Wear 267 (2009) 976–990
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Characteristics of electro-co-deposited Ni–Al2 O3 nano-particle reinforced metal matrix composite (MMC) coatings H. Gül ∗ , F. Kılıc¸, S. Aslan, A. Alp, H. Akbulut Sakarya University, Engineering Faculty, Department of Metallurgical & Materials Engineering, Esentepe Campus, 54187 Sakarya, Turkey
a r t i c l e
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Article history: Received 1 September 2008 Received in revised form 2 December 2008 Accepted 2 December 2008 Keywords: Electro-co-deposition Nano-reinforcement MMC Microstructure Friction Wear
a b s t r a c t In the present work, Ni/Al2 O3 metal matrix composite (MMC) coatings were prepared from a modified Watt’s type electrolyte containing nano-␣-Al2 O3 particles by direct current (DC) plating method to increase the surface hardness and wear resistance of the electrodeposited Ni. For these purposes, particle concentration in electrolyte, the effect of surfactant concentration, current density, and bath pH, etc. were investigated for optimization to obtain high quality coatings. Al2 O3 nano-powders with average particle size of 80 nm were co-deposited with nickel matrix on the steel substrates. The depositions were controlled to obtain specific thickness (between 50 and 200 m) and particle volume fraction in the matrix (between 0.03 and 0.12). The characterization of the coatings was investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD) facilities. The hardness of the resultant coatings was also measured and found to be 280–641 Hv depending on the particle volume in the Ni matrix. The effects of the surfactant on the zeta potential, co-deposition, and distribution of Al2 O3 particles in nickel matrix, and tribological properties of composite coatings were investigated. The results showed that the wear resistance of the nano-composites was approximately 2–3.5 times increased compared with unreinforced Ni deposited material. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Researches into the production of nano-composite coatings by electrolytic co-deposition of fine particles with metal from plating baths has been investigated by numerous investigators. Recently, researches and developments on metal matrix composite (MMC) coatings have come into prominence. Composite electroplating is a method of co-depositing fine particles of metallic, non-metallic compounds or polymers in the plated layer to improve material properties such as wear resistance, lubrication, or corrosion resistance [1–5]. Particle-reinforced MMCs generally exhibited wide engineering applications due to their enhanced hardness, better wear, and corrosion resistance when compared to pure metal or alloy [6–10]. They find application as coatings of engine cylinders, high-pressure valves, dies, in the production of musical instruments, drill fittings, car accessories, small aircraft microelectronics, microelectromechanical systems, precision engineering, aerospace, medical devices, marine, mining, agriculture and nuclear fields [10–15]. Research on electrodeposition of nano-composite coatings has been directed towards the determination of optimum conditions for their production, i.e. current density, temperature, particle concentration and bath composition [1–5]. In the literature, the
∗ Corresponding author. Tel.: +90 264 295 57 62; fax: +90 264 295 56 01. E-mail address:
[email protected] (H. Gül). 0043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.12.022
ceramic particles used for reinforcement is reported such as Al2 O3 , SiC, Cr2 O3 , TiO2 , MoS2 , WC, ZrO2 , diamond, etc. [2–20]. Ultrafine particles may agglomerate in the electrolyte bath due to high surface free energy. Furthermore, high ionic strength of electrolyte and high concentration of inert particles in electroplating bath make particles tend to get agglomerated. The agglomeration of particles in coatings would bestow poor mechanical properties to composite coatings. Hence, how to increase the co-deposition content and improve the distribution of ultrafine particles in composite coatings become to a crucial problem. Attempts to alleviate the above problem, the use of various physical and chemical methods have been reported by many researchers [5,12,16]. Kuo et al. [21] demonstrated that the homogeneity of the composite coating was promoted by decreasing the ionic concentration of the electrolyte solution and the use of specific ultrasonic energy treatment. Among these methods, the addition of metal cationic accelerants and organic surfactants in an electrolytic bath improved the amount and the distribution of co-deposited particles effectively [5]. The solid particles in the nickel plating bath affect the crystallization of the metal: they disturb the regular growth of nickel crystals and cause new nucleation sites to appear. The structure of the forming coating becomes increasingly more finely crystalline [11,22,23]. Jiaqiang et al. [24] have found that the crystallization temperature of the composite coating decreased due to the incorporation of nanometer alumina particles. Nano-particles at the grain boundaries prevent from dislocation movements and
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recrystallization procedures at elevated temperatures. Therefore, microhardness and thermal stability increase markedly. The good wear resistance mainly results from the fine-crystalline microstructure and the increased microhardness [25]. The purpose of this study is to deposit a layer, composed with soft Ni and hard nano-sized ceramic particles. Since the Ni coating has poor wear resistance it was suggested to increase the hardness of the Ni coating and wear resistance by optimizing process parameters of surfactant concentration, particle content in electrolyte and current density.
