Surface mechanical properties of CuNi and FeAl films produced by dynamic ion mixing

Materials Science and Engineering, A l l 5 (1989) 267-271

267

Surface Mechanical Properties of CuNi and FeAI Films Produced by Dynamic Ion Mixing* J. VON STEBUT L.S.G. 2M/L. G.M. Ecole des Mines, 54042 Nancy Cedex (France) J. R RIVII~RE. J. D E L A F O N D and C. SARRAZIN

Laboratoire de M~mllurgie l'hysique, UA 131 CNRS, 40 A venue du Recteur Pineau, 86022 Poitiers (France)

S. MICHAUX UNIREC, B.P. 50, 42702 Firrniny (France)

(Received September 16, 1988)

Abstract Dynamic ion mixing is a new method to produce thick and adherent coatings using a highenergy ion beam combined with a deposition technique. We investigate the mechanical properties of two coatings of Cu~oNiso and Fe~jAl4o deposited on stainless steel substrates. Surface failure analysis by means of scratch as well as ball-on-disc friction and wear testing indicates that for both systems studied the coating adhesion on stainless steel substrates is' excellent. Overall plastic deformation in scratch testing is essentially related to the substrate hardness. Both in scratch testing and during friction and wear FeAl films deform in a perfectly ductile manner. Failure of CuNi films is controlled by their intrinsic brittleness and wear is essentially abrasive.

In order to produce thick and adherent coatings we have developed the dynamic ion mixing (DIM) method combining continuous deposition with simultaneous bombardment by a highenergy ion beam [1, 3]. At the present time most successful applications of ion beam-assisted deposition techniques have used low-energy ions of a few kiloelectronvolts [4-6]. It appears that there would be several advantages in using higher-energy ion beams [7]. The first is that the dose required to produce the same energy deposition is greatly reduced, and a second is the important mixing effect in the growing film and at the film-substrate interface, resulting in superior adhesion performance. In addition it has been observed that ion beam-assisted deposition of metal films has the possibility of relieving the stress stored in the film during deposition even for low values of the ion:atom arrival rate ratio

1. Introduction

[8, 9].

There is a growing interest in the application of energetic ion beams as a technique for changing the mechanical or physical properties of a surface. Alloys of predetermined composition and structure can be produced for instance by ion mixing of a multilayer; their adhesion is generally excellent due to the mixing effect through the film-substrate interface [1, 2]. However the major limitation of conventional ion mixing is its shallow depth of treatment which is insufficient for many practical applications.

We present two examples of improved adhesion and tribological properties of CusoNis0 and Fe60A140 films deposited on stainless steel substrates. The CusoNis0 alloy is used as a surface coating for high power laser mirrors. The Fe60Al40 alloy exhibits a good resistance to hightemperature oxidation and corrosion and can be used as a protective coating.

*Paper presented at the Sixth International Conference on Surface Modification of Metals by Ion Beams, Riva del Garda, Italy, September 12-16, 1988. 0921-5093/89/$3.50

2. Experimental procedure 2.1. Sample preparation Stainless steel plates 2.5 cm x 2.5 cm mechanically mirror polished and ultrasonically cleaned were used as substrates. Both Fe60A140 and © Elsevier Sequoia/Printed in The Netherlands

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Cu~0Nis0 coatings were deposited at room temperature up to 1 and 1.3 p m in thickness. The dynamic mixing apparatus has been described elsewhere [3, 10]. It is composed of an ultrahigh vacuum chamber equipped with two 8 kW electron beam evaporators on line with a 200 kV ion implanter. The pressure during evaporation is always lower than 1-2 10 -t' Pa. The deposition rates from both sources (i.e. iron and aluminium or copper and nickel) are automatically and independently controlled via two calibrated quartz crystal oscillators. Ion mixing was performed in both alloys with 60 keV Ar + ions. The total dose up to the final thickness of 1-1.3 p m was 1.5 x 1016 c m -2 and the ion:atom arrival rate ratio was about 2 x 10- 4. A set of thinner films only 100 nm thick was deposited on freshly cleaved NaCl crystals for transmission electron microscopy characterization and determination of the final composition by X-ray microanalysis.

