Journal of Minerals & Materials Characterization & Engineering, Vol. 11, No.6, pp.569-586, 2012 Printed in the USA. All rights reserved
Characterisation and Corrosion-Erosion Behaviour of Carbide
based Thermal Spray Coatings
Sukhpal Singh Chatha a,*, Hazoor S. Sidhu a, Buta S. Sidhu b
a Yadavindra College of Engineering, Punjabi University Guru Kashi Campus, Talwandi Sabo,
Bathinda, Punjab-151302, India
b Punjab Technical University, Jalandhar, Punjab-144601, India
*Corresponding Author:
Thermal spraying has emerged as a suitable and effective surface engineering technology and is
widely used to apply wear, erosion and corrosion protective coatings for various kinds of
industrial applications. Cr3C2-based coatings have been applied to a wide range of industrial
components. Cr3C2–NiCr coatings offer greater corrosion and oxidation resistance, also having
a high melting point and maintaining high hardness, strength and wear resistance up to a
maximum operating temperature of 900 °C. The corrosion resistance is provided by NiCr matrix
while the wear resistance is mainly due to the carbide ceramic phase. This paper reviews the
performance, developments and applications of Cr3C2–NiCr thermal spray coatings for
corrosion/erosion-corrosion under different types of environments and outlines the
characterization of Cr3C2–NiCr coatings with respect to their microstructure and mechanical
properties, together with some brief characterisation work by the author for HVOF sprayed
75Cr3C2-25NiCr coating on T91 boiler steel.
Keywords: Thermal spray, Microstructure, Mechanical properties, Corrosion-erosion,
Degradation of metals and alloys due to hot corrosion or erosion has been identified as a serious
problem for many high temperature aggressive environment applications, such as boilers,
570 Sukhpal Singh Chatha, Hazoor S. Sidhu Vol.11, No.6
internal combustion engines, gas turbines, fluidized bed combustion, and industrial waste
incinerators [1, 2]. Due to depletion of high-grade fuels and for economic reasons, residual fuel
oil along with coal is used extensively in the energy generation systems such as turbines, boilers,
and industrial waste incinerators. The impurities namely, Na, V, K and S present in the fuel oil,
react together to form low melting point compounds on the surface of materials and induce
accelerated oxidation (hot corrosion). The degradation of material occurs when these molten
compounds dissolve the protective oxide layers that naturally form on materials during boiler/gas
turbine operation, inability to either totally prevent the hot corrosion or its detection at an early
stage has resulted in several accidents, leading to loss of life and/or destruction of
engines/infrastructures [3]. A dynamic surface layer consisting of a mechanical mixture of
particles from the boiler gases and oxide scale growing from the base metal is formed on boiler
tubes. This layer is constantly refurbished and removed at a rate that results in an essentially
constant thickness of the deposit/scale layer during steady state operation of the boiler. The
accumulation of these low melting-point salts from flue-gas containing sulfates of sodium and
potassium on the fire side surface of boiler tubes induces hot corrosion and is considered a root
cause for the severe wastage of tube materials used for superheaters and reheaters in
‘‘advanced’’ steam-generating systems [4]. The continuing development of modern energy
generation systems requires materials capable of withstanding increasing operating temperatures,
mechanical loads and chemical degradation [5]. It is not possible for a single material to have
different properties to meet the demand of today’s industry. Therefore, a composite system of a
base material providing the necessary mechanical strength with a protective surface layer
different in structure and/or chemical composition can be an optimum choice in combining
material properties [6, 7]. Conventional methods such as carburizing, nitriding, electroplating are
being used over a century to protect tools, however, the development of the hard protective
coatings in the narrower sense, started in the sixties with the discovery of chemical vapor
deposition and physical vapor deposition techniques. In recent times, modern deposition
techniques such as thermal spray and laser cladding have become more popular and can give
large throughput in shortest possible time [8]. Thermal spraying as a convenient way has been
reported by several researchers. Thermal spraying offers an effective and economic way to make
the coating without affecting any other properties of the component [9]. Thermal spray coatings
are produced by rapidly heating the feedstock material in a hot gaseous medium and
simultaneously projecting it at a high velocity onto a prepared surface where it builds up to
produce the desired coating [10]. Fig.1 shows the typical thermal spray process and coating.
