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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 34, NO. 4, AUGUST 2006 1223

Oxide-Layer Thickness Effect for Surface RoughnessUsing Low-Pressure Arc

Toru Iwao,Member, IEEE, Yoshihiro Inagaki, Atsushi Sato, Student Member, IEEE, andMotoshige Yumoto,Member, IEEE

AbstractLow-pressure arc cleaning is a process for removingan oxide layer. Currently, chemical and mechanical means are typ-ically used to remove such layers. However, both methods presentdifficulties such as a liquid waste, dust, and noise. Regarding thelow-pressure arc cleaning, waste comes from one source: the oxidelayer. In addition, the cathode spot has very high temperaturesthat are sufficient to remove the oxide layer. This paper describesthe removal of the nanometer-thick oxide layer from a thin metalplate. An oxide layer of 27157 nm was removed, thereby, obtain-ing a smooth surface whose respective arithmetical mean height

(Ra

) and average length of an outline curve element (Rsm

) are0.04 and 6.4 m. In that case, Ra and Rsm increased with anincreasing oxide-layer thickness at 3971680 nm. Those resultsdepend on the oxide-layer thickness. Therefore, although the sur-face is cratered and rough after a cathode-spot treatment on thechemical oxide layer (6.7 nm), a smooth surface is obtainable afterthe cathode-spot treatment on the thermal oxide layer (27, 66, and157 nm). Surface roughness depends on the processing time toproduce one crater, which depends on the oxide-layer thickness.

Index TermsCathode spots, low-pressure arc, oxide layer,processing time, smooth surface, vacuum arc.

I. INTRODUCTION

O XIDE LAYERS accrete stably on steel-material surfacesduring manufacturing. Low-pressure arc cathode spotscan remove the oxide layers from metallic surfaces [1], [2].

After removal of the oxide layers, those surfaces become

rough because the high-temperature cathode spot melts the

surface. Cathode spots are characterized [1], [3] by: 1) nu-

merous arc spots; 2) vapor emission of the cathode material

because of extremely intense arc power; 3) rapid, apparently

random movement of the spots on the cathode surface [1];

and 4) preferential formation on oxides [1]. Oxides under

the cathode spots are vaporized and removed instantaneously.

Application of a low-pressure arc creates neither noise nor

sludge, because the treatment is conducted in an enclosedspace. Although few reports have addressed the cleaning action

of the cathode spots in the process of the low-pressure plasma

spray (LPPS) [3], [4], few reports have described the smooth-

surface treatment. Removal characteristics of the oxide layer

on a stainless steel have been reported [1]. The present study

Manuscript received December 4, 2005; revised March 31, 2006.T. Iwao and M. Yumoto are with the Faculty of Engineering, Musashi

Institute of Technology, Tokyo 158-8557, Japan.Y. Inagaki andA. Sato are with theGraduateSchool of Engineering, Musashi

Institute of Technology, Tokyo 158-8557, Japan.Color versions of Figs. 25 are available online at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TPS.2006.877745

Fig. 1. Schematic illustration of the experimental setup.

elucidated the smooth-surface treatment of a nanometer-thick

oxide layer on a SUS430 stainless steel using the cathode

spots produced using a low-pressure arc. Results were evaluated

using an Auger electron spectroscopy (AES) and laser mi-

croscopy to determine the effect of the oxide-layer thickness on

the arithmetical mean height(Ra), and the average length of theoutline curve element(Rsm). In addition, the relation betweenthe oxide-layer thickness and the processing time to produce

one crater were investigated to determine the effect for the

surface roughness.

II. EXPERIMENTALA RRANGEMENT D ETAILS

A. Experimental Setup

Fig. 1 shows that the experimental setup comprises a vac-uum chamber containing electrodes and a water-cooled copper

electrode as an anode, along with a dc power supply (no-load

voltage of 200 V) for the arc, and an evacuation system with

a rotary pump. The SUS430 workpiece surface has an oxide

layer. The workpiece is connected to the negative pole of the

power supply and serves as a cathode. The anode and cathode

surfaces are maintained at 40-mm separation. The chamber with

the specimen was evacuated to 30 Pa. Argon gas was introduced

into the chamber at 0.1 MPa; it was evacuated again to 30 Pa.

