The composition and bonding characteristics of the alloying elements in electrodeposited NiCr alloys

THE COMPOSITION OF THE ALLOYING

AND BONDING CHARACTERISTICS ELEMENTS IN ELECTRODEPOSITED Ni-Cr ALLOYS

I. LE R. STRYDOM

and G. N. VAN WYK*

Department of Physics, University of the Orange Free State, Bloemfontein, 9300, Republic of South Africa (Received

4 January

1983; in revised form 9 May 1983)

Abstract-Electrodeposited Ni-Cr alloys of different compositions were studied by Auger Electron Spectroscopy (AES)and Electron Spectroscopy for Chemical Analysis (ESCA). It was found that all the chromium in the alloy was present as Cr,O,, while nickel was present in the metallic form. The influence of plating parameters on the compositions of the alloy and the efficiency of the deposition process was investigated in detail. The nature of inclusions in the deposit was also studied and quantitatively compared with the impurities found in thermally prepared Nixr alloys.

lNTRODUffION It is well known that many physical properties of electrodeposited alloys are quite different from conventionally prepared alloys of similar composition[ I]. Variations in the deposition conditions may cause substantial changes in the composition and structural properties of the alloys[2]. Contaminants from the plating bath often appear as inclusions in the deposit and influence the segregation and diffusion mechanisms of the alloying elements. The enhancement or inhibition of the diffusitivity of the alloying elements as well as the inclusions has a bearing on several properties such as oxidation, corrosion, hardness and wear resistance. A better understanding of the segregation phenomena in these electrodeposited alloys will contribute a great deal to the study of mechanical or electrical properties of the alloys. A study was undertaken to investigate segregation and diffusion of the alloying elements and inclusions in electrolytically prepared Ni
l

To whom all correspondence

should b-e addressed.

on the composition of the alloy comparing with the results of Shenoi and Gowri[4]. In this paper these results are reported and discussed in terms of the existing models. The usefulness of AES and ESCA in the characterization of electrodeposited materials will also be illustrated.

EXPERIMENTAL (i) Specimen prrparation

In a general review of the deposition of chromium alloys, Spencer[5] described several non-successful NiCr dating baths. Brenner et al.r61 reoorted that the dedosit obtained from a chromkacid bath contained very little chromium and thaj the deposit was not free from non-metallic inclusions. Only recently a successful Ni-Cr plating bath was developed by Shenoi and Gowri[4] from a trivalent chromium bath yielding sound, adherent deposits. Therefore it was decided to use it in this investigation. The composition is as follows: KCr(S04),

45OgI-‘,

Ni(COOH),

-lOOgl~‘,

Na,C,H,O,

-5OgI-‘,

WO,

-50

g1-‘,

NaF

-13g1-1,

C&N%

-25

gl-‘.

The functions of the constituents of the bath are described in an article by Gowri et al.[7]. It is interesting to note that with a trisodium citrate concentration of lOOgl_‘, which is the upper limit suggested by Shenoi, it was not possible to obtain a deposit with a high chromium content. The pH of the solution was maintained at a value of 2.6, however pH is not considered as a critical parameter as far as the composition of the alloy is concerned[4]. The current density was varied between 2 and 27 Adm-’ and the temperature between 15and 62°C. The

1817

I.

1818

LE

R.

STRYWM

dependence of the composition of the alloy on current density was measured at an electrolyte temperature of 20°C and the temperature dependence was determined at a current density of 8 Adn-“. The electrolyte was constantly agitated during the plating process by means of a small magnetic stirrer. The alloys were plated on circular cathodes cut from rolled Cu foil which were chemically etched to remove surface contaminants. X-ray measurements revealed a (I 10) preferential orientation in the Cu foil. A single nickel anode was used because of the unavailability of Ni-Cr alloys of suitable compositions. This, however, did not alter the composition of the solution appreciably because of the short periods of actual plating. To fully utilize the advantages of studying the specimens by means of AES and ESCA, a number of layers were deposited on the substrate consecutively. The difference between the subsequent layers is that the deposition parameters such as current density or temperature were varied from one layer to the other. The final specimen therefore consisted of a multiple “sandwich” of 3-5 layers of thickness 100 nm but with different compositions (see Fig. 1). The layers were then studied in the UHV system by sputtering with Ar+ ions through the different layers and analyzing the newly exposed surface by means of ESCA and AES. A typical depth profile from a layered specimen is shown in Fig. 2. The advantages of AES for the investigation of layers with different compositions are clearly illustrated.

