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Applied Surface Science 258 (2012) 5583–5592
Contents lists available at SciVerse ScienceDirect
Applied Surface Science
journal homepage: www.elsevier.com/locate/apsusc
On microstructure and flexural strength of metal–ceramic composite cladding
developed through microwave heating
Apurbba Kumar Sharma ∗ , Dheeraj Gupta
Department of Mechanical and Industrial Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, India
a r t i c l e
i n f o
Article history:
Received 10 July 2011
Received in revised form
20 December 2011
Accepted 6 February 2012
Available online 3 March 2012
Keywords:
Surface modification
Microwave heating
Composite cladding
Microstructure
Flexural strength
a b s t r a c t
A domestic multimode microwave applicator was used to develop carbide reinforced (tungsten-based)
metal–matrix composite cladding on austenitic stainless steel substrate. Cladding was developed through
microwave irradiation of the preplaced clad materials at 2.45 GHz for 420 s. Clads show metallurgical
bonding with substrate by partial dilution of materials. Back scattered images of clad section confirm
uniformly distributed reinforced particles in the metallic matrix. Presence of WC, W2 C, NiSi, NiW and
Co3 W3 C phases was detected in the clad. Flexural characteristics show two distinct load transitions
attributable to deformations of the matrix and the reinforced particles. Clads fail at the upper transition
load; further load is taken by the SS-316 substrate. Clads exhibit good stiffness and good adhesion with
the substrate. Multi directional cracks were observed at the clad surface; on further loading, cracks get
propagated into the clad thickness without getting peeled-off. Mechanism of clad development has been
introduced.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Austenitic stainless steel is widely used in many mechanical
components and structures in spite of its poor wear performance.
The classical approach to tackle wear problem is to design a new
wear resistant material depending on the dominating wear modes.
Wear is a surface related phenomenon and therefore, replacement
of the bulk component body by a newly designed material, often
may not be a cost-effective solution. However, in order to meet the
increasingly demanding operating conditions, the functional surface(s) of the concerned component may be redesigned (treated)
in such a way that they sustain in the aggressive environment. It is
thus possible to apply surface treatment selectively to the surface
of interest of a component while without compromising on other
requirements of the bulk material, for example, strength and corrosion resistance of austenitic stainless steels. Cladding is one of
the most commonly used pragmatic surface treatment methods in
which the chemistry of the target surface is changed with a material
system of desired properties. Studies show that hard materials, if
cladded on a soft substrate surface can effectively prevent ploughing on the rubbing surface and hence reduce wear [1]. Huang et al.
reported that cladding of hard carbides with metallic binder in the
form of a metal–matrix composite (MMC) can improve the wear
∗ Corresponding author. Tel.: +91 1332 285421; fax: +91 1332 285665.
E-mail addresses: [email protected], [email protected]
(Apurbba Kumar Sharma), [email protected] (D. Gupta).
0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsusc.2012.02.019
properties of a functional surface [2]. Traditionally, tungsten carbide (WC) is one of the most effective wear resistant materials,
although its economical processing has been a challenge for the
material processing community.
Thermal spraying and arc welding are commonly used techniques to produce WC reinforced composite coatings on a substrate.
The spraying techniques have high efficiency, but they cannot provide metallurgical bonding between the hard phase material and
the substrate. Thus, the spray deposits are, normally, poor in flexural loading. In contrast, arc-welding techniques can provide good
metallurgical bonding, because a melt-pool is formed on the surface of the substrate. However, in such processes the dilution of the
composite coating is so high (due to excessive energy input) that
the substrate might get severely deformed. Further, as observed
by Zhou et al., WC particles remain largely dissolved in the material system leading to comprehensive reduction in the mechanical
properties [3].
The electromagnetic energy in the form of laser is being widely
used to develop metal–matrix-composite (MMC) cladding mainly
due to (i) better control over dilution level and (ii) higher cooling
rate obtainable in the process to develop fine microstructure. In
spite of these facts, the laser cladding technique has some limitations like development of high thermal stress that causes cracking
of the clad during processing and distortion. Moreover, laser processing is not a very cost effective technique for cladding of large
areas [3]. However, application of electromagnetic energy in the
form of microwaves as a heating source can potentially overcome
many problems associated with laser cladding. One such technique,
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termed as microwave cladding, has been developed by the authors
[4–6].
