Adnane 1

Composite layers in Ni±P system containing TiO2 and PTFE

B. èosiewicz, A. SteËpienÂ, D. Gierlotka, A. Budniok*

Institute of Physics and Chemistry of Metals, University of Silesia, 40-007 Katowice, Bankowa 12, Poland

Received 16 July 1998; received in revised form 13 January 1999; accepted 9 February 1999

Abstract

Composite Ni±P±TiO2 and Ni±P±TiO2±PTFE layers were prepared by simultaneous electrodeposition of nickel and titanium dioxide

(anatase) with an addition of polytetra¯uoroethylene on a copper substrate from a solution in which TiO2 and PTFE particles were suspended

by stirring. The electrodeposition was carried out under galvanostatic conditions at a temperature of 293 K and current densities of j � 5 and

30 A/dm2. The phase composition of the layers was investigated by the X-ray diffraction method. The surface morphology of the layers was

examined by means of a metallographic microscope, a scanning microscope and a morphometric Supervist system. The percentage volume

fraction of TiO2 and PTFE as composite components and also their stereometric parameters were determined. It was found that the presence

of PTFE has an effect on reduction of the embedded TiO2 mean area in the Ni±P±TiO2±PTFE layers in comparison with the size of TiO2

grains in the Ni±P±TiO2 layers. q 1999 Elsevier Science S.A. All rights reserved.

Keywords: Amorphous materials; Electrochemistry; Titanium oxide; X-ray diffraction

1. Introduction

The modern production technology of composite layers

has been well known since the 1980s. The substance of such

layers is a system containing at least two coexisting phases

independently nascent during a composite formation

process. The composite layers can be obtained by electro-

chemical methods which allow the full use of course para-

meters of production process having an effect on the

properties of the obtained layer. Composite layers ®nd prin-

cipal use in tribology as abrasion-resisting layers and also as

electrode materials for catalysis of electrochemical reac-

tions. Additionally they are excellent for corrosion resis-

tance [1].

The application of electrode materials containing ruthe-

nium and titanium (e.g. ruthenium±titanium oxide anodes

[2], iridium±titanium oxide anodes [3], iridium±ruthenium±

titanium oxide anodes [4,5] and also ceramic oxide electro-

des based on a Ti/TiO2 [6±13] system) was the inspiration

for electrolytic formation of composite layers with similar

dispersed oxides. Many composite coatings are character-

ized by an amorphous or crystalline nickel matrix into

which oxides, NiO [14], Sc2O3 [15], Fe2O3 [16], RuO2

[17±19] or Al2O3 [20±22], were incorporated. As composite

components other chemical compounds can also be code-

posited with the nickel matrix, e.g. SiC [23], BN [24,25] or

PTFE [26±28]. The latter possibility of PTFE incorporation

into a composite layer is of particular importance because of

tribological applications. PTFE particles change the condi-

tions of electrode surface wettability. In this way PTFE can

improve the productivity of organic compounds during elec-

trochemical processes [23,29±31].

Recently, electroactive Ni±RuO2 materials were obtained

by codeposition of nickel in a Watts bath in which RuO2

particles were suspended by intensive mixing [32,33]. The

catalytic activity of those materials appeared to depend on

the RuO2 content in the layer, and the overpotential of

hydrogen evolution on such electrode materials was not

large. The feasibility of utilizing such electrodes for chlor-

alkali electrolysis was indicated on a laboratory scale.

The type of composite components and particle size

dispersed into the composite matrix determine utilizable

properties of nickel composite layers. Sajfullin [34] divided

these particles according to their size into ultramicro

(d � 1±100 nm), micro (d � 0:1±10 mm) and macro

(d . 10 mm). During the deposition of composite layers

usually micro- and macro-particles are used. The possibility

of incorporation of suspended particles from a galvanic bath

into the alloy structure is not the same for all types of parti-

cles. The size of particles embedded in the layer and their

quantity depends on the electrodeposition conditions. In the

case of such coatings, an analysis of the effect of current

conditions on size and number of particles incorporated into

Thin Solid Films 349 (1999) 43±50

0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.

PII: S0040-6090(99)00175-3

* Corresponding author. Fax: 1 48-596-929.

E-mail address: [email protected] (A. Budniok)

the layer has not been de®ned yet. The computer analysis of

microscopic surface images of electrolytic layers yields a

chance to ®x a correlation between parameters of production

and morphometric parameters of the layers.

The present study was undertaken in order to obtain the

amorphous Ni±P±TiO2 and Ni±P±TiO2±PTFE composite

layers. Our main aim was the determination of their struc-

ture and a comparative morphometric analysis.

