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W alloys
Applied Surface Science 200 (2002) 1±14
STM/AFM studies of the evolution of morphology of electroplated Ni/W alloys L. Zhua,1, O. Younesb, N. Ashkenasya,2, Y. Shacham-Diamanda, E. Gileadib,* a
School of Engineering, Tel-Aviv University, Ramat-Aviv 69978, Israel School of Chemistry and Faculty of Engineering, Tel-Aviv University, Ramat-Aviv 69978, Israel
b
Received 20 November 2001; accepted 16 April 2002
Abstract The surface morphology evolution of Ni/W alloys was studied, as a function of the alloy composition. Using the modi®ed plating baths developed in our laboratory recently, electroplated Ni/W alloys with different W content, in the range of 7±67 atom percent (a/o), can be obtained. This was found to lead to different structures, ranging from polycrystalline fcc-Ni type structure to amorphous, followed by orthorhombic with increasing W content in the alloy. Powder XRD was studied to determine the crystal structures. Ex situ STM, AFM and SEM were used to study in detail the surface morphologies of the different alloys, and their evolution with increasing W content. The important ®ndings are that a mixture of two crystalline forms can give rise to an amorphous structure. Hillocks that are usually a characteristic of epitaxial growth can also exist in the amorphous alloys. Oriented scratches caused by stress can also be formed. Up to 20 a/o of W is deposited in the alloys in crystalline form, with the fcc-Ni type structure. Between 20 and about 40 a/o an amorphous structure is observed, and above that an orthorhombic crystal structure is seen, which is characteristic of the NiW binary alloy. Careful choice of the composition of the plating bath allowed us to deposit an alloy containing 67 a/o W, which corresponds to the composition NiW2. # 2002 Elsevier Science B.V. All rights reserved. Keywords: NiW alloy; Tungsten alloy; Amorphous alloys; STM; AFM; Alloy plating
1. Introduction Conventional Ni/W alloys are known as high-temperature alloys. One of the important uses of these alloys is to protect and extend the service life of * Corresponding author. E-mail address: [email protected] (E. Gileadi). 1 Present address: Institute of Chemical Processing, National Research Council of Canada, 1500 Montreal Rod, M-12, Rm220 Ottawa, Ont. Canada K1A 0R6. Tel.: 1-972-3-640-8694; fax: 1-972-3-9293. 2 Current address: Scripps Research Institute, La Jolla, CA, USA.
turbine blades, operating under conditions of high temperature and erosion. Usually, the concentration of W in the plated alloys is rather low, in the range of 5±15 atom percent (a/o), but it has a major in¯uence on the mechanical and chemical properties, such as hardness, wear-resistance and improved corrosion resistance at high temperature [1]. By increasing the tungsten content above 20 a/o, an amorphous structure is obtained [2±6]. Electroplated Ni/W alloys with amorphous structure exhibit excellent properties, such as high hardness, high thermal stability and high corrosion resistance [7]. It has been reported recently that the corrosion rate of an amorphous Ni/W deposit
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 8 9 4 - 2
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is only 1/40 that of typical 304 stainless steel (UNS S30400) used in the industry [8]. Recently developed electroplated Ni/W alloys with amorphous/nanocrystalline structure, showed that Ni/W alloys can also be used in the molding process of microfabrication system using the lithographic galvanic deposition technique (LIGA) [9]. Moreover, Ni/W coating may serve in future applications as the barrier layer between copper and silicon in the next generation of ultra large scale integration (ULSI) circuits and microelectromechanical system (MEMS) [10,11]. A typical bath for a Ni/W plating system consists of an aqueous solution of NiSO4, Na2WO4, and Na3Citrate. Sulfuric acid and ammonia are added to adjust the pH. As mentioned above, using this system, the tungsten content in the electroplated alloys is low. In order to increase it, a modi®ed plating bath has been developed recently [2], in which the W content of the alloy can be controlled in the range of 5±67 a/o [1±5]. The most signi®cant aspect of this modi®cation, from the material science point of view, is that it offers a perfect template for variation of the crystalline structure, from the typical fcc form characteristic of Ni, to amorphous, followed by the orthorhombic structure, with the increase of W content. Surface morphology is an index of the crystalline state of electroplated alloys. Detailed understanding of this property is essential, since it helps to de®ne the deposition conditions under which these materials can offer the expected crystalline structure, leading to the desired physical, chemical and mechanical characteristics. It is known that scanning tunneling microscopy (STM) and atomic force microscopy (AFM) can provide high resolution for observing the surface morphology. Applying these methods, it is possible to observe and analyze the precursors of the crystallization state and microstructure variations that may be exploited in new advanced materials applications. In this study we used STM/AFM and SEM to observe the morphological evolution of Ni/W alloys with increasing tungsten content and related it to the crystallographic structure determined by XRD. 2. Experimental Electroplating was conducted in baths containing NiSO4 (0.01±0.1 M), Na3Citrate (C6H5Na3O72H2O)
(0.6 M) and Na2WO4 (0.4 M). The pH was adjusted to the range of 6±8, using H2SO4 and NaOH. It should be noted that the concentrations were chosen so that the sum of the molar concentrations of NiSO4 and Na2WO4 never exceeded that of sodium citrate. A gold cylinder, 0.2 cm diameter and 1.0 cm length, rotated at 2000 rpm, served as the working electrode (cathode). A Platinum wire counter electrode and an Ag/AgCl/Cl (1 M) reference electrode were used. The applied current density was constant at 15 mA/ cm2 and deposition was conducted at room temperature. Before turning on the current, the baths were deserted with puri®ed nitrogen for 40 min. The composition of electroplated ®lm was determined with an energy-dispersive spectroscopy probe (EDS, Link Corp.) attached to a Model 6300 SEM (Jeol), using ZAF calculations. The accelerating voltage was 20 kV and the measuring distance from the sample was 15 mm. Each sample was measured at ®ve different locations to check for uniformity. Samples with low and medium W content (corresponding to fcc-Ni like and to amorphous structures, respectively), were 1.0± 10 mm thick. Samples with high W content (corresponding to the range of NiW up to NiW2) were only 0.2±0.4 mm thick. These differences in thickness were due to the widely different Faradaic ef®ciencies obtained during deposition of the alloys from different solutions. Uniformity of composition with depth was con®rmed by XPS and by SIMS measurements. XPS measurements were conducted with a 5600 MultiTechnique system (PHI). Time-of-¯ight secondary ion mass spectroscopy (TOF-SIMS) measurements were carried out employing a TRIFT III (Physical Electronics Inc.). X-ray diffraction patterns were obtained with copper Ka radiation on a 2Y powder diffractometer (Scintag). STM experiments were conducted ex situ, in constant current mode, with NanoScope II (Digital Instrument) at room temperature. Commercial Pt-Ir nanotips were employed in these measurements. Prior to the experiments, the instrument was calibrated using standard highly oriented pyro graphite (HOPG, Union Carbon). AFM experiments were conducted ex situ, in the contact mode, using a NanoScope III (Digital Instrument). Plated samples were washed in double distilled water (18 MO cm) and placed rapidly in a vacuum dessiccator, to minimize oxidation.
