argon-hydrogen. The arc current is 650A, the
argon flow rate 33 L/min and the hydrogen volume
percentage 25%. An alumina powder and nickel
aluminium powder with particle sizes between 40
and 50 µm are used in this study. These spray
conditions result in fully melted particles with
velocities at impact around 200 m/s. Lamellas are
collected on a smooth (Ra=0.06 µm) 304L substrate
at a distance of 110 mm downstream of the nozzle
exit.
The measuring system comprises:
- In-flight measurement of droplet velocity by a two
points measuring optical detector and a fast
(response time = 50ns) dichromatic pyrometer to
follow the particle temperature during its flattening
and cooling. The two wavelengths of the
bichromatic pyrometer are 690 µm and 710 µm
respectively.
-Imaging techniques which allow following the
flattening of the particle. On each camera are fixed a
macro lens with a focal distance of 200 mm and two
focal doublers. One camera is disposed orthogonal to
the substrate and the other one tangential to it.
With different exposures times and a variable time
delay compared to the impact time, it is possible to
observe the flattening at different times but for
different particles (assuming they have the same
parameters at impact)11.
2.3. Substrates
Substrates are made of stainless steel (304L) and
titanium alloy (Ti-6242), are mirror polished by
using SiC paper 4000 and disposed at 110 mm from
the nozzle exit. They are fixed on copper supports
heated up to 200 °C with a heating rate of 0.5 °C/s
by two small resistances (each one with a power of
150 W). A monochromatic pyrometer (Ircon 5 µm,
10 ms response time) controls the substrate
temperature during the preheating stage.
2.4. Flattening velocity
The measurement of the flattening velocity is
complex. Indeed for droplets, it is not possible to
follow the particle flattening in real time. It is thus
necessary to make some hypotheses to determine the
flattening velocity. The only solution, to determine
the flattening time, is to measure the time necessary
to reach the maximum flattening diameter. So it
must be to consider that during flattening the cooling
is negligible. To confirm this assumption, it can be
noticed in images presented in the figure 1 that the
maximum of the pyrometer signal is reached at the
maximum flattening.
the maximum flattening diameter, to calculate this
mean velocity.
To compare drop and droplet flattening, it is
necessary to have a dimensionless approach
considering important diameter and velocity
variations. Thus, results are normalized by using the
factors “ξmax” and “a”, defined by the following
expressions:
- Factor of maximum flattening:
ξmax= Dmax/Do
where: Dmax: diameter of maximum flattening.
Do: diameter of the incident particle.
- Normalized flattening velocity:
a=ve/vo
Figure 1: Pictures of droplet flattening, on a cold stainless steel
substrate for alumina and nickel aluminium droplets. The
shaded area on the pyrometer signal represents the opening
time of the camera shutter.
where: ve: mean flattening velocity
vo: particle impact velocity
3. Results
Immediately afterwards the ejections and the film
rupture occurs11. By measuring the maximum
flattening diameter (Dmax), thanks to the image of the
camera 1 and the time corresponding to the
maximum of the pyrometer signal at a given
wavelength, it is possible to calculate a mean
flattening velocity.
3.1. Case of Al2O3 on 304L
In figure 3 are presented the maximum flattening
factor (ξmax=Dmax/Do) and the normalized flattening
velocity velocity (a=ve/vo) for substrates heated to
various temperatures respectively for alumina drops
(millimeter sized) and droplets (micrometer sized)
impacting on a stainless steel substrates.
For drops, as shown in figure 2, two flattening
velocities can be observed.
For droplets, when the factor “a” increases, the
droplet flattens much more. In the drop case, the
experimental points are rather dispersed (see figure
3) when the impact velocity is modified (from 3 to
10 m/s). The ratio is similar but when the kinetic
energy at the impact is larger, the particle flattens
more.
Figure 2: Evolution of flattening diameter for an alumina drop
impacting on a stainless steel at room temperature
In order to simplify the comparisons, only the mean
flattening velocity is considered. By using images of
the fast camera, it is possible to deduce the time of
Normalized flattening velocity velocity, noted “a”,
seems to depend on the substrate temperature. That
is confirmed in figure 3 (b) for drops and droplets.
