23. - 25. 10. 2012, Brno, Czech Republic, EU

WEAR BEHAVIOUR OF HVOF COATINGS ENGINEERED FROM NANOSTRUCTUREDWC-

15CO AND CONVENTIONAL WC-10CO4CR POWDERS

Iosif HULKAa,Viorel Aurel ŞERBANb, Dragoș UȚUc,Heli KOIVULUOTOd, Petri VUORISTOe, Kari

NIEMIf

“Politehnica” University of Timişoara, Timişoara, Romania, EU,

a hulka_iosif@yahoo.com,

b viorel.serban@rectorat.upt.ro,

c dragosutu@yahoo.com

Tampere University of Technology, Tampere, Finland, EU

dheli.koivuluoto@tut.fi,

e petri.vuoristo@tut.fi,

f kari.niemi@tut.fi

Abstract

Tungsten carbide base coatings deposited by High Velocity Oxy-Fuel (HVOF) technique are the most

common materials deposited in order to protect the components surface against wear and corrosion. The

purpose of this study is to investigate the sliding and abrasion wear and corrosion behaviour of HVOF

coatings engineered from nanostructured WC-15Co and WC-10Co4Cr powders. A 5% addition of

nanostructured WC-15Co powder was added to a conventional WC-10Co4Cr powder in order to improve the

properties of the coating. The coatings have been characterized by scanning electron microscope (SEM)

equipped with energy dispersive X-ray spectroscopy (EDAX) analyzer, X-ray diffraction (XRD) as well as the

microhardness testing was performed. The wear behavior was evaluated by means of rubber wheel abrasion

and ball-on-disk tests. The worn surfaces have been investigated by SEM microscope and optical

profilometer. The results showed that the nanostructured powder had a positive influence on sliding wear

and corrosion behavior of the coating in comparison with the conventional coating.

Keywords: HVOF, WC based coatings, nanostructures, wear

1. INTRODUCTION

Wear is the most common and unavoidable problem of mechanical parts in many industrial fields which

reduces the lifetime and performances of components [1]. To avoid this problem, wear resistant alloys like

WC based cermet coatings are deposited by thermal spray processes to enhance surface properties. WC-Co

powders are widely used to by various thermal spray processes to deposit protective coatings in a large

variety of applications where abrasion, erosion or other form of wear exist [2]. Due to the Cr content

agglomerated and sintered WC-CoCrpowders are used to manufacture coatings which besides a good wear

resistance are also suited for corrosive environments [3].One of the most popular spraying methods for WC

cermet coatings is the HVOF (High Velocity Oxygen Fuel) spraying process due to high flame velocities up to

2000 m/s [4] and lower process temperatures in comparison with plasma spraying which leads to less

decomposition of carbide phase during spraying [5]. In the HVOF spraying, oxygen and fuel are mixed and

burnet in a combustion chamber at flow rates and pressured in order to produce high temperatures and high

speed gas jet [6]. The high kinetic energy ensures that the powder particles will form lamellar splats and

allows the production of dense coatings with reduced porosity. Some authors observed that the WC-Co

cermet with reduced carbide grains in the nanometer range have an increased wear resistance. The problem

with nanostructured coatings is the higher decomposition of carbide during spraying due to higher surface

area to volume ratio of carbide grains in the feedstock powder [7]. The aim of this study is to investigate the

influence of nanostructured WC-15Co addition to a conventional WC-10Co-4Cr coating and compare the

microstructure and properties of HVOF deposited coating with a conventional WC-10Co-4Cr coating.

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2. EXPERIMENTAL PROCEDURE

2.1. Materials

Two types of feedstock powders were used in this study. WC-10Co-4Cr agglomerated and sintered powder

manufactured by Sulzer Metco with grain size in the range of -30+10 µm.The second one isa powder mix

composed of 5% WC-Co nanocomposite powder (Nanocarb, Nanodyne) with a grain size less the 15µm,

addition to an agglomerated and sintered WC-10Co-4Cr powder (H.C. Stark) with grain size in the range of

45+15 µm.

2.2. HVOF spraying

The powders were deposited by HVOF process and a Diamond Jet Hybrid 2700 gun at Tampere University

of Technology. The third generation HVOF gun is equipped with converging-diverging delaval type nozzles

which enhance the velocity of powder particles reducing oxidation. The coatings were sprayed from a

standoff distance of 230 mm, using as process gasses 70 l/min propane, 230 l/min oxygen and 370 l/min air.

The traverse speed of the gun was 11 mm/s and powder feed rate 60 g/min. Nitrogen was used a carrier gas

with a 13 l/min flow. The substrates were low carbon steel plates (50x20x4) and disks with a diameter of 65

mm. The substrates were grit blasted with Alumina grit to roughen the surface. During the deposition the

substrates were cooled with compressed air.

