International Journal of Engineering Trends and Technology (IJETT) – Volume17 Number 9–Nov2014
Hard Coating on Steel – A Review
Ms. HiteshriJadhav#1, Mr.Parthiv Trivedi*2 ,
#1
PG Scholar, Mechanical Engineering Department, Gujarat Technological University, India
Asst. Prof., Mechanical Engineering Department, Gujarat Technological University, India
*2
Abstract- Now a days various coating techniques are being used
in different fields for a large number of applications. This paper
consists of different effects and its respective changes on the steel
substrate material coated by High Velocity Oxy Fuel (HVOF)
coating. It helps enhance the properties such as micro structure,
abrasive wear, wear resistance etc. and various techniques such
as SEM, XRD, Pin-On-Disc etc. are used to check and analysis
these properties. On hard coating steel the material may increase
its service life and perform its intended function successfully.
Keywords—High Velocity Oxy Fuel (HVOF), Scanning Electron
Microscopy (SEM), X-Ray Diffraction (XRD), abrasive wear,
Pin-On-Disc.
I.
INTRODUCTION
Thermal spray coatings are increasingly used for surface
protection, primarily against high-temperature corrosion and
wear [1]. Wear and Friction are responsible for many
problems and large costs in a modern civilization and
engineers and designers always must take these factors into
account when constructing different equipments. The
technology of thermal spraying enables to create the surface
coating approximately 50 µm thick, providing the functional
surface protection of the coated parts. The HVOF technology
offers the possibility of creating the coatings of materials
based on the principle of hardmetals (cermets) with high wear
resistance and favourable sliding properties. Such a
combination predestines the HVOF sprayed coatings for
sliding applications, such as pistons of combustion engines,
pumps and other hydraulic devices. In practice the producers
and users of thermally sprayed coatings face the problem of
the interaction of the coatings and their counterparts with
presence of other media, fuels, or in the case of sliding wear
more often, the lubricants [4]. Coatings are commonly applied
to hot section gas turbine components, to midspan dampers on
fan and compressor blades in aero turbines engines, and are
being investigated for use in advanced fossil energy plants. In
all cases, the longevity and durability of the coating are of
paramount importance to its primary function of protecting the
substrate component.
II.
LITERATURE REVIEW
T.C. Totemeier, R.N. Wright et al [1]studied that X-ray
based residual stress measurements were made on type 316
stainless steel and Fe3Al coatings that were high-velocity oxyfuel (HVOF) sprayed onto low-carbon and stainless steel
substrates. Nominal coating thicknesses varied from 250 to
1500 mm. In this paper effect of substrate thickness on
residual stresses were determined. Residual stresses have been
shown to play an important role in the cracking, adhesion, and
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spallation behaviour of coatings. Residual stress measurement
has primarily been performed on plasma-sprayed ceramic
coatings. Higher spray particle velocities produced coatings
with more compressive residual stresses, higher hardness, and
decreased fractions of oxide and porosity. Two coating
materials were examined—Fe3Al and AISI type 316 stainless
steel. Three different spray conditions were investigated for
each material; the spray particle characteristics (size
distribution, velocity, and temperature) and relative deposition
efficiency were assessed for each material and spray
conditions. Coatings prepared at identical velocity and
temperature and similar powder particle sizes should result in
similar properties, regardless of the particular HVOF torch
used. Particle temperature is dependent on the temperature of
the combustion gas flow and the residence time of the particle
in the gas. Maximum deposition efficiency obtained for both
powders at an intermediate velocity. Stresses measured on the
surface of coatings applied to thin substratesbecome less
compressive with increasing thickness, while stresses for thick
substrates are relatively constant. The stress increase results
from increased peening effects imparted by higher-velocity
particles. Residual stresses measured on the surface of
unpolished Fe3Al coatings were tensile, reflecting the lack of
peening in the last layer of coating deposited.
