Ijmet 10 01_145

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International Journal of Mechanical Engineering and Technology (IJMET)
Volume 10, Issue 01, January 2019, pp. 1427-1436, Article ID: IJMET_10_01_145
Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=10&IType=01
ISSN Print: 0976-6340 and ISSN Online: 0976-6359
© IAEME Publication Scopus Indexed
EROSION RATE OF HVOF SPRAYED CRC-
NICRFESIBCOC (80%-20%) COATINGS OF
COMPARISON WITH SUBSTRATE METAL SS-
316
Navinesh BC, Dr.Somasundar B and Mamatha.M.P
School of Mechanical Engineering, REVA University, Bengaluru, Karnataka, India
ABSTRACT
In the present study, coatings were deposited on CrC-NiCrFeSiBCoC(80%-20% )a
Fe based SS316 steel substrate to reduce the damage caused by erosion boiler
applications. Erosion studies were conducted on uncoated as well as HVOF coated steels.
The erosion experiments were carried out using an air-jet erosion test rig according to
ASTM G-76 standard at a velocity of 30 m/s and at different impingement angles of 30°,
60° and 90°. The HVOF spraying leads to a high retention of CrC in the coating matrix
accompanied with lower porosity The high velocity oxy-fuel (HVOF) sprayed CrC-
NiCrFeSiBCoC(80%-20% )coatings on 316SS boiler tube steal exhibit composite ductile
or brittle modes of erosion under angular alumina sand erodent of size 50 impacted
at 30m/s. The HVOF spraying leads to a high retention of CrC in the coating matrix
accompanied with lower porosity.
Key words: HVOF, Solid particle erosion, SEM, Surface analysis and boiler tubes.
Cite this Article: Navinesh BC, Dr.Somasundar B and Mamatha.M.P, Erosion Rate of
Hvof Sprayed Crc-Nicrfesibcoc (80%-20%) Coatings of Comparison with Substrate Metal
Ss-316, International Journal of Mechanical Engineering and Technology, 10(01), 2019,
pp.1427–1436
http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=10&Type=01
1. INTRODUCTION
Erosion is the progressive loss of material from a solid surface as a result of mechanical
interaction between the solid surface and a multi-component fluid or impacting solid particles or
liquid. Erosion occurs when solid particles entrained in a fluid stream (gaseous or liquid) strike a
surface Manifestations of solid particle erosion in actual service conditions usually in the form of
thinning of components, a macroscopic scooping following the gas or particle flow, surface
roughening, lack of the directional grooving characteristic of abrasion and in some cases, the
formation of ripple patterns on metal surface. Erosive wear is caused in the solid bodies by the
action of sliding or impact of solids, liquids, gases or a combination of these [1]. Manifestations
of solid particle erosion in service usually include thinning of components, a macroscopic

