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113 Vol. 35, No. 2 (2013) 113‐122 Tribology in Industry www.tribology.fink.rs Tribological Behavior of Thermal Spray Coatings, Deposited by HVOF and APS Techniques, and Composite Electrodeposits Ni/SiC at Both Room Temperature and 300 °C A. Lanzutti a , M. Lekka a , E. Marin a , L. Fedrizzi a a Università di Udine, Dipartimento di Scienze e Tecnologie Chimiche, Via del Cotonificio 108, 33100 Udine, Italy. Keywords: Thermal spray coatings Nanocomposite electrodeposits Ni/SiC Microcomposite electrodeposits HVOF APS Dry sliding ABSTRACT The Both the thermal spray and the electroplating coatings are widely used because of their high wear resistance combined with good corrosion resistance. In particular the addition of both micro particles or nanoparticles to the electrodeposited coatings could lead to an increase of the mechanical properties, caused by the change of the coating microstructure. The thermal spray coatings were deposited following industrial standards procedures, while the Ni/SiC composite coatings were produced at laboratory scale using both microand nanosized ceramic particles. All the produced coatings were characterized regarding their microstructure, mechanical properties and the wear resistance. The tribological properties were analyzed using a tribometer under ball on disk configuration at both room temperature and 300 o C. The results showed that the cermet thermal spray coatings have a high wear resistance, while the Ni nanocomposite showed good anti wear properties compared to the harder ceramic/cermet coatings deposited by thermal spray technique. © 2013 Published by Faculty of Engineering Corresponding author: A. Lanzutti Università di Udine, Dipartimento di Scienze e Tecnologie Chimiche, Via del Cotonificio 108, 33100 Udine, Italy Email: [email protected] 1. INTRODUCTION The thermal spray coatings are widely used for many industrial applications [1‐13] because of the possibility to deposit different type of materials, ranging from different metal alloys to ceramics, and their technological properties, in particular the high wear resistance even if they are used also as corrosion barriers at both high temperature degradation or wet corrosion. The thermal spray coatings are mainly used for high temperature applications (oxidation resistance or fused salts resistance). Usually these types of coatings are deposited with the addition of rare earths in order to inhibit the oxidative degradation processes [1‐3]. Some technological processes are used to reduce the porosity of the coating and thus increase both mechanical properties and the barrier effect to oxidative environments. Sidhu et al [2] have RESEARCH CORE Metadata, citation and similar papers at core.ac.uk Provided by Directory of Open Access Journals

Transcript of 3. Tribological Behavior of Thermal Spray Coatings ... · material to the substrate [4‐6]. The...

Page 1: 3. Tribological Behavior of Thermal Spray Coatings ... · material to the substrate [4‐6]. The NiCr 80/20 alloy is also used as metal matrix to produce composite coatings reinforced

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Vol.35,No.2(2013)113‐122

TribologyinIndustry

www.tribology.fink.rs

TribologicalBehaviorofThermalSprayCoatings,DepositedbyHVOFandAPSTechniques,andCompositeElectrodepositsNi/SiCatBothRoom

Temperatureand300°CA.Lanzuttia,M.Lekka

a,E.Marina,L.Fedrizzia

a

UniversitàdiUdine,DipartimentodiScienzeeTecnologieChimiche,ViadelCotonificio108,33100Udine,Italy.

Keywords:

ThermalspraycoatingsNano‐compositeelectrodepositsNi/SiCMicro‐compositeelectrodepositsHVOFAPSDrysliding

ABSTRACT

TheBoththethermalsprayandtheelectroplatingcoatingsarewidelyusedbecause of their high wear resistance combined with good corrosionresistance. In particular the addition of both micro particles or nano‐particles to the electrodeposited coatings could lead toan increaseof themechanicalproperties,causedbythechangeofthecoatingmicrostructure.Thethermalspraycoatingsweredepositedfollowingindustrialstandardsprocedures, while the Ni/SiC composite coatings were produced atlaboratory scaleusingbothmicro‐andnano‐sized ceramicparticles.Alltheproducedcoatingswerecharacterizedregardingtheirmicrostructure,mechanicalpropertiesandthewearresistance.Thetribologicalpropertieswereanalyzedusingatribometerunderballondiskconfigurationatbothroomtemperatureand300oC.The results showed that the cermet thermal spray coatingshaveahighwear resistance, while the Ni nano‐composite showed good anti wearpropertiescomparedtotheharderceramic/cermetcoatingsdepositedbythermalspraytechnique.

