Thin Solid Films, 73 (1980) 189-191 © ElsevierSequoiaS.A., Lausanne---Printedin the Netherlands
M O D I F I C A T I O N OF M E C H A N I C A L PROPERTIES BY ION IMPLANTATION* H. HERMAN,W. W. HU AND C. R. CLAYTON Department of Materials Science and Engineering, State University of New York, Stony Brook, N.Y. 11794 (U.S.A.) J. K. HIRVONENAND R. KANT U.S. Naval Research Laboratory, Washington, D.C. 20375 (U.S.A.)
R. K. MACCRONE Department of Materials Engineering, Rensselaer Polytechnic Institute, Troy, N. Y. 12181 (U.S.A.)
(ReceivedApril 21, 1980;acceptedApril 23, 1980) 1. INTRODUCTION There has been considerable activity in recent years concerned with the effects of nitrogen ion implantation on the mechanical properties of steel. The experiments have generally involved the implantation of either atomic or molecular nitrogen, positively charged and accelerated to energies ranging from 50 to 200 keV. The properties that have been modified include friction, wear, fatigue lifetime, cavitation erosion and internal friction. The last three effects, studied principally under a joint Stony Brook-Naval Research Laboratory program, have recently been reviewed 1. We should like here to update our work briefly as well as to present other recent results on interstitial modification in principally ferrous alloys. 2. FATIGUE The work of the Harwell group 2 initiated interest in the extension of fatigue lifetime. Further studies by Hu et al. s have confirmed the earlier Harwell studies and have established a quantitative basis for the phenomenon. In addition, a postimplantation aging effect was discovered which, for the implantation of atomic nitrogen into AISI 1018 steel (0.18 wt.~ C), resulted in a major improvement in fatigue life. The aging can in fact be carried out "naturally" (in the terminology of metallurgy) at ambient temperature or "artificially" by heat treatment for 6 h at 100 °C. This thermal treatment, which also yields a significant improvement in the resistance to cavitation erosion, is believed to give rise to an effective "tempering" of the implanted structure and results in a high density of metastable nitrides, such as FelrN2, which can be observed by transmission electron microscopy (TEM). An association of these particles with near-surface dislocations has been proposed as the explanation for the fact that more modifications than expected occurred in mechanical propertiesl. More recently, Lo Russo et al. 4 have found that, for mild steel, an implanted 30 keV nitrogen dose of 2 x 1017 ions cm-2 gives a maximum improvement in fatigue behavior. The origin of this dependence on a non-monotonic dose is as yet unclear. * Extendedabstract of a paper presentedat the InternationalConferenceon MetallurgicalCoatings,San Diego, California,U.S.A., April 21-25, 1980.
H. HERMANet al.
Fatigue lifetimes of materials other than ferrous alloys have also been extended by implantation, e.g. carbon into titanium alloy (Naval Research Laboratory) and boron into copper (Bell Laboratories). Furthermore, the rolling contact fatigue performance has been significantly improved through nitrogen implantation (Harwell and The Polytechnic of Wales). 3.
The complexity associated with fatigue behavior can to some extent be resolved through internal friction experiments. In these studies we sought to explore the mechanical absorption mechanisms, as affected by dislocation vibrations. We attributed the influence of ion implantation on strengthening mechanisms to the interactions between interstitial and precipitate dislocation. Internal friction should therefore be a sensitive indication of such interactions. In fact after ion implantation the internal friction decreases by some 409/0 near room temperature, with the occurrence of further but less significant decreases on subsequent aging. This remarkable effect, resulting from the implantation of nitrogen to only 800 A below the surface, attests both to the great sensitivity of the method and to the highly influential effect of implanted nitrogen on the near'surface mechanical properties. Internal friction, together with high voltage TEM, is currently being employed in the program to determine the way in which the nitrogen is able to influence the dislocation structure in the vicinity of the surface. 4.
