Microstructure and Cavitation Erosion Behavior of WC–Co–Cr Coating on 1Cr18Ni9Ti Stainless Steel by HVOF Thermal Spraying 2012 International Journal of Refractory Metals

8/11/2019 Microstructure and Cavitation Erosion Behavior of WCCoCr Coating on 1Cr18Ni9Ti Stainless Steel by HVOF Ther

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Microstructure and cavitation erosion behavior of WCCoCr coating on 1Cr18Ni9Tistainless steel by HVOF thermal spraying

Yuping Wu a,,1, Sheng Hong a, Jianfeng Zhang b, Zhihua He a, Wenmin Guo a, Qian Wang a, Gaiye Li a

a College of Mechanics and Materials, Hohai University, Nanjing 210098, Chinab Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

a b s t r a c ta r t i c l e i n f o

Article history:

Received 4 October 2011Accepted 7 January 2012

Keywords:

WCCoCr coatingHVOF thermal sprayCavitation erosion

A WCCoCr coating was deposited by a high velocity oxy-fuel thermal spray (HVOF) onto a 1Cr18Ni9Tistainless steel substrate to increase its cavitation erosion resistance. After the HVOF process, it was revealedthat the amorphous phase, nanocrystalline grains (CoCr) and several kinds of carbides, including Co3W3C,Co6W6C, WC, Cr23C6, and Cr3C2were present in the coating. The hardness of the coating was improved tobe 11.3 GPa, about 6 times higher than that of the stainless steel substrate, 1.8 GPa. Due to the presence ofthose new phases in the as-sprayed coating and its higher hardness, the cavitation erosion mass loss erodedfor 30 h was only 64% that of the stainless steel substrate. The microstructural analysis of the coating after thecavitation erosion tests indicated that most of the corruptions took place at the interface between the un-melted or half-melted particles and the matrix (CoCr), the edge of the pores in the coating, and the bound-ary of the twin and the grain in the stainless steel 1Cr18Ni9Ti.

2012 Elsevier Ltd. All rights reserved.

1. Introduction

Cavitation erosion is a common phenomenon in the hydrauliccomponents such as valves, propellers, hydraulic pumps and dieselengines, which are mostly made of metal/alloy materials [1]. Intheir operation processes, these components are often kept in con-tact with a fast-owing or vibrating liquid with a uctuating pres-sure. Pressure uctuation results in generation and collapse ofbubbles, exerting stress pulses on solid surfaces nearby and leadingto cavitation erosion of the metal surface [2]. The preparation ofappropriate surface coatings on the metal/alloy substrates is oftennecessary to reduce cavitation damage of the hydraulic components.For example, the WC-based coatings can be used to increase thewear, oxidation and erosioncorrosion resistance of the metal/alloymaterials[36]. The reason is that the hard WC particles in the coat-ings lead to high coating hardness and high wear resistance, whilethe metal binder (Co, Ni, or CoCr) supplies the necessary coatingtoughness[7] . However, in the traditional thermal spray technolo-gies, the WC phase tends to decompose into W2C with a lowhardness and a higher brittleness, which usually deteriorates thehardness, oxidation and cavitation erosion resistance of the coatings[4].

High velocity oxy-fuel (HVOF) technology has attracted much at-tention for coating preparation in the past decades because it can pro-

vide a high quality coating with a good adhesion quickly. For the caseof WC- and Cr3C2-based coatings, it has more obvious predominanceover traditional plasma spray. The reason is that, with two great char-acteristics of lower temperature (19003000 K) and higher velocityof around 550 m s1, HVOF technology reduces efciently the phasetransformation and oxidization of carbide particles during the coatingprocess (decarburization)[810].

At present, a WCCoCr cermet coating is prepared by the HVOFthermal spraying on a stainless steel 1Cr18Ni9Ti substrate, which iswidely used for hydraulic machinery. The microstructures, phasecomposition and transformation of the carbide of coating are identi-ed by X-ray diffraction (XRD), optical microscope (OM), scanningelectron microscopy (SEM) and transmission electron microscopy(TEM). The Vickersmicrohardness of the coating and the substrateis tested by the indentation method. The cavitation erosion behaviorof the WCCoCr coating and the reference austenite stainless steel1Cr18Ni9Ti is investigated using vibratory cavitation apparatus. Theeroded surfaces are examined by means of SEM and the possible ero-sion resistance mechanism is discussed.