AND G. N.

VAN WYK

5-

rnPuyER

I

0

20

I

40

SECONDLAYER

I

60

80

1

100

I

I20

SPUTTERING TIME~minutes)

Fig. 2. A depth profile of one of the specimens consisting of 4 layers is shown, obtained by plotting the peak-to-peak height of the Auger signal from the elements shown US the time sputtered with 4 kV Ar+ ions. This time-scale thus corresponds with the depth-scale measured from the surface of the specimen. Note the correspondence of the oxygen peakto-peak height, with the chromium peak-to-peak height, indicating that the amount of oxygen present is to be associated with the chromium content in each layer.

RESULTS AND DISCUSSION (i) Formation

ofchemicalbonds

involving the alloying

(ii) Measuring apparatus

elements

The specimens were mounted in the UHV chamber of a physical Electronics Model 545 Auger/ESCA system which was interfaced with a minicomputer for data analysis. The ultra high vacuum (lo- lo torr) was obtained by adsorption roughing pumps and an ion pump assisted by a titanium sublimation pump. A double-pass cylindrical mirror analyzer was used to measure the energies of the Auger and photoelectrons. A 5 keV Ar+ ion gun was used to sputterclean the surface of the specimen prior to analysis and to sputter-through the different layers to do the depth profiling.

The binding energies of the nickel and chromium 2p,,, electrons in electrodeposited Ni-Cr were determined by ESCA measurements and compared with the corresponding binding energies in a conventionally prepared Ni
ELECTRON BEAM I A:-ETCHEDORATER / LAY

ml ER3

J Fig. 1. A diagram showing the deposited layers of different compositions on the Cu-substrate. The Ar*-etched crater exposing the different layers to be analyzed, is also shown. The thickness of each deposited layer is approx. 100 nm.

form. The situation

is different for the binding energy of the Cr 2p,,, electrons. The conventional alloy yielded a value of 574 eV but for the electrolytic alloy a value of 577 eV was measured, ie a shift of 3 eV. This binding energy, as well as the shape of the ESCA peak (Fig. 3(b)) corresponds empirically with the situation of chromium in the form of Cr,O,[S]. The ESCA measurements also revealed an oxygen peak at an energy value of 530.7 eV which corresponds more closely to the expected value of 530.3 eV for Cr,O, than the expected value of 529.7 eV for Ni0[8]. The presence of oxygen in the bulk of the electrolytic alloys was confirmed by AES depth profiting of the different layers. The oxygen peak was observed at a kinetic energy value of 511 eV. The peak-to-peak height of the Auger signal (APPH) was measured for oxygen, Ni and Cr and the Palmberg method[9] was used to calculate the relative percentages of the elements. The results indicated no correlation between the nickel and oxygen concentrations, but a chromium to oxygen ratio of roughly 2 : 3 was found throughout

Alloying elements in electrodeposited

NiCr

1819

alloys

lxygen Is -peaks

Ni-0

~slectrolytic~

CP3 (Rafenncna)

\

o?drencc8)

BINDING ENERGY(a’/ )

I

BINDING ENERGY (eV)