Thermal spraying has been widely considered to apply protective coatings on the boiler tubes of
the steam generating plants to counteract the erosion–corrosion problems [11]. Thermal spraying
is a common family of hardfacing techniques, which, compared to other processes (like welding
techniques), are characterized by flexibility in coating material choice, low substrate thermal
input and virtually no substrate dissolution [12]. Various thermal spray processes, such as
detonation-gun, plasma and high velocity oxy-fuel (HVOF) spraying methods are mostly used to
Vol.11, No6 Characterisation and corrosion-erosion behaviour 571
apply coating to impart a wear resistance against abrasion and erosion in corrosive environment
at high temperature up to 900 °C [3]. Among the available thermal spray processes, the HVOF
technique is one of the most promising methods. Due to higher gas jet velocities and lower flame
temperatures than many other thermal spray processes, HVOF spraying enables to produce
sophisticated, wear resistant cermet coatings with exceptional high quality regarding
microstructure and surface finishing. These coatings generally feature a very low porosity, low
oxidization and low carbide decomposition or carbide-matrix dissolution, resulting in high
hardness and high abrasion resistance [13]. In HVOF, the powder is injected into the flame of a
gun and sprayed on a substrate due to the high velocity and temperature achieved in the
combustion reaction of different gases. Several torch passes generates a coating that can be
characterised by low porosity, similar to the initial powder chemistry. The corrosion resistance of
the coating generated is better than other thermal spray techniques such as atmospheric plasma
(APS) and arc or flame spraying [14]. HVOF process has the advantage of being a continuous
and most convenient process for applying coatings to industrial installations at site. Several
HVOF sprayed coatings have been subjected to corrosion testing in seawater, including cermets
and anti-corrosion alloys [15].
Fig.1.Typical thermal spray process and coating [1]
The coatings properties are influenced not only by the properties of the powders used but also
significantly by the spray process and spray parameters. It has been reported that the abrasive
wear rate for the cermet coatings is controlled by several factors like the morphology of the
starting powder, the size and distribution of the carbide particles, hardness of the carbide
particles relative to the abrasive, properties of the matrix and its volume fraction, and the coating
process, which determines the coating characteristics like the phases, density, microhardness,
bonding strength and the residual stresses [16]. Good adherence of a coating to its substrate is
one of the most important aspects of a successful coating. It has been speculated that an excellent
coating is obtained when bonds between the film and the substrate is produced before a strong
572 Sukhpal Singh Chatha, Hazoor S. Sidhu Vol.11, No.6
bond within the film is formed. Strong bonding between the coating and the substrate has a
unique significance when the coated metals are used in aggressive corrosive environment [17]. In
general, the corrosion-erosion resistance of thermal spray coatings depends on the chemistry and
morphology of coatings. Microstructural features which enhance corrosion-erosion resistance
include high density and low porosity, small grain size, good adhesion and absence of cracks
[18]. The corrosion resistance is also associated to the surface roughness, in a way in which the
higher surface roughness, the higher the corrosion attack due to higher surface area [19].
The materials used as coatings must ensure an effective protection against oxidation as well as
have a high thermal conductivity (heat-exchangers) in order to provide good service behaviour
and an effective and economical maintenance layout [20]. The primary aim of the
coating/surface treatment is the ability to produce a stable, slow-growing surface oxide scale
providing a barrier between the coated alloy and the environment [21]. Thermally sprayed
coatings based on hard carbides embedded in a metallic matrix are being considered as an
important option to replace galvanic chromium deposits on many industrial components in the
petrochemical industry and offshore applications [14]. Cermet coatings are a combination of a
ceramic hard phase embedded in a ductile matrix, typical coating systems consist of WC–Co,
NiCr–Cr3C2 and Fe-CrAlY–Cr3C2. In thermal spray technology, Cr3C2-NiCr cermet coatings
have been extensively used to mitigate abrasive and erosive wear at high temperatures up to 850
°C [13, 22]. The corrosion resistance is provided by NiCr matrix while the wear resistance is
mainly due to the carbide ceramic phase. FE-SEM micrograph of 75Cr3C2-25(Ni20Cr) powder is
shown in Fig.2. It is evident from the micrographs of the coating powder particle that the Cr3C2
NiCr powder gives a clear contrast between the carbide and NiCr matrix phases, the powder
particle show irregular shaped morphology [3]. Cr3C2-NiCr coatings offer greater corrosion and
oxidation resistance, also having a high melting point and maintaining high hardness, strength
and wear resistance up to a maximum operating temperature of 900 °C. In addition to these
features, the coefficient of thermal expansion of Cr3C2 (10.3×106 °C1) is nearly similar to that
of iron (11.4×106 °C1) and nickel (12.8×106 °C1) that constitute the base of most high
temperature alloys. This minimizes stress generation through thermal expansion mismatch
during thermal cycles [23]. Although satisfactory results have been obtained with cermet coating
systems, a great deal of debate still persists over the best hard phase matrix ratio for optimum
erosion resistance. The erosion resistance of cermet coatings increases with an increase in
chromium carbide in the pre-sprayed powder. Researchers agree that carbide-based coatings
provide excellent erosion protection, but disagree on the optimum amount of carbide for
maximum erosion resistance [24]. 75Cr3C2–25NiCr coatings are primarily designed for wear
applications either at elevated temperatures, or at room temperature in corrosive environments.