2. Experimental procedure

2.1. Materials and HVOF thermal spraying process

The starting powder used for the coating on the austenitic stain-less steel (1Cr18Ni9Ti) in the present study was about 2050m ingrain size with a composition of 40 wt.% W36 wt.% Cr20 wt.% Co

Int. Journal of Refractory Metals and Hard Materials 32 (2012) 2126

Corresponding author. Tel./fax: +86 25 83787233.E-mail address:[email protected](Y. Wu).

1 Present address: College of Mechanics and Materials, Hohai University, XikangRoad 1, Nanjing 210098, China.

0263-4368/$ see front matter 2012 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ijrmhm.2012.01.002

Contents lists available at SciVerse ScienceDirect

Int. Journal of Refractory Metals and Hard Materials

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and a balance of C.Fig. 1shows the SEM morphology of this startingpowder at a low magnication of 100 (Fig. 1(a)) and a high magni-cation of 1000 (Fig. 1(b)), respectively.Fig. 1(b) shows that thepowder was composed of several small particles about 15m in an

agglomerated and a slightly sintered state.Before the coating process, the screw specimen made of stainless

steel (1Cr18Ni9Ti) was degreased and grit blasted to get a roughnessof 17m. Then the screw was coated by the WCCoCr coating fromthe above starting powder using a JP5000 spray system. Kerosene andoxygen were used as the fuel gases with ow rates of 0.02 m3 min1

and 1.85 m3 min1, respectively, whereas argon was used as the car-rier gas with a ow rate of 0.01 m3 min1. The powder feed rate wasxed at 10 g min1 with the aid of a computerized powder feederstation. After 15 passes of the spray gun, a coating was obtainedwith a deposit thickness of 500m.

2.2. Cavitation erosion test

Cavitation erosion tests were carried out using a vibratory cavita-tion apparatus, the detailed procedure of which can be found else-where in literature[11].In brief, before the cavitation erosion tests,the screw specimen with the WCCoCr coating on it was pretreatedby grinding and mechanical polishing. Then, the coated sample wasattached to the free end of the horn. The tip of the screw was im-mersed about 3 mm in the water held in a 1000 ml beaker and thesystem kept in a resonant condition with a frequency of 191 kHzand double amplitude of 605m by controlling the output powerof the ultrasonic generator. In the testing process, the beaker was sur-rounded by the owing cooling water to keep the water inside it at2530 C. In every 30 min, the well-handled water in the beakerwould be replaced by unused water. After the set time of erosion

(1.5 h), the samples were cleaned and weighted by a balance with

an accuracy of 0.1 mg. The reference austenitic stainless steel1Cr18Ni9Ti was also tested in the same condition for comparison.

2.3. Microstructural characterization

The crystal phase of the starting powder and the coating wereidentied by X-ray diffraction (XRD; Geigerex, Rigaku Corp.) withCuKradiation. The microstructure of the uneroded and eroded coat-

ing was observed by scanning electron microscopy (SEM, Hitach:S-3400N). Some ner-scale microstructure features of the coatingwere investigated using a transmission electron microscopy (TEM,

JEOL: 2000EX)). Vickers hardness (Hv) at room temperature was eval-uated by a hardness tester (HXD-1000TC) at a load (P) of 1.96 N for15 s and was averaged by 20 measurements along the mediumcross-sections of the coating and the austenitic stainless steel(1Cr18Ni9Ti).

3. Results and discussion

3.1. Phase composition

The X-ray diffraction patterns for the starting powder and the as-sprayed coating are shown inFig. 2. In the WCCoCr coating and thestarting powder, different phases of Co3W3C, Co2C, WC, CrCo andchromium carbide were detected. The results indicate that higherame velocity and lower ame temperature of the HVOF processwould effectively limit WC decomposition process [12]. Fig. 2 alsoshows the presence of a distinct diffuse diffraction halo centredaround 243 and 65 in the traces suggested that there is a certainproportion of amorphous phase within the powder and the coating,and it is more intense in the XRD data from the coating.

3.2. Characterization of the coating

Fig. 3(a)(d) presents the polished transverse surfaces of the coat-ing.Fig. 3(a) shows that the coating is very dense and has a good con-tact with the substrate. This indicates that the coating does have a

tight adherence to the substrate due to the higher velocity of HVOFthermal spraying (Fig. 3(a)). The porosity value for as-sprayed WCCoCr coating is less than 1% from Fig. 3(b), which correlates withthe results of the Fe-based coating by HVOF[11]and high velocityaxial plasma spraying (HVAPS) [13]. Scrivani et al. [14] proposedthat the high impact velocity of the coating particles, which causedhigh density and high cohesive strength of individual splats, maylead to the low value of porosity and high density of the coating.Fig. 3(b) shows the presence of the un-melted or half-melted particlein the coating with a near spherical morphology. A similar

Fig. 1.SEM micrographs of the powder morphology at a magnication of (a) 100 and(b) 1000.

