Relationship between pack chemistry and growth of silicide coatings on Mo-TZM alloy

Article abstract of DOI:10.1149/1.2987954

A theoretical model equation has been derived to relate the growth kinetics of silicide coating with the pack chemical composition and other processing conditions for siliconizing of Mo-TZM (Mo-0.5Ti-0.1Zr-0.02C) alloy to improve its oxidation resistance at high temperatures. A series of experiments conducted with varying pack Si (1-10 wt %) and N H₄ F (2-20 wt %) content, time (1-25 h), and temperature (800-1200°C) confirmed the validity of the model. MoSi₂ was the main coating layer formed during the siliconizing process. Optimum processing conditions were derived for doping of Al in MoSi₂ to form Mo (Si,Al)₂ in the outer layer of the coating.

Full text of DOI:10.1149/1.2987954

D734
Journal of The Electrochemical Society, 155 ͑12͒ D734-D741 ͑2008͒
0013-4651/2008/155͑12͒/D734/8/$23.00 © The Electrochemical Society
Relationship Between Pack Chemistry and Growth of Silicide
Coatings on Mo–TZM Alloy
Sanjib Majumdar,a,z Indrakumar Sharma,a Indradev Samajdar,b and
Parag Bhargavab
aBhabha Atomic Research Centre, Materials Processing Division, Mumbai-400085, India
bDepartment of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay,
Mumbai-400076, India
A theoretical model equation has been derived to relate the growth kinetics of silicide coating with the pack chemical composition
and other processing conditions for siliconizing of Mo–TZM ͑Mo–0.5Ti–0.1Zr–0.02C͒ alloy to improve its oxidation resistance at
high temperatures. A series of experiments conducted with varying pack Si ͑1–10 wt %͒ and NH4F ͑2–20 wt %͒ content, time
͑1–25 h͒, and temperature ͑800–1200°C͒ confirmed the validity of the model. MoSi2 was the main coating layer formed during
the siliconizing process. Optimum processing conditions were derived for doping of Al in MoSi2 to form Mo͑Si,Al͒ in the outer
2
layer of the coating.
© 2008 The Electrochemical Society. ͓DOI: 10.1149/1.2987954͔ All rights reserved.
Manuscript submitted May 27, 2008; revised manuscript received August 11, 2008. Published October 7, 2008.
There is an increasing demand for materials capable of with-
standing environments with respect to temperature, stress, radiation,
oxidation, and liquid metal corrosion for a prolonged duration. Mo-
lybdenum and niobium base alloys are the most promising materials
for such kinds of applications.1-3 Mo–TZM ͑Mo–0.5Ti–0.1Zr–
0.02C͒ is a molybdenum base alloy containing 0.5 wt % Ti,
0.1 wt % Zr, and 0.02 wt % C. Solid solution and carbide strength-
ening improve the high-temperature mechanical properties of the
alloy making it suitable as a potential candidate material for ad-
vanced high-temperature nuclear reactors operated beyond 1000°C.4
However, the major problem with Mo base alloys is their inadequate
oxidation resistance due to the formation and rapid volatilization of
the oxide MoO3 around 704°C that also melts at 794°C.5,6
In order to improve the oxidation resistance of the Mo base al-
loys, formation of a protective coating is necessary.7 Coating of
molybdenum di-silicide ͑MoSi2͒ on the outer surface of the alloy
component is the most promising because it has an outstanding oxi-
dation resistance up to a temperature of 1700°C.8,9 The excellent
oxidation resistance of MoSi2 at high temperatures is due to the
formation of a self-healing, glassy silica ͑SiO2͒ layer that protects
the substrate from further attack by oxygen. However, for porous
MoSi2 product, the SiO2 layer formed in the lower temperature
range of 400–600°C is not continuous, leading to the oxygen pen-
etration and pest disintegration.10-13 Recently, a lot of studies have
been done to improve the oxidation behavior of MoSi2 by doping
boron in the multiphase Mo–Si–B alloy.5,14,15 The other ways of
preventing the low-temperature pesting are formation of theoreti-
cally dense MoSi2 or addition of Al to form Mo͑Si,Al͒2.16 In
Mo͑Si,Al͒2, Al2O3 formation is preferred to that of SiO2, involving
a minor volume expansion of 4.9% compared to 85.6% in the case
of SiO2.17 Maruyama and Yanagihara18 conducted detailed oxidation
800–1200°C in an inert atmosphere for 8–36 h. The pack is com-
posed of a master alloy or pure element powder to be enriched at the
substrate surface, a halide salt activator, and inert filler. The halide
activator decomposes at high temperature to produce volatile halide
vapors of the elements. The chemical potential gradient drives the
gas phase diffusion of the metallic halides to result in surface depo-
sition followed by solid-state diffusion, which is considered as the
rate-limiting step.31 The process is easy to operate, inexpensive, can
accommodate substrates of varying size and shape, and can produce
a uniform and smooth coating of desired thickness. Formation of
alumoreinide32 and silicide33,34 coating on Mo–Si–B alloy using this
process has been reported. Our recent studies on the continuous and
cyclic oxidation behavior of MoSi2 and Mo͑Si,Al͒ coated Mo–
2
TZM alloy showed the protective nature of both the coatings under
oxidizing conditions up to 1300°C.35
The studies on formation of aluminide coating have been exten-
sively focused in comparison to the effect of the processing condi-
tions on growth kinetics.21,24 More systematic studies have been
reported in the literature on the formation of aluminide coating on
nickel base super alloys,24 steels,21 etc., by the pack aluminizing
process in relation to determining the effects of processing condi-
tions on the growth kinetics. Formation of aluminide and silicide
coatings on molybdenum base alloys using pack cementation pro-
cess with some specific conditions has been reported earlier.32,33,35
Systematic studies on the formation of silicide coatings on Mo-base
alloy by pack cementation process are nonexistent in the literature.
