-
siat
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Keywords:
Liquid Impingement erosion
Metalmatrix composites
ui
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eel
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generally observed that the material removal process in
cavitation and Liquid Impingement erosion is
alike. In this paper, Liquid Impingement erosion performance of
composite coatings of nickel and
alumina, mixed in various proportions, has been investigated and
compared with the uncoated 13Cr4Ni
severof theent in
de-silt
bubbles, which implode upon reaching high-pressure region.
used. Recently,riants of jet typeoperated in un-testing
environ-more aggressivet type rig. On thersion based upon
well as, by the cavitation erosion is considered to be
identical
Contents lists available at SciVerse ScienceDirect
.e
Wea
Wear 301 (2013) 424433micro-jets in cavitation and minute
droplets in liquid [email protected],
[email protected] (H. Singh).[912]. Several authors have shown that
materials behave identi-cally under both droplet impingement and
cavitation erosion[9,1215]. In both the cases, material is
considered to be removedby the pressure waves generated during the
impacts by the
0043-1648/$ - see front matter & 2013 Elsevier B.V. All
rights reserved.
http://dx.doi.org/10.1016/j.wear.2013.01.063
n Corresponding author. Tel.: 91 1881 242177; fax: 91 1881
223395.E-mail addresses: [email protected] (H.S. Grewal),into
existence due to the presence of low-pressure zones in theow eld.
This results in the transformation of liquid into vapor
the principle of liquid droplet impingement. In principle,
thedamage produced by both the liquid droplet impingement, asvanes,
nozzles and other erosion affected components of turbineshave also
been attempted. However, there is a limit to theimprovement, which
can be obtained by improving the designof turbine components.
De-silting plants are not fully efcient toremove the silt from the
river water. Very small sand particleso300 mm are not effectively
screened out by de-silting plant andthus pass through turbines and
interact with the componentsurfaces [5]. Cavitation is a severe
form of erosion which comes
ultrasonic vibration test rig (ASTM G-32) and subrig (ASTM
G-134) have been more frequentlySoyama [7] and Oka et al. [8]
presented two varig. Both of these rigs are capable of
beingsubmerged condition, providing a convenientment. The rig
suggested by Soyama [7] presentsconditions in comparison with the
submerged jeother hand, Oka et al. [8] suggested a simplest
veintegrated with the hydropower plants to reduce the silt load.On
the other hand, improvements in the design of impellers,
advanced materials under cavitating environment, different
typesof test rigs have been employed by researchers. Among
them,
merged jet type1. Introduction
Performance of hydroturbines istion and slurry erosion [14].
Onethese problems comprises improvemnents and plant. On one
hand,steel. Coatings were deposited using the High Velocity Flame
Spray (HVFS) technique. Effects of micro
hardness, fracture toughness, work-hardening index, residual
stresses and porosity of coatings on the
Liquid Impingement erosion performance are discussed. In depth
study of the erosion mechanism in
each coating and their work hardening capabilities were studied
using SEM/EDS and micro-hardness
tester. It was observed that fracture toughness and work
hardening index of coatings correlate well
with the erosion resistance. The effect of alumina content on
erosion mechanism of the developed
coatings was also investigated. The content of alumina was found
to be having signicant effect on the
erosion response and the degradation mechanism of the
coatings.
& 2013 Elsevier B.V. All rights reserved.
ely affected by cavita-approaches to controlthe design of
compo-
ing plants have been
When this implosion takes place near the solid surface, the
highvelocity micro-jets formed out of imploded bubbles impact
thesolid surface, causing the removal of material [6].
Another possible approach to protect the components from
theerosion is the use of advanced materials having high resistance
toerosive conditions. For evaluating the performance of
theseThermal spray coatings
Power generationUnderstanding Liquid Impingement
eronickelalumina based thermal spray co
H.S. Grewal, H. Singh n, Anupam Agrawal
School of Mechanical, Materials and Energy Engineering, Indian
Institute of Technology
a r t i c l e i n f o
Article history:
Received 28 August 2012
Received in revised form
10 January 2013
Accepted 24 January 2013Available online 1 February 2013
a b s t r a c t
Hydroturbines and other
environment resulting in t
Although hydroturbine st
degrades severely under
against slurry erosion, ho
journal homepage: wwwon behaviour ofings
par, Nangal Road, Rupnagar, Punjab 140001, India
d machineries are generally subjected to cavitation and slurry
erosion
egradation of impellers, vanes, nozzle, spear, labyrinth seal,
and buckets.
