Investigation of the formation mechanism of combustion synthesized Fe-TiC clads on steel
Abtin Rahimi-Vahedi, Mandana Adeli, Hassan Saghafian
School of Metallurgy and Materials Engineering, Iran University of Science and Technology,
Narmak, Tehran 1684613114, Iran
Corresponding author: Mandana Adeli (E-mail: adelim@iust.ac.ir, Tel.:+9821 7745 91 51,
Fax: +9821 77240 4 80).
Abstract
The application of combustion synthesis process in thermal explosion mode to form Fe-TiC
composite layers on a steel substrate was investigated. Starting powder mixtures of iron,
titanium, and carbon with different percentages of iron were cold-pressed onto a steel hollow
disk and exposed to high heating rates under an argon atmosphere. A self-sustaining, exothermic
reaction took place in all samples. Samples with lower Fe amounts (higher amount of Ti+C)
underwent too intensive reactions, resulting in poor clad/interface adhesion due to high amount
of pores, while samples with higher Fe contents demonstrated increasingly better adhesion.
Microstructural studies on samples with lower Fe amounts showed that complete melting and
dissolution of elements occurred as a result of exothermic heating. Following a dissolutionprecipitation mechanism, the final microstructure was composed of granular TiC particles which
were dispersed in a Fe-rich matrix. Samples with higher amounts of Fe showed partial reaction
on Ti particles, which resulted in reaction products with layers of varying chemical composition.
Due to the acting of iron particles as diluent, the heat released by the exothermic reactions was
absorbed by iron, leading to a deficiency of exothermic heat for the completion of reaction. The
distribution of hard particles throughout the softer iron matrix can improve the overall hardness
of the clad layer.
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Keywords: Combustion synthesis; Cladding; Microstructure; Fe-Ti-C composite; Mechanism;
Scanning Electron Microscopy
Introduction
TiC is a well-known, highly stable ceramic material with low friction coefficient, high hardness,
very high melting temperature, and low density [1, 2]. When dispersed in a metallic matrix, TiC
particles can improve wear resistance by performing as bearings for sliding surfaces and
reducing the friction coefficient. Therefore, it has been used as a reinforcement material in a
wide variety of metal matrix composites (MMCs) [3-11]. Among these, Fe-based MMCs offer
several advantages such as availability and lower cost in many applications [3]. Besides, they are
chemically compatible with steels and can be used to provide the surface with improved wear
resistance, hardness, and stiffness when applied as coatings.
The production methods for Fe-TiC composites such as conventional melting and casting,
powder metallurgy, thermite reduction, combustion synthesis, and carbothermic reduction of
ilmenite have been reviewed by Das et al. [12]. However, most of these methods are not suitable
to form a Fe-TiC composite coating on a Fe-based substrate. Combustion synthesis is one of the
methods which can be used to apply a composite layer on a substrate. The process which takes
use of the exothermic reactions between starting elements, has proved to be a fast, simple, and
energy saving method for producing various refractory compounds including composites and
cermets [13-16].
The implementation of the combustion synthesis process to produce Fe-TiC composites as well
as TiC-Fe cermets has been studied in several research works. Saidi et al. [17] investigated the
production of TiC and Fe-TiC via the combustion synthesis process, and measured ignition and
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combustion temperatures exceeding 1500°C and 2500°C, respectively, in the reaction between Ti
and C. They added different amounts of iron as diluent to Ti+C equimolar mixture and found out
that by adding even small amounts of iron a significant decrease in the ignition temperature was
observed. They concluded that in the presence of iron, the ignition temperature was dictated by
the eutectic temperature of the Fe-Ti system, not solid state reaction between Ti and C. An
increase in the amount of iron resulted in a decrease in the calculated combustion temperatures
because of the consumption of exothermic heat by iron. Fan et al. [18] investigated the formation
of TiC-Fe cermets using the combustion synthesis reaction between Ti and C in the presence of
30% wt.%Fe. They suggested that in the presence of high amounts of Ti+C, carbon diffuses into
iron resulting in the formation of lower-melting Fe-C alloys. The melting accelerates the
dissolution of Ti followed by the precipitation of TiC particles out of the saturated melt. Feng et
al. [19] synthesized TiC-Fe cermets under the action of an electric field. They found out that
under the effect of electric field, both Ti+C=TiC and 2Fe+Ti+Fe2Ti can take place at relatively
low temperatures. With increase in temperature, a part of Fe2Ti may decompose. The product
microstructure was consisted of Fe, TiC, and Fe2Ti. Similar mechanisms are also reported by
other researchers [20-22]. Most of these papers discuss the production of bulk Fe-TiC
composites or TiC-Fe cermets. To the best of authors’ knowledge, the process has never been
used to apply hard Fe+TiC coatings or clads on Fe-rich substrates, although other methods such
as laser cladding or plasma spraying have already been tried to form such coatings. Emamian et
al. [23] used the laser cladding process parameters to form a Fe-TiC clad on AISI 1030 steel.