Vol.11, No6 Characterisation and corrosion-erosion behaviour 573
Fig.2. FE-SEM of Cr3C2–NiCr powder particle [3]
Cr3C2–25NiCr coatings with conventional grains obtained by thermal spraying exhibit excellent
wear and corrosion resistance properties [9]. The reduction of the feedstock powder grain size to
submicron or nanometric scale is of key interest in the thermal spray technology to develop
novel fine structured or nanostructured cermet coatings with further enhanced structural and
functional properties. Such coatings do not only show a significantly lower porosity than
conventional coarse-structured coatings but also feature a higher surface quality. This enables the
application of thin, near-net-shape coatings. However, in contrast to conventional coarse-grained
powders, the feeding and processing of fine particles <10 µm involves major difficulties [13].
The microstructure of a coating is as important as the chemical composition of the coating
material. In cermet coatings prepared by thermal spray techniques, substantial variation in
composition and microstructure is possible due to the exposure to the high temperature
accelerating gas of the coating process [22]. The thermal sprayed coatings consist of oxides and
voids originating from spraying process that are found at the splat boundaries, through which the
coatings were mainly attacked [3]. Therefore, the coatings should be carefully sprayed so that the
molten salts do not penetrate into the coating layer [25]. Fig. 3 shows the optical microstructure
of HVOF deposit 75Cr3C2-25NiCr coating on T91 boiler tube steel substrates. Due to the higher
velocity, relatively lower temperatures, and higher impact of the powder particles on the
substrate material, the coatings with dense microstructure and less porosity are formed in HVOF
process. The HVOF sprayed coatings have shown a typical splat surface morphology with
distinct splat boundaries (Fig.3), due to the deposition and re-solidification of molten or semi
molten droplets. Microstructure of the coating is highly lamellae with distinct splat boundaries
and splats oriented parallel to the substrate surface. Also, few pores were detected in the coating,
thereby confirming the higher density of the Cr3C2-NiCr coating. A limited number of unmelted
particles can be observed in the microstructures of the coatings. The coatings possess some voids
574 Sukhpal Singh Chatha, Hazoor S. Sidhu Vol.11, No.6
and oxide inclusions that are typical characteristics of the HVOF sprayed coatings. The oxides
may form due to the in- flight oxidation during spraying process and/or preexisting in the feed
material [26]. Fig.4 shows BSE micrograph from the cross-section of HVOF sprayed 75Cr3C2-
NiCr coating on T91 boiler tube steel in as-sprayed condition. The porosities (black areas) and
oxides (circular black areas) can be seen in the image (Fig. 4a). A high magnification of the back
scattered images of the coating cross-sections can also be seen in Fig. 4b. Examination of the
microstructure (Fig. 4b) of the coatings also reveals the presence of three different zones. The
first zone is dark which indicates the presence of primarily Cr and C, revealing a Cr3C2 phase.
The second zone is of grey colour, besides Cr and C this area also contains Ni. A third zone is
white and consists of a NiCr phase, primarily.
Porosity is inversely proportional to thermal conductivity, and will also be susceptible to
penetration by corrosive reagents during operation on account of their open structure.
