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Preheated particles make superior plasma coatings The application of sprayed powders has been limited to the use of relatively low
quality of coatings due to an incomplete in-flight melting of the coating
powders. A new process using preheating of powders dramatically improves the
quality of spraying coatings in general but especially that of thick coatings
deposited from particles with diameters greater than 50 µm. This process also
achieves a much greater quality of coatings made from metal powders. The
quality improvement is especially remarkable in achieving a coating porosity
that is lower by a factor of 2.5 to 8. Plasma
or conventional flame torches are used to deposit molten ceramics or metals
on substrate surfaces. This process is called thermal projection. The
materials to be projected onto the substrate have in most cases a very
limited grain size and are fed at a controlled rate into the plasma or flame
jets from a powder dispenser. The powder is transported by a gas carrier,
typically at 5 to 10 liters per minute, in a flexible hose to an injector on
the projection torch. The
powder is injected into the jet, which has a very high temperature (up to 14
kK for electric plasma jets) and high speed. The powder is heated up to its
partial or total fusion temperature and accelerated in the jet. Upon impact
on the substrate the particles collapse from their spherical shape and
solidify into layer to form the coating. The quality of the coatings depends
in part on good contact between the grains in the coating. This contact is
determined by the state of particles impacting on the substrate. Good contact
between particles is reached when all particles are fully melted before the
impact. The
state of a particle at impact depends on its initial injection temperature
and its size. This initial temperature is ambient in all currently used
processes. The size of the injected particles is also limited to give them
opportunity to melt in the available flight-time and heat of the jet. The state of the particles
also depends on their trajectories in the jet. The maximum temperature is
found at the jet's central axis and is considerably lower in the radial
direction of the fringes of the jet. A small particle's trajectory brings it
to the outer part of the jet center… A too large particle, anywhere in the
jet, is insufficiently heated and impacts on the substrate to coat while
still being solid or only partially melted… To avoid
this dilemma we propose a small electric
device to preheat powders to high temperatures just before their injection
into the jet. Thus, the heating
of the particles to beyond their melting temperature becomes much less
dependent on their trajectory and heating time in the jet, and their melting
is now not influenced by the temperature gradients in the jet. Due to this
preheating the fraction of molten particles impacting on the substrate or
partially formed coating can be increased to virtually 100%. This drastically improves coating properties. In addition, the size
of the particles to be projected by the jet can be increased;
hence the intensification of the thermal projection (spraying, coating)
process can be achieved. Two different types of
reactors can preheat large particle powders before they are injected into the
thermal spray equipment: a GlidArc-I device heats
the particles up to 500°C or a tubular Joule-based device can heat them up to
900°C (see below). Both reactors can easily be connected to conventional
powder-feeding thermal spray equipment, see French patent No. 2773500.
Example 1 Calculated power to
preheat a flow of Al2O3 powder at 50 g/min, carried in
nitrogen at 5 L(n)/min, from 25 to 725°C is 0.74 kW. Our 1 kW Joule-based
device is capable to provide that power. The same device can also
continuously preheat other powders in other gas carriers such as air, O2,
Ar, H2, CO2 or CH4. Some fruitful tests of
such a controlled preheating of different powders of 22 µm to 1 mm diameter
have been achieved with an alumino-silicate (0.16 mm), nickel (0.21 mm), Cr2O3
("Amperite 740.1", 22 - 45 µm), SiO2 (0.2 - 1 mm) and
SiC (0.16 - 0.20 mm). The preheat temperature can reach 880°C, which is 1/2
or 1/3 of the final melting point temperatures of these powders. Example 2 The same preheater as in
Example 1 was installed between a powder dispenser and a METCO plasma torch.
The use of a CASTOLIN-EUTECTIC powder feeder achieved the transport of the
same powder Cr2O3 (see Example 1) to the preheater. The
exit of the preheater was closely connected to the usual torch injector in a
particular way that permits good electric current passage. The heat leaks
between the preheater and the cold body of the torch are however reduced.
Indeed, one of difficulties of our invention's realization rested in a risk
of preheated gas/powder mixture cooling if exposed to the cold torch
injectors. We also improved the electric safety of the system by putting the
preheater/torch attachment point to the ground potential, the same as the
plasma torch body. The projection has been done with the following
parameters:
A layer
of Cr2O3 was projected onto a steel roller of 30 mm
diameter on 4 tracks of about 10 cm length each. The roller surface was
sanded to remove all scale and to make it free of grease. During the
projection the roller rotated at 300 rpm whereas the speed of the torch
translation in relation to the roller was 0.36 m/min. The temperature of the
preheated powder/gas mixture 600°C was measured at the exit of the preheater
with a thermocouple. Data pertaining to these tests and some resulting coating
properties parameters are shown below:
* * * * * Results of tests show
the veritable advantages of the preheater. It permits the necessary
temperature elevation for the fusion of particles which gives:
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___________________________________________________________________________________ Contact us: echph@wanadoo.fr |
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