(13Cr4Ni) provides sufcient resistance against cavitation,
however, it
rry erosion. Generally thermal spray coatings are used for
protection
er their performance under cavitation erosion is not
appreciable. It is
lsevier.com/locate/wear
r
-
impingement test rig. Moreover, the liquid droplet
impingementtest rig presents the most aggressive conditions amongst
all therigs used for cavitation erosion testing [16].
Although, stainless steels used as a structural material
inhydroturbines possess sufcient resistance against
cavitationerosion, however, these steels do not provide sufcient
resistanceagainst slurry erosion. Therefore, coatings are mostly
used toprotect the steels against slurry erosion. It has been
learnt fromthe literature, that surface coatings could improve the
slurryerosion of such materials. Amongst the several coating
depositionmethods, thermal spraying processes have attracted
considerableattention during recent times. However, it is observed
that thereis need to evaluate the cavitation erosion behavior of
thermalspray coatings [1722].
WC based thermal spray coatings can offer signicant resis-tance
against slurry erosion [5,23], however, studies on
cavitationerosion behavior of these coatings are limited. Espitia
and Toro[24] investigated the ame sprayed WC/CoFeNiCr and Cr O
Ni40%Al2O3 coatings on Inconel-600. They showed that althoughthe
coating performed better under slurry erosion, their
cavitationerosion performance was not satisfactory. Lower bonding
strengthalong with inferior cavitation resistance of Ni was
identied asmajor factors, responsible for the degradation of the
coating.Moreover, the micro-hardness of the coating was only
1.52.5times higher than that of substrate. However, in one of the
recentinvestigation by the authors, it was found that thermal
sprayingcould be a superior choice over the cold spraying, which
couldsignicantly improve the microstructure and mechanical
proper-ties of NiAl2O3 coatings [33]. The main objective of the
presentstudy is to ll this gap with regard to cavitation erosion
behavior ofthermal sprayed NiAl2O3 based coatings. A High Velocity
FlameSpraying (HVFS) technique has been used to develop
NiAl2O3based coatings. The content of alumina was varied in the
coatingsystem and its effect upon erosion response was
investigated.
2. Experimentation
2.1. Materials
A hydroturbine steel, 13Cr4Ni (ASTM A743) was used as
thesubstrate material in the current study. Three different
coating
H.S. Grewal et al. / Wear 301 (2013) 424433 4252 3
coatings and found that microstructural defects such as
porosity,un-melted or partially melted particles affected the
performanceof these coatings. Although these coatings showed higher
resis-tance against slurry erosion; however, their performance
undercavitation erosion was not satisfactory [25]. Even high
velocityoxy-fuel (HVOF) sprayed WC/Co and CrC coatings were
severelydamaged under cavitating erosion conditions [25]. On the
otherhand, WC20 wt% Cr3C27 wt% Ni coating showed higher cavita-tion
resistance in comparison to SS410 steel both in terms ofincubation
period, as well as, the amount of material removed[26]. Sang and Li
[27] also demonstrated a higher cavitationerosion resistance for
NiB based ame spray welded coatings.In particular, Ni17 wt% Cr3.5
wt% B coatings showed ve timeshigher resistance in comparison to
17Cr9Ni steel. Sugiyama et al.[28] evaluated the cavitation erosion
performance of series of WCbased coatings sprayed by HVOF and ame
spray techniques. Theame sprayed and fused 41WCNiCrCo coating was
the bestperformer among the batch of 15 coatings and 13Cr4Ni
baresteel. Other WC coatings did not improve the cavitation
erosionresistance of the bare steel. Yuping et al. [29] evaluated
HVOFsprayed FeCrSiBMn coatings and showed that these
coatingsperformed better in comparison to uncoated
ZG06Cr13Ni5Momartensite stainless steel. The enhancement in
resistance wasattributed to the presence of complex carbides in the
coatings andtheir higher hardness.