In the present work, model calculations of the pack cementation
process has been performed for deriving the corelation between the
growth of silicide coatings on Mo–TZM alloy substrate with pack
composition and other processing conditions. The theoretical model
equation corelating the coating thickness with pack composition and
processing conditions has been obtained from the thermodynamic
and kinetic analysis. The validity of the analysis is subsequently
confirmed by experiments. Process optimization has also been done
on formation of alumino-silicide coating on the alloy substrate.
studies of Mo͑Si,Al͒ at different temperatures. Formation of pro-
2
tective Al2O3 scale below 1595°C, which is the eutectic temperature
in the SiO2–3Al2O3 2SiO2 system, has been reported. Above the
eutectic temperature, a liquid scale of SiO2–Al2O3 ͑mullite͒ rich in
Al2O3 content forms.18-20 Therefore, a coating of the dense MoSi2 or
Mo͑Si,Al͒2 compound on Mo–TZM alloy will increase its oxidation
resistance at high temperatures.
Pack cementation process is most widely used for applying pro-
tective coatings on materials, such as steels,21-23 superalloys,24
TiAl,25-27 and refractory metals alloys28,29 suitable for high-
temperature applications. It is a diffusion coating process30 involv-
ing embedding the substrate into a sealed or vented refractory con-
tainer with a powder mixture called the pack, which is then heated at
Experimental
Initially pure molybdenum powder was prepared by conducting
hydrogen reduction of MoO3 at 900°C for 2.5 h. Mo powder was
milled with pure Ti, Zr, and C powder of required quantity. The
powder mix was subsequently compacted to 20 mm diam pellets
and sintered at 1000°C for 4 h. Finally Mo–TZM alloy was pro-
duced by arc melt consolidation of the sintered pellets prepared from
a milled powder mixture containing Mo, Ti, Zr, and C. Alloy was
hot rolled into sheets and annealed at 1650°C for 1 h followed by
cutting into small buttons of dimensions 15 ϫ 10 ϫ 2 mm. The sur-
faces of the buttons were ground and polished to 1 ␮m diamond
finish. The specimens were then degreased and cleaned by ultrasonic
z E-mail: sanjib@barc.gov.in

Journal of The Electrochemical Society, 155 ͑12͒ D734-D741 ͑2008͒
D735
0.0
-2.5
cleaning. The cleaned samples were weighed accurately and placed
in the pack. The pack mixtures were prepared by mixing appropriate
amounts of Si, Al2O3 ͑filler͒, and NH4F ͑activator͒ powder. The
pack cementation experiments were conducted by following the
standard techniques.35 The average particle size of powder of each
material was Ͻ75 ␮m. The packs were prepared by filling and tap-
ping the powder mixture around the Mo–TZM substrates in a cylin-
drical alumina crucible of 36 mm diam and 48 mm length. The cru-
cible was then sealed with an alumina lid using alumina base
cement. The cement sealing was dried for 24 h at room temperature
followed by controlled heating inside a tubular furnace under argon
flow. Initial curing treatment was given at 150°C for 2 h followed
by heating at 250°C for 2 h to facilitate further curing of the cement
and to remove any residual moisture. Subsequently, the temperature
of the furnace was raised to the actual coating temperature at a rate
of 4°C min−1 and held for a desired time. The furnace was subse-
quently cooled to room temperature at a slow rate. Experiments
were conducted by varying process parameters, such as temperature
͑800–1200°C͒, dwell time ͑1–25 h͒, pack Si and activator ͑NH4F͒
contents, and different activators ͑NH4F, NaF, and NH4Cl͒. A simi-
lar set of experiments were carried out to form an adherent and
uniform alumino-silicide coating of sufficient thickness. The pre-
liminary set of experiments was conducted at five different tempera-
tures between 800 and 1200°C for 10 h using a fixed pack compo-
sition of Al2O3-80%, Si-10%, and NH4F-10%, by weight. Applying
the same pack composition, the next set of experiments was carried
out at 1100°C by varying the dwell time from 1 to 25 h. Subsequent
experiments were conducted at 1100°C for 10 h with varying Si and
NH4F content in the pack, keeping constant NH4F and Si percent-
ages in respective cases. Studies were also conducted using the other
activators ͑NH4Cl and NaF͒ of same weight percent at the similar
coating conditions. The experiments of Al doping in the silicide
coating were conducted using different pack compositions, and the
pack composition producing adherent and uniform coating of suffi-
cient thickness at 1100°C was found to be Al2O3-60%, Si-10%,
Al-10%, and NH4F-20% by weight. Using this pack composition,
the effects of time and temperature on alumino-silicide coating were
studied by carrying out a series of experiments. After cooling and
taking the samples out of the furnace, the coated samples were
washed in a water jet to remove any residual powder from the sur-
face. Weight and thickness measurements of the coated specimens
were conducted. The specimen weight gain in milligrams per centi-
meters squared was estimated from the weight difference and the
surface area of the specimens. The coated surface was examined by
X-ray diffraction ͑XRD͒ ͑Pananalytical X’pert Pro͒ for identifying
the phases. The cross-sectional microstructure and chemical compo-
sition of the coated specimens were analyzed using optical ͑Olym-
pus͒, scanning electron microscopy ͑SEM͒ ͑Hitachi͒, and energy
dispersive X-ray spectroscopy ͑EDS͒ ͑Thermo Electro Corporation͒.