In the recent years, microwave processing of materials has
emerged as one of the fastest material processing techniques.
Microwave processing of materials is fundamentally different from
the conventional thermal processing methods. Microwave energy
heats the material on molecular level, which leads to uniform bulk
heating. While in the conventional heating systems, the material
gets heated from the surface to the interior with thermal gradient
[7–9]. The inverse temperature profile and volumetric nature of
heating in microwave processing create a number of opportunities
in material processing, which include rapid heating of materials
without overheating the surface, significant reduction in the possibility of solidification cracking, reduced porosity, reduced residual
stresses and thermal distortion, improvement in product quality
and yield. These features make microwave heating attractive in
material processing industries.
In the last two decades, microwave energy is being widely used
in the area of processing of ceramics, ceramic composites and polymeric composites. Several studies on processing of ceramics using
microwave energy have been reported [8–14]. However, adaptability of microwave energy in processing metallic material is difficult
owing to the fact that microwave absorption coefficient for metals at 2.45 GHz radiation is significantly less at room temperature
[15] and the likely thermal instabilities can potentially lead to the
phenomenon of temperature runaway [16]. This makes extremely
difficult to heat metallic materials using microwave without hybrid
heating [7,17]. Microwave heating of bulk metallic materials is even
more challenging because of negligible microwave penetration into
it at room temperature. Sintering of hardmetals (for example, mixture of W, Co and C powder) using microwave radiations at 2.45 GHz
with power 1 kW was first demonstrated and reported by Rodiger et al. [18]. The study paved the way for a new application of
microwaves in material processing. In 1999, an US research group
first reported successful sintering of metallic materials with a composition of Fe + Cu (2%) + graphite (0:8%). The sintering was carried
out at 1200 ◦ C with an exposure of microwave radiation for 30 min
which resulted in excellent sintered density [19]. Later, several
authors have reported sintering of pure metallic materials or their
alloys through microwave heating [20–24].
However limited work has been reported on processing of composites using microwaves. In microwave processing, the heating
rate depends on the constituents of the target material. This aspect
is further discussed in Section 3. Microwave heating will take place
through the interaction of electric field or/and magnetic field with
the microwave absorbing constituent(s). Thus, in a metal–ceramic
system, heating will take place due to both electric and magnetic
fields; on the other hand, when both the constituents are either
metal or semi-metal, they get heated only in the magnetic field [25].
Thus, the variance in absorption of microwave energy may result in
non uniform heating leading to non-uniformity in microstructure
and consequent properties. In spite of such perceived difficulties,
few works on processing of composites using microwave has been
reported. Padmavathi et al. have reported microwave sintering
of yttrium–aluminum–garnet (YAG)-reinforced 316L composites
[26]. Zhu et al. have reported sintering of ZrB2 –B4 C particulate
ceramic composite and with density in excess of 98% of the relative density at the processing temperatures as low as 1720 ◦ C [14].
Further, use of multimode microwave systems and hybrid heating
in sintering of composites could be found in the works of Rajkumar and Aravindan (copper–graphite composites) and Luoa et al.
(W–Cu composites) [27,28]. Application of microwave energy for
curing of CNFs/EPON-862 nanocomposites has been reported by
Rangari et al. [29].
Use of microwave heating in metal-based material processing
has also been reported by Chiu et al. (cladding of NiTi on AISI 316L
stainless steel), Cho and Lee (metal recovery from stainless steel
mill scale), Takayama et al. (production of pig iron), Borneman and
Saylor (development of coating of friction reducing alloys using
CuNiIn powder on Ti–6Al–4V substrate), and Sharma et al. (joining of bulk metallic materials) [30–35]. The present authors have
successfully demonstrated development of metallic and non metallic cladding on metallic substrate using microwave hybrid heating
(MHH) at 2.45 GHz [4]. The authors have also shown the mechanism
of microwave clad formation elsewhere [5]. It was found that the
developed WC10Co2Ni clads on austenitic stainless steel (SS-316)
through MHH were crack free and contain porosity approximately
0.89%, which is less than that usually observed with other popular
cladding processes. The microwave clads also exhibited significantly high dry sliding wear resistance [6].
