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Investigation of Tungsten DC Vacuum Arc Characteristics.Technological Application1

I.B. Stepanov, I. A. Ryabchikov, S.V. Dektyarev

Nuclear Physics Institute, 2a Lenin ave., Tomsk, 634050, RussiaPhone (7-3822) 423963, Fax: (7-3822) 423934, E-mail: raduga@npi.tpu.ru

1 The work was supported by awards of CRDF RR2-994, RR2-1030 and award of the Ministry of Education of RF for

the Federal Goal Program “Integration” - № Л0059.

Abstract - The paper presents the results ofinvestigation of regularities of generation of Wvacuum arc plasma continuous flows and theirapplication for deposition of coatings with highadhesion strength.

The developed construction of the vacuum arcevaporator (VAE) ensures dc mode of W plasmageneration in the current range of 350 A – 750 A atarc voltage drop of more than 40 V. It is shown thatat discharge current of 750 A W coating depositionrate can achieve 80 µm/h, which offers attractiveperspectives for a number of technologicalapplications of W plasma.

We also investigated some regularities of changesin element composition, adhesion strength, hardness,morphology and structure of W coatings formedfrom plasma of a dc vacuum arc discharge (VAD)depending on regimes of coating deposition. It isshown that use of Ti plasma for generation of a thicktransition and damping layer under the regime ofplasma immersion ion implantation and coatingdeposition under intensive ion mixing allows increaseadhesion strength of W coatings by several times.

1. Introduction

Application of VAD for generation of metal plasma iswidely used for technologies of plasma deposition ofprotective and strengthening coatings. Presently Ti, Al,Cu, Cr and a number of other metals are very popular forgeneration of vacuum arc plasma flows [1]. Combinationof the above-mentioned materials with other materials (incomposite cathodes) or with reaction gases allows obtainnitrides, oxides, carbides, borides, intermetalliccompounds, as well as other composite compounds,which expands application fields for PVD coatings.

At the same time a number of practical applicationsrequire coatings with a complex of such physico-chemical properties as low chemical activity, elementpurity, homogeneous structure, high corrosion- and heatresistance. The most attractive way to solve this problemseems to be use of plasma of heat-resistant materials. Fora number of reasons generation of vacuum arc plasma ofsuch materials using standard technological equipmentespecially in a dc mode is regularly difficult (complicatedmechanical treatment of cathodes, stabilization of

cathode spots on cathode operational surfaces, highcurrents and high discharge voltages, high level of innertensions in generated coatings, etc.)

The presented paper aims at creation oftechnological equipment adapted to operation usingtungsten cathodes and investigation of characteristicsof generated plasma flows and discharges, andregularitiesof W-based coating deposition.

2. Generation of tungsten plasma continuous flows

Fig. 1 shows schematic of the experimental set-upused for deposition of W coatings, equipped with twoVAEs, one with W cathode, another with Ti cathode.VAE with Ti cathode has a filter for cleaning vacuumarc plasma from microparticle fraction [2]. VAEdischarge gap consists of a massive water-cooledcathode and hollow cylindrical anode. The cathode isfixed on the holder, which ensures immediate contactbetween the cathode and cooling liquid and excludescomplicated mechanical treatment of material.

Trigger electrode

Trigger electrode

Plasma filter

holderCatode

Catode

Anode

Shield

Ti

Samples

Bias voltage

W

Fig. 1. Schematic of the experimental set-up.

Discharge in VAE with Ti cathode is ignitedconventionally, by applying discharge onto the dielectric

2

0,0 0,5 1,0 1,5 2,0 2,50

10

20

30

Z, µm

X, mm

surface. The schematic of discharge ignition with Wcathode is based on electrical breakdown of the vacuumgap, formed when the triggering electrode is pulled off thecathode. The triggering schematic allows initiate thedischarge with the frequency of up to 25 Hz on the frontsurface of the cathode. Stability of discharge formation isensured by emission of energy of up to 15 J on the gap.

