Thermally sprayed tungsten-carbide (WC) coatings have proven to be one of the most wear resistant coatings available and a respectable replacement for hard-chromium coatings. They are used in paper machine parts such as calender rolls. However, improved lifetime and performance are continuing considerations, as well as finding more economical alternatives. This study researched the wear phenomena of tungsten-carbide coatings in a paper machine environment. To achieve this, five different feedstock materials and coatings manufactured from these were compared by electron microscopy as well as dry abrasion-, high-speed slurry abrasion- and cavitation erosion tests. Improvements in ductility by changing the matrix material were found, while changing the particle strength had no effect on the behavior of the coatings. The findings suggest further research on altering the matrix of the feedstock could lead to overall improvements in coating quality and component lifetime

Eronen, V

Kiilakoski, J

Vuoristo, P

Tribologia - Finnish Journal of Tribology

English

Journal.fi

J. Kiilakoski et al.: Wear properties of thermally sprayed tungsten-carbide coatings in paper machine environments29TRIBOLOGIA - Finnish Journal of Tribology 2 vol 33/2015WEAR PROPERTIES OF THERMALLY SPRAYEDTUNGSTEN-CARBIDE COATINGS IN PAPER MACHINEENVIRONMENTSJ. Kiilakoski1, V. Eronen2, P. Vuoristo11Department of Materials Science, Tampere University of TechnologyP.O. Box 589, 33101 Tampere, FinlandTel. +358503004445Email: jarkko.kiilakoski@tut.fi2Valmet Technologies Inc.P.O. Box 587, 40101 Jyväskylä, FinlandABSTRACTThermally sprayed tungsten-carbide (WC) coatings have proven to be one of the most wear resistant coatingsavailable and a respectable replacement for hard-chromium coatings. They are used in paper machine partssuch as calender rolls. However, improved lifetime and performance are continuing considerations, as wellas finding more economical alternatives. This study researched the wear phenomena of tungsten-carbidecoatings in a paper machine environment. To achieve this, five different feedstock materials and coatingsmanufactured from these were compared by electron microscopy as well as dry abrasion-, high-speed slurryabrasion- and cavitation erosion tests. Improvements in ductility by changing the matrix material were found,while changing the particle strength had no effect on the behavior of the coatings. The findings suggestfurther research on altering the matrix of the feedstock could lead to overall improvements in coating qualityand component lifetime.INTRODUCTIONWear-resistant coatings are used in papermachine components to lengthen their life-spans,  and research on the wear properties ofnew materials is continuous. An example of acommon component wherein carbide coatingsare  used  is  the  calender  roll  [1,2].  Theenvironment for components in a papermachine is harsh: speeds of up to 2000m/min, loads of over 3 kN/m, temperatures ofup to 200°C  and  an  abrasive  slurry  areconstantly present [3,4]. The demanding wetconditions in a paper machine have beenfound to wear the coating out unexpectedlyfast in some cases.Although tungsten-carbide coatings arewidely used for wear resistance in dryconditions, further examination is required inorder  to  find  out  the  cause  of  theirdeterioration in paper machines. The wearmechanism in tungsten-carbide coatings iscomplex and a result of many variables [5].These variables include the volume fractionand size of the carbides as well as the matrixmaterial. The wear properties of hard coatingshave been widely studied from differentperspectives [6,7], but so far no resultsdescribe their wear in paper machines.For this reason, we aimed to design morespecific experiments to simulate theconditions  as  closely  as  possible  to  gainknowledge of the behavior of the coatings. InJ. Kiilakoski et al.: Wear properties of thermally sprayed tungsten-carbide coatings in paper machine environments30TRIBOLOGIA - Finnish Journal of Tribology 2 vol 33/2015this study, high-speed slurry abrasion andcavitation erosion were used to simulate papermachine conditions, while dry abrasion(modified ASTM G65) was used as areference test. The compared coatings werehigh-velocity flame sprayed (HVFS) tungstencarbide coatings with varying particlestrengths, carbide sizes and matrixcompositions.METHODSMaterialsSeveral different tungsten carbide powdersfrom different suppliers were compared. Thepowders differ in composition, carbide sizeand  particle  strength.  Powder  1  was  areference HVFS WC-Co-Cr powder. Powder2 had low- and powder 3 high particlestrengths as reported by the manufacturer.Powder 4 had a more corrosion resistantHastelloy® C-276 matrix (18%NiMoCrFeCo) and a larger carbide size andpowder 5 had a dual carbide distribution. Allpowders were manufactured by agg-lomerating and sintering and their nominalparticle  size  distribution  was  10-25  µm.  