In the current investigation, microstructural and flexural strength aspects of metal–ceramic composite cladding on
austenitic stainless steel substrate developed through microwave
hybrid heating technique have been presented.
2. Experimental procedure
A multimode 900 W microwave applicator was used in the
present work for developing composite cladding of Ni-based EWAC
with 20% WC10Co2Ni cermet powder on stainless steel (SS-316)
substrate. The following sections briefly describe the development
and characterization of the clads.
2.1. Material details
Nickel is a characteristically tough metal and has high oxidation and corrosion resistance at room and elevated temperatures;
while WC based materials have high hardness and wear resistance. Thus, a composite cladding of such materials is expected
to withstand high tensile and compressive stresses. In such a system, the Ni-base metallic phase will form the tough matrix and
the metal carbides remain dispersed (often in agglomerated form)
as the reinforced hard phase. In order to develop cladding on
austenitic stainless steel, EWAC powder and WC10Co2Ni powder
of average grain size ∼40 ␮m were used. The 80% EWAC powder
and 20% WC10Co2Ni powder by weight were mixed in a powder
mixing device. The chemical compositions of the substrate SS316, EWAC and WC10Co2Ni powders are shown in Table 1, while
other relevant properties are shown in Table 2. Typical morphology and XRD spectra of the raw EWAC and WC10Co2Ni powders
used are illustrated in Figs. 1 and 2, respectively. The raw powders have largely spherical morphology. While the EWAC powder
is Ni dominated, the reinforcement powder primarily contains hard
tungsten carbide phase with marginal presence of Ni and Co phases
(2% and 10%, respectively) which are evident in Fig. 2. Austenitic
stainless steel (SS-316) plates were machined to the dimensions
35 mm × 12 mm × 6 mm to be used as the substrate material.
2.2. Development of cladding
Preparation of raw powder and substrate is critical in development of cladding. In the present work, the substrates were cleaned
in alcohol in an ultrasonic bath prior to deposition. The EWAC and
WC10Co2Ni particles were preheated at 100 ◦ C for 24 h in a conventional muffle furnace. Preheating removes possible moisture
content in the powder. The mixed powder was preplaced manually on the SS-316 substrate maintaining an approximately uniform
thickness. Melting of the preplaced powder layer was achieved in
a domestic microwave applicator in 2.45 GHz at 900 W power.
Microwave heating is the result of interaction of electromagnetic (EM) radiation with the irradiated material. However, all
materials do not interact with microwaves in the similar way.
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Table 1
Chemical composition (wt%) of the materials used.
Material
Elements
Fe
Substrate (SS-316)
WC based powder
EWAC powder
Bal.
–
–
Cr
17.3
–
0.17
Ni
Mo
W
13.1
2.0
Bal.
2.66
–
–
–
Bal.
–
Co
–
10.0
–
C
Si
Mn
Others
0.02
5.0
0.2
0.73
–
2.8
1.71
–
–
0.5
–
–
Table 2
Some important properties of the materials used.
Material
Substrate (SS-316)
WC
EWAC (Ni) powder
Properties
Thermal conductivity, k (W/(m K))
Heat capacity, c (J kg−1 K−1 )
Density, (kg/m3 )
16.3 [36]
84 [37]
90 [39]
500 [36]
167 [38]
461 [39]
7960 [36]
15,800 [37]
8910 [39]
The EM waves at the same frequency may be transmitted (transparent material), reflected (opaque material) or fully/partially
absorbed (absorber material) by different materials. Thus, the incident microwaves have to penetrate into the target material for any
interaction to take place. The penetration of microwaves into a
material body is expressed in terms of ‘skin depth’—the larger the
skin depth, higher is the microwave–material interaction, hence
heating.
Metals are opaque to microwaves at 2.45 GHz and ordinary conditions. The cermet tungsten carbide too exhibits almost similar
response to microwave irradiations. The calculated values of the
skin depths for nickel and tungsten carbide are ∼0.12 ␮m [5] and
∼4.7 ␮m at 2.45 GHz [18], respectively. The skin depth in WC is
approximately forty times higher than that of the Ni powders, yet
both the depths are found to be significantly less than the particles sizes (∼40 ␮m) of the powders used in the trials. Skin depths,
however, increase at elevated temperature. It indicates that the target particles cannot directly interact with microwave radiation at
room temperature. In order to overcome this problem, clads were
developed by microwave hybrid heating technique using a suitable susceptor. In order to avoid possible contamination of the clad
by the susceptor powder used in the MHH, a 99% pure graphite
sheet was used as a separator between the susceptor and the powder mix (EWAC + 20% WC10Co2Ni) as shown in Fig. 3. The SS-316
Fig. 1. Morphology of raw powder: (a) EWAC and (b) WC10Co2Ni.