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3. Results 3.1. XRD detection The XRD diffraction patterns shown in Fig. 1 represent the crystal structures of Ni/W alloys in the range of 7±67 a/o W. It can be seen that alloys with 7± 20 a/o W have the polycrystalline structure of fcc-Ni. The XRD diffraction patterns of alloys with 22±37 a/o W exhibit a broad peak (halo), which is typical of amorphous metals, at the position of 2y 438. At still higher W content, an orthorhombic structure is indicated. In addition, two transition regions can be observed. For an alloy containing 20 a/o W, the intensity of the characteristic peak of fcc-Ni drops dramatically, as compared to that for 7 a/o W, and the peak at 2y 43 is broadened. This indicates that at this W concentration a partial transition from the fcc to the amorphous crystal structure has already taken place. The XRD patterns obtained for a Ni/W alloy with 40 a/ o W consists of lines typical of the orthorhombic structure of NiW and a broad halo peak typical of the amorphous alloy, showing a transition from one predominant structure to the other. 3.2. SEM measurement SEM images were taken for alloys with different concentrations of W and different crystalline structures. The SEM images of alloys containing 7 and 10 a/o W are shown in Fig. 2. It can be seen that the
Fig. 1. Total XRD diffraction patterns of the Ni/W alloys with different crystal structures.
surface is rather smooth, showing morphology typical of a polycrystalline surface. The alloy contains cracks. The cracks can cross the grains, indicating that they may be due to hydrogen embrittlement. The surface morphology changes dramatically when the W content of the alloy is higher, in the range of 22±37 a/o W, where the alloys have an amorphous structure, as shown by the XRD measurements in Fig. 1. The surfaces shown in Fig. 3 consist of globular regions with gaps in between. This indicates that no real
Fig. 2. SEM images of the Ni93W7 and Ni90W10 alloys with fcc crystal structure.
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Fig. 3. SEM images of the Ni78W22, Ni76W24, Ni74W26 and Ni66W34 alloys with amorphous structure.
boundaries exist among the globular regions, which is characteristic of amorphous materials. The small globules superimposed on the bigger ones show that the surface is nonuniform and is composed of different size domains. As the tungsten content in the alloy is increased, the surface is found to be smoother, revealing an initial tendency for crystallization. Cracks can be observed, but their density is low, compared to the alloys with fcc structure, since it is dif®cult for the cracks to propagate in amorphous materials lacking long-range order. The surfaces of alloys with more than 40 a/o tungsten change gradually back to the smooth-feature characteristic of crystalline alloys, as seen in Fig. 4. This is in agreement with the XRD results, showing a transition of the alloy from the amorphous to the
orthorhombic structure. Oriented scratches emerge on the surface, despite the fact that the surface was not polished, implying the existence of stress. The scratches may also be associated with plastic deformation of grains, due to lattice distortion, caused by an increase in the amount of W dissolved in the Ni lattice. It should be noted that all the alloys shown in Fig. 4 were deposited at a very low Faradaic ef®ciency, with abundant hydrogen evolution occurring as a side reaction. Thus, strain introduced by hydrogen absorption is to be expected. Electroplating of an alloy with a composition of NiW (i.e. 50 a/o W) has been reported for the ®rst time in our recent publications [2,3]. An alloy with even higher W content, corresponding to the composition of NiW2, (i.e. 67 a/o W) as shown in Fig. 4 was also prepared in our laboratory recently [5].
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Fig. 4. SEM images of the Ni50W50 and Ni33W67 alloys with orthorhombic structure.
The composition of this alloy, which is formed at very low Faradaic ef®ciency, was determined by EDS and XPS, but the characteristic crystal structure has not been con®rmed so far (cf. Fig. 1). 3.3. STM measurement 3.3.1. Morphology examined at the magni®cation of 270 nm 270 nm The dependence of surface morphology of Ni/W alloys on their W content was studied by STM on the nanoscale. A series of STM images obtained for Ni/W alloys with different W contents are shown in Fig. 5. The surface of Ni93W7 shown in Fig. 5a looks relatively smooth. The maximum difference of surface height is 4.5 nm and the standard deviation of surface
roughness is only 0.7 nm. It should be noted that the grain size in the crystalline region of the ®lm surface is usually in the range of 0.1±1 mm, and appears in the form of islands, clusters and even humps. The maximum height difference and the surface roughness are much larger than those mentioned above, indicating the uniformity as well as the nanosized grains in Ni/W ®lm. This is in good agreement with the XPS and XRD results published elsewhere [4]. Some micro-trenches marked with arrows can be observed. The depth of these micro-trenches is about 0.6±1.3 nm, as determined by a STM cross-section. These micro-trenches are the micro-cracks formed in the alloy, due to stress. The surface morphology of a Ni80W20 alloy is shown in Fig. 5b. The surface is ®lled with globular regions of different sizes, as compared with the
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Fig. 5. STM images of the morphologies of Ni/W alloys in the scanning scale of 270 nm 270 nm, (a) Ni93W7, (b) Ni80W20, (c) Ni67W33, (d) Ni60W40, (e) Ni50W50, (f) Ni33W67.