2.3. Microstructural characterization

Philips XL-30 scanning electron microscope equipped with EDAX analyzer was used to investigate the

morphology of powders and coatings in this study. Morphology of nanostructured powder in cross-section

and worn tracks after ball-on-disk test were characterized using a field emission scanning electron

microscope, FESEM, Zeiss Ultra plus, for higher magnification. The phase composition of feedstock powders

and coatings were investigated by a Siemens Diffract 500 diffractometer with Cu-Kα radiation (1.5406 Å)

using a 0.02° step size and 0.2 s dwell time. Image analysis technique was used to quantify the porosity in

the coatings using Image Tool 3.00 software on 7 BSE-SEM micrographs at 1000x magnification.

Microhardness values were measured on cross sections of the coatings. The Vickers method was used with

300 g loads(Matsuzava MMT Vickers tester). 10 indentations were performed per sample.

2.4. Wear testing

The rubber wheel abrasion wear tester was used to determine the wear resistance of coatings using dry

quartz sand as an abrasive with grain size range of 0.1-0.6 µm. the tests were performed on ground

samples. The diameter of the rubber wheel was 660 mm, the test load was 22 N and the total wear length

was 5904 m. Wear losses were measured after every 12 min. Before measurements the samples were

cleaned with compressed air to remove sand particles. The sliding wear test was performed using a CETR

UMT tribometer using a WC-Co ball with diameter of 6.3 mm as a counterbody. The applied normal force

was 10 N and sliding speed 120 rev/min. The sliding distance was 3000 m at room temperature with a 22

mm diameter track radius. Wear tests were performed there times on polished surfaces with 0.03 µm Ra

final value.

2.5. Corrosion testing

The corrosion behavior and denseness of the coatings were tested with electrochemical open cell potential

measurements. The electrochemical cell consisted of a plastic tube of 20 mm diameter and volume 12 ml,

glued on the surface of the coated sample. The tubes were filled with 3.5 wt.%NaCl solution for 4 weeks.

Measurements were taken with a Fluke 79 III multimeter. A silver/silver chloride (Ag/AgCl) was used as a

reference electrode.

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3. RESULTS

3.1. Microstructure of powders

The conventional powders present typical agglomerated and sintered manufacturing process characteristics

with rounded particles, some pore entrances and a certain degree of porosity (Figure 1d). The powders have

an improved flowability due to rounded shapes in comparison with Nanocarb powder which contains a

reduced amount of spherical particles besides powder particles with irregular morphology with sharp facets.

The bigger powder particles presented in Figure 1c, are typical binder-induced agglomerates bounded during

the aspray conversion manufacturing process. The particle size distribution of powders was determined by a

laser difractometer (SympatecHelos) and the carbide sizes were measured on high magnification

SEM/FESEM images. The Sulzer Metco powder had carbides in the range of 0.3-1 µm and grain sizes

between 13.81 and 37.74 µm. The carbide sizes in the H.C. Stark powder were in the range of 0.25 µm and

0.93 µm with grain sizes between 16.5 and 36.7 µm. According to the manufacturer the Nanocarb powder

had grain sizes under 15 µm which could not be measured with the laser difractometer due to agglomerates.

The carbides are less than 300 nm, which classify the powder as near nanostructured due to the fact that the

carbides are bigger than 100 nm.

Fig. 1 SEM micrographs of WC-10Co-4Cr and WC-15Co powders: a) Sulzer Metco powder, b) H.C. Stark

powder, c) Nanocarb powder, d) cross-section of conventional powder, and e) cross section in

nanostructured powder

3.2. Microstructure of coatings

The polished surface morphology of conventional coating and conventional with addition of 5% Nanocarb

coating are presented in Figure 2. It can be noticed that both coatings possess a dense structure with small

pores distributed in the coatings. The conventional coating had a thickness about 150 µm and the one with

Nanocarb addition 300 µm.

a b c

d e

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Fig 2 SEM cross-sectional images showing the morphology and distribution of WC in the metallic matrix: a)

WC-10Co-4Cr plus 5% WC-15Co nano-structured coating, and b) WC-10Co-4Cr coating

The micro-hardness values of HVOF sprayed coatings are shown in Tab. 1. The micro-hardness

measurements showed that both coatings have similar hardness values. Ten roughness measurements

were performed per coating using a SJ-301 Mitutoyo tester and the average results are summarized in Table

1. The porosity measurements indicated that both coatings are dense with low degree of porosity.

Tab. 1 Values of hardness, roughness and porosity measurements

Coating material Microhardness

(HV0.3)

As sprayed roughness

(Ra µm)

Porosity %

WC-10Co-4Cr+5%

Nanocarb

1081±74.37 4.33 1.19

WC-10Co-4Cr 1100±52.58 3.34 1.24

The X-ray diffraction patterns of the powders and coatings are presented in Figure 3. Additional crystalline

reflections corresponding to tungsten hemicarbide (W2C) phase is present in both HVOF coatings in

comparison with the x-ray pattern of powders. The peaks corresponding to W2C are more pronounced in the

conventional coating with Nanocarb which might be due to an increased degree of carburization. Due to

Nanocarb powder addition the peaks corresponding to Co3W3C and Co have an increased intensity in

comparison with conventional powder.

