mechanism of material removal during cavitation …cavitation phenomena resulting in cavitation...

International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 6, Issue 3, March 2016)

259

Mechanism of Material Removal during Cavitation Erosion of

HVOF Coatings T. Pramod

1, R. K. Kumar

2, S. Seetharamu

3, P. Sampath Kumaran

4

Materials Technology Division, Central Power Research Institute, Prof. Sir C.V Raman Road, Bangalore, India

Abstract -Hydraulic machineries such as turbine, pump etc.

used in the generation of electric power, are exposed to

different water conditions. As an account of bubble formation

and subsequently collapsing of bubble cavitation erosion of

hydro part takes place. Cavitation erosion phenomenon

depends on both the cavitation intensity and the resistance of

the material to withstand the weight loss. To mitigate the

severe erosion due to cavitation, development and evaluation

of a new erosion resistant coating is one of the possible

solution to improve the life of parts. Presently, the turbine and

pump components are given a hard coating of 300 to 500

microns using High Velocity Oxy Fuel (HVOF) technique and

the life achieved for one full season.

The present paper highlights the results of cavitation

erosion resistance of four different tungsten carbide coatings

processed through HVOF. The properties of the coatings such

as chemical composition, hardness, porosity were evaluated.

The indentation toughness of coatings was evaluated at a test

load of 10 kg on the cross section of coatings. The weight loss

of coating of cavitated surface was measured at different

intervals. The progression of surface damage morphology

after different cavitation exposure periods of the coating was

analyzed using Scanning Electron Microscope. The effect of

porosity was studied with regard to the metal loss during

cavitation. The mechanism of metal removal has been

identified and studied from the examination point of view of

surface degradation.

Keywords - HVOF, Cavitation erosion, Porosity, Surface

topography, Surface roughness

I. INTRODUCTION

Erosion of underwater parts is a serious problem

encountered in hydro power plants. In Indian context, the

problem is much more serious and grave in the perennial

Himalayan rivers which have a very high (quartz) silt

content, ranging between 5000 ppm to 20000 ppm in

monsoon season alone. About 80% of hydro power is

coming from Himalayan region and the silt orientation in

this region constitute to about 60% of quartz Continuous

demand of electrical energy in India has necessitated a

smooth function of hydro power plants, mainly in the

Himalayan region [1]. Presently, the hydro part

components are made from martensitic stainless steel

material. Hard coatings processed through thermal spray

processes are widely employed to achieve improved service

life of these components.

The trailing end of runner blades as well as draft tube

cone components are affected predominantly due to

cavitation phenomena resulting in cavitation erosion loss.

To reduce the loss different types of hard coatings are

adopted for hydro power applications. Thus the need for

evaluation of these hard coatings becomes necessary.

Various techniques have been reported in the literature [2].

Hard coatings are the most suited materials for

machinery parts for providing excellent wear

characteristics, including cavitation erosion resistance in

view of high hardness [3, 9, 10]. A combination of the

development and evaluation of materials specifically

erosion resistance and the appropriate technique for the

application of these materials, as a coating would be the

optimum solution. Thermal spray coating represents an

important and cost effective technique for tailoring the

surface properties of engineering components with a view

to enhancing their durability and performance under a

variety of operating conditions. Among the various thermal

spray coating technique, the high velocity oxy fuel spray is

considered to provide hard, dense and consequently wear

resistant coatings and it has been moved up a good demand

in recent years [4,5].

A. Thermal spray technique

The HVOF process consists of a high pressure water

cooled HVOF combustion chamber and long nozzle. Fuel

(kerosene, acetylene, propylene and hydrogen) and oxygen

along with powder form of a feeder use fed to the

pressurized combustion chamber with the help of a nozzle

by forcing hot gases to pass through to increase its velocity.

The ability to produce dense coatings with low amount

of degradation, oxidation of metallic materials, and phase

transformations is the main feature of the HVOF process

[6]. The heated powder particles leave the nozzle at high

velocities in excess of 700 m/s strikes the substrate surface

forms dense coating in successive layers.The cavitation

performance of these coatings are affected by porosity,

hardness and toughness properties and thus different

coatings prepared under specific process conditions need to

be assessed through simulated test rigs for comparative

performance.



260

While the WC-CoCr based coatings are widely accepted

coating for achieving the silt erosion resistance properties,

a combination of silt and cavitation properties are possible

through modification of the matrix composition. Also, CrC

based hard coatings, having similar characteristics as that

of WC-CoCr are widely used for erosion resistance under

high temperatures as well as corrosion resistance

applications.

The process parameters adopted during the coating

process are given in Table I.

