EROSION BEHAVIOUR OF HVOF SPRAYED (CR3C2 …...HVOF processing did not degrade significantly the composition of the consumable and has been shown to produce coatings with low porosity,

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International Journal of Mechanical Engineering and Technology (IJMET) Volume 8, Issue 8, August 2017, pp. 1124–1134, Article ID: IJMET_08_08_112

Available online at http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=8&IType=8

ISSN Print: 0976-6340 and ISSN Online: 0976-6359

© IAEME Publication Scopus Indexed

EROSION BEHAVIOUR OF HVOF SPRAYED

(CR3C2-35%NICR) +5%SI COATINGS

B. Somasundaram, N. Jegadeeswaran and Madhu .G

School of Mechanical Engineering, REVA University, Bengaluru

M. R. Ramesh

Department of Mechanical Engineering, National Institute of Technology,

Karnataka, Surathkal

ABSTRACT

In the present study, (Cr3C2-35%NiCr) +5%Si cermet coatings were deposited on

a Fe based T22 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°. Silica sand particles of size ranging between

150 to180 μm were used as erodent. Analysis of weight-loss data and volumetric

steady state erosion rates for different coatings and substrate alloys are evaluated.

The bare T22 steel followed ductile erosion mode where as (Cr3C2-35%NiCr) +5%Si coating exhibited mixed behaviour respectively.

Keywords: HVOF, Solid particle erosion, Cermet’s, Surface analysis and boiler tubes.

Cite this Article: B.Somasundaram, N. Jegadeeswaran, Madhu.G and M.R.Ramesh,

Erosion Behaviour Of Hvof Sprayed (Cr3c2-35%Nicr) +5%Si Coatings, International

Journal of Mechanical Engineering and Technology 8(8), 2017, pp. 1124–1134.

http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=8&IType=8

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 [1]. 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 [2] It is generally

believed that the most erosive species in the fly ash are quartz, which is a crystalline form of

SiO2 and mullite. More than one quarter of all the boiler tube failures worldwide are caused

by fly ash erosion [3-4].

B.Somasundaram, N. Jegadeeswaran, Madhu.G and M.R.Ramesh


Coatings provide a way of extending the limits of use of materials at the upper end of their

performance capabilities by allowing the mechanical properties of the substrate materials to

be maintained while protecting them against wear or hot corrosion [5]. The role of coating as

a corrosion barrier is similar to the role of oxide layers. The coating is protective, if it prevents

outward diffusion of metal cations and inward diffusion of elements that could react with the

substrate material. Oxide layers forming on metals in reactions with atmosphere are self-

healing to a certain extent. If the oxide layer wears of or breaks up, a new oxide layer with

almost equivalent properties will form [6].

Thermal spraying has emerged as an important tool of increasingly sophisticated, surface

engineering technology. Various properties of the coating, such as wear and hot corrosion

resistance, thermal or electrical insulation can be achieved using different coating techniques

and coating materials [7]. The term thermal spray describes a family of processes that use

chemical or electrical energy to melt (or soften) and accelerate particles of a material which is

then deposited on a surface [8]. The quality of the coatings obtained by thermal spray

techniques is related to the nature of the process and the processing parameters. On the other

hand, thermal spray coatings are a good option to repair components and prevent excessive

wear because during the deposition process no significant changes to the microstructure of

substrates or excessive deformation are promoted [9]. The principle of thermal spray is to

melt material feedstock (wire or powder), to accelerate the melt to impact on a substrate

where rapid solidification and deposit build up occurs [10].

The high-velocity oxy-fuel (HVOF) process belongs to the family of thermal spraying

techniques, and is widely used in many industries to protect the components against erosion,

corrosion and wear. Particle degradation and open porosity are the two important factors that

affect corrosion and erosion resistance. HVOF processing did not degrade significantly the

composition of the consumable and has been shown to produce coatings with low porosity,

low oxide content, better density, better coating cohesive strength and bond strength than

many thermal spray processes [11-12].

Cr₃ C₂ –NiCr and WC based thermal spray coatings using HVOF technique, appear to be

the best alternative to hard chromium plating in most cases mainly when good wear or hot

corrosion resistance is required [13]. With the HVOF spraying technique low porosity of

metallic and ceramic–metallic (cermet) coatings can be achieved, having good oxidation

resistance and adhesion properties as well as faster deposition rates compared with other spray

and coating processes [14].

Uusitalo et al. [15] and Warren [16] also suggested that coatings for combined erosion-

corrosion protection, complex formulations containing Ni, Al, Cr, Mo, Si, borides and other

minor elements may also be used. Nickel-based materials are more resistant than iron-based

materials. Chromium is considered as the most beneficial alloying element for sulfidation

resistant steels. Aluminum and silicon are also reported to increase sulfidation resistance of

steels. Depending on the partial pressures of oxygen and sulfur, oxide formers, like chromium

and silicon, may form oxides also in reducing combustion atmospheres, whereas iron and

nickel form mainly sulfides.

In the present investigation, the combination of (Cr3C2-35%NiCr) with 5%Si 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.

Erosion Behaviour Of Hvof Sprayed (Cr3c2-35%Nicr) +5%Si Coatings


2. EXPERIMENTAL PROCEDURE

2.1. Substrate Material and Development of Coating

Fe based T22 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 (Cr3C2-35%NiCr) +5%Si 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.

