ELECTRIDEPOSITION AND WEAR BEHAVIOR OF NANO-STRUCTURED Cr-WC COMPOSITE COATINGS FROM A TRIVALENT CHROMIUM BATH

Transcription

1 2nd International Conference on Ultrafine Grained & Nanostructured Materials (UFGNSM) International Journal of Modern Physics: Conference Series Vol. 5 (2012) World Scientific Publishing Company DOI: /S ELECTRIDEPOSITION AND WEAR BEHAVIOR OF NANO-STRUCTURED Cr- COMPOSITE COATINGS FROM A TRIVALENT CHROMIUM BATH A. AMADEH School of Metallurgy and Materials Engineering, University college of Engineering, University of Tehran, Tehran, Iran amadeh@ut.ac.ir S. NADALI Islamic Azad University, Tehran South branch. Tehran, Iran Nadeali.said@gmail.com S.M. LARI BAGHAL School of Metallurgy and Materials Engineering, University college of Engineering, University of Tehran, Tehran, Iran mohlari2005@yahoo.com H. MORADI Maham Industries. Tehran, Iran Moradi.hadi@gmail.com Electrodeposition of nano-structured Cr- had been carried out from a trivalent chromium bath using a square shaped pulse current. The average size of particles was 70 nm. The effect of sodium saccharin and sodium dodecyl sulfate (SDS) additives as well as pulse electroplating parameters such as current density, duty cycle and frequency on the amount of incorporated particles and morphology of the coatings was investigated. The structure and morphology of the coatings were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. In addition, the hardness and tribological behavior of the coatings were investigated by microhardness and pin on disk methods, respectively. The results showed that although the addition of saccharin and SDS decreased the amount of particles in the coating, but reduced the size of agglomerates which, in turn, increased the wear resistance of the coatings. Also, the optimum wear resistance was achieved at the current density of 8 A dm 2, duty cycle of 50% and frequency of 10 Hz. Keywords: Electrodeposition, Wear, Cr-, Trivalent, pulse current. 737

2 738 A. Amadeh et al. 1. Introduction Ceramic particles-reinforced composite coatings have been widely developed over the past decades due to their high hardness, good wear and corrosion resistance compared to pure metal or alloy coatings. 1 Electroplating is one of the suitable methods for producing the nano-structured composite coatings because it provides a smooth surface, good bonding between the metal matrix and ceramic particles, and higher microhardness and wear resistance. 2 In this method the ceramic particles suspended in electroplating bath enter to the coating during plating process. 3 The properties of composite coatings mainly depend on both the matrix phase and also, the amount and distribution of co-deposited particles. 3 The agglomeration of nanoparticles in the coatings will results in poor mechanical properties. 4 In order to have a good dispersion of ceramic particles in the electrolyte, a change in the inter-particle interfacial region by either chemical or physical methods is necessary. For this purpose different alternatives such as ultrasonic vibration, reduction in the concentration of ionic solution, reduction in volumetric percentage of ceramic particles in the electrolyte, and addition of the surfactants have been used. 5 In this study, the effect of some additives such as Sodium Dodecyl Sulfate (SDS) and saccharin as well as pulse electrodeposition parameters namely current density and duty cycle on morphology, hardness and wear behavior of nano-structured Cr- composite coatings has been investigated. 2. Experimental procedure Electrodeposition of Cr- nano-composite coatings was carried out in an aquous electrolyte containing 100 g/l Cr 2 (SO 4 ) 3, 10 g/l CrCl 3, 40 g/l NaCOOH, 15 g/l KCl and 40 g/l H 3 BO 3. The average particle size of used powder was 70 nm. Sodium saccharin and SDS were added to the electrolyte as grain refiner and surfactant agents, respectively. The anode was 95% lead-5% Antimony and the cathode St14 steel. Before plating, the cathode samples were ground on SiC papers from 400 to 1200 grits and then cleaned in a solution of 200 cc phosphoric acid, 100 cc ethanol and 100 cc distilled water. During electroplating process, the solution ph was maintained at 2.5 and its temperature at 27 C. Electroplating time was 100 minutes and the thickness of the coatings was 8 µm. The morphology of the coatings was studied using scanning electron microscope (SEM) and the amount of tungsten carbide nano-particles in the coating was measured by EDS analyzer. The hardness of the coatings has been measured using a Vickers microhrdness indenter under 25 g load. The wear tests were carried by means of pin on disk tribotester under 5 N load at the speed of 0.75 m/s. The 6 mm diameter pin was made of AISI steel. The pin ran over the disk on a circular path of 3 cm diameter and the total sliding distance was 200 m.

3 Electrodeposition and Wear Behavior of Nano-Structured Cr- Coatings Results and discussion 3.1. Effect of SDS addition Fig. 1 shows the SEM image of surface morphology of nano-composite coatings at two different concentrations of SDS and Fig. 2 presents the effect of SDS content on incorporation in the coating. As observed, with increasing the SDS concentration up to 1 g/l, the amount of particles in the coating increases, but it remains approximately constant by further increase in SDS concentration. Moreover, the agglomeration of particles can be clearly observed in the absence of SDS in the bath. In fact, the presence of surfactant agent not only prevents the agglomeration of ceramic particles, but also encourages the content in the coating. When the SDS concentration exceeds 1 g/l, a part of it remains on ceramic particles and generates a repulsive force that prevents the presence of more particles on cation surface. a Fig. 1. Surface morphology of Cr- composite coatings in the presence of a) 0, b) 2 g/l SDS (1g/L saccharin, current density= 8 A/dm 2, frequency= 10 HZ, Duty cycle= 50% and concentration= 10 g/l). b Fig. 2. Effect of SDS concentration on amount of particles in Cr- coatings.

