Tribological Behavior of Thermal Spray Coatings, Deposited by ...

A. Lanzutti at al., Tribology in Industry Vol. 35, No. 2 (2013) 113‐122coatings with the anti‐wear properties of the Nicomposite coating, highlighting the importantproperties of the composite coatings producedwith a simple and cheaper technique comparedto the thermal spray process.2. EXPERIMENTALa. Samples productionFor all types of the deposits ASTM 387 F22 steelplates (7×10 cm) and discs (d=5 cm) have beenused as substrates (chemical composition inTable 1).Table 1. Chemical composition of steel substrateASTM 387 F22.C Si Mn P Cr Mo Fe0.11 0.31 0.5 0.025 2.2 0.9 Bal.The thermal spray coatings have been depositedusing industrial procedures. The depositedcoatings were: NiCr 80/20 and NiCr 80/20 +Cr 2 O 3 deposited by APS (Air Plasma Spray)technique and WC CoCr 18/4 deposited by HVOFtechnique.Regarding the Ni matrix coatings, three types ofdeposits have been prepared: pure Ni (to beused as reference). Ni containing microparticlesof SiC and Ni containing nanoparticles of SiC.The electroplating bath used was a high speednickel sulfammate plating bath having thefollowing composition: 500 g/l Ni(SO3NH 2 ) 2.4H 2 O, 20 g/l NiCl 2.6H 2 O, 25 g/l H 3 BO 3 , 1ml/l surfactant (CH 3 (CH) 11 OSO 3 Na basedindustrial product. The deposition was carriedout using a galvanic pilot plant (12 l platingtank) under galvanostatic control at 4 A/dm 2 ,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 irregularand sharp shape, while the nano‐particles havea mean diameter of 45 nm [21].b. Samples characterizationThe specimens characterization includesmicrostructure, chemical composition,microhardness, wear resistance at both roomtemperature and 300 °C and corrosionresistance in two different environments.The microstructure of the specimens have beenanalysed by SEM (Zeiss Evo‐40) + EDXS (Oxfordinstruments INCA) in cross section. Both the SiCcontent and the coatings’ porosity werecalculated using an image analysis software [13].For nano composite coating The SiC content wasmeasured through the measurements of RFGDOES (HR‐Profile, Horiba Jobin Yvon),calibrated using 28 CRM (Certified ReferenceMaterial) samples. The system was set up usingan Ar pressure of 650Pa and a applied power of50 W. The micro‐composite coating were notanalysed by the GDOES because of some issuesrelated to the plasma erosion of the reinforcingparticles [21].Micro‐hardness measurements (HV 0,3 ) 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.The testing parameters are summarized in Table2. The volume loss has been evaluated using astylus profilometer (DEKTAK 150 Veeco). Thewear rate K [10 −6 mm 3 /Nm] has been calculatedusing the equation described in [26].Table 2. Wear test parameters.Counter material WC sphere (d 9.5 mm)Applied load70 NTest radius18 mmRotation speed300 rpmSliding speed0.565 m/sTest duration60 min3. EXPERIMENTAL RESULTSa. Microstructural characterizationIn Fig. 1 is shown the microstructure of the steelsubstrate. The Gr 22 steel presents a ferriticmicrostructure with some carbides precipitatedin the metal matrix, that leads to the high creep115