scooping appearance following the gas particle flow field, surface roughening, and lack of the
directional grooving characteristic of abrasion and in some but not all cases, the formation of
ripple patterns on metals [2].
Solid particle erosion is an important material degradation mechanism encountered in a
number of engineering systems such as thermal power plants, aircraft gas turbine engines,
pneumatic bulk transport systems, coal liquefaction/gasification plants and ore or coal slurry pipe
lines [3,4].Power plants are one of the major industries suffering from severe corrosion and
erosion problems resulting in the substantial losses. Erosion results from impact of particulates,
such as coal ash, dolomite and unburned carbon particles o the surface of heated boiler tubes. It
is generally believed that the most erosive species in the fly ash are quartz, which is a crystalline
form of Al2O3 and mullite. More than one quarter of all the boiler tube failures worldwide are
caused by fly ash erosion [5, 6].
Figure 1 Erosion on Metal Surface
In the present investigation, the combination of CrC-NiCrFeSiBCoC(80%-20% ) has been
HVOF sprayed on boiler tube steel. The deposited coatings are characterized based on
microstructures and physical properties and further evaluated for its performance under solid
particle erosion conditions
2. EXPERIMENTAL PROCEDURE
2.1. Substrate Material and Development of Coating
Fe based SS316 steel substrate which is used as material for boiler tubes in some coal fired
thermal power plants in northern part of India has been used as a substrate in the study. The
specimens with approximate dimensions of 30mm × 30mm × 5mm were cut from the tubes for
erosion studies. Samples were grinded with SiC papers down to 180 grit and grit-blasted with
Al2O3 (Grit 45) before being HVOF sprayed to develop better adhesion between the substrate
and the coating.
The composite coating powder of CrC-NiCrFeSiBCoC(80%-20% ) was used to spray to
deposit coatings using HVOF process. HVOF spraying was carried out using a HIPOJET 2700
equipment (M/S Metallizing Equipment Co.Pvt.Ltd, Jodhpur, India), which utilize the supersonic
jet generated by the combustion of liquid petroleum gas (LPG) and oxygen mixture. LPG fuel
gas is cheap and readily available as compared to other fuels used for HVOF spraying. The
spraying parameters employed during HVOF deposition were listed in Table 1. All the process
parameters, including the spray distance were kept constant throughout coating process.

Erosion Rate of Hvof Sprayed Crc-Nicrfesibcoc (80%-20%) Coatings of Comparison with Substrate
Metal Ss-316
Table 1 Spray parameters employed for HVOF spray process
Oxygen Flow Rate 200l/mm
Fuel (LPG) flow rate 50 l/mm
Air flow rate 750 l/mm
Spray distance 100mm
Powder feed rate 36g/min
Fuel pressure 500kpa
Oxygen pressure 750kpa
Air pressure 535kpa
Model no of the gun 5220
Diameter of the gun 11mm
Jet number 182/191
Table 2 Erosion test conditions
Sl no Test parameter Detail
1 Air
Pressure 200Kn/mm2
Velocity 30 m/s
2 Erodent
Material and size 50 micron, Al2O3
Discharge 1-5 g/min
3 Specimen
Size 25x25x5mm and 25x20x5mm
Temperature RT
Angle of impingement 30,0
,600
and 900
4 Nozzle
Size ID:1.5mm/OD:15mmx50mm
Material 99.9% pure Alumina
Table 3 Coating Composition
Sl No Name of Coatings Compositions
1
CrC-NiCrFeSiBCoC CrC Ni Cr Fe Si B CoC
Ratio(80%-20%) 80 10.69 5.219 1.2 1.2 1.07 0.614
2.2. Erosion Studies
Room temperature erosion test was carried out using air jet erosion test rig (Figure 1) as per
ASTM G76-02 standard at M S R Institute of Technology, Bangalore, India. The erosion studies
were performed on uncoated as well as coated samples for the purpose of comparison. The
erosion test conditions utilized in the present study were listed in Table 2. The velocity of the
eroding particles was determined by a rotating double-disc method as described by Ruff and Ives
[17]. The sample was first cleaned in acetone using an ultrasonic cleaner, dried and then weighed
using an electronic balance with least count of 0.01 mg. The sample was then fixed to the sample
holder of the erosion test rig and eroded with alumina sand at the predetermined particle feed
rate, impact velocity and impact angle for a period of about 5 min. The sample was then removed,
cleaned in acetone and dried and weighed to determine the weight loss. This weight loss
normalised by the mass of the alumina particles causing the weight loss (i.e., testing time x
particle feed rate) was then computed as the dimensionless incremental erosion rate.