L. Zhu et al. / Applied Surface Science 200 (2002) 1±14
surface of the Ni93W7 alloy. This is consistent with the XRD result, which shows the beginning of a transition from crystalline to amorphous structure. The surface morphology of a Ni67W33 alloy shown in Fig. 5c is similar to the images of typical amorphous alloys reported in the literature [12,13]. It can be seen that the grains are distinct from each other and randomly distributed, showing boundary-free features without an ordered arrangement of grains, i.e. the alloy has the features typical of an amorphous alloy. In Fig. 5d, the globular regions characteristic of an amorphous alloy are still evident, but in addition, some areas (as shown in the right part of the image), have grains that are ®ne and well ordered, emerging on the surface. Two kinds of morphologies exist simultaneously on the surface of this alloy. This is in good agreement with XRD result, which also shows a transition from the amorphous to the crystalline orthorhombic structure at this W content. An STM image obtained for an alloy containing 50 a/o W is shown in Fig. 5e. The surface morphology is evidently different from the morphologies shown in Fig. 5a-d. Finely oriented, well-ordered grains are seen, typical of a crystalline structure. Note that the corresponding XRD pattern shows an orthorhombic crystalline structure. Fig. 5f, showing the surface morphology of a Ni33W67 alloy, exhibits features similar to those seen in Fig. 5e. Moreover, ¯ake structures of deposits on which the ®ne grains are arranged in rows are formed. 3.3.2. Morphology examined at the magni®cation of 10 nm 10 nm In order to observe the surface morphology evolution in more detail, a series of images at a higher magni®cation of 10 nm 10 nm were taken, for the same set of alloys, as shown in Fig. 6. Fig. 6a exhibits the surface of a crystalline Ni93W7 alloy. Diagonal arrangement of grains can readily be observed. Fig. 6b shows the surface of a Ni80W20 alloy, in which the transition from the fcc polycrystalline to the amorphous structure has just begun, as discussed above. It can be seen that the grain arrangement has no orientations. Grains with different sizes are mixed with each other. Fig. 6c demonstrates the surface of an amorphous Ni67W33 alloy. Ordered arrangement of domains or preferred directions were not observed. Fig. 6d exhibits the surface of a Ni60W40 alloy, where
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a second transition, from amorphous to crystalline (orthorhombic) structure has just started to emerge. Some regions with ordered grains can already be seen. However, the ordered grains do not cover the whole deposition area. This agrees with XRD result and the STM observation at the magni®cation of 270 nm 270 nm. Fig. 6e shows the morphology of a Ni50W50 alloy. In order to display the arrangement of grains clearly, the view pitch was changed in this particular case to be 608 during imaging. Well arranged ®ne grains are seen in the right part of this image. In the left part, the grain arrangement is distorted, indicating that strain and stress may exist and in¯uence the surface grain arrangement. Finally, in Fig. 6f we show the surface morphology of a Ni33W67 alloy. Slight aggregation of nanosized grains arranged in-line can be observed, which is in good agreement with the XRD result, showing that the crystal structure of Ni33W67 is orthorhombic. The curved rows of grains, which can be observed in almost the entire area of the surface, indicate strong surface strains and stresses. 3.4. AFM observation The surface of Ni/W alloys was also studied by AFM. The importance of using this technique is to verify the STM results and to show the real topography of the alloy surfaces. Fig. 7a and b show the surface topographies of Ni93W7 and Ni76W24 alloys, respectively. In Fig. 7a some periodic arrangements of the deposited grainaggregates can be seen in the 3D-view image. A diagonal surface orientation is indicated. This is in good agreement with the STM observation (cf. Fig. 5a). It is also consistent with the XRD data for this alloy. In Fig. 7b the large aggregates, observed also on the surface of the alloy with 7 a/o W, have the same diagonal orientation. However, ®ne aggregates emerge among the large grains. This indicates a mixed arrangement of the grains on the surface. Apparent ``hills'' and ``valleys'', which may be caused by structural transition [27], can also be observed. The surface topography of the two alloys is different, in agreement with the STM images and the XRD results. The image observed for the Ni67W33 alloy is presented in Fig. 8a, on a larger scale of 1:0 mm 1:0 mm. The humps seen near the sharp points consist of clusters of different sized domains, packed in a random fashion.
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Fig. 6. STM images of the morphologies of Ni/W alloys in the scanning scale of 10 nm 10 nm. Same compositions as in Fig. 5.
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Fig. 7. AFM 3D-view images of topographies of (a) a Ni93W7 alloy with fcc crystal structure, (b) a Ni76W24 alloy with amorphous structure, scanning size is 270 nm 270 nm.
This agrees well with the corresponding STM image. In addition, some hard dots (sharp points), represented by the peaks and columns, also exist on the surface, indicating that the surface is not uniform.
Fig. 8b shows an AFM image of a Ni63W37 alloy. Humps with the diameters ranging from less than 100 nm down to a few nanometers can be resolved. On the humps, regions of ¯uctuating small sized
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Fig. 8. AFM 3D-view images of topography of the alloys with amorphous structure, (a) Ni67W33, scanning size is 2 mm 2 mm, (b) Ni63W37, scanning size is 270 nm 270 nm.
deposits are seen. The height variation is less than 4 nm, re¯ecting the ¯atness of the surface of this alloy, as reported in the literature for amorphous alloys [28±30]. The depressions between the humps, which are typical of amorphous material, can be clearly observed [28±
30]. Even though no obvious periodic structure can be discerned, the in-line arrangement of grains, as well as the much higher grain density than that observed for alloys containing less tungsten, indicates the tendency for crystallization. Alloys with a tungsten content above
L. Zhu et al. / Applied Surface Science 200 (2002) 1±14
40 a/o have an orthorhombic crystalline structure. The AFM image for the alloy with 50 a/o W is shown in Fig. 9a, demonstrating a dramatic grain growth and a large increase of roughness. The boundaries between the grains are more pronounced on the surface, as expected for crystalline materials. Fig. 9b, taken of a Ni33W67 alloy, also shows a structure of ®ne grains, arranged in a few directions,
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similar to the feature observed in the STM image, indicating that the alloy has a crystalline structure, in agreement with the XRD results. 4. Discussion 4.1. Scratches on the surface An interesting feature that can be observed in the SEM images is the existence of well-oriented scratches on the surface of orthorhombic alloys. Usually, scratches can be found on polished surfaces. However, the samples in this work have not been polished. The well-oriented scratches appear mainly on the surface of alloys having an orthorhombic structure, but some are also seen on the surfaces of amorphous alloys with more than 30 a/o W. The scratches can generally be attributed to surface stress. The orientation of the scratches should follow the stress distribution. Suresh [14] regarded this kind of scratch as resulting from plastic deformation that is initiated at points with higher stress. When the surface is very ductile, such as the surface of fcc-Ni/W alloys, the existence of stress cannot affect the surface morphology to any signi®cant extent. In amorphous alloys, when surface stress density exceeds a certain limit, it cannot be relaxed completely via the free-boundary channel. This could explain why this kind of scratch also appears in amorphous alloys. It has already been shown in the present work that the grain size decreases with increasing W content, while the grain density increases. Thus, more plastic deformation occurs, leading to more scratches on the surface of the alloy. 4.2. Characteristics of Ni/W amorphous alloys, as revealed by STM and AFM
Fig. 9. AFM 3D-view images of topography of the alloys with orthorhombic structure, (a) Ni50W50, scanning size is 2 mm 2 mm (b) Ni33W67, scanning size is 2 mm 2 mm.