The thicknesses of silicide and aluminosilicide layers were esti-
mated from Si and Al diffusion profiles measured by EDS.
SiF
SiF3
H2
SiF2
SiF4
-5.0
-7.5
-10.0
-12.5
1100
1200
1300
1400
1500
1600
Temperature (K)
Figure 1. Equilibrium partial pressures of the major vapor species in the
10Si–10NH4F–80Al2O3 ͑in weight percent͒ pack.
Di ⌬Pi
Jv =
͓1͔
ͩ ͪ
i
RT ⌬x
where Di is the gas phase interdiffusion coefficient, R is the gas
constant, T is the temperature, and the derivative is the partial pres-
sure gradient at the substrate surface. The change in partial pressure
⌬Pi for each species “i” is defined as the partial pressure of the
species in the bulk pack ͑aSi = 1͒ minus the partial pressure of the
species at the substrate. Levine and Caves24 have shown that for this
kind of migration and deposition process, the amount of element
͑Si͒ being deposited on the metal or alloy surface of a unit area ͑m
in milligrams per centimeters squared͒ at a time interval t can be
given by
2␳␧MSi
m2 =
D ⌬P t
͓2͔
ͩ
ͪ
i
i
͚
lRT
where ␳ is the Si concentration ͑in grams per cubic centimeter͒ in
the pack, ␧/l is the correction factor due to the pack porosity and
pore length, and MSi is the atomic weight of Si. From Eq. 1 and 2,
it is clear that the difference in the partial pressures ͑⌬Pi͒ of silicon
fluoride vapors is the driving force for the migration of the vapors to
the outer surface of the substrate and formation of the coating by
deposition or reduction at the surface.
The metal-deficient fluoride SiF4 is assumed to be a depleted
species, and its deposition is thermodynamically unfavorable, rather
it carries fluorine back to the pack to reform the lower fluoride
species that support deposition. Considering total pressure inside the
pack as one atmosphere and applying mass balance approach, the
equilibrium partial pressure of different fluorides in a pack mixture
consisting of 10 wt % Si, 10 wt % NH4F, and 80 wt % Al2O3 at
different coating temperatures have been calculated and plotted in
Fig. 1. Among the fluorides, the partial pressure of SiF2 is the maxi-
mum in this temperature range. The standard free energies for the
formation of different fluorides are presented in Table I. Like SiF4,
SiF3 is also a very stable fluoride and as per Fig. 1 SiF has a very
low vapor pressure. Therefore, the coating process is mainly dictated
by the migration and dissociation of SiF2. Now, considering ␳ is
proportional to the pack Si content in weight percent ͑WSi͒, Eq. 2
can be written as
Results
Process model.— In the pack cementation process, the fluoride
activator ͑NH4F, NaF, etc.͒ reacts with Si and forms a number of
gaseous fluorides such as SiF4, SiF3, SiF2, and SiF. NH4F activator
initially dissociates into NH3 and HF at ϳ300°C. Subsequent reac-
tions between HF and Si lead to the formation of the fluoride vapors.
It is assumed that a depletion zone of depth ⌬x exists near the
substrate surface where the Si is completely consumed. The chemi-
cal potential gradient developed due to the formation of the deple-
tion zone drives the gas phase diffusion of silicon fluorides to result
in surface deposition. The flux Jvi of the migrating fluoride vapor is
given by
2␧MSiDSiF
2
m2 =
W P
t
͓3͔
ͩ
ͪ
Si SiF
2
lRT
SiF2 is formed as per the chemical reaction Si + 2MF = SiF2 + M
͑M = NH4,Na, etc.͒. The partial pressure of SiF2 is given by

D736
Journal of The Electrochemical Society, 155 ͑12͒ D734-D741 ͑2008͒
Table I. The standard free energies „⌬G°… of formation in kilojoules per molecule for different fluorides at different temperatures.
T ͑K͒
SiF4
SiF3
SiF2
SiF
AlF3
AlF2
AlF
1100
1200
1300
1400
1500
1600
−1457.08
−1442.69
−1428.31
−1413.94
−1399.58
−1385.22
−1037.8
−1033.22
−1028.62
−1023.98
−1019.32
−1014.63
−622.103
−624.685
−627.201
−629.651
−632.036
−634.359
−132.837
−142.578
−152.246
−161.844
−171.372
−180.834
−1141.77
−1134.12
−1126.42
−1118.66
−1110.85
−1102.99
−722.925
−723.693
−724.356
−724.924
−725.401
−725.795
−351.771
−357.754
−363.627
−369.397
−375.072
−380.659
2
2
coating growth rate and pack chemistry for the formation of silicide
coatings on molybdenum base alloys is not available in the litera-
ture. Our experimental results presented in the following sections
strongly supported this derived relationship ͑Eq. 10͒, confirming the
validity of the assumptions for the coating process.
PSiF = k͓MF͔ = kWMF
͓4͔
2
where k is the equilibrium constant and WMF is the weight-percent
activator ͑NH4F or NaF͒ used. Substituting Eq. 4 into Eq. 3, one
obtains
2␧MSiDSiF
K
Coating formation.— In general, the surfaces of the coated
samples were smooth and free from the entrapment of pack par-
ticles. The appearance of the silicide coating specimens was metallic
whitish gray. The cross-sectional SEM image of a representative
silicide coated sample is presented in Fig. 2a. The coating ͑Fig. 2a͒
was prepared at 1100°C for 17.5 h using a pack composition of
10Si–10NH4F–80Al2O3 ͑in weight percent͒. The specimen weight
gain ͑m͒ and coating thickness ͑h͒ for this coating experiment was
ϳ16.95 mg cm−2 and 82.68 ␮m, respectively. The diffusion profile
͑Fig. 2b͒ of major elements measured by EDS along the line marked
on the micrograph revealed the formation of a thick ͑ϳ79 ␮m͒
outer layer followed by a thin ͑ϳ3.7 ␮m͒ inner layer in the coating.