©2013PublishedbyFacultyofEngineering

Correspondingauthor:

A.LanzuttiUniversitàdiUdine,DipartimentodiScienzeeTecnologieChimiche,ViadelCotonificio108,33100Udine,ItalyE‐mail:[email protected]

1. INTRODUCTION

The thermal spray coatingsarewidelyused formany industrial applications [1‐13] because ofthe possibility to deposit different type ofmaterials,rangingfromdifferentmetalalloystoceramics, and their technological properties, inparticular thehighwear resistanceeven if theyareusedalsoascorrosionbarriersatbothhightemperaturedegradationorwetcorrosion.

The thermal spraycoatingsaremainlyused forhigh temperature applications (oxidationresistance or fused salts resistance). Usuallythese types of coatings are deposited with theaddition of rare earths in order to inhibit theoxidative degradation processes [1‐3]. Sometechnological processes are used to reduce theporosity of the coating and thus increase bothmechanical properties and the barrier effect tooxidative environments. Sidhu et al [2] have

RESEARCH

CORE Metadata, citation and similar papers at core.ac.uk

Provided by Directory of Open Access Journals

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found that the laser remelting process increaseboth the mechanical properties and theoxidation resistance and leaves only a smallamountofporositytothecoating(<1%).Sighetal[3],instead,showedthattheNiCr(80/20)hasa good resistance to hot corrosion in moltensaltsat900°C.The literature about NiCr wear performance isscarce. Usually this kind of coating is used asbond coat for ceramic or cermet coatings inordertopromotetheadhesionof thedepositedmaterial to the substrate [4‐6].TheNiCr80/20alloy is also used as metal matrix to producecompositecoatingsreinforcedwithcarbides[7‐9]. The WC‐Co coatings, instead, were widelyused and analysed bymany research teams [7‐14].Theinterestaboutthesetypesofcoatingsisrelated to their high mechanical property thatleads to high wear resistance of the coatedsystem.Fedrizzietal[7]performedsometribo‐corrosiontestsoncermetcoatingsandobservedtheirgoodwearresistance,relatedtotheirhighhardness, but low corrosion resistance, in wetwear tests.Thisbehaviour is related to the lowtoughnessthatpromotesthecrackenucleationsandthusthepermeationofcorrosionmediathatenhances the undermining corrosion of thecoating.Tomaetal[8]showedthattheadditionofCrtometalmatrixincreasedtheabrasionandcorrosion resistance. Fedrizzi et al [9] showedthat these type of coatings can be used as analternative to hard chromium and theirperformancearealsoincreasedifareusednano‐sized powders. Murthy et al [11] showed thatthe coatings grinded have an higher wearresistancebecauseof theproductionofanhardlayer on surface. Other researcher [12] foundoutthatthedecreaseinpowdersizeincreasethecoating performance because of both reductionof the porosity and increase of the mechanicalproperties. This enhances both corrosion andwearsprotection.The ceramic coating Cr2O3 was subjected tonumerous studies in the areaof bothwear andcorrosionprotection. ItwasshownbyAhnetalthat, in the reciprocating wear tests, the wearmechanism is a plastic deformation of weardebristhatinfluenceboththefrictionandwearbehavior. This is related to a formation of CrO2layer under hertzian loads [4]. Bolelli et al [5]performed wear tests at room temperature ondifferent plasma spray ceramic coatings. He