They found out that the formation as well as morphology of TiC is highly influenced by the laser
process parameters. Wang et al. [24] used laser cladding to form in situ TiB2+TiC/Fe coatings on
AISI 1045 steel from initial B4C-TiO2-Al powders and reported an excellent wear resistance for
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the coating. In some research works, laser-synthesized Fe-TiC coating on steel was formed by insitu reaction of ilmenite or ferrotitanium with graphite [25, 26]. Algodi et al. [27] used the EDC
(Electric Discharge Coating) process to apply a Fe-TiC cermet coating on 304 stainless steel.
They developed localized melt pools between a TiC electrode and the steel substrate, which
resulted in the coverage of steel by a cermet layer. Cliché and Dallaire [28] and Li et al. [29]
used simultaneous synthesis and deposition of TiC-Fe coatings by plasma-spraying and reactive
flame spraying, respectively. Licheri et al. [30] used the combustion synthesis process to produce
TiC-Fe composite, and subsequently employed it as a coating on two types of steel by plasma
spraying. Wang et al. [31] used the GTAW multi-layers process to form a Fe-TiC composite
layer from a mixture of graphite and ferrotitanium (FeTi), and obtained very high values of
hardness as well as excellent wear resistance for the coating.
Since the potential of combustion synthesis to form Fe-based composites coatings on steel has
never been studied, the following study was undertaken to study the applicability of the thermal
explosion mode of combustion synthesis to directly form in situ Fe-TiC layers on 316 stainless
steel. The conditions of obtaining a good clad-to-substrate adhesion as well as the
microstructural evolution and reaction mechanisms using different amounts of iron in the
reacting mixture were studied.
Material and Methods
Titanium (45µm ave., 99.9%) and graphite (<10µm, 99.9%) powders were mixed at an equiatomic ratio. Different quantities of iron (<60 µm, 99.5%) were added to obtain Fe weight
percentages of 50, 60, 70, 80, and 90. The powder mixture was thoroughly hand-mixed. Hollow,
cylindrical steel SS316 disks with external diameter of 25mm and height of 5mm were used as
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substrate. The cavity in the cases was 23mm in dia. and 3mm in depth. The inner surface of the
steel case was polished, completely degreased, and washed with acetone. The case was filled
with the powder mixture, which was cold-pressed in the hole with a force of ~50kN using
matching steel punch and matrix as schematically shown in Fig. 1.
The samples were then placed in a tube furnace already heated to 1200°C. A mild flow of argon
gas (1 l/min) was maintained during the synthesis process. It took only a few seconds for the
powder mixture to react inside the steel case. Immediately after the beginning of the reaction, the
furnace was turned off to allow the reaction to continue in a self-sustained manner. The samples
were cooled under argon gas and then transversally cut. The cross sections were ground,
polished, and examined using Scanning Electron Microscopy (SEM). A Vega-TESCAN SEM
equipped with Energy Dispersive Spectrophotometer (EDS) was used to perform microstructural
investigations and analyses on the samples. A Jeol JDX-8030 X-ray diffractometer using Cu kα
radiation (λ~1.5418Å) was exploited to determine the product phases. Microhardness tests were
conducted on the composite layer using a Vickers Microhardness tester with a 300g load to study
the reinforcing ability of the reaction products.
Results and Discussion
The percentage of Ti+C in the starting powder mixture was adopted as 10, 20, 30, 40, and 50%
by weight. Heating the samples at a high heating rate gave rise to the combustion synthesis
reaction, which started almost immediately after the samples were placed in the furnace. This is
because of the high enthalpy of the reaction between titanium and carbon to form TiC (-184
kJ/mol.Ti [17]). Observations showed that regardless of the composition of the powder mixture,
the beginning of the exothermic reaction was accompanied with intense glowing of the sample
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and rising of fumes from the reacting powders. The heat released as a result of the exothermic
reactions was sufficient not only to make the reaction propagate throughout the whole mixture,
but also to melt (or partially melt) iron particles, form a continuous matrix, and establish bonds
between the clad layer and the substrate. Although it was reported in previous studies that Fe-TiC powder mixtures with iron contents of more than 60% would not show a self-sustaining
behavior during combustion synthesis [30, 32], in our study the combustion-like reaction was
initiated and finished regardless of the percentage of iron in the powder mixture. This may be
because of the containment of the powder mixture inside the steel case, which maintains the
conditions close to adiabatic; under these conditions, the heat loss by radiation and convection is
exceeded by that generated as a result of exothermic reactions. No deformation occurred in
samples containing 60-90wt.% Fe; however, in the sample with 50wt.% Fe the reaction was too
intense, causing “swelling” in the central part of the clad. The appearance of the products with
50 wt.% and 60 wt.% Fe are compared in Fig. 2, in which the swelled part is shown by an arrow.