A. Lanzutti at al., Tribology in Industry Vol. 35, No. 2 (2013) 113‐122strength of material. The carbides are mainlyproduced by Cr and Mo. The hardness of thematerial is about 180±20 HV 0,3 and the ferriticgrain size is about 45±15 µm.c)Fig. 2. SEM images and microstructuralcharacterization of thermal spray coatings: a) NiCr80/20, b) WC CoCr 18/4 and c) NiCr 80/20+ Cr 2 O 3 .Fig. 1. Microstructure of gr. 22 steel.In Fig. 2 are shown the SEM micrographsobtained for Thermal spray coatings and therelative data acquired by mechanicalcharacterization and image analysis. In Tab. 3the thermal spray coatings’ properties are listed.a)b)Table 3. Results of thermal spray coatings’characterization.Coating Thickness Porosity Hardness HV0,3[µm] vol.%NiCr 98±16 6.5 359±18WCCoCr 105±15 3.45 1027±21NiCr+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, is higher for the coatings deposited byAPS technique compared to the HVOF deposits.This difference could be related to both powderssize and impact velocity that is lower in APStechnique with respect to HVOF. Indeed, thedifference in kinetic energy of the moltenpowders, that is higher in HVOF technique, leadsto a different density on deposited coating. Thehardness acquired is associated to the materialdeposited and the values acquired are similar todata available in scientific literature for thermalspray coatings [1‐13].The SEM micrographs obtained on cross sectionof Ni/SiC composite coatings previously etched(acetic acid: nitric acid 1:1) are shown in Fig. 3.In Table 4 are listed the electrodepositedcoatings’ properties.Table 4. A result of Ni/SiC electrodepositscharacterization.Coating Thickness[µm]SiC wt.% HardnessHV0.3Ni 78±7 ‐ 172±7Ni/µSiC 73±8 0.8 247±8Ni/nSiC 75 ± 5 0.15 270 ±9116

A. Lanzutti at al., Tribology in Industry Vol. 35, No. 2 (2013) 113‐122a)b)micro‐particles leads to a microstructurecolumnar with a slight modification oforientation, caused probably by the deviation ofelectrical field in proximity of ceramic particlesthat are in non‐conductive material. On thecontrary, the addition of nanoparticles gives agrain refinement and a multi‐orientation ofcolumns. The SiC amount is higher in the microcompositecoatings compared to the nanocompositeone. The addition of particles leads toa noticeable microhardness increase due to boththe presence of the particles and the grainrefinement.All the analysed samples showed a surfaceRoughness Ra of about 0.5µm, obtained after thesurface’ grinding.b. Tribological characterizationAll the wear tracks obtained for both bare steel,thermal spray and composite electrodeposits atboth room temperature and 300 °C are shown inFigs. 4‐6.c)In Fig. 4 the top views of the wear tracksobtained for gr. 22 steel are shown, tested atboth room temperature and 300 °C.Room temperature 300°CFig. 3. SEM images and microstructuralcharacterization of electrodeposited coatings:a) pure Ni, b) Ni/µSiC and c) Ni/nSiC.The microstructure of the electrodeposits iscolumnar. In the case of the pure Ni the metalcolumns are oriented along the direction ofelectrical fields. The addition of SiC microparticlesleads to a slight modification of the Nicolumns orientation and size. On the other hand,the codeposition of SiC nano‐particles leads tothe formation of a fine grained deposit in whichthe Ni columns are not oriented. The addition ofFig. 4. SEM images of the wear tracks obtained for thebare steel at both RT and 300 °C.The steel is subjected to triboxidative wear atboth room temperature and 300 °C. At roomtemperature the oxide produced is adherent tometal substrate and very homogeneus. At 300 °Cthe oxide produced is concentrated on the weartrack’ sides. This phenomenon is related to theloss of mechanical properties of the substratesthat permits the countermaterial to destroy theoxide layer that is thus deposited on the sides ofthe wear track. At 300 °C are highlighted the117

A. Lanzutti at al., Tribology in Industry Vol. 35, No. 2 (2013) 113‐122debris of both oxide and steel along the bordersof wear track.In Fig. 5 the top views of the wear tracksobtained for thermal spray coatings are shown,tested at both room temperature and 300 °C.For the metal matrix deposits the degradationmechanism is a triboxidation at both roomtemperature and 300 °C, which intensity isvarying in function of the coating material.In particular the thermal spray coatings showedalso other degradation mechanisms which arerelated to their microstructure.Room temperature 300°Ctests the detachment is decreased due to apossible hardness decrease of the materialwhich allowed the sealing of porosity, thusreducing the contact fatigue failure. The cermetcoatings were all subjected to triboxidation ofmetal matrix, more intense at high temperature.For the ceramic material (Cr 2 O 3 ) the degradationmechanism at room temperature is similar tothe NiCr coating while at high temperature testthe degradation becomes more intense. This iscaused by a phase change of chromium oxidewhich enhances the wear ratio and reduces atthe same time the friction coefficient [4].In Fig. 6 are shown the top views of the weartracks obtained for Ni/SiC electrodeposits testedat both room temperature and 300 °C.Room temperature 300°CNiNiCrWC CoCrNi/µSiCNiCr+Cr2O3Ni/nSiCFig. 5. SEM images of the wear tracks obtained for thethermal spray coatings at both RT and 300 °C.The Ni/Cr showed a material detachmentoriginated by contact fatigue phenomenon,aggravated by its porosity. At high temperatureFig. 6. SEM images of the wear tracks obtained for theNi/SiC deposits at both RT and 300°C.118