Fig. 2.XRD patterns of the as-powder and as-sprayed coating.

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morphology has been observed in our previous investigations of Fe-based alloy coatings[11, 15, 16]. It is proposed here that the metalbinder (CoCr) was partly or fully melted, while the most of the car-bide particles remained in the solid state during the HVOF thermal

spraying[17].Fig. 3(c) and (d) shows that the coating also consistedof the matrix (CoCr) and carbide particles. A small amount of oxideappears with the lightest grey contrast, whereas no oxide formationhas been observed in the coating by the XRD analysis due to thevery low oxide content. A similar result has also been observed byMagnani et al.[18].

TEM was also used to obtain more detailed information about themicrostructure of the WCCoCr coating in the present study. Thecorresponding TEM images were shown inFig. 4, from which it canbe detected that the coating consists of carbides, nanocrystallinephase and amorphous phase. The diffraction patterns, taken withthe selected area aperture centered over the amorphous region,showed the expected diffuse halo with diffuse diffraction spots aris-ing from crystalline grains within the selected area (Fig. 4(b)).

These diffuse characters present in the TEM coincide with the XRD re-sults inFig. 2. The size of the nanocrystalline grains in the coating is inthe range of 10 to 30 nm shown inFig. 4(c), and the nanocrystallinegrains were identied as the bcc Cr-based phase from the polycrystal-line selected area diffraction (SAD) pattern shown in Fig. 4(d). Be-cause the cooling rate of a droplet could be high enough to give ahigh rate of nucleation and the recrystallization of the originallyamorphous region during successive passes of the thermal spraying,the nanocrystalline grains are able to form. The latter explanationwas the same as the literature[19]. The WC phase noted in the XRDspectrum ofFig. 2was also observed by TEM shown inFig. 4(e) and(f), which has an orthogonal structure with a massive shape and ahexagonal lattice structure.

The amorphous phase formation of the WCCoCr system is

closely related to the atomic structure besides the high cooling

velocity of ~107 K s1 [20], which is suitable for forming amor-phous phase. The base composition in the present coating is theCoCrWC system, the effective addition of Co, Cr, W and Ccauses the sequential change in the atomic size in the order of

W(1.41 )>Cr(1.30 )>Co(1.25 )>C(0.91 ) as well as the gen-eration of new atomic pairs with various negative heats of mixing. Thetopological structure and chemical short-range order are increased,leading to the formation of a highly dense, random packed structurewith low atomic diffusivity in the super cooled liquid[21]. That is tosay, the formation of the amorphous phase was attributed to the highcoolingvelocity of molteddroplets and theproperpowdercomposition.This view is similar to that in the literature[20].

Fig. 5 shows the microstructure of stainless steel 1Cr18Ni9Ti,which presents typically twin austenite. The microhardness of thecoating is 11.3 GPa at a load of 1.96 N, which is signicantly higherthan that of the comparing material stainless steel 1Cr18Ni9Ti,1.8 GPa at the same load of 1.96 N. The high hardness of the coat-ing is attributed to the complex structures of amorphous phase,

nanocrystalline grains and several kinds of the carbide hardenedphases, such as Co3W3C, Co6W6C, WC and chromium carbide, asshown inFig. 2 and Fig. 4. On the other hand, the lower porositycontent is benecial to the high hardness value of HVOF depositedWCCoCr coating[4].

3.3. Cavitation erosion characteristic

The cavitation erosion cumulative mass loss curves of the HVOFdeposited WCCoCr coating and reference stainless steel(1Cr18Ni9Ti) are shown inFig. 6. It is shown that the HVOF-sprayedWCCoCr coating exhibits apparently higher resistance than the ref-erence stainless steel 1Cr18Ni9Ti. After being eroded for 30 h, themass loss of the coating was 17.5 mg, only 64% that of reference stain-

less steel 1Cr18Ni9Ti (27.4 mg).

Fig. 3.SEM images of a transverse section of the as-sprayed coating: (a) a lamellar morphology; (b) pores and half-melted particles; (c) and (d) Co Cr matrix, oxide and carbidegrain.