2.2. Mechanical surface properties The friction and wear behaviour was studied on a standard bail-on-disc tribometer with a 100C6 (AISI 52100) steel ball 5 mm in diameter. The experiments were performed under dry sliding and normal atmospheric conditions at a sliding speed of 30 mm rain -J. The normal load F n applied to the ball was 0.5 N for the Cus0Nis0 and 1.7 N for the Fe60Al~0 films. Adhesion of the films was assessed by means of scratch testing using a Rockwell C diamond indenter under progressive loading conditions [11]. Acoustic emission (AE) and friction force (FF) failure analyses were monitored on-line for quick routine. Detailed damage mechanism diagnosis was done off-line by means of optical microscopy (OM) and scanning electron microscopy (SEM), electron microprobe (EMP) and three-dimensional profilometric mapping.

as approximately one track diameter beyond on both sides of the scratch scar.

3.1.2. Adhesion, friction and wear behaviour of CusoNiso films obtained by DIM 3.1.2.1. Adhesion. In Fig. l(b) we see that DIM considerably enhances coating adhesion, increasing not only the critical load for surface damage but also changing the basic failure mechanism. Over a scratch scar length identical to that of Fig.l(a) no adhesive failure is observed. The first damage consists of brittle cracking within the very scratch track at F o = 3 N (x=0.3 ram) accompanied by a local pile-up of the track borders (Figs. l(b) and 2(a)). However the track bottom (inverted plot, Fig. 2(b)) keeps "diving" into the surface without any step discontinuity comparable with that observed for the unimplanted coating (Fig. l(a)). This confirms that in addition to increasingly large cracks in the coating there is no spalling within the track itself but only along the groove edges. In Figs. 2(c) and 2(d) we show the A E and the EMP F e K a signals over the entire scratch distance x = 10 mm (Fn = 90 N). As can be expected with brittle failure the A E signal increases with the onset of brittle cracking. At Fn= 20 N (x= 2.2 mm) where these cracks have become increasingly large the corresponding F e K a signal is entirely due to the substrate. In spite of this severe surface damage caused by the ploughing action of the indenter there is still no evidence of adhesive failure. Instead we observe a scraping effect which finally leads to the complete coating removal at the groove

3. Results and discussion

3.1. CusoNiso coatings on stainless steel 3.1.1. Adhesion of co-evaporated films without any ion beam assistance As can be seen in Fig. l(a), adhesion of CusoNis0 films is very poor when prepared by coevaporation alone. Indeed Fig. l(a) shows that the film spalls off almost immediately from the start of a load F, = 1.5 N only. This adhesive, brittle failure extends over the entire track width as well

Fig. 1. Scratch testing of CusoNis. coatings on stainless steel. Comparison of films prepared (a) without DIM and (b) with DIM. (Scratch length 1 ram)

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270

the load probably being transmitted via a third body formed by the wear debris. EMP analysis confirms this interpretation and shows some coating persistance in the track centre as well as tribo-oxidation within the track. 3.2. FeooAl4ocoatings on stainless steel 3.2.1. Scratch testing on dynamic ion mixed films As discussed above for non-implanted CuNi films such Fe60Al60 coatings also have very poor adhesion and wear resistance and will not be discussed here. Conventional techniques such as AE, FF and OM in scratch testing of ion-mixed Fe60A140coatings do not allow for any failure analysis [11]. Such information may be obtained, however, by EMP analysis. Indeed Fig. 4(a) shows that above Fn=30 N the A I K a signal breaks down with considerable scatter building up. This proves the existence of a critical load for coating perforation. In fact three-dimensional mapping (Fig. 4(b)) also yields evidence of such damage. Above Fn=30 N the piled-up track edges are very irregular whereas below this value a smooth, con20

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tinuous build-up is observed. This differs from the smooth continuous increase in track depth (inverted plot) even after perforation. The most plausible explanation of these findings is that indentation depth in scratch testing is essentially related to the substrate's surface hardness. Perforation should occur as a consequence of the scratch diamond's ploughing action. The continuous increase in track depth beyond perforation proves that the ion-mixed Fe60A140coating is perfectly adherent on its stainless steel substrate in spite of an overall track depression of 7 /~m, over three times greater than the coating thickness. Beyond perforation the difference in hardness (HVs0=180 for the substrate and HVs0 = 400 for the coating) might well explain why, at the surface, the coating is crushed and pushed aside (Fig. 4(b), direct plot). Threedimensional surface mapping also allows for a detailed analysis of the volume displacement during scratch testing. In fact the volume expelled from the centre groove is equal to that piled up along and ahead of the track. This is clear evidence that the FeAl-stainless steel system is deformed in a perfectly plastic manner during scratch testing. As opposed to the CuNi-stainless steel system the FeA1 coating is sufficiently ductile to accommodate all the plastic strain generated during scratch testing without cracking and spalling.