found that the Cr2O3 coating is the hardest andmost anisotropic coating with high abrasionresistance,asconfirmedbyLeivoetal[6],whilein sliding condition the material forms acompact tribofilm. The high temperature dataforthistypeofcoatingsisscarce.TheNibasedelectrodepositsarewidelyusedasboth corrosion/wear barrier coatings in manyapplications ranging from high temperatureapplications to room temperature applicationsinbothdryandwetconditions[16‐23].In this work composite coatings are alsoanalyzed. The introduction of reinforcingparticlesisaimedtoenhanceboththewearandcorrosion properties, in particular if the nano‐particlesareembeddedtothemetalmatrixtheyproduceanano‐structuredmicrostructure.Garciaetal[16]showedthattheincreaseofwearproperties at room temperature for micro‐compositeNi/SiCcoatingsisafunctionofparticle’size. In particular the decrease of particle’ sizeleadstoanincreaseofanti‐wearproperties.The reaserch group of Zimmermann et al [17‐18]observedthattheadditionofsub‐microsizedparticles to the coating leads to an increase ofbothmechanical strength and toughness, if thereinforcing content is below the 2 wt%. Abovethe 2 wt% they showed that some particles’coalescence is possible, during the deposition,leadingtotcoatingembrittlement.Theytriedtoaddnano‐particlestothecoatingandobservedanotableincreaseofthemechanicalproperties.Beneaetal[19‐20]inmanyworksdemonstratethat the addition of SiC nano particles, in Nimatrix, leads to an increase of the wearproperties and they calculated the relationbetween the microstructure of the coating andthewearperformance.The wear properties of both micro compositeand nano composite Ni/SiC coatings at hightemperature were investigated by Lekka et al[21]. They found an increase of anti‐wearproperties of the composite coating at bothroomtemperaturetestand300°CcomparedtothepureNicoating.This work aimed to compare the wearperformance of the most used thermal spray

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coatingswiththeanti‐wearpropertiesoftheNicomposite coating, highlighting the importantproperties of the composite coatings producedwitha simpleandcheaper techniquecomparedtothethermalsprayprocess.2. EXPERIMENTAL

a. SamplesproductionForalltypesofthedepositsASTM387F22steelplates(7×10cm)anddiscs(d=5cm)havebeenused as substrates (chemical composition inTable1).Table 1. Chemical composition of steel substrateASTM387F22.

C Si Mn P Cr Mo Fe0.11 0.31 0.5 0.025 2.2 0.9 Bal.Thethermalspraycoatingshavebeendepositedusing industrial procedures. The depositedcoatings were: NiCr 80/20 and NiCr 80/20 +Cr2O3 deposited by APS (Air Plasma Spray)techniqueandWCCoCr18/4depositedbyHVOFtechnique.RegardingtheNimatrixcoatings,threetypesofdeposits have been prepared: pure Ni (to beusedasreference).Nicontainingmicroparticlesof SiC and Ni containing nanoparticles of SiC.Theelectroplatingbathusedwasahigh speednickel sulfammate plating bath having thefollowing composition: 500 g/l Ni(SO3NH2)2.4H2O,20g/lNiCl2.6H2O,25g/lH3BO3,1ml/l surfactant (CH3 (CH)11OSO3Na basedindustrial product. The depositionwas carriedout using a galvanic pilot plant (12 l platingtank) under galvanostatic control at 4 A/dm2,50 °C, under continuous mechanical stirring.The deposition time was 2.5 h in order toobtain 70–80 μm thick deposits. For theproduction of the composite coatings 20g/l ofmicro‐ or nano‐powders were added into theelectroplating bath, dispersed usingultrasounds (200 W, 24 kHz) for 30 min andthen maintained in suspension undercontinuous mechanical stirring during theelectrodeposition. The micro‐particles have amean dimension of 2μm and a very irregularandsharpshape,whilethenano‐particleshaveameandiameterof45nm[21].