Consequently, the need arises to employ denser coatings which degrade less due to their more
Fig.3. Optical micrograph (Original magnification 50X) showing cross-section
morphologies of HVOF sprayed 75Cr3C2-25NiCr coating on T91 boiler steels in as-sprayed
closely packed structure [27]. The porosity of the Cr3C2-NiCr coatings as reported by authors lies
in the range of 1–3.5 % (Table 1), which was found to be useful in developing hot corrosion
resistance in boiler tube steel [28, 29, 30]. The low value of porosity might be due to high kinetic
energy of the powder particles. The dense coatings are supposed to provide very good corrosion
resistance as compared to porous coatings, as porosities are the preferential corrosion paths
through which the corrosive species can penetrate the coatings to reach the substrate and may
cause rapid corrosion attack [26, 31]. It is standardized that coating thickness is approximately
300–500µm in the case of a cermet coating [14]. Thicker coatings permit the pass of the
electrolyte due to the stresses generated during coating deposition and the corresponding crack
formation between different layers. Thinner coated samples let the electrolyte go through the
Vol.11, No6 Characterisation and corrosion-erosion behaviour 575
coating because it is not thick enough to correctly protect the base steel. Optimisation of spraying
parameters and stress relaxation processes will be as important as thickness when protection of
the base steel is needed with cermet coatings [14]. The microhardness values of the Cr3C2-NiCr
coatings on different substrate materials as reported by different authors have been compiled in
Table 1. Higher values of hardness for the coatings may be due to the high density and cohesive
strength of the individual splats as a result of the high impact velocity of the coating particles as
suggested by Sidhu TS et al [26] and Verdon C et al [41]. The non-uniformity of the hardness
values of the coatings, along the distance is reported by many authors, this variation in hardness
value is attributed to the microstructural changes along the cross-section of the coatings. These
microstructural changes might be due to the presence of porosity; oxidised, melted, unmelted and
semi-melted particles in the coating structure [22] as observed in the optical micrograph (Fig.3).
Fig.4. BSEI micrograph showing cross-section morphology of HVOF sprayed 75Cr3C2-
25NiCr coating on T91boiler steels in as-sprayed condition.
Fig.5. SEM micrograph and EDAX analysis surface of HVOF sprayed 75Cr3C2-25NiCr
coating on T91 substrate.
a b
Element Wt.%
C 04.21
O 04.07
Cr 55.31
i 21.78
b 07.62
Mo 06.16
C 06.90
O 08.05
Cr 63.89
i 10.05
b 04.11
Mo 05.72
576 Sukhpal Singh Chatha, Hazoor S. Sidhu Vol.11, No.6
Author and Year Coatings Spray processPorosit
y value
Substrate material
Wang Y,1993[32] WC-12%Co D-gun 1219 MoCN315M low
carbon steel
WC-12%Co Plasma 1103
WC-20% Co D-gun 1003
TiC-20%Ni D-gun 865
D-gun 742
Plasma 382
HVOF <2% 750-900 Mild steel
Cr3C2 HVOF <2% 804-990 AISI1018 low carbon
HVOF 1-2% 614-810
WC-17Co HVOF <1% 850-930
HVOF 1-2% 614-810AISI1018 low carbon
Cr3C2 HVOF <2% 804-990 AISI1018 low carbon
HVOF <2% 614-810
Wang J,2000[36] 75Cr3C2
D-gun 1.9 Cr3C2 , Ni DIN 12CrMo44
Plasma 561 AISI 1043 steel
Plasma with
Laser remelting
2% 42 CrMo6 steel
(UNS G11200)
HVOF 2.5
850 Cr3C2, Ni,
Cr, NiC,
SA213-T-11 steel
12Co HVOF 1.5
Roy M,2006[9] Cr3C2
HVOF 2% 980Mild steel
Nano-crystalline grains
HVOF 1.5% 1140
HVOF <1.5% 870
950 Ni-
Cold spray843 4140 alloy steel
Vol.11, No6 Characterisation and corrosion-erosion behaviour 577
Table1. Mechanical and microstructural characteristics of carbide based coatings
The XRD diffractrograms of Cr3C2–NiCr coatings in as-sprayed conditions on different substrate
materials as reported by the authors is depicted in Table 1. The SEM micrographs and the
elemental compositions (surface) of HVOF Cr3C2-NiCr coatings on T91 boiler tube steel at
selected points are depicted in Fig. 5. The micro-structure consists of interlocked particles with
irregular morphology. There is a presence of some tiny particles in the coating, which might
have formed after fracturing of some of the powder particles during spraying. Some unmelted
and semimelted particles and pores can be seen in the coatings. Black spot in the microstructure
is identified as porosity. The regions marked ‘M’ in these micrographs might have formed from
the impact of fully molten feedstock droplets. SEM micrograph (Fig. 5) of as-sprayed Cr3C2
NiCr coating indicates the formation of the required composition for HVOF coatings, with NiCr
metallic binder phase corresponds to the white region in the micrograph and the dark grey
region as the carbides.
