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Keywords: Electro-thermal explosion spraying; WC/Co coatings; Performances

coatings for high porosity and poor bonding of the coatings.

10 K/s.Few researches have been done on the microstructure and

ings were conducted in a Philips Quant 200 scanning electronmicroscope (SEM) equipped with an EDAX apparatus. Thenano-hardness and nano-elastic modulus of the coatings wasinvestigated using the nano-indentation tester.

Materials Letters 61 (2007)tribological properties of EEDS coatings. Tamura studied theElectro-thermal explosion directional spraying (EEDS),a new spraying method, will be used widely due to its abilityto process coatings with compact microstructure, high micro-hardness, low porosity and good bonding. Ushakov [2], andTamura [3,4] investigated the characteristics of the EEDSprocess. They concluded that the EEDS technique was charac-terized by the duration of process of between 1 and 10 ms, aplasma temperature about 104 K, the speed of spraying particlesup to 34.5 km/s, and an average cooling rate of particles up to

9

The medium carbon steel of AISI 1045, 303010 mm insize, was selected as substrates for spraying treatment. Thesurface of samples was cleaned and degreased. The spayedmaterial was WC/Co with the nominal composition Co 6% andWC rest (YG6). The spayed material was machined to0.2 mm5 mm60 mm foil. The coatings were produced byEEDS [6]. The applied voltage was 4850 Vand the thickness ofcoating was about 100 m.

Observations of the cross-section morphology of the coat-1. Introduction

Cermet thermal spraying coatings are widely used in wearsituations because they combine several advantages such asresistance to abrasion, erosion, corrosive atmospheres and hightemperature [1]. WC/Co is one of the most useful cermetthermal spraying materials. But for the traditional thermalspraying technology, it's difficult to gain the high quality cermet

formation mechanism of ceramic powders [3,4]. Mizusako [5]investigated the microstructure of ZrOB graded coating.

This study is aimed to have deeper knowledge of the micro-structure and wear performances of EEDS WC/Co coatings.The relationship between mechanical and tribological propertieswas also discussed.

2. ExperimentsCharacterization of WC/Co

Guo Jin a,b,, Bin-shi Xu b, Hai-dou Wa Center for Biomedical Materials and Engineer

b National Key Laboratory for Remanufacturing, A

Received 14 December 2005Available onlin

Abstract

In the present work, WC/Co coatings were obtained by electro-thermmicrostructure, elements distribution, hardness, elastic modulus and wenano-indentation test, friction and wear test, respectively. Results showbond, high hardness and elastic modulus. The WC/Co coatings also h 2006 Elsevier B.V. All rights reserved. Corresponding author. National Key Laboratory for Remanufacturing,Armored Forces Engineering Institute, Beijing, China. Tel.: +86 451 82518173.

E-mail address: jg97721@yahoo.com.cn (G. Jin).

0167-577X/$ - see front matter 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2006.09.036oatings on metal substrates

ng b, Qing-fen Li a, Shi-cheng Wei b

, Harbin Engineering University, Harbin, Chinaored Forces Engineering Institute, Beijing, China

cepted 16 September 2006October 2006

explosion directional spraying technology. The performances includingesistance of the coatings were investigated by means of SEM, EDAX,at WC/Co coatings are characterized by compact construction, goodoutstanding wear resistance.

24542456www.elsevier.com/locate/matletFriction and wear tests were measured in a block-on-ring testwith coated blocks and TiCN cement ring under the appliedloads of 300 N. The lubrication oil was 650SN. The liner speed

using an optical microscope. Wear mechanism of the coatingswas determined by observing the worn morphologies. The AISI1045 steel was selected as compared material.

3. Results and discussions

3.1. Microstructures

The microstructures of WC/Co coatings are shown in Fig. 1. It can

Table 1EDAX results of WC/Co coatings

Element (wt.%) W Fe C

Area A 37.73 57.13 5.14Area B 22.55 65.23 12.22

2455G. Jin et al. / Materials Letters 61 (2007) 24542456was 0.42 m/s, and the sliding time varied from 10 min to 60 min.The variation of friction coefficient with sliding time wasrecorded automatically. The width of worn scar was measured

Fig. 1. SEMmicrographs of WC/Co coatings. (a) Coating area and (b) transitionarea.

