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  1. 1. Thin Solid Films 349 (1999) 43±50 Composite layers in Ni±P system containing TiO2 and PTFE B. èosiewicz, A. Stepien, D. Gierlotka, A. Budniok* Ë Â 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 ˆ 5 and 30 A/dm 2. 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 posited with the nickel matrix, e.g. SiC [23], BN [24,25] or PTFE [26±28]. The latter possibility of PTFE incorporation The modern production technology of composite layers into a composite layer is of particular importance because of has been well known since the 1980s. The substance of such tribological applications. PTFE particles change the condi- layers is a system containing at least two coexisting phases tions of electrode surface wettability. In this way PTFE can independently nascent during a composite formation improve the productivity of organic compounds during elec- process. The composite layers can be obtained by electro- trochemical processes [23,29±31]. chemical methods which allow the full use of course para- Recently, electroactive Ni±RuO2 materials were obtained meters of production process having an effect on the by codeposition of nickel in a Watts bath in which RuO2 properties of the obtained layer. Composite layers ®nd prin- particles were suspended by intensive mixing [32,33]. The cipal use in tribology as abrasion-resisting layers and also as catalytic activity of those materials appeared to depend on electrode materials for catalysis of electrochemical reac- the RuO2 content in the layer, and the overpotential of tions. Additionally they are excellent for corrosion resis- hydrogen evolution on such electrode materials was not tance [1]. large. The feasibility of utilizing such electrodes for chlor- The application of electrode materials containing ruthe- alkali electrolysis was indicated on a laboratory scale. nium and titanium (e.g. ruthenium±titanium oxide anodes The type of composite components and particle size [2], iridium±titanium oxide anodes [3], iridium±ruthenium± dispersed into the composite matrix determine utilizable titanium oxide anodes [4,5] and also ceramic oxide electro- properties of nickel composite layers. Sajfullin [34] divided des based on a Ti/TiO2 [6±13] system) was the inspiration these particles according to their size into ultramicro for electrolytic formation of composite layers with similar (d ˆ 1±100 nm), micro (d ˆ 0:1±10 mm) and macro dispersed oxides. Many composite coatings are character- (d . 10 mm). During the deposition of composite layers ized by an amorphous or crystalline nickel matrix into usually micro- and macro-particles are used. The possibility which oxides, NiO [14], Sc2O3 [15], Fe2O3 [16], RuO2 of incorporation of suspended particles from a galvanic bath [17±19] or Al2O3 [20±22], were incorporated. As composite into the alloy structure is not the same for all types of parti- components other chemical compounds can also be code- 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 * Corresponding author. Fax: 1 48-596-929. conditions on size and number of particles incorporated into E-mail address: (A. Budniok) 0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0040-609 0(99)00175-3
  2. 2. 44 B. èosiewicz et al. / Thin Solid Films 349 (1999) 43±50 Fig. 1. The surface morphology of the Ni±P±TiO2 layers obtained at j ˆ 5 A/dm 2 (a) and at j ˆ 30 A/dm 2 (c) at 293 K, and appropriate micrographs (b,d) used for the morphometric analysis. the layer has not been de®ned yet. The computer analysis of nized water were used for the solution. The suspension had a microscopic surface images of electrolytic layers yields a pH of 4.8±5.1. chance to ®x a correlation between parameters of production A copper plate substrate was mechanically polished on and morphometric parameters of the layers. abrasive paper and using diamond pastes, and next it was The present study was undertaken in order to obtain the treated for a few seconds with a dilute HNO3 solution (v/v amorphous Ni±P±TiO2 and Ni±P±TiO2±PTFE composite 1:3) in order to remove impurities, and then the substrate layers. Our main aim was the determination of their struc- surface was activated for a few seconds in a dilute HCl ture and a comparative morphometric analysis. 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- 2. Experimental deposition. The vessel diameter was 8 cm. The copper plates of one- In order to obtain composite Ni±P±TiO2 and Ni±P±TiO2± sided area 1 cm 2 were placed parallel to the bottom of the PTFE layers the following nickel plating bath was prepared vessel. Electrodeposition was conducted in the electrolytic (g/dm 3): 51 NiSO4´7H2O, 107 NH4Cl, 29 NaH2PO2´H2O, 10 cell containing 400 cm 3 of the solution. The other side of the CH3COONa, 8 H3BO3. To this was added 200 g TiO2 plates was covered with non-conducting resin. The distance (anatase) and 40 g PTFE, respectively. TiO2 and PTFE between the plates and the surface of the solution was 5 cm. particles originally incorporated in the bath were examined The counter electrode was made of platinum mesh with the by morphometric analysis. The mean areas of TiO2 and geometric area of 1 dm 2. The process of deposition for both PTFE grains were measured and calculated to be respec- types of layers was carried out at 293 K and at the current tively: 6.7 mm 2 and 4.9 mm 2. On the basis of computer densities of j ˆ 5 and 30 A/dm 2 for 80 and 15 min, respec- counting the morphometric parameters like mean area of tively. Under these conditions the solution mixing rate of TiO2 and similarly PTFE grains were determined. Reagents 300 rev/min was applied. The thickness of the deposited from POCh Gliwice (Poland) of analytical purity and deio- layers was found to be about 25 mm.
