0 Wear Performance of Sandwich Structured WC–Co–Cr Thermally Sprayed Coatings Using Different Intermediate Layers 2007 Wear

8/11/2019 0 Wear Performance of Sandwich Structured WCCoCr Thermally Sprayed Coatings Using Different Intermediate

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Wear 263 (2007) 691699

Short communication

Wear performance of sandwich structured WCCoCr thermallysprayed coatings using different intermediate layersM. Hadad a, , R. Hitzek b, P. Buergler b, L. Rohr a, S. Siegmann a

a EMPA, Swiss Federal Laboratories for Materials Testing and Research, 3602 Thun, Switzerland b Stellba Schweisstechnik AG, 5605 Dottikon, Switzerland

Received 1 September 2006; received in revised form 29 November 2006; accepted 12 December 2006Available online 23 May 2007

Abstract

Cermet based WCCoCr thermally sprayed coatings are known for their good wear and corrosion performance. Different intermediate layerswith various deposition processes were performed between cermet WCCoCr coatings to form a sandwich structure to enhance adhesion and todamp particle impacts during erosion. As the outer cermet layer exhibits high hardness against abrasion whereas the intermediate layer is ductile todamp the shock due to particle impact in erosion, sandwich coatings might have potential improvement in life time of coated tools used in mining,drilling, cutting and grinding.

Adhesion tests were performed and the tribological behavior of cermet coating and sandwich structured materials was mainly investigated withdry erosion and high pressure slurry jet erosion tests with 30 and 90 impinging angles to study wear mechanism and particle impact at interfacesbetween coatings. In this work, the adhesive strength of different coatings was measured for each combination. Cermet coating, combinationsof NiCr 8020 and Ni-plating showed high strength value. In spite of the low erosive wear rate NiCr 8020, the metallographic examinationrevealed many crackswithin this coating due to theparticle erosion at high pressuresthat hasbeen found to drive microcracks through the interfacesbetween splats leading to aking of the sprayed coatings. The electrochemically deposited interlayer showed better tribological performance undermetallographic observation, because of its homogeneous microstructure, since the electrochemical deposition does not provide splat formation.Consequently, the absence of discrete interfaces limits crack initiation within the coating and at the interface. However, the microstructure of theinterlayer is known to be related to coating deposition process such as microsplats andoxide formation in spraying process.Therefore, the inuenceof microstructure on wear mechanism was investigated. 2007 Elsevier B.V. All rights reserved.

Keywords: Wear mechanism; Cermet WCCoCr; Thermally sprayed coating; Erosion; Sandwich structure; Nickel plating

1. Introduction

Cermet based WCCoCr thermally sprayed coatings haveproven to be interesting wear resistant coating materials sincethe hard WC grains provides generally good bonding to themetallic matrix, e.g. CoCr. The WC particles in the coatinglead to high coating hardness and high abrasive wear resistance,while the metal binder CoCr supplies the necessary coatingtoughness [1,2]. The cermet sandwich structured coatings, con-sisting of a cermet outer layer and ductile bond layer, mighthave potential improvement in life time of coated tools usedin mining, drilling tunnels and grinding since the outer cermet

Corresponding author. Tel.: +41 332282963; fax: +41 332284490. E-mail address: [email protected] (M. Hadad).

layer exhibits high hardness against abrasion whereas the bondlayer is ductile to damp the impact shock of particle during ero-sion. A number of research papers reported a dependency of erosion damage on the mechanical properties such as materialhardness [3,4] if the erodent solicitation is perpendicular to thesurface, or material toughnessif theerosivematerialsare appliedin shearing solicitation. Subsequently, the abrasive wear resis-tance of materials was found to increase generally with bothincreasing hardness and fracture toughness [59]. The impinge-ment erosion resistance of HVOF coatings can be explained bythe microjets formed by dividing bigger jets into smaller jetsthat cannot penetrate easily the hard tungsten carbide particleswhere the repetitive high intensity of the striking jet weakensthe matrix, which is then removed easily by microjets [10]. Dryand slurry erosive wear mechanism of WCCoCr was reportedwhere the high impact of particles on the coating surface results

0043-1648/$ see front matter 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.wear.2006.12.057
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Fig. 1. SEM micrograph of WCCoCr powder and the histogram of grain size distribution.