Fig. 2. Nano-mechanical properties of WC/Co coatings.be seen from Fig. 1(a) that the coatings have compact microstructureand no lamellar structure. During the spraying process, the WC/Cofoils are melted firstly, then gasified and exploded when they absorbenough energy. The highest pressure at the centre of exploding gas isup to 710 GPa and the highest temperature is up to 104 K. And theflying speed of spraying droplets can be up to 30004500 ms1. Underthe condition of high pressure, high temperature and high speed, themelted spraying droplets impact the substrate, then deform, solidifyquickly and form compact fine grain coating [79].

The bond micrographs of WC/Co coatings are shown in Fig. 1(b).The coatings are well combined with the substrate and the transitionarea can be obviously seen. There exist columnar grain and isometriccrystal in the transition area. EDAX results show that the diffusion ofelementWand Cwithin the interface zone of coatings is obvious. At thesame time, the contents of W, Fe and C are different between the grainboundary and intracrystalline (Table 1). The content of W at thegrain boundary is higher than that of intracrystalline. It can be con-cluded that the bond between the coatings and the substrate is diffusiveand metallurgical one [10]. During the spraying process, the surface ofsubstrate is melted by spraying droplets with a temperature of 30004000 C. During the later solidification period, the columnar grain andisometric crystal are formed. The diffusion will take place due to theconcentration grads in the bond area [11] and the diffusion distancereaches 510 m. The diffusive and metallurgical bond will bebeneficial in increasing the bond strength and wear resistance.Fig. 3. Wear properties of WC/Co coatings.

etters 61 (2007) 245424563.2. Nano-mechanical properties

The nano-mechanical properties are shown in Fig. 2. The nano-hardness and nano-elastic modulus can be calculated [12,13] from thecurve of load with displacement. The nano-hardness of WC/Cocoatings and 1045 steel is 32.4 GPa and 2.4 GPa, respectively. Thenano-elastic modulus of the two materials is 414.8 GPa and 183.2 GPa,respectively. The hardness/modulus of elasticity ratio (H/E ratio) forthis EEDSWC/Co coating is higher than the laser WC/Ni coatings (theformer, 0.0781, the later, 0.0601 [14]). The high H/E ratio has somebeneficial effects on the wear resistance.

3.3. Analysis of wear resistance

Fig. 3 shows the variation of volume loss of WC/Co coating and theAISI 1045 steel as functions of sliding time. It can be seen that thevolume loss increases linearly with the increase in sliding time and thevolume loss of WC/Co coatings is always lower than that of AISI 1045steel. In the case of two surfaces in relative movement, a protuberancecan be considered a hard particle if its surface is harder than that of the

Fig. 4. Morphologies of worn surfaces.counterpart [15]. So, the high hardness of carbides presents anattractive solution for protecting surfaces against wear [1].

The worn micrograph of WC/Co coatings is shown in Fig. 4. It canbe seen that the main wear mechanism of WC/Co coatings is the pull-out of hard particles. This is because that the extrusive hard particlesendure the load during the process of wear. After the long-term actionof high load, the bond between the hard particles and the bindingmaterial will slacken, and finally the hard particles will be pulled out.

4. Conclusions

Microstructure observations show that the coatings possesssuch good properties as compact microstructure, low content ofoxidation and decomposition, high microhardness and goodbond. In the transition area, the diffusion of W, Fe and C isobvious and there exist the columnar grain and isometric crystal.The two phenomena show that the bond between the coating andsubstrate is a metallurgical and diffusive one. The microhardnessand elastic modulus is high for the fine grain strengthening. Thewear resistance of WC/Co coatings is much higher than that ofAISI 1045 steel. The main wear failure mechanism is the pull-outof hard particles.

Acknowledgements

The research is supported financially by 863 Project(2003AA331130). The authors thank professor Liu Zong-defromNorth China Electric Power University China for preparingthe coatings and professor Miao He-zuo from TsinghuaUniversity China for supplying cermet samples.

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Characterization of WC/Co coatings on metal substratesIntroductionExperimentsResults and discussionsMicrostructuresNano-mechanical propertiesAnalysis of wear resistance

ConclusionsAcknowledgementsReferences