Thepowders are listed below in table 1 and SEM-images are presented in Figure 1. Powder 1 isa currently used commercial powder while theothers are experimental powders indevelopment. Some matrix-rich areas can beseen in Figure 1 a) along with some porosity.From Figure 1 b) and c) the effect of particlestrength can be compared: the higher particlestrength of powder 3 gives in general a denserstructure, but it leads to some breakage ofparticles which are seen as large pores orcavities in the cross-section. Both powders 4and 5 exhibit a dual distribution of carbidesize and the larger amount of matrix inpowder 4 is evidenced by the higher cohesionof the particle. Powder 5 on the other hand isquite porous and exhibits large metallicregions.Table 1. Characteristics of the WC powders.Particle strengths “n/a” were not disclosed.No. Matrix (WC-) Carbidesize (µm)ParticlestrengthCoatingthickness(µm)1 10Co-4Cr 0.5 n/a 3352 10Co-4Cr 0.5 Low 4203 10Co-4Cr 0.5 High 4644 18NiMoCrFeCo 1 n/a 3945 10Co-4Cr 0.5+2.5 n/a 412The coating specimens studied in this workwere thermally sprayed with an HVFS systemusing the same spray parameters on stainlesssteel (AISI 316L) substrates. Sample sizes forthe tests were 50x50x5 mm for abrasion,50x20x20 mm for high-speed slurry abrasionand 20x20x5 mm for cavitation erosion.CharacterizationThe coatings and powders were characterizedwith a Scanning Electron Microscope (SEM),either a Philips XL30 or a Field-EmissionScanning Electron Microscope (FESEM, CarlZeiss  Microscopy  GmbH,  Germany).  TheVickers hardnesses of the coatings weremeasured from the cross-section with a LeicaVMHT 30A hardness-tester using a load of300 grams.The powders were characterized by thepowder manufacturers, who supplied theparticle strength and carbide-size information.The carbide size was verified visually bySEM.Dry-sand abrasionThe surfaces of the samples were ground witha P1200 diamond grinding disk to eliminatethe effect of as-sprayed surface quality. Thiswould also be the case in real-lifeapplications.J. Kiilakoski et al.: Wear properties of thermally sprayed tungsten-carbide coatings in paper machine environments31TRIBOLOGIA - Finnish Journal of Tribology 2 vol 33/2015Figure 1.  SEM-images of the cross-sections of the used powders 1 (a), 2 (b), 3 (c), 4 (d) and 5 (e).In the test the samples were pressed against arotating  rubber  wheel  with  a  force  of  30  Nand dry silica (Nilsiä Quartz, particle size 0.1-0.6 mm) was fed between the samples and thewheel to act as an abrasive. The weight of thesamples was measured before the experimentand after 30 minutes utilizing a SartoriusCP224  S  scale  to  the  precision  of  0.1  mg  ±0.05 mg.High-speed slurry abrasionIn  the  experiment  a  radial  velocity  of  2000m/min  was  used  with  a  tangential  force  of77.3 N pressing each sample against a rubberroll.  An  abrasive  slurry,  15  wt.  %  of  kaolinpigment  Ansilex  93  and  water,  was  fedbetween the sample and the roll with a rotatorpump with  a  frequency  of  1  Hz.  The  particlesize  of  the  kaolin  was  less  than  2  µm.  Eachexperiment lasted for 20 hours; the longestpossible time within practical limitations. Thetest was run twice for each sample to verifythe result.The samples were ground and polished for thetest to a mirror-like surface finish of Ra value0.02 m using grinding discs and finishingwith  3  m  diamond  slurry.  The  finer  surfacefinish compared with the abrasion test waspreferred due to increase the applicability ofthe  test  in  real-life  conditions,  thus  givingmore reliable comparisons between coatingsand better understanding of the test method.The samples were weighed before and afterthe  test  with  a  Sartorius  CP224  S  scale.  Theprecision of the scale was 0.1 mg ± 0.05 mg.The samples were cleaned before eachweighing ultrasonically in ethanol and driedin a kiln overnight at 70°C.Cavitation erosionThe cavitation erosion experiment wasperformed in deionized water using theindirect cavitation method described inASTM  G32  standard.  The  test  duration  waschosen as the time when the sample hadclearly passed the cavitation incubation timeand when the erosion rate was linear. The testspecifications are presented in table 2.J. Kiilakoski et al.: Wear properties of thermally sprayed tungsten-carbide coatings in paper machine environments32TRIBOLOGIA - Finnish Journal of Tribology 2 vol 33/2015Table 2. Test specifications for the indirectcavitation experiment.Amplitude (µm) 50.16Time (h) 3Temperature (°C) 17Tip diameter (mm) 13The samples were fine ground and polished toa mirror-like surface finish of Ra value 0.02µm.  