It is well known that STM is not an indicative analysis method used to determine whether the alloy is amorphous, unlike XRD and TEM. However, it is a powerful tool to be used to interpret amorphous nature on the nano/atomic scale [15±17]. Starting from the principle of STM, one can consider that in amorphous materials, during tip scanning, the topmost atoms do not lie on a common plane, as in crystalline materials. Elevated atoms hide their direct, lower-lying neighbors and show up in the z(x, y) direction as irregularly spaced ``small humps''. In addition, localized states
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may be formed in the vicinity of protruding atoms, thus enhancing the contrast [16]. For AFM operated in the contact mode, each randomly distributed individual grain yields almost the same scanning environment as its neighbor. Therefore, nonoriented surface can be observed in an AFM image. Based on literature [16±22] and our own ®ndings, some characteristic features describing the surface of amorphous materials can be identi®ed. The surface of Ni/W alloys with an amorphous structure can be observed when the tungsten content is between 20 and 40 a/o. One of the main surface features observed for several compositions is the globular feature and randomly distributed regions of grains, which exists on the surface, indicating the precursor of nonde®ned boundary and disordered grain arrangement (c.f. Fig. 5b and c). Humps and hillocks are also seen on the surface and are well
pronounced in the AFM images, as shown in Fig. 8. The surface cross-sections, measured for amorphous materials, should show nonperiodic features, as compared to crystalline structure. However, this does not ®t all the cases. In the case of 27 a/o W, shown in Fig. 10, the surface indeed shows much less periodicity than that measured for an alloy with 7 a/o W, which has the fcc structure (c.f. Fig. 1). The STM image shows a periodicity in the order of 10 nm, which is roughly consistent with the calculation of the effective particle size derived from the half-height width of the maximum in the X-ray diffraction scan by means of the Scherrer equation [4,31]. There is, however, no information regarding the crystalline structure in this analysis. Previous investigations on glassy metals prove that nanocrystals or small crystalline domains commonly form in the amorphous material matrix, while
Fig. 10. STM cross-section pro®le and the surface morphology image of a Ni73W27 alloy. Scanning size is 270 nm 270 nm. Hillocks presented by the excess shoulder part of each grain peak can be clearly seen in the cross-section pro®le.
L. Zhu et al. / Applied Surface Science 200 (2002) 1±14
the XRD diffraction only shows the amorphous halo [32±37]. The surface cross-section shows also a mixed con®guration. Hillocks, which appear frequently in epitaxial growth, are coupled with periodic peaks, which usually represent crystalline growth. The two types of grains merge together to form the common microstructure of amorphous alloys. This indicates that the amorphous structure can be formed by a mixture of two different structures: the fcc-Ni structure, which is observed in the alloys with low a/o of W and the Ni50W50 orthorhombic structure that is observed at high concentration of W. The depression and pinch regions between the bump/humps can also be regarded as precursor of the transition from amorphous to crystalline structure. 4.3. Where is W in the fcc alloy and where is NiW2 in the orthorhombic alloy? The XRD data shown in Fig. 1 for alloys having the fcc-Ni like structure have no peaks related to tungsten. Careful measurements of the shift in the Ni peaks with W content indicates that about 2 a/o of W may be in the form of a solid solution [1]. The rest may be associated with vacancies in the Ni crystal and possibly with deposition in grain boundaries. It is well known that elements segregated at the grain boundary are hard to detect by XRD. According to the phase diagram [23,38], the peak corresponding to the NiW2 phase should show up in the XRD pattern of Ni33W67 alloy, but these were not observed. This could be due to the combination of two factors: the location of these peaks is very close to the peaks for the orthorhombic NiW structure and the two may overlap [24±26]. Additionally, the ®lm of Ni/W alloy with 67 a/o W is very thin, due to a very low Faraday ef®ciency, making this phase hard to detect by XRD. 5. Conclusion A good template for studying the variation of crystalline structures in Ni/W alloys has been presented. The crystalline structure of Ni/W alloys changes from crystalline (fcc) to amorphous, and back to crystalline (orthorhombic) with increasing W content in the alloy. As an index of crystal structure, STM,
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AFM and SEM have been used to study the surface morphologies of alloys and their evolution with the increase of tungsten content. The morphology of the alloys with fcc structure shows cracks, probably caused by hydrogen embrittlement occurring during electrodeposition and microcracks due to the stress that exist on the surface. The surface morphology of alloys with amorphous structure exhibits globular regions that are separate from each other and disordered. An important ®nding is that a mixture of two crystalline forms can give rise to an amorphous structure. Hillocks that are usually a characteristic of epitaxial growth can also exist in the amorphous alloys. The surface morphology of the alloys with orthorhombic structure demonstrates well arranged ®ne grains with high density. The grain density of the surface increases with increasing W content in alloy. Well-oriented scratches caused by stress are also formed. Acknowledgements The authors wish to thank Dr. D. Porath of the School of Physics, Tel-Aviv University for useful discussion and Dr. Y. Rosenberg of the Wolfson Center for Materials Research of TAU for performing the XRD analysis. Financial support from the Ministry of Science Sports and Fine Art is gratefully acknowledged.