Fig. 3 XRD patterns of: a) conventional powder and HVOF coating and b) conventional powder and coating

with addition of 5% Nanocarb

3.3. Wear resistance

The influence of abrasive silica sand on the grinded samples showed that both coatings have a good

abrasion wear resistance as it is indicated in Figure 4. In literature it has been reported that the abrasion

wear resistance of thermally sprayed WC based cermet coatings decrease as the level of WC react or

a b

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decompose during spraying increase [7]. In the present study the coating with Nanocarb addition showed a

reduced abrasion wear resistance in comparison with the conventional coating due to an increased

decarburization. In both cases the material removal was characterized by carbide pull outs, grooving and

scars along the direction of abrasive flow.

Fig. 4 Abrasion wear graphs for the HVOF coatings

The tribological behavior under dry conditions was evaluated using a ball-on-disk tribometer. The tests were

carried out by sliding the pin against polished disks at constant linear speed. The wear of WC based cermet

coatings is considered to be a function of carbide size and content and binding strength between the

carbides and matrix [8]. From Fig. 5 can be noticed that the sliding track was slightly narrower on the coating

with Nanocarb addition and also the tribo-layer, contains free form of graphite and oxides. The tribo-layer

formed during the test at elevated temperature is more pronounced and this increased the sliding wear

resistance. The high magnification images revealed material removal, formation and propagation of cracks

and grooves in the metallic matrix. WC pull outs can be noticed as well. The grooves and pits in both cases

are partially covered with particle debris, which also acts as an abrasive between the ball and coating. The

EDS spectra present similar results for both coatings and the presence of oxygen is confirmed in the tribo-

layer.

Fig. 5 Sliding wear results: a) worn track of WC-10Co-4Cr, b) worn surface of WC-10Co-4Cr coating, c) EDS

of WC-10Co-4Cr worn surface, d) worn track of WC-10Co-4Cr+Nanocarb, e) worn surface of WC-10Co-

4Cr+Nanocarb coating, f) EDS of WC-10Co-4Cr+Nanocarb worn surface

a b c

d e f

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3.4. Corrosion resistance

At the beginning of open cell potential measurements the electrode started to move to a more negative

direction after the exposure in the NaCl solution which indicated an active corrosion behavior. After the first

day of test on the conventional coating small corrosion products appeared which increased by time. This

occurred due to interconnected pores, splat boundaries and microcracks found in the coating [9]. The open

cell potential graph and corrosion products developed on coatings are presented in Figure 6. It can be

noticed that the coating with Nanocarb addition exhibited better corrosion resistance after 4 weeks in

comparison with conventional coating. This might be due to better bounding between splats indicating a

better structural durability.

Fig. 6 Open cell potentials of HVOF coatings and as function of exposure time in 3.5% NaCl solution and

corrosion behavior of a) WC-10Co-4Cr coatings and b) WC-10Co-4Cr plus Nanocarb coating

4. CONCLUSIONS

Two types of cermet coatings a conventional WC-10Co-4Cr and WC-10Co-4Cr with 5% WC-15Co Nanocarb

addition have been deposited by HVOF spraying in order to study their wear and corrosion resistance. The

coatings presented similar morphology, hardness and porosity. Due to an increased dissolution process the

coating with Nanocarb addition exhibited lower abrasion wear resistance but somewhat better sliding wear

behavior in comparison with the conventional coating due to the appearance of graphite and oxides during

the wear test between the ball and coating. After 4 weeks of immersion in 3.5% NaCl solution, the coating

WC-10Co-4Cr and Nanocarb addition exhibited a better corrosion resistance indicating a better structural

durability. The addition of Nanocarb powder to the conventional WC-10Co-4Cr powder slightly increased the

sliding wear and corrosion resistance of HVOF deposited coating.

ACKNOWLEDGEMENTS

This work was partially supported by the strategic grant POSDRU 2009 project ID 50783 of the

Ministry of Labor, Family and Social Protection, Romania, co-financed by the European Social Fund-

Investing in People.

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