C.-J. Li, Y.-Y. Wang et al [2] in this paper the relationship
between abrasive wear resistance and the size and content of
carbide was introduced. The correlation of the theoretical
relationship with experimental data was examined. Four types
of commercially available WC-Co powders were used also a
commercial Cr3C2-25%NiCr powder (YF-25, Zigong) of
particle size from 10 to 45 mm was also used here. WC-Co
coatings were deposited on blasted mild steel surface with JetKote spray gun. A gas mixture of C2H2-30%C3H6 was used
as fuel gas operating at a pressure of 0.35MPa. The pressure
of oxygen was kept at 0.55MPa and spray distance 150mm.
Cr3C2-25NiCr coatings were deposited with CH-2000 HVOF
spray system. Propane was used as fuel gas operating at a
pressure of 0.4MPa. Oxygen was operated at a pressure of
0.55MPa. Nitrogen gas was used as powder carrier gas
operating at a fixed condition. Both the coatings were coated
at different working conditions with fifteen set of
combinations of spraying parameters. The microstructure of
cross-sections of both powders and deposited cermet coatings
was examined by a scanning electron microscopy (SEM).
Abrasive wear of WC-Co coatings was estimated using a
Suga-type abrasive wear tester while that of Cr3C2-NiCr
coatings was carried out by dry rubber wheel wear tester. To
eliminate the effect of surface conditions, all test pieces were
polished before the test. The worn surface of typical coatings
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was examined using SEM. Then the relation between the
abrasive wear and size and content of carbide was derived
according to which the wear of cermet coating is proportional
to the square root of the carbide particle in the coating and
reversely proportional to the volume fraction of carbide phase
content. Experimental correlations were carried out for
Carbide particle size and wear weight loss of WC-Co coatings,
Carbide particle size and wear weight loss of Cr3C2-NiCr
coatings and Correlation of relative wear with relative carbide
particle size is shown in fig:1
Fig: 2 Schematic diagram of D-Gun Spray process [3]
Fig: 1Relation between the relative wear and (dc/dcr)1/2(fcr/fc) [2]
This paper concluded that the correlation using HVOF WCCo and Cr3C2-NiCr coatings proved reasonably experimental
evidence for the proposed relationship. As the results, the
abrasive wear of a thermally sprayed cermet coating will be
determined by carbide particle size and carbide content. The
abrasive wear resistance of cermet coating will be increased
inversely proportional to the square root of carbide size and
positively to carbide content.
Kamaljeet Singh, Charanjeet Singh Sandhu et al
[3]discussed that wear and friction arise in almost all parts,
interfaces and equipments which cause a great impact on the
gross production so needs to be reduced and various measures
are given in order to overcome this problem. Coating is the
best and most appropriate method used. . Major thermal spray
processes include Wire Spraying Detonation Gun Deposition,
Plasma Spray, and High Velocity Oxygen Fuel. Here in this
paper the detonation gun spray process is used in which a
water cooled barrel with some gases such as oxygen and
acetylene along with a charge of powder is introduced and a
spark is ignited to flow down an impinge on the surface to be
coated as shown in fig:2
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EN8 steel is used as the substrate material with which small
cylindrical pins were made to perform pin on disk experiment.
Two types of coating powders namely (1) Tungsten Carbide
(WC) + (10%) Cobalt (Co) + (4%) Chromium (Cr), (2)
Aluminium Oxide (Al2O3) + (13%) Titanium Oxide (TiO2)
was selected and were successfully deposited. A uniform
thickness coating of 250 mm ± 10 mm was deposited in all the
cases of WC–10Co-4Cr and Al2O3-13TiO2 powders for EN8
substrate steel. Dry sliding wear tests for the uncoated and
detonation sprayed EN8, were conducted using a pin- on –disc
machine. The wear test for coated as well as uncoated
specimens was conducted for sliding wear and wear kinetics
which concluded that 1) Improvement in wear resistance of
EN8 was observed after the deposition of detonation sprayed
Al2O3-13TiO2 and WC– 10Co-4Cr coatings. 2) WC – 10Co 4Cr coating has better wear resistance than that of Al 13TiO2
coatings. 3) Detonation spray process provides the possibility
of deposition of Al2O3-13TiO2, WC – 10Co -4Cr powders on
the EN8 steel. A uniform coating thickness of 250+10 micro
meter was achieved.