It is further concluded from the literature that the
slurry/cavitation erosion behavior of the NiAl2O3 based coatings
hasnot been evaluated extensively. Recently Hu et al. [30] studied
theslurry and cavitation erosion performance of cold sprayedFig. 1.
SEM micrographs of (a) Al2O3 andcompositions were prepared by
blending Al2O3 (corundum) powderin three different proportions; 20,
40 and 60 wt% with Ni powder. Thescanning electronmicroscopy
(SEM;Make: JEOL, Model: JSM 6610LV)micrographs of the feedstock
powders is shown in Fig. 1. It can beobserved that Ni particles, in
general, exhibit a regular roundmorphology, whereas, alumina
particles seem to be formed fromagglomeration of smaller particles
and have blocky appearance.A High Velocity Flame Spray (HVFS)
system is a proprietary productof Metallizing Company Pvt. Ltd.,
Jodhpur, India. This system wasused for the deposition of the
coatings. Table 1 shows the designationsystem for the developed
coatings. The various HVFS process para-meters are given in Table
2. As-sprayed, as well as, the uncoated steelsamples were polished
using emery paper down to 1500 grit,followed by cloth-wheel
polishing using slurry of 1 mm aluminapowder.
Table 1Designation system used for the developed ame sprayed
coatings on 13Cr4Ni steel.
Coating Ni20%Al2O3 Ni40%Al2O3 Ni60%Al2O3
Designation Ni20A Ni40A Ni60A(b) nickel powder used as
feedstock.
-
eroded samples were sectioned using slow speed diamond saw
and
3.1. Coating characterization
Detailed discussion on the characterization of the coatings
hasbeen given elsewhere [33]. Here, only a brief discussion is
presentedfor the sake of completeness. The surface morphology of
the as-sprayed coatings is shown in Fig. 3. The difference between
Ni andAl2O3 splats could be clearly observed. The SEMmicrographs
showingthe cross section of the coatings are shown in Fig. 4. From
these
H.S. Grewal et al. / Wear 301 (2013) 424433426The coatings were
characterized using SEM, equipped with energydispersive
spectroscopy (EDS; Make: Oxford) and X-ray diffraction(XRD; Make:
PANalytical, Model: X0pert Pro) technique. Residualstress in the
coatings was also measured using the XRD technique.Determination of
the residual stress was done using the standardsin2C method with
the help of expression given in Eq. (1). Here sis the residual
stress to be determined, E is Youngs modulus taken as200 GPa and n
is Poissons ratio taken as 0.3. The parameter m is theslope of line
obtained by tting the data points between measuredlattice
parameters and sin2C, where C is the angle between thenormal of the
sample and normal of the diffracting plane, measuredwith the help
of XRD. The indentation technique was used for thedetermination of
microhardness and fracture toughness of the coat-ings using a
microhardness tester (Make: Wilson, Model: MV 402D).Value of
fracture toughness (KIC) was calculated usingEq. (2) given by Evans
and Wilshaw [31], where P is the indentationload in N, whereas, c
and
s E1n
m: 1
K IC 0:079P
a
3=2log 4:5
a
c
2
a are the crack length from the center of the indent and
half-diagonallength respectively. The reported values of the
microhardness andfracture toughness are the average of 20 readings
obtained from theindents, made at random locations for each sample.
For determina-tion of hardness, load of 0.1 Kgf was used. Density
of the coatings wasmeasured using Archimedess principle according
to ASTM standard B311-08 and porosity using the image analysis
method. More detailsabout these measurements may be found elsewhere
[33].
2.2. Liquid Impingement erosion
Liquid Impingement erosion testing was conducted on both
Table 2HVFS process parameters used for deposition of
NiAl2O3, composite coatings on 13Cr4Ni steel.