Fig. 2. Typical XRD spectra of (a) EWAC powder and (b) WC10Co2Ni powder.
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Fig. 3. Schematic of the microwave hybrid heating set-up used for developing
claddings.
Fig. 4. Schematic arrangement for 3-point bend test.
substrate was masked inside a base material (for example,
refractory brick) so as to avoid direct exposure to the incident
microwaves. In the present investigation, experimental trials were
carried out at 900 W power with a processing time of 420 s in atmospheric condition. Details of development of cladding using MHH
with suitable examples have been discussed at length elsewhere
[4].
2.3. Characterizations of the clad
The developed composite clads were subsequently washed
thoroughly with acetone in an ultrasonic cleaner prior to proceeding for characterization. Clads were cut along the thickness at the
center using a low speed diamond cutter (Model: BAINCUT – LSS,
Make: Chennai Metco, India). The cut sections of the clad specimens were cold mounted using resins for polishing. The mounted
specimens were polished first by emery paper of 320 grit, followed
by further polishing using emery papers of grades 600, 800, 1×, 2×,
4×, and finally with 1 ␮m diamond paste in a cloth wheel machine.
Specimens were then water cleaned and dried in hot air.
The XRD patterns were obtained at room temperature in a
Bruker AXS diffractometer with Cu-K␣ X-ray. The scan rate used
was 1◦ min−1 ; the scan range was from 20◦ to 80◦ . The analysis of clad microstructures and chemical composition of the clads
were carried out in a field emission scanning electron microscope
at an acceleration voltage of 20 kV equipped with energy dispersive X-ray detector (Model: Quanta 200 FEG). Microhardness on
the transverse surface of clad and substrate was measured by a
Vicker’s microhardness tester (Mini load, Leitz, Germany) at the
load of 50 g applied for 30 s. The indentations for Vicker’s hardness
measurements were made at an interval of 80 ␮m starting from the
substrate material up to the top of the clad surface.
2.4. Flexural strength test
Flexural strength of the clads was evaluated through standard
three-point bend test. The test was carried out in a Hounsfield Monsanto facility (H25KS/05) in air at room temperature. The crosshead
displacement rate during the testing was kept at 1 mm min−1 . The
arrangement for experiments is shown schematically in Fig. 4. The
span of inner and outer dowel pins was 27 mm. The samples were
tested in a configuration to place the cladding in tension. Before
testing, the samples were polished by 0.5 ␮m diamond paste to
remove any possible oxide layer on the clads. The clads were subjected to tensile loading while point load was applied to the SS-316
substrate as illustrated in Fig. 4. Strength values were calculated
from the test data; average of three samples in the same conditions
was considered. Load and displacement were recorded for each
measurement. Fractured specimens were examined using a field
emission scanning electron microscope (FESEM) (Model: Quanta
200 FEG).
3. Mechanism of clad development
Microwave heating is the result of microwave–material interaction, if the EM radiation penetrates into the mass of the target
material. Thus, the heating should be ideally volumetric in nature.
Now, for a fixed mass of workload inside an applicator, the amount
of EM energy injected is given by Eq. (1) [40].
E=
T
p dt
(1)
0
where E = total energy injected into the workload (J), p = applied
power to the workload, may be variable with time (W), and T = total
time of the process (s).
The injected energy E is responsible for causing thermal changes
in the workload. In the present work, the target composite powder can be considered as a partial workload as the susceptor and
the separator graphite sheet would also absorb microwave energy;
while the bulk metallic substrate can be fairly considered as a
reflector. In composite, the matrix (Ni-based) powder (M) and the
reinforced WC10Co2Ni particles (R) are assumed to be placed as
shown in Fig. 5. In an ideal situation of volumetric heating, energy
is dissipated uniformly throughout the workload, and hence the rise
in temperature in the ‘M’ and ‘R’ particles in Fig. 5(a) is expected to
be uniform. However, the same does not take place in the present
system of material. Assuming, in the first instance, that the workload is dry and passive and remains in the same state throughout
the process, the rate of rise of temperature (d/dt) (K s−1 ) is related
to the power dissipation in the workload P (W), the mass of the
Fig. 5. Microwave hybrid heating during composite cladding.
