Dependence of hardness on modulation amplitude in electrodeposited Cu-Ni compositionally modulated thin films

Saipta Metallurgica et Materialia, Vol. 32, No. 4, pp. 583-588, 1995 Copyright 0 1994 Fhevier Science Ltd Printed in the USA. All rights reserved 0956-716x195 $9.50 + .oO

DEPENDENCE OF HARDNESS ON MODULATION AMPLITUDE IN ELECTRODEPOSITED Cu-Ni COMPOSITIONALLY MODULATED THIN FILMS R.R. Oberle* and R.C. Cammarata Department of Materials Science and Engineering The Johns Hopkins University Baltimore, MD 21218-2689 U.S.A. (Received August 30,1994) (Revised September 23,1994)

Introduction Compositionally modulated metallic and ceramic thin films have displayed a variety of interesting and often enhanced mechanical properties (for recent reviews, see references l-3). For example, dramatic increases in the ultimate tensile strength of Cu-Ni multilayered films (of overall composition Cun.lnNio.gn) produced by electrodeposition were observed when the copper layer thickness was reduced below 400 nm (4). Similar enhancements in the tensile strength of vapor deposited Al-Cu multilayers have also been reported (5). Recently, low load indentation testing has become a common method of investigating the plastic properties of modulated films (l-3). Almost all of these indentation studies have involved investigating the hardness as a function of layer thickness or composition modulation wavelength. A variety of theories have been offered to model the hardening mechanisms in thin layered and compositionally modulated structures (6-12). Extending an analysis by Head (13) Koehler (7) proposed that when two materials with significantly different shear moduli are used as ultrathin layer materials in a laminated composite with sharp interfaces, the strength of this composite material could be of order p/100, where l.t is the lower of the two shear moduli. This enhanced strength would result from image forces that retard dislocation generation and mobility. One of the model systems suggested by Koehler to display this behavior was Cu-Ni. Krzanowski (lo), expanding on previous work by Fleischer (14), has performed detailed calculations for hardness enhancements owing to composition modulations with diffuse interfaces. Effects of sinusoidal, trapezoidal, and triangular wave profiles were investigated. Predicted hardness enhancements were linearly proportional to the composition modulation amplitude, and the effect greatly increased when the interfaces were made sharper. In this paper, an experimental study of the hardness dependence on the composition modulation amplitude of electrodeposited Cu-Ni multilayered thin films is presented. The amplitude was reduced by annealing the films, resulting in interdiffusion between the layers. The hardness enhancement over that for a completely intermixed film was measured as a function of composition modulation amplitude A. It was found that for films with a sine wave composition profile, the hardness enhancement AH varied as Al.*+O.l, in good agreement with the model given by Krzanowski (10). In addition, a film with sharper interfaces displayed a more dramatic decrease in AH as the film was annealed and the interfaces became more diffuse. *Present address: Ethone-OMI, West Haven, CT 06508 583

ELECI’RODEPOSIT@D THIN FILMS

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Experimental

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Procedure

Polycrystalline Cu-Ni (100) compositionally modulated thin films with equal layer thicknesses and with modulation wavelengths between 2.5 and 50 nm were electrodeposited from a nickel sulfamate-based plating solution (12). The electrolyte composition was 120 g/l nickel as metal, 25 g/l boric acid ( H3B03) and 4 g/l copper sulfate (CuSO4 . 5H20). Plating was carried out at room temperature with an overall deposition rate of approximately 0.5 urn/h. Substrates were polished polycrystalline copper, and the total film thickness was about 10 pm. After deposition of the multilayered film, a silver capping layer was plated. The substrate was then electrochemically dissolved in a chromic-sulfuric stripping solution, revealing the surface of the multilayered film on which hardness measurements were performed. The composition modulation wavelengths were determined by measuring the angular difference in the diffraction pattern between the positions of the (200) main Bragg peak and the first order satellite peak (16). Transmission x-ray diffraction was used (17) to determine the modulation wavelength associated with the transition from coherent to semicoherent interfaces (i.e., the smallest wavelength where misfit dislocations at the interfaces first appeared). This wavelength was found to be approximately 15 mn. [Another study involving Cu-Ni(100) multilayered films produced by the same electrodeposition method as used here found this wavelength occurred at approximately 20 nm WI.

The Knoop microhardness of the films was measured with an Anton Paar low load microhardness tester using a 5 g load. Each hardness value given in this paper represents the average of five to ten measurements taken from widely spaced indents. The hardness of the asdeposited films as a function of modulation wavelength are shown in Figure 1. A peak in the hardness occurred at a composition modulation wavelength of approximately 12 nm. Similar

P 200

0

I.

I.

I.

I.

I.

10

20

30

40

50

MODULATION

WAVELENGTH

FIG. 1. Knoop hardness of as-deposited Cu-Ni compositionally function of composition modulation wavelength.