The above procedure was repeated till the incremental erosion rate attained a constant value
independent of the mass of the erodent particles or, equivalently, of testing time. This constant
value of the incremental erosion rate was defined as the steady-state erosion rate. The incremental
erosion rate was converted into volume wear rate to take into account the different densities of
the coating material and the substrate.
3. RESULTS AND DISCUSSION
3.1. Erosion Rate as a Function of Impingement Angle
The camera photographs and schematic diagram showing the erosion scar produced on the eroded
surface at different impact angles of 30°, 60° and 90° are shown in Figure 2. The centre portion
of the eroded scar (A) represents localized region of material removal and it is surrounded by a
region of elastically loaded material (B). The loss in weight of the sample after each 5 minutes is
measured and using weight loss and mass of the erodent, erosion rate is measured as follows
Erosion rate (g/g) = Cumulative weight loss of sample/ Mass of erodentAn erosion rate curve is
drawn as a plot of erosion rate versus cumulative mass of the erodent, for each erodent impact
angle. Steady state volume erosion rate is estimated as follows Steady state volume erosion rate
(cm3/g) = Average of constant value of incremental erosion rate/ Density.
Figure 2 Camera Macrographs showing the erosion scar of uncoated SS316 substrate (top row, in
sequence for 30°, 60° and 90º) and CrC-NiCrFeSiBCoC(80%-20% ) coating (Bottom row, in sequence
for 30°, 60° and 90º)

Metal Ss-316
Figure 3 Schematic diagram showing the erosion scar produced in general on the eroded surface at
minimum and maximum impact angle. Mark “A” represent localized region of material removed and
“B” is the peripheral region of elastically loaded material.
Figure 4 Histogram illustrating the steady state volume erosion rate of uncoated SS316 steel at different
impact angles (30°, 60° and 90°).

Figure 5 Variation of the Incremental erosion rate with the cumulative weight of the erodent for CrC-
NiCrFeSiBCoC (80%-20%) coatings at 30°, 60° and 90° impact angle
Figure 6 Histogram illustrating the steady state volume erosion rate of CrC-NiCrFeSiBCoC (80%-20%)
coatings at different impact angles
The erosion rate curves along with the bar chart indicating the steady state volume erosion
rate for uncoated steel are shown in Figures 3 and 4. The steady state volume erosion rate of the
SS316 steel (Figure 4) at 30° impingement is higher than that at 90° which is a characteristic
behavior of the ductile materials, where material removal takes place predominantly by plastic
deformation. It is observed that variation of erosion rate with respect to impact angle of 30°, 60°
and 90º is marginal, which indicates that the erosion rate is independent of impact angle for SS316
steel In general, the incremental erosion rate curves follows the same trend as that for the ductile
steels at 60° and 90°, having a low initial rate, reaching a peak after 42 g of impacting particles
and, subsequently, reaching a steady state erosion rate which is considerably lower than the peak
rate
In the present work, the SS316 substrate steel demonstrate lower erosive loss when compared
to the HVOF sprayed coatings under the same test conditions (Figure 5 and 6). The embedment

