Surfaceand CoatingsTechnology,32 (1987)273 - 284 273

A NEW TYPE OF ATOMIZED COATING POWDERFOR PROTECTIONAGAINST WEAR AND CORROSION*

E. LUGSCHEIDER,A. KRAUTWALD, H. ESCHNAUERandJ. WILDEN

Lehr- und ForschungsgebietWerkstoffwissenschaften,Rheinisch-WestfdlischeTechnischeHochschuleAachen,Templergra ben55, D-5100Aachen(F.R.G.)

H. MEINHARDT

HermannC. StarckBerlin GmbHand Co.KG, NiederlassungLaufenburg,Postfach12 29,D-7887 LaufenburgiBaden(F.R.G.)

(ReceivedFebruary28, 1987)

Summary

Conventionallytailored hard alloys are dominatedby binary boridesandcarbidesor complexcarbides.This paperdiscussesa new typeof nickel-basealloy, the propertiesof which are governedby the presenceof acom-plex ternaryboride.

This boride hasa face-centredcubic structureandacompositionrangeof Ni20 - 21M2 - 3B6, where M is agroup IV - Va metal usedas a stabilizer.Extensive investigationsof phaseequilibria and diagramshave shownthatthese so-calledr-borides can be stabilized in chromium-containingalloysof nickel, refractorymetal and boron as well as in ternaryNi—M--B systems(whereM is titanium,vanadium,niobiumor tantalum).

Optimized alloys were chosen for powder production via vacuummelting and inert gasatomization.Finally, high quality powdersreadyforindustrialapplicationin varioussprayingtechniquesweremadeavailable.

The paperdiscussesthe propertiesof the alloys with regard to wearresistance,oxidationbehaviourandcorrosion.

1. Introduction

In almost all industries,intensive efforts are made to reduce lossesattributed to wear and corrosion, and hard alloys basedon iron, cobaltand nickel are playing an importantpart in this. Theyare mainly usedascoatings,manufacturedby thermal spraying,and thereforemust be avail-ableas powders.

*paper presentedat the 14th International Conferenceon Metallurgical Coatings,SanDiego,CA, U.S.A., March 23 - 27, 1987.

0257-8972/87/$3.50 © ElsevierSequoia/Printedin The Netherlands

274

Hard alloys with iron and cobalt as the main constituentweredevel-oped in the 1920sand 1930srespectively.They essentiallycontain complexcarbidesas hard material, mainly of the M7C3 type. In nickel hard alloys,however,binary borides and suicides crystallize as hard phases.This classof alloyshasbeenknown sinceabout 1940.

So-called carbidic nickel-basedhard alloys basedon the Ni—Cr—W—Csystem were developed in 1980 at the Materials ScienceDivision of theTechnical University in Aachen. Their improved propertiesare attributedto stabilizedcomplexcarbides[1J.

Another new type of hard alloy wasdevelopedat the sameestablish-ment in 1986 in cooperationwith HermannC. Starck Berlin. In this typeof alloy, the characteristicwear-resistanthard material is acomplexternaryboride containing stabilizing metals. Theseso-calledr-boridesare recognizedas typical hardphases.

This paper describesthe structure of the r-boride alloys and theirproperties, the production of spray powdersand their characteristics,andthe wear, corrosionandoxidationbehaviour.

2. Experimentaldetails andresults

2.1. Structure of thecoatingalloysThe compositionof nickel r-borides is given by the formula Ni2021-

M23B6, where M standsfor the stabilizingmetalsshownin Fig. 1.r-borides based on nickel stabilized by group IVa and Va elements

of the periodic system are of specialinterest in wearcorrosionand oxida-tion protection.

The phaseequilibria of the Ni—M—B systems(M Ti, V, Nb, Ta) wereexplained by Stadelmaier and coworkers [2 - 4J. The melting points andthe regions of homogeneityof the r-borideswere also known at that time.Further investigationsrevealedexcellent values for microhardnessof theseborideswhich are isotypicto Cr23B6.

In developing new alloys in this field, careful considerationmust begiven to the corrosion behaviour. Therefore the niobium and tantalumr-borideswerealloyedwith 10 at.% and20 at.% Cr [5, 6].