Components of power station boiler, gas turbines, internal combustion engines and industrial
waste incinerators are subjected to frequent degradation by erosion–corrosion problems relevant
to reliability and economics of these installations. The environment is characterized by high-
temperature conditions together with aggressive atmospheres, leading to corrosive deposits
adhere to the surface and to erosion processed due to the ash particles. Attempts to decrease the
maintenance costs of these components have increased interest in shielding them with protective
coatings [42].
Guilemany JM et al [14] investigated the role of coating thickness in the corrosion behaviour of
steel protected with different numbers of HVOF sprayed Cr3C2–NiCr layers. . Electrochemical
25NiCr HVOF 1-2%
897 AISI 4340 steel
10Ni HVOF 1-2% 906
WC12Co HVOF 1218 Carbon steel
WC12Co APS 1021
Cr3C225(Ni20Cr) HVOF 945
Cr3C225(Ni20Cr) APS 788
(Aerospray 150)
1036 Cr3C2,
Mild steel
1163 Cr3C2,
powders (28+2 µm)
HVOF 1.7%1004 C45 steel
578 Sukhpal Singh Chatha, Hazoor S. Sidhu Vol.11, No.6
measurements, such as Open-Circuit Potential, Polarisation Resistance and Cyclic Voltammetry,
were performed in an aerated 3.4 NaCl media (%wt.). Electrochemical Impedance Measurements
were also done in order to obtain a mechanism that explains the corrosion process. Results show
that the corrosion resistance of the complete system is mainly influenced by the substrate
behaviour. The application of a higher number of deposited layers did not substantially increase
their anticorrosive properties. Stress generation during the spraying deposition process plays an
important role in the behaviour of the coated steel against corrosion phenomena.
Wang BQ et al [18] carried the elevated temperature erosion tests on cooled AlSl 1018 low-
carbon steel and four thermal spray-coated mild steel specimens using a nozzle-type elevated
temperature erosion tester. The thermal-sprayed coatings included a HVOF 75Cr3C2-25NiCr
cermet coating, HVOF Cr3C2, low velocity flame-sprayed Cr2O3 ceramic coatings and an arc-
sprayed FeCrSiB coating. Test conditions attempted to simulate the erosion conditions at the in-
bed tubes of fluidized bed combustors. The specimens were water cooled on the backside.
Material wastage was determined from thickness loss measurements of the specimens. Erosion
test results were compared with those from testing isothermal specimens. It was found that the
cooled specimens had greater material wastage than the isothermal specimens. The effect of
cooling the specimen on the erosion wastage for 1018 steel was greater than that for the thermal-
sprayed coatings. The lower material wastage of coated specimens compared to 1018 steel was
related to the coatings composition and morphology. Among the four coatings tested the two
hard coatings (HVOF Cr3C2, and low velocity flame-sprayed Cr2O3 ceramic coatings) exhibited
lower erosion wastage than the other two coatings. The HVOF Cr3C2 coating exhibited the
lowest erosion wastage due to its favorable composition and morphology. Which was about:
three to 12 times lower than the erosion wastage of 1018 steel and the other three coatings. It had
a finer structure and smaller splat size than the other three coatings. Further Wang BQ et al [43]
carried out series of hot erosion tests on several carbide–metal composite coatings using a nozzle
type elevated temperature erosion tester. The coatings included HVOF sprayed Cr3C2 cermet
system and WC cermet coatings. 1018 low carbon steel and two arc-sprayed iron-base coatings
were also tested for references. Bed ashes retrieved from different boilers were used as the
erodent materials for tests at both shallow and steep impact angles. It was found that the all
HVOF carbide cermet coatings showed much higher erosion resistance than 1018 steel and arc-
sprayed iron-base coatings. Among the coatings tested the WC–17CrCo coating exhibited the
highest erosion resistance. Among the HVOF Cr3C2 cermet system coatings the composite
powder sprayed coatings showed higher erosion resistance than the blend powder sprayed
coatings. The erosion behavior of the coatings is closely related to their microstructure,
composition and the properties of erodent particles.