4 740 A. Amadeh et al. Fig. 3 reveals the variation of wear rate and hardness of the coatings as a function of SDS concentration. An increase in SDS concentration from 0.2 to 2 g/l increases the coating s microhardness from 595 to 680 HV which can be attributed to the increase in the co-deposition of particles in the coating. It can also be seen in this figure that with increasing the SDS concentration from 0.2 to 1 g/l, the wear rate decreases from 0.22 mg/m to 0.16 mg/m, but further increase in SDS concentration reduces the wear resistance of the coating. Fig. 3. Variation of wear rate and hardness of the coatings as a function of SDS concentration 3.2. Effect of saccharin addition Fig. 4 shows the SEM image of surface morphology of Cr- nano-composite coatings at two different concentrations of saccharin. Fig. 5 depicts the effect of saccharin concentration on weight percent of particles in the coating. It can be observed that the amount of particles in the coating reduces by increasing the saccharin content in electrolyte. Because saccharine cause to smooth the surface and remove the preferred places for settling, restraint and of particles and particles expose in fluid current and move hydrodynamic shear force and separate from coat surface. On the other hand, with increasing the saccharin concentration from 0.5 to 3 g/l the hardness of the coating increases from 620 HV to 660 HV due to grain refining effect of saccharin which reduces the grain size of chromium matrix. a b Fig. 4. SEM image of surface morphology of Cr- nano-composite coating in the presence of a) 0.5, b) 3 g/l saccharin (SDS=1g/L, concentration=10 g/l, current density=8 A/dm 2, Frequency= 10 Hz and Duty cycle= 50%).

5 Electrodeposition and Wear Behavior of Nano-Structured Cr- Coatings 741 Fig. 5. Effect of saccharin content on amount and hardness of Cr- composite coatings Effect of current density Fig. 6 demonstrates the surface morphology of Cr- composite coatings obtained at different current densities and Fig. 7 shows the effect of current density on content and wear rate of Cr- coatings. An increase in current density up to 15 A/dm 2 increases weight percent in the coating whereas further increase in the same parameter has an inverse effect. According to Celis model 6, at low current densities, the charge transfer stage controls the precipitation kinetics which leads to slow reduction of chromium ions on cathode surface. Therefore, the detachment of loosely adsorbed particles is quite possible and their incorporation in the coatings decreases. The decrease in content beyond 15 A/dm 2 should be attributed to the evolution of hydrogen at cathode surface which prohibits the access of both free and ceramic containing chromium ions to the cathode. 7 With increasing the current density up to 8 A/dm 2, the wear rate reduces from 0.24 mg/m to 0.12 mg/m (Fig. 7) that is in accordance with coating hardness and Archard s law. In fact, an increase in current density results in the presence of more which, in turn, increases the coating s hardness. The reduction on wear resistance of the coatings beyond 8 A/dm 2 could be attributed to the presence of microcracks which are frequently existent in chromium coatings specially at high current densities as can be observed in Fig. 6d.

6 742 A. Amadeh et al. a b c d Fig. 6. Surface morphology of Cr- nano-composite coatings at different current densities: a) 2A/dm 2, b) 8A/dm 2, c) 15A/dm 2, d) 20A/dm 2 (SDS=1g/L, saccharin=1g/l, concentration=10 g/l, duty cycle=50% and frequency=10hz). Fig. 7. Effect of current density on content and wear rate of Cr- composite coatings (SDS=1g/L, saccharin=1 g/l, concentration 10 g/l, frequency=10 Hz and duty cycle=50%) Effect of duty cycle Fig. 8 shows the microhardness and wear rate of the coatings obtained at different duty cycles. As seen, with increasing duty cycle up to 90% the coating s hardness decreases from 680 HV to 610 HV due to the presence of more particles in the coating at low frequencies. EDS analysis revealed that at the duty cycle of 10% the weight percent of particles is 2.6% which decreased to 1% at the duty cycle of 90%. At high duty cycles, the on-time period is long and so, the free chromium ions can access the cathode

7 Electrodeposition and Wear Behavior of Nano-Structured Cr- Coatings 743 surface faster than ceramic particle carrier ions. Therefore, content in the coating increases as the duty cycle reduces. The presence of more particles according to Archard s law enhances the microhardness and wear resistance of the coating. Fig. 8. Variation of microhardness and wear rate of Cr- coatings as a function of pulse frequency composite (SDS=1g/L, saccharin=1 g/l, concentration=10 g/l, current density=8 A/dm 2 and frequency=10 Hz). 4. Conclusions 1- Addition of SDS encouraged the co-precipitation of particle in the coating, but the addition of saccharin had an inverse effect. 2- An increase in current density up to 8 A/dm 2, enhanced the wear resistance of Cr- nano-composite coating, but further increase in this parameter had an inverse effect. 3- By increasing the duty cycle, the amount of particles as well as hardness and wear resistance of the coating reduced. References 1. B. Du, B. Xu, S. Dong, H.Yang and Y. Wu, Surf. Coat. Technol. 192, 311 (2005). 2. A. Karimpoor, M.Sc. Thesis, Toronto University, Department of Materials Science and Engineering, 18 (2001). 3. W. Wang, F.Y. Hou, H. Wang and H.T. Guo, Scrip. Mater. 53, 613 (2005). 4. A.F. Zimmerman, G. Palumbo, K.T. Aust and U. Erb, Mater. Sci. Eng. A 328, 137 (2002). 5. S. L. Kuo, Y.C. Chen and M.D. Ger, Mater. Chem. Phy. 86, 5 (2004). 6. J. Celis, J Roos and C.Buelens, J. Electrochem. Soc. 134, 1402 (1987). 7. Z. Abdelhamid and I.M. Ghayad, Surf. Interf. Analy. 37, 573 (2005).