Cavitation erosion can result in the severe deterioration of a surface through the formation and collapse of a very high density of bubbles at rates so great that sh6ck waves are transferred into the adjacent,surface. This phenomenon of surface attack occurs very commonly in industrial processes and in the marine industry. Traditionally, a major defense from Cavitation erosion attack is to harden the surface. We have used ion implantation of nitrogen to modify cavitation erosion in mild plate steel 5. In general, the degree of improvement is analogous to that encountered in fatigue where implantation, especially with subsequent aging, can yield dramatic improvements in lifetime. A measure of a material's resistance to cavitation erosion is the duration of an incubation time prior to measurable weight loss. For our cavitation erosion experiments the incubation time is significantly extended, as seen in Table I. This effect, as for fatigue, is believed to reside in the implantation strengthening of the ferrite phase. Similar studies on other implanted systems are in progress. 5. CONCLUSION
Wear, fatigue and cavitation erosion are surface-controlled properties. A surface which is modified, either thermomechanically or by coating processes, gives rise to a hardened and thus a crack-resistant surface. It is this enhanced surface strengthening which is achieved by the implantation of nitrogen into steel. In Table I
M E C H A N I C A L P R O P E R T I E S M O D I F I C A T I O N BY I O N I M P L A N T A T I O N
TABLE I EFFECTS OF IMPLANTATION ON FATIGUE LIFETIME, CAVITATION EROSION AND INTERNAL FRICTION
Unimplanted Implanted Implanted and aged at 100 °C for 6 h
Fatigue lifetime (cycles) at 54 kgf in- 2
Cavitation erosion incubation time (rain)
Internal friction Q-
~ 10 6
0 10 50
1.68 0.98 0.84
2 x 106 > 107
( x 10- a)
For implantation with 150 keV N 2+ to a dose of 2 x 10~7 ions era- 2. we review the effects of i m p l a n t a t i o n o n fatigue lifetime, c a v i t a t i o n e r o s i o n a n d i n t e r n a l friction. T h e r e exist r e l a t i o n s between the s t r e n g t h a n d the d i s l o c a t i o n structure. A l l o y i n g a n d c o l d d e f o r m a t i o n can result in the i m m o b i l i z a t i o n of dislocations. It is k n o w n , for e x a m p l e , t h a t b u l k m e c h a n i c a l p r o p e r t i e s c a n be c o n t r o l l e d b y the e s t a b l i s h m e n t of a fine d e b r i s layer which effectively acts to b l o c k d i s l o c a t i o n s m o v i n g on slip p l a n e s f r o m exiting at the m a t e r i a l surface 6. This effect acts as a s t r e n g t h e n i n g m e c h a n i s m and, we suggest, can be e n h a n c e d b y ion i m p l a n t a t i o n . T h e c e n t r a l question, however, is the d e t a i l e d m e c h a n i s m by which the i m p l a n t effects this strengthening. ACKNOWLEDGMENT This research at S t o n y B r o o k was s p o n s o r e d by the U.S. Office of N a v a l Research. REFERENCES 1 W.W. Hu, H. Herman, C. R. Clayton, J. Kozubowski, R. A. Kant, J. K. Hirvonen and R. K. MacCrone, Surface related mechanical properties of nitrogen-implanted 1018 steel, Proc. Materials Research Society Annu. Syrup., Cambridge, Massachusetts, November 1979, in the press. 2 G. Dearnaley, Practical applications of ion implantation, Proc. Materials Research Society Annu. Syrup., Cambridge, Massachusetts, November 1979, in the press. 3 W.W. Hu, C. R. Clayton, H. Herman and J. K. Hirvonen, Scr. Metall., 12 (1978) 697. 4 S. Lo Russo, P. Mazzoldi, I. Scotoni, C. Tosello and S. Tosto, J. Appl. Phys., to be published. 5 W.W. Hu, C. R. Clayton, H. Herman and J. K. Hirvonen, J. Mater. Sci. Eng., to be published. 6 I.R. Kramer, Trans. A1ME, 222 (1963) 1003.