2. Experimental

In order to obtain composite Ni±P±TiO2 and Ni±P±TiO2±

PTFE layers the following nickel plating bath was prepared

(g/dm3): 51 NiSO4´7H2O, 107 NH4Cl, 29 NaH2PO2´H2O, 10

CH3COONa, 8 H3BO3. To this was added 200 g TiO2

(anatase) and 40 g PTFE, respectively. TiO2 and PTFE

particles originally incorporated in the bath were examined

by morphometric analysis. The mean areas of TiO2 and

PTFE grains were measured and calculated to be respec-

tively: 6.7 mm2 and 4.9 mm2. On the basis of computer

counting the morphometric parameters like mean area of

TiO2 and similarly PTFE grains were determined. Reagents

from POCh Gliwice (Poland) of analytical purity and deio-

nized water were used for the solution. The suspension had a

pH of 4.8±5.1.

A copper plate substrate was mechanically polished on

abrasive paper and using diamond pastes, and next it was

treated for a few seconds with a dilute HNO3 solution (v/v

1:3) in order to remove impurities, and then the substrate

surface was activated for a few seconds in a dilute HCl

solution (v/v 1:3). After deposition of the layer the samples

were rinsed with water, acetone and then dried. The mass

increment of the layer was measured and estimated on the

basis of the mass difference before and after layer electro-

deposition.

The vessel diameter was 8 cm. The copper plates of one-

sided area 1 cm2 were placed parallel to the bottom of the

vessel. Electrodeposition was conducted in the electrolytic

cell containing 400 cm3 of the solution. The other side of the

plates was covered with non-conducting resin. The distance

between the plates and the surface of the solution was 5 cm.

The counter electrode was made of platinum mesh with the

geometric area of 1 dm2. The process of deposition for both

types of layers was carried out at 293 K and at the current

densities of j � 5 and 30 A/dm2 for 80 and 15 min, respec-

tively. Under these conditions the solution mixing rate of

300 rev/min was applied. The thickness of the deposited

layers was found to be about 25 mm.

B. èosiewicz et al. / Thin Solid Films 349 (1999) 43±5044

Fig. 1. The surface morphology of the Ni±P±TiO2 layers obtained at j � 5 A/dm2 (a) and at j � 30 A/dm2 (c) at 293 K, and appropriate micrographs (b,d) used

for the morphometric analysis.

Using the stereometric quantitative microscopy method

with a Nikon Alphaphot metallographic microscope

(magni®cation 450 £ ) and a computer Supervist system

for morphometric analysis, the surface morphology of the

Ni±P±TiO2 and Ni±P±TiO2±PTFE layers was examined.

The stereological parameters of those layers with a metric

character obtained by measurements (e.g. mean radius of

grain, mean perimeter of grain, mean area of grain, Martin's

diameters) and the ones with a topological character

obtained by computer counting (e.g. number of grains per

area unit, sum of grain boundaries per area unit, shape

factors, percentage volume fraction of composite compo-

nents TiO2 and PTFE), were measured and calculated.

The principle of the morphometric analysis is based on a

program which can make a digital conversion of a `living'

image of the layer surface observed under a microscope by

means of a digital camera and appropriate computer card

[35]. From the `living' image the area in the optical plane

containing subjectively the largest quantity of phase/phases

observed was chosen. Computer analysis of the digital

image yields a chance to display that image as a luminance

with 256 grey shades. It also allows us to select the searched

phase/phases with a precise grey shade grade from the

observed area. In this way the selected objects with an iden-

tical brightness threshold were marked in one colour and cut

out from the `frozen' image and then subjected to the

morphometric measurements. The micrographs used for

the morphometric analysis with the selected phases for the

Ni±P±TiO2 and Ni±P±TiO2±PTFE layers are shown in Figs.

1 and 2. Such prepared images allowed us to execute the

arithmetical and logical operations. The calculations were

given as average values. The measurements for each layer

were repeated 5 times. The measuring error for the obtained

results was about 3±5%.

The coating cross-sections were investigated using a digi-

tal scanning microscope (magni®cation 500 £ ). The phase

compositions of the layers were examined by X-ray diffrac-

tion using a Philips diffractometer and Cu Ka radiation.

3. Results and discussion

The composite Ni±P±TiO2 layer electrodeposited at

j � 5 A/dm2 (Fig. 1a) exhibits many microcracks in contrast

to the same layer obtained under high current conditions of

j � 30 A/dm2 (Fig. 1b). In both cases the TiO2 particles are

uniformly embedded into the nickel matrix with a tendency

to agglomeration.

