-
Formation of Fe-Based Amorphous Coating Films
by Thermal Spraying Technique
Masahiro Komaki1, Tsunehiro Mimura1, Yuji Kusumoto1,Ryurou
Kurahasi1, Masahisa Kouzaki2 and Tohru Yamasaki3
1Nakayama Steel Works, Ltd., R&D Division, Osaka 551-8551,
Japan2Nakayama Steel Works, Ltd., Osaka 551-8551, Japan3University
of Hyogo, Himeji 671-2280, Japan
Some amorphous Fe-Cr-P-C coating films having high hardness and
high corrosion resistance have been produced by a newly
developedthermal spraying technique. In order to control the
temperatures of the powder particles in the flame spray and the
substrate, a newly developedcylindrical nozzle, with external
cooling nitrogen gas, was mounted to the front end of the thermal
spraying gun. Fe70Cr10P13C7 films withvarious external cooling gas
velocities between 20m/s and 40m/s exhibited entire amorphous
structure without oxides and/or unmeltedparticles.
Corrosion-resistance of the films was observed in immersion tests
using various corrosive liquids. An amorphous film was formed onthe
surface of the shaft sleeve of the slurry pump by using the
cylindrical nozzle. This shaft sleeve was installed in the slurry
pump of chemicalfertilizer maker’s production line and the life
test was done under the real operation condition for two
months.[doi:10.2320/matertrans.MAW201022]
(Received April 27, 2010; Accepted June 1, 2010; Published
August 25, 2010)
Keywords: iron-chromium-phosphorus-carbon, amorphous coating
film, thermal spraying method, rapid cooling, hardness,
corrosion
resistance
1. Introduction
Thermal spraying is a superior technique for producingamorphous
alloy coating films with large area on variousindustrial
materials.1–4) Especially, Fe-Cr based amorphousalloys have high
hardness and high corrosion resistance.5–8)
However, the Fe-Cr based amorphous alloys having highmelting
temperatures of about 1000�C have not yet beenwell produced by
previous thermal spraying methods. Theimportant problems in these
methods may be due to thecontamination of oxide and the density of
pores in thethermal sprayed films.1,2,9–13) Recently, there are
someresearch papers that were able to inhibit the oxide formationby
the shield nozzle installed at the front end of theatmospheric
plasma spray gun or the high velocity oxygenfuel gun.14,15)
In the present study, formation of some amorphous Fe-Cr-P-C
coating films having high hardness and high corrosionresistance
have been demonstrated by a newly developedthermal spraying
technique. In order to control the temper-atures of the powder
particles in the flame spray and thesubstrate, a newly developed
cylindrical nozzle with externalcooling nitrogen gas was mounted at
the front end of thethermal spraying gun. Cooling rates of the
sprayed sampleson SUS316L substrates were estimated to attain
about106�C/s by measuring the temperature gradient of thespraying
flame.
2. Experimental Procedures
The thermal spraying equipment used in this study wascomposed of
a gas flame spraying gun and a newly developedcylindrical nozzle.
The thermal spraying material was gas-atomized powder (grain size:
38–63 mm) of Fe70Cr10P13C7(at%) with a melting temperature of
997�C. A SUS316L(100mm� 50mm� 3:2mm) substrate, shot blasted by
alumina powder, was used. In order to keep a reductionatmosphere
in the flame, an acetylene-rich gas fuel mixturewas used. Nitrogen
gas was introduced into the cylindricalnozzle to prevent the
formation of oxides in the coating film.The thermal spray gun was
mounted on a robot arm, scannedat the rate of 300mm/s in feed rate
and 10mm in sprayingpitch, and an alloy layer of up to 500 mm in
film thickness wasproduced. Structural analysis of the coating
films was doneby X-ray diffraction (Cu K� radiation with a
graphitemonochromator, 40 kV-200mA) and transmission
electronmicroscope (TEM:HF2000-200 kV). The amorphous alloycoating
films, separated from the substrate, were immersed invarious kinds
of corrosive liquid for the maximum period of4-weeks. For
comparison, standard specimens of Hastelloy Cand commercially
produced pure titanium were used. Anamorphous film was formed on
the surface of the shaft sleeveof the slurry pump by using the
cylindrical nozzle. This shaftsleeve was installed in the slurry
pump of chemical fertilizermaker’s production line and the life
test was done under thereal operation condition for two months.
Figure 1 shows a schematic illustration of cylindrical-nozzle
type thermal spray gun. In order to control thetemperatures of the
powder particles in the sprayed flame andthe substrate, and to
prevent the formation of oxides in thecoating film, a newly
developed cylindrical nozzle withexternal cooling nitrogen gas was
mounted at the front end ofthe thermal spraying gun.