SarkaHoudkovaa Iva Zabranskab et al [4] in this paper
tests were performed on Cr2C3-25%NiCr coating, sprayed by
the HP/HVOF JP-5000® (TAFA) device. High impact energy
of the molten particles leads to the reduction of tensile stress
in the coating, even enables to create the coatings with
compressive stress, while the low temperature prevents the
overheating of the coating material and lowers the amount of
undesirable structure phase changes. The coating was sprayed
using the standard preparation procedure on the grit blasted
substrate of carbon steel and the previously optimized
spraying parameters. The microstructure of the HVOF
sprayedCr2C3-25%NiCr coating (Fig no. 3) is slightly
heterogeneous, with less than 1 % of porosity.
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losses were achieved in almost all coating types. Coating 1
350 cracked after few thermal cycles. Resistance to erosive
wear of all coatings is approximately the same.
Fig: 3 Microstructure of the Cr3C2-NiCr coating[4]
Fig: 4 Adhesion of coatings after thermal cycles [5]
The sliding wear of thermally sprayed coatings was tested on
the CSM High Temperature Tribometer according to ASTM
G-99, based on the principle of “pin-on-disc” test. The
measurements were performed on the Cr3C2-NiCr without
and with lubricants. The lubricants were the commercially
available MOGUL 15W-40 mineral engine oil, the semisynthetic MOGUL 10W-40 engine oil and the synthetic
MOGUL 5W-40 engine oil. After this test they concluded that
the presence of the lubricant decreases the co-efficient of
friction (CoF) mean value more than 5 times, the CoF value
scatter and the wear of the Cr3C2-NiCr HVOF sprayed
coating.
J. Brezinova and A. Guzanova et al [5] studied that
thermally-sprayed coatings have excellent properties such as
high wear resistance, corrosion resistance and resistance
against high temperatures. Different coating combinations
provide a vast area of applications and required strengths. On
application of cyclic thermal stresses their properties are
evaluated. The steel substrate with mechanical properties of
tensile strength 740 – 880 MPa, yield strength ≥ 440 MPa was
used for coating. Air grit blasting was performed on the
material as the pretreatment. Three types of coatings based on
WC-Co, WC-Co-Cr and Cr3C2-25NiCr were deposited then
the adhesion and thickness of the coatings was evaluated. 10
Cyclic thermal loading cycles were applied in the electric
chamber furnace at 900 degrees with 20 minutes of dwell and
cooled at still air. After 1 or 2 cycles each the material was
evaluated for adhesion and its construction structure and
chemical analysis was done by SEM. Thickness of the
coatings as sprayed, were as follows: 1 343 – 234 µm, 1 350 –
356 µm and 1 375 – 393µm. The highest micro hardness
values was shown by coating 1 350. Then the EDX analysis
was carried out to give the final results which showed that the
adhesion was decreased by third cycle only (fig: 4). Also
during the thermal cyclic loading the coating 1 343 showed a
fracture in the coating, at higher impact angle higher weight
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Fig: 5 3D appearance of the worn surface of coating 1 343 [5]
ThiyagarajanSundararajan, Seiji Kurodaet al [6]in their
paper used 9Cr-1Mo type ferritic steel as a substrate specimen
with dimensions 1 cm2 area with 4mm thick specimens coated
with 50Ni-50Cr powder evaluated in the temperature range of
873–1023 K. Microstructural changes during the oxidation
tests were the point of focus for this study. The HVOF coating
process yielded the thickness of around 60 mm and the
coating appears to be free from through porosity. The
hardness studies revealed that though the carbon migration
occurred from the near substrate region to the
coating/substrate interface to form the chromium carbide, the
carbon depleted zones showed the hardness values very close
to the core substrate. Below 1023K the chromium carbide
does not form a continuous layer, which allows the Ni in the
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coating to diffuse into the substrate resulting in a more
complicated microstructure. When the carbide formed a
continuous layer, the Ni diffusion from the coating to the
substrate was essentially blocked, whereas, when the carbide
precipitates were dispersed and non-continuous, Ni diffused
from the coating to the substrate. The carbon required to form
the chromium carbide migrated from the substrate adjacent to
the coating. The carbon depletion in the segment of substrate
resulted in the ferrite microstructure with lower hardness.