A. Lanzutti at al., Tribology in Industry Vol. 35, No. 2 (2013) 113‐122The Ni electrodeposits showed, at roomtemperature, a triboxidation with a descaling ofoxide 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 steelsubstrate was reached. For the composite Ni/SiCcoatings it seems that the oxide produced ismore adherent to steel substrate at hightemperature. Probably both the grainrefinement and the presence of ceramic particlesare linking the oxide to the metal matrix. Itseems that some counter‐material wastransferred to the coating surface, due to slightadhesion phenomena.The wear rate of all tested coatings at both roomtemperature and 300 °C are shown in Fig. 7.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.The ceramic coating (Cr 2 O 3 ) showed a high wearrate at 300 °C tests due to phase change ofchromium oxide under tribological contact.The electrodeposits showed good wearresistance at room temperature, higher for thenano‐composite coating. This reduction in wearrate could be related to the grain refinement ofmicrostructure of the metal matrix, whichincreases also the mechanical properties of thecoating. This effect is not visible in the microcompositecoatings because the reinforcementparticles are usually detached from the metalmatrix leading to intensive abrasive wear causedby hard particle third body contact betweencounter material and surface of the specimen. Athigh temperature the mechanical behaviour ofthe coating is reduced, probably because of thehardness decrease. In this case the pure Nicoating is completely removed while the microcomposite coating showed a better wearresistance, compared to the pure Ni one, but thewear 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 matrixcompared to the other electrodeposits.Fig. 7. Wear rates graph at both room temperatureand 300 °C for all the coatings tested.All the coatings protect the steel substrateduring the test, except for the pure Ni coatingthat showed, at 300 °C, the highest wear rate. Itis possible to observe that the cermet coatinghas the lowest wear rate compared to the othercoatings 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.For this coating the wear resistance is associatedto the carbide component and the particlesbinding is related to the metal matrix which hasa high toughness. On the other hand, the NiCrcoating showed a lower wear rate at hightemperature tests, compared to the roomIn Figs. 8‐9 the COF (Friction Coefficients of 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, apartthe ceramic coating that showed a lower COF athigh temperature due to the change phase ofceramic oxide under hertzian loads. The NiCrcoatings showed a noisy COF graph because ofthe coating material 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 samevalues of the test at room temperature. Probablyduring the start of the test the surface of thesample was covered by a oxide layer producedduring the heat up of the system. The presence119

A. Lanzutti at al., Tribology in Industry Vol. 35, No. 2 (2013) 113‐122of oxide decreased the surface energy inproximity of the hertzian contact of the twomaterials, reducing the friction coefficient. Whenthe oxide was broken the contact between thetwo materials was between the WC and theNi/Cr slightly oxidized.COF [-]RT test300°C testTime [s]a)COF [-] COF [-]Time [s]a)b)Time [s]COF [-] COF [-]RT test300°C testRT test300°C testTime [s]Time [s]Fig. 8. COF graphs at both room temperature and 300°C for the thermal spray coatings: a) NiCr 80/20, b)Wc Co Cr (18/4), c) NiCr 80/20+ Cr 2 O 3 .b)c)Fig. 9. COF graphs at both room temperature and 300°C for the Ni/SiC composite coatings: a) Roomtemperature test, b) 300 °C.For the WC‐CoCr coating the COF are slightlydifferent and this is caused mainly by thenumber of third body particles produced duringthe test. Indeed is possible that at hightemperature the amount of abrasive particles,that are taking part to the hertzian system, arehigher due to the intense triboxidation thatcause probably a high amount of descaled oxide.At the end of the test part of the particles areevacuated from the wear track reaching a COFvalue comparable with the room temperature test.The COF acquired from the test performed at 300°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. This leads to have an high amount of thirdbody particles that increase the COF value, at thebeginning, that is decreasing, during the test,because of particle’ evacuation form the wear trackcaused by the relative motion of the two materials.120