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Table 1 Bath compositions and electrodeposition conditions for nano-Al2 O3 reinforced MMC production. Nickel sulphate (Ni2 SO4 ·6H2 O) (g/l) Nickel chloride (NiCl2 ·6H2 O) (g/l) Boric acid (H3 BO3 ) (g/l) Sodyumdodecyl sulphate (g/l) Hexadecylpyridinium bromide (HPB) (mg/l) Alumina (Al2 O3 ) (g/l) (80 nm) pH Temperature (◦ C) Current density (A/dm2 ) Plating time (h)
300 50 40 0.1 0, 100, 200, 300 5, 10, 20, 30 4 45 1, 3, 6, 9 2
2. Experimental procedure The plating electrolyte for the electrodeposition of the nanoparticle reinforced MMCs was a Watt’s-type bath. The basic composition of the electrolyte and the plating conditions are shown in Table 1. The temperature of the electrolyte during deposition of nano-particle reinforced MMCs was controlled at 45 ◦ C (±2) and the pH was fixed at 4 (±0.2). The average particle size of the ␣Al2 O3 used in the experiment as reinforcing phase is 80 nm. Prior to deposition, zeta potential of the nano-particle suspended solution was measured with Malvern Zetasizer Nano Series Nano-ZS model instrument. In the electrodeposition experiments, three different surfactant (HPB) concentrations of 100, 200 and 300 mg/l and four different particle concentrations in electrolyte (5–10–20–30 g/l) were studied to obtain optimum conditions for homogeneous
microstructures and wear resistance. Plating time was 2 h for each electroplating run. The process steps are: (i) polishing of substrate, (ii) preparation of electrolyte with nano-particles, (iii) magnetic stirring for 20 h, (iv) ultrasonic dispersing for 0.5 h and (v) performing electro co-deposition for 2 h. Microstructural investigations were performed by JEOL-JSM 6060LV instrument. Rigaku D/MAX/2200/PC model device was used for X-ray analysis at speed of 1◦ /min and range between 10◦ and 100◦ . From the XRD pattern results, lattice distortion, and the grain size of the matrix material were calculated. The hardness of the coatings was measured by using a Vicker’s microhardness indenter (Leica VMHT) with a load of 50 g. Wear and friction tests were performed with a reciprocating ball-on disk CSM tribometer at room temperature with a rela-
Fig. 1. Cross sectional SEM micrographs of MMCs co-depositions showing distribution of Al2 O3 particles coated with the HPB concentrations of: (a) 0 mg/l, (b) 100 mg/l, (c) 200 mg/l and (d) 300 mg/l.
