siat

Ro

Keywords:

Liquid Impingement erosion

Metalmatrix composites

ui

he d

eel

slu

wev

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 dropletharpreetsingh@iitrpr.ac.in, hnr97@yahoo.com (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: harpreetsg@iitrpr.ac.in (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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Acknowledgment

Authors thankfully acknowledge the nancial assistance pro-

0

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10

15

20

25

1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

(Ni20A)

(Ni40A)

(Ni60A)

Porosity (%)

Nor

mal

ized

Ero

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Res

ista

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Ren

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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