Parameter Value
Flame temperature (1C) 2700Acetylene ow rate (lpm) 70
Oxygen ow rate (lpm) 42
Air pressure (Kg/cm2) 4.5
Powder feed rate (g min1) 12Spraying distance (mm) 20
Particle size, alumina (mm) 20100Particle size, nickel (mm)
2060the coated and uncoated samples using an experimental
set-upsuggested by Oka et al. [8]. A high-pressure plunger pump
drivenby 3 KW electric motor was utilized for pumping the
waterthrough 0.4 mm diameter diverging nozzle. A spray type
nozzlemade of SS 316 was used [8], which is capable of transforming
thejet of water into small droplets. Various testing parameters
usedfor the experimentation are given in Table 3. A schematic
showingthe principle of Liquid Impingement erosion testing is given
inFig. 2. Velocity was measured with the help of the
dischargemethod. Testing was conducted for a total time of 100 s
for thecoatings and 600 s for the bare steel, being interrupted
regularlyafter every 5 s. The exposed samples were washed with
acetoneprior to the weight measurement. Weight change
measurementswere made with an accuracy of 0.01 mg.
The eroded samples were analyzed using SEM, EDS and
micro-hardness tester. In order to investigate the cross-sectional
features ofwere polished following standard metallurgical sample
preparationpractice. Work-hardening index of the coating was also
calculatedfrom the indentation testing using Meyers Index given by
[32]
P Kdn 3where, d is the diagonal of the indent and K is the
proportionalityconstant. According to this expression, the
work-hardening iscorrelated with the value of the index, n. For
non-hardeningmaterials, the value of n is 2, whereas, if n42, the
material is saidto be work hardened [32]. For obtaining the value
of n, loglog plotbetween the load, P, and diagonal length, d, was
plotted and thusthe value of index, n was obtained from the slope
of the line.
3. Results and discussionthe exposed samples at the center of
the erosion scar, the sampleswere sectioned and subjected to SEM
analysis. For indentation testing,
Table 3Set of parameters used for cavitation erosion test-
ing of materials in current investigation.
Parameter Value
Velocity (m/s) 14773Pressure (MPa) 14
Stand-off-distance (mm) 200
Impingement angle (deg.) 90
Nozzle diameter (mm) 0.4
Fluid Tap water
pH of uid 7.2
Cycle time (s) 5
Testing time (s) 100
High-pressure pump Nozzle
Water droplets
Specimen
Fig. 2. Schematic illustration of experimental set-up employed
for Liquid Impin-gement erosion testing.micrographs, the typical
lamellar type structure of the coatings couldbe easily observed.
The dark phased, conrmed as Al2O3 by EDS, isobserved to be
distributed throughout the coatings. The presence ofsome
micro-pores and un-melted particles could also be seen in
thecoatings (Figs. 3 and 4). These un-melted particles are Al2O3
phase,identied using EDS. The splats of Ni seem to be in fully
melted state,holding the alumina splats/particles in place. The
bonding betweenthe splats of Ni and Al2O3 seems to be satisfactory.
However, bondingbetween the Al2O3 particles and Ni splats does not
appear to besatisfactory, as is perceptible from Fig. 5. The high
temperature at theinterface of the Al2O3 and Ni splats could have
resulted in theimprovement of the bonding. On the other hand,
temperature atthe interface of the Ni splat and Al2O3 particle
might not be highenough to facilitate the bonding. Area fractions
of the Al2O3 phase inthree coatings measured using the optical
micrographs were around25%, 39% and 58% for Ni20A, Ni40A and Ni60A
coatings, respectively.
-
Pore
Ni splat
Al2O3 splat
Al2O3 particle
50 m
Pore
50 m
Pore
Al2O3 splat 50 m
Fig. 3. Surface SEM micrograph of high velocity ame sprayed (a)
Ni20%Al2O3, (b) Ni40%Al2O3 and (c) Ni60%Al2O3 coating deposited
onto 13Cr4Ni steel [33].
elted
Pore
CoatingSubstrateb
Pore
Pore
Un-melted
Coating Substrate Substrate Coating
Pore
Pore
Fig. 4. Cross-sectional SEMmicrographs of high velocity ame
sprayed (a) Ni20%Al2O3, (b) Ni40%Al2O3 and (c) Ni60%Al2O3coating
deposited onto 13Cr4Ni steel [33].