(c) Fig. 3.(a) The shape of the Ni 2p,,, ESCA peaks with the bindingenergypositionof eachpeak indicated.(b) The shape of tbe Cr Zp,,, ESCA peaks. (c)The shape of the 0 1s ESCA peaks. No oxygen peak was observed for the thermally

prepared Ni-Cr alloy. ihrumium 2p3 -e 4

57%2av I

Ni-Cr

BINDING ENERGY (eV)

the bulk of the material, indicating once more that the form of the oxide is Cr,O,. The results described above show convincingly that nickel exists as a pure metallic form in the electrodeposited alloy, but that chromium forms the oxide Cr,O,. No trace of pure metallic chromium was observed, indicating that the chromium was fully oxidized to Cr,O,. A study of the available literature revealed no evidence of this type of behaviour for this particular alloy. It is well known that a Cr,O, film forms on the surface of conventionally prepared alloys, but does not persist throughout the bulk. Brenner[lO] reported small amounts of Cr,O, (m 2%) in electrodeposited Fe-0 alloys, indicating that most of the chromium is in a pure form for this alloy. Subsequent high temperature studies[3] revealed that the oxide persists at temperatures of up to 700°C which is a further indication ofa stable metal oxide in the deposit. The presence of chromium oxide in the bulk of the material is believed to be connected to the low current efficiency of chromium deposition. The remainder of the current is carried by hydrogen ions which discharge at the surface of the cathode in the form of hydrogen gas. This increase in hydrogen ion concentration Causes a sharp decrease of pH near the surface of the cathode. Precipitation of basic compounds of chromium then occurs on the surface. The chromium,

I. LE R. STRYDOMAND G.

1820

N.

VAN WYK

Table 1. The measured binding energies of the Cr and Ni Zp,,, and 0 Is photoelectrons from electrodeposited and thermally prepared Ni-Cr alloys in comparison with values for pure Ni and Cr, Cr,Oa and NiO given hy[8] Cr &‘,,z

0 1s

853.1 eV

517.2 eV

530.7 eV

852.9eV 852.3eV

574.5eV 574.I eV (Cr)

530.3eV(Cr,O,)

576.6 eV(Cr,O,)

529.7 eV (NiO)

Ni &JWZ

Electrodeposited Ni-Cr Thermally prepared Ni-Cr Ref.[8]

rather than nickel, ions bind with the oxygen ions present in the solution because of the lower electronegativity of chromium relative to nickel. This marked difference in chemical bonded states between electrodeposited and thermally prepared NiXr could be the cause of differences in physical properties and should be considered in discussions on the behaviour of these alloys. (ii) The inq7uence ofplating parameters on composition (a) Current density.

The composition of the alloy, ie the percentage of each metal present in the deposit, as a function of current density, is shown in Fig. 4. The chromium content increases with increasing current density; a 5s-50 composition occurs at 11.5 A dm-‘. These results are in accordance with the behaviour of alloys of the regular codeposition group (as classified by Brenner) and are also in agreement with results[4] reported earlier. The increase in chromium content with increasing current density is explained by the fact that chromium is the less noble metal of the two constituents, ie it has the more negative electrode potential. An increase in current density shifts the electrode potential of the system to a more negative value, thereby favouring the deposition of chromium. (b) Temperature ofthe electrolyte. The variation of the composition of the alloy with temperature is shown

in Fig. 5. An increase in temperature increases the nickel content in the deposit. This occurs because an increase in temperature decreases the overpotential of nickel to a greater extent than that of chromium due to the dependence of the Nemst equation on temperature and concentration. The difference between the nickel and the chromium electrode potentials increases and thus favours nickel deposition. These results are in agreement with the work of Shenoi. (c) The cathode current e@ciencies. The cathode efficiencies of Ni, Cr and the alloy were calculated by means of Brenner’s form&( 11) current

E,(alloy) = T z,

I

where E,is the current efficiency of the alloy, P, is the weight percentage, mi the mass and Qi the electrochemical equivalent of metal i. I and t represent the current and deposition time respectively. In Fig. 6 the cathode current efficiencies are shown as a function of current density and in Fig. 7 as a function of temperature. It is clear that an increase in current density causes an increase in the current efficiency of the chromiumcontent, while that of nickel remained constant, thereby increasing the efficiency of deposition of the alloy. Our results in this respect differ from those of Shenoi and Gowri[4], as they found that the efficiencies of both nickel and chromium remained

Cr

Ni

CURRENT DENSITY (A/d&

Fig. 4. The relative percentages of each metal present in the deposit normalized to 100% is shown as fuctions ofcurrent density. Note the sharp increasein Cr content at higher

current densities.