Erosion resistance of FeCrAlY–Cr3C2 and NiCr– Cr3C2 cermet coatings with carbide levels
ranging from 0–100% (in the pre-sprayed powder) was investigated by Stein KJ et al [24] in
order to determine the optimum ceramic content for the best erosion resistance. Erosion testing
Vol.11, No6 Characterisation and corrosion-erosion behaviour 579
was carried out using a particle accelerator apparatus set at a velocity of 40 m/s, a mass flow of
80 g/min, and 400°C with 450 mm Al2O3 particles at 30° and 90° impact angles. Image analysis
revealed the carbide levels in the pre-sprayed powder are much higher than in the final as
sprayed coating. The lower carbide levels in the coatings are caused by a combination of poor
carbide spray efficiency and reduction and oxidation of the carbide particles in the HVOF jet,
resulting in the formation of various oxides and metal rich carbides. Erosion testing of the
sprayed coatings found that decreasing the carbide content, and overall hard phase content
(oxides and carbides), decreased the erosion rate for 90° impact. In addition, for 30° impact the
erosion rate remained fairly constant regardless of carbide or hard phase content. In a
comparative study of HVOF sprayed Cr3C2–NiCr and Ni–20Cr coatings on a Ni-based
superalloy in a molten salt environment of Na2SO4–60%V2O5 at 900 °C under cyclic conditions,
Sidhu TS et al [15] found that the hot corrosion resistance of Ni–20Cr coating was better than
Cr3C2–NiCr coating. The presence of oxides of nickel and chromium and their spinels NiCr2O4,
and uniform fine grain microstructure of the as sprayed coating might have contributed for
better hot corrosion resistance of HVOF sprayed Ni–20Cr wire coating. These oxides might have
blocked the pores and splat boundaries, and acted as diffusion barriers to the inward diffusion of
oxidizing species.
Kamal S et al [23] investigated the hot corrosion resistance of detonation gun sprayed Cr3C2
NiCr coatings on Superni 75, Superni 718 and Superfer 800H superalloys. Thermogravimetry
technique is used to study the high temperature hot corrosion behavior of bare and Cr3C2–NiCr
coated superalloys in molten salt environment (Na2SO4–60% V2O5) at high temperature 900 C
for 100 cycles. Results show that Cr3C2–NiCr coating can be successfully deposited by
Detonation gun spraying process on Ni and Fe-based superalloy substrates, with uniform,
adherent and dense microstructure with porosity less than 0.8%. Cr3C2–NiCr coatings on Ni- and
Fe-based superalloy substrates are found to be very effective in decreasing the corrosion rate in
the given molten salt environment at 900 C. Particularly, the coating deposited on Superfer
800H showed a better hot corrosion protection as compared to Superni 75 and Superni 718. The
coatings serve as an effective diffusion barrier to preclude the diffusion of oxygen from the
environment into the substrate superalloys. It is concluded that the hot corrosion resistance of the
D-gun sprayed Cr3C2–NiCr coating is due to the formation of desirable microstructural features
such as very low porosity, uniform fine grains, and the flat splat structures in the coating. Further
Kamal S et al [3] investigation hot corrosion behaviour of Cr3C2–NiCr cermet coatings deposited
on two Ni-based superalloys, namely superni 75, superni 718 and one Fe-based superalloy
superfer 800H by detonation-gun thermal spray process. The cyclic hot-corrosion studies were
conducted on uncoated as well as D-gun coated superalloys in the presence of mixture of 75
wt.% Na2SO4+25 wt.% K2SO4 film at 900 °C for 100 cycles. It was observed that Cr3C2–NiCr-
coated superalloys showed better hot-corrosion resistance than the uncoated superalloys in the
presence of 75 wt.% Na2SO4+ 25 wt.% K2SO4 film as a result of the formation of continuous and
protective oxides of chromium, nickel and their spinel, as evident from the XRD analysis. The
580 Sukhpal Singh Chatha, Hazoor S. Sidhu Vol.11, No.6
surface oxide scale developed on coated superalloys is massive, crack free, with dense clusters
mainly consisting of oxides of Cr and Ni, in which, irregular disc shaped Cr2O3 distributed
throughout the top surface of the scale. Cr3C2–NiCr-coated and uncoated superalloys superni 718
and superfer 800H showed the lowest and highest weight gains, respectively, in the given
In another study Kamal S et al [44] investigated the cyclic oxidation behavior of detonation-gun-
sprayed Cr3C2–NiCr coating on three different superalloys namely Superni 75, Superni 718 and
Superfer 800H at 900 °C for 100 cycles in air under cyclic heating and cooling conditions. The
kinetics of oxidation of coated and bare superalloys was analysed, using thermogravimetric
technique. It was observed that all the coated and bare superalloys obey a parabolic rate law of
oxidation. The results on the Cr3C2–NiCr-coated superalloys showed better oxidation resistance
due to the formation of a compact and adhesive thin Cr2O3 scale on the surface of the coating
during oxidation. The scale remained intact and adherent to the partially oxidised coating during
cyclic oxidation due to its good compatibility and similar thermal expansion coefficient between
Cr3C2–NiCr coating and the superalloy substrates. In all the coated superalloys, the chromium,
iron, silicon and titanium were oxidised in the inter-splat region, whereas splats which consisted
mainly of Ni remained unoxidised.