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(W) and W2 C phase, which can be explained by the relations (3)
and (4).
WC ↔ W + C
(3)
2WC ↔ W2 C + C
(4)
The residual free carbon might further reacts with binder cobalt
and free W element and forms the complex carbide phase Co3 W3 C
which can be explained by the relation (5).
3W + 3Co + C → Co3 W3 C
The major constituents (Ni and Si) of the EWAC powder at elevated temperature formed the intermetallic NiSi. The Ni present
in the clad powder as shown in Table 2 (and Fig. 2) combines with
free tungsten as in Eq. (4) and forms the observed NiW phase. However, a considerable amount of the hard phase WC remains in the
clad structure as can be seen clearly by the phases (corresponding to 2 values 35.80◦ and 48.41◦ ) in Fig. 6. This WC phase along
with other carbides (W2 C, Co3 W3 C, etc.) in the clad structure act
as the particulate reinforcements in the developed clad as in the
composites.
Fig. 6. Typical XRD spectrum of composite cladding developed through MHH.
workload M (g) and its mass specific heat c (J kg−1 K−1 ) as given by
Eq. (2) [40].
d
P=M c
dt
(5)
(2)
It is seen in Table 2 that the properties of the materials used for
the proposed composite are significantly different, for example, the
mass specific heat c values. Consequently, the rise in temperature
(d/dt) within the material system (metal based matrix + cermet)
will vary significantly, hence the power dissipation. In the present
work, the susceptor and graphite absorb microwave radiation initially and get heated up. This heat is transferred to the powder
particles by the conventional modes of heat transfer as shown in
Fig. 5. At elevated temperature, however, the ‘R’ particles will start
absorbing the injected microwave energy and get heated causing
further rise in the temperature of the powder layer (‘M’ particles also get heated) as illustrated schematically in Fig. 5(b). The
‘M’ particles with low melting point (nickel ∼1400 ◦ C) get melted
consequently. Further, the reinforced powder particles (R) during
microwave interaction reach to a semi molten stage and remain
dispersed in the molten metallic matrix. The heated powder layer
will cause the interface layer of the substrate to melt due to conventional heat transfer and get diluted with the molten clad material.
However, substrate melting will remain confined to a significantly
thin layer due to the poor thermal conductivity of SS-316 substrate.
A detailed discussion on mechanism of clad formation has been
reported elsewhere [5].
4. Results and discussion
The composite clads developed through microwave heating
were subjected to various test environments in order to characterize them. Results are discussed in the following sections with
relevant illustrations.
4.1. X-ray diffraction study
A typical XRD spectrum of the composite clad developed
through microwave hybrid heating is shown in Fig. 6. The XRD
spectrum shows the presence of phases of free tungsten, WC, W2 C,
NiSi, NiW and Co3 W3 C. It is observed with reference to Fig. 2 that
some of the phases identified in the microwave clad are the result
of transformations during microwave-heating as a consequence of
microwave–material interaction. Once the WC10Co2Ni particles
reach a critical temperature, it gets decomposed into free tungsten
4.2. Study of clad microstructure
Important characteristics of a good clad may be identified as (i)
good bonding of the clad material with the substrate in terms of
partial dilution and (ii) free from metallurgical defects like porosity and cracks. Typical backscattered electron micrographs of cross
section of the developed composite clad of EWAC + 20% WC10Co2Ni
are shown in Fig. 7. The clad of thickness ∼500 ␮m shows good
metallurgical bonding with the substrate by partial mutual diffusion of elements. The clad–substrate interface can be clearly seen in
Fig. 7(a) without any visible discontinuities at this magnification.