60

(nm)

modulated thin films as a

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ELECTRODFPOSFIEDTHINmLMS

peaks in hardness have been observed in various metallic and ceramic multilayered thin films (l3). However, these results are in contrast to those obtained from equal layer thickness Cu-Ni (111) multilayered films produced by sputtering, where a monotonic increase in hardness was observed as the modulation wavelength was reduced to 1.6 nm (19). Films with modulation wavelengths of 4.8,8.8, 11.6 and 18.8 m-nwere used to investigate the change in hardness with annealing. X-ray diffractometer scans produced by these films in their as-deposited state displayed three orders of satellite peaks about the (200) Bragg peak, indicating the presence of a composition modulation. During the early stages of annealing, the intensity of the second and third order satellite peaks quickly decayed. As a result, the composition modulation profile of the annealed films could be taken as a single sine wave along the modulation direction (16). Before each anneal the multilayered films were encapsulated in a glass tube that was evacuated to a pressure of about 1 x 10-d Pa before sealing. Samples were annealed for various times at 4OOOC,resulting in interdiffusion between the copper and nickel layers. After each anneal the hardness was measured, and x-ray diffraction was used to obtain the integrated intensity of the first order satellite peak. This first order satellite peak intensity was normalized with respect to the integrated intensity of the (111) Bragg peak of the silver backing layer. In order to determine the hardness enhancement AH = H - & owing to the sine wave composition modulation, where H was the hardness of the film and Ho was the hardness of a totally intermixed film of the same composition, it was necessary to completely interdiffuse the films to obtain the value of Ho- A film was determined to be fully interdiffused when the satellite peaks completely disappeared and the hardness of the film did not change with further annealing. Total annealing times of approximately 200 hours were used in order to transform the films into completely interdiffused alloys. In addition to examining films with sine wave composition modulation profiles, the behavior of a 15.8 run modulation wavelength film during the early stages of annealing was investigated. As with the other films, the intensity of the second and third order peaks decayed much faster than the first order peaks. Hardness measurements were performed on this film while the higher order satellite peaks were still observable in order to qualitatively investigate how the hardness enhancement was affected as the interfaces became less sharp. Results and Discussion In each film it was found that after a 30 min anneal the hardness increased slightly even though the intensity of the first order satellite peak decreased, indicating that some interdiffusion had occurred (11). The reason for this behavior is not completely clear. Further annealing resulted in a decrease in the hardness of every sample. If the hardness enhancement AH is related to the intensity I of the first order satellite peak by the expression AH = Im, then the exponent m can be obtained from the slope of the plot of ln(AH) versus m(1). Since I = A2, where A is the amplitude of the composition modulation (13), the exponent n in the expression AH = An relating the hardness enhancement to the composition modulation amplitude is equal to 2m. Plots of ln(AH) versus m(1) for films with composition modulation wavelengths of 8.8 and 11.6 nm are shown in Figures 2 and 3, respectively. (Total annealing times associated with lowest intensity values were 65.5 h for the 8.8 m-n wavelength

585

586

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ELECIRODJ3POSIlED TTUN FILMS

72

-1

0

2

1

In (Normalized

3

4

Intensity)

FIG. 2. Natural logarithm of the hardness enhancement AH plotted versus the natural logarithm of the normalized first order x-ray satellite peak intensity I for the 8.8 nm wavelength film. The solid line represents the least squares best fit.

6

-2

.~.a.,.,.

-1

0

In (Normalized

1

2

3

Intensity)

FIG. 3. Natural logarithm of the hardness enhancement AH plotted versus the natural logarithm of the normalized first order x-ray satellite peak intensity I for the 11.6 nm wavelength film. The solid line represents the least squares best fit.

587

ELJXTRODEPOSlT@DTHINFILMS

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film and 176.5 h for the 11.6 nm wavelength film.) The large scatter seen for the smallest ln(AH) values in Figure 2 can be attributed to the fact that in this region AH was of the order of the uncertainty in the hardness measurements. Plots of this type were obtained for each film, and the slopes were determined from least squares best fits. Table 1 lists the as-deposited hardness, the hardness Ho of the completely interdiffused sample, and the exponent n for each film. The fact that the values of Ho were somewhat different for each sample was presumably a result of differences in microstructure (for example, differences in grain size or dislocation density). It is noted that the measured exponent n was close to 1 for all of the films, in agreement with the theory given by Krzanowski (10).

TABLE 1. As-deposited Knoop hardness number, completely interdiffused hardness Ho, and exponent n relating hardness enhancement to composition modulation amplitude for compositionally modulated Cu-Ni films of different modulation wavelength.

Wavelength (run)

As-Deposited Hardness

Ho

n

4.8 8.8 11.6 18.8

366 470 524

195 230 284

1.1 1.2 1.2 1.0

387

The 15.8 nm modulation wavelength film was annealed for a total of 361 minutes during which time the intensity of the second and third order satellite peaks decayed much faster than the first order peaks. This indicated that in addition to a reduction in the amplitude of the composition modulation, the interfaces became much more diffuse during the anneals. The exponent n associated with the amplitude of the first order peak was measured to be 2.8. It is not meaningful to attach too much significance to the precise value of n, but the fact that it was significantly larger than 1 indicated that the hardness enhancement resulting from the layering became much less pronounced when the interfaces became less sharp. Although the results obtained in this study were in good agreement with the model proposed by Krzanowski (lo), it is possible that effects other than those owing to image forces may play an important role in the hardness behavior. For example, it has been shown that coherency strains in three-dimensionally modulated materials characteristic of spinodally decomposed alloys can produce forces on dislocations that may critically affect the mechanical behavior (6,8). In the model presented by Kato et al. (8), this strengthening was predicted to be linearly proportional to the composition modulation amplitude. Thus, while the experimental results presented here are consistent with a model based on image force effects, they may also be explained by other mechanisms.

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Acknowledgments The authors thank Professor J.P. Hirth for useful discussions, and J.B. Savader, T.M. Trimble, and N.A. Vaught for critical readings of the manuscript. Financial support of the National Science Foundation through grant number ECS-920222 and the Office of Naval Research through grant number NOOO14-91-J-1169 is gratefully acknowledged. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

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