The analysis of plasma nitrided steels can be divided to two main categories - structural and chemical. Structural analysis can provide information not only on the hardening mechanisms but also on the fundamental processes involved. Chemical analysis can be used to study the kinetics for the nitriding process and its mechanisms. In this paper preliminary results obtained by several techniques of both categories are presented and the applicability of those techniques to the analysis of plasma-nitrided steels is discussed

Ferrante, J.

Hoffman, R., Jr.

Honecy, F.

Salik, J.

English

NASA Technical Reports Server

- L 
NASA Technical Memorandum 898 15 
Analysis of Plasma-Nitrided Steels 
( N A S A - T B - 8 9 8  15) ANALYSIS  CP E L A S l A - U I T R I D E D  
S l E E L S  (hASA) 72 p C S C L  11F 
J.  Salik 
University of Arizona 
Tucson, Arizona 
J. Ferrante and F. Honecy 
Lewis Research Center 
Cleveland, Ohio 
and 
R.  Hoffman, Jr. 
Case Western Reserve University 
Cleveland, Ohio 
Prepared for the 
Ion Nitriding Conference 
sponsored by the American Society for Metals 
Cleveland, Ohio, September 15-17, 1986 
N87-2 1078 
Unclas 
G3/26 43588 
https://ntrs.nasa.gov/search.jsp?R=19870011645 2020-03-20T11:59:55+00:00Z
* 
4 
ANALYSIS OF PLASMA-NITRIDED STEELS 
J. Salik 
University of Arizona 
Tucson, Arizona 85721 
J. Ferrante and F. Honecy 
National Aeronautics and Space Administration 
Lewis Research Center 
Cleveland, Ohio 441 35 
and 
R. Hoffman, Jr. 
Case Western Reserve University 
Cleveland, Ohio 44106 
SUMMARY 
The analysis of plasma nitrided steels can be divided to two main 
categories - structural and chemical. Structural analysis can provide infor- 
mation not only on the hardening mechanisms but also on the fundamental pro- 
2 cesses Involved. Chemical analysis can be used to study the kinetics of the 
nitriding process and its mechanisms. In this paper preliminary results 
L: obtained by several techniques of both categories are presented and the appli- 
cability of those techniques to the analysis of plasma-nitrided steels Is 
discussed. 
INTRODUCTION 
Product analysis has an Important roll in many operations. In the indus- 
try It Is essential to quality control and trouble shooting. From the scien- 
tific aspect it can yield valuable information about the fundamental processes 
involved. This seems to be especially important In the field of plasma chem- 
istry where even the basic principles are yet to be well understood and very 
few efficient research tools are available. Even the mechanisms involved in 
plasma nitriding, which is one of the simplest plasma operations, are still 
subject to myths, rules of thumb, and irrational hypotheses. These reasons 
prompted us to devote effort to the analysis of plasma-nitrided steels. This 
work Is i n  its very early stages, though, and thus what we are going to present 
here is just some preliminary results and a few guidelines to further progress 
in this area. 
The traditional analytical procedure used in both industry and research 
is determination of the case depth. This is done by cross-sectioning a sample 
of the nitrided material and measuring the hardness along a line. This method 
is destructive and requires considerable work. An example demonstrating it is 
presented in fig. 1 taken from the Metals Handbook (ref. 1). Although this 
method has been instrumental in the study of the kinetics and efficiency o f  
nitriding processes for many years, the information it provides is limited. 