A. Lanzutti at al., Tribology in Industry Vol. 35, No. 2 (2013) 113‐122The COF acquired for the Ni/SiC electrodeposits islower compared to the pure Ni electrodeposit. Thiscould be related to both different mechanicalproperties of the composite material respect to thepure Ni and possible interactions of SiC particleswith countermaterial that could lower the surfaceenergy and interaction of the 2 surfaces.At high temperature the COF graphs are verysimilar and this behaviour could be related to thechange of contact, compared with roomtemperature test that is between Ni oxide and theWC sphere, instead of Ni slightly oxidized and WC.4. CONCLUSIONSIn this work different type of coatings have beenanalysed deposited either by thermal spraytechniques or by electrodeposition. The coatingsdeposited by thermal spray are: NiCr (APS), WCCoCr (HVOF) and NiCr+Cr 2 O 3 (APS) . Theelectrodeposits are Ni/SiC coatings with nano‐ ormicro‐sized particles embedded in metal matrix.The analysed coatings showed differentmicrostructure that depends on both depositedmaterial and deposition technique.Regarding the wear properties, the steel substrateshowed the worst wear resistance at both roomtemperature and 300 °C. This behaviour is relatedto the low mechanical properties of this steel thatare decreasing as the temperature increases.All the tested metal matrix coatings underwenttriboxidation that was increased at hightemperature test. The triboxidation behaviourdepends on metal oxidation resistance. The ceramiccoating 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 metalmatrix coatings was present a third body abrasioncaused mainly by both oxide descaling and ceramicreinforcement detachment from the metal matrix.Observing the wear rates, the WC CoCr coatingshowed the highest wear resistance at bothroom temperature and 300 °C. This behaviour isrelated to the microstructure of the deposit: thereinforcing particles (WC) give high hardnessalso 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 wearbehaviour that is comparable with the WC CoCrone. For the nano‐composite electrodeposits thesynergy of both grain refinement and nanoparticlesembedding leads to an increase ofhardness at both room temperature and 300 °C.This effect probably enhances the wearresistance of the Ni metal matrix that issubjected to hertzian loads.The COF values are strongly dependent on thematerial analysed but it was observed, forthermal spray coating, similar COF valuesbetween the room temperature test and the 300°C tests. The ceramic coating showed the lowestCOF values at high temperature caused mainlyby the production of brittle CrO 2 phase. Theelectrodeposits showed some differences in theCOF values between the high temperature testsand the room temperature tests caused mainlyby 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.This effect is related to the different mechanicalproperties of the coating and the possibleinteraction of reinforcing particles with thecountermaterial in the hertzian contact/motion.REFERENCES[1] N.F. Ak, C. Tekmen, I. Ozdemir, H.S. Soykan, E.Celik: NiCr coatings on stainless steel by HVOFtechnique, Surface and coatings technology, Vol.173‐174, pp. 1070‐1073, 2003.[2] B.S. Sidhu, D. Puri, S. Prakash: Mechanical andmetallurgical properties of plasma sprayed andlaser remelted Ni‐20Cr and stellite‐6 coatings,Journal of Materials Processing Technology, Vol.159, pp. 347‐355, 2005.[3] H. Singh, D. Puri, S. Prakash: Some studies on hotcorrosion performance of plasma sprayedcoatings on Fe‐based superalloy, Surface &coatings technology, Vol. 192, pp. 27‐38, 2005.[4] H.S. Ahn, O.K. Kwon: Tribological behaviour ofplasma‐sprayed chromium oxide coating, Wear,Vol. 225‐229, pp. 814‐824, 1999.[5] G. Bolelli, V. Cannillo, L. Lusvaraghi, T.Manfredini: Wear behaviour of thermally sprayed121

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