This operation was repeated. Adjusting the needle valve of the

gas inlet controlled the chamber pressure. At this time, the

pressure was adjusted to 100 Pa. A transferred arc was ignited

0093-3813/$20.00 2006 IEEE

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1224 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 34, NO. 4, AUGUST 2006

Fig. 2. Input voltage.

using an RF igniter. Then, the arc current was adjusted to 20 Ain the constant-current mode. The arc time was 1 s.

B. Oxide Layer on the Workpiece

The SUS430 workpieces were 30 mm 30 mm with a

0.5-mm height. They were heated in an electrical furnace to

their respective heating temperatures. The thermal-oxides

thickness, Do, was analyzed using the AES.The workpiecesurfaces were covered uniformly with a black oxide. The oxide

layers on the workpieces treated using the cathode spot of the

low-pressure arc were 6.7 (chemical oxide), 27, 66, 157, 397,

667, and 1680 nm. Chemical oxide forms after an exposure to

air. The thermal oxide, called a black oxide, is produced during

a processing in the electrical furnace.

III. RESULTS ANDD ISCUSSION

A. Crater Size and Oxide Layer Thickness

The cathode spots move, split, and recombine on the cathode

during the processing [5]. Fig. 2 shows the input voltage used

for 157-nm oxide-layer thickness. The respective mean voltages

are 33.3, 32.6, and 32.7 V in the cases of 6.7, 157, and 1680 nm

of the oxide-layer thickness. There is no great difference

attributable to the oxide-layer thickness. Fig. 3(a)(g) showsthe surface images taken using a laser microscope after the

low-pressure arc treatment in the cases of 6.7-(chemical

oxide), 27-, 66-, 157-, 397-, 667-, and 1680-nm oxide-layer

thickness. These are the different sample surfaces. When the

cathode spot of the low-pressure arc moves, some craters are

formed, as shown in Fig. 3(a), because of the high energy

density of the spots. However, a few small craters exist, as

shown in Fig. 3(b)(d). Then, some craters of Fig. 3(e)(g)

become larger than those of Fig. 3(b)(d). Therefore, the

surface condition is inferred to depend on the oxide-layer

thickness. The low-pressure arc can remove the oxide layer

in this experimental condition. A chemical oxide layer exists

after the treatment because the sample is exposed to an oxygenwhen it is taken from the vacuum chamber.

B. Roughness and Smooth Surface

Fig. 4 shows the surface roughness of each sample measured

using a laser microscope. Although high peaks exist at 6.7 nm,

low peaks exist at 27, 66, and 157 nm. The peaks increase with

an increasing oxide-layer thickness. In addition, although the

length between the peaks becomes great at 6.7 or 1680 nm, it

is less at 27, 66, or 157 nm. Fig. 5 shows the arithmetical meanheight(Ra)and the average length of the outline curve element(Rsm) measured using a laser microscope after the treatment.The Ra and Rsm, each becomes high at 0.53 and 23 mwhen the oxide-layer thickness is 6.7 nm. However, when its

thickness is 27, 66, or 157 nm, theRa andRsm, respectively,becomes low and constant at 0.04 and almost 6.4 m: Therespective Raand Rsmare 0.03 and 13m before the treatment.When the oxide-layer thickness is 1 680 nm, the Ra andRsmbecome high at 0.32 and 14.5 m, respectively, because oflarge craters created by the cathode spot. Subsequently, Rsmdecreases, thereby, a smoother surface is obtained with this

cathode-spot treatment using the low-pressure arc. Numeroussmall indentations exist on the surface when the Ra andRsmare small. For that reason, to achieve a smooth surface, it is

important to form small indentations. Therefore, the smooth

surface requires a small Ra and Rsm. One reason is that thehigh-temperature cathode spot melts the surface even if the arc

is in a low-pressure condition. Consequently, the cathode spot

presented a thermal-plasma condition. When the cathode-spot

size and number are constant, the cathode-spot energy densities

are equal.

C. Model of Crater Formation

In this experimental condition, the current and voltage are

constant. Fig. 6(a)(d) shows the model of the formed crater

under a consideration of the above energy-density condition.