Today many analytical techniques are available which can give a much broader 
picture. 
The analys is  o f  n i t r i d e d  s tee l s  can be d i v i d e d  t o  two main categor ies - 
s t r u c t u r a l  and chemical. S t r u c t u r a l  ana lys is  can y i e l d  va luab le  i n fo rma t ion  
about the  phases formed as a r e s u l t  o f  n i t r i d i n g  and t h e i r  d i s t r i b u t i o n .  
i n  tu rn ,  may shed l i g h t  no t  on ly  on the  hardening mechanisms bu t  a l s o  on the  
fundamental processes invo lved.  Chemical ana lys is ,  on the  o ther  hand, i s  
expected t o  provide i n fo rma t ion  concerning the  k i n e t i c s  and mechanisms o f  these 
processes. Such In format ion,  besides i t s  s c i e n t i f i c  value, may lead t o  
improvements i n  i n d u s t r i a l  n i t r i d i n g  operat ions.  
T h i s ,  
STRUCTURAL ANALYSIS 
The bas ic  technique o f  s t r u c t u r a l  ana lys i s  i s  x-ray d i f f r a c t i o n  ( X R D ) .  A 
d i f f r a c t i o n  pa t te rn  obtained from a 4140 s t e e l  sample which was n i t r i d e d  by 
means o f  a glow discharge process i s  shown i n  f i g u r e  2, where a d i f f r a c t o g r a m  
obtained from an as-received sample o f  the  same m a t e r i a l  i s  a l s o  shown f o r  
comparison. These d i f f rac tograms enable us t o  i d e n t i f y  the  phases formed i n  
the  process. Thus, i t  i s  seen t h a t  i n  t h i s  case the  n i t r i d i n g  r e s u l t s  i n  t h e  
format ion o f  the  two i r o n  n i t r i d e s  - Fe2-3N and Fe4N; presence and i n t e n s i t i e s  
o f  these n i t r i d e  peaks i s  c l o s e l y  r e l a t e d  t o  an impor tant  t echn ica l  f a c t o r ,  
namely the  wh i te  layer ,  which i s  undesi rab le f o r  most app l i ca t i ons .  I t  i s  now 
w e l l  es tab l i shed t h a t  i t  I s  t h i s  l a y e r  which conta ins those n i t r i d e s .  This i s  
demonstrated i n  f i g s .  3 and 4 which show respec t i ve l y  a meta l lograph and a d i f -  
f r a c t i o n  p a t t e r n  obtained from a s u p e r n i t r a l l o y  sample which was n i t r i d e d  by 
means o f  t he  recent ly  developed ion-beam n i t r i d i n g  process ( r e f .  2 ) .  As can be 
seen no 
wh i te  l aye r  i s  formed i n  t h i s  process and consequently no i r o n  n i t r i d e  peaks 
are  present I n  the  d i f f rac togram.  Thus, x-ray d i f f r a c t i o n  can serve as a non- 
d e s t r u c t i v e  technique f o r  determin ing the  presence or ,  a f t e r  proper c a l i b r a -  
t i o n ,  the  thickness o f  the  wh i te  l aye r .  
This,  however, i s  n o t  the  only  i n fo rma t ion  which can be der ived from x-ray 
d i f f r a c t i o n .  C l o s e r  examination o f  t he  d i f f rac tograms i n  f i g s .  2 and 4 revea ls  
t h a t  i n  both cases the  n i t r i d i n g  resu l ted  i n  the  disappearance o f  some o f  t he  
d i f f r a c t i o n  l i nes  o f  i r o n  such as (200) and (220) .  This imp l i es  t h a t  the  
n i t r i d i n g  has also caused some s t r u c t u r a l  change i n  the  i r o n  ma t r i x .  The 
na ture  o f  t h i s  s t r u c t u r a l  change i s  no t  known ye t  bu t  i t  seems t h a t  o ther  ana- 
l y t i c a l  techniques such as e l e c t r o n  microscopy may reso lve  t h i s .  
CHEMICAL ANALYSIS 
Auger E lec t ron  Spectroscopy 
N i t r i d i n g  1s a surface process and i t  i s  t h e r e f o r e  n a t u r a l  t o  employ f o r  