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[23] Phase diagram, ASM internation, The Material Information Society, vol. 3, 1992. [24] E. Epremian, D. Harker, Trans. AIME 185 (1949) 267. [25] J.M. Walsh, M.J. Donachie Jr., Metal. Trans. 4 (1973) 2854. [26] K.E. Poulsen, S. Rubek, E.W. Langer, Scr. Metal. 8 (1974) 1297. [27] H.X. Li, W.J. Wang, H.Y. Chen, J.F. Deng, J. Non-Cryst. Solid 281 (2001) 31±38. [28] A. Slawska-Waniewska, A. Witek, A. Reich, Mater. Sci. Eng. A133 (1991) 363±366. [29] Y. Watanabe, Y. Nakamura, J. Mater. Res. 7 (1992) 2126. [30] J.D. Ayers, J.H. Konnert, P. Dantonio, A. Pattnaik, C.L. Vold, H.N. Jones, J. Mater. Sci. 30 (1995) 4492. [31] A. Guinier, X-ray Diffraction, Freeman, San Francisco, 1963. [32] B.W. Corb, M. Ringger, H.-J. GuÈntherodt, Appl. Phys. Lett. 50 (1987) 353. [33] R. Walz, R. Wiesendanger, L. Rosenthaler, H.-J. GuÈntherodt, Mater. Sci. Eng. 99 (1988) 501. [34] R. Wiesendanger, M. Ringger, L. Rosenthaler, H.R. Hidber, P. Oelhafen, H. Rudin, H.-J. GuÈntherodt, Surf. Sci. 181 (1987) 46. [35] J.K. Gimzewski, R. MoÈller, S. Myhra, R.R. Schlittler, E. Stoll, J. Non-Cryst. Solids. 116 (1990) 253. [36] L. Scandella, U. Staufer, D. Brodbeck, P. Reimann, H.-J. GuÈntherodt, R. Zehringer, R. Hauert, E.M. Moser, Mater. Sci. Eng. A133 (1991) 601. [37] K. Habib, A. Abdullah, J. Mater. Sci. Lett. 9 (1990) 1055. [38] P. Nash (Ed.), Phase Diagrams of Binary Nickel Alloys, ASM International, Material Park, 1991.
STM/AFM studies of the evolution of morphology of electroplated Ni/W alloys L. Zhua,1, O. Younesb, N. Ashkenasya,2, Y. Shacham-Diamanda, E. Gileadib,* a
School of Engineering, Tel-Aviv University, Ramat-Aviv 69978, Israel School of Chemistry and Faculty of Engineering, Tel-Aviv University, Ramat-Aviv 69978, Israel
b
Received 20 November 2001; accepted 16 April 2002
Abstract The surface morphology evolution of Ni/W alloys was studied, as a function of the alloy composition. Using the modi®ed plating baths developed in our laboratory recently, electroplated Ni/W alloys with different W content, in the range of 7±67 atom percent (a/o), can be obtained. This was found to lead to different structures, ranging from polycrystalline fcc-Ni type structure to amorphous, followed by orthorhombic with increasing W content in the alloy. Powder XRD was studied to determine the crystal structures. Ex situ STM, AFM and SEM were used to study in detail the surface morphologies of the different alloys, and their evolution with increasing W content. The important ®ndings are that a mixture of two crystalline forms can give rise to an amorphous structure. Hillocks that are usually a characteristic of epitaxial growth can also exist in the amorphous alloys. Oriented scratches caused by stress can also be formed. Up to 20 a/o of W is deposited in the alloys in crystalline form, with the fcc-Ni type structure. Between 20 and about 40 a/o an amorphous structure is observed, and above that an orthorhombic crystal structure is seen, which is characteristic of the NiW binary alloy. Careful choice of the composition of the plating bath allowed us to deposit an alloy containing 67 a/o W, which corresponds to the composition NiW2. # 2002 Elsevier Science B.V. All rights reserved. Keywords: NiW alloy; Tungsten alloy; Amorphous alloys; STM; AFM; Alloy plating
1. Introduction Conventional Ni/W alloys are known as high-temperature alloys. One of the important uses of these alloys is to protect and extend the service life of * Corresponding author. E-mail address: [email protected] (E. Gileadi). 1 Present address: Institute of Chemical Processing, National Research Council of Canada, 1500 Montreal Rod, M-12, Rm220 Ottawa, Ont. Canada K1A 0R6. Tel.: 1-972-3-640-8694; fax: 1-972-3-9293. 2 Current address: Scripps Research Institute, La Jolla, CA, USA.
turbine blades, operating under conditions of high temperature and erosion. Usually, the concentration of W in the plated alloys is rather low, in the range of 5±15 atom percent (a/o), but it has a major in¯uence on the mechanical and chemical properties, such as hardness, wear-resistance and improved corrosion resistance at high temperature [1]. By increasing the tungsten content above 20 a/o, an amorphous structure is obtained [2±6]. Electroplated Ni/W alloys with amorphous structure exhibit excellent properties, such as high hardness, high thermal stability and high corrosion resistance [7]. It has been reported recently that the corrosion rate of an amorphous Ni/W deposit
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 8 9 4 - 2
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is only 1/40 that of typical 304 stainless steel (UNS S30400) used in the industry [8]. Recently developed electroplated Ni/W alloys with amorphous/nanocrystalline structure, showed that Ni/W alloys can also be used in the molding process of microfabrication system using the lithographic galvanic deposition technique (LIGA) [9]. Moreover, Ni/W coating may serve in future applications as the barrier layer between copper and silicon in the next generation of ultra large scale integration (ULSI) circuits and microelectromechanical system (MEMS) [10,11]. A typical bath for a Ni/W plating system consists of an aqueous solution of NiSO4, Na2WO4, and Na3Citrate. Sulfuric acid and ammonia are added to adjust the pH. As mentioned above, using this system, the tungsten content in the electroplated alloys is low. In order to increase it, a modi®ed plating bath has been developed recently [2], in which the W content of the alloy can be controlled in the range of 5±67 a/o [1±5]. The most signi®cant aspect of this modi®cation, from the material science point of view, is that it offers a perfect template for variation of the crystalline structure, from the typical fcc form characteristic of Ni, to amorphous, followed by the orthorhombic structure, with the increase of W content. Surface morphology is an index of the crystalline state of electroplated alloys. Detailed understanding of this property is essential, since it helps to de®ne the deposition conditions under which these materials can offer the expected crystalline structure, leading to the desired physical, chemical and mechanical characteristics. It is known that scanning tunneling microscopy (STM) and atomic force microscopy (AFM) can provide high resolution for observing the surface morphology. Applying these methods, it is possible to observe and analyze the precursors of the crystallization state and microstructure variations that may be exploited in new advanced materials applications. In this study we used STM/AFM and SEM to observe the morphological evolution of Ni/W alloys with increasing tungsten content and related it to the crystallographic structure determined by XRD. 2. Experimental Electroplating was conducted in baths containing NiSO4 (0.01±0.1 M), Na3Citrate (C6H5Na3O72H2O)
(0.6 M) and Na2WO4 (0.4 M). The pH was adjusted to the range of 6±8, using H2SO4 and NaOH. It should be noted that the concentrations were chosen so that the sum of the molar concentrations of NiSO4 and Na2WO4 never exceeded that of sodium citrate. A gold cylinder, 0.2 cm diameter and 1.0 cm length, rotated at 2000 rpm, served as the working electrode (cathode). A Platinum wire counter electrode and an Ag/AgCl/Cl (1 M) reference electrode were used. The applied current density was constant at 15 mA/ cm2 and deposition was conducted at room temperature. Before turning on the current, the baths were deserted with puri®ed nitrogen for 40 min. The composition of electroplated ®lm was determined with an energy-dispersive spectroscopy probe (EDS, Link Corp.) attached to a Model 6300 SEM (Jeol), using ZAF calculations. The accelerating voltage was 20 kV and the measuring distance from the sample was 15 mm. Each sample was measured at ®ve different locations to check for uniformity. Samples with low and medium W content (corresponding to fcc-Ni like and to amorphous structures, respectively), were 1.0± 10 mm thick. Samples with high W content (corresponding to the range of NiW up to NiW2) were only 0.2±0.4 mm thick. These differences in thickness were due to the widely different Faradaic ef®ciencies obtained during deposition of the alloys from different solutions. Uniformity of composition with depth was con®rmed by XPS and by SIMS measurements. XPS measurements were conducted with a 5600 MultiTechnique system (PHI). Time-of-¯ight secondary ion mass spectroscopy (TOF-SIMS) measurements were carried out employing a TRIFT III (Physical Electronics Inc.). X-ray diffraction patterns were obtained with copper Ka radiation on a 2Y powder diffractometer (Scintag). STM experiments were conducted ex situ, in constant current mode, with NanoScope II (Digital Instrument) at room temperature. Commercial Pt-Ir nanotips were employed in these measurements. Prior to the experiments, the instrument was calibrated using standard highly oriented pyro graphite (HOPG, Union Carbon). AFM experiments were conducted ex situ, in the contact mode, using a NanoScope III (Digital Instrument). Plated samples were washed in double distilled water (18 MO cm) and placed rapidly in a vacuum dessiccator, to minimize oxidation.