H.S. Grewal et al. / Wear 301 (2013) 424433 427
-
me
H.S. Grewal et al. / Wear 301 (2013) 424433428Al2O3
Loosely bonded particles
Fig. 5. Higher magnication SEM cross-sectional micrographs of
high velocity a
a b c
Ni - Nickel NiO - Nickel oxide , phases of
Al2O3The XRD proles of the coatings are shown in Fig. 6,
whichindicate that with an increase in Al2O3 content in the
feedstock, theproportional intensity of Al2O3 peaks in the coatings
is also increasing.Some low intensity peaks of NiO could also be
observed in thesecoatings, indicating the possibilities of
oxidation of Ni powder duringthe coating process. The presence of
both a and g phases of Al2O3 wasrevealed in the coatings.
The microhardness and fracture toughness of the coatings
aregiven in Table 4. Optical micrograph of the indented Ni20A
coatingused for determination of the fracture toughness is shown in
Fig. 7.Crack lengths and half diagonal lengths were measure from
suchmicrographs, which were used for the estimation of fracture
tough-ness using the expression given by Evans and Wilshaw [31]. It
can beobserved that microhardness of the coatings improves with
theaddition of the Al2O3; however, the fracture toughness showed
amaxima for Ni40A coating. Furthermore, the density and
porosityvalues for the coatings are also presented in Table 4.
Density andporosity also showed an approximately linear trend.
Ni20A coatingsshowed a maximum density whereas, porosity was found
to bemaximum for Ni60A coating. Residual stress values for the
coatingsare also given in Table 4. Trend in residual stress data
shows that withthe rise in Al2O3 content, the compressive stress in
the coatingsdecreases.
3.2. Liquid Impingement erosion testing
The variation in volume loss (mm3) for the investigated
sampleswith testing time is plotted in Fig. 8, whereas, Fig. 9
presents plots ofvolume loss rate (mm3/s) versus exposure time. It
can be observedthat the coatings did not show any incubation
period, whereas, thesteel is still in its incubation period, even
till the end of total exposure
Fig. 6. XRD proles of high velocity ame sprayed (a)
Ni20%Al2O3,(b) Ni40%Al2O3 and (c) Ni60%Al2O3 coating deposited on
13Cr4Ni steel.time. The coatings showed high erosion rates during
initial period ofstudy as observed from Fig. 9. Subsequently, the
erosion rates haveshown the tendency to become uniform. It is
pertinent to mentionthat such trends predict protective nature of
the investigated coatingsduring longer duration of usage. Small
de-acceleration (dip in erosionrate) period was observed for the
coatings during initial 515 s.Furthermore, it has been observed
that none of the coatings couldslow down the erosion rate of the
steel. The Ni60A coating wasseverely affected and got completely
detached from the substrate.The erosion rate for this coatingwas
exceptionally high in comparisonwith the other investigated cases
as observed from Fig. 9. This may beattributed to its relatively
higher brittleness, low toughness andcompressive residual stress
(Table 4). Upon impact by high velocitydroplets, it is possible
that some cracks might have initiated in thecoatings along its
thickness. These cracks upon reaching the coatingsubstrate
interface would have tendency to propagate along
thecoatingsubstrate interface, since high crack resistance of steel
mightnot have allowed cracks to progress further along the
thickness. Thismay have eventually resulted in the complete
detachment of thecoating. The low value of residual stress (almost
tensile) could also beresponsible for rather easy propagation of
cracks in Ni60A coating.