For a surface at the room temperature, factors “a”
are close for the two scales. But when the substrate
surface is heated, the factor “a” falls down much
faster for droplets than for drops. That means that on
a hot surface, the droplet flattening is slowed down
faster than in the case of a drop. The particles
cooling are not identical at the two scales. The
kinetic energy variation of the flattening droplet is
more important according to the viscosity energy,
which takes part in the flow deceleration. This
phenomenon is less important for drops, where a
bigger quantity of matter is present. Thus at
micrometer scale, flattening is controlled by the
boundary layer between the flattening particle and
the substrate, whereas this boundary layer is
negligible for drop flattening.
For drops, the flattening velocity rises according to
the increase of the maximum flattening factor and
decreases with the increase in the substrate
temperature. However, in the case of a droplet
impacting on a titanium alloy substrate, no real
evolution of the factor “a” is detected, both with the
maximum flattening factor and the substrate
temperature. It can be due to the weak variation of
the diameter when the substrate is heated.
(a)
(a)
(b)
(b)
Figure 3: Evolutions of normalized flattening velocity, noted
“a”, (a=ve/vo), for an alumina melted particle on a stainless
steel substrate, in function of (a) the maximum flattening factor
“ξmax” (ξmax=Dmax/Do), (b) the substrate temperature
“Tsubstrat”
Figure 4: Evolutions of normalized flattening velocity, noted
“a”, (a=ve/v0), for an alumina melted particle on a titanium (Ti6242) substrate, in function of (a) the maximum flattening factor
“ξmax” (ξmax=Dmax/Do), (b) the substrate temperature
“Tsubstrat”
3.2. Case of NiAl on Ti-6242
In figure 4 are presented the maximum flattening
factor (ξmax=Dmax/Do) and the normalized flattening
velocity velocity (a=ve/vo) for substrates heated to
various temperatures respectively for nickel
aluminum drops (millimeter sized) and droplets
(micrometer sized) impacting on titanium alloy (Ti6242) substrates.
An important point must also be underlined: in all
impacts of NiAl droplets on titanium alloy
substrates, it was not possible to obtain perfect disk
shaped lamellas. All lamellas presented either a film
or a peripheral phenomenon of ejection, indicating
the presence of kinetic energy excess, which this
translated here by the important (around 0.8) value
of the factor “a”. It is probable that if it had been
possible to heat the substrate at higher temperatures,
the factor “a” would have more decreased.
3.3. Conclusions of experiments
The comparison of the flattening velocity evolutions
with the maximum flattening diameter and substrate
temperature highlights, the differences, between a
metal particle and a ceramic one.
For a ceramic droplet, the range of normalized
flattening velocities is important (from 0.2 to 1.1)
whereas for a NiAl droplet, it is weak (from 0.8 to
1.1). The important variation observed in the case of
ceramic droplets can be related to the evolution of
the liquid viscosity according to the cooling
velocity. Indeed, for alumina drops, cooling on cold
and hot substrates are overall close together and the
variation of normalized flattening velocities “a” is
weak (from 1 to 1.3), whereas for droplet, cooling
velocities are very different and the variation of “a”
is important (0.2 to 1.1). As expected, the faster is
the cooling, the more the liquid flow on the substrate
is slowed down.
For drops, on cold substrates, alumina and NiAl
have the same flattening velocity, but it is no more
the case on hot substrates. The NiAl lamellas are
adherent on the substrate, whereas it is not the case
of Al2O3 ones: the latters have not a sufficient
contact with the substrate, to drastically influence
the flattening liquid flow, in spite of desorption of
adsorbates and condensates at hot substrate surfaces.
Indeed, in spite of the substrate temperature beyond
the transition temperature and circular lamellas,
ejections are always present in their periphery for
alumina (see Table 1,15).
necessary to carry out a dimensionless approach of
our results.
4. Discussion
4.1.
Variation
of
flattening
velocity
according to the surface tension, and the
viscosity of the particle.
In order to find a link between drops and droplets,
the flattening diameter evolution is presented
according to the physical parameters of the particle.
Thus, Reynolds and Weber numbers are calculated
according to flattening parameters and not with the
impacting parameters. For these dimensionless
numbers, it is necessary to know characteristic
dimensions of the flow.
(a)
(b)
Table 1: Alumina drop flattening on polished stainless steel for
an impact velocity of 10 m/s and an subtrate temperature of
200°C.
t=0ms
t=3.2ms
t=6.4ms
In conclusion, for drops and droplets, to avoid the
peripheral ejection phenomenon, it is necessary to
achieve normalized flattening velocities lower than
0.8. But it exist some differences between the
alumina and nickel aluminum case. It is now
Figure 5: Evolutions of the flattening particles (metal and
ceramic) at two scales, with (a) the Reynolds (Re=ρveRmax/ µ),
(b) the Weber (We=ρve2Rmax/σ), the velocity taken into account
being that of the liquid on the substrate and characteristic
dimension being equal to the radius of maximum flattening.