The observed wavy substrate interface is due to localized meltpool current during microwave heating. This melt-pool current is
responsible for dilution of the melted clad material and the SS-316
substrate. The transition from the substrate to the clad zone appears
defect-free. The clad is free from any visible pores and interfacial
cracks as evident from the observed microstructure even at higher
magnification (500×) as shown in Fig. 7(b). As microwave started
to interact with preplaced powder particles, the low melting temperature EWAC powder (melting point of the major constituent Ni
is 1455 ◦ C) starts melting first. The non-uniform rise in temperature
in the powder-substrate system due to the differences in the thermal and other properties of the constituent materials as discussed
in Section 3; a thermal imbalance is set up inside the powder layer
which eventually causes a localized melt-pool current. The convection current set up inside the melt pool is sufficient to make the
particles partially agglomerated. The hard carbide particles remain
distributed uniformly inside the soft matrix as can be clearly seen
in Fig. 7.
The volumetric heating in the preplaced powder resulted in
low thermal gradient in the microwave exposed surface. The
effect of volumetric heating on the clad formation is also evident from the homogeneous microstructure of the developed clads.
The slow solidification rate of the melt pool results in defect-free
clads as seen in Fig. 7. Different metallic carbides, primarily the
tungsten carbide and the observed complex carbide, get partially
agglomerated due to the melt-pool current and remain uniformly
distributed. The distributed carbides could provide strength to the
clad structure and thus would act as a reinforcement as in composites. Undissolved free carbons segregated at the cell boundaries
are also clearly seen in the clad microstructure as illustrated in
Fig. 7(a) and (b). The typical solidification pattern associated with
microwave processing results in cellular like structure as illustrated
in Fig. 7(b). The volumetric nature of heating of microwave causes
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Fig. 7. Typical backscattered electron micrographs of the transverse section of the developed microwave induced composite clad.
Fig. 8. Typical EDS spectra of (a) clad matrix and (b) white phase.
cellular growth in the developed clads and their direction is controlled by the direction of heat flow so that the cells will grow in
the direction of the maximum thermal gradient. Cell to dendritic
transition is not seen in the developed clad because the cellular
structure is often grown in the direction of heat flow.
In order to ascertain the elemental constituents of different
phases present in the microstructure, energy dispersive X-ray
spectroscopy (EDS) was also carried out at different locations.
Typical EDS spectra of the clad matrix and the white phase
as observed in Fig. 7 are illustrated in Fig. 8. It is observed
that the matrix phase is dominated by the relatively soft nickel
and cobalt, while the reinforcement (white) phase primarily
consists of carbides of tungsten and nickel. Free tungsten and
carbon are also observed in the matrix phase as discussed in
Section 4.1.
4.3. Microhardness study
Vicker’s microhardness of the clad layer along the cross-section
was monitored. The distance between two indentations was kept
at minimum 80 ␮m with an additional indentation at the fusion
line. The distribution of microhardness is illustrated in Fig. 9(a); the
Fig. 9. (a) Vicker’s microhardness profile along the typical composite clad cross-section, inset: schematic of the measurement and (b) typical Vicker’s indentation geometries
on different phases.
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Fig. 10. (a) Load–displacement characteristics of the microwave induced composite clad system during 3-point bend test; zoomed views of the characteristics with reference
to displacements: (b) up to 1 mm, (c) between 1 mm and 3 mm, and (d) 3 mm onwards.
corresponding indentation geometries are presented in Fig. 9(b).
The average microhardness of the clad layers was observed to be
∼416 ± 20 Hv, which is significantly higher than that of the SS-316
substrate (∼200 Hv).
It is seen in Fig. 9(a), that the clad hardness is fairly uniform
throughout the clad cross section. It is attributed to the facts that the
reinforced particles are uniformly distributed in the tough matrix
and volumetric heating does take place in the preplaced powder
layer owing to microwave–material interaction. The indentation
morphology on the clad matrix (I1 ) presents almost an ideal indentation on a metallic phase; while the observed localized higher
microhardness at few measurement locations is due to indentations on a reinforced particle (or, on a partially agglomerated
particle (I2 )) as identified in Fig. 9(b).
4.4. Studies on flexural strength
Flexural strength is one of the most important utility indicators
of many mechanical components. In order to assess the flexural
strength of the developed microwave clads, they were subjected
to standard three-point bend test as discussed earlier. The monitored load–displacement characteristic is illustrated in Fig. 10(a).