It is further observed that the erosion rates for the Ni20A
andNi40A coatings were 70 and 40 times higher than that of the
steel,respectively. Among the coatings, Ni40A coating showed the
highestresistance against erosion. The erosion resistance (Re) of
coatingscalculated as the inverse of steady state erosion rate
(s/mm3) isplotted in Fig. 10, where Re of Ni60A coating is used for
the normal-ization (Ren). It can easily be observed that erosion
resistance of Ni40Acoating is 24 times that of Ni60A coating,
whereas, a correspondingmultiple for Ni20A coating is 14. Several
other investigators have alsoobserved that the thermal spray
coatings do not provide muchprotection against cavitation erosion
[24,25,28,30] with an exceptionof few cases as discussed in Section
1. On the other hand, coating
Ni Al2O3
Well bonded splat
sprayed (a) Ni20%Al2O3 and (b) Ni40%Al2O3 coating onto 13Cr4Ni
steel [33].deposited using laser and welding systems has
demonstrated higherresistance to cavitation erosion
[1820,22,34,35]. This inferior perfor-mance of thermal spray
coatings toward Liquid Impingement erosionmay be due to their
lamellar structure. Apart from hardness, LiquidImpingement erosion
also depends on the crack resistance of thematerial. The splat
boundaries in the thermal spray coatings could actas a preferential
site for the generation and propagation of the cracks.Along with
these splat boundaries, the presence of pores, un-meltedparticles
could also act as stress raisers, resulting in the generation
ofcracks. The SEM micrographs presented in the next section
providesome evidence of these facts.
3.3. Erosion mechanism
The possible mechanism responsible for the degradation of
thecoatings is discussed in this section. As reported earlier,
Ni60Acoating completely failed during the erosion testing. The
SEM
-
eel.
tou
)7
1
9
2
H.S. Grewal et al. / Wear 301 (2013) 424433 429Table 4Various
properties of high velocity ame sprayed NiAl2O3 coatings on 13Cr4Ni
st
Coating Coating thickness
(mm)7SDMicrohardness
(HV0.1 Kgf)7SDFracture
(MPaOm
Ni20A 684738 563790 1.470.1Ni40A 628754 714778 1.670.0Ni60A 5837
24 11517108 0.970.1micrographs of the eroded surfaces of Ni20A,
Ni40A coatings andbare steel are shown in Fig. 11. These
micrographs were takenduring initial stage of the erosion, after an
exposure for 30 s,whereas, Fig. 12 shows the corresponding
micrographs afteroverall exposure for 100 s. It is perceptible from
the analyses ofFigs. 11 and 12, that nature of damage has not
changed signi-cantly for the coatings, as the exposure time was
increased from30 s to 100 s. This is an indicator of steady state
of erosion after anexposure for 30 s. This observation is also
supported by the
crack
Indent
Al2O3
Ni
200 m
Fig. 7. Optical micrograph of the indented high velocity ame
sprayed Ni20%Al2O3 coating used for determination of the fracture
toughness.
Fig. 8. Liquid Impingement erosion volume loss versus time plot
for the uncoatedand coated 13Cr4Ni steel.ghness
SD
Density
(Kg/m3)7SDPorosity
(%)7SDResidual stress
(MPa)7SD
71007100 1.370.20 1227146350768 1.870.16 937105880783 2.570.25
3176erosion rate plot (Fig. 9), whereas, the inuence of exposure
timeon bare steel could be observed from the increase in the
numberof pits by the end of 100 s. A high magnied image of the
erodedsteel shown in Fig. 13 indicates the signature of plastic
deforma-tion along with those of fatigue. In the gure, one could
observestriation marks in the crater, which could be an indication
of thefatigue process.
The mechanism of material removal for the coatings, as
per-ceived from Figs. 11 and 12 appears to be predominantly
debond-ing of the splats. The presence of smooth surfaces craters
inmicrographs of both the coatings is the indication of the loss
ofmaterial through the removal of splats. As discussed earlier,
splat
Fig. 9. Liquid Impingement erosion volume loss rate versus time
plot for theuncoated and coated 13Cr4Ni steel.
Fig. 10. Erosion resistance of ame sprayed coatings in
comparison to Ni60A coating.
-
1Pit
Pit
Cracks
Site of detachedsplat
Pit
100 m
Pit Fragmented
splat
CracksSite of detachedsplat
100 m
Pits
Lip
5 m
Fig. 11. Surface SEM morphology of the eroded surface of high
velocity ame sprayed (a) Ni20A coating, (b) Ni40A coating and (c)
bare steel after 30 s of LiquidImpingement erosion testing.
