fl84- 21? 19

NASA Technical Memorandum 83599

Ion-Beam Nitriding of Steels

Joshua Salik

Lewis Research Center

Cleveland, Ohio

Prepared for the

Eleventh International Conference on Metallurgical Coatings

sponsored by the American Vacuum Society

San Diego, California, April 9-13, 1984

NASA
ION-BEAM NITRIDING OF STEELS

Joshua Salik^

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135

SUMMARY

The application of the ion-beam technique to the nitriding of steels is

described. Preliminary results indicate that the technique can be successfully

applied for this purpose. Some of the structural changes obtained by this

technique are similar to those obtained by ion-nitriding. The main difference

is the absence of the iron nitride diffraction lines. The dependence of the

resultant microhardness on beam voltage for super nitralloy was found to be

different from that of 304 stainless steel.

INTRODUCTION

Nitriding of steel is a well-established surface treatment used extensively in the metal industry. The main purpose of this process is to increase

hardness and wear resistance without sacrificing the bulk properties. Traditionally this process has been done at high temperatures (around 540° C) using

either molten salt bath or reactive gases. These processes are costly as

well as time consuming. This is the reason which led to the development of an

alternative process called ion-nitriding. This process is accomplished in a

vacuum chamber at a pressure of several torrs of the nitriding gas which is

usually a mixture of N? and KL. A voltage (either rf or negative dc) is

applied to the workpiece thus causing a discharge which results in nitriding

of the workpiece. Although relatively few studies of this process have beer.

9—fi

reported ~ it has found industrial

^NRC-NASA Research Associate.
application as it decreases the cost and process time (from some 20 to 6

hr) and also allows for lower substrate temperatures during ion-nitriding.

The process described herein, namely nitriding by means of ion beams is

different from the ion-nitriding process in several aspects. The ion-beam

technique was used extensively for deposition of thin films but was rarely

7 R employed for chemical modification of surfaces. Here preliminary results

obtained upon applying it to the nitriding of steels are reported.

Some Features of the Ion-Beam Nitriding Process

The process is carried out in a vacuum system sketched in Figure 1 in an

Np atmosphere at a pressure of 3*10 torr, i.e. four orders of magnitude

lower than that employed in ion-nitriding. This low pressure will have a beneficial effect in that the introduction of undesirable impurities into the

workpiece will be reduced. This may be of significance since ion-nitriding is

usually done in a N,,-^ atmosphere. The combination of relatively high

Hp partial pressure with high workpiece temperature may cause diffusion of

hydrogen into it with the detrimental result of embrittlement.

Production of ions is done by the use of the ion source rather than by

discharge throughout the whole chamber as is the case in ion-nitriding. This

may be also significant in reducing the introduction of impurities into the

workpiece.

Another advantage of the use of ion source is avoidance of the discharge

instability problem often encountered in ion-nitriding. ' This problem is

due to the fact that ion-nitriding is done in the region of abnormal glow discharge where voltage and current increase simultaneously thus tending to change

into arc discharge. Since arc discharge is concentrated around isolated spots
it can cause overheating, melting and thereby result in damage to the

workpiece.

Yet antoher advantage of the ion beam technique is the ability to control

beam voltage and current independently and thus reach optimal conditions.

This is not possible in ion-nitriding where voltage and current are interdependent at a given pressure.

MATERIALS

The nitridability of steel depends on its alloy content. Although most

steels are capable of yielding iron nitride if exposed to nitrogen at suitable

temperatures, the best results are obtained with steels which contain one or

more of the nitride-forming alloying elements. Among the common alloying

elements the strongest nitride formers are aluminum, chromium, molybdenum and

vanadium.

For the present study two types of steel containing different nitride

forming elements were selected: Super nitralloy which is a grade specifically

manufactured for nitriding in which the nitride forming element is aluminum

and 304 stainless steel in which the nitride forming element is chromium.

The nominal compositions of these steels are given in Table II.

RESULTS AND DISCUSSION

Several techniques were used to characterize the nitrided samples.

Microscopy

Metallographic cross sections of the two types of steel are presented in

Figs. 2 and 3. It is seen in Fig. 3 that in the case of 304 stainless steel

the interface between the nitrided layer and the core is sharp and that the

nitrided layer is shallow and brittle. This, however, is by no means
unique to the present technique and was reported for steels containing more

9 10 than 5 wt. percent chromium upon both conventional and ion-nitriding.

