FeAl and Mo-Si-B Intermetallic Coatings Prepared by Thermal Spraying Page: 4 of 8
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An effect of HVOF particle velocity on the microstructure and properties of FeAl coatings was observed. The effects were
essentially identical to those previously described for HVOF-sprayed Fe3Al coatings [4,5]-increasing particle velocity results in
decreased porosity and oxide inclusion levels, increased microhardness, more compressive residual stresses, and increased elastic
modulus. These effects all stem from the increased peening effect of higher-velocity particles. It is not currently known why
porosity and unmelted particle fractions increase in the FeAl coatings going from 660 m/s to 700 m/s, when these fractions
monotonically decrease in Fe3Al coatings. Residual stresses in the FeAl coatings range from just barely compressive (10 to 30
MPa) to highly compressive (180 to 200 MPa). The curvature results are supported by the agreement between the average
coating stress calculated from curvature and the XRD-measured stress near the coating surface.
The microstructures of HVOF FeAl coatings reported in  are similar to those reported here, although a small fraction of Fe3Al
was observed in the earlier study. The reported hardness values (290 and 390 DPH) are considerably lower than those observed
in the present study. Although no particle velocity or temperature data were presented in Ref. , it is likely that the lowered
hardness results from a lower spray particle velocity. The coating CTE values are similar to those of wrought FeAl . As
previously reported for HVOF Fe3Al coatings, the expansion behavior in the first heating cycle is significantly different than in
subsequent cycles. Distinct drops in mean CTE are observed at 320 and 7600C, and a net shrinkage of 0.5% is observed after
cooling to room temperature. For Fe3Al , similar drops in CTE (at 400 and 8200C) and net shrinkage were observed. These
observations are consistent with recovery and recrystallization of the as-sprayed coating microstructure. The elastic modulus
values for the FeAl coatings show good agreement with wrought FeAl of a similar composition, 150-160 GPa for the coatings
compared to 167 GPa for wrought material . The modulus of the coating prepared with a particle velocity of 540 m/s was
somewhat lower than those at higher velocites, which likely results from porosity present in the low-velocity coating.
The effect of spray particle velocity on APS Mo-Si-B coatings was more marked-microstructure and chemistry were both
strongly affected. Spherical droplets associated with poor interparticle bonding and local porosity were present in the low-
velocity coating, while the high-velocity coating exhibited a microstructure of dense, apparently well-bonded lamellar splats.
XRD analysis of the low-velocity coating surface indicates the conversion of the powder microstructural constituents to a-Mo and
amorphous SiO2; the initial constituents were substantially retained in the high-velocity coating, although a-Mo is also observed.
EDS analysis verifies that the coating sprayed at low velocity lost considerable Si during the spray process (56% of the original
amount), in comparison with the high-velocity coating, for which Si was less reduced (14%).
These microstructural effects are related to particle oxidation, the extent of which increases with decreased spray particle velocity.
This effect is two-fold: lower velocities result in both a longer particle time-of-flight and a higher mean particle temperature.
Hence both the oxidation time and temperature are affected. Higher particle temperatures at slower velocity result from an
increased residence time in which heat transfer from the plasma to the spray particles occurs. The magnitude of the effect is
significant-the mean particle temperature at 180 m/s was 26000C, versus 21300C for 350 m/s. In the Mo-Si-B alloy, Si
preferentially oxidizes, forming amorphous SiO2. At the spray temperatures the SiO2 is liquid and will ablate or evaporate from
the particles. Hence higher temperature and more time at temperature result in greater Si loss and a-Mo formation. The
spherical particles observed in the low-velocity coating are believed to be redeposited droplets formed by "splashing" of the spray
particles upon surface impact. In contrast, the viscosity of the cooler, higher-velocity particles will be greater, reducing the
tendency for splashing. Since the melting point of the T1 phase (21800C) is near the mean particle temperature, the higher-
velocity spray particles may in fact have been semi-solid rather than fully molten.
As with FeAl coatings, increasing spray particle velocity leads to more compressive residual stresses. In the case of plasma
spraying, however, the quench component of the residual stress is high due to the larger temperature difference between the spray
particle and the substrate [11,15]. The net coating residual stress therefore remains tensile, even with relatively high-velocity
particles impacting the surface. The difference in residual stress between the coatings prepared with high- and low-velocity
particles indicates that peening effects can play a significant role in plasma spraying.
As observed for FeAl and Fe3Al, the CTE for the first heating cycle of the Mo-Si-B coatings was lower than in subsequent runs,
and a small net shrinkage was observed after the first run. As expected, CTE data for the coatings lie between literature data for
Mo5Si3 and MoSi2. The Mo-Si-B coating elastic modulus appeared to decrease in each loading cycle, likely indicating
microcracking under load, which is expected given the poor interparticle bonding and the brittle nature of the constituent phases,
as seen in the vicinity of hardness indents. Similar dramatic reductions in effective modulus have been observed for plasma-
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Totemeier, T.C.; Wright, R.N. & Swank, W.D. FeAl and Mo-Si-B Intermetallic Coatings Prepared by Thermal Spraying, article, April 22, 2003; Idaho Falls, Idaho. (digital.library.unt.edu/ark:/67531/metadc781673/m1/4/: accessed April 25, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.