FeAl and Mo-Si-B Intermetallic Coatings Prepared by Thermal Spraying Page: 2 of 8
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This paper reports on the preparation of FeAl and boron-modified Mo5Si3 coatings using HVOF and air plasma spray (APS)
processes, respectively. Coatings were sprayed onto low-carbon and stainless steel substrates. As in the previous work [4,5],
coatings were prepared using a range of spray particle velocities, and the effects of particle velocity on the coating characteristics
were assessed. The coatings were characterized in terms of microstructure, residual stress, microhardness, elastic modulus, and
Gas-atomized FeAl powder with a nominal composition of Fe-24.1Al-0.5Mo-0.1Zr (wt.%) was obtained from Ametek Specialty
Metal Products Division. The size distribution was such that essentially all of the particles were less than 38 pm in diameter (-
270 mesh). FeAl coatings were prepared by HVOF thermal spraying in air using a Hobart-Tafia Technologies JP-5000 system.
Coatings were produced at an equivalence ratio of one (a stoichiometric mixture of kerosene and oxygen) and three torch chamber
gage pressures: 340, 520, and 620 kPa. Measurement of the particle temperature and velocity characteristics (using an
integrated Doppler velocimeter and high-speed two-color pyrometer) was performed for each gage pressure. As shown in Table
1, the mean particle velocity increases with increasing chamber pressure while the particle temperature remains essentially
constant. Coatings approximately 250 to 1500 pm thick were built upon low-carbon and stainless steel substrates in 45 pm thick
layers in a raster deposition scheme at a standoff distance of 355 mm. A more detailed description of the HVOF coating
apparatus is given in Ref. .
Mo-Si-B powder with a nominal composition of Mo-13.4Si-2.6B was obtained from Ames Laboratory. Coatings were prepared
by plasma spraying in air at atmospheric pressure using a SG100 torch. Particle velocities and temperatures were measured using
the same system as described above; the spray parameters and resulting particle characteristics are shown in Table 2. Coatings
500 pm thick were deposited onto thin low-carbon steel substrates at both conditions for curvature-based residual stress
measurement and microstructural evaluation. Additional coatings up to 1500 pm thick were produced at the higher velocity
conditions for further residual stress measurements and free-standing physical property specimens.
Residual stresses in the coatings were characterized by curvature measurements on coating-substrate couples. Average coating
and substrate stresses were calculated from the measured curvature using the simple beam bending model described in Ref. .
The required physical parameters (elastic modulus)were measured on free-standing coating specimens (described below). As a
check on the validity of the curvature based model, residual stresses in the FeAl coatings were directly measured using XRD
techniques. Prior to measurement, 100 pm was removed from the coating surface by grinding and polishing. Microstructural
characterization was performed on transverse coating sections in the as-polished condition using metallography, microhardness
measurements, XRD-based dislocation density analysis, and scanning electron microscopy (SEM) coupled with energy-dispersive
X-ray spectroscopy (EDS).
The mean coefficient of thermal expansion (CTE) was measured using a dilatometer. Free-standing coating specimens 6.3 mm
long were sectioned from 1.5 mm thick coatings; a Pt standard was used as the reference. Three sequential runs were performed
on each specimen to observe changes in expansion behavior with thermal cycling. Elastic modulus measurements were made on
free-stranding coatings with strain gages.
3.1 Coating Microstructure
Typical microstructures are shown in Fig. 1 for FeAl coatings prepared at 540 and 660 m/s. All coatings showed B2 ordering in
XRD analysis, and no significant differences in chemistry were observed between the coatings and the feedstock powder. As
identified in the figure, the coatings exhibited measureable fractions of porosity, oxide inclusions, and unmelted particles. The
porosity of coatings sprayed 540 m/s was 5-6%; while essentially zero porosity was observed at the higher velocities. Similarly,
the fraction of oxide varied from 5-6% to 1% with increasing velocity. The coating microhardness increased with spray velocity
from 310 DPH at a spray velocity of 540 m/s to 580 DPH at a spray velocity of 700 m/s. The microhardness values translate to
yield stresses ranging from approximately 700 to 1,300 MPa. Dislocation densities computed from X-ray peak broadening were
essentially constant at 2 x 10" cm-2
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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/m1/2/: accessed October 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.