Applied Surface Science 254 (2008) 5470–5474

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Applied Surface Science

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Erosion behavior of EEDS cermet coatings

Jin Guo a,b,c,*, Xu Bin-shi c, Wang Hai-dou c, Yin Liang a, Li Qing-fen b, Wei Shi-cheng c, Cui Xiufang d

a State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, Chinab Center for Biomedical Materials and Engineering, Harbin Engineering University, Harbin 150001, Chinac National Key Laboratory for Remanufacturing, Armored Forces Engineering Institute, Beijing 100072, Chinad School of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China

A R T I C L E I N F O

Article history:

Received 30 October 2007

Received in revised form 23 February 2008

Accepted 24 February 2008

Available online 6 March 2008

Keywords:

Erosion

Structure

TEM

EEDS

Cermet coating

A B S T R A C T

This paper investigates the erosion performance of electro-thermal explosion directional spraying (EEDS)

cermet WC/Co coatings using an air solid particle erosion rig. The influences of the different parameters

such as impact angle, impingement velocity, environment temperature, particle diameter, on the erosion

property of the coatings were studied. The eroded surfaces were examined by scanning electron

microscopy (SEM) equipped with energy dispersive spectrometry (EDS) and the erosion mechanisms

were discussed. The structure and bond characters of the coatings were also determined by transmission

electron microscopy (TEM), X-ray diffraction (XRD) and SEM.

The results indicate that the EEDS WC/Co coatings are characterized by fine grain structure, good

metallurgical bond and brittle erosion character. The erosion rates of the coatings decrease with

temperature increasing and increase with impact angle and impingement velocity increasing. At elevated

temperature, the oxidation happens on the coatings surface, which affects the erosion behavior.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

Fluidized bed combustion boilers, turbines and engines arenormally exposed to erosive environments and the erosion has ledto many accidents [1–4]. The ceramic coatings are considered aspowerful barriers against deterioration of machine parts exposedto particulate flow at high temperatures [5]. Recently, the cermetcoatings are used to further increase the erosion resistance throughincreasing the toughness of the coatings [6–11]. The erosionresistance of the coatings is influenced by the impact angle,particle velocity and environment temperature, but it is stronglydependent on the coating process [12–15].

Electro-thermal explosion directional spraying (EEDS) tech-nology is a new technique [16] based on the explosion from highvoltage impulsive discharge and quick solidification of the hightemperature droplets. This technique is characterized by theduration of 1–10 ms, the plasma temperature about 104 K,the speed of spraying droplets up to 3–4.5 km/s, the averagecooling rate of particles up to 109 K/s and the specific energy ofmore than 1 MJ/kg [17,18]. The gas and droplet with hightemperature and high pressure, which are produced by the

* Corresponding author at: Center for Biomedical Materials and Engineering,

Harbin Engineering University, No. 145-11 Nantong Street, Harbin 150001, China.

Tel.: +86 10 62795148.

E-mail address: jg97721@yahoo.com.cn (J. Guo).

0169-4332/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2008.02.098

exploding of WC/Co foils, impact the substrate surface withextremely high speed and solidify quickly to form coatings withsuch good properties as compact microstructure and metallur-gical bonding [19–22].

The present work is to investigate the erosion resistance of theEEDS cermet coatings at different conditions, such as temperature,impact angle and particle velocity. The structure and bondcharacters of the coatings are also presented. The relationshipbetween the erosion behavior and the structure of the coatings areanalyzed.

2. Experimental procedures

2.1. Materials and coating process

Prior to spray, the non-sprayed surface of AISI 1045 steelsubstrate (25 mm � 16 mm � 4 mm) was treated by powercalorizing (power: 15% Al, 84% Al2O3, 0.5% NH4Cl and 0.5%KHF2; process condition: 900 8C � 4 h) to reduce the influence ofoxidation at high temperature on the erosion test. Then thesubstrate was cleaned, degreased and surface finished. The sprayedmaterial of WC/Co cermet foils (0.2 mm � 5 mm � 60 mm) wasalso cleaned, degreased and then deposited on the substrate usingEEDS system (described in detail in our former article [20]). Thespraying circuit consisted of a large capacitor, an explodingchamber, and many other elements. The applied voltage was4850 V and the thickness of coating was about 100 mm.

Fig. 1. Sketch map of elevated temperature erosion apparatus.