Results of research on the quaternarysystemsare reported. Phasediagrams that were unknown at the beginning of this work can now bepublished.

Fig. 1. Stabilizing metals for T-borides.

275

All investigatedalloys were melted in a Tamannfurnaceandhomog-enized at 800°Cfor 175 h, using argonas the protectinggas.Afterwards,quenchedsampleswere investigatedby metallography, X-ray diffraction,thermoanalysisand electron beam microanalysis.Theseresults led to thephasediagrams shown in Figs. 2 and 3. The structureof the isothermalsectionsis determinedby the CrB and r-boride phaseswhich are in equi-librium. In both cases,the two-phasearea CrB plus r-boride shifts in the20 at.% Cr sectionto higherboroncontents.

Some alloys were selectedfor spray powder production by referenceto thesephasediagrams,alloy hardnessandcorrosionbehaviour.

lOat-%Er 2Out-%Cr

~50A ‘b50A

CDt40

/ B’CrBt

2~//1 (0593’ ~ 20~ ~

/~I~Y///~irs~.~ \ F/ ~N~N~)To~CrSB

3\I ~ I~!!II/ //F ~~ \ 10

0 NC~2B~ ~N~3To- Cr5930./~~\~“v\ ~ ~ _____________________________________________________________________________________________________________ I~~~ ~ NN~To’C~2BNi 0 10 20 30 40 50 Ni 0 10 20 30 40 50at-°!, Ta at’!0 TaFig. 2. Isothermal sectionsof the Ni—Cr—Ta—B systemat 800 °C:left, 10 at.% Cr; right,20 at.% Cr.

lOat-%Cr 2Oat-%Cr

~ 50 ~ 50

40 40

N~ ‘C~B T

20 :‘~CDT20 C~B.1

10 ~ f

N . N~3Nb0 Cr B.

SNi 0 10 20 30 40 50 Ni 0 10 20 30 40 50

at /N Nb at-‘I, Nb 0

Fig. 3. Isothermal sectionsof the Ni—Cr—Nb—B systemat 800 °C:left, 10 at.% Cr; right,20 at.% Cr.

276 :2.2. Powderproduction by inert gasatomization :

Mechanicalsize reduction and atomizationprocessesare two differentmethods,usedmainly for the productionof metallic powders.

Figure 4 shows a schematicdrawing of the HCST atomizing (Starck)equipment. It comprises the following main components:the induction-melt and pour furnace(1) within an evacuablewater-cooledmelting cham-ber, which also contains a heatablepouring cup (2); the atomizing device(3) placed at the bottom of the melting chamber;the atomizationtower (4)which servesas a dropping spacefor the solidification of the melt dropletsproduced into spherical powder; the vacuum pumps(5) which generateanatmosphere almost oxygen-free; the controlling device (6); the cyclonseparating(7) the very fine powder particles; collecting vessels for thepowder(8).

Alloying elements or prealloys are melted in a ceramic crucible andthen poured into a ceramic pouring cup. This pouring cup has a circularbottom outlet of known diameter.The melt emergesfrom the outlet as aconstantmelt streamand entersthe atomizing nozzle. The atomizinggas,e.g. argon, reachesthe annulargap of the nozzlewith acertainpressureanddispersesthe metal stream into more or less fine droplets. Thesedropletssolidify into sphericalparticlesduring their fall within the atomizingtower.Then the powder is collected (under protective gas) and cooled down inspecialcontainers.

L~i~5 / 7~

~jL\$/ I

277

Free Fall Confined

//\\Fig. 5. Atomizing nozzlesof variousdesignused in alloy powderproduction: left, free-fall; right, confined.

The particlesize anddistribution which canbe receiveddependsamongother things on the viscosity of the alloy, surfacetension, pouring tem-perature,metal flow rate, pressureof the atomizing medium,the velocityof the atomizingmedium, the massflow rateof the atomizingmediumandthe nozzlegeometry.

Two different atomizing systemsareused,andareshownschematicallyin Fig. 5.

Gas atomization using the confined system tends to produce finerpowdersthanthat obtainedby the free-fall system.