Pit
LspLocaplat
ationn off deb
Pit
bon
t
10
nded
00
d
m
Locdeb
P
catibond
Cra
Pit
50
on oded
ack
0 m
of spl
m
lat
PPit
Cra
5
ack
50 m
Crack
Fig. 12. Surface SEM morphology of the eroded surface of high
velocity ame sprayed (a) Ni20A coating, (b) Ni40A coating and (c)
bare steel after 100 s of LiquidImpingement erosion testing.
H.S. Grewal et al. / Wear 301 (2013) 424433430
-
boundaries can provide an easy passage for the propagation of
thecracks. Therefore, during impingement of droplets, the
cracksmight have originated at the splat boundary, pore and/or
un-melted particle. Once the cracks are formed, the bonding of
thesplats with the matrix gets weakened during further
impacts,ultimately leading to the detachment of splats. In addition
to theloss of material in the form of splats, the presence of pits
in thematrix phase of the coatings was also revealed by the SEM
analysis.
The high intensity pressure droplet upon impact with softer
matrixcould result in the formation of such pits, resulting in the
loss ofmaterial. The pores, un-melted particles and pits formed at
theinitial erosion, could act as stress raisers, due to which the
failure ofsplats of Ni or Al2O3 could also take place through
fatigue inducedcracking as shown in Figs. 11a and 12b. Small
fragments of coatingmaterial would form through such kind of
cracking. The cross-sectional micrographs of the exposed samples
are given in Fig. 14aand b. The presence of cracks beneath the
eroded scars is quiteevident from these micrographs. Presence of
cracks along the splatboundaries could be observed. In these
micrographs, the origina-tion of cracks due to presence of
un-melted particle and pore couldbe observed. To conclude, it is
believed that both the coatingsexhibited a brittle mode of erosion
behavior, along with presenceof fatigue-induced cracks.
3.4. Correlation with coating properties
In order to understand the correlation between different
proper-ties of the coatings and the Liquid Impingement erosion
performanceof the coatings, the dependence of normalized erosion
resistance (Ren)of NiAl2O3 coatings on hardness, fracture
toughness, residual stress,porosity and work hardening rate was
evaluated. The dependence ofRen on hardness and fracture toughness
is presented in Fig. 15,whereas, correlation with residual stress,
and porosity in Fig. 16.
It is observed from Figs. 15 and 16 that erosion resistance
Renhas shown a linear dependence on fracture toughness, that is,
Ren isdirectly proportional to fracture toughness. Therefore, it
can beconcluded that in order to improve Re of the coatings,
fracture
Sifat
ignatigu
atureue
e offLip
5
p
5 mm
Fig. 13. Higher magnication view of the eroded surface of the
bare steel after100 s of testing.
Braanchhingg
CCrackks
Souurcee
10 mm
Fig. 14. Cross-sectional SEMmicrographs of the high velocity ame
sprayed (a) Ni20A and
20
30
(Ni40A)
ion
Res
ista
nce,
Ren
0
H.S. Grewal et al. / Wear 301 (2013) 424433 4310
10 (Ni20A)
(Ni60A)
Nor
mal
ized
Ero
s
Hardness (HV)600 800 1000 120Fig. 15. Correlation of (a)
hardness and (b) fracture toughne1.0 1.2 1.4 1.6
0
10
20
30
(Ni40A)
(Ni20A)
(Ni60A)
Nor
mal
ized
Ero
sion
Res
ista
nce,
Ren
Fracture toughness, KIC (MPa m-1/2)CCrack
Sou
ks al
urce
ong
e
splat bouundaaries Craack in
so
1
nitiaource
0 m
ation e
m
at
(b) Ni40A coating subjected to Liquid Impingement erosion after
an exposure of 100 s.ss with the erosion resistance of the NiAl2O3
coatings.
-
vided by the Council of Scientic and Industrial Research
(CSIR),
of Cavitation Erosion in Hydraulic Turbines, ASME, Albuquerque,
NM, USA,1985, pp. 5361.
