Investigation of the silicon loss in APS MoSi{sub 2} under the range of typical spray conditions Page: 4 of 15
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coatings thus creating the potential for premature coating failure. The porous structure
also results in inferior mechanical properties for the coating. Another potential failing of
APS MoSi2 coatings is the loss of Si from the particles in flight or on the hot substrate
after being deposited. As indicated above, the lesser silicides such as Mo5Si3 are
undesirable constituents of the coating since they oxidize quickly at high temperatures
without forming SiO2. It is the loss of Si during spraying and the Mo5Si3 content in the
resultant coatings which is addressed in this paper.
In order to best understand the conditions responsible for Si loss during spraying
the temperature, velocity, trajectory, and size distribution of the particles in flight must
be measured. Such particle data under conditions which span the range of typical spray
parameters is beneficial for correlating to the Si lost during spray processing.
Additionally, an understanding of the effect of spray parameters on particle
characteristics for this particular material system will be beneficial for optimizing spray
conditions in both research and production environments.
Data Collection
The first stage in the investigation was to assess the effect of argon flow, helium
flow, and arc current on the temperature and velocity of the particles. Once this
parameter space mapping was completed, spray conditions were chosen to span the range
of particle temperatures and velocities. Coatings were produced under the extreme high
and low temperature and velocity conditions and also a medium condition. The sprayed
samples were used to evaluate the Si loss as a function of spray parameters.
The MoSi2 powder used in the study was supplied by Cerac Inc., and was given
the size classification of -200/+325 mesh (-75 m/+45 m) by the manufacturer. A
particle size analysis which was performed on a Coulter LS-100 particle sizer revealed
that the actual mean particle size was 68 m with 10 vol% greater than 100km and 10
vol% finer than 40 m. The fine particles extend all the way down to single micron sizes.
The broad particle size distribution presented some problems during spraying due to
clogging of the powder port on the torch by the small particles. Powder port enlargement
and periodic cleaning of the opening minimized any effect of port clogging on the
experimental results.
The powder was sprayed using a Miller Thermal SG-100 plasma spray torch in a
spray cell at Sandia National Laboratories. The torch used a model 730 anode, 720
cathode, and 112 gas injector. Particle velocity was measured using a laser 2-focus
velocimeter which detects the reflected laser pulses from a particle as it passes through
two parallel laser beams separated by a known distance. The time interval between the
first and second received reflected laser pulses along with the known distance between the
beams allows for the calculation of the particle velocity. See Ref. 9 for a more detailed
description of the velocimeter. The mean velocity for each position in the particle plume
was used as the velocity value reported. Particle temperatures were also measured in
flight. A 2-color, optical fiber linked pyrometer was used for measuring particle
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Hollis, K. J.; Castro, R. G.; Bartlett, A. & Neiser, R. Investigation of the silicon loss in APS MoSi{sub 2} under the range of typical spray conditions, report, September 1, 1996; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc682978/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.