L.P. Ward, B. Hinton et al [7]HVOF technology was used
for three WC based cermet coatings to be deposited on ferritic
stainless steel substrate. Salt spray testing and
potentiodynamic
studies
were
conducted.
Also
characterization of coating structure, composition and
morphology was carried out. The coated samples exposure
time ranged between 19 hours upto 558 hours. A higher level
of porosity was distributed across the whole of the coating
cross section according to the micrographs revealed. The
surface topography is extremely rough and suggests the
presence of an inhomogeneous structure, containing both
granular dispersed and matrix phases separated by regions of
high porosity / voids. Initially in Salt Spray Test slight
discolouration of WC-based coating was evident. Time to red
rust (TTRR) was first observed on the WC-12Ni coating
surface after 114 hours exposure in the salt spray chamber.
The lack of red rust spots on the WC-12Ni coating, prior to
114 hours exposure time was possible due to the fact that
stainless steel initially showed high resistance to corrosion and
/or the coating was acting as a barrier to corrosion. The
corroded surface of the WC-20Cr2C3-7Ni coating showed the
presence of quite large micro cracks, as previously observed
in the WC-12Ni coatings. Potentiodynamic scanning studies
revealed poor corrosion performance of the coatings when
compared with the stainless steel substrate, with increased
corrosion current Icorr values and more negative corrosion
potentialEcorr values. This was attributed to the poor structure
and possible galvanic coupling effects between the substrate
and the coating. Corrosion of the coating substrate system
may have been accelerated by dissolution and / or erosion of
specific phases within the coating, resulting in the formation
of micro-channels and increased number / size of voids.
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Fig. 6 Optical micrograph of WC-12Ni coating after 114 hours exposure to
the salt spray test [7]
III.
CONCLUSION
The Hard Coating of different steel substrate materials
increases the microstructure, wear resistance, abrasion
properties of the material. This increases the service life and
enhances the various mechanical and surface properties of the
material.
REFERENCES
[1] T.C. Totemeier, R.N. Wright, and W.D. Swank, “Residual Stresses in
High-Velocity Oxy-Fuel Metallic Coatings” Metallurgical and Materials
Transactions A 2004 Vol-35A pg-1807-1814
[2] C.-J. Li, Y.-Y. Wang, G.-C. Ji“Relation between Abrasive Wear and
Microstructure of HVOF Cermet Coatings” ASM International, 2003, pg435-441
[3] Kamaljeet Singh , Charanjeet Singh Sandhu and RakeshGoyal,
“Comparision of wear properties of WC-10Co-4Cr and Al2O3-13TiO2 alloy
powders on EN8 steel by detonation spray process” Mechanica confab, 2014,
vol-2 no-7pg-19-25
[4] SarkaHoudkova, Iva Zabranskab, Frantisek Zahalkac, “The influence of
lubricants on the friction properties of HVOF sprayed coatings suitable for
combustions engines” Metal 2009.
[5] J. Brezinova, A. Guzanova, “Possibility of utilization high velocity oxygen
fuel (HVOF) coatings in conditions of thermal cyclic loading” Metalurgija 51,
2012, pg-211-215
[6] ThiyagarajanSundararajan, Seiji Kuroda and Fujio Abe “Steam Oxidation
Studies on 50Ni-50Cr HVOF Coatings on 9Cr-1Mo Steel: Change in
Structure and Morphology across the Coating/Substrate Interface” Materials
Transaction 2004 Vol-45 No.-4 pg-1299-1305
[7] L.P. Ward, B. Hinton, D. Gerrard and K. Short “Corrosion Behaviour of
HVOF Sprayed WC Based Cermet coatings on Stainless steel” Journal of
Minerals and Materials Characterization and Engineering 2011 Vol-10 No11 pg-989-1005
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