Elemental analysis and XRD studies confirmed that the thick outer
2
m2 =
W W2MFt
͓5͔
ͩ
ͪ
Si
lRT
or
km
T1/2
1/2
m =
W
WMFt1/2
͓6͔
Si
where km is a constant at constant temperature. As the final stage of
the coating growth is inward diffusion of Si in Mo base matrix, the
linear relationship between m and t1/2 at constant temperature and
other parameters is obvious and also observed by many
researchers21,24 during the diffusion controlled pack cementation
process. Now, if there is no loss of substrate material into the pack
and the silicide compound of well-defined stoichiometry ͑MoxSiy͒
forms in the coating layer, the relationship between the substrate
weight gain ͑m in milligrams per centimeters squared͒ and coating
thickness ͑h in microns͒ is given by
10Mc
h =
m = k1m
͓7͔
yMSi␳
c
k1 is a constant incorporating MSi atomic weight of Si, Mc, and ␳
c
are molecular weight and the density ͑in grams per cubic centime-
ters͒ of the coated compound MoxSiy, respectively. Substituting Eq.
7 into Eq. 6 one gets
kh
T1/2
1/2
h =
W
WMFt1/2
͓8͔
Si
where kh is a constant at constant temperatures. The variation of kh
with temperature can be given by
Ea
k = k exp −
͓9͔
ͩ ͪ
h
0
RT
where Ea is the activation energy for the coating growth process and
k0 is a constant. Finally, substitution of Eq. 9 into Eq. 8 yields
k0
T1/2
Ea
1/2
h =
W
WMFt1/2 exp −
͓10͔
ͩ ͪ
Si
RT
Equation 10 is the ultimate kinetic equation for the coating growth
process, which relates the growth rate of the coating with tempera-
ture ͑T͒, time ͑t͒, and pack composition, such as Si ͑WSi͒ and fluo-
ride activator content ͑WMF͒ of the pack. The main assumption
made in deriving Eq. 10 is constant composition in the coating lay-
ers with the variation of pack Si and activator contents, time, and
temperature of coating experiments. From Eq. 10, it is clear that
thickness of the coating would increase if the pack Si and activator
content is increased at constant T and t. Equation 10 suggests that
1/2
coating thickness would increase linearly with W at constant T, t,
Si
and activator content in the pack. Most importantly, a linear rela-
tionship between the coating thickness ͑h͒ and pack activator con-
tent ͑WMF͒ is predicted by Eq. 10. Such a relationship between the
Figure 2. ͑Color online͒ ͑a͒ Cross-sectional SEM image and ͑b͒ diffusion
profiles of major elements in the coating layer formed during silicide coating
formation at 1100°C for 17.5 h.

Journal of The Electrochemical Society, 155 ͑12͒ D734-D741 ͑2008͒
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120
silicide
alumino-silicide
100
80
60
40
20
0
0
1
2
3
4
5
6
Time1/2 (h1/2)
Figure 4. ͑Color online͒ Kinetics of silicide and alumino-silicide coating
growth at 1100°C.
is consistent with the prediction of Eq. 10 with constant temperature
͑T͒ and pack Si and NH4F content. The straight line in Fig. 5 is a
least-squares fit to the data points, which gives
h = 16.97t1/2 + 14.12
͓11͔
where h is in microns and t in hours. A relatively high offset value of
14.12 was probably due the slow heating and cooling rates, and a
relatively higher temperature ͑1100°C͒ for the coating process.
Linear relationship was also observed while h was plotted
against t1/2 ͑Fig. 4͒ for alumino-silicide coating experiments con-
Figure 3. ͑Color online͒ ͑a͒ Cross-sectional SEM image and ͑b͒ diffusion
profiles of major elements in the coating layer formed during alumino-
silicide coating formation at 1100°C for 17.5 h.
ducted at 1100°C using
a constant pack composition of
10Si–10Al–20NH4F–60Al2O3 ͑in weight percent͒ for the time in-
tervals of 1, 5, 10, 17.5, and 25 h. The least-squares fit of the data
points yielded the following linear relationship
h = 10.94t1/2 + 21.99
͓12͔
layer was formed by molybdenum di-silicide ͑MoSi2͒ and the inner
layer consisted of Mo5Si3. The concentration of other alloying ele-
ments like Ti was found to be uniform ͑ϳ0.6 wt %͒ throughout the
cross section.
The lower slope ͑10.94͒ of the line for alumino-silicide coating
compared to that ͑16.97͒ for silicide coating indicated a lower
growth rate of the coating for the alumino-silicide coating process.
For the formation of alumino-silicide coatings, different pack
compositions were tried at 1100°C with varying time. The pack
composition yielding a sufficiently thick and desired coating com-
position was found to be 10Si–10Al–20NH4F–60Al2O3 ͑in weight
percent͒. Like silicide coating specimens, the coated surface was
smooth and much brighter in appearance. Specimen weight gain of
14.83 mg cm−2 corresponding to a coating thickness of ϳ61.67 ␮m
was observed at 1100°C for 17.5 h. Figures 3a and b shows the
cross-sectional SEM image and the concentration profiles of the Si,
Al, and Mo measured by EDS along the marked line. The outer
Effect of temperature.— The
͑800–1200°C͒ was studied using
effect
of
temperature
a pack of composition
10
9
layer ͑ϳ52 ␮m͒ consisted of Mo͑Si,Al͒ phase with a constant Al
2
8
concentration of ϳ9.89 at %. Reduction in Si ͑59.74–20.3 at %͒
concentration and increase in Al ͑9.89–34.35 at %͒, and Mo ͑30.54–
45.35 at %͒ percentage was observed in the inner layer ͑ϳ10 ␮m͒.