The low-pressure arc forms a large crater in the case of the

chemical oxide layer, as shown in Fig. 6(a), because it can

form a crater not only at the oxide layer, but also at the bulk

layer. However, it forms a very small crater in the case of the

thin oxide layer, as shown in Fig. 6(b) and (c), because it can

form the crater mostly at the oxide layer and slightly at the bulk

layer. A thick oxide layer takes a long time for removal. The

bulk metal was melted because of a thermal conduction. At this

time, the bulk metal was gush as shown in Fig. 6(d). Therefore,

Fig. 5 shows that,Raand Rsmare small in the cases of 27, 66,and 157 nm.

Fig. 7 shows the relation betweenRaandRsm for a smoothsurface. In this result, a smooth surface is obtainable after the

treatment of the thin oxide layer. The Ra andRsm should behigh values; alternatively, Ra should be high when a roughsurface is desired. In this case, the oxide-layer thickness can

be changed.

D. Cathode-Spot Size

Fig. 8 shows the relation between the oxide-layer thick-

ness and the processing time to make one crater under anexperimental case to determine the effect for the surface

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Fig. 3. Surface image by a laser microscopy. (a) 6.7 nm (chemical oxide). (b) 27 nm. (c) 66 nm. (d) 157 nm. (e) 397 nm. (f) 667 nm. (g) 1680 nm.

Fig. 4. Surface roughness.

roughness: 20-A current; two cathode spots; 5-m cathode

crater; 33.3, 32.6, 32.7-V mean voltage; 1.51, 0.00, 0.87-mestimated depths of the oxide; and a bulk layer that has been

Fig. 5. Roughness measured using a laser microscope.

melted in the case of 6.7, 157, and 1680 nm of the oxide-

layer thickness, respectively. The oxide layer of the SUS430

is considered to be Cr2O3. The boiling point, specific heat,

and specific gravity of Cr2O3 are 2400 K, 460 J/(kg K), and

6900 kg/m3, respectively. Because the value for a specific heatof Cr2O3 is unknown, that of Fe is used. The melting point,

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Fig. 6. Model of a crater formation. (a) 6.7 nm. (b) 27 nm. (c) 127 nm. (d) 1680 nm.

specific heat, and specific gravity of SUS430 bulk (Fe) are

1510 K, 460 J/(kg K), and 7700 kg/m3, respectively

E= iV/Ns (1)

Qcathode= Qvapor+ Qelec+ Qsub+ Qrad (2)

Qcathode= (iV) (3)

Qvapor = s

dD

dt

Hv (4)

Qelec = i (5)

Qsub= s

dT

dx

x=0

(6)

Qrad= sT4v (7)

t= 1+ 2+ 3 (8)

1=

Tv2E

2(9)

2= 1

E2

22T2v

+ D0EHv

+ 2Tv

2T2v + D0EHv

(10)

3= 1 + 2

. (11)

Fig. 7. Relation betweenRaand Rsmfor a surface.

In these equations: Qcathode(W/m2) is the heat flux tothe cathode spot from the arc; Qvapor(W/m

2) is the surface-vaporization energy; Qelectron(W/m

2) is the cooling energy

of the electrode with an electron emission; Qradiation isthe radiation loss (Blackbody) (W/m2); Qsubstrate(W/m

2)

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Fig. 8. Relation between the oxide-layer thickness and processing time to

make one crater for a surface roughness.

represents the thermal conductivity to a bulk (one dimen-

sion); i(A) is the current of each cathode spot; I(A) is thearc current; V(V) shows the arc voltage; (= 1/3) [3] isthe ratio of the cathode spot for all input energy; (eV) isthe work function of the oxide layer; = /(Cp) is thethermal diffusivity;T denotes temperature (K); [W/(m K)]is the thermal conductivity; (kg/m3) represents the surfacemass density; Cp[J/(kg K)] is the specific heat at a con-stant pressure; Tv(K) is the boiling point of the surface; t(s)

represents the total processing time (treating oxide and bulklayer); 1(s) is the time to reach the boiling temperature ofthe oxide-layer surface; 2(s) is the time to evaporate theoxide layer; 3(s) is the time to treat the bulk layer;