Stabilization of the cathode spot on the front surfaceof the cathode is ensured by outer magnetic field of up to12 mT generated by two coils with current. In order toavoid the shift of the cathode spot onto the cathode lateralsurface and holder elements, protective cylindrical screenwas used in VAE with W cathode.

Fig. 2 (curve 1) shows the dependence of changein VAD life on current. The presented data show thatthe discharge dc regime is realized at the current ofmore than 350 A for a given VAE construction. Forthis current value characteristic voltage drop at the arcis about 40 V (curve 2).

200 400 600 80020

30

40

50

60

0,1

1

10

100

40

45

5032

1

Ii, A

Id, A

t, sec

Ub, V

Fig. 2. Parameters of a dc VAD with W cathode:1 – Dependence of discharge glow time on VAD current;2 – Volt-ampere characteristics of the VAD; 3 – Saturationion current from plasma depending on VAD current.

Changes in ion current from Ii plasma depending ondischarge current Id were measured at pressure in thevacuum chamber equal to 4⋅10-4 Pa and outer magneticfield equal to 10 mT. Collector with negative potentialwas located at the source output. The dependencepresented at Fig. 2 (curve 3) shows that for W plasma wehave Ii ≅ (0,08÷0,09) Id , and plasma flow is generatedwith pronounced increase in ion current density ji near itsaxis. This phenomenon can be explained by the fact thatwe employed some means to compulsorily stabilize thecathode spot on cathode front surface, as well as by theproper magnetic field generated at discharge currentsequal to several hundreds of amperes.

Fig. 3 shows the pictures and profile of W cathodesurface. The data confirm that after many hours ofoperation the cathode surface has low roughness, whichmeans low coefficient of cathode erosion. We see thatunder the given experimental conditions W erosioncoefficient does not exceed 40⋅10-9 kg/C.

a)

b)

Fig. 3. Pictures (a) and profiles (b) of W cathode surfaceafter 8 hours of operation.

Absence of sharp fringes on the cathode surfacemeans that during operation the temperature of thecathode surface was near the melting temperature.Paper [3] shows that a number of droplets producedby W arc cathode spot per one second is 3⋅103, whichis one order of magnitude less than for Al.

Despite high melting temperature of W, the largestportion of microparticle fraction observed in ourexperiments had spherical droplet shape, which showsthat it was formed from the liquid phase. Maximumsize of observed droplets didn’t exceed 30 µm. At thesame time we discovered impurities with the size of50 µm with sharp fringes at the surface of theanalyzed sample. Shape of these particles ischaracteristic of the microparticle fraction formedfrom the solid phase. As a rule, the mechanism of suchmicroparticle fraction formation can be explained byappearance of high mechanical tensions in cathodematerial. It was noticed that probability ofmicroparticle fraction formation in solid aggregativestate decreases with the increase in VAD glowduration.

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3. Deposition of tungsten coatings and investigationof their properties

Tungsten coatings were deposited onto steel 3 sampleswith the diameter of 50 mm. Before deposition thesample surface was mechanically polished andburnished using abrasive pastes АСН 40/28, 14/10 upto the roughness Rz ≈ 1,28 µm and chemically cleanedin the ultrasonic bath.

Ti plasma cleaned from microparticle fraction wasused to prepare the surface for W coating deposition.We executed the following stages: ion etching,formation of a surface layer with large amount ofactive sorption centers, heating of samples, formationof a substrate-coating transition layer, and depositionof a damping layer.

High intensity ion treatment of the surface wasrealized under high frequency generation of shortnegative voltage pulses on samples [4]. Pulsedgeneration of a negative potential allows avoidformation of erosion centers on a sample surface andensures stable operation of the power supply system.Various regimes of ion-beam and ion-plasma materialtreatment are realized at the change of the amplitudeand pulse duty factor in the range (0,3 ÷ 2) kV and 0,1÷ 0,66, respectively. The coating was deposited whenwe switched on the VAE with W cathode facingsample operational surface. The pressure in the vacuumvolume was ensured by a diffusion pump and was equalto 10-3 Pa. The temperature was (400 ÷ 450) 0С.