J. Guo et al. / Applied Surface Science 254 (2008) 5470–5474 5471

2.2. Structure observation

Scanning electron microscopy (SEM) characterization wascarried out using a Philips Quant 200 microscope equipped withenergy dispersive spectrometry (EDS). Transmission electronmicroscopy (TEM) was performed with an H-800 microscope

Fig. 2. SEM morphologies of erosion particles (inset is the EDS result). (a) SiO2 (300–90

operating at 150 kV. Indexation of diffraction spots was performedthrough the Bragg formula

R ¼ lL

d(1)

where R is the distance measured between the spot and the centerof the pattern, l is the electron wavelength (here l is 0.00295 nm),L is the camera length (here L is 800 mm) and d is the reticulardistance. The WC/Co coatings were further investigated by X-raydiffraction (XRD) with Cu Ka (l = 0.154 nm) radiation on a D8-Advance apparatus. The crystallite size was determined by TEMphotograph and XRD results, respectively.

2.3. Erosion test

The erosion tests were carried out using a gas–sand elevatedtemperature erosion apparatus (shown in Fig. 1). Before erosionthe samples were preheated for about 15 min and the sand andgas were heated instantaneously. The sand velocity wasmodulated by air pressure as the velocity at different air pressurehad been determined by velocimeter. The impingement anglewas adjusted by rotating the sample. The test conditions in thispaper are listed in Table 1 and if there is no special mention thetest condition is 90 8C, SiO2 (130–210 mm), 450 8C, 130 m/s,90 min. And the sand photographs and compositions are shownin Fig. 2.

The mass loss was determined by weighing the samples beforeand after the test using an electronic analytical balance (TG328B)

0 mm), (b) SiO2 (130–210 mm), (c) Al2O3 (50–130 mm), and (d) SiC (30–110 mm).

Fig. 3. XRD spectra of the EEDS WC/Co coatings (inset is the small-angle XRD

results).

Table 2Crystallite sizes of the EEDS WC/Co coatings

Phase 2u (8) B (rad) d (nm)

WC 38.02 0.0038 38.39

WC2 39.56 0.0035 42.13

Table 1Erosion test conditions

Temperature ( 8C) 30, 450, 600

Velocity (m/s) 62–130

Angle (8) 30–90

Sand material SiO2, Al2O3, SiC

Sand size (mm) 30–900

Sand mass (mg) 200

Time (min) 90

J. Guo et al. / Applied Surface Science 254 (2008) 5470–54745472

precision balance with an accuracy of �0.1 mg. The erosion rate e(mg/100 g) was determined by the following formula:

e ¼ M1

ðM2=100Þ (2)

where M1 (mg) is the mass loss of the sample and M2 (g) is the massof erosion sand.

The patterns, compositions of the eroded surface were alsoinvestigated by SEM and EDS.

3. Results and discussion

3.1. Structures

Fig. 3 presents the XRD patterns of the EEDS WC/Co coatings.From the large-angle scanning results, it can be seen that the main

Fig. 4. TEM micrographs of the EEDS WC/Co coatings (inset is the electron diffraction pat

phases are WC and W2C. The small-angle scanning results arealso shown in Fig. 3 and the crystallite size of the coatings canbe calculated from the most intense peak using Scherrer formula[22]

d ¼ klB cos u

(3)

where d is the average crystallite size, k is a constant (here k is 0.9),l is the X-ray wavelength (here l is 0.154 nm), B (rad) is the linewidth at half the maximum intensity corrected for instrumentaland strain broadening and u (8) is the Bragg’s angle. The obtainedresults are shown in Table 2 and the crystallite size of EEDS coatingis about 40 nm.

The detailed phase structures of the coatings observed by TEMare given in Fig. 4. The WC phase ([2 1 2]) can be determined inFig. 4(a) and the crystallite size is within the range of 50–100 nm.Using the electron diffraction pattern shown in Fig. 4(b), the W2Cphase ([2 1 1]) can also be determined. These phase structureresults are coincident with the XRD results.

The cross-sectional micrographs of the coatings illustrated inFig. 5(a) shows that the coatings are characterized by uniform,compact microstructure and good bond. The columnar grain andisometric crystal are presented in the transition area. It may beconcluded that the bond between the coating and substrate ismetallurgical. In order to validate the above conclusion, TEM wasused to further investigate the microstructure of the coating bondarea, and the results are shown in Fig. 5(b). It can also be seen thatthe bond between the coating and substrate is good. And a newphase, Fe2W4C ([1 1 0]), has been detected in the interface.Considering the compositions of WC/Co and AISI 1045 steel, itcan be concluded that the Fe2W4C phase is the reaction product ofthe above two materials, which evidently approves the metallur-gical bond.

During the spraying process, the temperature of sprayingdroplets was as high as 3000–4000 8C and the WC was partlydecomposed to W2C. When the high temperature, high pressure,high speed molten spraying droplets impacted the substrate, the

tern corresponding to the bright field image). (a) a-WC phase and (b) a-W2C phase.