The Ni±P±TiO2±PTFE layer electrodeposited at j � 5 A/

dm2 exhibits stresses causing separation from the copper

B. èosiewicz et al. / Thin Solid Films 349 (1999) 43±50 45

Fig. 2. The surface morphology of the Ni±P±TiO2±PTFE layers obtained at j � 5 A/dm2 (a) and at j � 30 A/dm2 (c) at 293 K, and appropriate micrographs

(b,d) used for the morphometric analysis.

substrate (Fig. 2a). PTFE particles create large agglomerate

clusters on the surface of the layers. In contrast, the same

layer obtained at j � 30 A/dm2 shows good adherence to the

copper substrate. The PTFE particles are homogeneously

embedded in the amorphous nickel matrix (Fig. 2b).

Composite Ni±P±TiO2 and Ni±P±TiO2±PTFE layers are

mat-grey with a visible white tarnish on the surface. Both

types of coatings obtained under low current conditions

exhibit microcracks on the surface. The increase in current

density improves the adherence to the copper substrate and

the uniform embedding of composite components into the

nickel matrix.

The microscopic cross-section image of a Ni±P±TiO2

layer electrodeposited at j � 5 A/dm2 is characterized by

more compact and homogeneous character of the coating

in comparison with the cross-section image obtained for the

same type of layer under high current conditions (Fig. 3a,b).

The microscopic cross-section image of Ni±P±TiO2±PTFE

layer electrodeposited at the current density of j � 5 A/dm2

indicates an incorporation of PTFE particles into the layer

causing an increase in its real surface (Fig. 4a). In the

instance of the same layer electrodeposited at j � 30 A/

dm2 the presence of PTFE in the layer does not cause

such considerable surface development as found in low

current density deposition (Fig. 4b). Consistently, an

increase in electrodeposition current density causes a

decrease in a degree of real surface development of Ni±P±

TiO2±PTFE layer. The cross-section of composite layers

with modi®ed PTFE indicates the presence of built-in

PTFE particles into the amorphous Ni±P matrix containing

also TiO2 as a composite component causing an increase in

real surface of these layers in comparison with the Ni±P±

TiO2 coatings. It is irrespective of electrodeposition current

conditions observed.

Besides, the morphology of the Ni±P±TiO2±PTFE layers

reveals a greater number of microcracks, which probably

in¯uence the increase in the surface roughness, than the

Ni±P±TiO2 layer morphology. The PTFE presence in the

Ni±P±TiO2±PTFE layer has an effect on the microscopic

shape inhomogeneity of embedded particles, their size and

distribution in the composite matrix in comparison with the

Ni±P±TiO2 inhomogeneity. That presence also increases the

compact Ni±P±TiO2 layer cracking, which loads to the

formation of dendritic Ni±P±TiO2±PTFE coating. The

PTFE incorporation in the amorphous matrix with titanium

dioxide also causes the growth of the sum of grain bound-

aries for all particles incorporated in the matrix. The Ni±P±

TiO2 layers contain a smaller number of titanium dioxide

B. èosiewicz et al. / Thin Solid Films 349 (1999) 43±5046

Fig. 3. Microscopic cross-section of the Ni±P±TiO2 layers obtained at

j � 5 A/dm2 (a) and at j � 30 A/dm2 (b) at 293 K.Fig. 4. Microscopic cross-section of the Ni±P±TiO2±PTFE layers obtained

at j � 5 A/dm2 (a) and at j � 30 A/dm2 (b) at 293 K.

grains per mm2 than the coatings with embedded PTFE. The

embedded particle mean area in case of the Ni±P±TiO2±

PTFE layer is smaller than the mean area of TiO2 particles

embedded in the Ni±P±TiO2 layer. The other stereological

parameters like mean radius of grain, mean perimeter of

grain, Martin's diameters of embedded particles in the

layer containing PTFE are also characterized by smaller

values than corresponding parameters values of the Ni±P±

TiO2 layer. In consequence of this fact, the Ni±P±TiO2±

PTFE layer microstructure distinguishes a very probable

increase in the real surface development.

The phase composition analysis of Ni±P±TiO2 and Ni±P±

TiO2±PTFE layers revealed that the structure of these layers

is characterized by the existence of an amorphous Ni±P

matrix with crystalline titanium dioxides as composite

components in the range of angles 2u corresponding to

the Ni±P system (Fig. 5). The diffraction pattern obtained

for Ni±P±TiO2±PTFE composite layer is cheracterized by

higher background level. This fact can be caused by the

presence of embedded PTFE in the amorphous Ni±P matrix.