TEMFERAlURER3 Fig. 5. The relative percentages of each metal present in the

deposit normalized to 100% is shown as functions of the electrolyte temperature. Note here the increase in Ni content at higher temperatures.

Alloying elements in electrodeposited

Ni-Cr alloys

1821

Within detection limits, no carbon or sulphur could he found in a thermally prepared Ni-Cr alloy containing 80% Ni and 20 % Cr. This indicates that the impurities detected in the electrodeposited alloys are non-metallic inclusions originating from the plating bath.

CONCLUSIONS

Ni 20

IO

I 3f

CURRENT DEN!51TY(A/drn2) Fig. 6. The calculated cathode current efficiencies for both metals and the alloy as fimctions of the current density. The efficiency of Cr deposition shows an increase with current density because the electrolyte was agitated during plating.

TOMPERATURE(“C)

(i) The relative ease with which AES and ESCA are used to study the nature of the chemical states in electrodeposited alloys in comparison with alloys prepared thermally, is clearly illustrated. (ii) The electrodeposition of NiXr alloys is shown to be of the regular codeposition type regarding the influence of current density and temperature on the composition of the alloy. (iii) Development ofa plating bath for NiXr alloys having much greater current efficiency than the present bath, can alter the measured phase structure of CrtO, and metallic nickel appreciably and warrants further investigation. (iv) The presence of basic inclusions in the deposit, which could have a considerable influence on the diffusitivity of the alloying elements and the codeposits, is easily recognized using the AES technique. At present the segregation of several elements in Ni-43 deposits of different composition is being investigated and compared with results[3] from thermally prepared Ni-Cr alloys. Acknowledgements-The financial assistance from the Central Research Fund of the University of the Orange Free State as well as the Department of University Research of the Council for Scientific and Industrial Research is gratefully

acknowledged. Fig. 7. The calculated cathode current efficiencies for both metals and the alloy as functions of the electrolyte

The National Physical Research Laboratory of the C.S.I.R. is also thanked for financially supporting one of us (I.S.).

temperature.

with current density. They did not, however, mention that they agitated the electrolyte, whereas we did during plating. It is therefore suggested that a further increase in the efficiency of alloy deposition can be obtained by introducing rotating, cylindrical elecconstant

trodes, thereby agitating This could also diminish bubbles at the cathode.

the electrolyte uniformly. the formation of hydrogen

The calculated current efficiencies as functions of temperature once again are in good agreement with Shenoi’s[4]

results.

(ii!) Inclusions

in the deposits

of oxygen which was described in the previous section, the specimens were also analysed for other inclusions. The detection limit of the Auger technique is in the order of 1 %, therefore only impurities present in quantities of over 1% are reported. With this restriction in mind, our results indicate the presence of only carbon (12 at.%) and sulphur (1 at. %). Apart

from

the

presence

REFERENCES 1. K. M. Gorbunova and Y. M. Polukarov, Electrodeposition of alloys, in Advances in Elrctrwhrmistry and Electrochemical Engineering (Edited by C. W. Tobias) Vol. 5, p. 249. Interscience, New York (1967). 2. C. L. Faust, Principles of alloy plating, in Modern Electroplaling (Edited by F. A. Lowenheim). John Wiley, New York (1963). 3. I. le R. Strydom and G. N. van Wyk, S. Afr. J. Phys., in

press. 4. B. A. Shenoi and S. Gowri, Metol Finish. 70, 96 (1972). 5. L. F. Spencer, MetalFinish. 60, 48 (1962). 6. A. Brenner, P. Burkbardt and C. Jennings, 3. Res. Nar. Bur. Stds 40, 31 (1948).

7. S. Gowri, P. L. Elsie and B.A. Shenoi. Metal Finish. 65,67 (1967). 8. Handbook ofX-ray Photoelectron Spectroscopy. PerkinElmer Corporation (19791. 9. Handbook of Auger Electron Spectroscopy. PerkinElmer Corporation (1976). 10. A. Brenner, Electrodeposition of Alloys, Vol. 11, p. 116. Academic Press, New York (1963). 11. A Brenner, ibid. p, 149.