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3. Results 3.1. XRD detection The XRD diffraction patterns shown in Fig. 1 represent the crystal structures of Ni/W alloys in the range of 7±67 a/o W. It can be seen that alloys with 7± 20 a/o W have the polycrystalline structure of fcc-Ni. The XRD diffraction patterns of alloys with 22±37 a/o W exhibit a broad peak (halo), which is typical of amorphous metals, at the position of 2y 438. At still higher W content, an orthorhombic structure is indicated. In addition, two transition regions can be observed. For an alloy containing 20 a/o W, the intensity of the characteristic peak of fcc-Ni drops dramatically, as compared to that for 7 a/o W, and the peak at 2y 43 is broadened. This indicates that at this W concentration a partial transition from the fcc to the amorphous crystal structure has already taken place. The XRD patterns obtained for a Ni/W alloy with 40 a/ o W consists of lines typical of the orthorhombic structure of NiW and a broad halo peak typical of the amorphous alloy, showing a transition from one predominant structure to the other. 3.2. SEM measurement SEM images were taken for alloys with different concentrations of W and different crystalline structures. The SEM images of alloys containing 7 and 10 a/o W are shown in Fig. 2. It can be seen that the
Fig. 1. Total XRD diffraction patterns of the Ni/W alloys with different crystal structures.
surface is rather smooth, showing morphology typical of a polycrystalline surface. The alloy contains cracks. The cracks can cross the grains, indicating that they may be due to hydrogen embrittlement. The surface morphology changes dramatically when the W content of the alloy is higher, in the range of 22±37 a/o W, where the alloys have an amorphous structure, as shown by the XRD measurements in Fig. 1. The surfaces shown in Fig. 3 consist of globular regions with gaps in between. This indicates that no real
Fig. 2. SEM images of the Ni93W7 and Ni90W10 alloys with fcc crystal structure.
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Fig. 3. SEM images of the Ni78W22, Ni76W24, Ni74W26 and Ni66W34 alloys with amorphous structure.
boundaries exist among the globular regions, which is characteristic of amorphous materials. The small globules superimposed on the bigger ones show that the surface is nonuniform and is composed of different size domains. As the tungsten content in the alloy is increased, the surface is found to be smoother, revealing an initial tendency for crystallization. Cracks can be observed, but their density is low, compared to the alloys with fcc structure, since it is dif®cult for the cracks to propagate in amorphous materials lacking long-range order. The surfaces of alloys with more than 40 a/o tungsten change gradually back to the smooth-feature characteristic of crystalline alloys, as seen in Fig. 4. This is in agreement with the XRD results, showing a transition of the alloy from the amorphous to the
orthorhombic structure. Oriented scratches emerge on the surface, despite the fact that the surface was not polished, implying the existence of stress. The scratches may also be associated with plastic deformation of grains, due to lattice distortion, caused by an increase in the amount of W dissolved in the Ni lattice. It should be noted that all the alloys shown in Fig. 4 were deposited at a very low Faradaic ef®ciency, with abundant hydrogen evolution occurring as a side reaction. Thus, strain introduced by hydrogen absorption is to be expected. Electroplating of an alloy with a composition of NiW (i.e. 50 a/o W) has been reported for the ®rst time in our recent publications [2,3]. An alloy with even higher W content, corresponding to the composition of NiW2, (i.e. 67 a/o W) as shown in Fig. 4 was also prepared in our laboratory recently [5].
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Fig. 4. SEM images of the Ni50W50 and Ni33W67 alloys with orthorhombic structure.
The composition of this alloy, which is formed at very low Faradaic ef®ciency, was determined by EDS and XPS, but the characteristic crystal structure has not been con®rmed so far (cf. Fig. 1). 3.3. STM measurement 3.3.1. Morphology examined at the magni®cation of 270 nm 270 nm The dependence of surface morphology of Ni/W alloys on their W content was studied by STM on the nanoscale. A series of STM images obtained for Ni/W alloys with different W contents are shown in Fig. 5. The surface of Ni93W7 shown in Fig. 5a looks relatively smooth. The maximum difference of surface height is 4.5 nm and the standard deviation of surface
roughness is only 0.7 nm. It should be noted that the grain size in the crystalline region of the ®lm surface is usually in the range of 0.1±1 mm, and appears in the form of islands, clusters and even humps. The maximum height difference and the surface roughness are much larger than those mentioned above, indicating the uniformity as well as the nanosized grains in Ni/W ®lm. This is in good agreement with the XPS and XRD results published elsewhere [4]. Some micro-trenches marked with arrows can be observed. The depth of these micro-trenches is about 0.6±1.3 nm, as determined by a STM cross-section. These micro-trenches are the micro-cracks formed in the alloy, due to stress. The surface morphology of a Ni80W20 alloy is shown in Fig. 5b. The surface is ®lled with globular regions of different sizes, as compared with the
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Fig. 5. STM images of the morphologies of Ni/W alloys in the scanning scale of 270 nm 270 nm, (a) Ni93W7, (b) Ni80W20, (c) Ni67W33, (d) Ni60W40, (e) Ni50W50, (f) Ni33W67.