b. SamplescharacterizationThe specimens characterization includesmicrostructure, chemical composition,microhardness, wear resistance at both roomtemperature and 300 °C and corrosionresistanceintwodifferentenvironments.Themicrostructureof thespecimenshavebeenanalysedbySEM(ZeissEvo‐40)+EDXS(OxfordinstrumentsINCA)incrosssection.BoththeSiCcontent and the coatings’ porosity werecalculatedusinganimageanalysissoftware[13].FornanocompositecoatingTheSiCcontentwasmeasured through the measurements of RFGDOES (HR‐Profile, Horiba Jobin Yvon),calibrated using 28 CRM (Certified ReferenceMaterial)samples.ThesystemwassetupusinganArpressureof650Paandaappliedpowerof50 W. The micro‐composite coating were notanalysed by theGDOES because of some issuesrelated to theplasmaerosionof the reinforcingparticles[21].Micro‐hardness measurements (HV0,3) havebeen performed on cross section of thespecimens.Wear tests have been performed using a CETRUMT tribometer in a ball‐on‐disc configuration[25] at both room temperature and at 300 °C.ThetestingparametersaresummarizedinTable2. The volume loss has been evaluated using astylus profilometer (DEKTAK 150 Veeco). ThewearrateK[10−6mm3/Nm]hasbeencalculatedusingtheequationdescribedin[26].Table2.Weartestparameters.

Countermaterial WCsphere(d9.5 mm)Appliedload 70NTestradius 18mm

Rotationspeed 300rpmSlidingspeed 0.565m/sTestduration 60min

3. EXPERIMENTALRESULTSa. MicrostructuralcharacterizationInFig.1isshownthemicrostructureofthesteelsubstrate. The Gr 22 steel presents a ferriticmicrostructurewithsomecarbidesprecipitatedinthemetalmatrix,that leadstothehighcreep

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strength of material. The carbides are mainlyproduced by Cr and Mo. The hardness of thematerial is about180±20 HV0,3 and the ferriticgrainsizeisabout45±15µm.

Fig.1.Microstructureofgr.22steel.In Fig. 2 are shown the SEM micrographsobtained for Thermal spray coatings and therelative data acquired by mechanicalcharacterization and image analysis. In Tab. 3thethermalspraycoatings’propertiesarelisted.

Fig. 2. SEM images and microstructuralcharacterization of thermal spray coatings: a) NiCr80/20,b)WCCoCr18/4andc)NiCr80/20+Cr2O3.Table 3. Results of thermal spray coatings’characterization.Coating Thickness

[µm]Porosityvol.%

HardnessHV0,3

NiCr 98±16 6.5 359±18WCCoCr 105±15 3.45 1027±21

NiCr+Cr2O3 (38+187)±25 5.5+10.1 (341+1118)±24As can be observed, the three types of thermalspray coatings present different thickness andporosity. The porosity, acquired by imageanalysis, ishigher for thecoatingsdepositedbyAPS technique compared to theHVOFdeposits.Thisdifferencecouldberelatedtobothpowderssize and impact velocity that is lower in APStechnique with respect to HVOF. Indeed, thedifference in kinetic energy of the moltenpowders,thatishigherinHVOFtechnique,leadstoadifferentdensityondepositedcoating.Thehardness acquired is associated to thematerialdepositedandthevaluesacquiredaresimilartodataavailableinscientificliteratureforthermalspraycoatings[1‐13].TheSEMmicrographsobtainedoncrosssectionof Ni/SiC composite coatings previously etched(aceticacid:nitricacid1:1)areshowninFig.3.In Table 4 are listed the electrodepositedcoatings’properties.Table 4. A result of Ni/SiC electrodepositscharacterization.

Coating Thickness[µm]

SiCwt.% HardnessHV0.3

Ni 78±7 ‐ 172±7Ni/µSiC 73±8 0.8 247±8Ni/nSiC 75±5 0.15 270±9

a)

b)

c)