It was concluded that too high amounts of Ti+C in the powder mixture can result in too much
exothermicity, too intense reactions as a result of more effective contact between Ti and C,
entrapment of released gases at the coating/substrate interface, and deformation of the clad layer.
The surface oxidation also seems to be more significant in samples with higher amounts of Ti+C.
Typical XRD patterns of products of the combustion synthesis process are presented in Fig. 3.
XRD analysis showed the presence of Fe and TiC as the dominant phases in the sample with
70wt.%Fe. TiC peaks were detected in sample with 80wt.% Fe as well, although their intensity
seemed to have decreased while the Fe peaks appeared stronger. In sample with 90wt.% Fe, TiC
peaks were not observed, which could be due to the small percentage of TiC. The formation of
the desired TiC phase in some of the products shows that the combustion synthesis can be
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successfully accomplished in such situation; however, some minor peaks are also detected in the
patterns which may belong to other phases present in the microstructure as discussed later.
SEM micrograph showing the microstructure of the highly porous combustion-synthesized layer
in the sample with 60wt.% Fe is presented in Fig. 4. EDS analysis showed the presence of grey,
Ti-rich “islands” (grey regions) in an iron-rich matrix (bright regions) (Fig. 4(a)). The
composition of Point 1 is ~97.5wt.%Fe + ~2.5%Ti. Analysis of Point 2 as well as other points in
the regions inside the islands shows the co-existence of a high percentage of Ti along with
carbon and iron; however, because of the fineness of the layers and particles the accurate
percentage of elements as reported by EDS may not be reliable in these regions. There are also
particles of TiC formed both in the form of granules in boundaries between islands (white arrow
in Fig. 4(a)) and porous, isolated phases as shown in Fig. 4(c). Based on these, it is concluded
that Fe and Ti can form low-melting eutectic phases capable of dissolving other elements such as
C. As also suggested by other researchers [17, 33], intermetallic compounds such as Fe2Ti with a
low melting point (1085°C) can readily be formed as a result of solid-state diffusion between Fe
and Ti. By forming the first droplets of liquid phase, gradual dissolution of Ti and C particles in
the molten phase creates a ternary liquid in which considerable amounts of Ti and C can be
dissolved on further increase in temperature. Upon cooling, the reaction between Ti and C forms
stable TiC particles which precipitate from the ternary melt and result in a uniform distribution
of very fine, granular TiC in the Fe-rich matrix, and the remaining iron-rich part of the melt is
the last to solidify. Where appropriate contact between Ti and C is provided and the local
temperature is sufficiently high, solid-state reaction between Ti and C can also result in the
formation of individual TiC particles similar to that shown in Fig. 4(c). The mechanism is similar
to that proposed by Fan. et al. [34] as a dual-dissolution-precipitation mechanism, the result of
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which is the precipitation of TiC particles out of a saturated Fe phase, the fusing of Fe and Ti
particles, and a final microstructure consisting of TiC particles in an Fe-rich matrix. The results
show that although the temperature may not be as high to initiate the solid-state reaction between
Ti and C throughout the powder mixture, Fe promotes the reaction by the formation of ternary
liquid phases.
Inspection of the clad/substrate interface in the sample with 60wt.%Fe in the starting powder
showed that limited, discontinuous bond was established between the clad and the substrate as
typically shown in Fig. 5. The micrographs confirm the formation of liquid pools which advance
into the substrate as shown in the figure. TiC is sometimes precipitated in these regions in the
form of clusters. In other places, excessive amount of pores and release of gases during the
reactions results in poor clad-to-substrate adherence. The appearance of the interface is similar to
some plasma-sprayed TiC-Fe coatings on steel [27].