Fig. 4 shows dependencies of W coatingcondensation velocity changes along plasma flow crosssection. The data were obtained by measuring thethickness of the obtained coating using the method of“spherical section” [5]. The presented data show thatwhen the samples were located at the distance of 450 mmfrom the source, coating deposition rate could reach80 µm/h. Since the plasma flow expands quickly,homogeneity of formed coating thickness distributionreaches 82 %, which together with W coating formationrate offers attractive perspectives for a number oftechnological applications.

-10 -5 0 5 10

5

70

75

υc, µm

L, cm

Fig. 4 Change in coating deposition rate across plasmaflow cross-section: Id = 650 A Ub = 500 V.

Fig. 5 shows the results of morphologicalinvestigation of W coating surface using the MicroMeasure 3D Station (STIL) [5]. The presented datashow that roughness of the formed coating is 4,12 µmalong Rz. Increase in coating roughness compared tosample initial state is related to presence of a smallamount of microparticle fraction in plasma flow andformation of phases with a large size of grains (up to12 µm). More homogeneous structure and decrease insurface roughness up to 2,88 µm along Rz wereobtained for coatings generated at lower dischargecurrent and increased bias potential (Fig. 5b).

a)

0 200 400 600 800 1000

-1

0

1

Z, µ

m

X, µm

b)

Fig. 5. 3D image and profile of W coating surfacewith the thickness of 4 µm: a) Id = 750 A, Ub =500 V,b) Id = 550 A, Ub =1000 V.

It was experimentally shown that W condensationvelocity and bias potential on samples influence bothchange in coating surface morphology and coatingsurface structure. In particular, porosity of coatings

0 200 400 600 800 1000-2-101234

Z, µ

m

X, µm

4

formed under 1 - Id = 750 А, Ub = 500 V and 2 - Id =500 A, Ub = 1000 V was changed. We calculatedporosity as ratio of coating density 1- 14,3⋅103 kg/m3

and 2- 16,9⋅103 kg/m3 and W polycrystal density -18,8⋅103 kg/m3. In first case coating porosity was 76 %,in second – 90 %.

Coating hardness was measured using the NanoHardness Tester (CSM) [5]. Hardness of the coatingformed under the first coating deposition regime (Fig. 6,curve 2) was HU = 3227 MPa. Coating hardness increasedup to HU = 4703 MPa with decrease in discharge currentand increase in bias voltage. Curve 1 shows change inconditions of indenter loading and unloading.

0 200 400 600 800

1020304050

500 750 1000 125030354045

a)

b)

21

Nor

mal

forc

e, m

N

Penetration depth, nm

Ub, V H

ardn

ess x

102 , M

Pa

Fig. 6. a) Indenter loading depending on its penetration: 1-,Id = 550 A, Ub = 1000 V, 2 - Id = 750 A, Ub =500 V.b) Dependence of change in coating hardness onsample potential.

Fig. 6b shows dependence of increase in W coatinghardness on increase in bias potential. This effect canbe explained by the regime of coating formation underintensive ion mixing accompanied by changes in phase-formation conditions, intensive diffusion processes, andsurface ion sputtering processes.

Coatings from materials with high meltingtemperatures are characterized by high inner tensionsformed both at the stage of coating deposition in the fieldof temperature gradients and during coating exploitationbecause of difference between the coefficients of thermalexpansion of the coating and substrate [6].

In order to decrease inner tensions of W coating, agradient structure between the substrate and thecoating was formed. The structure was composed of athick transition layer formed along the depth of themodel sample and thin damping layer at its surface.Fig. 7 shows distribution of element compositionalong the depth of the formed coating and modelsample surface layer.

The presented data show that the transition Tilayer with the thickness of ∼ 700 nm was formed alongthe depth of the sample surface layer. The layer wasformed under the regime of high intensity short-pulsedhigh frequency plasma immersion ion implantation, whichstimulated diffusion of titanium deeper into the layer [4].