This technique i s  very powerfu l  i n  p r o v i d i n g  i n f o r -  
i t s  study one o f  the  numerous surface a n a l y t i -  c a l  techniques developed i n  the  
l a s t  two decades ( r e f .  3 ) .  Among these the most es tab l i shed one i s  Auger e lec-  
t r o n  spectroscopy (AES). 
mat ion on the  elemental composition o f  surface l aye rs .  Auger spect ra o f  
s u p e r n i t r a l l o y  i n  the  as-received s t a t e  and f o l l o w i n g  n i t r i d i n g  by means o f  
the  ion-beam technique a re  g iven i n  f i g u r e  5, where the  n l t r o g e n  peak r e s u l t i n g  
f rom the  n i t r i d i n g  i s  c l e a r l y  seen. 
I 2 
4 
Unfor tunate ly ,  however, very l i t t l e  i n fo rma t ion  can be r e a d i l y  der ived  
from these spectra.  
F i r s t ,  a l though the  i n t e n s i t y  o f  the var ious peaks i n  an Auger spectrum depends 
on the  concentrat ions o f  t he  corresponding elements, t h e r e  i s  no s imple method 
o f  t r a n s l a t i n g  these i n t e n s i t i e s  t o  absolute concentrat ions,  and c a l i b r a t i o n  
w i t h  samples o f  known compositions i s  necessary. The Auger technique i s  a l s o  
not very h e l p f u l  i n  ob ta in ing  t h e  other  parameter i n  which we are  i n te res ted ,  
namely the d i s t r i b u t i o n  o f  n l t rogen  i n  the  sample. This  I s  due t o  t h e  f a c t  
t h a t  the  Auger s igna l  comes f rom t h e  very few ( u s u a l l y  n o t  more than 1 0 )  outer-  
most atomic l aye rs  whereas the  thickness o f  the  n i t r i d e d  l a y e r  i s  o f  the  order 
of microns. We t r i e d  t o  overcome t h i s  problem by means o f  depth p r o f i l i n g  
us ing  the  sput te r -e tch ing  technique. Yet ,  even a f t e r  severa l  hours o f  spu t te r -  
e t ch ing  no change i n  the  i n t e n s i t y  o f  the n i t r o g e n  peak was observed. 
This i s  due t o  two fea tures  o f  t he  Auger technique. 
Rutherford Back Sca t te r i ng  
The obvious way t o  overcome t h i s  l i m i t a t i o n  o f  AES i s  by us ing  an a n a l y t i -  
c a l  method which i s  n o t  l i m i t e d  t o  few sur face layers  b u t  i s  capable o f  p rov id -  
i n g  In fo rma t ion  about the  composition o f  deeper layers  as w e l l .  One such 
method i s  Ruther ford back-scat ter ing (RBS)(ref .  4 ) .  I n  t h e  RBS technique the  
sample sur face i s  bombarded w i th  l i g h t  ions  which were acce le ra ted  t o  k i n e t i c  
energies o f  hundreds o r  even thousands o f  KeV. This r e s u l t s  i n  pene t ra t i on  
depth o f  the  order  o f  microns and thus t h i s  technique seems t o  i d e a l l y  s u i t  
our needs. 
RBS spectra o f  as-received and ion-beam n i t r i d e d  samples o f  s u p e r n i t r a l l o y  
( a c t u a l l y ,  t he  same samples t h a t  were used f o r  AES) a re  presented i n  f i g u r e  6. 
One can r e a d i l y  see the  main shortcoming o f  the  RBS technique i n  our case, 
namely i t s  i n s e n s i t i v i t y  t o  n i t rogen.  The reason f o r  t h i s  i s  t h a t  t h e  RBS 
s igna l  o f  an element depends on i t s  s c a t t e r i n g  cross-sect ion which, i n  tu rn ,  
i s  p ropor t i ona l  t o  the  square o f  i t s  atomic number. This i s  un for tunate  i n  
s i t u a t i o n s  such as ours where one wishes t o  determine the  concent ra t ion  o f  a 