SEM micrographs showing the cross section of samples with 70, 80, and 90wt.%Fe are presented
in Fig. 6. These figures show that in order to obtain an acceptable metallurgical bond between
the substrate and the clad, higher amounts of iron should be present in the composition of the
composite layer. There are similarities in the microstructure of the samples with 70wt.% and
80wt.%Fe. The 90wt.%Fe sample shows good adhesion even at the corners of the steel case;
however, the microstructural details seem to be slightly different. Examination of the samples by
SEM also reveals the presence of pores throughout the microstructure, especially in 70wt.%Fe
sample. Porosity, which is characteristics of combustion- synthesized products, can have
different sources: the pores in the green, compressed powder mixture, the difference in the molar
volumes of reactants and products, and the vaporization and expulsion of volatile impurities [35].
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The microstructures seem to be essentially consisted of reinforcing particles (grey Ti-rich
phases) dispersed in a Fe-rich matrix. As shown in Fig. 7 for the 70wt.%Fe sample, some of the
grey particles (as shown by black arrow in Fig. 7(a)) seem to be the product of complete
combustion reaction between Ti and C.
EDS analysis confirms the presence of Ti, Fe and C in this phase, although determining the exact
amount of C was again not possible. As shown by EDS analysis, there is also a certain amount of
iron in this phase, showing the approximate ratio of Ti/Fe to be 86/14 (excluding carbon). It is
also seen that unlike the sample with 60wt.%Fe, the exothermic reactions between constituents
are no longer sufficient for complete melting of iron and intermixing of the elements, and iron
particles seem to have only been sintered.
There also exist some larger particles with a layered structure in the microstructure of the same
sample, as typically shown in Fig. 8. EDS analysis of the dark and light grey layers as well as the
bright core is presented in Fig. 9.
EDS analysis showed that these particles are embedded in a Fe-rich matrix (Point 1 in Fig. 8(a):
99wt.%Fe, Point 2: 98wt.%Fe, 2%Ti). The outermost layer is consisted of Ti, C, and Fe with an
approximate Ti/Fe ratio of 86/14 (excluding carbon). The bright core shows the same ratio of Ti
and Fe with no carbon. The intermediate layer which has nucleated and grown on the inner side
of the carbide layer is highly pure Ti, with a very small amount of iron and carbon. The analysis
of Point 6 in Fig. 8(a) also shows almost the same amount of Fe as before (~14wt.%Fe) in TiC
particles. It can be deduced that the reaction invariably takes place by the diffusion of Fe in Ti
followed by the formation of local low-melting liquids, which can dissolve carbon and form a Ti
carbide layer with some dissolved Fe. The growth of carbide layer then continues towards the
core of Ti Particles, and will depend on the diffusion of titanium and carbon atoms from inside
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and outside of the carbide layer, respectively. Where finer Ti particles are involved, diffusion
processes are facilitated and the formation of TiC is completed (as denoted by 6 in Fig 8(a)). In
larger Ti particles, Ti-rich phase (α-Ti according to Ti-Fe binary phase diagram [22]) nucleates
on the inner boundaries of TiC layer upon cooling of the system, and the remaining melt with
chemical composition of β-Ti solidifies at the particle core.
Similar phenomena were observed in the sample with 80wt.% Fe. Fig. 10 shows the products of
incomplete carbide formation as well as X-ray elemental maps for the same region. The chemical
compositions of the layers are exactly the same as described for 70wt.%Fe sample. These are in
also in accordance with findings of other researchers investigating the combustion synthesis of
TiC-Fe cermets [36], who analyzed the mechanism of combustion synthesis of TiC-Fe cermet.
They also reported that the reaction in Ti occurs earlier than Fe, and the diffusion rates of atom
in Ti are higher than that into Fe. They suggested that a solid state reaction occurs in Ti particle
by ternary-reaction-diffusion of C and Fe into Ti. The reaction starts from Ti surface and
propagates towards the core of Ti particles, resulting in the formation of TiC and a Ti-rich
binder. In our case, we encountered both completely- and partially-reacted particles.
Closer view of the microstructure of the 90wt.%Fe sample (Figs. 6(c) and (d)) is shown in Fig 11
along with X-ray elemental map of the same zone. The TiC layer has begun to form by diffusion
of carbon to the Ti-Fe molten liquid. Since the duration of exposure of the samples to high
temperatures was very short and the exothermic reactions were frustrated shortly after their
ignition, sufficient time for the completion of the diffusion processes and the formation of a
continuous carbide layer around the Ti particle could not be provided. However, the iron-lean
line around the particles, the accumulation of carbon behind this line, and the formation of a
porous line around the Ti particle, shows that the formation of molten phase had started around
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the Ti particle. The released heat has been enough to produce a sintered, continuous iron matrix.