In order to compensate inner tensions in W layer,Ti layer with the thickness of ∼ 500 nm was depositedonto the model sample surface under the regime of ionassistment.

0,0 0,5 1,0 1,50

20

40

60

80

100

Con

cent

ratio

n, %

at

Depth, µm

W C Ti O Fe

Fig. 7. Distribution of element composition along thedepth of W coating and sample surface layer.

Formation of the transition layer between Ti and W wasexecuted at simultaneous treatment of the sample by Tiand W plasma flows. Formation of the second transitionlayer was also executed under high-intensity ion mixing.

Adhesion strength of coatings was measured usingScratch testing methods and the Micro-Scratch Tester(CSM) [5]. W-coated samples with the thickness of 8 µmformed at Ub = 500 V and T = 400 0С were analyzed.Before testing the samples were heated and cooled downin the temperature range of 20 0С - 400 0С during severalhours. Fig. 8 and 9 show the results of investigations in theform of the output signal from acoustic emission sensorand pictures of the indenter track.

0,5 1,0 1,5 2,00

10

20

30

40 a)

Sign

al le

vel,

%

Normal force, N

0,5 1,0 1,5 2,00

10

20

30

40 b)

Sign

al le

vel,

%

Normal force, N

Fig. 8. Level of acoustic emission from loading onindenter.

Fig. 8a shows the processes of destruction of Wcoating formed in the absence of damping Ti layer. Theresults show that the process of film destruction began atthe loading of ∼ 0,5 N. Fig. 9a shows the areas of the

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delaminated coating formed at critical indenterloading. Destruction characteristics reveal high innertensions in the material.

a) b)

Fig. 9. Image of W coating surface near the indentertrack a) in absence of damping Ti layer; b) withdamping Ti layer.

We obtained different data for W coating with adamping Ti layer. Changes in Fig. 8b show that alongthe whole track from the indenter we can observe lowlevel of acoustic emission, which means increase incoating adhesion strength. Coating destruction starts atloading equal to 1,35 N, which is confirmed by theimage at Fig. 9b. For this case coating destructionscharacteristic of high level inner tensions are absentnear the indenter track as well. The obtained dataconfirm positive influence of the formed Ti layer oncompensation of inner tensions arising at thermocycliccoating loadings.

During investigation of regularities of W coatingformation using plasma of dc VAD, it was found outthat increase in intensities of ion treatment causesincrease in coating adhesion strength. Increase ofsample temperatures in the range of 300 0С - 450 0Сdoes not cause decrease in adhesion strength offormed coatings. At the same time at highertemperatures of coating deposition one can expectdecrease in coating adhesion strength due to increasein temperature gradients during formation of coatings.

It was experimentally found out that increase incoating generation rate decreases coating adhesionstrength. However, to investigate regularities of theprocess in details, an additional research is required.

References

[1] I.I. Aksenov, V.M. Horoshih, Particle flows andmass transfer in vacuum arc, Kharkov, publ. byPhysico-Technical Institute, 1984, p. 57.

[2] A.I.Ryabchikov, I.B.Stepanov, Patent RU2097868 C1, 1998.

[3] I.I. Udris, Rus. J. Radio engineering andelectronics 6, 1057 (1962).

[4] A.I. Ryabchikov, I.A. Ryabchikov, I.B. Stepanov,“High-Frequency Short-Pulsed Plasma-ImmersionIon Implantation into Metals Using Filtered DCVacuum-Arc Plasma”, presented at 7th

International Conference on Modification ofMaterials with Particle Beams and Plasma Flows,Tomsk, Russia, 2004.

[5] I.B. Stepanov, S.V. Dektyarev, A.I. Ryabchikov,I.А. Ryabchikov, D.О. Sivin, I.А. Shulepov, inProc. 6th Int. Conf. on Modification of Materialswith Particle Beams and Plasma Flows, 2002,pp.649-653.

[6] V.А. Barvinok, Control of strained state ofmaterials, Moscow, Science, 1992, p.346.