However, there are crystalline titanium dioxides together

with the amorphous Ni±P matrix as was found in typical

diffraction patterns of Ni±P±TiO2 layers [36].

In a morphometric analysis the stereometric parameters

for both types of testing coatings such as number of grains,

percentage volume fraction of composite phases TiO2 and

PTFE, area of grain, radius of grain, perimeter of grain, sum

of grain boundaries, number of grains and their perimeter

per mm2, Martin's diameters and shape factors were

measured and evaluated. The percentage volume fractions

of TiO2 and PTFE particles embedded into the layers were

also determined.

Irrespective of deposition current conditions of Ni±P±

TiO2 layers it was ascertained that in the composite layer

conglomerates of TiO2 are built in with a mean area of

surface about a dozen times higher than TiO2 particles

primarily existing in the bath (Table 1). This tendency to

conglomeration appears even more at lower current density

of electrodeposition. An argument for this fact could be

ascertained in a smaller number of TiO2 grains and a smaller

sum of all grain boundaries per 1 mm2 occurring on the

surface.

The values of Martin's diameters V and H (vertical and

horizontal lengths of sections dividing a grain into two parts

with the same area) and shape factors k1 and k2 of titanium

dioxide grains are comparatively contained in a limit of

error and in both cases point at regular, spherical shape of

built-in TiO2 particles. The micrographs of Ni±P±TiO2

surface layers also con®rm this.

Also for Ni±P±TiO2±PTFE layers it was found that inde-

pendently of the deposition current such layer conglomer-

ates of TiO2 1 PTFE are built in with mean area of surface

about a dozen times higher than the mean surface area of

TiO2 grains and simultaneously repeatedly higher than the

surface area of PTFE grains originally incorporated into the

bath (Table 1). However, the mean surface area of TiO2 1PTFE grains embedded into Ni±P±TiO2±PTFE layer is

considerably smaller than the corresponding size of TiO2

particles incorporated into Ni±P±TiO2 layer. Besides, the

fact that the tendency to conglomeration for Ni±P±TiO2±

PTFE layers is smaller than in the case of Ni±P±TiO2 layers

is surely caused by the in¯uence of PTFE presence. Consis-

tently, in spite of a comparable volume fraction of compo-

site component in both types of layers, the number of grains

per area unit in the case of Ni±P±TiO2±PTFE layer is

considerably higher than for Ni±P±TiO2 layer. The increase

B. èosiewicz et al. / Thin Solid Films 349 (1999) 43±50 47

Fig. 5. X-ray diffraction patterns of Ni±P±TiO2 and Ni±P±TiO2±PTFE layers.

in deposition current density of Ni±P±TiO2±PTFE layer

causes ®rst of all the reduction of percentage contents of

composite components, which contributes to the diminution

of grain number per area unit. This smaller number of grains

shows an almost identical value of sum of grain boundaries

per area unit. On this basis we assume that for Ni±P±TiO2±

PTFE layers at higher deposition currents a conglomeration

effect occurs. The proof of this fact is higher values of grain

areas and values of Martin's diameters at higher deposition

current density of Ni±P±TiO2±PTFE layer. Composite Ni±

P±TiO2 layers are characterized by a higher number of TiO2

than comparable Ni±P±TiO2±PTFE layers which can be

explained by the higher electrical resistance during the elec-

trodeposition process of Ni±P±TiO2±PTFE layer in conse-

quence of the presence of non-conducting PTFE particles

present in the electrolyte.

Based on the morphometric analysis of both types of

layers the detailed classi®cation of built-in TiO2 and PTFE

grains with regard to their area size was done. For Ni±P±

TiO2 layer electrodeposited at j � 5 A/dm2 over 48% of

TiO2 grains with mean surface area below 15 mm2 and

simultaneously about 30% of the same type of grains and

their agglomerates with the area size over 100 mm2 were

found (Fig. 6). In the instance of the same layer obtained

at the current density of j � 30 A/dm2 the number of grains

from the range below 15 mm2 shows a gain of 20%, while

the number of built-in large grains drops down to 25%.

From these results it is unequivocally evident that the

increase in deposition current density results in a higher

number of built-in grains with the least area of 15 mm2

creating in this way a more homogeneous structure of the

layer surface. The smaller are the particles of the dispersed

phase the easier are the conditions of their embedding into

the amorphous nickel matrix. This effect can be connected

with the fact that smaller particles move faster in an electric

®eld and the motion of the larger particles is slower.