  3. 3. 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 ˆ 5 A/dm 2 (a) and at j ˆ 30 A/dm 2 (c) at 293 K, and appropriate micrographs (b,d) used for the morphometric analysis. Using the stereometric quantitative microscopy method out from the `frozen' image and then subjected to the with a Nikon Alphaphot metallographic microscope morphometric measurements. The micrographs used for (magni®cation 450 £ ) and a computer Supervist system the morphometric analysis with the selected phases for the for morphometric analysis, the surface morphology of the Ni±P±TiO2 and Ni±P±TiO2±PTFE layers are shown in Figs. Ni±P±TiO2 and Ni±P±TiO2±PTFE layers was examined. 1 and 2. Such prepared images allowed us to execute the The stereological parameters of those layers with a metric arithmetical and logical operations. The calculations were character obtained by measurements (e.g. mean radius of given as average values. The measurements for each layer grain, mean perimeter of grain, mean area of grain, Martin's were repeated 5 times. The measuring error for the obtained diameters) and the ones with a topological character results was about 3±5%. obtained by computer counting (e.g. number of grains per The coating cross-sections were investigated using a digi- area unit, sum of grain boundaries per area unit, shape tal scanning microscope (magni®cation 500 £ ). The phase factors, percentage volume fraction of composite compo- compositions of the layers were examined by X-ray diffrac- nents TiO2 and PTFE), were measured and calculated. tion using a Philips diffractometer and Cu Ka radiation. 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 3. Results and discussion means of a digital camera and appropriate computer card [35]. From the `living' image the area in the optical plane The composite Ni±P±TiO2 layer electrodeposited at containing subjectively the largest quantity of phase/phases j ˆ 5 A/dm 2 (Fig. 1a) exhibits many microcracks in contrast observed was chosen. Computer analysis of the digital to the same layer obtained under high current conditions of image yields a chance to display that image as a luminance j ˆ 30 A/dm 2 (Fig. 1b). In both cases the TiO2 particles are with 256 grey shades. It also allows us to select the searched uniformly embedded into the nickel matrix with a tendency phase/phases with a precise grey shade grade from the to agglomeration. observed area. In this way the selected objects with an iden- The Ni±P±TiO2±PTFE layer electrodeposited at j ˆ 5 A/ tical brightness threshold were marked in one colour and cut dm 2 exhibits stresses causing separation from the copper
  4. 4. 46 B. èosiewicz et al. / Thin Solid Films 349 (1999) 43±50 Fig. 3. Microscopic cross-section of the Ni±P±TiO2 layers obtained at Fig. 4. Microscopic cross-section of the Ni±P±TiO2±PTFE layers obtained j ˆ 5 A/dm 2 (a) and at j ˆ 30 A/dm 2 (b) at 293 K. at j ˆ 5 A/dm 2 (a) and at j ˆ 30 A/dm 2 (b) at 293 K. substrate (Fig. 2a). PTFE particles create large agglomerate current density deposition (Fig. 4b). Consistently, an clusters on the surface of the layers. In contrast, the same increase in electrodeposition current density causes a layer obtained at j ˆ 30 A/dm 2 shows good adherence to the decrease in a degree of real surface development of Ni±P± copper substrate. The PTFE particles are homogeneously TiO2±PTFE layer. The cross-section of composite layers embedded in the amorphous nickel matrix (Fig. 2b). with modi®ed PTFE indicates the presence of built-in Composite Ni±P±TiO2 and Ni±P±TiO2±PTFE layers are PTFE particles into the amorphous Ni±P matrix containing mat-grey with a visible white tarnish on the surface. Both also TiO2 as a composite component causing an increase in types of coatings obtained under low current conditions real surface of these layers in comparison with the Ni±P± exhibit microcracks on the surface. The increase in current TiO2 coatings. It is irrespective of electrodeposition current density improves the adherence to the copper substrate and conditions observed. the uniform embedding of composite components into the Besides, the morphology of the Ni±P±TiO2±PTFE layers nickel matrix. reveals a greater number of microcracks, which probably The microscopic cross-section image of a Ni±P±TiO2 in¯uence the increase in the surface roughness, than the layer electrodeposited at j ˆ 5 A/dm 2 is characterized by Ni±P±TiO2 layer morphology. The PTFE presence in the more compact and homogeneous character of the coating Ni±P±TiO2±PTFE layer has an effect on the microscopic in