Electrocrystallization of magnetic Co–W–Mn films

Surface and Coatings Technology 135 Ž2000. 34᎐41

Electrocrystallization of magnetic Co᎐W᎐Mn films L. OrlovskajaU , E. Matulionis, A. Timinskas, V. ˇ Sukiene ˙ Institute of Chemistry, A. Gostauto st. 9, 2600 Vilnius, Lithuania Received 31 March 2000; accepted in revised form 5 June 2000

Abstract Magnetic Co᎐W᎐Mn films were deposited in a sulfate electrolyte in both the presence and the absence of sodium thiosulfate. When sodium thiosulfate is added, the cathodic process is depolarized, the rate of coating deposition increases and the phase composition of coatings, as well as their morphology, changes. In addition, very fine-grained Co᎐W᎐Mn films are characterized by enhanced luster, corrosion resistance, hardness, and lower coercive force and magnetic squareness. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: Electrodeposition; Magnetic Co᎐W᎐Mn films; Coating properties and structure

1. Introduction Among the various magnetic substances, coatings of cobalt alloys deposited both electrochemically w1᎐8x and by the electroless-plating method w9᎐12x are still of great interest, because it is possible to use them in the production of high-density information carriers. Magnetic Co᎐W᎐Mn coatings electrodeposited from sulfate electrolytes are distinguished for their high coercivity and the coefficient of hysteresis loop rectangularity w13᎐15x. However, they are electrodeposited at a low current efficiency ŽCE. and at low Ždown to 2.5᎐3.0 A dmy2 . cathodic current density ŽCD.. These coatings are not distinguished by a high corrosion resistance, hardness or by other physical and chemical properties necessary to use them as media for magnetic recording. The addition of sodium thiosulfate ŽT. to the electrolyte is known to have a positive effect on the properties of various electroplates, including metals of iron group w16,17x and their alloys w18x. The aim of the present work was to study the effect of sodium thiosulfate ŽNa2 S2 O3 ⭈ 5H2 O. additive on the process of magnetic U

Corresponding author. Fax: q370-261-70-18. E-mail address: [email protected] ŽL. Orlovskaja..

Co᎐W᎐Mn plate electrodeposition and on the properties of these coatings. It was also intended to obtain information on the composition and structure of the Co᎐W᎐Mn deposits developed in the study.

2. Experimental Magnetic Co᎐W᎐Mn films were plated from a bath containing cobalt sulfate Ž50 g ly1 .; manganese sulfate Ž100 g ly1 .; sodium tungstate Ž20 g ly1 .; magnesium sulfate Ž100 g ly1 .; boric acid Ž30 g ly1 .; and sodium thiosulfate, as an additive Ž0.1᎐1 g ly1 .. The bath Ž1 l volume. was maintained at pH 5, 40⬚C and was agitated with compressed air Ž150 l hy1 .. The coatings were plated onto electropolished plates of copper foil Žbrand M1. with a working surface of 1 = 1 cm as cathodes Žto record polarization curves and to study the phase composition of electrodeposits. and copper plates 0.7= 5 cm Žto study magnetic properties of the coatings.. High purity cobalt Žbrand KO. sheets were used as anodes. The coatings were plated at current densities Ž ic . chosen within the range from 0.1 to 8 A dmy2 . The thickness of the films was 0.4 ␮m Žto study magnetic

0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 7 3 0 - 1

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properties, luster and structure . and 30 ␮m wto study the coating corrosion resistance and Vickers microhardness Ž HV .x. The current efficiency Ž CE . of Co᎐W᎐Mn coatings was determined by coulogravimetry by means of a copper coulometer connected in series to the circuit. The Co᎐W᎐Mn coatings were deposited onto copper foil Žsurface area 12 cm2 . under constant current density. The equation CE s

QCo q QW q QMn 100% Q

Ž1.