in cracks by fatigue at the subsurface leading to micro-crackedcoating. This is the mainly found dominating wear mechanismof cermet coatings [1115] . Different deformation zones havebeen determined causedby theimpact of singlesolidparticlesonlaminated structure where bond coat is taken in sandwich [16].A common feature of all thermal spray coatings is their lamel-lar grain structure resulting from the rapid solidication of smallglobules, attened from striking a cold surface at high velocities.This structure is interspersedwith oxide inclusions andporosity.Most metallic coatings suffer oxidation during normal thermalspraying in air [17]. However, we do not nd paper describingwear mechanism at the interface between cermet coating andbond coat, where the adhesion and tribological performanceof bond coat are associated for long life service of compo-nents. Certain coatings cannot be used for some applicationsbecause of their low and insufcient adhesive strength. Among

the most widespread adhesion test methods used are indentationtests [18,19] , shear tests [2023] and tensile adhesive strengthaccording to ASTM C633, ASTM F1147, ISO 14916 or EN582 [2426] . We should also note that adhesion is not a constantin practical applications, but rather a complicated property thatdepends on loading conditions, coating thickness [16] and dif-ferent parameters such as grit blasting to roughen the substratesurface [2731] . Furthermore, the residual stresses due to themismatch in thermal and mechanical properties between coat-ings andsubstrates areof special importance [3236] . Thiswork

explored the tribologicalbehavior of cermet coating and variousbond coats within sandwich structured cermet coatings underdifferent erosion conditions. Although that cermet coating andcombination with the interlayer NiCr 8020 showed quantita-tively good wear and adhesion performance, the metallographicobservations revealed cracksparallel to interface in cermetcoat-ing, and propagating through the interfaces between splats inthe intermediate layer NiCr 8020. Theses cracks might leadto coatings delaminating in life service. Combinations 4 and 5with Ni-plating revealedquantitativelyand qualitatively a betterwear and adhesion performance since no cracks were observedwithin this coating. This is mainly due to its dense and homoge-neous microstructure related to the deposition process that doesnot provide discrete interfaces within the coating.

2. Cermet and bond coating materials

Cermet based WCCrCo coating was deposited by highvelocity oxy-fuel (HVOF) spraying on steel substrate (DIN1.4313) that has been grit blasted with Al 2O3 grit of no. 36mesh size to increase the substrate roughness to approximatelyRa=5.5 m promoting a good coating adhesion. Specimenswere degreased immediately prior to deposition. The specimensdimension was 40 mm 40 mm. Details of the HVOF param-eters are given in Table 1 . As the grain size and morphologyof WCCoCr powder have high impact on mechanical proper-

Fig. 2. Micrographs of Ni-plating within cermet coatings: (a) the grit blasted Ni-plated coating (combination 5) and (b) the Ni-plated coating as deposited and not

grit blasted (combination 4) between WCCoCr coatings.


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Table 1Spraying conditions of HVOF process

Spray gun Top-gunKerosene pressure (bar), ow rate (l/h) 2123, 2024O2 pressure (bar), ow rate (l/min) 2021, 8001000N2 pressure (bar), ow rate (l/min) 910, 1517Spraying distance (m) 0.30.5

ties of the coatings. Scanning electron microscopy (SEM) andPowder-shape system by scanner [37,38] have been used inorder to characterize the grain morphology and statistical grainsize distribution that found to be 2050 m as shown in Fig. 1.The porosity within the cermet coating was estimated by imageanalysis to be around 2%. Different bond layers were depositedon the rst cermet coating, and then a cermet top coating wassprayed to form thesandwich structure. NiCr 8020 andCoCrwere deposited by HVOF spraying on the rst cermet coating.

Nickel was electrochemically plated on the cermet coating withoptimized parameters to achieve a uniform and continuous Nilayer. The thickness of the Ni layer was approximately 60 m.The cermet coating surface previously exhibited a roughnessof Ra=5 m. After having deposited the Ni plating, half of the Ni-electroplated samples were slightly grit blasted to reachagain a roughness of Ra up to 4.5 m, so-called Ni-plating-Xcombination 5 in Table 2 . The other Ni-electroplated sampleswere not grit blasted to perform the combination 4 shown in thecross-section in Fig. 2. Lastly, a cermet coating as top layer hasbeen deposited on all interlayer coatings with the same sprayingconditions shown in Table 1 . The sandwich structure and inter-layer morphology of combinations are shown in Fig. 3 in the assprayed condition. The mechanical properties of the steel sub-strate, WCCoCr and interlayer coatings such as hardness andYoungs modulus have beendeterminedby low-load indentationtechniques [39,40] and are summarized in Table 2 .

Fig. 3. Micrographs of cross-sectioned samples as sprayed. Micrograph of (a) cermet based WCCoCr sandwich structured coatings using an intermediate layer(example cermet/NiCr/cermet), (b) NiCr 8020 HVOF intermediate layer morphology presenting some oxide layers between splats, (c) CoCr HVOF intermediatelayer morphology with some oxide layers between splats (dark lines), (d) Ni-plating morphology showing a dense and homogeneous coating and (e) cermet based

WCCoCr HVOF coating morphology.


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Table 2Mechanical characteristics of substrate, cermet and bond coating materials

Combinations and nomenclature Characteristics of substrate and bond coatings

Interlayer Coating system Thickness ( m) Youngs Modulus (GPa) Hardness, Hv 0.3

Mean S.D. Mean S.D.