comparison with the cross-section image obtained for the shape inhomogeneity of embedded particles, their size and same type of layer under high current conditions (Fig. 3a,b). distribution in the composite matrix in comparison with the The microscopic cross-section image of Ni±P±TiO2±PTFE Ni±P±TiO2 inhomogeneity. That presence also increases the layer electrodeposited at the current density of j ˆ 5 A/dm 2 compact Ni±P±TiO2 layer cracking, which loads to the indicates an incorporation of PTFE particles into the layer formation of dendritic Ni±P±TiO2±PTFE coating. The causing an increase in its real surface (Fig. 4a). In the PTFE incorporation in the amorphous matrix with titanium instance of the same layer electrodeposited at j ˆ 30 A/ dioxide also causes the growth of the sum of grain bound- dm 2 the presence of PTFE in the layer does not cause aries for all particles incorporated in the matrix. The Ni±P± such considerable surface development as found in low TiO2 layers contain a smaller number of titanium dioxide
  5. 5. 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. grains per mm 2 than the coatings with embedded PTFE. The TiO2 layers it was ascertained that in the composite layer embedded particle mean area in case of the Ni±P±TiO2± conglomerates of TiO2 are built in with a mean area of PTFE layer is smaller than the mean area of TiO2 particles surface about a dozen times higher than TiO2 particles embedded in the Ni±P±TiO2 layer. The other stereological primarily existing in the bath (Table 1). This tendency to parameters like mean radius of grain, mean perimeter of conglomeration appears even more at lower current density grain, Martin's diameters of embedded particles in the of electrodeposition. An argument for this fact could be layer containing PTFE are also characterized by smaller ascertained in a smaller number of TiO2 grains and a smaller values than corresponding parameters values of the Ni±P± sum of all grain boundaries per 1 mm 2 occurring on the TiO2 layer. In consequence of this fact, the Ni±P±TiO2± surface. PTFE layer microstructure distinguishes a very probable The values of Martin's diameters V and H (vertical and increase in the real surface development. horizontal lengths of sections dividing a grain into two parts The phase composition analysis of Ni±P±TiO2 and Ni±P± with the same area) and shape factors k1 and k2 of titanium TiO2±PTFE layers revealed that the structure of these layers dioxide grains are comparatively contained in a limit of is characterized by the existence of an amorphous Ni±P error and in both cases point at regular, spherical shape of matrix with crystalline titanium dioxides as composite built-in TiO2 particles. The micrographs of Ni±P±TiO2 components in the range of angles 2u corresponding to surface layers also con®rm this. the Ni±P system (Fig. 5). The diffraction pattern obtained Also for Ni±P±TiO2±PTFE layers it was found that inde- for Ni±P±TiO2±PTFE composite layer is cheracterized by pendently of the deposition current such layer conglomer- higher background level. This fact can be caused by the ates of TiO2 1 PTFE are built in with mean area of surface presence of embedded PTFE in the amorphous Ni±P matrix. about a dozen times higher than the mean surface area of However, there are crystalline titanium dioxides together TiO2 grains and simultaneously repeatedly higher than the with the amorphous Ni±P matrix as was found in typical surface area of PTFE grains originally incorporated into the diffraction patterns of Ni±P±TiO2 layers [36]. bath (Table 1). However, the mean surface area of TiO2 1 In a morphometric analysis the stereometric parameters PTFE grains embedded into Ni±P±TiO2±PTFE layer is for both types of testing coatings such as number of grains, considerably smaller than the corresponding size of TiO2 percentage volume fraction of composite phases TiO2 and particles incorporated into Ni±P±TiO2 layer. Besides, the PTFE, area of grain, radius of grain, perimeter of grain, sum fact that the tendency to conglomeration for Ni±P±TiO2± of grain boundaries, number of grains and their perimeter PTFE layers is smaller than in the case of Ni±P±TiO2 layers per mm 2, Martin's diameters and shape factors were is surely caused by the in¯uence of PTFE presence. Consis- measured and evaluated. The percentage volume fractions tently, in spite of a comparable volume fraction of compo- of TiO2 and PTFE particles embedded into the layers were site component in both types of layers, the number of grains also determined. per area unit in the case of Ni±P±TiO2±PTFE layer is Irrespective of deposition current conditions of Ni±P± considerably higher than for Ni±P±TiO2 layer. The increase