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Fig. 3. SEM images and microstructuralcharacterization of electrodeposited coatings:a)pureNi,b)Ni/µSiCandc)Ni/nSiC.The microstructure of the electrodeposits iscolumnar. In the case of the pure Ni themetalcolumns are oriented along the direction ofelectrical fields. The addition of SiC micro‐particles leads toaslightmodificationof theNicolumnsorientationandsize.Ontheotherhand,the codeposition of SiC nano‐particles leads totheformationofafinegraineddepositinwhichtheNicolumnsarenotoriented.Theadditionof

micro‐particles leads to a microstructurecolumnar with a slight modification oforientation,causedprobablybythedeviationofelectrical field inproximityof ceramicparticlesthat are in non‐conductive material. On thecontrary, the addition of nanoparticles gives agrain refinement and a multi‐orientation ofcolumns.TheSiCamountishigherinthemicro‐composite coatings compared to the nano‐compositeone.Theadditionofparticlesleadstoanoticeablemicrohardnessincreaseduetoboththe presence of the particles and the grainrefinement.All the analysed samples showed a surfaceRoughnessRaofabout0.5µm,obtainedafterthesurface’grinding.

b. TribologicalcharacterizationAlltheweartracksobtainedforbothbaresteel,thermal sprayandcompositeelectrodepositsatbothroomtemperatureand300°CareshowninFigs.4‐6.

In Fig. 4 the top views of the wear tracksobtained for gr. 22 steel are shown, tested atbothroomtemperatureand300°C.

Roomtemperature 300°C

Fig.4.SEMimagesoftheweartracksobtainedforthebaresteelatbothRTand300°C.The steel is subjected to triboxidative wear atboth room temperature and 300 °C. At roomtemperature the oxide produced is adherent tometalsubstrateandveryhomogeneus.At300°Ctheoxideproducedisconcentratedontheweartrack’ sides. This phenomenon is related to theloss of mechanical properties of the substratesthatpermits the countermaterial todestroy theoxidelayerthatisthusdepositedonthesidesofthe wear track. At 300 °C are highlighted the

c)

b)

a)

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debrisofbothoxideandsteelalongthebordersofweartrack.In Fig. 5 the top views of the wear tracksobtained for thermal spray coatings are shown,testedatbothroomtemperatureand300°C.For the metal matrix deposits the degradationmechanism is a triboxidation at both roomtemperature and 300 °C, which intensity isvaryinginfunctionofthecoatingmaterial.Inparticularthethermalspraycoatingsshowedalso other degradation mechanisms which arerelatedtotheirmicrostructure. Roomtemperature 300°C

NiCr+Cr

2O3

WCCoCr

NiCr

Fig.5.SEMimagesoftheweartracksobtainedforthethermalspraycoatingsatbothRTand300°C.The Ni/Cr showed a material detachmentoriginated by contact fatigue phenomenon,aggravatedby itsporosity.Athigh temperature

tests the detachment is decreased due to apossible hardness decrease of the materialwhich allowed the sealing of porosity, thusreducing thecontact fatigue failure.Thecermetcoatings were all subjected to triboxidation ofmetalmatrix,moreintenseathightemperature.Fortheceramicmaterial(Cr2O3)thedegradationmechanism at room temperature is similar totheNiCr coatingwhileathigh temperature testthe degradation becomesmore intense. This iscaused by a phase change of chromium oxidewhich enhances the wear ratio and reduces atthesametimethefrictioncoefficient[4].In Fig. 6 are shown the top views of the weartracksobtainedforNi/SiCelectrodepositstestedatbothroomtemperatureand300°C. Roomtemperature 300°C

Ni/nSiC

Ni/µSiC

Ni

Fig.6.SEMimagesoftheweartracksobtainedfortheNi/SiCdepositsatbothRTand300°C.

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The Ni electrodeposits showed, at roomtemperature,a triboxidationwithadescalingofoxide which forms a third body between thecounter material and the wear track, thusleading to the formation of secondary tracksrelated to abrasive wear. At high temperaturethe coatings showed a strong triboxidation. ByEDXS analysis on pure Ni coatings, it wasdetected that during the wear test the steelsubstratewasreached.ForthecompositeNi/SiCcoatings it seems that the oxide produced ismore adherent to steel substrate at hightemperature. Probably both the grainrefinementandthepresenceofceramicparticlesare linking the oxide to the metal matrix. Itseems that some counter‐material wastransferred to the coating surface, due to slightadhesionphenomena.Thewearrateofalltestedcoatingsatbothroomtemperatureand300°CareshowninFig.7.

