A R C H I V E S O F M E T A L L U R G Y A N D M A T E R I A L S

Volume 57 2012 Issue 2

DOI: 10.2478/v10172-012-0053-0

M.W. RICHERT∗, A. MAZURKIEWICZ∗∗, J.A. SMOLIK∗∗

THE DEPOSITION OF WC-Co COATINGS BY EBPVD TECHNIQUE

OSADZANIE POWŁOK WC-Co TECHNIKAMI EBPVD

The WC-Cocarbides are widely used to deposit protective coatings on engineering surfaces against abrasion, erosionand other forms of wear existence. The nanostructure coatings offer high strength, a low frictioncoefficient and chemical andthermal stability. WCcoatings were deposited using EBPVD technique realized in original technological process implementedin the hybrid multisource device, produced in the Institute for Sustainable Technologies – National Research Institute inRadom (Poland). The different kind of precursor sources was used. Depending on the source of precursors nanostructure ofcoatings forms continuous film or consist from nano-carbides. Nanocrystalline WC-Co coatings show hardness in the range of510-1266 HV. The microstructure of coatings were observed by transmission electron microscopy (TEM). The phase consistencewere determined byBrucker D8 Discover-Advance Diffractometer. The paper presents the original technological equipment,methodology, and technological parameters for the creation of the nanocomposite coatingsWC.

Keywords: electron beam evaporation, WC-Co coatings, microstructure

Węgliki WC-Co są szeroko używane do osadzania na powierzchniach inżynierskich jako ochrona przeciwko tarciu, erozjii innym formom zużycia. Powłoki nanostrukturalne wykazują wysoką wytrzymałość, niski współczynnik tarcia oraz chemicznąi termiczną stabiność. Powłoki WC-Co zostały osadzane techniką EBPVD przy użyciu oryginalnego technologicznego procesuzhybrydowym źródłem prekursorów, w Instytucie Technologii Eksploatacji w Radomiu (Polska). Użyto różnego rodzaju źró-deł prekursorów. W zależności od rodzaju użytego źródła prekursorów nanostrukturalne powłoki były zbudowane z ciągłychwarstw lub z nano-węglików. Mikrotwardość nanokrystalicznych powłok mieściła się w zakresie 510-1266 mHV. Mikrostrukturapowłok była obserwowana transmisyjnym mikroskopem elektronowym (TEM). Skład fazowy powłok określono za pomocą apa-ratu rentgenowskiego Brucker D8 Discover-Advance Diffractometer. Artykuł prezentuje oryginalne technologiczne urządzenie,metodologię i technologiczne parametry pozwalające na wytworzenie nanokompozytowej powłoki WC.

1. Introduction

Tungsten carbide (WC) hard coatings are materialsof great interest due to their excellent properties andgreat technological applications. The tribological appli-cations require high hardness and high strength, a lowfriction coefficient and chemical and thermal stability[1,2]. WC-Co and WC coatings offer a great opportuni-ty to develop such properties. This kind of coatings haverelative high hardness (about 2200HV) and high meltingtemperature (about 2800◦C). The WC exhibited extreme-ly high modulus of elasticity, well above 700 GNm−2, avalue exceeded only diamond, and has a high thermalconductivity of 1,2 J (cm.s.K)−1[3].

It is difficult to obtain a pure WC phase, as W-Cphase diagram shows a narrow range of homogeneity

for WC. The different methods are used for tungstencarbide deposition; PVD, CVD, hybrid method as wellas thermal spraying methods . Same investigations in-dicated the dependence of WC coatings on the PVDdeposition parameters, for example gas composition orsputtering time [4]. This results proof that the depositionparameters can strongly influence on the microstructure,composition and properties of coatings. The influence ofgas parameter flow on WC coatings was also investigat-ed in the work [5]. It was found that the nanostructuredWC-Co coatings show better mechanicaland wear prop-erties in comparison to coatings with the convention-al microstructures [1]. The observations performed byTEM revealed that the nanoparticles of WC were embed-ded in the amorphic matrix [6]. It suggests that the coat-ings could be mixture of nanomaterials and amorphic

∗ AGH UNIVERSITY OF SCIENCE AND TECHNOLOGY, FACULTY OF NON-FERROUS METALS, 30-059 KRAKÓW, 30 MICKIEWICZA AV., POLAND∗∗ INSTITUTE FOR SUSTAINABLE TECHNOLOGIES – NATIONAL RESEARCH INSTITUTE, 26-600 RADOM, 6/10 PUŁASKIEGO STR., POLAND

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materials. The detailed investigations of microstructurethorough the WC cross section coatings were presentedin the work [7].