1(s) isthe time to reach the melting temperature of the bulk-layer

surface; 2(s) is the time to melt the bulk layer; E(W/m2)

is the energy density of the cathode spot; D(t)(m) denotesthe time function of the oxide-layer thickness; D0(m) is theinitial value of the oxide layer; t(s) denotes time; tu(s) isthe time necessary for removal of the oxide layer; L(m) isthe melt depth;[W/(m2 K4)]denotes the Stefan-Boltzmannconstant;Nis the number of cathode spots; s is the cathode-

spot area(m

2

); andHv(J/m

3

)represents the vapor energy perunit volume.Equation (2) shows the energy balance of a heat flux to the

cathode spot from the arc. Each energy is expressed by (3)(7).

Here,Qradis ignored because of the low radiation. Equations(9) and (10) are derived from (2). Parameters of this experiment

are substituted for (9) and (10). In this calculation, the cathode

spot evaporates the oxide layer and melts the bulk layer. The

total processing times to produce one crater, (=1 + 2 +3), are2.42 103,6.94 104,5.24 103 s in the casesof 6.7, 157, 1680 nm of the oxide-layer thickness. The time to

reach the boiling temperature of the oxide-layer surface and

time to evaporate the oxide layer; 1+ 2, are 2.86 104,

6.94 104, and 3.78 103 s. The time to treat the bulklayer, 3, is2.14 10

3, 0,1.46 103 s. When the oxide-

layer thickness increases, the and 3 to make one craterincrease, except in the case of 6.7 nm (chemical oxide-layer

thickness). Surface roughness is determined by 3, becausethe increment of 3 depends on the bulk depth.Therefore,the surface roughness depends on the processing time, which

depends on the oxide-layer thickness to produce one crater.

In the case of 157 nm, the total processing time is veryfast, with a low peak, because the cathode spot only melts

the oxide layer. Therefore, the roughness becomes extremely

smooth, as shown in Fig. 5. In the case of 1680 nm, it takes

a long time to evaporate the oxide layer; the bulk is melted

through the thermal conduction. Therefore, the roughness is

determined by melting the bulk thickness, and the surface

becomes rough, as shown in Fig. 5. In addition, the cathode-

spot movement becomes fast with decreasing the oxide-layer

thickness at the experiment. This result is explainable by this

calculation. Therefore, this processing time of the oxide layer,

which depends on the oxide-layer thickness, is important to

treat the surface.

IV. CONCLUSION

Removal of a nanometer-thick oxide layer on a thin metal

plate was described. The main results are described below.

1) The oxide layer is removed, and a smooth surface whose

arithmetical mean height (Ra) and average length ofoutline curve element (Rsm) are obtainable as 0.04 and6.4 m, respectively, using an oxide layer of 27 nm.These values are identical to those before treatment.

2) Roughness depends on the oxide-layer thickness. Craters

form, because of the cathode spot of high energy densitywhen the oxide-layer thickness is small: ca. 6.7 nm.

When the oxide-layer thickness is 27, 66, or 157 nm,

the crater becomes quite small, and the roughness de-

creases. However, when the oxide-layer thickness is

greater than 157 nm, the crater becomes large, and rough-

ness increases.

3) A smooth surface is obtained using this cathode-spot

treatment with a low-pressure arc. One reason is that the

high-temperature cathode spot melts the surface, even if

the arc is in a low-pressure condition. For that reason, the

cathode spot presents a thermal-plasma condition.

4) Low-pressure arc treatment forms large craters in a chem-

ical oxide layer. The treatment can produce a crater

not only in the oxide layer, but also in the bulk layer.

However, it forms a small crater in the case of a thin oxide

layer because, it can form the crater at the oxide layer and

a small bulk layer.

5) Surface roughness depends on the processing time,

which depends on the oxide-layer thickness to produce

a crater; the roughness is determined by the melting bulk

thickness.

ACKNOWLEDGMENT

The authors would like to thank Prof. T. Inaba andM. Hara of Chuo University for their fruitful suggestions.