According to the elemental composition, the inner coating layer
could be constituted with a Mo͑Al, Si͒ type of phase. The compo-
sition of the coated layers varied drastically at different processing
conditions as two elements of different chemical nature deposited
simultaneously during the coating process.
7
6
5
Effect of time on coating thickness.— The effect of time
͑1–25 h͒ on silicide coating thickness was studied at a constant
temperature ͑1100°C͒ using
10Si–10NH4F–80Al2O3 ͑in weight percent͒. The time was varied
from 1 to 25 h. A linear relationship was obtained when the coating
thickness ͑h͒ was plotted against t1/2 presented in Fig. 4. This result
6
7
8
9
10
a
pack composition of
10000/T
Figure 5. Dependence of silicide coating thickness on temperature at con-
stant coating composition and time of 10 h.

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Journal of The Electrochemical Society, 155 ͑12͒ D734-D741 ͑2008͒
70
80
60
40
20
0
60
50
40
30
0
1
2
3
4
2
4
6
8
10
1/2
WSi
NH4F (wt%)
1/2
Figure 6. Dependence of coating thickness ͑h͒ on Si content ͑W ͒ of the
Si
Figure 7. Linear dependence of coating thickness on NH4F ͑maximum up to
10 wt %͒ content in the pack.
pack for siliconizing at 1100°C for 10 h using constant NH4F content.
Effect of NH4F content in the pack.— The effect was studied by
varying the pack NH4F content from 2 to 20 wt % while keeping
the pack Si content constant at 10 wt %. All the coating experiments
were conducted at 1100°C for 10 h. The coating thickness increased
with increasing NH4F content up to 10 wt %. A lower coating thick-
ness was obtained for 20 wt % NH4F in the pack. As the NH4F
content was increased, a lot of NH3 and HF were generated with
increasing vapor pressure at higher temperatures. This caused cracks
in the cement layer at the top of the crucible, and most of the vapors
went out with the argon flow. Therefore, the quantities of halide
vapors available for the coating to grow were very small. Coating
thickness ͑h͒ was plotted against NH4F content up to 10 wt % and
presented in Fig. 7, which confirms the linear relationship predicted
by Eq. 10. The straight line in Fig. 7 is a least-squares fit of the
experimental data points producing the best fit ͑R2 = 0.9994͒ com-
pared to the other results presented in the previous sections, giving
10Si–10NH4F–80Al2O3 ͑in weight percent͒ at constant duration of
10 h. The pack composition used for these experiments was main-
tained as 10Si–10NH4F–80Al2O3 ͑in weight percent͒. The coating
thickness increased with increase in temperature. Si concentration in
the outer layer was constant, and there was a marginal increase in
the thickness of the inner Mo5Si3 layer. As confirmed by XRD, the
outer layer formed at all temperatures was found to be MoSi2.
Therefore, the temperature affected only the coating thickness or the
growth rate, not the Si concentration at the outer and the inner layer
of the coating. It was observed that the temperature had a more
significant effect than coating time ͑t͒, especially at higher tempera-
tures. Equation 10 could be rewritten as
Ea
1
ln͑T1/2h͒ = −
+ ln k0 + ln͑WSit͒ + ln͑WNH F
͒
͓13͔
4
RT
2
Hence, at constant t, WSi, and WNH F, the activation energy of the
4
h = 2.95WNH F + 32.57
͓15͔
4
silicide coating growth process in the temperature range of
800–1200°C can be obtained from the slope of the ln͑T1/2 h͒ vs 1/T
plot. Figure 5 shows the good linear fit between ln͑T1/2 h͒ and 1/T
͑by least-squares method͒ to the experimental data points. The acti-
A positive offset could be due to a significantly high growth rate of
the coating even at lower NH4F content in the pack.
Considering all the results presented in the previous sections, the
average value of k0 was calculated and found to be
984.89 ␮m h−1/2 K1/2. Thus for the formation of silicide coating on
Mo–TZM alloy using NH4F ͑Ͻ10 wt %͒ activated pack in the tem-
perature range of 800–1200°C, the final relationship of coating
thickness with other process parameters becomes
vation energy obtained from the slope was 48.06 Ϯ 4.85 kJ mol−1
.
This is a qualitative value for the overall coating process. Equation
13 could not be applied for alumino-silicide coating process due to
the variation in composition observed in the outer and the inner
layers of the coating formed at different temperatures.
984.89
T1/2
5781.2
1/2
h =
W
WNH Ft1/2 exp −
͓16͔
ͩ ͪ
Effect of Si content in the pack.— To study the effect of the
pack Si content, the experiments were conducted at 1100°C for 10 h
using a fixed NH4F ͑10 wt %͒ composition. Si content was varied
between 1 and 10 wt %. In general, coating thickness was increased
with increasing Si content, but the compositions at the outer and the
inner surfaces were the same as Fig. 2a. Therefore, the variation in
Si content in the pack did not affect the composition of the coating
which was one of the key assumptions in deriving Eq. 10. The thick
outer layer consisted of MoSi2 and the inner layer was of Mo5Si3.