Fig.7.Wear rates graph at both room temperatureand300°Cforallthecoatingstested.All the coatings protect the steel substrateduring the test, except for the pure Ni coatingthatshowed,at300°C,thehighestwearrate.Itis possible to observe that the cermet coatinghasthelowestwearratecomparedtotheothercoatings at both test temperatures. This isrelated to the high amount of WC which isbonded by a metal matrix that has highoxidation resistance at the test temperatures.Forthiscoatingthewearresistanceisassociatedto the carbide component and the particlesbindingisrelatedtothemetalmatrixwhichhasa high toughness. On the other hand, the NiCrcoating showed a lower wear rate at hightemperature tests, compared to the room

temperature one and this could be related toboth high oxidation resistance of the materialthat contains a high amount of Cr and to thereduction of material detachment thatconsequently reduces the abrasive phenomena.Theceramiccoating(Cr2O3)showedahighwearrate at 300 °C tests due to phase change ofchromiumoxideundertribologicalcontact.The electrodeposits showed good wearresistance at room temperature, higher for thenano‐compositecoating.Thisreduction inwearratecouldberelated to thegrainrefinementofmicrostructure of the metal matrix, whichincreases also themechanical properties of thecoating. This effect is not visible in the micro‐composite coatings because the reinforcementparticles are usually detached from the metalmatrixleadingtointensiveabrasivewearcausedby hard particle third body contact betweencountermaterialandsurfaceofthespecimen.Athigh temperature the mechanical behaviour ofthe coating is reduced, probablybecause of thehardness decrease. In this case the pure Nicoating is completely removedwhile themicrocomposite coating showed a better wearresistance,comparedtothepureNione,butthewear rate values were still higher than thethermal spray coatings wear rates. The higherwear resistance of the nano‐composite coatingat 300 °C is probably related to the highermechanical properties of the metal matrixcomparedtotheotherelectrodeposits.

InFigs.8‐9theCOF(FrictionCoefficientsof thetested materials) are shown. For all the testperformed on thermal spray coatings, it ispossible to observe that the COF values, at theend of the test, are comparable between thetests performed at different temperature, aparttheceramiccoatingthatshowedalowerCOFathigh temperature due to the change phase ofceramic oxide under hertzian loads. The NiCrcoatings showed a noisy COF graph because ofthe coatingmaterial detachment that producedabrasive particles that dissipated more energy,required to move the particles in the hertziansystem. At high temperature there is a start atlow COF and, at regime, it reached the samevaluesofthetestatroomtemperature.Probablyduring the start of the test the surface of thesamplewas covered by a oxide layer producedduring theheatupof the system.Thepresence

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of oxide decreased the surface energy inproximity of the hertzian contact of the twomaterials,reducingthefrictioncoefficient.Whenthe oxide was broken the contact between thetwo materials was between the WC and theNi/Crslightlyoxidized.

Fig.8.COFgraphsatbothroomtemperatureand300°C for the thermal spray coatings: a)NiCr80/20, b)WcCoCr(18/4),c)NiCr80/20+Cr2O3.

Fig.9.COFgraphsatbothroomtemperatureand300°C for the Ni/SiC composite coatings: a) Roomtemperaturetest,b)300°C.For the WC‐CoCr coating the COF are slightlydifferent and this is caused mainly by thenumberofthirdbodyparticlesproducedduringthe test. Indeed is possible that at hightemperature the amount of abrasive particles,that are takingpart to the hertzian system, arehigher due to the intense triboxidation thatcauseprobablyahighamountofdescaledoxide.At the end of the test part of the particles areevacuated from the wear track reaching a COFvaluecomparablewiththeroomtemperaturetest.TheCOFacquiredfromthetestperformedat300°C is lower compared to the value acquired atroom temperature test. This behaviour is relatedto the change phase of ceramic oxide thatdecreased the contact energy and thus the COF.The friction coefficient values are higher at thestart of the test because of possible partialfragmentation of the material caused by brittlecontact between the countermaterial and thecoating.Thisleadstohaveanhighamountofthirdbodyparticles that increase theCOFvalue,at thebeginning, that is decreasing, during the test,becauseofparticle’evacuationformtheweartrackcausedbytherelativemotionofthetwomaterials.

