Effect of alumina particle size on properties of electrolessly deposited composite Ni-P/Al2O3

608 Journal of Chemistry, Vol. 47 (5), P. 608 - 612, 2009 Effect of alumina particle size on properties of electrolessly deposited composite Ni-P/Al2O3 Received 5 January 2009 HA MANH CHIEN1, MAI THANH TUNG1, DANG VIET ANH DUNG1, HOANG VAN HUNG2 1Dep. of Electrochemistry and Corrosion Protection, Hanoi University of Technology 2LILAMA Hot Deep Galvanizing Company Abstract In this study, effect of particle size on morphology, alumina content and hardness of electrolessly deposited composite Ni-P/Al2O3 layers was examined with diffrent sizes of codeposited alumina particles. Results on Scanning Electron Microscopy (SEM), Energy Dispersed Spectroscopy (EDS), X-ray Diffraction (XRD) and hardness measurements showed that as particles size increased composition of NiP matrix did not change remarkbly e.g the composition of the obtaiend matrix ranged from Ni90P10 to Ni91P9. Meanwhile, Al2O3 content in the composites increased as particle size decreased. As a result, hardness of the obtained layers increased with decreasing particle size and maximum hardness was achieved with particle size of 0.6μm. I - INTRODUCTION Autocatalytically deposited Ni-P amorphous alloys have many industrial applications because of their paramagnetic properties, excellent resistance to wear and corrosion, and high hardness [1 - 4]. Codepositing another metallic or non-metallic elements or abrasive/lubricative particles or combination of in binary Ni-P matrix can futher enhance these properties. Recently, electroless nickel composite coatings have gained more attention in reseach community due to their ability to produce coatings that posses improved wear, abrasion and lubrication properties than Ni-P deposits [3-5]. Several particles have been incoporated in the nickel matrix, and among them, the combinations that have received considerable attention are electroless nickel with SiC, B4C, Si3N4, Al2O3 and PTFE and the particles used are of micron size. Codeposition of the paricles depends on the size shape, density, concentration and method of suspension in the bath. In addition, it is very much dependent on the charge present on the particle [6, 7]. Aim of the present investigation is to study the particle size effect on properties of the deposits. Hence, in this study Ni-P/Al2O3 composites were prepared using alumina powders of sizes 0.6 μm, 8 μm and 15μm and their surface morphology, structure and hardness were investigated. II - EXPERIMENTAL Mild steel specimens (2.5 cm × 2.5 cm × 0.08cm) were used for plating electroless NiP and nanocomposite coatings containing alumina particles. Specimens were ultrasonically cleaned in acetone, cathodically cleaned in 10% sodium hydroxide solution at 1.5 A/dm2 for 5 min. then specimens were thoroughly rinsed with deionized water and immersed in 50 vol.% sulphuric acid solution for deoxidization for 30 s. After deionized water rinse, specimens were transferred immediately to the plating solution. 609 Electroless nickel bath was used for preparing the Ni-P and composite coatings. Bath contains nickel sulphate 21 g/l, propionic acid 3 g/l and small amounts of lead nitrate and operated at pH 4.0 - 4.5 and temperature 90±2 oC. Alumina powders used were of the size 0.8 μm, 6 μm and 15 μm (Acola) and the alumina concentration in solution was kept at 30 g/l. The plating time is 2 hours and the obtained layers have thickness of about 50 μm (thickness determined by electrochemical stripping techniques). After plating, the specimens were taken out then thoroughly rinsed with deionized water and air dried at room temperature. Then these specimens were used for further characterization. Surface morphology of the obtained electrolessly deposited layers was investigated using scanning Electron Microscopy (SEM). The composition of the alloys was determined by Energy Dispersion Spectroscopy (EDS). Texture and phase formation of the films were analyzed by X-ray Diffractometer (XRD). III - RESULTS AND DISCUSSION Figs. 1 and 2 show SEM images and corresponding EDS analyses of the electrolessly deposited composite Ni-P/Al2O3 with different particle sizes of 0.6 μm, 8 μm and 15μm. It can be seen that in all cases, the sedimentation of Al2O3 on the deposited layers due to high interaction energy between freshly formed Ni-P layer and Al2O3 particles in the solution do not occur. Since Al signals appear in EDS spectra (Fig. 2), it can be concluded that the Al2O3 particles are already trapped in the deposited layers. Composition of the deposited Ni-P alloys and content of dispersed Al2O3 particles in the layers obtained from EDS analyses are summarized in Tab. 1. Results show that the alumina particle size does not influence on composition of the Ni-P matrix e.g the composition of the Ni-P is in the range of Ni90P10 to Ni89P11. Meanwhile, Al2O3 content decreases with increasing alumina