Table 3Erosion rates of EEDS WC/Co coatings at different conditions using 130–210 mm

SiO2

T ( 8C) y (m/s) u (8) e (mg/100 g)

30 130 90 178.1

45 88.9

30 44.4

450 62 90 20.3

89 90 57.4

111 90 103.3

130 90 163.8

45 107.6

30 55.3

600 130 90 96.8

Fig. 5. Cross-sectional micrographs of the EEDS WC/Co coatings. (a) SEM and (b) TEM (inset is the electron diffraction pattern corresponding to the bright field image).

J. Guo et al. / Applied Surface Science 254 (2008) 5470–5474 5473

surface of substrate was molten and blended with the sprayingdroplets. The metallurgical bond was gained in the latersolidification period. At the same time, the columnar grain andisometric crystal were formed. The good bond was in favor of theheat transfer between the droplets and the substrate, so the hightemperature droplets cooled and solidified quickly to form the finegrain microstructure. These microstructure characters such asmicrometer/nanometer grain microstructure and metallurgicalbond can all improve the erosion resistance.

Fig. 6. SEM micrographs of the eroded WC

3.2. Erosion resistance

The erosion rates of EEDS WC/Co coatings at differentconditions using 130–210 mm SiO2 are described in Table 3. Itcan be seen that the erosion rate at different ejecting velocity isfitted to the following formula:

e ¼ 2:05 � 10�4y2:79 (4)

The coatings represent brittle erosion character that is to say theerosion rate increases with increasing the ejecting velocity, but itsbrittleness is lower than pure ceramic materials [23].

The influence of impingement angle on the erosion rate at 30 8Cand 450 8C is also shown in Table 3. It can be seen that the erosionrate both at 30 8C and 450 8C increases with increasing theimpingement angle. The following formula is mostly used toexpress the erosion rate at different angle:

e ¼ A cos2 b sin ðnbÞ þ B sin2 b (5)

where e is the erosion rate, b is the impingement angle, n is aconstant, and A, B are also constants to describe the brittle andplastic, respectively, for typical brittle material A = 0, for typicalplastic material B = 0, and for the other material, the plastic itemdisplays the main effects at low impingement angle while that isthe brittle item at high impingement angle. That is to say, thebrittle materials have high erosion rate at high impingement

/Co coatings (a) 450 8C and (b) 600 8C.

Table 4EDAX results of the eroded EEDS WC/Co coatings

Temperature (8C) Composition (wt.%)

W C Co O

30 87.99 8.52 3.49 0

450 90.58 3.04 0 6.38

600 87.51 0 0 12.49

Table 5Erosion rates of the EEDS WC/Co coatings with different particles

Materials SiO2 Al2O3 SiC

Size (mm) 300–900 130–210 50–130 30–110

e (mg/100 g) 216.7 178.1 139.2 198.3

J. Guo et al. / Applied Surface Science 254 (2008) 5470–54745474

angle, but low erosion rate at low impingement angle, and it isreversed for plastic materials. The eroded results shown in Table 3indicate that the coatings are brittle both at 30 8C and 450 8C, butthe brittleness of the later is slightly lower. The micrographs of theeroded scars are shown in Fig. 6. The brittle erosion characters arefound on the eroded scar. With the temperature increasingthe erosion rate decreases and it decreases less from 30 8C to450 8C, while it decreases more from 450 8C to 600 8C. The massgain by oxidation influences the results of the mass loss byerosion. The EDAX results of eroded coatings are shown in Table 4.The Co is absent after high temperature erosion for burning loss.The C content decreases while the O increases from 30 8C to600 8C. Besides this, the hardness of the erosion particlesdecreases at high temperature, which also decreases the erosionrate.

Table 5 shows the erosion rate of the coatings impinges bydifferent particles at room temperature. It can be seen that thelarger particles cause higher erosion rate for their higher impactenergy. And the more cuspate particles with similar dimensioncause higher erosion rate, which is mainly because that theseparticles can cut and plough the coatings easily.

4. Conclusions

(1) T

he air solid particle erosion resistance of the EEDS cermetcoatings is sensitive to the impact angle, impingement velocityand environment temperature.

(2) T

he coatings exhibit uniform and compact microstructure, finecrystalline grain and excellent bond of metallurgical onedetermined by TEM, XRD and SEM.

(3) T

he coatings present brittle erosion mechanisms both at roomtemperature and elevated temperature. At room temperature,the mass loss is mainly for the fatigue flake and brittle fracture,while at elevated temperature, it is the interaction results ofwear and oxidation.

Acknowledgements

The work supported financially by China Postdoctoral ScienceFoundation (20070420032), 863 Project (2003AA331130,2007AA04Z408) of China and 973 Project (2007CB607605) ofChina. The authors thank professor Liu Zong-de from North ChinaElectric Power University China for preparing the coatings.

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