Souza RC [19] compared the influence of Cr3C2–25NiCr and WC–10Ni coatings applied by
HVOF process and hard chromium electroplating on the fatigue strength, abrasive wear and
corrosion resistance of AISI 4340 steel. S–N curves were obtained in axial fatigue tests for base
material, chromium plated and HVOF coated specimens. HVOF coated specimens showed
higher axial fatigue resistance and better performance in wear weight loss in comparison to
electroplated chromium. Further for Cr3C2–25NiCr HVOF coating, results indicate clearly the
higher salt spray resistance and higher microhardness than for chromium electroplated.
Porosity or voids in the coating micro structure is an important issue in thermal spraying, as due
to this physical property, corrosion resistance of different thermal spraying coatings differs [45].
To reduce the internal stresses, interconnected porosity and inter splat boundaries, coatings are
post treated by various methods such as heat treatment, sealing, laser remelting etc [46]. In a
study by Zhang S-H et al [16] compared to the original HVOF WC–CoCr coating, the laser
treatment has essentially eliminated the porosity, providing a more homogeneous and densified
microstructure. After laser heat treatment, the thickness of the coating has decreased from 300
μm to 225 μm. As a result, the average porosity in the laser-treated material has been reduced by
a factor of approximately five compared to the nonlaser heat-treated material. The laser treatment
has produced a considerable increment in the hardness of the coating near the surface where the
Vol.11, No6 Characterisation and corrosion-erosion behaviour 581
average value increases from Hv0.2=1262.4 in the HVOF-sprayed coating to Hv0.2=1818.7 in
the laser-treated coatings.
Wang BQ et al [47] evaluated high temperature erosion resistance of several thermal sprayed
coatings under as-deposited, post heat-treated and sealed conditions using a nozzle type elevated
temperature erosion tester. The coatings included high velocity continuous combustion (HVCC)
arc sprayed metal and ceramic coatings, and high velocity oxygen fuel (HVOF) sprayed metal
and carbide cermet coatings. The heat treatment was performed at 500°C for 24 hours, after
heating the samples were cooled in air. For sealing coating a high temperature aluminum
phosphate based sealant was used. It was found that post heat-treating and sealing of coatings
increased the elevated temperature erosion resistance of coatings. Sealed coatings with aluminum
phosphate based sealant also proved beneficial in increasing the erosion resistance from a few
percent up to twice for both metal and carbide cermet coating.
Matthews S et al [48] investigations the variation in microhardness in high velocity sprayed
75Cr3C2–25NiCr coatings after exposure to heat treatment at 900 °C for a period of up to 60 days
in air and argon as a function of microstructural development. The results showed that with
treatment, matrix phase supersaturation was reduced, while widespread carbide nucleation and
growth generated an expansive carbide skeletal network. An initial softening of the coatings
occurred through matrix phase refinement, the subsequent hardness recovery was a function of
carbide development. The changes in hardness with heat treatment were accompanied by
concurrent changes in microstructure—namely the precipitation of fine carbides and their
subsequent growth; and the recrystalisation of the matrix. Treatment in air generated further
hardness increases as a result of internal oxidation Further Matthews SJ et al [22] investigated
the influence of structure on erosion performance of high velocity air fuel and high velocity
oxygen fuel sprayed Cr3C2–NiCr coatings under industrial turbine conditions. Coated samples
were heat treated in air at 900 °C for 30 days. Erosion testing of as-sprayed and heat treated
coatings was conducted at ambient and elevated temperature. Single impacts were characterised
using Scanning Electron Microscopy in order to determine the erosion mechanism. Results show
that at ambient temperature as-sprayed HVOF and HVAF Cr3C2–NiCr coating deformation
occurred through brittle cracking and fracture of the matrix phase, along with extensive brittle
crack propagation along the carbide–matrix interfaces. The coating splat structure was a
preferential path for crack propagation in the HVOF coating but less significant in the HVAF
coating. During erosion at 800 °C the ductility of the as-sprayed coatings significantly increased.