was used to calculate the total current efficiency ŽCE. of deposited Co, W, and Mn; where Q was the quantity of electricity passed through the electrolyte and QCo , QW , QMn were the quantities of electricity necessary to reduce Co2q, W6q and Mn2q to the metallic state. QCo , QW and QMn were calculated on the basis of the data obtained by analytical determination of Co, W and Mn w19,20x in the magnetic films. This is the reason for the presumption that in separate experiments CE calculated in this way can exceed 100% on account of the formation non-metallic compounds. Polarization curves were recorded by a PDK-4 x᎐y chart potentiometer with the help of a P-5827 M potentiostat. The cathodic potentials, E, were measured relative to the silver᎐silver chloride reference electrode and recalculated to the normal hydrogen electrode. The elemental composition of freshly deposited Co᎐W᎐Mn coatings was studied by X-ray photoelectron spectroscopy ŽXPS. on an ESCALAB MK-11 spectrometer w21x using characteristic Mg K␣ radiation Žwith an energy of 1253.6 eV.. The surface of some samples was etched to a depth of 2᎐20 nm using Arq ions Žan ultra-high pure Arq beam at the etching rate of 2 nm miny1 .. The measured bond energies Ž Eb . of the elements were compared with their standard values w22x. The surface microstructure of the Co᎐W᎐Mn deposits was studied by an JXA-50A scanning microscope. Magnetic properties of the films were determined with a ferrometer by means of the oscillogram of the static hysteresis loop. Vickers microhardness was measured with a hardness meter PMT-3 under a 50-g load. The luster Ž L. of the films was measured by a FB-2 lustermeter. The corrosion rate Ž C . of the coatings was determined by dissolving them at 20⬚C in a 1:1 mixture of H2 SO4 Ž1%. and H3 PO4 Ž1%. in distilled water as well. 3. Results and discussion The introduction of sodium thiosulfate ŽT. additive to the electrolyte has a depolarizing effect on the process of Co᎐W᎐Mn electrodeposition ŽFig. 1a., as in the case of silver electrodeposition w23x. While

Fig. 1. Ža. Polarization curves recorded when depositing magnetic Co᎐W᎐Mn films on the Cu cathode at a potential scan rate of 2 mV sy1 . Žb. current efficiency Ž CE . as a function of current density Ž ic . in: Ž1. basic electrolyte; Ž2. electrolyte with 0.1 g ly1 sodium thiosulfate; and Ž3. electrolyte with 0.5 g ly1 sodium thiosulfate.

Co᎐W᎐Mn electroplates in the basic electrolyte start depositing at Es y0.53 V ŽFig. 1, curve 1., this process takes place in the electrolyte with additive T, with concentrations of 0.2 Žcurve 2. and 0.5 Žcurve 3. g ly1 at Es y0.43 V and y0.39 V, respectively, i.e. the cathodic process proceeds more easily. It should also be noted that the additive has a considerable effect on the current efficiency ŽCE. ŽFig. 1b.. While the CE does not exceed 60% in the basic electrolyte ŽFig. 1b, curve 1., and the electrodeposition of Co᎐W᎐Mn coating stops with an increase in ic to 3 A dmy2 , the current efficiency reaches 90% in the electrolyte with sodium thiosulfate additive ŽFig. 1b, curves 2 and 3., and it even exceeds 100% and more in other experiments. This fact is indicative of the inclusion of insoluble compounds into the electroplates. It should be noted that electrodeposition of Co᎐W᎐Mn coatings does not discontinue with an increase in ic to 3 A dmy2 Žcurves 2 and 3. and occurs at ic higher than 6 A dmy2 when sodium thiosulfate is added. Thus, the range of ic of electrodeposition of Co᎐W᎐Mn coatings increases up to more than 6 A dmy2 . It should also be noted that black powdery electroplates with a luster of L s 0% ŽFig. 2, curves 2᎐5. are deposited at the cathode in the zone of low ic Ž ic F 0.75 A dmy2 . from the electrolyte with sodium thiosulfate additive of G 0.2 g ly1 . However, the luster of Co᎐W᎐Mn coatings deposited from the additive-free

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L. Orlo¨ skaja et al. r Surface and Coatings Technology 135 (2000) 34᎐41

Fig. 2. The dependence of luster Ž L. on the cathodic current density for Co᎐W᎐Mn films Žthickness 0.4 ␮m. electrodeposited from: Ž1., the basic electrolyte; Ž2., 0.1 g ly1 sodium thiosulfate; Ž3., 0.2 g ly1 sodium thiosulfate; Ž4., 0.5 g ly1 sodium thiosulfate; and Ž5., 1 g ly1 sodium thiosulfate added.