Table.I

Process parameters used during HVOF spraying

Sl.No. Parameters Values

1 Primary gas flow (Nitrogen) 50 lpm

2 Kerosene flow 15 lph

3 Nozzle diameter 10 mm

4 Oxygen flow 1100 scfh

5 Transverse speed 175 mm/s

6 Approx. thickness of coating per pass 6-8 µm

In this work, comparative evaluation of WC-CoCr

coating mixed with CrC+NiCr and spray atomized

NiCrBSi powder were studied. Also the cavitation

performance of 74-20-6 (CrC-Ni) based coating was

studied and the coding for the coatings are given in Table

II. Table.II

Coding of different types of coatings

Sl.No. Coating Type Coding

1 86-10-4 C1

2 90-10 NiCrBSi M1

3 74-20-6 C2

4 90-10CrC M2

II. EXPERIMENTAL PROGRAM

A. Materials Studies

Four tungsten carbide coatings were made on to 420

grade base stainless steel of size 100x100x8 mm by HVOF

technique.

Fig.1 Spay Powders a)WC-CoCr spray powders -45 +15µ size, b) NiCrBSi spray atomized powders (~25 µ), c) 74-20-6 spray powder -45+15µ size

and d) CrC spray powders -45 +25µ size

JP5000 equipment with Praxair spray powder, having an

average grain size of 1.6 micron was used for spraying. The

morphologies of the feed stock powder used for coating

process are shown in Fig.1.

Small specimens of size 15x15x8 mm were cut by EDM

process from the plates prepared. The specimens were

polished using different grits of diamond grinding discs

(220 to 1200 grit) followed by cloth polishing using 9µ, 6µ

and 1µ diamond suspension spray. The surface roughness

(Ra) value of as polished coatings was measured to be 0.2

µm.

B. Porosity and Toughness measurement

Thermal spray coatings are susceptible to formation of

porosity due to lack of fusion between sprayed particles

and expansion of gases generated during the spray process.

The determination of area of porosity was carried out on

the coating cross section by ASTM E2109.

The average porosity value was reported based on

average of five readings is given in Table 3.

The indentation toughness of coatings was measured in

the cross section at 10 kg load using Vickers indentation.

The average toughness value was calculated based on crack

length measured using the formula

Kc= 0.0193* Hv*D* (E/Hv)(2/5)

* a(-0.5)

…….. [7]

Where, Kc is Fracture Toughness (MPa√m), Hv is

Vickers hardness (GPa), D is Half the diagonal (µm), E is

Young‘s Modulus (GPa) and ‗a‘ is the crack length (µm).

C. Cavitation Erosion Test Setup

Cavitation erosion resistance was measured in a

vibratory cavitation test rig and the detail of the cavitation

test rig is shown in Fig.2. The frequency of 20 KHz with an

amplitude of 100 micron (peak to peak) was used. The end

of the horn is attached with a replaceable titanium tip. The

water level above the tip was maintained at 25 mm.



261

Fig.2 Cavitation test setup

The water level above the tip was maintained at 25 mm.

Under the conditions of high frequency oscillations within

the water medium, the pressure at the horn tip reaches

below the vapor pressure of the water, giving rise to the

formation of bubbles. The bubbles so formed gets

accelerated during the positive pressure cycle and implode

on to the sample test surface, leading to localized erosion

damage. The cavitation testing was carried out as per the

guidelines of ASTM G32. The test specimen was placed at

a small distance of 1.0 mm below the tip of the ultrasonic

probe.

The test was conducted for a total duration of 9 hours

and weight loss was measured at different intervals. The

samples were cleaned with acetone, dried, weighed to an

accuracy of 0.1mg using an electronic balance, to

determine weight loss. The eroded surfaces were studied in

SEM for identifying the mechanism of material damage in

these coatings.

III. RESULTS AND DISCUSSION

The chemical compositions of the coatings were

evaluated by Energy Dispersive X-Ray analysis and the

results are given in Table III.

Fig.3 shows the cross sectional view of the coatings seen

through an optical microscope. The coating thickness was

observed to be in the range of 400 to 830 microns.

While, the addition of 10% NiCrBSi to C1 powder,

result in increase in matrix content from 14% to 22.6% in

the sprayed coating, the addition of 10% CrC to the powder

decreases the matrix content to 6%.

Table III.

Chemical composition of the coatings

The hardness value was measured by Vickers scale at a

test load of 0.3 kg and the average of eight indentation

values were reported. The results of the porosity, toughness

and hardness are given in Table IV.

The phase composition analysis of the coatings and the

feed stock powders was analysed using Panalytical

x‘pert‘pro diffractometer with Cu-Kα radiation in the

30º≤2θ≤90º. The integral intensities of WC and W2C were assessed

corresponding to (100) and (101) peak respectively.