The  samples  were  then  mounted  to  asample holder that was immersed in the liquidalong with the ultrasonic head and a coolingcoil made of copper. The ultrasonic processorwas a Sonics VCX 750.The samples were ultrasonically cleaned inethanol, dried with hot air and weighed beforeand after the test with a Mettler AE 163 scale.The precision of the scale was 0.1 mg ± 0.05mg.Figure 2. SEM-images of the coatings: 1 (a), 2 (b), 3 (c), 4 (d) and 5 (e).RESULTSSEM-images of the coatings are presented inFig.  2.  All  coatings  1,  2  and  3  retained  theirfine carbide size. Coating 1 has quite manypores, which is in agreement with the powderstructure seen in Figure 1. In colder spraymethods such as HVFS the coating structureoften is almost a direct result of the powderstructure due to low melting degree. Sameagreement can be seen in coatings 2 and 3,although coating 2 does seem denser due topullout of small particles from coating 3.Coating 4 seems very dense while coating 5had numerous pull-outs of larger carbides.However, both retained their bimodal carbide-size distribution. The larger amount of metalin coating 4 can be seen in Fig. 2 d) whichwas the case also in the feedstock material.From  Fig.  2  e)  it  is  clear  that  the  unevendistribution  of  carbides  in  the  powder  ismirrored in the coating as well. In all coatingsthe carbides are quite angular, indicating lowheating during spraying.J. Kiilakoski et al.: Wear properties of thermally sprayed tungsten-carbide coatings in paper machine environments33TRIBOLOGIA - Finnish Journal of Tribology 2 vol 33/2015Table 3 presents the results of the experimentsand Figure 3 plots them in the same graph.The hardness values of the coatings are verysimilar, which was to be expected since thematerials have nearly equal carbide contentsand the HVFS system reliably produces high-quality coatings. The only outlier is coating 4,which has a smaller fraction of hard phaseand,  therefore,  a  lower  hardness  as  well.  Forthe rest of the coatings, the variation is minuteand within standard deviation.Table 3. Summary of results of the dry-sandrubber wheel (DSRW) abrasion, high-speedslurry abrasion (HSSA) and cavitationerosion experiments along with hardnessvalues of the coatings.Sample Hardness(HV)DSRW(mm3)HSSA(mm3)Cavitation(mm3)1 1150±76 0.39 4.24 1.042 1067±81 0.59 5.06 1.363 1145±108 0.42 4.69 1.124 947±78 1.12 3.27 0.615 1043±73 0.51 4.11 1.37Steel - 170 586 0.49For the wear tests, volume loss was calculatedpresuming zero-porosity from theoreticaldensities and elemental ratios. The dryabrasion wear seems to follow a similar trendto  hardness;  only  coating  4  differs  from  therest, wearing more. In contrast, in high-speedabrasion the WC-18NiMoCrFeCo coatingseems to be the most resistant to wear. This isalso the case in cavitation erosion, where itfares almost as well as the reference sampleof stainless steel.Figure 3. The volume loss of the coatings inthe dry abrasion-, slurry wear- and cavitationerosion wear tests.DISCUSSIONCompared to sample 1, the reference coatingin this study, the powders with low- and highparticle strength seem to offer no advantages.Between the two, though, the higher particlestrength gives a slightly better resistance towear  in  these  tests.  It  is  possible  that  duringthe relatively cool spraying process the moreporous  particles  did  not  heat  and  soften  asthoroughly as the denser particles, resulting inpoorer cohesion of the coating.The coating with Hastelloy matrix was softerand more abradable than the othercompositions, as expected due to its higherbinder content. Poorer adhesion of the matrixwith the carbides compared with cobalt alsohinders its wear resistance [8]. However, thelarger amount of matrix does make thecoating more ductile, which can be seen asimproved cavitation erosion resistance.Furthermore, the mild corrosive nature of theJ. Kiilakoski et al.: Wear properties of thermally sprayed tungsten-carbide coatings in paper machine environments34TRIBOLOGIA - Finnish Journal of Tribology 2 vol 33/2015high-speed slurry abrasion wear test gives anedge for the corrosion resistant Hastelloy-matrix compared with the coatings withcobalt-chromium matrices. The poorerabrasion resistance and higher wet slurryabrasion resistance of WC-Hastelloycompared to WC-CoCr is also documented byHoudková et al. [9] where corrosion by waterwas determined to be the differentiatingfactor. The synergestic effect of hardness,toughness and corrosion resistance on wearrate of cermet coatings has also beenconfirmed elsewhere. [10, 11]In coating 5 the dual carbide size was thoughtto have an advantage by combininghomogeneity and shorter binder mean freepath, due to the small carbides, with wearresistance, emanating from the large carbides.However, these characteristics were notachieved as no significant changes in itsperformance were detected comparing withthe reference