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surface of the Ni93W7 alloy. This is consistent with the XRD result, which shows the beginning of a transition from crystalline to amorphous structure. The surface morphology of a Ni67W33 alloy shown in Fig. 5c is similar to the images of typical amorphous alloys reported in the literature [12,13]. It can be seen that the grains are distinct from each other and randomly distributed, showing boundary-free features without an ordered arrangement of grains, i.e. the alloy has the features typical of an amorphous alloy. In Fig. 5d, the globular regions characteristic of an amorphous alloy are still evident, but in addition, some areas (as shown in the right part of the image), have grains that are ®ne and well ordered, emerging on the surface. Two kinds of morphologies exist simultaneously on the surface of this alloy. This is in good agreement with XRD result, which also shows a transition from the amorphous to the crystalline orthorhombic structure at this W content. An STM image obtained for an alloy containing 50 a/o W is shown in Fig. 5e. The surface morphology is evidently different from the morphologies shown in Fig. 5a-d. Finely oriented, well-ordered grains are seen, typical of a crystalline structure. Note that the corresponding XRD pattern shows an orthorhombic crystalline structure. Fig. 5f, showing the surface morphology of a Ni33W67 alloy, exhibits features similar to those seen in Fig. 5e. Moreover, ¯ake structures of deposits on which the ®ne grains are arranged in rows are formed. 3.3.2. Morphology examined at the magni®cation of 10 nm 10 nm In order to observe the surface morphology evolution in more detail, a series of images at a higher magni®cation of 10 nm 10 nm were taken, for the same set of alloys, as shown in Fig. 6. Fig. 6a exhibits the surface of a crystalline Ni93W7 alloy. Diagonal arrangement of grains can readily be observed. Fig. 6b shows the surface of a Ni80W20 alloy, in which the transition from the fcc polycrystalline to the amorphous structure has just begun, as discussed above. It can be seen that the grain arrangement has no orientations. Grains with different sizes are mixed with each other. Fig. 6c demonstrates the surface of an amorphous Ni67W33 alloy. Ordered arrangement of domains or preferred directions were not observed. Fig. 6d exhibits the surface of a Ni60W40 alloy, where
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a second transition, from amorphous to crystalline (orthorhombic) structure has just started to emerge. Some regions with ordered grains can already be seen. However, the ordered grains do not cover the whole deposition area. This agrees with XRD result and the STM observation at the magni®cation of 270 nm 270 nm. Fig. 6e shows the morphology of a Ni50W50 alloy. In order to display the arrangement of grains clearly, the view pitch was changed in this particular case to be 608 during imaging. Well arranged ®ne grains are seen in the right part of this image. In the left part, the grain arrangement is distorted, indicating that strain and stress may exist and in¯uence the surface grain arrangement. Finally, in Fig. 6f we show the surface morphology of a Ni33W67 alloy. Slight aggregation of nanosized grains arranged in-line can be observed, which is in good agreement with the XRD result, showing that the crystal structure of Ni33W67 is orthorhombic. The curved rows of grains, which can be observed in almost the entire area of the surface, indicate strong surface strains and stresses. 3.4. AFM observation The surface of Ni/W alloys was also studied by AFM. The importance of using this technique is to verify the STM results and to show the real topography of the alloy surfaces. Fig. 7a and b show the surface topographies of Ni93W7 and Ni76W24 alloys, respectively. In Fig. 7a some periodic arrangements of the deposited grainaggregates can be seen in the 3D-view image. A diagonal surface orientation is indicated. This is in good agreement with the STM observation (cf. Fig. 5a). It is also consistent with the XRD data for this alloy. In Fig. 7b the large aggregates, observed also on the surface of the alloy with 7 a/o W, have the same diagonal orientation. However, ®ne aggregates emerge among the large grains. This indicates a mixed arrangement of the grains on the surface. Apparent ``hills'' and ``valleys'', which may be caused by structural transition [27], can also be observed. The surface topography of the two alloys is different, in agreement with the STM images and the XRD results. The image observed for the Ni67W33 alloy is presented in Fig. 8a, on a larger scale of 1:0 mm 1:0 mm. The humps seen near the sharp points consist of clusters of different sized domains, packed in a random fashion.
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Fig. 6. STM images of the morphologies of Ni/W alloys in the scanning scale of 10 nm 10 nm. Same compositions as in Fig. 5.
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Fig. 7. AFM 3D-view images of topographies of (a) a Ni93W7 alloy with fcc crystal structure, (b) a Ni76W24 alloy with amorphous structure, scanning size is 270 nm 270 nm.
This agrees well with the corresponding STM image. In addition, some hard dots (sharp points), represented by the peaks and columns, also exist on the surface, indicating that the surface is not uniform.
Fig. 8b shows an AFM image of a Ni63W37 alloy. Humps with the diameters ranging from less than 100 nm down to a few nanometers can be resolved. On the humps, regions of ¯uctuating small sized
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Fig. 8. AFM 3D-view images of topography of the alloys with amorphous structure, (a) Ni67W33, scanning size is 2 mm 2 mm, (b) Ni63W37, scanning size is 270 nm 270 nm.