Figure 6 shows powder particles of a Ni—Cr—Ta—B alloy for plasmaspraying.

Fig. 6. Atomized r-boride powder for Ni—Cr—Ta—B alloy. (Magnification, 275x.)

2.3. PlasmasprayingThe atomizedT-boride powderswere usedfor extensivesprayingtests.

The parameterswere optimized and the coatings were investigatedby dif-ferent methods. Altogether, five alloys were plasmasprayed.The particle 0

size range of the powders was from 5.6 to 45 ~tm. Figures 7(a) and (b) 0

show plasma-sprayedcoatings of r-boride alloys, revealing a porous-freecoatingwith a typical lamellarstructure.

278

(a) (b)

Fig. 7. Optical micrograph of plasma-sprayedr-boride hard alloys: (a) Ni—Cr--Nb--B; (b)Ni—Cr--Ta—B. (Magnifications, 170X.)

2.4. Coatingproperties2.4.1. AdhesionstrengthThe adhesionstrengthof thermally sprayedcoatingsis oneof the most

important criteria for quality. Methods of determining adhesionstrengthdata are laid down in DIN 50160 (ASTM C 633-79). Samplesof all fivealloysweretestedaccordingto theseregulations.

Figure 8 shows the scatteringof the measureddata, and the meanvaluesfor niobium andtantalumr-borides.

N Adhesion StrengthTest according

~‘ 50 V771 to DIN 50160

3 — mean value

0 20 spread10 Ta T alloys

_______________ Nb-t- alloys

Fig. 8. Adhesionstrengthof plasma-sprayedr-boride coatings.

Average values of 40 N mm2 for the adhesionstrength of niobiumr-borides and 43 N mm2 for that of tantalum r-alloys were determined.In adhesionstrength tests,the coatingsfailed throughoutbecauseof cracksin the r-boride layer, so the strengthof the coating itself is responsibleforfailure. Figures 9(a) and(b) show scanningelectronmicrographsof adhesionstrengthsamples.Both picturesclearly reveal the fracture surfaceoccurringin the layer.

2.4.2. Oxidation resistanceThe oxidation resistanceof alloys can be determinedby using a ther-

mobalance.

279

(a) (h)

Fig. 9. Scanningelectron micrographsshowing the fracture surface of plasma-sprayedr-boride coatings: (a) Ni—Cr—Nb—B (magnification, 800x); (b) Ni—Cr—Ta—B (magnifica-tion, 2400x).

A thermobalanceenablesmeasurementof this very slight increaseinweight.The oxidation behaviourof the coatingsonly needsto be measuredduring the tests. It wasthereforenecessaryto separatethe coating andthesubstrate.Using the variouscoefficientsof thermalexpansion,the coatingswere detached by thermal stress.From the separatedlayers, small speci-mens with a reactivesurfaceof about 1 cm2 were prepared.In the eval-uations, it had to be rememberedthat the specimensin thethermobalance,stimulated by their arrangement,react with their completesurface.All thespecimenswere submitted to two load cycles: (i) continuousexposureto1000 C for 5 h; (ii) cyclic oxidation between800 C (5 h) and 1000 C(6 h), at intervalsof 1 h for five cyclesin eachcase.

All the alloysshowedsimilar behaviour. Initially, the weight increasedlinearly with time. After about 40%of the testingtime, the weight remainedat a constantlevel.This behaviouralso occurredundercyclic load, for whichFig. 10 showsthe results.

As expected,niobium T-alloys are more oxidation-resistantthan thetantalum r-borides are, becausetantalum is more reactive. The changeinthe weight gain with time indicatesthat oxide layerswhich aroseinitiallyare very denseand preventfurther oxidation. The establishedweight gainis very small. This meansthat the alloy examinedpossessesan outstandingoxidationresistance.

E~ Ta-T-alloysNb-c-alloys

fl T~J—-~,o1o

005 6hrsatl000 C______________ 5hrsot 800’C

Fig. 10. Oxidation behaviourof plasma-sprayedr-boridecoatings.