The mean flattening velocity is considered, and for
the characteristic dimension it is the maximum
flattening radius, in order to obtain curves presented
in figure 5.
No link exists between Re and the maximum
flattening when the material (metal or ceramic) and
diameters (40 µm => 5 mm) of the impacting
particle are modified. But, the evolution of the
lamella diameter seems to be strongly related to the
evolution of We. Indeed, the flattening curves
corresponding to various conditions of the substrate
(cold, hot) and of particles (metal, ceramic) are close
to each other. This means that the maximum
flattening is controlled by the competition between
the surface tension and inertia energies. That also
means that the effects of the surface tension are
preserved during the modification of the particle
scale, due to the weak variation of the surface
tension when the liquid is cooling. It is not the case
of the viscous effects which are strongly related to
liquid cooling.
4.2. Comparison with literature
As introduced previously, the only relationship
previously developed is the factor of Fukumoto
noted Kf 14. This factor corresponds to the following
expression:
In the case of NiAl droplets impacting on a titanium
alloy substrates, the transition temperature could not
be reached (due to the system limits) and the value
of Kf is higher than 7. However, the shape of the
lamellas is close to that of perfect disks (see figure 6,
case at 350°C,). For drops, however it was possible
to obtain lamellas with very few ejections
(Tsubstrate=300°C), and Kf parameter lower than 7.
(a)
(b)
K f = 0.5 ∗ a 1.25 ∗ Re 0−0.3 ∗ K
where K is the Sommerfeld parameter defined by
K= Weo0.5Reo0.25
In this case, the dimensionless numbers are
calculated by using the parameters of the impacting
particle. Fukumoto uses drops to find the critical
value of Kf. This critical value, noted K cf , is defined
as the value below which only circular lamellas are
obtained. He found that K cf =7, in the case of metal
drops on various materials. However, Fukumoto
could not confirm this value for droplets, being
unable to measure the flattening velocity at this
scale. However here, it is possible to calculate Kf
parameter at both scales and figure 6 are obtained.
For droplets, the alumina lamellas presenting disk
shapes and very few ejections (Tsubstrate=200°C)
obtained on a stainless steel substrate have a Kf
value lower than 7. For alumina drops, in spite of a
substrate temperature over Tt and disk shaped
lamellas, the values of Kf are higher than 7, and
many ejections are present.
Figure 6: Evolutions of the factor Kf according to the substrate
temperature for two types of particles (a) ceramic (Al2O3) (b)
metal (NiAl). According to 14, below Kfc=7, only circular
lamellas are obtained.
In conclusion, it seems that, when Kf is lower than 7,
some disk shaped lamellas are obtained. But below a
value of 7, lamellas present a jagged form or
ejections. This is in good accordance with results
found by Fukumoto.
5. Conclusions
The comparison of the two flattening scales, to
understand the impact phenomena, shows many
similarities (in the flattening process and the ejection
phenomena), but also some important differences.
Indeed, the thermal thickness of boundary layer of
the flattening melted particles plays a more
important role with droplets than with drop. Such
finding involves modifications in the flattening
process, which can be classified according to the
substrate temperature:
- for a cold substrate, the desorption of the
adsorbates and condensates generates a gas cloud at
the interface flattening particle-substrate. This gas
cloud has not the effect at both scales. For
micrometer-sized droplets, the flattening particle
cannot solidify because it is lifted over the substrate.
Because of low thickness of the liquid film, gases
generated at the interface can cross it, destabilize it
and cause de-wetting. It is not the case for drops
where the cloud effect is negligible due to the
important thickness of liquid flow, which is
solidified without de-wetting. This difference is
observed for the two types of materials.
5.
6.
7.
8.
9.
10.
11.
12.
- for a hot substrate, the phenomenon of
solidification at the interface allows dissipating the
flattening energy and avoid ejections. For drops, due
to the more important flowing liquid thickness, the
upper part of the flattening drop can be always liquid
whereas the bottom part in contact with the substrate
is solid. This phenomenon induces the presence of
ejections at the time of the maximum flattening and
an important recoil phenomenon in the direction of
the flattening drop center, while for droplets,
because of thickness of the flattening droplet and the
cooling velocity, most of the lamella is probably
solidified at the time of maximum flattening.
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