Figure 2 shows the cooling-gas flow rate, over thedistance from
the cylindrical nozzle head to the substrate,measured by a pitot
tube. The cooling-gas flow rate can becontrolled up to 60m/s by
adjusting the gas pressure in thecylindrical nozzle. To keep the
optimal temperature and flowstates of the spraying flame, the flow
rate of the cooling gaswas set to the nearly equal to that of the
spraying flame.In this study, optimal flow rate of the sprayed
flame was30�40m/s.16)
Materials Transactions, Vol. 51, No. 9 (2010) pp. 1581 to
1585#2010 The Japan Institute of Metals
http://dx.doi.org/10.2320/matertrans.MAW201022
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Figure 3 shows the thermal gradients of the sprayingflame, over
the distance from the cylindrical nozzle head tothe substrate,
measured by a thermocouple. In this diagram,the 0mm gradient curve
indicates the thermal gradient at theflame center, while the 5mm
and 10mm gradient curvesindicate the thermal gradient at the
positions outlying fromthe center of flame. Although the center of
flame hits thesubstrate at a temperature of approximately 1000�C,
theflame temperature decreased sharply to about 300 to 500�Cfor the
10mm and the 5mm respectively positions.
Figure 4 shows the compositions of the thermal sprayingflame,
over the distance from the cylindrical nozzle head to20mm and 70mm,
measured by the orsat apparatus. In thisexperiment, nitrogen gas or
compression air were used as acooling gas. In order to keep a
reduction atmosphere in thespraying flame, an acetylene-rich gas
fuel mixture was used.When nitrogen gas was used, oxygen was
contained in thespraying frame, 0.2% and 0.8% at positions of 20mm
and70mm away from the cylindrical nozzle head, respectively.On the
other hand, when compression air was used, oxygenwas contained 8%
and 14% in the spraying frame at samepositions.
3. Results
3.1 Structure of the coating filmsFigure 5 shows the optical
micrographs in cross-section of
the thermal spray coating films with and without the
newlydeveloped cylindrical nozzle. When the thermal spray
gunwithout the cylindrical nozzle was used, many inclusions
ofoxides have been observed in the coating film as shown in
C2H2
O2
Flame
Molten range Quenching range
Spraying film
Cooling gas N2
Cooling gas N2Spray powder material (Carrier gas: N2)
Cylindrical nozzle head
Internal cooling gas
Cylindrical nozzle (twofold cylinder)
Front end of thermal spray-gun nozzle
Substrate
Fig. 1 Schematic illustration of cylindrical-nozzle type thermal
spraying gun.
Coo
ling-
gas
flow
rat
e, v
/m/s
Distance from the cylindrical nozzle head, l/mm
0.60MPa
0.35MPa
0.25MPa
0.15MPa
0 20 40 60 80 100
70
60
50
40
30
20
10
0
Position of the substrate
Fig. 2 Cooling-gas flow rate.
Spra
yed
flam
e te
mpe
ratu
re, T
/ °C
Distance from the cylindrical nozzle head, l/mm
0mm
5mm
10mm
0
200
400
600
800
1000
1200
0 20 40 60 80 100
1400
5mm
10mm
Cylindrical nozzle
Cross section of the flame0mm position
Position of the substrate
Fig. 3 Flame temperature gradient.
Com
posi
tion
of th
e sp
rayi
ng f
lam
e (v
ol.%
)
Distance from the cylindrical nozzle head, l/mm
0
5
10
15
20
0
5
10
15
20
0
5
10
15
20
CO
20mm 70mm 20mm 70mm
O2
CO2
Cooling Gas:N2 Cooling Gas:Air
CO
O2
CO2
Fig. 4 Compornent of the spraying flame.
1582 M. Komaki et al.
-
Fig. 5(a). On the other hand, shown in Fig. 5(b), when
thethermal spray gun with the cylindrical nozzle was used,
high-quality amorphous sprayed film without the inclusion ofoxides
can be produced.
Figure 6 shows the X-ray diffraction patterns of thethermal
spray coating films with and without the cylindricalnozzle. When
the thermal spray gun without the cylindricalnozzle was used,
crystalline peaks from many kinds of oxideswere observed. On the
other hand, when the thermal spraygun with the cylindrical nozzle
was used, X-ray diffractionpeaks are well-broadened, indicating
structure of the film isthe amorphous.