coating. Erosion due tocavitation was the highest, stemming from thelarge carbides acting as nucleation sites forthe cavitating bubbles.CONCLUSIONSIn this study five different tungsten-carbidecoatings were compared in three wear tests:dry abrasion, high-speed slurry abrasion, andcavitation erosion.While the coatings performed very similarly,there were two significant findings. First, thesuperior  performance  of  the  WC-18NiMoCrFeCo coating in cavitation erosionand high-speed slurry abrasion revealing thebeneficial effect of a ductile- and corrosionresistant matrix against wear in wet as well ascavitating conditions. Second, higher particlestrength seems beneficial when selectingcoatings against wear. This increasedperformance likely stems from the morethorough heating during spraying and hence amore cohesive coating structure. While thiscannot be seen in the cross-sections directlythe particle strength is the only differencebetween coatings 2 and 3 and, therefore, thelogical explanation. In summary, themanipulation of particle properties seems tobe of paramount importance when designingfeedstock for thermal spray, especially forcold methods such as high-velocity flame-spraying.ACKNOWLEDGEMENTSThe  work  was  funded  by  ValmetTechnologies Inc. (formerly Metso PaperInc.).REFERENCES1. P. Vuoristo, Thermal spray coatingprocessing in Comprehensive MaterialsProcessing (Editor-in-Chief: SaleemHashmi),  Vol.  4  Coatings  and  Films,Elsevier Ltd, Oxford, UK, 2014, 60 p.,ISBN: 978-0-08-096532-1.2. V. Eronen, S. Ahmaniemi, K. Niemi, P.Vuoristo, Microwear in thermally sprayedhard coatings by different abrasives,Proceedings of ITSC 2005, Basel,Switzerland, 20053. VTT Technical Research Centre ofFinland and Prowledge Oy (2009),KnowPap 11.0, [Electronic learningenvironment for papermaking andautomation]4.  HengAn sets speed record at 2,000 m/minfor Advantage ViscoNip press at Weifangmill, Metso press release, 02.12.20115. H. M. Hawthorne and Y. Xie, An Attemptto Evaluate Cohesion in WC/Co/CrCoatings by Controlled Scratching,Meccanica, vol. 36, no. 6, pp. 675–682,Nov. 2001.J. Kiilakoski et al.: Wear properties of thermally sprayed tungsten-carbide coatings in paper machine environments35TRIBOLOGIA - Finnish Journal of Tribology 2 vol 33/20156. R.  Nieminen,  P.  Vuoristo,  K.  Niemi,  T.Mäntylä, G. Barbezat, Rolling contactfatigue failure mechanisms in plasma andHVOF sprayed WC-Co coatings, Wear,vol. 212, Issue 1, 1997, pp. 66-777. P. Vuoristo, J. Laurila, T. Mäntylä, K.Niemi,  S.  Rekola,  S.  Ahmaniemi,  Surfacechanges in thermally sprayed hardcoatings by wear of different abrasives.Thermal Spray 2004: Advances inTechnology and Application, May 10-122004, Osaka, Japan.8. H. Exner, Physical and chemical nature ofcemented carbides, Int. Met. Rev., 243,Issue 4, 1979, pp. 149-1739. Š. Houdková, F. Zahálka, M. Kašparová,L.M. Berger, Comparative study ofthermally sprayed coatings under differenttypes of wear conditions for hardchromium replacement, Tribol. Lett. 43,2011, pp. 139–154.10. L.  Fedrizzi,  L.  Valentinelli,  S.  Rossi,  S.Segna, Tribocorrosion behaviour ofHVOF  cermet  coatings,  Corros.  Sci.  49,2007, pp. 2781–2799.11. V.A.D. Souza, A. Neville, Aspects ofmicrostructure on the synergy and overallmaterial loss of thermal spray coatings inerosion–corrosion environments, Wear.263, 2007, pp. 339–346.

Wear properties of thermally sprayed tungsten-carbide coatings in paper machine environments

Thermally sprayed tungsten-carbide (WC) coatings have proven to be one of the most wear resistant coatings available and a respectable replacement for hard-chromium coatings. They are used in paper machine parts such as calender rolls. However, improved lifetime and performance are continuing considerations, as well as finding more economical alternatives. This study researched the wear phenomena of tungsten-carbide coatings in a paper machine environment. To achieve this, five different feedstock materials and coatings manufactured from these were compared by electron microscopy as well as dry abrasion-, high-speed slurry abrasion- and cavitation erosion tests. Improvements in ductility by changing the matrix material were found, while changing the particle strength had no effect on the behavior of the coatings. The findings suggest further research on altering the matrix of the feedstock could lead to overall improvements in coating quality and component lifetime.Peer reviewe

Kiilakoski, J.

Eronen, V.

Vuoristo, P.

Trepo - Institutional Repository of Tampere University

Wear Properties of Thermally Sprayed Tungsten-Carbide Coatings in Paper Machine Environments

https://journal.fi/tribologia/article/download/69256/30722

Wear properties of thermally sprayed tungsten-carbide coatings in paper machine environments

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