The kind of the precursor source could be the one ofthe parameters influencing the microstructure of WC-Cocoatings deposited by PVD method. In the presentedwork two kinds of precursors were used in the electronbeam physical vapour deposition method(EBPVD). Theeffects of their influence on microstructure and proper-ties of WC coatings were investigated.

2. Esperimental

The tungsten carbide coatings have been depositedat aluminium alloy substrate by electron beam physicalvapor deposition method (EB PVD). The coatings wereprepared in multicatode hybrid evaporation system in In-stitute of Suitable Technologies in Radom, Poland.Thetwo ways of precursor deposition were used. The firstcoating process was performed form precursor sourceconsisting from WC-Co compacted powderevaporated inargon. The second process of deposition was performedby evaporation of tungsten in C2H2 atmosphere. Beforethe deposition the substrate was heating by arc etchingusing the Cr cathode.

The microstructure of coatings was observed by us-ing transmission electron microscope JEM 2010 ARP.Thin foils, to microstructure observations, were pre-

pared by the cross sections technique using Gathan ma-terials and equipment. The phase composition of coat-ings was investigated by Brucker D8 Discover-AdvanceDiffractometer with copper tubing (40 kV, 30 mA,A = 1,540598 A◦). The method of superficial layermeasurement and Diffract Plus Evaluation software wereused to examine the layers composition X-ray results.Thescanning electron microscopy SU-70 with field emissiongun thermally aided was used for investigation of chem-ical composition of deposited coatings. The microscopywas equipment with analyzers EDS UltraDry and WDSMagnaRay produced by Thermo Ecientific. The hardnessof coatings was measured by Vickers method.

3. Results and discussion

Figure 1 presents the microstructure of WC-Co coat-ing prepared from WC-Co compacted powder evaporat-ed in argon. It could be visible that the microstructureof coating is very differentiate and consists from brightand dark layers (Fig. 1a). The higher magnificationsshow that layers are built from nanometriccircular inshape particleswith the characteristic cafe grain contrast(Fig. 1b, Fig. 1c). In dark layers particles have largerdimension than inside bright ones. Mostly particles havespherical shape, however also some elongated particleswere also found (Fig. 1b).

Fig. 1. WC-Co coating deposited by EBPVD technique, a) coating microstructure observed by transmission electron microscopy, b) outerlayer of deposition, c) characteristic spherical particles

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The investigations performed by EDS techniqueidentified close to substrate face the Cr layer, which re-sultsfrom ion surface cleaning by Cr source in the PVDchamber, before the deposition (Fig. 2). The next layerof coating contains Co and C, respectively in proportion:C Kα–44,54% at to Co L – 55,46% at. The outer layeris enriched in carbon and corresponds to dark layers inFig. 1b and Fig. 1c consisting from spherical black par-ticles. The performed analysis indicates that the coatingis generally build from the mixture of Co and C particles(Fig. 3). The tungstenappears on the very low level, asit informs the number of counts presented in the upperright corner of figures. In the case of tungsten the only

37 counts appeared (Fig. 2e) in comparison to the 1027counts for Co (Fig. 2d).

The X- ray investigations, which results are present-ed in Fig. 4., show high level of background typical foramorphic phase. The picks from Al and Si backgroundare only visible in the diagram. There were not foundpicks from WC phase, which confirms presented chemi-cal identification of coating performed by the SEM EDS.It is possible that inside the Co layer exists some part ofWC carbides but in very insignificant quantity.

The average 510 HV hardness of the deposited lay-ers was found. It is rather lower value in comparison tothe hardness of WC-Co coatings, which usually have thelevel of about 1800-2100 HV [8].