Compressive Residual stress (MPa)
sity
H.S. Grewal et al. / Wear 301 (2013) 424433432toughness should
be enhanced. Interestingly, the correlationbetween hardness and Ren
is not, as straightforward as, betweenfracture toughness and Ren.
From Fig. 15, it can be observed thatNi20A coating having hardness
of the order of 580 HV has shown14 times higher Ren in comparison
with the Ni60A, which hashardness of the order of 1151 HV. This
higher Re of the Ni20coating is attributed to its higher toughness
and lower porositycontent. It can be observed from Table 4 that
Ni20A coating haslower porosity in comparison to Ni60A coating. As
discussed in theprevious section, the presence of porosity could
lead to higherstress concentrations, and could eventually lead to
greater loss ofmaterial.
On the other hand, the correlation of residual stresses with
Renof the coatings seems to be again non-linear. It is expected
thatcompressive stresses inside the coatings could provide
someassistance in confronting the generation and propagation of
thecracks. However, Ni20A coating having 1.3 times higher
compres-sive residual stress has shown 1.7 folds lower Ren in
comparisonto Ni40A coating. Work-hardening index calculated using
Meyer0sexpression given as Eq. (3), was also considered while
correlatingvarious properties of coatings with erosion resistance.
The hard-ening index, n, obtained for as-sprayed Ni20A, Ni40A, and
Ni60Acoatings was 2.04, 2.09 and 2.01 respectively. The index of
all theinvestigated coatings correlates well with their erosion
resis-tances, Re. For fully strain hardened material, the value of
n isaround 2, whereas for annealed materials it is around 2.5.
There-fore, the investigated coatings showed only a marginal
tendency
Fig. 16. Correlation of (a) residual stress and (b) poro0
10
20
30
(Ni40A)
(Ni20A)
Nor
mal
ized
Ero
sion
Res
ista
nce,
Ren
(Ni60A)
0 50 100 150of work hardening. The values obtained are very much
near to thefully hardened condition. Thus, it can be concluded that
thesecoating do not exhibit tendency for further hardening.
4. Conclusion
Liquid Impingement erosion performance of thermal sprayedNiAl2O3
coatings with varying content of Al2O3 was evaluated. Itwas
observed that content of alumina present in the coatingsignicantly
affected the erosion behavior and mechanism of thecoatings. Among
the investigated Ni/Al2O3 coatings, a maximumresistance was
provided by the coating containing 40 wt% Al2O3.Although none of
the coatings could enhance the erosion resistanceof 13Cr4Ni steel
for the given duration of 100 s. However, theoutcome of the study
would help in designing and developingcoatings for cavitation
erosion applications. It is anticipated thatNi20A and Ni40A
coatings could provide protection over longerduration of usage,
which can be concluded from the fact that thesehave the tendency to
achieve steady state erosion rate after 20 s.[2] J. Santa, J.
Baena, A. Toro, Slurry erosion of thermal spray coatings
andstainless steels for hydraulic machinery, Wear 263 (2007)
258264.
[3] P. Kumar, R.P. Saini, Study of cavitation in hydro turbinesa
review,Renewable and Sustainable Energy Reviews 14 (2010)
374383.
[4] M.K. Padhy, R.P. Saini, A review on silt erosion in hydro
turbines, Renewableand Sustainable Energy Reviews 12 (2008)
19741987.
[5] H.S. Grewal, S. Bhandari, H. Singh, Parametric study of
slurry-erosion ofhydroturbine steels with and without detonation
gun spray coatings usingtaguchi technique, Metallurgical and
Materials Transactions A 43 (2012)India, under project title
Development of Slurry Erosion ResistantCoatings for Hydroturbines
(File no. 22(0604)/12/EMR-II).
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0
5
10
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1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6
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H.S. Grewal et al. / Wear 301 (2013) 424433 433
Understanding Liquid Impingement erosion behaviour of
nickel-alumina based thermal spray
coatingsIntroductionExperimentationMaterialsLiquid Impingement
erosion
Results and discussionCoating characterizationLiquid Impingement
erosion testingErosion mechanismCorrelation with coating
properties
ConclusionAcknowledgmentReferences