l i g h t  element ( n i t r o g e n  i n  our case) i n  a m a t r i x  o f  a much heavier  one ( i n  our 
case i r o n )  s ince the  s igna l  due t o  t h e  former i s  masked by t h a t  due t o  t h e  
l a t t e r .  A poss ib le  s o l u t i o n  t o  t h i s  problem may be the  use o f  another a n a l y t i c  
technique, r e l a t e d  t o  RBS, which i s  known as nuc lear  r e a c t i o n  ana lys is .  
Energy-Dispersive X-ray Analys is  
Having f a i l e d  t o  ob ta in  q u a n t i t a t i v e  r e s u l t s  by means o f  the  AES and RBS 
techniques we turned t o  ye t  another a n a l y t i c a l  method which was a v a i l a b l e  t o  
us - energy-dispersive x-ray analys is  (EDS) .  I n  t h i s  technique, l i k e  i n  AES, 
e lec t rons  a re  accelerated towards the sample surface. I n  t h e  case o f  EDS, how- 
ever, one detects  the  x-rays emit ted from t h e  sur face as opposed t o  t he  slow 
Auger e lec t rons  which one de tec ts  i n  AES. Since the  absorp t ion  o f  x-rays by 
the  sample i s  much less than the  absorpt ion o f  t he  Auger e lec t rons ,  t he  depth 
analyzed by EDS i s  much greater ,  namely o f  t he  order o f  microns. I t  should be 
po in ted  out t h a t  due t o  the low atomic number o f  n i t rogen  i t s  ana lys i s  requ i res  
t h a t  the  EDS analyzer inc lude l i g h t  element c a p a b i l i t y .  Many l a b o r a t o r i e s  
a l ready  have t h i s  c a p a b i l i t y .  
EDS spectra o f  4140 s t e e l  both as-received and f o l l o w i n g  n i t r i d i n g  by 
means o f  a glow discharge process are presented i n  f i g u r e  7. A t  f i r s t  the  
r e s u l t s  look d isappo in t ing  - i t  seems almost impossible t o  even de tec t  t he  
n i t r o g e n  peak, l e t  a lone measure i t s  i n t e n s i t y .  Standard programs supp l ied  
3 
w i t h  the  analyzer a l l o w  us t o  examine d i f f e r e n c e  spectra.  Thus, we can sub- 
t r a c t  t h e  two spectra o f  f i g u r e  7 f rom each other  a f t e r  a d j u s t i n g  t h e  he igh t  
o f  t he  i r o n  peak i n  both o f  them and o b t a i n  the  d i f f e r e n c e  spectrum. The 
r e s u l t  i s  shown i n  f i g u r e  8, and as can be seen i t  a l lows us t o  measure the  
area underneath the n i t r o g e n  peak and thus have a q u a n t i t a t i v e  measure o f  t h e  
amount o f  n i t rogen  i n  t h e  sample. This c lea rs  the  way t o  k inemat ic s tud ies o f  
n i t r i d i n g  processes. 
Another way i n  which t h e  EDS technique can be used i s  mapping. An example 
o f  a SEM micrograph o f  4140 s t e e l  n i t r i d e d  by means o f  a glow discharge process 
a!nn~s!d~ t h e  corresponding n i t r o g e n  map i s  shown I n  f i g u r e  9. This map can be 
e a s i l y  u t i l i z e d  t o  determine the  n i t r o g e n  d i s t r i b u t i o n  I n  t h e  sample. 
CONCLUSIONS 
Both s t r u c t u r a l  and chemical analyses a re  capable o f  p r o v i d i n g  impor tant  
i n fo rma t ion  about t h e  mechanisms invo lved  i n  n i t r i d i n g  o f  s t e e l s .  For chemical 
ana lys i s  energy-dispersive x-ray ana lys i s  seems t o  have the  g rea tes t  p o t e n t i a l  
w h i l e  e l e c t r o n  microscopy i s  probably the  most promising s t r u c t u r a l  ana lys i s  
technique. 
REFERENCES 
1. T. Lyman, ed.: Metals Handbook, vol. 2, 8 t h  E d i t i o n ,  American Society  f o r  
Metals, Metals Park, OH, 1964, p.  162. 