deposits are seen. The height variation is less than 4 nm, re¯ecting the ¯atness of the surface of this alloy, as reported in the literature for amorphous alloys [28±30]. The depressions between the humps, which are typical of amorphous material, can be clearly observed [28±
30]. Even though no obvious periodic structure can be discerned, the in-line arrangement of grains, as well as the much higher grain density than that observed for alloys containing less tungsten, indicates the tendency for crystallization. Alloys with a tungsten content above
L. Zhu et al. / Applied Surface Science 200 (2002) 1±14
40 a/o have an orthorhombic crystalline structure. The AFM image for the alloy with 50 a/o W is shown in Fig. 9a, demonstrating a dramatic grain growth and a large increase of roughness. The boundaries between the grains are more pronounced on the surface, as expected for crystalline materials. Fig. 9b, taken of a Ni33W67 alloy, also shows a structure of ®ne grains, arranged in a few directions,
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similar to the feature observed in the STM image, indicating that the alloy has a crystalline structure, in agreement with the XRD results. 4. Discussion 4.1. Scratches on the surface An interesting feature that can be observed in the SEM images is the existence of well-oriented scratches on the surface of orthorhombic alloys. Usually, scratches can be found on polished surfaces. However, the samples in this work have not been polished. The well-oriented scratches appear mainly on the surface of alloys having an orthorhombic structure, but some are also seen on the surfaces of amorphous alloys with more than 30 a/o W. The scratches can generally be attributed to surface stress. The orientation of the scratches should follow the stress distribution. Suresh [14] regarded this kind of scratch as resulting from plastic deformation that is initiated at points with higher stress. When the surface is very ductile, such as the surface of fcc-Ni/W alloys, the existence of stress cannot affect the surface morphology to any signi®cant extent. In amorphous alloys, when surface stress density exceeds a certain limit, it cannot be relaxed completely via the free-boundary channel. This could explain why this kind of scratch also appears in amorphous alloys. It has already been shown in the present work that the grain size decreases with increasing W content, while the grain density increases. Thus, more plastic deformation occurs, leading to more scratches on the surface of the alloy. 4.2. Characteristics of Ni/W amorphous alloys, as revealed by STM and AFM
Fig. 9. AFM 3D-view images of topography of the alloys with orthorhombic structure, (a) Ni50W50, scanning size is 2 mm 2 mm (b) Ni33W67, scanning size is 2 mm 2 mm.
It is well known that STM is not an indicative analysis method used to determine whether the alloy is amorphous, unlike XRD and TEM. However, it is a powerful tool to be used to interpret amorphous nature on the nano/atomic scale [15±17]. Starting from the principle of STM, one can consider that in amorphous materials, during tip scanning, the topmost atoms do not lie on a common plane, as in crystalline materials. Elevated atoms hide their direct, lower-lying neighbors and show up in the z(x, y) direction as irregularly spaced ``small humps''. In addition, localized states
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may be formed in the vicinity of protruding atoms, thus enhancing the contrast [16]. For AFM operated in the contact mode, each randomly distributed individual grain yields almost the same scanning environment as its neighbor. Therefore, nonoriented surface can be observed in an AFM image. Based on literature [16±22] and our own ®ndings, some characteristic features describing the surface of amorphous materials can be identi®ed. The surface of Ni/W alloys with an amorphous structure can be observed when the tungsten content is between 20 and 40 a/o. One of the main surface features observed for several compositions is the globular feature and randomly distributed regions of grains, which exists on the surface, indicating the precursor of nonde®ned boundary and disordered grain arrangement (c.f. Fig. 5b and c). Humps and hillocks are also seen on the surface and are well
pronounced in the AFM images, as shown in Fig. 8. The surface cross-sections, measured for amorphous materials, should show nonperiodic features, as compared to crystalline structure. However, this does not ®t all the cases. In the case of 27 a/o W, shown in Fig. 10, the surface indeed shows much less periodicity than that measured for an alloy with 7 a/o W, which has the fcc structure (c.f. Fig. 1). The STM image shows a periodicity in the order of 10 nm, which is roughly consistent with the calculation of the effective particle size derived from the half-height width of the maximum in the X-ray diffraction scan by means of the Scherrer equation [4,31]. There is, however, no information regarding the crystalline structure in this analysis. Previous investigations on glassy metals prove that nanocrystals or small crystalline domains commonly form in the amorphous material matrix, while
Fig. 10. STM cross-section pro®le and the surface morphology image of a Ni73W27 alloy. Scanning size is 270 nm 270 nm. Hillocks presented by the excess shoulder part of each grain peak can be clearly seen in the cross-section pro®le.
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the XRD diffraction only shows the amorphous halo [32±37]. The surface cross-section shows also a mixed con®guration. Hillocks, which appear frequently in epitaxial growth, are coupled with periodic peaks, which usually represent crystalline growth. The two types of grains merge together to form the common microstructure of amorphous alloys. This indicates that the amorphous structure can be formed by a mixture of two different structures: the fcc-Ni structure, which is observed in the alloys with low a/o of W and the Ni50W50 orthorhombic structure that is observed at high concentration of W. The depression and pinch regions between the bump/humps can also be regarded as precursor of the transition from amorphous to crystalline structure. 4.3. Where is W in the fcc alloy and where is NiW2 in the orthorhombic alloy? The XRD data shown in Fig. 1 for alloys having the fcc-Ni like structure have no peaks related to tungsten. Careful measurements of the shift in the Ni peaks with W content indicates that about 2 a/o of W may be in the form of a solid solution [1]. The rest may be associated with vacancies in the Ni crystal and possibly with deposition in grain boundaries. It is well known that elements segregated at the grain boundary are hard to detect by XRD. According to the phase diagram [23,38], the peak corresponding to the NiW2 phase should show up in the XRD pattern of Ni33W67 alloy, but these were not observed. This could be due to the combination of two factors: the location of these peaks is very close to the peaks for the orthorhombic NiW structure and the two may overlap [24±26]. Additionally, the ®lm of Ni/W alloy with 67 a/o W is very thin, due to a very low Faraday ef®ciency, making this phase hard to detect by XRD. 5. Conclusion A good template for studying the variation of crystalline structures in Ni/W alloys has been presented. The crystalline structure of Ni/W alloys changes from crystalline (fcc) to amorphous, and back to crystalline (orthorhombic) with increasing W content in the alloy. As an index of crystal structure, STM,
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AFM and SEM have been used to study the surface morphologies of alloys and their evolution with the increase of tungsten content. The morphology of the alloys with fcc structure shows cracks, probably caused by hydrogen embrittlement occurring during electrodeposition and microcracks due to the stress that exist on the surface. The surface morphology of alloys with amorphous structure exhibits globular regions that are separate from each other and disordered. An important ®nding is that a mixture of two crystalline forms can give rise to an amorphous structure. Hillocks that are usually a characteristic of epitaxial growth can also exist in the amorphous alloys. The surface morphology of the alloys with orthorhombic structure demonstrates well arranged ®ne grains with high density. The grain density of the surface increases with increasing W content in alloy. Well-oriented scratches caused by stress are also formed. Acknowledgements The authors wish to thank Dr. D. Porath of the School of Physics, Tel-Aviv University for useful discussion and Dr. Y. Rosenberg of the Wolfson Center for Materials Research of TAU for performing the XRD analysis. Financial support from the Ministry of Science Sports and Fine Art is gratefully acknowledged.
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