Fig.3 Coating cross section a) C1, b) M1, c) C2 and d) M2

(a) (b) (c) (d)



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Table IV

Mechanical properties of the coatings

The phase analysis of the coatings was carried out using

XRD and the result has shown a small peak of metallic

tungsten and CrC peaks for WC-CoCr (C1) and CrC-Ni

(C2) as seen in Fig.4.

Fig.5 shows the porosity and indentation toughness

measured through optical microscope.

Fig.4 Phase analysis of a) C1 and b) C2 coatings respectively

Fig.5 Optical micrograph showing porosity and Indentation toughness across the cross section of the coatings a) C1, b) M1, c) C2 and d) M2

A. Cavitation Loss Measurements

The variation of weight loss of different coatings with

cavitation time is shown in Fig. 6. All the coatings undergo

metal loss right from the beginning of cavitation exposure

and increases linearly with time [8].

The existence of inherent coating, which act as

nucleation areas for the cavitation damage and thus

undergoes metal loss from the initial period of cavitation.

The porosity regions undergo bubble imploding force

causing the inter-splat regions weak enough to dislodge the

hard carbides out of the matrix region. Hence the

phenomenon of incubation period is not observed in these

coatings.

Fig.6 Metal loss with cavitation time

(a) (b) (c) (d)



263

The surface damage during the initial period of

cavitation included the occurrence of plastically

deformation of the matrix layer, cracking along WC grain

boundaries as well as cracking on the interlayer regions.

Fig.7 Rate of weight loss with cavitation time

The rate of weight loss (dw/dt) calculated based on

cumulative metal loss shown in Fig.7. The rate of metal

loss of C2 and C1 coatings is observed to be nearly same.

However the addition of 10% of NiCrBSi marginally

increased the rate of weight loss compared to base C1

coating. This appears probably due to the comparable

toughness achieved in this coating. However M2 coatings

has shown exponentially decreasing rate of weight loss till

5hrs exposure and reaches a steady state in the subsequent

exposure periods. The combined effect of high porosity and

reduction in toughness of the coating (up to 45%) caused

an increase in the rate of cavitation loss. The rate of metal

loss due to cavitation of C2, M1 and M2 coatings was

observed to be 1.6, 3.3 and 5.2 times higher than C1

coating.

B. Change in Surface Roughness during cavitation

The analysis of surface roughness was measured using

Talyor Hobson roughness machine at three different

locations for an evaluation length of 4mm as can be seen in

Fig.8 and 9.

Fig.8 Roughness measurement of the different coatings

The change in surface damage morphology after

different periods of cavitation was assessed in terms of

change in surface roughness value (Ra) after different

cavitation intervals. The average of five measurements

performed on randomly selected areas of each sample was

reported. The surface roughness increases linearly with

time up to 7 hrs and decreases subsequently in the C1, C2,

M1 & M2 respectively with a decrease in fracture

toughness. The Ra value of M2 has shown highest value for

all the exposure periods evaluated. The extensive severity

of surface damage observed in terms of gross removal of

coating layers giving indication of large & deep pit like

appearances supports this point of view.

As can be seen from the Fig.6 & 8, between the time

variation curves of cumulative weight loss and surface of

the M2 coating assumes the highest volume loss with major

surface changes while compared to the other three coatings.

The observed deep valleys complied with the extensive

cracking in the vicinity of the damaged lend support to this

view.

The surface roughness developed after cavitation is

readily affected by the hardness of the coating. Hence the

higher hardness value gives rise to lower roughness values.

The variation of profile peak height (Rp), valley depth

(Rv), mean spacing between the peaks (Rsm) and height of

profile (Rz) values were compared for all the coatings.

While, the maximum depth of the coating affected due to

the cavitation (Rz) gives an indication of the extent of

inter-layers, the width of the profile generated after

cavitation gives the information about of efficiency of the

matrix, in providing the required reinforcement.



264

The roughness profile of all the four coatings had shown

the prevalence of multiple sharp valleys (Rv) and the

measured values was in the range of 20 to 35µm at initial

exposure of 1hr. For all the coatings the profile width

measured along the mean line indicated a value close to

100µm for 3hrs cavitation. However, the width of profile

increases rapidly after 3hrs and in particular the M2, the

relative increase in width was observed to be 1.8 times

more than that of other coatings. The observed high mass

loss in this coating thus indicative of the gross loss of

matrix and carbide detachment.

For the initial exposures, Rp and Rv were sharp and

narrow because of the micro surface degradation area,

which later increased to wider and deep valleys where the

Ra varied from 11um to 14um due to the macro surface

damage at an exposure of 9hrs as can be clearly seen in

Fig.9.