Therefore the main effect of in-flight degradation of the coatings was to reduce the carbide
concentration. Heat treatment reduced the extent of the brittle matrix response, improving the
matrix ductility and the overall coating toughness. It also led to sintering of the splats reducing
the influence of the splat structure and increasing the influence of the microstructure. Carbide
precipitation occurred with heat treatment to form an extensive network which resisted relative
582 Sukhpal Singh Chatha, Hazoor S. Sidhu Vol.11, No.6
movement of the carbides during impact, while also enabling the applied load to be distributed
over a larger volume of the coating, forming a more erosion resistant microstructure.
Matthews S et al [49] investigated relative erosion rate of Cr3C2-NiCr coatings at 7000 C and
8000 C under impact velocity of 225-235m/s, under simulated conditions of turbine environment.
HVAF and HVOF sprayed coatings produced comparable erosion rates in both as sprayed and
heat treated coatings, which can be related to the complex transition in influence of the brittle
matrix phase in the as-sprayed state and impact of the three dimensional carbide network
following in heat treated samples. However the ductility of the NiCr matrix phase significantly
increased for both coatings at 8000 C, but the erosion rate of heat treated samples increased due
to the increasing constraint imposed upon the matrix by the developing carbide microstructure.
Suarez M et al [50] investigated the corrosion resistance of, as coated and post treated Cr3C2
NiCr coating of 450 μm thickness, deposited by a vacuum plasma spray process (VPS) on a steel
substrate. The post-heat treatment of coating was done by heating as-deposited coating in Ar at
400 °C and 800 °C, respectively. Electron probe micro analyzer (EPMA) with wavelength
dispersive X-ray spectrometers (WDS) was used to characterize the coatings. Results showed
that no significant changes were produced by heat treatment at 400 °C whereas the coating
annealed at 800 °C has a better corrosion resistance than the as-deposited coating due to the
microstructural changes that take place in the coating and the diffusion of Ni into Fe at the
coating–substrate interface, which ensures the presence of a metallurgical bond.
In the present work detailed literature review have been done, along with some brief
characterisation work by the author for HVOF sprayed 75Cr3C2-25NiCr coating on T91 steel and
following conclusions are made:
1. Degradation of metals and alloys due to hot corrosion or erosion has been identified as a
serious problem for many high temperature aggressive environment applications.
2. Carbide coatings can be successfully deposited over the substrate material by HVOF,
Detonation gun and Plasma Spray processes. Cr3C2–NiCr coatings offer excellent corrosion and
oxidation resistance, also having a high melting point and maintaining high hardness, strength
and wear resistance at high temperature.
3. It has been learnt from literature that detailed studies are needed to optimize the
composition of cermet coatings for erosive-corrosive environments. The erosion resistance of
cermet coatings increases with an increase in chromium carbide in the pre-sprayed powder.
Researchers agree that carbide-based coatings provide excellent erosion protection, but disagree
on the optimum amount of carbide for maximum erosion resistance.
4. The thermal sprayed coatings consist of oxides and voids originating from spraying
process that are found at the splat boundaries, through which the coatings were mainly attacked.
Consequently, the need arises to employ denser coatings which degrade less due to their more
Vol.11, No6 Characterisation and corrosion-erosion behaviour 583
closely packed structure. Further post treatments such as heat treatment and sealing of carbide
based coatings are reported with positive results, so need arises to post treat the coatings to
further enhance their life for different applications.
5. Although different authors have made the efforts to understand the mechanism of hot
corrosion using carbide based coatings in different aggressive environments in the laboratory but
still long term exposure of these coatings in actual industrial environment needs to be studied for
estimating the protection capabilities of these coatings.
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