electrolyte in the zone of low ic Ž0.2᎐0.5 A dmy2 . amounts to 72᎐75% ŽFig. 2, curve 1., whereas at higher ic , the luster of films becomes lower: at ic s 1.5 A dmy2 , L is approximately 58%, and at ic s 3 A dmy2 electrodeposition discontinues ŽFig. 1b, curve 1., as mentioned above. In the case of electrolyte containing T additive, the luster of electroplates deposited at higher ic Ž1᎐1.5 A dmy2 . with an increase in the additive concentration from 0.1 ŽFig. 2, curve 2. to 0.2 Žcurve 3. g ly1 , reaches 80%, and the range of ic for electrodeposition of bright films of good quality widens to 5 A dmy2 . A further increase in the concentration of sodium thiosulfate in the basic electrolyte widens the range of ic of black powdery electrodeposits, however, the brightness of coatings electroplated at ic G 1 A dmy2 does not improve ŽFig. 2, curve 5.. Thus there is no need to add more than 0.2 g ly1 Na2 S2 O3 ⭈ 5H2 O to the electrolyte. Making use of XPS to investigate the surface of Co᎐W᎐Mn coatings deposited in the additive-free electrolytes, it has been found that two peaks with a bond energy Eb s 778.1" 0.2 eV and 781.6" 0.2 eV are observed at the Co 2p 3r2 spectrum line ŽFig. 3a, curve 1.. The first peak corresponds to metallic cobalt, and the second one is related to compounds of CoO, CoŽOH.2 , or to a form of mixed CoWO4 oxide. Co hydroxides may appear at the coating surface due to alkalization of the electrolyte at the cathode layer as a result of hydrogen evolution in the course of electrolysis. The metallic cobalt peak alone is most often observed at a depth of 10 nm Žcurve 2.. According to the analysis data ŽTable 1, No. 1 and 2., 15.2᎐27.6 and 63.2᎐67.1% Co have been found on average at the surface and at a depth of 10 nm, respectively. Two peak-doublets with binding energies of Eb s 35.5" 0.2

and 37.0" 0.2 as well as Eb s 31.0" 0.2 and 33.5" 0.2 eV are observed at the W 4f electron spectrum line ŽFig. 3b. at the surface of coatings Žcurve 1. and at a depth of 10 nm Žcurve 2., respectively. The first peak is related to either WO3 andror compounds of CoWO4 , the second one is related to metallic W. The W content was found to be 4.2᎐9.7% at the coating surface and 4.2᎐12.1% at a depth of 10 nm ŽTable 1, No. 1 and 2.. The presence of metallic Co and W in the coating is most probably indicative of the formation of a hard W solution in Co. This has been observed in electrodeposition of other Co᎐W alloys w24᎐26x. It may be said that Mn in the Co᎐W᎐Mn coatings found in small Ž4.1᎐0.6%. quantities ŽTable 1. is in the form of MnO oxide, identified by an Eb s 341.5" 0.2 eV peak at the line of Mn 2p 3r2 ŽFig. 3c, curves 1᎐3, 2⬘., i.e. Mn does not, apparently, become a component of the alloy with cobalt and tungsten. The oxygen 01s spectrum with a binding energy peak at 531.3" 0.2 eV ŽFig. 3e, curves 1, 2. also confirms the presence of cobalt hydroxide and tungstate in the specimen. A considerable part of oxygen on the surface of Co᎐W᎐Mn coating Ž64.9 ᎐71.0%. may also be conditioned by its adsorption from the environment. A much lower amount of oxygen Ž24.1᎐28%. has been detected at a coating depth of 10 nm. The coatings under study seem to be fine grained Žcrystal size 0.05᎐0.1 ␮m, Fig. 4a.. In most cases, metallic cobalt has not been detected ŽFig. 3a, curve 3. on the surface of Co᎐W᎐Mn coatings deposited in the electrolyte containing sodium thiosulfate. Oxide᎐hydroxide Co compounds are identified by a peak on the line of Co 2p 3r2 spectrum with a binding energy of Eb s 781.6" 0.2 eV ŽTable 1, No. 3᎐6.. Metallic Co has been determined at a coating depth of 10 nm ŽFig. 3a, curve 4.. The amount increases with the increasing ic and concentration Ž cT . of the T additive ŽTable 1.. Neither metallic tungsten ŽFig. 3b, curves 3 and 4. nor metallic Mn ŽFig. 3c, curves 3 and 4. has been found in these coatings, at least at a depth of 10 nm. In accordance with photoelectron spectra peaks, the latter elements are in the form of oxides andror sulfides ŽTable 1.. Thus, they possibly do not form an alloy with cobalt and are only inclusions in the Co matrix and form composites w27x. It should be noted that quite a considerable amount of sulfur Ž2.1᎐6.5% at the coating surface and 4.6᎐5.8% at a depth of 10 nm. ŽFig. 3d. has been detected in these coatings ŽTable 1, No. 4᎐6.. The sulfur quantity ranges from 14.3 to 23.6% in black electrodeposits ŽTable 1, No. 3.. In addition, both sulfate sulfur Ž Eb s 168.5" 0.2 eV. and sulfide sulfur Ž Eb s 162.5" 0.2 eV. have been identified by the S 2p spectrum line on the coating surface. The latter peak confirms the possible presence of coat-forming sulfides ŽCo, W, Mn. ŽTable 1, No. 3. not only on the coating surface, but also at a depth of 10 nm ŽFig. 3d, curve 2..