The entire characteristic was further zoomed and presented in
three different segments as shown in Fig. 10(b)–(d) for displacement up to 1 mm, displacement between 1 mm and 3 mm and
displacement beyond 3 mm till the end of loading, respectively. Different loads and corresponding deformations at important stages
are presented in Table 3. The tested clad samples exhibit a 3-stage
load–displacement behavior as shown in Fig. 10.
In the stage-I of Fig. 10(a), the clad shows a uniform
load–displacement characteristic up to a load of ∼750 N. Further
loading causes a deviation in the load–displacement characteristic; there is an almost step increase in load from ∼750 N to ∼7850 N
within a corresponding deformation of 0.5 mm as described by the
stage-II (Fig. 10(a) and (b)). These two loads may be termed as
‘lower transition load’ and ‘upper transition load’, respectively as
indicated in Fig. 10(b). Following this, the deformation characteristic exhibits another characteristic pattern attributed to substrate
deformation (stage-III, Fig. 10(a)). The critical initial 1 mm deformation was further zoomed and presented in Fig. 10(b). In the
initial stage of loading, up to the ‘lower transition load’, the clad
matrix provides resistance to the applied load and undergoes the
observed deformation (∼0.15 mm, in the present case). However,
with the continuously increasing load, micro cracking on the top
clad surface takes place as indicated by the constant load deformation at lower transition load (around 750 N, in this case). At this
Table 3
Observations during flexural strength testing of microwave clad.
Sample set
Lower
transition load
(N)
Upper
transition load
(N)
Deformation at
upper transition
load (mm)
1
2
3
750
728
753
7880
7993
7751
0.75
0.81
0.73
Mean
744
7875
0.76
Maximum
load (N)
Maximum
deformation
(mm)
Deformation
Index [×10−5 ]
(mm N−1 )
630
639
619
14,415
14,760
15,140
5.505
6.42
6.30
6.1
6.7
5.8
629 ± 8
14,772
6.075
6.2
Flexural
strength (MPa)
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Fig. 11. Macrographs of typical fractured clad samples in 3-point bend test: (a) lateral view and (b) top view.
stage, a transition in the load bearing will take place and the hard
carbide particles start taking the load showing a steep load deformation characteristic as revealed by the stage-II. This deformation
can be considered as the beginning of plastic deformation in clad.
In the stage-I, initially, the relatively ductile metallic matrix phase
of the clad deforms elastically under the applied load. However,
as the applied load increases beyond the observed ‘lower transition load’ (marked in Fig. 10(b)), the matrix gets elongated under
the tensile stresses and the applied load is transferred to the reinforcement phases. The corresponding deformation in this stage
becomes plastic and the magnitude decreases sharply with increase
in load. However, microcracks start developing at this stage releasing the induced stresses as depicted by the minute kinks (staircase
profile) in the zoomed view in Fig. 10(b). These minor kinks are
also attributed to the localized debonding of the phases—matrix
material gets separated from the reinforced phase resulting in the
initiation of major cracks. However, as the applied load is increased
past a critical value, termed as ‘upper transition load’ (∼7880 N, in
this case in Fig. 10(b)), more plastic deformation results and the
microcracks get propagated into the depth of the clad. The clad
eventually fails at this load and the applied load is transferred to the
steel substrate. The steel substrate, then exhibits typical progressive deformation as characterized by the stage-III in Fig. 10. Loading
was stopped as the load started dropping as shown in Fig. 10(d).
The clads were further analyzed in terms of ‘Deformation Index’
(DI), an useful parameter defined for analyzing behavior of coatings
under flexural loading [41] and was subsequently used by others
[42]. The DIs were calculated using the relation (6).
DI =
ı
(mm N−1 )
P
(6)
Fig. 12. Scanning electron micrographs of fractured clad samples in 3-point bend test: (a) fracture surface topography, (b) crack propagation inside the clad depth, and (c)
secondary cracking.
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Apurbba Kumar Sharma, D. Gupta / Applied Surface Science 258 (2012) 5583–5592
where P = ±10% change in load at 50% of the rupture load P (N),
ı = corresponding change in flexural deformation (mm).