280 :2.4.3. WearresistanceDifferent kinds of stressinglead to varioustypesof wear.In this work,

the behaviourof sprayedcoatings under abrasionand adhesionweretested.Model tests were performed using pin-disc equipment. This equipmentneedscylindrical specimens,which are undera fixed load. Their circularsurfaceis pressedagainstthedisc which is rotating with definedvelocity.

From this test, the wearratecan be determinedby weighing the spec-imens after various test periods.To evaluatetheseexperiments,it is neces-saryto describethe whole tribological system[8].

2.4.4.AbrasionFor these tests, SiC abrasive papersof two different grit sizes (120

and 600) were used. The weardistanceswere 100 pm and 200 pm respec-tively and the relative velocities were adjustedto 20 m min~and 50 mmin~respectively.

The diameterof the specimenswas20 mm in all cases,andtheir weightwas 35 - 40 g. An additional load of 0.5 kg was usedfor all samples.Waterwas the intermediatesubstanceusedto flush out all abradedparticles.Thesurroundingmedium in all caseswas air. The test results show that resis-tance againstabrasionis a function of the coating hardness.The specimensfor hardnessmeasurementwere in the as-cast condition. Wear testswereperformedwith plasma-sprayedcoatings,and Fig. 11 showsthetest results.All investigations revealed a linear dependencebetween wear rate andabrasiondistance.

90 disc .120 grit SiC3 mg distance 250 m

H speed 5Dm/mmH H [~ load 05kg60 JJ]j [~somplee;2o mm

400 500 6(X) 700 Ta-T-allaysHardness HV 50 ... Nb-2- alloys

Fig. 11. Correlation betweenwearrateand hardnessof T-boridealloys.

2.4.5. AdhesionwearThis type of wear is caused by relative motion of two parts under a

standardload. Using pin-disc equipment,a relative motion canbe simulatedif pin (specimen) and disc are moved against each other. X21OCr12(1.2436) was used as the disc material; its surfacewaspolished.The diam-eter of the specimenwas 20 mm and the load of the sampleswas 0.5 kg.The surroundingmedium was air. Various superimposedwearmechanismslead to completely different surfaces,as seenafter abrasiontesting. Figure12 shows the dependenceof the wear rateson the distanceof adhesionwear.

The wear rate diminishes,since the abrasivestressdoes not dominatein this case. These results are attributed to chemical reaction products

281

dac X~0Cr~

30 ~ speed l5Ornfrnin

2 5 10 km 20 —-— To-V-alloysDistance —Nb-V-alloys

Fig. 12. Correlationbetweenwear rateandtest distance.

which changethe tribological structureof the systemby layer building. Inthis case,too, the niobium r-boride is superiorto tantalumr-alloys.

2.4.6. Corrosion resistance[7]Potentiodynamicexperimentsare well known tests for determining

the corrosionbehaviourin aqueoussolutions.In thesetests,the specimenas

medium1,2 0,ln H

2S04

as cast

1p,~ 75/9

2,8 ‘~ —— 75/10

Oh ~

U — ~——~ I

-~ 0 ~ 4~ 6(X) 800 ~) 1200 1400 1600 1800potentiaL LmVCQ(I

(a)

medium:

1,2 0,1 n H2S04

CoatingI —75/9/2

——75/10/2/:,

~ I / 0

Q2” / \ _‘~~“ 0

0 4~’~ } _____________________

-400 -200 0 ~ 400 ~ 800 ~O 12(0 1600 16(0potential tmV~0tI

(b) :Fig. 13. Corrosion behaviourof Ni—Cr—Nb—B alloys in 0.1 N H2S04: 7 5/9/2, Ni—l9Cr--9Nb—28; 7 5/10/2, Ni—l4Cr—lONb—3B.

282

an electrode is surroundedby an electrolyte and loaded in stagesby ahigher voltage [9]. The resulting current is determinedand standardizedto the specimensurface.

Plasma-sprayedr-boride coatings 250 pm thick are as corrosionresis-tant as cast alloys are. Theseresults are shownin Figs. 13(a) and (b). Theessential feature of a corrosion-resistantalloy is a high breakdownvoltage(that potential which causesa rapid riseof currentdensitywithout a furtherincreasein voltage).