The growth rate of the inner layer was marginal with respect to that
Si
4
T
where h is in microns, WSi and WNH F in weight percent, t in hours,
4
and T in degrees Kelvin. Equation 16 may be used to predict the
growth of a coating thickness at any specified values of Si ͑weight
percent͒, NH4F ͑weight percent͒, time ͑in hours͒, and temperatures
of Ͻ1200°C.
Relationship between h and m
.— Coating thickness ͑h͒ and
specimen weight gain ͑m͒ obtained from various experiments on
formation of silicide coating has been plotted against each other in
Fig. 8 ͑open squares͒. The solid line is a least-squares fit ͑R2
= 0.994͒ of the experimental data points, which gives
of the outer layer. Figure 6 shows the linear relationship between h
1/2
and W as predicted by Eq. 10. The straight line is a least-squares
Si
fit of the experimental data points, which gives
1/2
h = 4.85m + 2.08
͓17͔
h = 24.10W − 9.86
͓14͔
Si
Despite a little offset, the slope ͑4.85 ␮m mg−1 cm2͒ is close to the
The negative offset could be due to the slower initiation of the
coating process.
theoretical value of 4.24 ␮m mg−1 cm2 calculated from Eq. 7 using

Journal of The Electrochemical Society, 155 ͑12͒ D734-D741 ͑2008͒
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100
80
60
40
20
0
silicide
alumino-silicide
0
5
10
15
20
25
Weight gain (mg/cm2)
Figure 8. ͑Color online͒ Coating thickness against specimen weight gain of
silicide and alumino-silicide coatings.
a theoretical density value of 6.24 g cm−3 for MoSi2. It is observed,
in all the coating conditions, that the thickness of the inner Mo5Si3
layer is close to 5% of the total coating thickness. Applying this
condition and using equivalent values of Mc, ␳c, and y in Eq. 7, the
theoretical value of the slope ͑k1͒ is calculated to be
4.62 ␮m mg−1 cm2, which is closer to the experimental value
͑4.85 ␮m mg−1 cm2͒. This result also confirms the validity of the
proposed model in Eq. 10.
Figure 8 ͑solid circles͒ also shows the plot of h vs m for alumino-
silicide coating experiments. It is evident from the figure that the
least-squares linear fit is not very good ͑R2 = 0.935͒ and yields
Figure 9. Cross-sectional optical image of etched silicide and alumino–
silicide coated samples.
h = 3.38m + 15.85
͓18͔
which has a very high offset value. This could be due to the differ-
ence in vapor pressures of Si and Al fluorides producing the com-
position difference in the coating layers under different experimental
conditions. As the coating composition and thickness of different
layers vary with temperature, the validity of Eq. 7 has not been
tested for alumino-silicide coating.
section of the silicide-coated sample, which shows that MoSi2 layer
has a columnar microstructure due to a preferred direction of growth
parallel to the diffusion direction. Mo–TZM substrate shows the
annealed microstructure of average grain size 44 ␮m adjacent to the
thin Mo5Si3 inner layer.
Discussion
Assumptions made during derivation of Eq. 10 were validated
from the experimental results. Equation 10 was derived on the basis
of two major assumptions, namely: ͑i͒ the constant phase formation
in the coating layer for all the experimental conditions and ͑ii͒ SiF2
as the major vapor species responsible for Si migration and deposi-
tion of Si. Both the conditions were found to be satisfied from the
experimental results. Therefore, Eq. 16 could be used to predict the
thickness of the coating to be formed in a particular pack composi-
tion, time, and temperature of interest. A small deviation from the
actual results may be observed for lower Si and NH4F contents
͑Ͻ1 wt %͒ due to the offsets in Eq. 14 and 15, which is inherent to
any actual processes.
Codeposition of Si and Al on the Mo–TZM substrate resulted in
formation of Mo͑Si,Al͒2 and Mo͑Al, Si͒ phases in the outer and the
inner layers, respectively ͑Fig. 9b͒. The condition for codeposition
of two ͑Al plus Si͒ or more elements by pack cementation is that the
partial pressures of gaseous species in the pack for the elements to
be deposited have to be comparable. Figure 10 shows the calculated
equilibrium partial pressures of all Si and Al fluorides for a pack
composition of 10Al–10Si–10NH4F–70Al2O3 ͑in weight percent͒ at
different temperatures. For the fluoride-activated process, AlF is re-
sponsible for deposition of Al on the metal surfaces.21 It is evident
from Fig. 10 that the partial pressures of SiF2 and AlF are compa-
rable at all the temperatures. But AlF is less stable compared to SiF2
as dictated by the standard free energy of formation for different
In the pack cementation process, the chemical potential gradient
is the main driving force for diffusion of halide vapors ͑e.g., SiF2͒
from the surroundings to the substrate surface in the pack. It was
observed in the case of silicide coating ͑siliconizing͒ experiments
that the Si concentration in the outer layer remained constant at all
time intervals. This indicates that Si concentration reaches a con-
stant value within a very short time of the coating process, main-
taining a thermodynamic equilibrium with the partial pressure of
SiF2 vapors at the vapor/coating interface during the process. After a
certain time interval, a constant source of Si exists at the substrate
surface forming a diffusion couple between Mo and Si. There are
three silicide phases, Mo3Si,Mo5Si3, and MoSi2, identified on the
Mo–Si binary equilibrium phase diagram.16 Tortorici and
Dayananda36,37 did a detailed investigation on the Mo–Si diffusion
couple and reported that the Mo3Si layer could not form below
1350°C due to nucleation difficulties. The thickness of the Mo5Si3
layer formed near Mo surface was very small. The growth rate of the
MoSi2 was much faster than that of Mo5Si3 in the temperature range
of 900–1350°C. Our results were consistent with these earlier re-
sults. Thickness of the inner Mo5Si3 layer was Ͻ5% of the major
coating layer ͑MoSi2͒ in most of the coated specimens. Absence of
pack particles in the coating ͑MoSi2͒ layer suggests that the inward
diffusion of Si along tetragonal MoSi2 phase is the dominant diffu-
sion mechanism. Figure 9a is an etched optical image of the cross

D740
Journal of The Electrochemical Society, 155 ͑12͒ D734-D741 ͑2008͒
0
SiF
SiF
SiF
SiF
H2
AlF43
AlF32
AlF2
-5
-10
-15
1100
1200
1300
1400
1500
1600
Temperature (K)
Figure 10. Equilibrium partial pressures of the major vapor species in the
10Al–10Si–10NH4F–70Al2O3 ͑in weight percent͒ pack.