Metal Ss-316
of alumina particles into the substrate steel imparts the shielding effect against further material
loss.
The Scanning electron micrographs obtained on the eroded SS316 (Figure 7) clearly shows
the embedment of alumina sand particles into the substrate steel and the mechanism of wear is
due to indentation induced severe plastic deformation. The embedment of alumina sand onto the
surface also results in variation in erosion rate with impact of cumulative mass of erodent. It is
observed from the SEM micrographs (Figure 7) of the eroded surface at 30° impact angle that the
alumina sand particles deform the surface by ploughing, lip due to severe plastic deformation of
the material. With the successive impacts, these extremely strained lips are susceptible to be
detached as micro-platelets. The crater formed by ploughing and lips at the rim of the crater are
clearly seen in the micrograph. As the erodent particles are being in contact for extended time on
the surface during sliding, the mass loss is more. At 60° impact angle material damage is in the
form of ploughing, groove formation and craters. Possibly, grooves are formed due to falling off
of entrapped erodent particle.
At normal impact, the substrate material undergoes severe plastic deformation and there is
less mass loss. The alumina sand particles impinge onto the substrate and extrude forming a big
crater as shown in Figure 7. Small platelets are formed at the rim of the crater while the alumina
erodent is extruded. These platelets are further compressed to critical plastic strain by the impact
of the subsequent erodent particles and are then detached from the rim of the crater as micro
platelets. The embedment of the alumina particles into the substrate material is shown in Figure
7. The erodent impacting at 90° will make the ductile metal to undergo work hardening and hence
the further impact of the particle will penetrate less. Thus, a ductile material at 90° shows lower
erosion rate.
3.2. Erosion Mechanism
It is known that materials that consist of both brittle and ductile constituents can behave in either
a ductile or a brittle manner. The erosion rate curves (Figure 5) indicates that after the initial
incubation period the erosion rate reaches a steady state in general for all the three impact angles
under study. The steady state volume erosion rate is found to be maximum for 60° impact angle
(Figure 6). This suggests that the CrC-NiCrFeSiBCoC (80%-20%) coatings behaves neither as
ductile, where the maximum erosion is expected at 30° nor purely brittle where maximum erosion
is expected at 90° and has a composite behavior but also is influenced by the erosion conditions
and erodent particles and hence suggest that the terms brittle and ductile in the context of erosion
should therefore be used with caution. This leads to the further detailed microscopic analysis.
The surface morphologies of eroded coatings at 30º and 60º impact angles (Figure 8) shows
the evidence of grooves and ridges (lips) as the material ahead of the erodent is removed by
cutting and ploughing mechanism. Also material removal may occur in the form of platelets from
the ridges around the grooves by cutting and ploughing with the repeated impact of erodent. The
groove formation in the softer binder region act as failure initiating regions and this may also
result in undercutting of the carbide grains, which may get loosened and eventually pulled out,
whereas the major mechanism of material removal is by ploughing (crater formation).The pull-
out of the carbide grains (Figure 8) can also be seen in some regions.
At higher impact angle (90°), indentation impressions (Figure 8) due to the impingement of
erodent on the surface are clearly seen. The material around the grooves are generally deformed
manifest in the form of lips. The severity of deformation of the binder matrix, dislodge the carbide
particle from the surface and leads to the higher erosion loss. The impacts of erodent also damage
the chromium carbide spalts, where microcracks are clearly seen. The carbide particles as a result
of propagation of cracks within it, with further impact of erodent, are removed from the surface
as fragments or chips. Thus, the surface morphology indicates that the predominant mechanisms

are grooving of binder phase, cratering, microcracks and pull-out of carbide particles that are
prevalent in the coatings. These mechanisms are responsible for the composite erosion mode.
Figure 7 SEM micrographs showing the eroded surface morphology of SS316 steel eroded at various
impact angles (a) and (b) at 30° impact angle (c) and (d) at 60° impact angle (e) and (f) at 90° impact
angle
Figure 8 Surface morphology of CrC-NiCrFeSiBCoC (80%-20%) coated steels eroded at various
impact angles (a) and (b) at 30° impact angle (c) and (d) at 60° impact angle (e) and (f) at 90° impact
angle

Metal Ss-316
4. CONCLUSIONS
High velocity oxy-fuel thermal spraying with oxygen and liquid petroleum gas as the fuel gases
have been used successfully to deposit CrC-NiCrFeSiBCoC (80%-20%) alloy coatings on boiler
tube steels.
1. The CrC-NiCrFeSiBCoC (80%-20%) coating material behaves neither as purely
ductile nor purely brittle as a function of impact angle and has a composite behavior
whereas the morphology of the eroded surface point out grooving of binder phase,
cratering. Platelet formation and particle pull-out that is prevalent in the coatings. The
grooves in the binder region act as failure initiating concentrators and small carbide
grains crumble off uncrushed, whereas the main mechanism of large grains failure is
chipping.
2. Substrate SS316 steel exhibit lower steady state volume erosion rate in comparison to
all the HVOF coatings under similar test conditions. The higher hardness ratio
between alumina erodent particle and substrate steel might have caused the
penetration of alumina particles into the surface which bestow some shielding effect
against impacting particles leading to lower wear loss.
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