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Fig. 7shows the surface SEM micrographs of the stainless steel1Cr18Ni9Ti samples after being eroded for 30 h. There is some defor-mation in the reference stainless steel 1Cr18Ni9Ti after the cavitationtest.Fig. 7(a) shows the central part of the sample that presented the

most severe cavitation erosion characteristic and some particles fromplastic deformed material were torn off. But the perimeter of the sam-ple surface showed no trace of erosion characteristic. Therefore, thereis a dividing region between the eroded and uneroded regions, which

Fig. 4.TEM images (bright eld) of typical microstructure of coating: (a) a region of amorphous phase, (b) SAD ring pattern of amorphous region, (c) a region of nanocrystallinegrains, (d) SAD ring pattern of nanocrystalline region, (e) carbide, nanocrystalline grains and amorphous phase, (f) SAD ring pattern of WC phase.

Fig. 5.Microstructure of stainless steel 1Cr18Ni9Ti, at a magnication of 1600. Fig. 6.Cumulative mass losses vs. cavitation erosion time.

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is shown inFig. 7(b). The characters of slip bands and twins weremore obvious than that of original structures (Fig. 5). The microcracks

are initiated at the connection part of the twin lamella and the aus-tenitic grain boundary.

Fig. 8shows the cavitation erosion characteristic of the coatingsample eroded 30 h. There is no obvious damage phenomenon onthe coating surface in the SEM image (Fig. 8(a)). The surface of thecoating only shows little material desquamation, and there are stillsome polished regions undamaged. As shown in Fig. 8(b, c), themass loss began at the interface between the un-melted or half-

melted particles and the matrix or the edges of pores, then extendto the general edges of the pores and even over the surface, whichis in agreement with the result of HVOF spraying Fe-based coatingin our previous investigation[11]. A larger magnication morphology(Fig. 8(d)) shows that the coating was removed by delamination.Fractography (Fig. 8(d)) seems to have a fatigue character, which isbecause of the presence of fatigue strip.

The materials subjected to the cavitation erosion can be destroyedby repeated short-time impacts. Therefore, cavitation erosion ofmetals in some previous works was interpreted as a particular caseof cyclic microimpact-load destruction[22]. For the sake of repeatedimpact load and very small contact area, degradation mechanismcaused under the action of cavitation erosion could be also describedby repeated nanoindentation loading[23, 24]. The WCCoCr HVOFcoating in the present study also had a fatigue character, as showninFig. 8. In our experimental procedure, the cavitation erosion wasinterrupted for weight measurement. After that, the cavitation ero-sion continued until the next weight measurement. This may contrib-ute to the fatigue character inFig. 8.

It should be pointed out nally that the erosion resistance was ex-tremely sensitive to the quantity of microstructural defects [25].Some literatures have proved that the strength and hardness of mate-rials decreased as the porosity increases[26]. It may thus suggest thatthe pores and un-melted particles could weaken the capability of ma-terial. Therefore, cavitation erosion resistance can be improved by the

Fig. 7. Cavitation erosion characteristic of 1Cr18Ni9Ti stainless steel (eroded 30 h):(a) cavitation erosion region, magnication: 2500; (b) the boundary of the cavitationerosion region, magnication: 500.

Fig. 8. Cavitation erosion characteristics of the WCCoCr coating: (a) magnication: 100; (b) and (c) un-melted or half-melted particles and pores, magnication: 2000;

(d) fatigue strip, magnication: 10,000.

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optimization of spray parameters for a more homogeneous coatingwith a smaller number of un-melted particles and pores. Such workis still underway and will appear in a further paper.

4. Conclusion

A WCCoCr coating is obtained by HVOF spraying for cavitationerosion resistance applications. The main conclusions are as follows:

(1) The HVOF-sprayed WCCoCr coating has a uniform micro-structure and tight metallurgical bonding to the substrate.The phases of the coating consist of the amorphous phase,the nanocrystalline grains and several kinds of carbide. TheVickers hardness values of the coating (11.3 GPa) are muchhigher than that of the stainless steel 1Cr18Ni9Ti (1.8 GPa).

(2) The cavitation erosion resistance of the coating is higher thanthat of 1Cr18Ni9Ti stainlesssteel as a result of its high hardnessand ner structure.

(3) The mass loss took place at the interfaces of different compo-nents. In the coating, the mass loss began at the interface be-tween the un-melted or half-melted particles and the matrix,the edge of the pores and the interface of different phases.The microcracks in austenitic stainless steel 1Cr18Ni9Ti are ini-

tiated at the connection part of the twin lamella and austeniticgrain boundaries.

Acknowledgements

The research was supported by the Innovation Foundation ofHohai University, China Postdoctoral Science Foundation (No.20100471371) and the Fundamental Research Funds for the CentralUniversities (No. 2009B16314).

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Microstructure and Cavitation Erosion Behavior of WC–Co–Cr Coating on 1Cr18Ni9Ti Stainless Steel by HVOF Thermal Spraying 2012 International Journal of Refractory Metals