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Fig. 3. The XPS spectrum lines for: Ža., Co 2p 3r2; Žb., W 4f; Žc., Mn 2p 3r2; Žd., S 2p; and Že., O 1s, recorded at the surface Žcurves 1 and 3. and at a depth of 2 nm Žcurve 2⬘. and 10 nm Žcurves 2 and 4. in Co᎐W᎐Mn coatings electrodeposited from additive-free electrolyte Žcurves 1,2 and 2⬘. and the same electrolyte with sodium thiosulfate ŽŽNa2 S2 O3 ⭈ 5H2 O. additive 0.2 g ly1 Žcurves 3 and 4..

Compact bright coatings ŽFig. 2, curves 2᎐5. are deposited at higher ic values Ž1.5᎐7 A dmy2 . in the electrolyte containing a sodium thiosulfate additive. The lines of the S 2p photoelectron spectrum recorded on the surface of the latter coatings and at a depth of 10 nm are similar to curve 2 shown in Fig. 3d, i.e. sulfate sulfur has not been detected. A high sulfur content and the appearance of black electroplate at the cathode at low ic values may apparently be accounted for by the formation of insoluble sulfides of the co-deposition metals ŽCo, W, Mn.. It is known w16,28x that a multi-step decomposition of sodium thiosulfate involving sulfite formation occurs in acid medium. Sulfite S may be reduced at the cathode to a sulfide ion w16x, which is capable of developing a slightly insoluble CoS, together with Co2q. The de-

crease in the sulfur content with the increase in deposition ic may be accounted for by the fact that, for example, the rate of reaction of electrochemical Co2q reduction at the cathode predominates over the rate of reaction of CoS formation. Fig. 4b᎐4f shows the changes in morphology of Co᎐W᎐Mn coatings deposited from the electrolyte containing sodium thiosulfate, depending on ic . Fairly big Ž2᎐3 ␮m. spherolytic crystallites were observed at the surface of black coatings deposited at ic s 0.2 A dmy2 . They were formed from smaller Ž0.02᎐0.1 ␮m. oval crystallites. When ic was increased to 0.7 A dmy2 , oval crystallites Ž0.05᎐0.3 ␮m. which had not grown together were observed at the surface of the coatings obtained. When ic was increased to 1.5 and 5 A dmy2 , the coatings deposited were so fine-grained that it was

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Table 1 Elemental and phase compositions of the surface layer of Co᎐W᎐Mn films, determined by X-ray photoelectron spectroscopy ŽXPS. No

c T Žg ly1 .

i c ŽA. dmy2 .

Element

On the surface

At a 10-nm depth

Element conc. Ž%.

Identified compounds and metals

Element conc. Ž%.