DI gives a close indication of the deformation behavior of
the clad system or the system stiffness. It indicates the compliance of the system. The computed DIs for the tested clads are
shown in Table 3. The upper transition load was considered as
the rupture load for clads. The observed low DI values (mean
value = 6.2 × 10−5 mm N−1 ) indicate higher resistance of clads to
deformation. The parameter (DI) can be used for optimization.
4.5. Fractographic analysis
Fractographic analyses of the 3-point bend tested clad samples
were carried out in order to assess the mode of fracture. Typical macrographs and scanning electron micrographs are illustrated
in Figs. 11 and 12, respectively. As discussed earlier, the continued loading causes severe plastic deformation in the clad owing to
which micro cracks grew in longitudinal as well as in transverse
directions in the clad with corresponding release of loads. This is
seen clearly in Figs. 11(a) and 12(a). Major primary cracks indicating
the clad failure are seen wide open and parallel to the width of the
clad samples. This is characteristic comb cracking of clad/coating
under tensile load. The primary cracks grew under continued loading and reached the substrate as can be seen in Fig. 11(b). Following
this, the substrate started taking the applied load beyond the upper
transition load. However, it is interesting to note that the highly
tenacious composite clads did not get peeled-off under the continued loading as can be seen in Fig. 11, which indicates good dilution
(hence good adhesion) of the clad material and the stainless steel
substrate.
The scanning electron micrographs presented in Fig. 12 reveal
that tensile shear deformation in the clad surface causes multidirectional macro crack formation. The cracks propagate into the
clad depth till the interface through primary cracks as loading continues (Fig. 12). The study, in general, shows a ductile behavior of
the developed microwave clads mainly due to the presence of Ni
based matrix. The hard and relatively brittle reinforcing (metallic
carbides) phases, on the other hand, act as crack initiation site at the
matrix–reinforcement interfaces as shown in Fig. 12(b) as debonding takes place while loading above lower transition load. The
matrix exhibits a valley-like fracture pattern in which the carbide
phases are seen deflecting the cracks as in the case of reinforced
composites (Fig. 12(b) and (c)). The ductile matrix acts as crack
inhibiter in the initial loading stage but in the later stages, cracks
propagate into the matrix and secondary microcracks appear as
seen in Fig. 12(c). However, the reinforcing particles were still seen
firmly held in the matrix. The secondary micro cracking is indicative
of decohesion in the matrix phase under tensile loading, however,
these cracks do not propagate beyond the matrix phase.
5. Conclusion
The present work has shown the viability of using microwave
heating for processing of metal-based composite material. Development of a new processing method for enhancement of surface
properties of soft metallic materials through microwave cladding
has been reported. The mechanism of the clad formation has been
briefly discussed. The process has the potential to grow as a new
surface treatment method for developing crack free clad having good metallurgical bonding with substrate. Major conclusions
drawn from the present work are:
1. It is possible to develop metal–ceramic composite clad on
metallic substrate using microwave energy. The cladding process is complete in terms of metallurgical bonding at the
2.
3.
4.
5.
6.
7.
8.
9.
10.
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interface, and clads are free from major processing defects like
interface cracking.
Microwave heating induces material phase transformation
in the powder layer—carbides like W2 C, Co3 W3 C and intermetallics (NiSi, NiW) are formed.
The reinforced carbide particles are uniformly distributed
inside the tough metallic (Ni and Co based) matrix.
The solidification patterns of clad result in cellular like structure
in the clad which is primarily due to the volumetric heating
associated with microwave processing.
The average Vicker’s microhardness of the developed
microwave clad is 416 ± 20 Hv, which is significantly higher
than the soft metallic substrate SS-316 (∼200 Hv).
Under the 3-point bend test condition, clads exhibit 3-stage
load-deformation characteristics. The outermost layer of the
clad cracks at the lower transition load and the substrate starts
taking the load beyond the upper transition load.
The average flexural strength of the clads is 629 ± 8 N and corresponding deformation is around 0.76 mm.
Deformation Index showed that the clads exhibit good deformation resistance.
Multi directional cracks are developed at the clad surface and
primary cracks propagate into the substrate as comb cracking.
Secondary cracks are due to the matrix decohesion and do not
propagate beyond the matrix phase.
The composite clads exhibit good tenacity and did not get
peeled-off from the substrate at the end of test (corresponding
to average load ∼ 14,772 N).
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