Figure 13(a) shows identical high breakdown voltages of 900 mV(Kalomel) for the niobium r-boride alloys 75/9 and 75/10 in the as-castcondition. Figure 13(b) showsdatafor thesamealloys in the plasma-sprayedcondition.

For both coatings, the breakdownvoltage was the sameas that mea-suredin the as-castcondition. This indicatesthat the r-boride alloys canbesprayed very well. The coatings produced are denseand very corrosion-resistant. However, the corrosion resistancedependsessentially on theconcentrationof alloying elements,and the relationshipbetweenthe twois shownin Fig. 14.

The corrosion resistanceof r-boride alloys is improved by increasingthe amount of refractory metal (in this case,tantalum). A comparisonofthe corrosiondataof the alloys 76/8 (36 wt.% Ta) and 76/6 (2.6 wt.%Ta)show this clearly. Alloys containing identical amountsof tantalum can beimproved further if the chromium content is optimized. A comparisonof76/3 and 76/8 indicates that the higher chromium content (14 wt.% in76/8) results in bettercorrosionresistance.

All the results describedin this paper,obtainedby investigatinga newtype of alloy, indicatethat r-boride alloys can be adaptedto various indus-trial demands.Their resistanceagainst wear, oxidation and corrosion de-pendson the chemicalcompositionwhich variesoverawide range.

medium —

1,2 I I 0,ln HCL

as castI I

-~ I ——76/2

I / I 76/3

/ I / ——76/6/ / / - -76/8

0,4 ~ //~ ~

0,2’~’ I /

o ~ ~ 8~ ~bo12~b0 1&~ 18~0potential (mi/cal]

Fig. 14. Corrosion behaviourof various Ni--Cr--Ta- B alloys in 0.1 N HC1: 76/2, Ni—9Cr--24Ta—2B; 76/3, Ni—8Cr—37Ta—0.5B; 76/6, Ni—l8Cr—3Ta—4B; 76/8, Ni—l4Cr—36Ta--0.5B (wt.%).

283

3. Conclusion

The paper describesa new class of nickel hard alloys containing aternary complex boride as the predominant hard phase. This complexboridecharacterizedby the formula Ni20 - 21M2 - 3B6 (the so-calledr-boride)is stabilizedby refractory metalsof the IVA and VA groupsof the periodicsystem of elements,especially by titanium, vanadium, niobium and tan-talum.

The developmentof new materials demandsextensiveeffort in thisfield. The structuresof quaternaryalloys of the Ni—Cr—Nb—B and Ni—Cr—Ta—B systemswere investigatedand clarified. Isothermal sectionsof thesystemsat 800 °Ccontaining 10 at.% and 20 at.% chromium were drawn;in addition, the melting behaviourandthe macrohardnessweredetermined.

Based on these fundamentalresults, five alloys of the Ni—Cr—Nb—Band Ni—Cr—Ta—B systems were selected and manufactured by plasma.spraying.The required powderswere produced by inert gas atomizationof the liquid alloys. Subsequentinvestigationsindicated that spraypowdersof high quality wereproduced.

Spraying parameterswere adjusted to these special alloys and themanufacturedcoatingswere tested.The adhesionstrength of the coatingsreached40 - 45 N mm

2. Examinationof theoxidationbehaviourindicatedthat r-boride coatings,after a short period of linear weight gain, build upa denseoxide layerwhich preventsfurther oxidation.

The wear behaviourof the r-boride coatings was analysedby modeltests using pin-disc equipment.The results show that r-borides are veryresistantto adhesionandabrasion.

Potentiodynamic corrosion tests indicated very good , resistanceofr-boride coatingsto 0.1 N nitric andsulphuricacids.

The new classof r-boride alloys canbe producedas powderswithin awide range of chemical composition. Becausethey are simple to handleand they haveexcellent properties,thesealloysshouldbe regardedasusefulalternativematerialsfor surfacehard-facing.

Acknowledgment

This work was supported by the Directorate-Generalfor Science,Researchand Developmentof the Commissionof the EuropeanCommu-nities undercontractsSUM-003-Dand SUM-004-D.

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