B. èosiewicz et al. / Thin Solid Films 349 (1999) 43±5048

Table 1

Stereometric parameters for the Ni±P±TiO2 and Ni±P±TiO2±PTFE layers obtained at 293 K

Type of layer Deposition

current

density (A/

dm2)

Percentage

volume

fraction of

TiO2 1

PTFE (%)

Area of

grain

(mm2)

Radius of

grain (mm)

Perimeter

of grain

(mm)

Number of

grains/

mm2

Perimeter

of grains/

mm2

Martin's diameters (mm) Shape factors

H V k1 k2

Ni±P±TiO2 5 39.16 127.63 3.62 38.28 2994.35 114631.4 7.59 6.81 1.82 2.69

30 37.39 105.65 3.91 38.31 3453.39 132330.5 7.32 7.81 1.96 2.44

Ni±P±TiO2±PTFE 5 32.08 72.27 1.81 19.39 5289.54 102580.5 3.31 3.71 1.77 3.05

30 24.35 80.23 3.12 29.88 3771.18 112707.6 6.04 6.12 1.80 2.77

Fig. 6. Histogram of TiO2 1 PTFE grain area embedded in the Ni±P±TiO2 and Ni±P±TiO2±PTFE layers obtained at 293 K.

The histogram obtained of Ni±P±TiO2±PTFE layer elec-

trodeposited at j � 5 A/dm2 con®rms an extension of

TiO2 1 PTFE grain number with the area contained in the

range below 15 mm2 up to 82% and the extenuation of grain

contents with the surface area larger than 100 mm2 to 8%. In

the case of the same layer obtained at higher current density

the grains with the least surface area embedded into the

matrix amount to 63% reducing the content of dispersed

TiO2 1 PTFE particles with a mean surface area over

100 mm2 up to 4%. On the basis of histograms characteriz-

ing the size of built-in grains in Ni±P±TiO2 and Ni±P±TiO2±

PTFE layers it can be inferred that the presence of PTFE in

the nickel bath containing crystalline TiO2 has an effect on

surface homogenizing and smoothing of the modi®ed layer

by means of PTFE in comparison with Ni±P±TiO2 layers

obtained under the same conditions. The incorporation of

non-polar PTFE particles into the bath in order to build in

the composite layer is conductive to the deposition of small

TiO2 1 PTFE grains from the surface range below 15 mm2.

On the basis of works of Diejniega and Ulberg [37] the

mechanism of polar particles being embedded into a metal-

lic composite matrix can be explained. The essence of the

assumed mechanism is an adsorption of metal ions on the

polar particles of dispersed phase in a bath. Consistently

with this mechanism after a voltage is applied to electrode

poles, the generation of an electric ®eld follows which

induces a dipole moment inside the dispersed phase. A

result of this is an orientation change and an interaction of

dispersed phase particles and nickel ions adsorbing on

polarized metal oxide particles inducing a dipole moment.

The electric ®eld action causes a movement of the polarized

phase with adsorbed nickel ions in the direction of the cath-

ode. Towards the cathode head free ions and also the

adsorbed ones on the dipole of dispersed phase on the

metal oxide. In this way on the surface of the cathode,

reduction process of nickel ions occurs and a conglomera-

tion of metal oxides into a composite structure. Similarly

PTFE particles embedding can be explained assuming that

TiO2 is a carrier transporting PTFE in the direction of the

layer.

4. Conclusions

As the result of electrolytic nickel deposition in the

presence of TiO2 suspension, composite Ni±P±TiO2 layer

was obtained and after the addition into the bath of PTFE

also the modi®ed composite Ni±P±TiO2±PTFE layer. Both

types of layer exhibit the composite structure and are typi-

®ed by the amorphous Ni±P matrix containing embedded

crystalline titanium dioxide (anatase) and additionally

PTFE particles in the case of Ni±P±TiO2±PTFE layers.

TiO2 and PTFE particles create conglomerates as compo-

site components embedded into the composite layer. This

tendency to conglomeration appears more at lower current

density of the layer deposition.

The presence of PTFE has an effect on the reduction of

the mean surface area of TiO2 grains incorporated into the

Ni±P±TiO2±PTFE layer in comparison with the grain size of

TiO2 embedded in the Ni±P±TiO2 layer. PTFE particles also

change the real surface development of Ni±P±TiO2±PTFE

layers in comparison with the layers which are devoid of

such component.

Acknowledgements

This research was ®nanced by the Polish Committee for

Scienti®c Research (Project 7TO8 027 10). The authors

wish to thank Dr H. Jehn and Dr A. Zielonka for helpful

discussions and cooperation with Forschungsinstitut fuÈr

Edelmetalle und Metallchemie, SchwaÈbisch GmuÈnd

(Germany) in the framework of TEMPUS (Project 9032-

95).

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