Figure 7 shows a TEM image and a selected areadiffraction
pattern (SADP) of the sprayed coating filmfabricated by using the
cylindrical nozzle. It consists ofamorphous phase in general. The
SADP with halo ringsalso supports that the coating film has an
amorphousstructure. Compositions of the coating film obtained byEDX
analysis are as follows; Fe: bal, Cr: 11.6 at%, P:13.7 at%,
indicating a similar composition to that of the gas-atomized
powder.
3.2 Corrosion testsThe amorphous alloy coating films separated
from the
substrates were immersed in various kinds of corrosiveliquid.
For comparison, standard specimens of Hastelloy Cand commercially
produced pure titanium were used. Table 1shows the results of the
corrosion tests. In the corrosionstandard for chemical plant
materials, a corrosion rate in theweight variation of between 0�0:5
g/m2 day is consideredgood.17) Corrosion of the amorphous sprayed
film scarcelyprogressed after an initial increase arising from the
gener-ation of an oxide layer. On the other hand, both Hastelloy
Cand titanium slightly corroded. Accordingly, the corrosiontests
have clarified that the sprayed amorphous film exhibiteda high
corrosion resistance, equal to that of Hastelloy C andcommercially
produced pure titanium, under most condi-tions. However, the
corrosion resistance test in hydrochloricacid did not show a
favorable result for the Fe70Cr10P13C7film. The corrosion liquids
containing strong oxidizers suchas nitric acid and sulfuric acid
etc. usually exhibit the highformability of the passive films on
the sample surface. Onthe contrast, the hydrochloric acid is the
nonoxidizing acid,
(a) (b)
Substrate (SUS316L)
Amorphous
Substrate (SUS316L)
Amorphous
Oxide
Fig. 5 Optical micrographs in the cross section of the coating
films: (a) Without the cylindrical nozzle; (b) With the cylindrical
nozzle.
20° 40° 60° 80° 100°2θ
With the cylindrical nozzle
Without the cylindrical nozzle
Inte
nsity
Fig. 6 X-ray diffraction patterns of the spray coating
films.
Diffracted pattern
Site being diffractedand EDX analyzed
Fig. 7 TEM image of the spray coating film with the cylindrical
nozzle.
Table 1 Results of the corrosion tests.
Corrosion rate in the weight variation (g/m2 day)
CorrosiveTemperature
Immersion AmorphousStandard materials
liquids period sprayed
filmHastelloy
CTitanium
Hydrochloric
acid, 35%6 h �7890 �4:56 �312
Nitric acid,
10%+0.065 0 �0:004
Sulfuric acid,
5%+0.049 0 �0:013
Sulfuric acid,
70%�0:011 �0:013 �0:117
Caustic soda,
48%
Room
temperature4 weeks
+0.024 �0:008 �0:004
Sodium
hypochlorite,
12%
+0.079 0 0
Salt water,
3%+0.154 0 0
Phosphoric
acid, 5%+0.100 — —
Formation of Fe-Based Amorphous Coating Films by Thermal
Spraying Technique 1583
-
indicating the formation speed of the passive film may beslower
than that of the other liquids.
3.3 Life test of the sprayed coated slurry pump shaftsleeve
An amorphous film was formed on the surface of the shaftsleeve
of the slurry pump by using the cylindrical nozzle.This shaft
sleeve was installed in the slurry pump of chemicalfertilizer
maker’s production line and the life test was doneunder the real
operation condition for two months.
Figure 8 shows the diagrammatic illustration of the slurrypump.
The shaft sleeve of the slurry pump was worn easilybecause of the
griding effects of slurry that sandwichedbetween the driving shaft
and the seal. Therefore, thedevelopment of the material of high
hardness and highcorrosion resistance has been demanded.
Table 2 shows the life test conditions for the shaft sleeveof
the slurry pump. Surface of the SUS304 shaft sleeve wasshot blasted
by alumina powder, and then NiCr undercoatlayer with about 50 mm
thickness was formed. The Fe-Cr-P-Camorphous film of 300 mm
thickness was coated on the NiCrlayer. Finally the surface of the
coated shaft sleeve wasground with a diamond grindstone and
finished up. As aresult, the thickness of the remained Fe-Cr-P-C
coating filmwas about 150 mm.
Figure 9 shows the wear track of the shaft sleeve afterlife test
for two months. There is no wear track in theamorphous thermal
spraying shaft sleeve though wear trackof 4 mm is admitted in
conventional SCS23 product (Fe: bal,
Cr: 20mass%, Ni: 30mass%, Mo: 3mass%, Cu: 3mass%).The amorphous
thermal spraying shaft sleeve indicatedhigh corrosion resistance
and high abrasion resistance(HV700�900).