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3.2.2. Friction and wear of ion-mixed FeAl coatings Figure 5(a) shows the evolution of friction during 400 revolutions of the ball-on-disc tribometer. Up to 100 turns the friction coefficient slowly increases with oscillations of approximately 15% around the mean value. Beyond 100 turns these oscillations increase considerably (about 50%) together with friction (/~ = 0.7 at 400 turns). This suggests that the ball has worn through the coating at 100 turns. EMP analysis altogether confirms such an analysis. After 50 turns the wear track appears smooth and neat without any C r K a signal (chromium being the substrate-specific element analyzed). Three-dimensional mapping shown in Fig. 5(b) gives evidence of a wear track approximately 0.5 /am deep with some wear debris along the track edges but no plastic pile-up. After 200 turns (/a -- 0.5) EMP analysis clearly shows that the coating is worn through with some persistence of coating residues within the wear

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Real friction and wear testing is the best way of testing the wear resistance of such coatings under conditions close to real service life. However scratch testing is a powerful method for quick and efficient failure analysis especially when backed up by specific surface-analytical tools such as electron microprobe and surface mapping. We have shown the ductile failure mode of the FeA1 coating and the essentially brittle, abrasive wear in the case of CuNi. Attention should be paid to the system's load-carrying capacity. Especially in the case of hard brittle coatings the substrate should be sufficiently hard to avoid brittle cracking due to plastic pile-up of the substrate.

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track and wear debris along the track edges. Tribo-oxidation is clearly confined to this coating debris. Three-dimensional surface mapping (Fig. 5(c)) shows the wear track to be approximately 3 /~m deep with about 1.5 /~m of material pile-up on both sides of the track. Thus the wear mechanism in the present case is ductile ploughing rather than brittle, abrasive wear as for the CuNi coating discussed above. Similar ductile wear has been found for other ion-implanted systems by Takadoum et al. [13] as well as by Singer [14]. It is worth mentioning that even though there is considerable surface damage practically no weight loss can be measured in such a case. 4. Conclusion Dynamic mixing during deposition considerably enhances coating adhesion as compared with simple co-evaporation.

Acknowledgments R. Rezakhanlou and J. P. Haeussler (L.S.G.2M./L.G.M) have contributed considerably to scratch testing and EMP analysis. The three-dimensional mapping device is essentially due to C. Roques-Carmes, J. E Champigny and M. Pequignot (L.M.S. Besanqon). References 1 P. Moine, O. Popoola, J. P. Villain, N. Junqua, S. Pimbert, J. Delafond and J. Grilhd, Surf. Coat. Technol., 33(1987) 479. 2 K. Kobs, H. Dimingen, H. Hfibsch and H. J. Tolle, Philips Tech. Rev., 44(1)(1988)24. 3 M. Jaulin, S. Pimbert-Michaux, G. Laplanche and J. Delafond, Surf. Coat. Technol., 37(1989) 225. 4 N. A. G. Ahmed, J. S. Colligon and A. E. Hill, Thin Solid Films, 129(1985) 223. 5 Y. Andoh, Y. Suzuki, K. Matsuda, M. Satou and E Fujimoto, Nucl. lnstrum. Methods B, 6 (1985) 111. 6 J. M. E. Harper, J. J. Cuomo, R. J. Gambino and H. R. Kauffman, Nucl. lnstrum. Methods B, 17/18 (1985) 886. 7 R. A. Kant and B. D. Sartwell, Mater. Res. Soc. Syrup. Proc., 27(1984) 525. 8 D. W. Hauffman and M. R. Gaerttner, J. Vac. Sci. Technol., 17(1980) 425. 9 E. H. Hirsh and I. K. Varga, Thin Solid Films, 69(1980) 99. 10 J. E Rivi~re, E Guesdon, J. Delafond and M. E Denanot, J. Less-Common Met., 145 (1988) 477. 11 J. von Stebut, Proc. 1st. Int. Conf. on Plasma Surface Engineering, Dr Riederer, Stuttgart, 1989. 12 I.L. Singer, Mater. Res. Soc. Symp. Proc., 27(1984) 585. 13 J. Takadoum, J. C. Pivin, J. Chaumont and C. RoquesCarmes, J. Mater. Sci., 20 (1985) 1480. 14 I. L. Singer, AppL Surf Sci., 18(1984) 28.