particle sizes. The maximum Al2O3 content 12%) was Fig. 1: SEM images of electrolessly deposited composite NiP-Al2O3 with different size of Al2O3 particles (a) without Al2O3;(b) d = 15 μm; (c) d = 6 μm; (d) d = 0.8 μm 610 obtained with particle size of 0.8 μm. This result can be explained based on the formation mechanism of composites. According to Hajdu, the formation of composite coatings consist of 3 steps: (i) transportation of Al2O3 particles from electrolyte to the surface through convection or diffusion (ii) adsorption of the particles on the surface and (iii) deposited Ni-P buries the particles [1, 2]. Thus smaller Al2O3 particles, which have higher specific surface energy compared to that of coarser ones, will tend to adsorb more on the surface. As a result, the amount of Al2O3 particles in the obtained Ni- P/Al2O3 composites will increase. Fig. 2: EDS spectra of electrolessly deposited composite Ni-P/Al2O3 with different size of Al2O3 particles (a) without Al2O3; (b) d = 15 μm; (c) d = 6 μm; (d) d = 0,8 μm Tab. 1: Composition of as-plated electroless Ni-P and composite coatings determined by EDS analysis Type of coatings Ni (wt.%) P (wt.%) Al2O3 (wt.%) Ni-P matrix Ni – P Ni – P – Al2O3 (0.8 μm) Ni – P – Al2O3 (6 μm) Ni – P – Al2O3 (15 μm) 89,0 79,2 71,9 82,8 11,0 8,8 8,1 8,2 0 12,0 10,0 9,0 Ni89P11 Ni90P10 Ni90P10 Ni91P9 Fig. 3 displays XRD patterns of the composite with alumina particle sizes of 0.8 μm, 6 μm and 15μm. It can be observed that the obtained Ni-P matrix has semi-amorphous structure, indicated by very low intensity of Ni(111) and Ni(111) signals. This result also agrees with the EDS analyses, which show that the phosphorus content in Ni-P layer is around 9 - 10%. According to the phase diagram, this composition corresponds to a partly amorphous structure of Ni-P and dispersed Ni crisstalline inside the amorphous matrix [1, 2]. This 611 structure also promotes a high corrosion resistant and high hardness of the Ni-P layer [1, 7]. It is also interresting to note that the Al2O3 peak intensities in XRD spectra also decrease with increasing particle size due to the increase of Al2O3 content in the composite layers. 0 100 200 300 400 500 600 700 800 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Cu {111} Ni {111}Al2O3 Al2O3 Ni {200} Cu 15μm 8μm 0.6μm Fig. 3: XRD patterns of electrolessly deposited composite Ni-P/Al2O3 with different size (d) of Al2O3 particles (a) without Al2O3; (b) d = 15 μm; (c) d = 8 μm; (d) d = 0.6 μm Table 2: Hardness of composite coatings Type of coatings HV hardness HB hardness Bare steel 216 216 Ni – P 238 226 Ni – P – Al2O3 (0.8 nm) 266 253 Ni – P – Al2O3 (6 μm) 272 258 Ni – P – Al2O3 (15 μm) 294 279 Hardness of composite layers with different sizes of co-depsosited alumina particles was measured and sumarized on Tab. 2. Results show that hardness increases with increasing alumina particle size. Since the composition of the matrix Ni-P does not change much as alumina particle size changes (Tab. 1), this result can be explained by the two effects occuring during the deposition process. The first effect is the decrease of the Al2O3 content in composite layers as particle size increases. The second reason is that as particles become smaller, the distribution of alumina particle will be more evently in the composite layers. IV - CONCLUSION Effect of alumina particle size on 612 morphology, particle content and hardness of electrolessly deposited composite Ni-P/Al2O3 layers was examined using alumina particles with sizes of 0.8μm, 6μm and 15μm. Results showed that as particles size increased, the composition of Ni-P matrix did not change remarkbly e.g the composition of the obtained matrix ranged from Ni90P10 to Ni91P9. Meanwhile, Al2O3 content in the composite increased as particle size decreased. As a result, hardness of the obtained layer increased with decreasing particle size and maximum hardness was achieved with particle size of 0.6m. Acknowledgement: The authors thank Ministry of Science and Technology for financial support of this work (project KHCB. 5.028.06). REFERENCES 1. Gllen Mallory. Electroless deposition technology, Surface Finishing Publisher, NewYork (2005). 2. Gugau. Funktionerelle Oberflaechen durch chemische Nickel, Eugen G Leuzer Verlag, Wurtt, (2006). 3. C. Sambucetti. Electroless Deposition, Society Publishers, Florida (1990). 4. Y. Okinaka, T. Osaka. Electroless Deposition Processes: Fundamentals and Applications, in Advances in Electrochemical Science and Engineering, VHC Publisher,Vol. 2, 55 - 116 (1994). 5. Sh. Alirezaei, S. M. Monirvaghefi, M. Salehi, A. Saatchi. Sur. Coat. Tech., 184, 170 - 175 (2004). 6. A. Grosjean, M. Rezrazi, P. Bercot. Surf. Coat. Technol., 130, 250 (2000). 7. I. Apachitei, J. Duszczyk, L. Katgerman, P. J. B. Overkamp. Scr. Mater., 38 (9), 1347 (1998). Corresponding author: Mai Thanh Tung Dep. of Electrochemistry and Corrosion Protection, Hanoi University of Technology.