4. Discussion
Figure 10 shows a schematic diagram explaining thetemperature
distribution of the flame just prior to the inpacton the substrate.
When the cylindrical nozzle is not used, thehigh temperature area
at about 1000�C is spread over theentire flame. In contrast, when
the cylindrical nozzle is used,the high temperature area is well
converged to the center ofthe flame as expected from the results of
Fig. 3.
Slurry inhalation
Slurry exhalation
Seal
Pump drive shaft
Impeller
LinerCover plate
Fe-Cr-P-C amorphouscoated shaft sleeve
Fig. 8 Diagrammatic illustration of the slurry pump.
Table 2 Test conditions and results of the varification test by
using the
amorphous coated and uncoated shaft sleeve of the slurry
pump.
Item Conventional product Material under test
Substrate: SUS304
Material SCS23Under coat: NiCr 50mmTop coat: Fe70Cr10P13C7
150mm
Test circumstanceSlurry pump of chemical fertilizer maker’s
production line
Required
performance
High hardness and high corrosion resistanc in
slurry of pH2
Depth of wear track
after two months4 mm 0mm
Wear track withslurry and seal
SCS23 material
Depth of wear track : 1.5 µ m
0 0
4
Fe70Cr10P13C7 coated materialµ m
µ m
µ m
Fig. 9 Wear track of the shaft sleeve after two months.
Lower temperature area
(a) (b)
High temperature areaabout 1000°C
Fig. 10 Schematic diagram explaining the temperature
distribution of the
flame just prior inpact on the the substrate: (a) Without the
cylindrical
nozzle; (b) With the cylindrical nozzle.
1584 M. Komaki et al.
-
Figure 11 shows the changes of the substrate temperatureas a
function of the thermal spraying time with and withoutthe
cylindrical nozzle. The thermal spray gun was mountedon a robot
arm, scanned at the rate of 300mm/s in feed rateand 10mm in
spraying pitch. Temperature of the substrate atthe surface center
point was measured by radiation ther-mometer. When the thermal
spray gun without the cylindricalnozzle was used, a hot area
expands over all the flame, thuscausing a rapid increase of
substrate temperature beyond thevitrification point (493�C) and,
accordingly, crystallization ofthe coating occured easily. In
contrast, when the thermalspray gun with the cylindrical nozzle was
used, the substratetemperature saturated gradually under the
vitrification tem-perature (400�C), indicating the high formability
of theamorphous film. Therefore, the high scanning speed of300mm/s
is very effective for rapid cooling of the coatingfilms, preventing
the crystallization of the former coatedamorphous layer.
5. Summary
Some amorphous Fe-Cr-P-C coating films having highhardness and
high corrosion resistance have been producedby a newly developed
thermal spraying technique. In order tocontrol the temperatures of
the powder particles in the flamespray and the substrate, a newly
developed cylindrical nozzlewith external cooling nitrogen gas was
mounted to the frontend of the thermal spray gun. When the thermal
spray gunwith the cylindrical nozzle was used, the substrate
temper-
ature increased gradually because of the lower temperaturearea
surrounding the high temperature area of about 1000�C.Cooling rates
of the sprayed samples on SUS316L substrateswere estimated to
attain about 106�C/s by measuring thetemperature gradient of the
spraying flame. Fe70Cr10P13C7films with various external cooling
gas velocities between20m/s and 40m/s exhibited an entirely
amorphous structurewithout oxides and/or unmelted particles.
Corrosion testinghas clarified that the sprayed amorphous films
exhibited ahigh corrosion resistance equal to that of Hastelloy C
andcommercially produced pure titanium. An amorphous filmwas formed
on the surface of the shaft sleeve of the slurrypump by using the
cylindrical nozzle. This shaft sleeve wasinstalled in the slurry
pump of chemical fertilizer maker’sproduction line and the life
test was done under the realoperation condition for two months. The
amorphous thermalspraying shaft sleeve indicated high corrosion
resistance andhigh abrasion resistance.
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0
200
400
600
800
0 60 120 180
Thermal spraying time, t/s
With the cylindrical nozzle
Without the cylindrical nozzle
Vitrification temperature(493°C)
Subs
trat
e te
mpe
ratu
re (
°C)
Fig. 11 Changes of the substrate temperature as a function of
the thermal
spraying time when the thermal spray gun scanned at the rate of
300mm/s
in feed rate and 10mm in spraying pitch.
Formation of Fe-Based Amorphous Coating Films by Thermal
Spraying Technique 1585