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REFERENCES

[1] I. I. Beilis, State of theory of vacuum arc, IEEE Trans. Plasma Sci.,vol. 29, no. 5, pp. 657670, Oct. 2001.

[2] A. E. Guile and B. Juttner, Basic erosion processes of oxidized and cleanmetal cathodes by electric arcs,IEEE Trans. Plasma Sci., vol. PS-8, no. 3,pp. 259269, 1980.

[3] K. Takeda and S. Takeuchi, Removal of oxide layer on metal surface byvacuum arc,Mat. Trans. JIM, vol. 38, pp. 636642, 1997.

[4] Y. Kubo, S. Maezono, K. Ogura, T. Iwao, S. Tobe, and T. Inaba, Pre-treatment on metal surface for plasma spray with cathode spots of lowpressure arc, inSurface and Coating Technology, vol. 200. Amsterdam,The Netherlands: Elsevier, 2005, pp. 11681172.

[5] B. Juttner, Cathode spots of electric arcs,J. Phys. D, Appl. Phys., vol. 34,no. 17, pp. R103R123, Sep. 2001.

Toru Iwao (S98A99M03) was born in Kana-gawa, Japan, on February 15, 1974. He receivedthe B.E., M.E., and Ph.D. degrees in electrical andelectronic engineering from Chuo University, Tokyo,Japan, in 1997, 1998, and 2000, respectively, and theB.A. degree in human development and educationfrom the University of the Air, Chiba, Japan, in 2002.

From 1997 to 1998, he was an Associate Re-

searcher with the Institute of Science and Engineer-ing, Chuo University. In 1999, he was a ResearchAssistant with the Institute of Science and Engineer-

ing, Chuo University. From 2001 to 2004, he was a Research Associate withthe Institute of Science and Engineering, Chuo University. Since April 2004,he has been a Lecturer with Musashi Institute of Technology, Tokyo, Japan.Since 2001, he has been a Visiting and Joint Researcher with Chuo Universityand Osaka University. From 2001 to 2002, he was a Visiting Scientist at TexasTech University, Lubbock. From 2002 to 2003, he was a Postdoctoral ResearchFellow at the University of Minnesota, Minneapolis. In 1998, he studied atCentral Washington University. His current research interests are plasma arcdischarge, thermal plasma, lightning, plasma treatment for hazardous wastes,pulsed power, education of experiment, and so on.

Dr. Iwao is a member of the Institute of Electrical Engineers of Japan, JapanSociety of Applied Physics, Japan Society of Plasma Science and NuclearFusion Research, Iron and Steel Institute of Japan, Institute of Engineers onElectrical Discharges in Japan, and Institute of Applied Plasma Science. Hereceived a Paper Presentation Award from the Institute of Electrical Engineersof Japan, in 2000, and a Best Paper Presentation Award from the Japan-Korea(JK) Symposium in 2003.

Yoshihiro Inagaki was born in Kanagawa, Japan,on October 5, 1982. He received the B.E. degreein electrical and electronic engineering in 2005from Musashi Institute of Technology, Tokyo, Japan,where he is currently working toward the M.E.degree.

His current research interests include removingoxide layers from metal layers using low-pressure

arc discharge.Mr. Inagaki is a member of the Institute of Electri-cal Engineers of Japan.

Atsushi Sato(S06) was born in Yamanashi, Japan,on May 23, 1983. He received the B.E. degreein electrical and electronic engineering in 2006from Musashi Institute of Technology, Tokyo, Japanwhere he is currently working toward the M.E.degree.

His current research interests include removingoxide layers from metal layers using low-pressure

arc discharge.Mr. Sato is a member of the Institute of Electrical

Engineers of Japan.

Motoshige Yumoto (A75S77A78M86) wasborn on January 17, 1950. He received the B.E.,M.E., and Ph.D. degrees in electrical engineeringfrom Musashi Institute of Technology, Tokyo, Japan,in 1972, 1974, and 1978, respectively.

He is a Professor in the Department of Electri-cal and Electronic Engineering, Musashi Institute ofTechnology, Tokyo, Japan, and a Chairperson of theTechnical Committee on Electrical Discharge.

Mr. Yumoto is a member of the Institute of Elec-trical Engineers of Japan.

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