Identified compounds and metals

1

0.0

0.2

Co W Mn O S

27.6 4.2 3.3 64.9 0.0

Co, CoŽOH. 2 , CoO W, WO 3 , CoWO4 MnO

67.1 4.2 0.7 28.0 0.0

Co W, WO 3 , CoWO4 MnO

2

0.0

1.5

Co W Mn O S

15.2 9.7 4.1 71.0 0.0

Co, CoŽOH. 2 , CoO W, WO 3 , CoWO4 MnO

63.2 12.1 0.6 24.1 0.0

Co W, WO 3 , CoWO4 MnO

3

0.2

0.2

Co W Mn O S

15.0 0.8 5.0 64.9 14.3

CoO, CoŽOH. 2 , CoS WO 3 , WS 2 , CoWO4 MnO, MnS

31.0 1.3 3.6 40.6 23.6

Co WO 3 , WS 2 , CoWO4 MnO, MnS

4

0.2

1.5

Co W Mn O S

15.1 2.0 1.4 75.9 5.6

CoO, CoŽOH. 2 , CoS WO 3 , WS 2 , CoWO4 MnO, MnS

51.3 2.1 1.0 39.8 5.8

Co WO 3 , WS 2 , CoWO4 MnO, MnS

5

0.2

4.0

Co W Mn O S

11.3 2.9 1.7 82.0 2.1

CoO, CoŽOH. 2 , CoS WO 3 , WS 2 , CoWO4 MnO, MnS

59.2 5.9 0.9 29.4 4.6

Co WO 3 , WS 2 , CoWO4 MnO, MnS

6

0.5

1.5

Co W Mn O S

15.1 1.4 2.1 74.9 6.5

CoO, CoŽOH. 2 , CoS WO 3 , WS 2 , CoWO4 MnO, MnS

60.8 7.2 0.7 25.9 5.4

Co WO 3 , WS 2 , CoWO4 MnO, MnS

impossible to measure the size of most crystals ŽFig. 4d,e.. These coatings are much more fine-grained than Co᎐W᎐Mn coatings deposited from an additive-free electrolyte ŽFig. 4a.. With ic s 6 A dmy2 , the luster of coatings decreases ŽFig. 2, curve 3., and separate, bigger crystallites of various forms appear on their surface ŽFig. 4f.. Changes in the chemical composition and structure of Co᎐W᎐Mn coatings under the influence of T additive in the electrolyte also lead to changes in magnetic properties and hardness ŽTable 2., as well as to changes in the corrosion resistance ŽFig. 5.. Fairly soft Ž HV s 168 kgf mmy2 . Co᎐W᎐Mn coatings with a coercive force Ž Hc . and magnetic squareness of 600 Oe and 0.81, respectively ŽTable 2, row 1. were deposited from the additive-free electrolyte. After adding sodium thiosulfate Ž0.1᎐2 g. ly1 . to the electro-

lyte, hard coatings Ž HV s 817᎐520 kgf mmy2 . were electrodeposited ŽTable 2, rows 2᎐6.. The Hc and S values for these are considerably lower Ž Hc s 354᎐151 Oe, S s 0.77᎐0.63. than those of the previous coatings. The corrosion resistance of the coatings being deposited changes as well. The corrosion rate Ž C . of the coatings electrodeposited in the additive-free electrolyte ŽFig. 5, curves 1 and 3. is several times ŽG 3. higher than that of the coatings in the electrolyte with additive Žcurves 2 and 4., i.e. the addition of sodium thiosulfate considerably increases the corrosion resistance of deposits. A sudden decrease in Hc and S in Co᎐W᎐Mn coatings deposited from the electrolyte with sodium thiosulfate additive can probably by accounted for by the change in both the composition of the coatings deposited and their morphology. Firstly, no metallic W

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Fig. 4. Micrographs of Co᎐W᎐Mn films electroplated at: Ža. ic s 1.5 A dmy2 in additive-free electrolyte; Žb. ic s 0.2 A dmy2 in electrolyte with 0.2 g ly1 sodium thiosulfate; Žc. ic s 0.7 A dmy2 in electrolyte with 0.2 g ly1 sodium thiosulfate; Žd. ic s 1.5 A dmy2 in electrolyte with 0.2 g ly1 sodium thiosulfate; Že. ic s 5 A dmy2 in electrolyte with 0.2 g ly1 sodium thiosulfate; and Žf. ic s 6 A dmy2 in electrolyte with 0.2 g ly1 sodium thiosulfate.

has been found in these coatings ŽTable 1, No. 3᎐6.. Secondly, sulfides of all the deposited metals have additionally been identified in the coatings. Table 2 The coercive force Ž Hc ., magnetic squareness Ž S ., microhardness Ž HV. of Co᎐W᎐Mn coatings, deposited at i c s 1.5 A. dmy2 under different concentrations of Na 2 S 2 O 3 ⭈ 5H 2 O Ž c T . in the bath No.

c T Žg ly1 .

