The application of the ion beam technique to the nitriding of steels is described. It is indicated that the technique can be successfully applied to nitriding. 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. It is found that the dependence of the resultant microhardness on beam voltage for super nitralloy is different from that of 304 stainless steel

Salik, J.

English

NASA Technical Reports Server

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
https://ntrs.nasa.gov/search.jsp?R=19840013651 2020-03-20T23:45:39+00:00Z
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 exten-
sively in the metal industry. The main purpose of this process is to increase
hardness and wear resistance without sacrificing the bulk properties. Tradi-
tionally 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 ben-
eficial 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 dis-
charge 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 inter-
dependent 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 10than 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 mis-
sing. It can thus be concluded that the structural change is due to the ni-
triding 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
_ ,
™
H0.°.-r |ON|L_° p SOURCE
ION Itfl
BEAM -"'I
•fl
THERMO- /
COUPLE L
LEADS— "' I
j j
i
[ffl^.-WORKPIECE
n=?-- HOLDER
\\
' ' ^— 1 l i e !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 5KPostal Manual) I*> Nut Return


Ion-beam nitriding of steels

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server