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Fig. 2. (a) Surfactant (HPB) concentration–Zeta potential relationship. (b) The volume percentage of co-deposited Al2 O3 particles in various concentrations of surfactant (HPB).
tive humidity of 55–65% under dry sliding conditions. Counterpart was a M50 steel ball (Ø 10 mm) suitable to DIN 50 324 and ASTM G 99-95a. The system allows measuring friction coefficient and time dependent depth profiles by using sensitive transducers. The depth transducer was vertically located on top of the sample. Since sub-micron sized hard particle reinforced Ni matrix composites are considered to use in some areas such as, small aircraft microelectronics, microelectromechanical systems, preci-
sion engineering, aerospace and medical devices, the wear test conditions was aimed to fit with these practical applications. Thus, the tests were performed at a constant applied load of 1.0 N with a sliding speed of 50 mm/s. The amount of wear of the composites after each tests were calculated by measuring the wear width and dept by using surface profilometry and low magnification optical micrographs. These measurements were also compared with the vertical transducer depth profiles and thus,
Fig. 3. Cross sectional SEM micrographs of MMCs co-depositions showing distribution of Al2 O3 particles coated with the different particle concentrations in electrolyte: (a) 5 g/l Al2 O3 , (b) 10 g/l Al2 O3 , (c) 20 g/l Al2 O3 and (d) 30 g/l Al2 O3 .
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the wear rate of the composites and the steel ball was determined.
venting agglomeration of particles resulted from electrophoretic migration.
3. Results and discussions
3.2. Effect of particle concentration in electrolyte on deposition
3.1. Effect of surfactant on deposition
The studies that were carried out for determining the optimum particle concentration in electrolyte was performed in a constant HPB concentration of 200 mg/l. Increasing the particle concentration in electrolyte resulted in increasing the amount of the nano-alumina particles, which are shown as dark impurities (detected by EDS), at the deposited layer without agglomeration. Fig. 3 shows the cross sectional SEM microstructures for different particle charge in electrolyte. The increment of the nano-alumina particles in the deposited Ni layer by increasing particle concentration in electrolyte is almost linear. This relation is shown in Fig. 4. However, the increase in the amount of co-deposited alumina particles is not quantitatively comparable with the amount of particles charged to the electrolyte. For example, by increasing the amount of particles from 5 to 30 g/l with six times, the particle concentration in the deposited layer was increased only 2.7 times. The co-deposition of Al2 O3 by the electrodeposition technique may be attributed to the adsorption of Al2 O3 particles on the cathode surface, as suggested by Guglielmi’s two-step adsorption model. Once the particle is adsorbed, metal begins building around the cathode slowly, encapsulating and incorporating the particles [26]. Fig. 4b shows comparable XRD-patterns depending on the particle content at the deposited layer. As previously indicated by SEM microstructures and quantitive analysis, XRD results also demonstrated that nano-Al2 O3 content is increased when the alumina particle charge was increased in the electrolyte. For comparison, unreinforced Ni coating was also deposited and unreinforced Ni coating exhibited a preferential growth on the crystal face (2 0 0), once nano-Al2 O3 was co-deposited, the growth orientation of the Ni/Al2 O3 composite coating was changed from crystal face (2 0 0) to (1 1 1). The co-deposition of Al2 O3 obviously affects the relative intensity corresponding to different crystal faces, with increase of Al2 O3 in the coatings, the relative intensity corresponding to crystal face (1 1 1) increases, but the relative intensity of crystal face (2 0 0) decreases. For composite coatings, the peak of crystal face (2 2 0) is not obvious; peak of (3 1 1) becomes strong with increasing of Al2 O3 content. Erler et al. [27] demonstrated that the XRD patterns of nickel nano-composite coatings indicate changes in texture of such coatings which are dependent on the plating current density and the particle content in electrolyte. Chang et al. [28] was also presented similar results.