The Rz of C2, M1 and M2 coatings shows consistent

and uniform variation of 1.70, 1.85 and 2.0 times higher

than C1 coating at all the exposure periods as can be seen

in Fig.10.

Fig.9 Profile evaluation of different coatings at the exposure periods of 3hr and 9hr respectively

Fig.10 Variation of total height of the profile

C. Change in surface morphology during Cavitation

The matrix of C2 coating shows uniform eroded surface

with absence of deep pits and volume of the affected area is

considerably low compared to C1 coating. The M2 shows

some lager radial pits on the perimeter region of the eroded

surface, which also exhibited the roughest macro-eroded

surface with many pits being visible which has also

reported in [11-12].

Fig.11 shows the macro eroded surface of all the

coatings at an exposure of 1hr where M1 (10% NiCrBSi)

shows clearly the radial damage compared to 90-10CrC

(M2) coating which is completed eroded at the initial

exposure. In C2 coating flat eroded surface with

considerable higher volumetric area compared to the WC-

CoCr coating.



265

The evaluation of surface damage in different coatings

after cavitation was observed through SEM and the

micrographs are shown in Fig.12 and 13.

The percentage of the original polished area affected

during the initial 1hr period was calculated at different

regions through image analysis and the average of five

locations is shown in Fig.12.

The C1 base coating has shown the lowest affected area

of 27% against 57.6% for M2 coating during the initial

period of one hour. This is in line with the highest peak

depth observed in this coating. The ability of NiCrBSi

matrix in sustaining the bubble implosion during the initial

cavitation is shown in Fig. 12(b). The morphology of

NiCrBSi particles has undergone morphological change

from spherical to irregular shape during the high velocity

impact in the HVOF process and also the percentage of

affected area is close to that of C2 and M2 coating.

The change in surface damage morphology after 9hrs

cavitation has shown distinctly changed structure in all the

coatings. The damage in terms of delamination and gross

plastic deformation of matrix (Fig.13 b&d) could be clearly

seen. While the morphology of C1 and C2 coatings was

similar, the relatively higher weight loss of C2 coating is

attributed to increased depth of damage observed in terms

of higher Rz values.

The M1 shows a relatively flat macro-eroded surface in

the centre, whereas C2 coating shows pits away from the

porous region at initial exposures and progresses during

continued exposure resulting in large craters in the affected

areas Similar observations have been reported in cavitation

erosion studies on WC-17Co cermet HVOF coatings [9-

10].

In C1 coating, minor craters have been observed and the

coating remains intact after the 9hrs of cavitation exposure

compared to C2 coating.

Fig.11 Macro eroded surface degradation at an exposure period of 1hr for all the coatings



266

Fig.12 Surface topography of cavitated tested coatings after 1hr period

Fig.13 Surface topography of cavitated tested coatings after 9hrs period

(d)

(b) (a)

(c)

(d) (c)

(a) (b)



267

IV. CONCLUSIONS

Four types of coatings with modified matrix content

were deposited by the HVOF spray processes. The phase

analysis, chemical composition, mechanical properties,

cavitation erosion resistances, surface degradation studies

of these coatings were investigated. The following

conclusions can be drawn from the present study:

The coating porosity readily affects the rate of metal

loss during cavitation and the metal loss in the initial

stage of cavitation occurs in porosity regions. While

the actual metal loss during the initial stages of

cavitation is comparable with different porosities, the

higher porosity coatings experience accelerated

surface damage during extended hours of cavitation.

Thus the rate of metal loss is considered the function

of final surface porosity of the coating.

The cavitation erosion damage in all the coatings

occurrence is mainly due to generation and

propagation of cracks induced by the cyclic micro-

impact loads, which led to a larger number of hard

carbide phase particles being continuously pulled off

the surface of the coating and progressive damage

occurs subsequently by fatigue.

The improved cavitation erosion resistance of C1

coating is attributed to higher toughness and low

porosity.

While the addition of CrC or NiCrBSi brings in an

increase in matrix content, they do not seem to play a

significant role in improving the cavitation resistance.

The main mass removal mechanisms observed in

thermal spray coatings after cavitation tests was brittle

fracture showing both delamination and detachment

of the WC grains.

The roughness of the cavitated surface progressively

increase with time and follows a similar trend with

that of metal loss.

The total height of the profile was found to increase

linearly with cavitation time in all the coatings.

During the initial period of cavitation the peaks of Rv

was sharp and narrow because of micro-craters

whereas at higher exposure the peaks were found to

be wide and deep due to formation of large surface

degradation.

Acknologment

The authors thankfully acknowledge the management of

CPRI for according permission to present this paper.

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mechanism of material removal during cavitation …cavitation phenomena resulting in cavitation...

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