fluorides presented in Table I. Formation of the inner layer of
Mo͑Al, Si͒ rich in Al is due to the preferential dissociation of AlF at
the initial stages of the coating process. The subsequent coating
growth takes place by solid-state diffusion of both Al and Si. The
final phase to form in the outer layer is of MoSi2 type ͓i.e.,
Mo͑Si,Al͒ ͔ because this is the most stable congruently melting
2
compound.38 The bright luster observed in the alumino-silicide–
coated specimens was due to the presence of Al in the outer layer.
As the composition of the inner layer varies with temperature35 and
pack composition, Eq. 10 is not strictly valid for multiple element
͑Si and Al͒ coating on Mo–TZM as observed under different experi-
mental conditions. However, as reported earlier, both the silicide and
alumino-silicide coatings showed excellent performance under oxi-
dizing environment, even up to 1300°C.35
Fluoride activators, especially NH4F, is the most suitable activa-
tor for growing silicide coatings on Mo base alloys. To study the
effect of different activators on the growth of the coating, the sili-
conizing experiments were conducted at 1100°C for 10 h using the
same quantities ͑10 wt %͒ of NaF and NH4Cl. Si content of the
pack was kept constant at 10 wt %. Figure 11 shows the cross sec-
tion of the coatings formed for different activators using the same
quantities ͑10 wt %͒. It is clear that NaF produced the lowest thick-
ness ͑ϳ19 ␮m͒ followed by NH4Cl ͑ϳ22 ␮m͒ and coating growth
was maximum ͑ϳ63 ␮m͒ for NH4F under identical processing con-
ditions. The lower partial pressure of the fluorides24 formed in the
case of NaF activated pack could be the reason for lower coating
thickness. Similarly, the partial pressures of the chloride vapors of Si
formed in NH4Cl activated pack are also lower compared to fluoride
vapors in the NH4F-activated pack.39 Si diffusion profiles for all the
coatings are presented in Fig. 12, which shows that the inner and the
outer layers are formed by Mo5Si3 and MoSi2 phases, respectively,
for all the activators. Figures 11 and 12 confirm that the thickness of
the inner ͑Mo5Si3͒ layer is maximum for the NaF-activated process
and minimum for the NH4F-activated coating. The results indicate
that in the NaF-activated process the supply of Si on the coating
surface is reduced after a certain time interval. This causes reduction
in the chemical potential gradient between the surface and the sub-
strate. The inner Mo5Si3 layer grows further due to the existence of
a concentration gradient of Si between MoSi2 layer and Mo–TZM
substrate. Whereas for the NH4F activated pack, a constant vapor
pressure of SiF2 is maintained at the vapor/substrate interface caus-
ing the growth of the outer MoSi2 layer through out the processing
time. This effect also confirms the validity of Eq. 10 for predicting
the coating growth kinetics for the NH4F-activated siliconizing pro-
cess.
Figure 11. ͑Color online͒ Cross-sectional optical images of silicide-coated
samples using different activators ͑a͒ NaF, ͑b͒ NH4Cl, and ͑c͒ NH4F of the
same quantity ͑10 wt %͒, 10 wt % Si, and coating at 1100°C for 10 h.
Conclusions
The kinetic model Eq. 10 can be used to predict the growth of
silicide coatings formed on Mo–TZM substrate by pack cementation

Journal of The Electrochemical Society, 155 ͑12͒ D734-D741 ͑2008͒
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0
Kamat, IIT Bombay, Mumbai, India, for his support in SEM and
EDS measurements. The constant help of S. Gavai, MPD, BARC,
during all the stages of coating experiments is also greatly appreci-
ated.
NaF
NH4Cl
NH4F
Bhabha Atomic Research Centre assisted in meeting the publication costs
of this article.
References
1. M. S. El-Genk and J. M. Tournier, J. Nucl. Mater., 340, 93 ͑2005͒.
2. J. J. Park, D. P. Butt, and C. A. Beard, Nucl. Eng. Des., 196, 315 ͑2000͒.
3. E. Glenny and J. E. Restall, J. Less-Common Met., 9, 367 ͑1965͒.
4. B. V. Cockeram, Metall. Mater. Trans. A, 36, A1777 ͑2005͒.
5. M. G. Mendiratta, T. A. Parthasarathy, and D. M. Dimiduk, Intermetallics, 10, 225
͑2002͒.
6. G. R. Smolik, D. A. Petti, and S. T. Schuetz, J. Nucl. Mater., 283–287, 1458
͑2000͒.