Time [s]

Time [s]

Time [s]

CO

F [

-]

CO

F [

-]

CO

F [

-]

RT test 300°C test

RT test 300°C test

RT test 300°C test

a)

b)

c)

CO

F [

-]

CO

F [

-]

Time [s]

Time [s]

a)

b)

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TheCOFacquiredfortheNi/SiCelectrodepositsislowercomparedtothepureNielectrodeposit.Thiscould be related to both different mechanicalpropertiesofthecompositematerialrespecttothepureNi and possible interactions of SiC particleswithcountermaterialthatcouldlowerthesurfaceenergyandinteractionofthe2surfaces.At high temperature the COF graphs are verysimilarandthisbehaviourcouldberelatedtothechange of contact, compared with roomtemperaturetestthatisbetweenNioxideandtheWCsphere,insteadofNislightlyoxidizedandWC.4. CONCLUSIONS

Inthisworkdifferenttypeofcoatingshavebeenanalysed deposited either by thermal spraytechniquesorbyelectrodeposition.Thecoatingsdepositedby thermal spray are:NiCr (APS),WCCoCr (HVOF) and NiCr+Cr2O3 (APS). TheelectrodepositsareNi/SiCcoatingswithnano‐ormicro‐sizedparticlesembeddedinmetalmatrix.The analysed coatings showed differentmicrostructure that depends on both depositedmaterialanddepositiontechnique.Regardingthewearproperties,thesteelsubstrateshowed the worst wear resistance at both roomtemperatureand300°C.Thisbehaviourisrelatedtothelowmechanicalpropertiesofthissteelthataredecreasingasthetemperatureincreases.All the tested metal matrix coatings underwenttriboxidation that was increased at hightemperature test. The triboxidation behaviourdependsonmetaloxidationresistance.Theceramiccoating was subjected to an intensive materialdetachment, caused mainly by the highinterconnected porosity of the thermal sprayedcoating. The detachment increased in function oftemperature because the ceramic oxide changedphase under the hertzian loads. For all the metalmatrixcoatingswaspresentathirdbodyabrasioncausedmainlybybothoxidedescalingandceramicreinforcementdetachmentfromthemetalmatrix.Observing thewear rates, theWCCoCr coatingshowed the highest wear resistance at bothroomtemperatureand300°C.Thisbehaviourisrelatedtothemicrostructureofthedeposit:thereinforcing particles (WC) give high hardness

also at high temperature and the metal matrix(CoCr) increases the toughness of the coatingand acts as binder for the reinforcing particles.The electrodeposits Ni/nSiC showed a wearbehaviourthat iscomparablewiththeWCCoCrone.Forthenano‐compositeelectrodepositsthesynergy of both grain refinement and nano‐particles embedding leads to an increase ofhardnessatbothroomtemperatureand300°C.This effect probably enhances the wearresistance of the Ni metal matrix that issubjectedtohertzianloads.The COF values are strongly dependent on thematerial analysed but it was observed, forthermal spray coating, similar COF valuesbetweentheroomtemperaturetestandthe300°Ctests.TheceramiccoatingshowedthelowestCOF values at high temperature causedmainlyby the production of brittle CrO2 phase. Theelectrodeposits showed somedifferences in theCOFvaluesbetween thehigh temperature testsand the room temperature tests causedmainlyby the change of hertzian contact from Nislightly oxidized, at room temperature, to Nistrongly oxidized, at 300 °C. At roomtemperature is visible a different in COF valuebetween pure metal and composite coatings.Thiseffectisrelatedtothedifferentmechanicalproperties of the coating and the possibleinteraction of reinforcing particles with thecountermaterialinthehertziancontact/motion.REFERENCES

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