2. J .  S a l i k :  Met .  Prog., 1985, v o l .  127, no. 4, pp. 55-56. 
3. J .  Ferrante:  Lubr. Eng. 1982, v o l .  38, pp. 223-236. 
4. W.K. Chu, J.W. Mayer, and M.A. N i c o l e t :  Backscat ter ing Spectrometry, 
Academic P r e s s ,  New York, 1978. 
4 
ORIGINAL PAGE IS 
OF POOR QUALITY 
FIGURE 1. - CASE DEPTH DETERMINATION BY MEANS OF HARD- 
NESS NEASUREHENTS (FROM REF. 1). 
12 
8 
4 
0 
6 
4 
2 
(211) 
(110) n I AS RECEIVED 
L 
01 
z a- 
01 a 
10 20 30 40 50 60 70 80 90 100 
20 
FIGURE 2. - X-RAY DIFFRACTION PATTERN OBTAINED FROM 4140 STEEL 
NITRIDED BY A GLOW-DISCHARGE PROCESS. 
5 
ORIGINAL PAGE IS 
OF POOR QUALITY 
L - - - - 
c I 
FIGURE 3. - MICROGRAPH OF SUPERNITRALLOY NITRIDED BY THE ION- 
BEAM PROCESS. 
6 
. 
2.5 
AS RECEIVED 
- 
li 
NITRIDED 
FIGURE 5. - AUGER SPECTRA OF AS-RECEIVED AND ION-BEAM NITRIDED 
SUPERN I TRALLOY . 
15.0 n 
I 
5.0 
7 
0 1 2 
I I 
c 
AS RECEIVED 
I 
N O F  
E 
S I :  I 
NITRIDED 
FIGURE 7. - EDS SPECTRA OF AS-RECEIVED AND PLASMA-NITRIDED 4140 
STEEL. 
8 
ORIGINAL PAGE IS 
OF POOR QUALITY 
4 
DIFFERENCE 
f IGURE 8. - SPECTRUfl OBTAINED BY SUBTRACTION OF THE TWO SPECTRA OF FIGURE 7. 
- - -  
FIGURE 9. - SEM MICROGRAPH AND NITROGEN MAP OF PLASMA-NITRIDED 41'40 STEEL. 
9 
1. Report No. 2. Government Accession No. 
NASA TM-89815 
4. Title and Subtitle 
Analys is  o f  Plasma-Nitrided Stee ls  
7. Author@) 
J .  Sa l i k ,  J .  Ferrante, F. Honecy, and R. Hoffman, J r .  
9 Performing Oraanization Name and Address 
Nat iona l  Aeronautics and SDace Admin i s t ra t i on  
Lewis Research Center 
Cleveland, Ohio 441 35 
-- 1 3. Reclplent's Catalog No. 
5. Report Date 
6. Performing Organization Code 
505-90-01 
- 
8. Performing Organization Report No. 
E-3442 
10. Work Unit No. 
11. Contract or Grant No. 
13. Type of Report and Perlod Covered 
20. Securlty Classif. f thls 21. No. of pages 
Unc'Pass!??ed 10 
9. Security Clas if. (of thl report) Uric\ ass I !I ed 
I Technical  Memorandum 2. Sponsoring Agency Name and Address 
22. Price' 
A02 
Nat iona l  Aeronautics and Space Admin ts t ra t ion  
Washington, D.C. 20546 
I 
14. Sponsoring Agency Code  
I 
I 
5. Supplementary Notes 
Prepared f o r  t h e  Ion  N i t r i d i n g  Conference, sponsored by the  American Society  f o r  
Metals, Cleveland, Ohio, September 15-17, 1986. J .  S a l i k ,  U n i v e r s i t y  o f  Arizona, 
Tucson, Ar izona 85721; 3 .  Ferrante and F. Honecy, NASA Lewis Research Center; 
R. Hoffman, J r . ,  Case Western Reserve U n i v e r s i t y ,  Cleveland, Ohio 44106. 
~ 
6 Abstract 
The ana lys i s  o f  plasma n i t r i d e d  s tee l s  can be d i v i d e d  t o  two maln ca tegor ies  - 
s t r u c t u r a l  and chemical. S t r u c t u r a l  ana lys i s  can prov ide  i n fo rma t ion  n o t  on l y  
on the  hardening mechanisms b u t  a l s o  on the  fundamental processes invo lved.  
Chemical ana lys i s  can be used t o  study the  k i n e t i c s  f o r  t he  n i t r i d i n g  process 
and i t s  mechanisms. I n  t h i s  paper p re l im ina ry  r e s u l t s  obta ined by severa l  tech- 
niques o f  bo th  categor ies a r e  presented and t h e  a p p l i c a b i l i t y  o f  those techniques 
t o  the  ana lys i s  o f  p lasma-ni t r ided s t e e l s  i s  discussed. 
7. Key Words (Suggested by Author@)) 
I on  n i t r i d i n g  
Steels  
18. Dlstributlon Statement 
Unc lass i f i ed  - u n l i m i t e d  
STAR Category 26 


Analysis of plasma-nitrided steels

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