Another feature to be noted in these micrographs is the absence of the

compound layer usually observed in conventionally and ion-nitrided

steels. This may be due to sputtering of this layer by the impinging ions.

As can be seen in Fig. 4 the process results in a severly etched surface and

it is possible that any compound layer which might have been formed was removed

by sputtering.

X-ray Diffraction

X-ray diffraction patterns obtained from the two types of steel under

several conditions are presented in Figs. 5 and 6. Examination of the patterns

obtained from super nitralloy (fig. 5) reveals that the (200) diffraction line

of a-Fe at 2e = 65.10° is missing in the nitrided sample, indicating that

the process causes some structural change in the near-surface layers. The

same phenomenon is revealed upon examination of the diffractograms reported by

Spalvins for ion-nitrided AISI 4140 steel. In order to check whether this

structural change is due to the nitriding reaction or to the bombardment of

the workpiece by the ions, another sample of the same material was bombarded

with argon rather than nitrogen ions and examined by X-ray diffraction. The

diffraction pattern obtained from this sample is presented in Fig. 5(c) and it

is seen that none of the lines which appear in the as-received sample are missing. It can thus be concluded that the structural change is due to the nitriding reaction. The same phenomenon is observed in the case of 304 stainless

steel (fig. 6) where nitriding results in the disappearance of three of the

Y-Fe diffraction lines - the (200) line at 2e = 50.46°, the (220) line at

2e = 74.56°, and the (311) line at 2e = 90.4°.
There is a major difference, however, between the diffraction patterns

reported by Spalvins for ion-nitrided AISI 4140 steel and the present

results. The diffractograms reported by Saplvins clearly indicate the presence

of two iron nitrides - Fe.N and e-Fex 3N whereas no such observaton was

made in our case. The reason for this discrepancy probably lies in the absence

in our case of the compound layer which was discussed earlier. It is probably

this layer which contains the iron nitrides and thus its absence in our case

is compatible with the absence of the iron nitride diffraction lines.

Microhardness

The microhardness of the nitrided surfaces was measured by means of a

Vickers tester with a load of 500 gm. The results obtained for the two types

of steel under different conditions are presented in Figs. 7 and 8. It is

seen that with increasing processing time the hardness first increases, reaches

a maximum and then decreases. The reason for this behavior has not been

studied yet.

Another striking feature of the results is the different effect of beam

voltage on the surface microhardness attained with the two types of materials.

This also is not yet understood.

CONCLUSIONS

The main conclusion of this study is that the ion-beam technique can be

successfully applied to nitriding of steels.

Some of the structural changes obtained by this technique are similar to

those obtained by ion-nitriding. The main difference is the absence of iron

nitride diffraction lines.
The dependence of the resultant microhardness on beam voltage was found

to be different for the two types of steel used.
REFERENCES

1. Metals Handbook, vol. 2, 8th edition, American Society for Metals, Metals

Park, Ohio, 1964.

2. M. Hudis, J. Appl. Phys. 44 (1973) 1489.

3. B. Edenhofer, Heat Treat. Met. !_ (1974) 23-28; 2_ (1974) 59-67.

4. 6. G. Tibbets, J. Appl. Phys. £5 (1974) 5072.

5. P. C. Jindal, J. Vac. Sci. Technol. 15^ (1978) 313.

6. A. M. Staines and T. Bell, Thin Solid Films 86 (1981) 201.

7. W. 0. Moore, S. R. Logan, L. C. Luther, and S. N. Brown in "Ionic

Bombardment Theory and Applications," ed. by J. J. Trillat, Gordon and

Breach, New York, 1964.

8. C. Weissmantel, G. Reisse, H. J. Erler, F. Henny, K. Bewilogua, U.

Eberslack and C. Schuer, Thin Solid Films 63_ (1979) 315.

9. B. J. lightfoot and D. H. Jack, Source Book on Nitriding, American

Society for Metals, Metals Park, Ohio, 1977, pp. 248-254.