1. Cermet/steel HVOF 5002. Cermet/NiCr/cermet NiCr 8020 HVOF 110 100 14 387 423. Cermet/CoCr/cermet CoCr HVOF 140 144 13 697 634. Cermet/Ni-plating/Cermet Ni-Plating Electroplating 50 243 15 516 315. Cermet/Ni-plating-X/cermet Ni-Plating-X Electroplating 50 243 15 516 316. Substrate DIN-1.4313 210 5 260 287. Cermet WCCrCo HVOF 250 266 12 1432 141

3. Experimental procedure

3.1. Tensile adhesive strength

According to the standard test EN 582, coated specimens of 25 mm diameter were joinedwith cylindricalcounter parts usingan adhesiveagent and cured at a temperature of 210 C. The ten-sile load was applied on a Universal Epprecht-Multitest tensilemachine. The mean adhesive strength values were calculatedfrom ve tests performed under the same conditions as:

max (MPa) =F

A, (1)

where F isthemaximumloadatruptureand A is thecross-sectionarea of the specimen.

3.2. Slurry erosion tests

A liquid jet impingement erosion test has been performedsimilar to the principle of ASTMG73 using sand particles intro-duced through thewaterjets nozzleby a powder feederas shownin Fig. 4. Two different incident angles of 90 and 30 betweenthe nozzle axis and the sample surface were used in order to sim-ulate normalandshear slurry erosive wear, respectively. Thetestparameters are shown in Table 3 .

3.3. Dry erosion tests

The dry erosion experiments were conducted using a grit-

blasting machine, which is based on pressurizedair to acceleratesand particles as shown in Fig. 5. A blocky shaped white Al 2O3wasusedas erodent particles. However, thedetails of test param-eters are given in Table 3 . Two incident angles of 90 and 30

between the nozzle axis and the sample surface were used in

order to simulate steep and at impact erosion, respectively.Wear rates from three erosion tests are averaged and expressedby mass loss per minute. The test samples were weighed beforeand after testing to an accuracy of 0.02 mg and were mounted atxed angles of 30 and 90 .

Fig. 4. Schematic presentation of liquid impingement erosion equipment.

Fig. 5. Schematic presentation of dry sand erosion test.

Table 3The test parameters of dry and slurry erosion experiments

Test Erodentmaterial

Erodentsize ( m)

Flow rate(water) (l/min)

Flow rate(erodent) (g/min)

Velocity(m/s)

Stand off distance (mm)

Nozzle diameter(mm)

Pressure(bar)

Exposure time(min)

Slurry erosion Al 2O3 75 + 45 13.6 9.52 147 100 1.4 250 38Dry erosion Al 2O3 1000 + 500 378 12 70 7.5 3.5 25


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Fig. 6. Determination of tensile adhesive strength according to the standardsEN 582 or ISO 14916 for different sandwich combinations (combinations of materials see Table 2 ).

4. Results and discussion

The tensile adhesive strength was calculated and presented

in Fig. 6. Sandwich combination 3 showed the lowest bondstrength, whereas the difference between the other sandwichcombinations was small. The failure fracture of coatingsoccurred adhesively within the glue and the substrate exceptcombination 3 hasfailed cohesively within theCoCr intermedi-ate layer. This is due to the presence of weak interfaces betweensplats and pre-existing oxide layers within the coating where theadhesive energyof coating at the interfaces was greater than thatstocked in between splats. The investigations on bond strengthand tribologicalbehavior of coatings allow better understandingthe mechanical and wearing performance.

Fig. 7. Results of (a) dry erosion and (b) slurry erosion rate with error bars priorto particle impact of 30 and 90 .

4.1. Dry and slurry erosion rate

The results of wear resistance in dry erosion shown in Fig. 7aunder the impact angle of 30 showed only small differencesbetween combinations 1, 2, 4 and 5. The combination 3 with

Fig. 8. SEM micrographs of the upper surface of sandwich combinations under dry erosion conditions. (a) WCCoCr, (b) Ni-Plating, (c) NiCr 8020 and (d)

CoCr.


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Fig. 10. Micrographs of the cross-sectioned sandwich combinations under slurry erosion with different angles of impact.

Thesurfaceexposedunder30 ( 7 mm2)wasuptofourtimesbigger than that of 90 (31 mm2). This behavior was not thecase underdryerosionbecause theexposedsurfaceunderbothangles covered the whole specimen of 40 40 of dimension.

The erodent content in water can play also a role where ithas found in similar conditions, the difference in wear rate at

the steep and normal angles was very slight [42].

Previous work under slurry erosion showed wear rate of cermetcoating at 30 and 90 in the same order of magnitude. The wearerosion rate in slurry erosion is about 10 times smaller than thatin dry erosion under both angles as shown in Fig. 7b, which is inagreement with the literature [11]. This difference is mainly dueto erodent size and to its kinetic energy. In dry rather in slurry

erosion, the difference in wear resistance between combinations


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0 Wear Performance of Sandwich Structured WC–Co–Cr Thermally Sprayed Coatings Using Different Intermediate Layers 2007 Wear