7. G. L. Miller and F. G. Cox, J. Less-Common Met., 2, 207 ͑1960͒.
8. R. Mitra and V. V. R. Rao, Mater. Sci. Eng., A, 260, A146 ͑1999͒.
9. R. Mitra, V. V. R. Rao, and V. A. Rao, Intermetallics, 7, 213 ͑1999͒.
10. A. Stergiou and P. Tsakiropoulos, Intermetallics, 5, 117 ͑1997͒.
11. A. Stergiou, P. Tsakiropoulos, and A. Brown, Intermetallics, 5, 69 ͑1997͒.
12. T. C. Chou and T. G. Nieh, Scr. Metall. Mater., 26, 1637 ͑1992͒.
13. T. C. Chou and T. G. Nieh, Scr. Metall. Mater., 27, 19 ͑1992͒.
14. T. A. Parthasarathy, M. G. Mendiratta, and D. M. Dimiduk, Acta Mater., 50, 1857
͑2002͒.
0
20
40
60
m)
80
Depth from surface (
µ
Figure 12. Si concentration profiles in the coating layers formed at 1100°C
for 10 h in different activators.
15. V. Supatarawanich, D. R. Johnson, and C. T. Liu, Intermetallics, 12, 721 ͑2004͒.
16. Y. Liu, G. Shao, and P. Tsakiropoulos, Intermetallics, 8, 953 ͑2000͒.
17. K. Yanagihara, T. Maruyama, and K. Nagata, Intermetallics, 4, 133 ͑1996͒.
18. T. Maruyama and K. Yanagihara, Mater. Sci. Eng., A, 239–240, A828 ͑1997͒.
19. T. Tabaru, K. Shobu, H. Hirai, and S. Hanada, Intermetallics, 11, 721 ͑2003͒.
20. K. Yanagihara, T. Maruyama, and K. Nagata, Intermetallics, 3, 243 ͑1995͒.
21. Z. D. Xiang and P. K. Datta, Acta Mater., 54, 4453 ͑2006͒.
22. Z. D. Xiang and P. K. Datta, Scr. Mater., 55, 1151 ͑2006͒.
23. Z. D. Xiang, S. R. Rose, and P. K. Datta, Scr. Mater., 59, 99 ͑2008͒.
24. S. R. Levine and R. M. Caves, J. Electrochem. Soc., 121, 1051 ͑1974͒.
25. Z. D. Xiang, S. R. Rose, and P. K. Datta, Mater. Sci. Technol., 21, 1111 ͑2005͒.
26. Z. D. Xiang, S. R. Rose, and P. K. Datta, Mater. Sci. Eng., A, 356, A181 ͑2003͒.
27. Z. D. Xiang, S. R. Rose, and P. K. Datta, Mater. Chem. Phys., 80, 482 ͑2003͒.
28. S. Majumdar, P. Sengupta, G. B. Kale, and I. G. Sharma, Surf. Coat. Technol., 200,
3713 ͑2006͒.
process at temperatures below 1350°C, where MoSi2 forms the
main coating layer. The NH4F-activated process yielded the maxi-
mum coating growth compared to other activators under identical
processing conditions. Equation 10 was found to be valid in the
temperature range of 800–1200°C and the inter-relation between the
coating thickness ͑h, in microns͒, siliconizing temperature ͑T,
in degrees Kelvin͒, time ͑t, in hours͒, pack Si content ͑WSi in
weight percent͒, and NH4F activator content up to 10% ͑WNH F, in
4
weight percent͒ can be given by h = 984.89T−1/2WS1/i2WNH Ft1/2
4
exp͑−5781.2/T͒.
The pack composition for growing Mo͑Si,Al͒ coating
2
29. A. Mueller, G. Wang, R. A. Rapp, E. L. Courtright, and T. A. Kircher, Mater. Sci.
Eng., A, 155, A199 ͑1992͒.
of sufficient thickness at 1100°C was found to be
10Si–10Al–20NH4F–60Al2O3. Both the diffusion-controlled sili-
cide and alumino-silicide coating growth processes obeyed the lin-
ear relationship between h and t1/2 at constant processing tempera-
ture ͑1100°C͒.
30. B. V. Cockeram, Surf. Coat. Technol., 76–77, 20 ͑1995͒.
31. A. Mueller, G. Wang, and R. A. Rapp, J. Electrochem. Soc., 139, 1266 ͑1992͒.
32. R. Sakidja, F. Rioult, J. Werner, and J. H. Perepezko, Scr. Mater., 55, 903 ͑2006͒.
33. R. Sakidja, J. S. Park, J. Hamann, and J. H. Perepezko, Scr. Mater., 53, 723 ͑2005͒.
34. K. Ito, T. Hayashi, M. Yokobayashi, and H. Numakura, Intermetallics, 12, 407
͑2004͒.
Acknowledgments
35. S. Majumdar, I. G. Sharma, S. Raveendra, I. Samajdar, and P. Bhargava, Mater. Sci.
Eng., A, 492, A211 ͑2008͒.
The authors thank Dr. S. Banerjee, Director, Bhabha Atomic Re-
search Centre, for his guidance and consistent encouragement for
pursuing the activities on surface coating on refractory metal alloys.
The authors are also thankful to Dr. A. K. Suri, Director, Materials
Group, BARC, for his constant support and advice, and to Dr. S. L.
36. P. C. Tortorici and M. A. Dayananda, Scr. Mater., 38, 1863 ͑1998͒.
37. P. C. Tortorici and M. A. Dayananda, Metall. Mater. Trans. A, 30, A545 ͑1999͒.
38. R. W. Walser and R. W. Bene, Appl. Phys. Lett., 28, 624 ͑1976͒.
39. S. Majumdar, I. G. Sharma, and A. K. Suri, Int. J. Refract. Met. Hard Mater., 26,
549 ͑2008͒.