HVOF process parameter Quantity

Oxygen flow rate 250 l/min

Fuel (LPG) flow rate 65-70 l/min

Air-flow rate 550 l/min

Spray distance 178 mm

Powder feed rate [Cr3C2-35%(NiCr)]+5%Si 28 g/min

Fuel(LPG) pressure 681 kPa

Oxygen pressure 981 kPa

Air pressure 588 kPa

Table 1 Spray parameters employed for HVOF spray process

Erodent material Silica sand

(Angular)

Erodent size (μm) 150-180

Particle velocity (m/s) 30

Erodent feed rate (gm/min) 2.2

Impact angle (°) 30,60 and 90

Test temperature Room Temperature

Test time (min) Cycles of 5 minutes

Sample Size (mm) 25 x 25 x 5

Nozzle inner diameter (mm) 1.5

Standoff distance (mm) 10

Table 2 Erosion test conditions

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 P. E. S 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 silica 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 silica 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.

Figure 1 Schematic view of an Air Jet Erosion Test Rig

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 erodent

An 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 T22 substrate (top row, in

sequence for 30°, 60° and 90º) and (Cr3C2-35%NiCr) + 5% Si coating (Bottom row, in sequence for

30°, 60° and 90º)

Figure 3 Variation of the Incremental erosion rate with the cumulative weight of the erodent for

uncoated T22 steel at 30°, 60° and 90° impact angle

Figure 4 Variation of the Incremental erosion rate with the cumulative weight of the erodent for

(Cr3C2-35%NiCr)+5%Si coatings at 30°, 60° and 90° impact angle



Figure 5 Histogram illustrating the steady state volume erosion rate of uncoated T22 steel at different

impact angles

Figure 6 Histogram illustrating the steady state volume erosion rate of (Cr3C2-35%NiCr) + 5% Si

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 5. The steady state volume erosion rate of

the T22 steel (Figure 5) 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. This is in agreement with the study of Tabakoff [18] for Superni 718 superalloy.

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 T22 steel.

Similar observation has been reported by Ninham [19] where erosion response of high

strength materials such as Superni 718, particularly when eroded by quartz, is weakly

dependent upon impact angle. Identical observations have been reported by Hidalgo et al. [20]

for plasma sprayed Ni-Cr coating. 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 22 g of impacting particles and, subsequently, reaching a steady state erosion rate

which is considerably lower than the peak rate[21-22].

In the present work, the T22 substrate steel demonstrate lower erosive loss when

compared to the HVOF sprayed coatings under the same test conditions (Figure 3 and 4). The

embedment of silica particles into the substrate steel imparts the shielding effect against

further material loss. The erosion mechanism is controlled by the hardness ratio which is

defined as the ratio of hardness of the erodent particles (Hp) to the target hardness (Ht) [23-



26]. The silica particles have an average hardness value of 880 Hv and, for the T22 steel, it is

196 Hv. The hardness ratio Hp/Ht is approximately 4.4, which cause the penetration of silica

particles into the target material. Hutchings [27] reported that the abrasive silica particles of

any shape will embed the surface and cause plastic scratching, only if Hp/Ht>1.2.

The Scanning electron micrographs obtained on the eroded T22 (Figure 6) clearly shows

the embedment of silica sand particles into the substrate steel and the mechanism of wear is

due to indentation induced severe plastic deformation. The embedment of silica sand onto the

surface also results in variation in erosion rate with impact of cumulative mass of erodent.

Mishra et al. [28] and Sidhu hazoor singh et al. [29] also reported regarding the silica particle

embedment on the surface of eroded superalloys and steels.

It is observed from the SEM micrographs (Figure 7) of the eroded surface at 30° impact

angle that the silica 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. Similar erosion

behavior of the ductile materials has also been reported by Brown et al. [30], Kumar et al.

[31]. 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 silica 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 silica

eroent 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 silica 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, as indicated by different parameters (Levy 1986). The

erosion rate curves (Figure 4) 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 Cr3C2-35%(NiCr) + 5% Si 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[27], Sundararajan and Roy [32] reported the dependence of the

erosion loss on the impact angle is not a characteristic of the material alone, 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. Similar observations of carbide pull out from the WC-Co, WC-Co-Cr and Cr3C2-

20NiCr coating surface at shallow angle of impact has been reported by Hawthorne et al. [33],

Feng and ball [34].

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 impact

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 T22 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

Lip

Groove crater

Silica

particle

Platele

(a)

(c)

(e)

(b)

(d)

(f)



Figure 8 Surface morphology of (Cr3C2-35%NiCr) + 5% Si 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

4. CONCLUSIONS

High velocity oxy-fuel thermal spraying with oxygen and liquid petroleum gas as the fuel

gases have been used successfully to deposit Cr3C2-35%(NiCr) + 5% Si alloy coatings on

boiler tube steels. LPG fuel gas is cheap and readily available as compared to other fuels used

for HVOF spraying.

1. The (Cr3C2-35% NiCr) + 5% Si 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 are 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.

Plough

Mark

Grove

Grove

Groove

formed due to

carbide pull

Crater

Indentation

Mark

Li

p

groove

(a)

(e) (f)

(d) (c)

(b)



2. Substrate T22 steel exhibit lower steady state volume erosion rate in comparison to all

the HVOF coatings under similar test conditions. The higher hardness ratio between

silica erodent particle and substrate steel might have caused the penetration of silica

particles into the surface which bestow some shielding effect against impacting

particles leading to lower wear loss.

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Documents

EROSION BEHAVIOUR OF HVOF SPRAYED (CR3C2 …...HVOF processing did not degrade significantly the composition of the consumable and has been shown to produce coatings with low porosity,