The clad layer shows very good adherence onto the substrate which shows that partial melting
and joining at the interface region has occurred despite the shortage of time and lower amounts
of exothermic reagents.
Vickers microhardness tests were executed to evaluate the hardness of the formed phases.
Microhardness traces in the reaction products as well as matrix for the 80wt.%Fe sample are
typically shown in Fig. 13. Obtaining a precise hardness value for the clad layer would not be
possible because of the presence of various phases as well as pores. The tests showed values
ranging from 350-450HV (Fe-rich matrix) to 800-1970HV (reaction products). This shows that
with using finer titanium particles and obtaining a uniform distribution of hardening particles,
coatings with considerable hardness values can be provided.
Conclusions
Combustion synthesis was used to apply a composite Fe-TiC layer on steel by igniting the
exothermic reaction in Fe+Ti+C powder mixture. The amount of iron varied in the mixture to
obtain composite layers with 50, 60, 70, 80, and 90 wt.%Fe. In powder mixtures with low
amounts of Fe and higher amounts of Ti+C, the exothermic reaction could be too intense, leading
to unacceptable levels of deformation in the clad layer as encountered in 50wt.%Fe sample.
Samples with higher Ti+C amount also showed poor clad/substrate adhesion due to more intense
exothermic reaction leading to high amounts of porosity and excessive entrapment of gases in the
boundary. On the contrary, samples with higher Fe contents showed increasingly better clad-tosubstrate adhesion. Based on the SEM observations and EDS results, a mechanism was proposed
for the formation of hard Ti-Fe-C particles throughout the structure. The formation of carbide
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particles initiates from the formation of low-melting liquid phases between Ti and Fe, dissolution
of carbon in this melt, and its reaction with Ti. In samples with lower amount of iron where
higher exothermic reactions and higher temperatures are involved, melting and intermixing of
elements take place, and a dissolution-precipitation mechanism governs the formation of fine,
granular TiC particles from a Ti-Fe-C melt. In low-exothermicity conditions, there is a shift in
mechanism, and the carbide particles tend to form following a reaction-diffusion mechanism.
With sufficiently small Ti particles, the completion of formation of carbide could be possible. It
was concluded that the degree of reaction and the morphology of products are highly influenced
by the amount of iron as diluent. Considering factors such as the clad-to- substrate adhesion and
microstructural features, composite clad layers with ca. 20wt.% Ti+C can be proposed as the
most appropriate composition in our study. With a fine and uniform distribution of hard particles
throughout the matrix, high values of hardness can be reached in the clad layer.
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Figure captions
Fig. 1. Schematic illustration of the cold-pressing procedure.
Fig. 2. Comparison of appearance of the clad layer after combustion synthesis in powder
mixtures with 50wt.% and 60wt.% Fe. White arrow shows the “swelled” region of the sample.
Fig. 3. Typical XRD patterns of the products of the combustion synthesis in powder mixtures
with 70, 80, and 90wt.% Fe.
Fig. 4. SEM micrographs of the combustion synthesis product with 60wt.% iron in the starting
powder mixture.
Fig. 5. SEM micrograph of the interface of 60wt.%Fe sample showing occasional bonds between
the clad and the interface.
Fig. 6. SEM micrographs of clad/substrate interface with varying amounts of Fe in the clad, (a)
70wt.%Fe, (b) 80wt.% Fe, (c) 90wt.%Fe (d) 90wt.%Fe at the corner of the steel case.
Fig. 7. SEM micrograph as well as EDS analysis of complete reaction products in the
microstructure shown in Fig. 6(a) (70wt.%Fe).
Fig. 8. SEM micrograph (a) and elemental X-ray maps (b) of the layered reaction products in the
microstructure shown in Fig. 6(a) (70wt.%Fe).
Fig. 9. EDS analyses of the successive layers of the reaction products in the 80wt.%Fe sample
(points 3, 4, and 5 in Fig. 8(a).
Fig. 10. SEM micrograph (a) as well as elemental X-ray maps (b) of the reaction products in the
microstructure shown in Fig. 6(b) (80wt.%Fe).
Fig. 11. SEM micrograph (a) as well as elemental X-ray maps (b) of the reaction products in the
microstructure shown in Figs. 6(c) and (d) (90wt.%Fe).
Fig. 12. Microhardness traces on the matrix and the reinforcement phase (80wt.%Fe sample).
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