10. T. Spalvins, Thin Solid Films 108 (1983) 157-163.
TABLE I. - COMPARISON OF THE ION-NITRIDING AND ION-BEAM NITRIOING PROCESSES

Pressure

Ion production

Voltage and

current control

lon-nitriding

Several torrs

Glow discharge

Voltage and

current are

interdependent

Ion-beam nitriding

3-10~4 torr

Ion-beam source

Voltage and current

are independently

controllable

Advantages

Lower impurity

(mainly hydrogen)

introduction

Avoidance of discharge

instability problems

Lower impurity

introduction

Better process control

TABLE II. - COMPOSITION OF WORKPIECE

MATERIALS

Al

Cr

Ni

C

Mn

Si

Mo

V

P

S

Fe

Super nitralloy

2.06

.58

5.16

.24

.25

.22

.26

.12

.005

.003

Balance

304 Stainless steel

18-20

8-12

0.08 max

2.00 max

1

0.045 max

0.03 max

Balance
l£AK

VALVE

< CONTROL

JNIT

TEMPERATURE

INDICATOR

_ ,

TM

H

0

.°.-r

 |ON

|L_° p SOURCE

ION Itfl

BEAM -"'I

•fl

THERMO- /

COUPLE L

LEADS— "' I

j j

i

[ffl^.-WORKPIECE

n

=?-- HOLDER

\

\

— 1 lie ' ' ^!

1 1 i1^

NITROGEN

ION GAUGE

LN2 TRAP

DIFFUSION

PUMP

Figure 1. - Schematic of the ion beam system.
50^171

Figure 2. - Metallgraphic cross-section of super nitralloy nitrided with ion

beams at 1000 V, 30mA for 50 minutes.

<.**,-.

•- ' , & " O *-"jrf

.dft

>•••

Figure 3. - Metallgraphic cross-section of 304 stainless steel nitrided with

ion beams at 1000V, 30 mA for 50 minutes.
50um

Figure 4 - Surface of super nitralloy nitrided with ion beams at 1000V,

30mA for 90 minutes.

(110)

(200)

^('>^

(220)

(a) As received.

(110)

(211)

./w.

(b) Ion-beam nitrided.

(110)

(211)

(220)

A

10 20 30 40 50 60 70 80

(c) Sputter etched with Ar ions.

Figure 5. - X-ray diffractograms of super nitralloy.

90 100 110
(Ill)

(200) (220) (311)

.-wtf^jW...du«MW<»^l'\»xa^^

(a) As received.

(Ill)

jL. u*rf^^

(b) Ion-beam nitrided.

(200) (220)

(311)

10 20 30 40 50 60 70 80

(c) Sputter etched with Ar ions.

Figure 6. - X-ray diffractograms of 304 stainless steel.

90 100 110

V mA

1500 30

20 40 60 80 100

NITRIDING TIME, min

120 140

Figure 7. - Microhardness of super nitralloy nitrided by ion beams

under various conditions.
V mA

1000 30

200

0 20 40 60 80

NITRIDING TIME, min

Figure 8. - Microhardness of 304 stainless steel

nitrided by ion beams under various conditions.
1. Report No.

NASA TM-83599

2. Government Accession No. 3. Recipient's Catalog

4. Title and Subtitle 5. Report Date

Ion-Beam Nitriding of Steels 6. Performing Organization Code

506-53-1B

7. Authors)

Joshua Salik

8. Performing Organization Report No

E-1992

10. Work Unit No.

9. Performing Organization Name and Address

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135

11. Contract or Grant No

12. Sponsoring Agency Name and Address

13. Type of Report and Period Covered

Technical Memorandum

National Aeronautics and Space Administration

Washington, D.C. 20546

14. Sponsoring Agency Code

15. Supplementary Notes

NRC-NASA Research Associate. Prepared for the Eleventh International Conference

on Metallurgical Coatings sponsored by the American Vacuum Society, San Diego,

California, April 9-13, 1984.

16. Abstract

The application of the ion-beam technique to the nitriding of steels is

described. Preliminary results indicate that the technique can be successfully

applied for this purpose. Some of the structural changes obtained by this

technique are similar to those obtained by ion-nitriding. The main difference

is the absence of the iron nitride diffraction lines. The dependence of the

resultant microhardness on beam voltage for super nitralloy was found to be

different from that of 304 stainless steel.

17. Key Word* (Suggested by Author(s))

Nitriding

Steel

Ion beam

18. Distribution Statement

Unclassified - unlimited

STAR Category 26

19. Security Classlf. (of this report)

Unclassified

20. Security Classlf. (of this page)

Unclassified

21. No. of pages 22. Price*

*For sale by the National Technical Information Service, Springfield. Virginia 22161
National Aeronautics and

Space Administration

Washington, D.C.

20546

Official Business

Penalty for Private Use, $300

SPECIAL FOURTH CLASS MAIL

BOOK

Postage and Fees Paid

National Aeronautics and

Space Administration

NASA-451

NASA POSTMASTER

If Undeliverahle (Section I 5K

Postal Manual) I*> Nut Return