  6. 6. 48 B. èosiewicz et al. / Thin Solid Films 349 (1999) 43±50 Table 1 Stereometric parameters for the Ni±P±TiO2 and Ni±P±TiO2±PTFE layers obtained at 293 K Type of layer Deposition Percentage Area of Radius of Perimeter Number of Perimeter Martin's diameters (mm) Shape factors current volume grain grain (mm) of grain grains/ of grains/ density (A/ fraction of (mm 2) (mm) mm 2 mm 2 dm 2) TiO2 1 PTFE (%) 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 in deposition current density of Ni±P±TiO2±PTFE layer grains with regard to their area size was done. For Ni±P± causes ®rst of all the reduction of percentage contents of TiO2 layer electrodeposited at j ˆ 5 A/dm 2 over 48% of composite components, which contributes to the diminution TiO2 grains with mean surface area below 15 mm 2 and of grain number per area unit. This smaller number of grains simultaneously about 30% of the same type of grains and shows an almost identical value of sum of grain boundaries their agglomerates with the area size over 100 mm 2 were per area unit. On this basis we assume that for Ni±P±TiO2± found (Fig. 6). In the instance of the same layer obtained PTFE layers at higher deposition currents a conglomeration at the current density of j ˆ 30 A/dm 2 the number of grains effect occurs. The proof of this fact is higher values of grain from the range below 15 mm 2 shows a gain of 20%, while areas and values of Martin's diameters at higher deposition the number of built-in large grains drops down to 25%. current density of Ni±P±TiO2±PTFE layer. Composite Ni± From these results it is unequivocally evident that the P±TiO2 layers are characterized by a higher number of TiO2 increase in deposition current density results in a higher than comparable Ni±P±TiO2±PTFE layers which can be number of built-in grains with the least area of 15 mm 2 explained by the higher electrical resistance during the elec- creating in this way a more homogeneous structure of the trodeposition process of Ni±P±TiO2±PTFE layer in conse- layer surface. The smaller are the particles of the dispersed quence of the presence of non-conducting PTFE particles phase the easier are the conditions of their embedding into present in the electrolyte. the amorphous nickel matrix. This effect can be connected Based on the morphometric analysis of both types of with the fact that smaller particles move faster in an electric layers the detailed classi®cation of built-in TiO2 and PTFE ®eld and the motion of the larger particles is slower. 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.
  7. 7. B. èosiewicz et al. / Thin Solid Films 349 (1999) 43±50 49 The histogram obtained of Ni±P±TiO2±PTFE layer elec- The presence of PTFE has an effect on the reduction of trodeposited at j ˆ 5 A/dm 2 con®rms an extension of the mean surface area of TiO2 grains incorporated into the TiO2 1 PTFE grain number with the area contained in the Ni±P±TiO2±PTFE layer in comparison with the grain size of range below 15 mm 2 up to 82% and the extenuation of grain TiO2 embedded in the Ni±P±TiO2 layer. PTFE particles also contents with the surface area larger than 100 mm 2 to 8%. In change the real surface development of Ni±P±TiO2±PTFE the case of the same layer obtained at higher current density layers in comparison with the layers which are devoid of the grains with the least surface area embedded into the such component. matrix amount to 63% reducing the content of dispersed TiO2 1 PTFE particles with a mean surface area over 100 mm 2 up to 4%. On the basis of histograms characteriz- Acknowledgements 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 This research was ®nanced by the Polish Committee for the nickel bath containing crystalline TiO2 has an effect on Scienti®c Research (Project 7TO8 027 10). The authors surface homogenizing and smoothing of the modi®ed layer wish to thank Dr H. Jehn and Dr A. Zielonka for helpful by means of PTFE in comparison with Ni±P±TiO2 layers È discussions and cooperation with Forschungsinstitut fur obtained under the same conditions. The incorporation of È Edelmetalle und Metallchemie, Schwabisch Gmund È non-polar PTFE particles into the bath in order to build in (Germany) in the framework of TEMPUS (Project 9032- the composite layer is conductive to the deposition of small 95). TiO2 1 PTFE grains from the surface range below 15 mm 2. 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