Fig. 2. SEM EDS mapping showing a) microstructure of coating, b) distribution of C element, c) distribution of Cr element, d) distributionof Co element, e) distribution of W and f) substrate element Al

Fig. 3. Co + C coating deposited from WC-Co compacted powder evaporated in argon

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Fig. 4. X-ray diagram showing high level of background typical for amorphic phase

The microstructure of coating obtained by deposi-tion from tungstensource in the C2H2atmosphere usingthe PVD technique, observed by transmission electronmicroscopy, is presented in Fig. 5. The columnar mi-crostructure was found. The detailed investigations showthat columnwidth has about 200-500 nm. The columnswere built from narrow bands consisting from gran-ules showing the nanometric dimensions (Fig. 4c andFig. 4d).

Figure 5 presents the exemplary EDS maps of chem-ical element distribution. The number of counts for car-bide and tungsten is comparable (C – 97, 71 – Fig. 5b,W – 81, 82 – Fig. 5e). It suggests that WC carbide exis-tence inside the investigated areas (Fig. 7a). The investi-

gations show that next the regionswith the WC carbides,the W2C carbides form, which example of occurrence ispresented in Fig. 7b.

Thecalculation of the stoichiometry composition ofcarbides was also performed on the base of intensityelements plot recorded during SEM investigations.

The X-ray investigations show that contrary to thedeposition from WC-Co compacted powder evaporatedin argon, coatings deposited from tungsten in C2H2 at-mosphere exhibitmixed amorphic and nanometric form,which can be concluded fromboth high level of back-ground and the broader peaks in phase diagrams (Fig. 6).The discrete picks from WC suggests some part of nano-metric phase.

Fig. 5. Microstructure of WC coating deposited by EBPVD, a) column microstructure, b) spatial distribution of columns, c),d) internalmi-crostructure of columns

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Fig. 6. The distribution of elements in coating, a) investigated area of coating, b) distribution of carbides, c) substrate element Al,d) distribution of Cr and e) distribution of tungsten

Fig. 7. WC coating deposited from tungsten source in the C2H2 atmosphere, a) area of occurrence of WC carbides, b) area of occurrence ofW2C carbides

Fig. 8. X-ray diagram of WC coating, deposited from tungsten source in the C2H2 atmosphere, showing high level of background, somepicks from Al, Si and discrete picks from WC are visible

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4. Summary

Considering the presented results,the mechanismof Cr interlayer deposition during the cleaning surfaceprocess was isolated. This is the reason of the Cr inter-layer occurrence. The results of the work [5] reportedthat interlayer of chromium can improve the adhesionof WC coatings. From this point of view existence ofCr interlayer is advantageous in the deposited coatingcomposition.

The microstructure and phase composition of coat-ing deposited from WC-Co compacted powder evapo-rated in argon in comparison to the coating obtained bydeposition of tungsten source in the C2H2 atmospheredemonstrate essential differences. In the first case the on-ly insignificant percentage of WC was found. The maindominating elements inside the coating was Co and car-bon. The mixture of Co and C particles was typical mi-crostructure in this analysed coating. Results presentedin the work [4] shown appearance of the WC carbidecoating deposited in argon gases onto carbon steel sub-strates. Therefore it is difficult to recognized the rea-son of the unsuccessful deposition of WC carbides fromWC-Co compacted powder evaporated in argon in theperformed work. Probably the vapour pressure of tung-sten was too low to deposition of this element in theenough quantity. The understanding of this phenomenonrequires additional investigations.

The deposition of tungstenin the C2H2 atmospherewas successful and WC and W2C carbides were found in-side the coating. Very characteristic is uniform structureof coating. The detailed investigations by TEM techniquerevealed that there exists very delicately visible internalcolumnar microstructure of coating having nanometricsize of columns.

On the basis of the experimental results of the X-rayinvestigations thephase consistence of coatings was char-acterized. In the both cases it has been found themixtureof the dominating amorphic phase and partly existenceof nanometricstate of coating. Especially in the case ofcoating deposited from tungsten source in the C2H2 at-mosphere probably larger part of coating characterizedthe nanometric state.

The hardness of coating containing Co and C (coat-ing deposited from WC-Co compacted powder) revealedlevel of about 510 HV. The hardness of coating contain-ing WC and W2C carbides is about twice time higher

(1266 HV). The essential increase of hardness in thesecond case is connected with the existence of hard car-bides, which were absent in the first kind of coating.

Acknowledgements

The work was financially supported by polish project N R150001 06.

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Received: 10 December 2011.