The effect of surfactant HPB on the co-deposited nano-alumina reinforced coatings was studied with constant particle concentration in electrolyte of 10 g/l and applied current of 3.0 A/dm2 conditions. Fig. 1 shows the cross sectional SEM micrographs of deposited layers. The dark constituents in Fig. 1 denote Al2 O3 particles which were characterized by EDS analysis. It appears that the volume fraction of co-deposited Al2 O3 particles increased with increasing surfactant concentration in the electrolyte. Introducing 100 mg/l HPB results to reveal inhomogeneous particle distribution compared with the electrodeposited layer without HBP addition. At this amount of HPB addition a few amount of nano-alumina particles were co-deposited and significant amount of segregation was observed. It was detected that co-deposited alumina particle concentration at the deposited layer was increased after a certain amount of HPB (approximately 200 mg/l) addition to the electrolyte. This increment in Al2 O3 vol.% by increasing HPB was also illustrated in Fig. 2b. When the concentration of HPB increased to 300 mg/l, it clearly appeared that amount of deposited particles and homogeneity was increased. From these results, it can be concluded that the addition of 100 mg/l is not enough for increasing the surface tension between the particles and electrolyte. As can be seen from the Fig. 2a, zeta potential of the electrolyte is very close to 0 mV at the condition when 100 mg/l HPB added to the electrolyte. Increasing the amount of the surfactant addition beyond 100 mg/l leaded to increase zeta potential value which is a key value for suspending the nano-particles and preventing agglomeration during the electrodeposition process. Similar results were also reported by different authors who studied electrodeposition but in different systems. For example, Chen et al. [5] demonstrated that the enhancement is considered to be associated with the modification of the surface charge of the particles by the absorbed molecules or ions, thereby promoting electrophoretic migration of the suspended particles. Naturally, ceramic particles surrounded with HPB molecules increased with increasing HPB concentration in electrolyte and, agglomeration trend of nano-particles was prevented. The microstructural and quantitive analysis, therefore, showed that increasing the amount of HPB resulted in increasing co-deposition of nano-alumina particles in nickel metal matrix because of pre-
Fig. 4. The effect of particle content in electrolyte on: (a) co-deposited Al2 O3 particles and, (b) XRD patterns.
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Fig. 5. Cross sectional SEM micrographs of MMCs co-depositions showing distribution of Al2 O3 particles coated with current densities: (a) 1 A/dm2 , (b) 3 A/dm2 , (c) 6 A/dm2 and (d) 9 A/dm2 .
3.3. Effect of current density on deposition Figs. 5 and 6 show the effect of current density on the volume percentage of Al2 O3 in coatings. In order to determine the current density on the microstructural and wear properties of the resul-
tant coatings, the surfactant (HPB) and particle concentration in electrolyte were kept constant as 200 mg/l and 20 g/l, respectively. It is observed that the volume percentage of Al2 O3 in coatings increases initially with the current density and reaches to the maximum at 3.0 A/dm2 . Beyond this current density, the co-deposited Al2 O3 content is almost stable. The observed maxima in the curve of current density versus volume percentage of Al2 O3 in coatings can be attributed to the transition from an activation-controlled metal deposition reaction to a diffusion-controlled of particles transfer. From these results it can be concluded that increasing current density beyond a certain value is not so significant for dispersing of nano-particles into Ni coating layer. As also reported by Chang et al. [28], increasing current density leads to decrease in the grain size of Ni matrix and does not significantly increase the amount of nano-particles in the deposited layer. Nevertheless, decreasing the matrix grain size shows beneficial effect in terms of increasing hardness, wear resistance, etc. 3.4. Microhardness of composite coatings
Fig. 6. The volume percentage of co-deposited Al2 O3 particles in various current densities.
Fig. 7 compares the microhardness of unreinforced Ni and Ni–Al2 O3 composite coatings. The microhardness of the coatings basically increases with increasing dispersed nano-particle content. The improvement in the hardness of composite coatings is related to the dispersion hardening effect caused by Al2 O3 particles in the composite matrix, which obstructs the shift of disloca-
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Fig. 7. Effect of: (a) surfactant concentration, (b) particle concentration in electrolyte and (c) current density on microhardness (numbers in brackets denotes standard deviation of microhardness).
tion in nickel matrix [26]. From Fig. 7, it can be seen that the microhardness of Ni–Al2 O3 composite coatings is higher than that of pure Ni coating and increases with the increase of the nanoAl2 O3 content in plating solution. There are three reasons for the increase in hardness [12,16]: particle-strengthening, dispersionstrengthening and grain refining. Particle-strengthening is related to the incorporation of hard particles and volume percent above 20%. Dispersion-strengthening is associated with the incorporation of fine particles (