Hc ŽOe.

S

HV Žkgf mmy2 .

1 2 3 4 5 6

0 0.1 0.2 0.5 1.0 2.0

600 354 275 206 199 151

0.81 0.77 0.76 0.69 0.64 0.63

168 817 941 652 640 520

Though fine-grained coatings ŽFig. 4a. are deposited from the additive-free electrolyte, their luster amounts to only 58% ŽFig. 2, curve 1.. More finely-grained and much more compact coatings ŽFig. 4d,e. with L s 80% have been deposited from the electrolyte with sodium thiosulfate additive at ic s 1.5᎐5 A.dmy2 . The colloidic parts of metals sulfides which adsorb onto the surface of the growing deposit, and which make the free growth of crystals more difficult w29x, are conducive to the formation of more lustrous and compact coatings. Colloidic CoŽOH.2 , which forms during electrolysis due to hydrogen evolution in alkalization of the precathodic layer w30x, acts in the same way. Bright galvanic coatings of most metals are known to be harder than mathic ones, and their hardness increases with the

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L. Orlo¨ skaja et al. r Surface and Coatings Technology 135 (2000) 34᎐41

; 10 nm. is in the form of oxide, and is held to be an inclusion in the coating. Metallic Co is found in the surface layers of coatings Žat a depth of ; 10 nm. deposited from the electrolyte containing sodium thiosulfate additive; however, W and Mn are present in the form of oxides andror sulfides. When a sodium thiosulfate additive was used, the Co᎐W᎐Mn coatings were characterized by an enhanced luster, corrosion resistance, hardness, and lower coercive force and magnetic squareness.

Acknowledgements Fig. 5. Corrosion rate Ž C . of Co᎐W᎐Mn coatings electroplated at ic s 1.5 A dmy2 from additive-free electrolyte Žcurves 1 and 3. and from the same electrolyte with sodium thiosulfate ŽNa2 S2 O3 ⭈ 5H2 O. additive 0.2 g ly1 Žcurves 2 and 4.; C was determined by dissolving the coating at 20⬚C in distilled water Žcurves 1 and 2. and in a 1:1 mixture of H2 SO4 Ž1%. and H3 PO4 Ž1%. Žcurves 3 and 4..

decrease in the crystal size of fine-grained deposit. Foreign insertions, which are abundant in bright coatings, also increase HV w29x. In the case of the Co᎐W᎐Mn coatings under investigation, Co, W, and Mn sulfides, oxides and hydroxides act as such insertions. Similar causes perhaps determine the lower dissolution rate C of the coatings being deposited, both in distilled water and in the solution of diluted sulfuric and phosphoric acids. Most of the said inserts wWS, WO3 , MnS, CoS, CoŽOH.2 , CoO, MnOx that are present in these highly compact coatings are quite, or nearly, insoluble in water and ŽWO3 , WS2 . in inorganic acids w31,32x. Hence, with the addition of sodium thiosulfate to the electrolyte, the increase of insoluble inserts in the deposits and the increase in compactness of the coatings, the dissolution rate of the Co᎐W᎐Mn coatings under investigation decreases, i.e. the corrosion resistance of these coatings increases.

4. Conclusions Magnetic Co᎐W᎐Mn plates in the sulfate electrolyte were deposited both in the presence and the absence of sodium thiosulfate ŽNa2 S2 O3 ⭈ 5H2 O. ŽT. additive. The introduction of T additive to the electrolyte has a depolarizing effect on the process of Co᎐W᎐Mn electrodeposition: the rate of coating deposition increases, and the phase composition, as well as the morphology changes. Metallic Co and metallic W form an alloy in Co᎐W᎐Mn coatings deposited from an additive-free electrolyte, and a negligible amount of manganese found at the surface layers of the coating Žat a depth of

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